Austroads - Guide to Bridge Technology Part 2 - Materials

Austroads GUIDE TO BRIDGE TECHNOLOGY

Part 2: Materials

Guide to Bridge Technology Part 2: Materials

Guide to Bridge Technology Part 2: Materials Summary The Guide to Bridge Technology, Part 2: Materials covers all aspects of the common building materials available to the engineer including concrete, metallic and non-metallic materials and timber. Part 2 of this guide discusses material characteristics, their properties, durability, construction issues when using such materials, and protection and preservation treatments. A detailed section on concrete reinforcing materials is also included. Keywords Bridge Materials, Concrete Materials, Concrete Characteristics, Concrete Durability, Concrete Steel Materials, Metallic Materials, Non-metallic Materials, Connections, Fibre Reinforced Polymers, Timber First Published September 2009 © Austroads Inc. 2009 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads. ISBN 978-1-921551-46-8 Austroads Project No. TP1564 Austroads Publication No: AGBT02/09 Project Manager Geoff Boully, VicRoads Prepared by Don Carter Ray Wedgwood

Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email: [email protected] www.austroads.com.au

This Guide is produced by Austroads as a general guide. Its application is discretionary. Road authorities may vary their practice according to local circumstances and policies. Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should rely on their own skill and judgement to apply information to particular issues.

Guide to Bridge Technology Part 2: Materials

Sydney 2009

Austroads profile Austroads purpose is to contribute to improved Australian and New Zealand transport outcomes by: 

providing expert advice to SCOT and ATC on road and road transport issues



facilitating collaboration between road agencies



promoting harmonisation, consistency and uniformity in road and related operations



undertaking strategic research on behalf of road agencies and communicating outcomes



promoting improved and consistent practice by road agencies.

Austroads membership Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government in Australia, the Australian Local Government Association, and New Zealand Transport Agency. Austroads is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its 11 member organisations:           

Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Transport and Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Planning and Infrastructure Northern Territory Department of Territory and Municipal Services Australian Capital Territory Department of Infrastructure, Transport, Regional Development and Local Government Australian Local Government Association New Zealand Transport Agency.

The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.

GUIDE TO BRIDGE TECHNOLOGY PART 2: MATERIALS

CONTENTS 1

INTRODUCTION AND GUIDE STRUCTURE................................................................ 1

1.1 1.2

Scope.............................................................................................................................. 1 Guide Structure............................................................................................................... 1

2

CONCRETE MATERIALS.............................................................................................. 3

2.1

Cement ........................................................................................................................... 3 2.1.1 Source .............................................................................................................. 3 2.1.2 Cement Reactivity – Setting Process ............................................................... 3 2.1.3 Cement and Durability ...................................................................................... 4 Supplementary Cementitious Materials (SCMs)............................................................. 5 Aggregates ..................................................................................................................... 5 2.3.1 Coarse Aggregate............................................................................................. 5 2.3.2 Fine Aggregates ............................................................................................... 6 2.3.3 Alkali Aggregate Reaction (AAR)...................................................................... 8 2.3.4 Lightweight Aggregates .................................................................................. 10 Admixtures.................................................................................................................... 10 Grouts and Mortars....................................................................................................... 12 2.5.1 Grouts ............................................................................................................. 12 2.5.2 Mortars............................................................................................................ 13 2.5.3 Mortar Pad Set Up .......................................................................................... 14 2.5.4 Mortar Pad Problems...................................................................................... 14

2.2 2.3

2.4 2.5

3

CONCRETE REINFORCING MATERIALS ................................................................. 15

3.1 3.2 3.3 3.4

Material Certification ..................................................................................................... 15 Heat Numbers............................................................................................................... 15 Country of Origin........................................................................................................... 15 Carbon Steel Reinforcement ........................................................................................ 16 3.4.1 Material Characteristics .................................................................................. 16 3.4.2 Method of Manufacture................................................................................... 17 3.4.3 Old Reinforcing Steels .................................................................................... 17 3.4.4 Packaging and Handling................................................................................. 18 3.4.5 Ductility ........................................................................................................... 18 3.4.6 Weldability ...................................................................................................... 19 3.4.7 Tack Welding .................................................................................................. 19 3.4.8 Welded Splices ............................................................................................... 20 3.4.9 Mechanical Splices ......................................................................................... 21 3.4.10 Mechanical Couplers ...................................................................................... 21 3.4.11 Rebending ...................................................................................................... 22 3.4.12 Protective Treatments..................................................................................... 22 3.4.13 Fire Damage to Steel...................................................................................... 22 Stainless Steel Reinforcement...................................................................................... 23 3.5.1 Construction Issues ........................................................................................ 23 3.5.2 Material Characteristics .................................................................................. 24 3.5.3 Supply............................................................................................................. 24 Prestressing Steel......................................................................................................... 25 3.6.1 Material Characteristics .................................................................................. 25 3.6.2 Material Certification ....................................................................................... 25 3.6.3 Material Properties.......................................................................................... 25 3.6.4 Modulus of Elasticity/Tangent Modulus .......................................................... 25 3.6.5 Secant Modulus .............................................................................................. 26

3.5

3.6

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3.7

3.6.6 Relaxation....................................................................................................... 27 3.6.7 Creep .............................................................................................................. 27 3.6.8 Anchorages..................................................................................................... 27 3.6.9 Construction Issues ........................................................................................ 28 Steel and Polymer Fibres ............................................................................................. 29

4

CONCRETE CHARACTERISTICS .............................................................................. 30

4.1

Design Issues ............................................................................................................... 30 4.1.1 Cover .............................................................................................................. 30 4.1.2 Congested Reinforcement .............................................................................. 31 4.1.3 Construction Joints – New Concrete Placed Against Old ............................... 31 4.1.4 Match Cast Joints ........................................................................................... 32 4.1.5 Re-entrant Corners ......................................................................................... 32 4.1.6 Over-specifying Concrete Strength................................................................. 32 4.1.7 Restricted Areas ............................................................................................. 33 4.1.8 Thin Elements................................................................................................. 33 Construction Issues ...................................................................................................... 33 4.2.1 Monitoring of Test Results .............................................................................. 33 4.2.2 Placement....................................................................................................... 34 4.2.3 Compaction..................................................................................................... 35 4.2.4 Finishing ......................................................................................................... 37 4.2.5 Bridge Decks .................................................................................................. 37 4.2.6 Deck Sealing................................................................................................... 39 4.2.7 Cold Weather Concreting ............................................................................... 39 4.2.8 Hot Weather Concreting ................................................................................. 40 Compaction and Density............................................................................................... 40 Curing ........................................................................................................................... 40 4.4.1 Moist Curing.................................................................................................... 41 4.4.2 Sealed Curing ................................................................................................. 41 4.4.3 Curing Compounds......................................................................................... 42 4.4.4 Plastic Sheeting .............................................................................................. 42 4.4.5 Self-curing Additives ....................................................................................... 42 4.4.6 Steam Curing.................................................................................................. 42 4.4.7 Heat Curing..................................................................................................... 43 Creep and Shrinkage.................................................................................................... 43 4.5.1 Shrinkage........................................................................................................ 44 4.5.2 Shrinkage Classification ................................................................................. 44 Cracking........................................................................................................................ 45 Crack Control................................................................................................................ 45 4.7.1 RTA Research Project .................................................................................... 48 4.7.2 Field Trial ........................................................................................................ 48 Investigation of Concrete Construction Quality............................................................. 49 4.8.1 Concrete Repair Techniques for Construction Defects .................................. 49 Special Concretes......................................................................................................... 50 4.9.1 Self-compacting Concrete .............................................................................. 50 4.9.2 Fibre Reinforced Concrete.............................................................................. 50 4.9.3 Reactive Powder Concrete ............................................................................. 51

4.2

4.3 4.4

4.5

4.6 4.7

4.8 4.9

5

CONCRETE DURABILITY........................................................................................... 52

5.1

Concrete Distress Mechanisms .................................................................................... 52 5.1.1 Reinforcement Corrosion................................................................................ 52 5.1.2 Damage Caused by Alkali Aggregate Reaction (AAR) ................................... 52 5.1.3 Delayed Ettringite Formation (DEF)................................................................ 53 Austroads 2009 — ii —

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5.2

5.3

5.1.4 Chloroaluminate Formation ............................................................................ 54 5.1.5 Carbonation .................................................................................................... 54 5.1.6 Chloride Attack ............................................................................................... 54 5.1.7 Sulphate Attack............................................................................................... 56 5.1.8 Acid Attack...................................................................................................... 56 5.1.9 Physical Damage............................................................................................ 57 5.1.10 Freeze/Thaw................................................................................................... 58 5.1.11 Fire Damage ................................................................................................... 58 Protection of Concrete in Adverse Environments ......................................................... 59 5.2.1 Curing ............................................................................................................. 59 5.2.2 Chemical Composition of Supplementary Cementitious Materials (SCMs)............................................................................................................ 60 5.2.3 SCM and Concrete Protection ........................................................................ 60 5.2.4 Waterproofing Membranes ............................................................................. 61 5.2.5 Protective Coatings......................................................................................... 61 5.2.6 Hydrophobic Impregnating Materials .............................................................. 62 5.2.7 Corrosion Inhibitors......................................................................................... 62 5.2.8 Cathodic Protection ........................................................................................ 62 Durability Assessment Techniques............................................................................... 63 5.3.1 Desk Top Investigation ................................................................................... 63 5.3.2 Visual Inspection............................................................................................. 63 5.3.3 Defect Mapping............................................................................................... 64 5.3.4 Measurement of Crack Development ............................................................. 64 5.3.5 Cover Surveys ................................................................................................ 64 5.3.6 In Situ Compression Testing........................................................................... 64 5.3.7 Ultrasonic Pulse Velocity ................................................................................ 64 5.3.8 Ground Penetrating Radar.............................................................................. 65 5.3.9 Permeability and Water Absorption ................................................................ 65 5.3.10 Concrete Sampling ......................................................................................... 65

6

METALLIC MATERIALS.............................................................................................. 66

6.1

Historic Development.................................................................................................... 66 6.1.1 Structural Iron and Steel ................................................................................. 66 6.1.2 Iron to Cast Iron to Wrought Iron to Steel ....................................................... 66 6.1.3 Cast Iron ......................................................................................................... 68 6.1.4 Wrought Iron ................................................................................................... 72 Structural Assessment of Existing Bridges ................................................................... 74 6.2.1 Yield Strength ................................................................................................. 74 6.2.2 Ultimate Tensile Strength ............................................................................... 75 6.2.3 Ductility ........................................................................................................... 75 6.2.4 Members and Connections............................................................................. 75 6.2.5 Weldability ...................................................................................................... 75 6.2.6 Fatigue............................................................................................................ 75 Structural Steel ............................................................................................................. 76 6.3.1 Modern Steel Properties ................................................................................. 76 6.3.2 Hardness ........................................................................................................ 76 6.3.3 Ductility ........................................................................................................... 79 6.3.4 High Strength Steel......................................................................................... 79 Aluminium ..................................................................................................................... 79 6.4.1 Material Properties.......................................................................................... 79 6.4.2 Non-heat Treatable Alloys .............................................................................. 80 6.4.3 Heat Treatable Alloys ..................................................................................... 80 6.4.4 Material Certification ....................................................................................... 81

6.2

6.3

6.4

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6.4.5 6.4.6

Welding........................................................................................................... 81 Fatigue............................................................................................................ 81

7

STEEL DISTRESS MECHANISMS.............................................................................. 82

7.1 7.2 7.3 7.4

Corrosion ...................................................................................................................... 82 Fatigue.......................................................................................................................... 84 Brittle Fracture .............................................................................................................. 84 Protective Coatings....................................................................................................... 85 7.4.1 History of Protective Coatings ........................................................................ 85 7.4.2 Present Protective Coatings ........................................................................... 86 Interior of Steel Members ............................................................................................. 89 7.5.1 Hermetically Sealed........................................................................................ 89 7.5.2 Hot Dip Galvanising ........................................................................................ 89 7.5.3 Steel Box Girders............................................................................................ 90 7.5.4 Steel Trough Girders ...................................................................................... 90

7.5

8

METALLIC MATERIALS – CONNECTIONS AND FABRICATION ............................ 91

8.1 8.2 8.3 8.4 8.5

Rivets............................................................................................................................ 91 Bolts.............................................................................................................................. 92 Proprietary Mechanical Fasteners ................................................................................ 94 Proprietary Chemical Fasteners ................................................................................... 95 Welding......................................................................................................................... 96 8.5.1 Welding Methods ............................................................................................ 96 8.5.2 Type of Welds................................................................................................. 98 8.5.3 Effects of Welding......................................................................................... 100 8.5.4 Construction Issues ...................................................................................... 100 8.5.5 Weld Categories ........................................................................................... 100 8.5.6 Weld Procedure Qualification ....................................................................... 101 8.5.7 Welders......................................................................................................... 102 8.5.8 Welding of High Strength Steels................................................................... 102 8.5.9 Weld Defects ................................................................................................ 102 8.5.10 Stud Welding ................................................................................................ 103

9

NON-METALLIC MATERIALS .................................................................................. 104

9.1

Elastomers.................................................................................................................. 104 9.1.1 Bridge Bearings ............................................................................................ 104 9.1.2 Construction Issues ...................................................................................... 104 9.1.3 Serviceability Issues ..................................................................................... 105 Fibre Reinforced Polymers (FRP)............................................................................... 105 9.2.1 History........................................................................................................... 105 9.2.2 Types of Materials ........................................................................................ 106 9.2.3 Material Characteristics ................................................................................ 106 9.2.4 Glass Transition Temperature of Polymers .................................................. 107 9.2.5 Resins and Moisture ..................................................................................... 107 9.2.6 FRP Bridge Applications............................................................................... 108 9.2.7 FRP Timber Member Replacements ............................................................ 109 9.2.8 Bridge Strengthening .................................................................................... 110 9.2.9 Design Issues – Strengthening..................................................................... 112 9.2.10 Strengthening Materials................................................................................ 113 9.2.11 Construction Issues ...................................................................................... 113 9.2.12 Monitoring ..................................................................................................... 114

9.2

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9.3

9.4

Polytetrafluoroethylene (PTFE) .................................................................................. 114 9.3.1 Chemical Resistance .................................................................................... 114 9.3.2 Working Temperature Range ....................................................................... 114 9.3.3 Coefficient of Friction .................................................................................... 114 9.3.4 Extrusion of PTFE......................................................................................... 114 9.3.5 Cleanliness of the PTFE Surface.................................................................. 114 Polystyrene ................................................................................................................. 114

10

TIMBER ...................................................................................................................... 116

10.1 Botanical Classification............................................................................................... 116 10.1.1 Softwood....................................................................................................... 116 10.1.2 Hardwood ..................................................................................................... 116 10.2 Moisture Content of Timber ........................................................................................ 117 10.3 Shrinkage.................................................................................................................... 118 10.4 Engineering Classification .......................................................................................... 118 10.4.1 Hardwood ..................................................................................................... 118 10.4.2 Softwood....................................................................................................... 118 10.4.3 Strength Properties....................................................................................... 118 10.5 Structural Grading - Australia ..................................................................................... 119 10.5.1 Visual Grading .............................................................................................. 119 10.5.2 Mechanical Grading...................................................................................... 119 10.5.3 Australian Standards .................................................................................... 120 10.6 Structural Grading – New Zealand ............................................................................. 121 10.6.1 Visual Grading .............................................................................................. 121 10.6.2 Machine Stress Graded Timber .................................................................... 121 10.6.3 New Zealand Standards ............................................................................... 122 10.7 Deterioration Mechanisms .......................................................................................... 122 10.7.1 Splits, Shakes and Checks ........................................................................... 122 10.7.2 Pipe............................................................................................................... 123 10.7.3 Knots............................................................................................................. 123 10.7.4 Wood Decay ................................................................................................. 123 10.7.5 Types of Fungal Decay................................................................................. 123 10.7.6 Effects and Indications of Fungal Decay ...................................................... 124 10.7.7 Indicators of Decay ....................................................................................... 124 10.8 Durability..................................................................................................................... 124 10.8.1 Termites........................................................................................................ 125 10.9 Preservative Treatments............................................................................................. 126 10.9.1 Treatment of Timber Bridge Components .................................................... 126 REFERENCES .................................................................................................................... 128

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TABLES Table 3.1: Table 6.1: Table 6.2: Table 6.3: Table 8.1: Table 9.1: Table 10.1: Table 10.2: Table 10.3: Table 10.4: Table 10.5: Table 10.6:

Strength and ductility of reinforcement ........................................................... 18 Typical properties of cast iron......................................................................... 69 Typical bridge material properties................................................................... 76 Typical properties of aluminium used in bridge applications .......................... 80 Bolting classification ....................................................................................... 93 Comparison of material properties of FRP to steel, concrete and timber ..... 106 Strength properties of green timber .............................................................. 119 Strength properties of seasoned timber........................................................ 119 Minimum target values for visually graded timber ........................................ 121 Minimum target values for machine stress-graded timber ............................ 121 Durability class and in-service life................................................................. 125 Preservative treatment and classification ..................................................... 126

FIGURES Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 3.1: Figure 3.2: Figure 3.3: Figure 3.4: Figure 3.5: Figure 3.6: Figure 3.7: Figure 3.8: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8: Figure 5.1: Figure 5.2: Figure 5.3: Figure 5.4:

Plastic shrinkage cracking in deck.................................................................... 7 Pier headstock cracking caused by AAR .......................................................... 7 Cross-section of concrete core showing expansive gel around aggregate....... 9 Vertical cracks below water in octagonal prestressed concrete pile................. 9 Damage to reinforcement due to poor quality tack welding ............................ 19 Butt splice ....................................................................................................... 20 Welded butt splices in column ........................................................................ 20 Welded lap splice............................................................................................ 21 Macros of welded lap splice to check the penetration of weld ........................ 21 Tangent modulus ............................................................................................ 26 Secant modulus .............................................................................................. 26 Barrel and wedges and seven wire strand...................................................... 28 Effect of excess cover on cantilever ............................................................... 31 Air void as a result of pouring concrete both sides of void former .................. 34 Vertical core through deck showing plastic cracking and voids – poor compaction ..................................................................................................... 36 Top surface of the cored deck showing severe plastic shrinkage cracking .... 37 Block cracking in bridge deck ......................................................................... 45 Relationship between strength ratio and density ratio .................................... 47 Influence of density on concrete strength ....................................................... 47 Influence of density ratio on fatigue life .......................................................... 47 Severe pile deterioration caused initially by AAR then DEF ........................... 53 Corrosion in precast culvert due to the use of calcium chloride...................... 55 Chloride attack in tidal channel....................................................................... 56 Acid attack ...................................................................................................... 57

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Figure 5.5: Figure 6.1: Figure 6.2: Figure 6.3: Figure 6.4: Figure 6.5: Figure 6.6: Figure 6.7: Figure 6.8: Figure 6.9: Figure 6.10: Figure 6.11: Figure 7.1: Figure 7.2: Figure 7.3: Figure 7.4: Figure 7.5: Figure 7.6: Figure 7.7: Figure 8.1: Figure 8.2: Figure 8.3: Figure 8.4: Figure 8.5: Figure 8.6: Figure 8.7: Figure 8.8: Figure 8.9: Figure 8.10: Figure 8.11: Figure 9.1: Figure 9.2: Figure 9.3: Figure 9.4: Figure 10.1: Figure 10.2: Figure 10.3:

Fire damage to Yowaka River bridge ............................................................. 59 Coalbrookdale cast iron bridge ....................................................................... 67 Graphitisation of cast iron ............................................................................... 70 Wrought iron caissons above ground – cast iron caissons below ground ...... 71 Cast iron columns on timber bridge pier ......................................................... 71 Cast iron shoe at lower end of timber truss member ...................................... 72 Wrought iron lattice truss ................................................................................ 73 Wrought iron plate showing laminar structure ................................................ 74 Increase in yield point by repetitive straining .................................................. 77 Effect of hardening and tempering.................................................................. 78 Tensile test of reinforcing bar – ductile failure ................................................ 79 Fracture in aluminium weld............................................................................. 81 Corrosion due to accumulation of dirt in member ........................................... 83 Crevice corrosion at steel/timber interface ..................................................... 83 Crevice corrosion at steel/steel interface ........................................................ 83 Schematic drawing of a standard impact testing apparatus ........................... 84 Brittle failure of King Street Bridge girder ....................................................... 85 Welding of hollow steel to base plate - full penetration weld compared to fillet weld ......................................................................................................... 89 Base plate showing corrosion of fillet weld ..................................................... 90 Power riveting ................................................................................................. 92 Markings for high strength bolts...................................................................... 93 Stud shear connectors used for composite action girder/slab ........................ 94 Stud Shear Connectors on top flange of a steel girder ................................... 95 Shielded manual metal-arc welding and submerged-arc welding................... 97 Metal inert gas welding and flux-cored arc welding ........................................ 98 Fillet weld terminology and dimensions .......................................................... 99 Butt weld terminology and dimensions ........................................................... 99 Partial penetration butt welds ....................................................................... 100 Macro – full penetration fillet weld flange to web .......................................... 102 Macro – butt weld (double sided).................................................................. 102 FRP span – bridge over Orara River at Coutts Crossing.............................. 108 Proof loading of FRP span for Coutts Crossing ............................................ 109 Trial FRP cross girder for timber truss bridge ............................................... 110 Shear strengthening of a reinforced concrete T- beam bridge with CFRP strips ............................................................................................................. 111 Softwood cell structure ................................................................................. 116 Hardwood cell structure ................................................................................ 117 Common timber faults................................................................................... 123

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1

INTRODUCTION AND GUIDE STRUCTURE

1.1 Scope The purpose of the Guide to Bridge Technology is to provide guidance to bridge owners and authorities on technology related issues relevant to bridge ownership, design procurement, vehicle and pedestrian accessibility and bridge maintenance and management practices, including the use and application of Australian and New Zealand bridge design standards. Bridge owners are a diverse group including state road authorities, toll road concessionaires, local governments, private landowners and businesses such as shopping centre owners. The guide has also been written with the young engineer in mind particularly those recently graduated, and looking at specialising in the design and construction of bridges. The Guide to Bridge Technology, Part 2: Materials covers all aspects of the common building materials available to the engineer including concrete, metallic and non-metallic materials and timber. Part 2 of this guide discusses material characteristics, their properties, durability, construction issues when using such materials, and protection and preservation treatments. A detailed section on concrete reinforcing materials is also included.

1.2 Guide Structure The Austroads Guide to Bridge Technology is published in seven parts and addresses a range of bridge technology issues, each of which is summarised below. Part 1: Introduction and Bridge Performance 

This part covers the scope of the Guide to Bridge Technology, includes factors affecting bridge performance, the relationship to the bridge design standards, and an understanding of the evolution of bridges and bridge loadings. Technical and non-technical design influences are also discussed along with the evolution of bridge construction methods and equipment. Specifications and quality assurance in bridge construction are also included in this Part.

Part 2: Materials 

The full range of bridge building materials is discussed in Part 2 including concrete, steel, timber and non-metallic components. It also discusses the material characteristics including the individual stress mechanisms.

Part 3: Typical Bridge Superstructures, Substructures and Components 

Included in discussion in this part are superstructure and substructure components - namely timber, steel, wrought iron, reinforced and pre-stressed concrete. Typical bridge types such as suspension, cable stayed and arched types are discussed. Included in this part is a section on bridge foundations.

Part 4: Design Procurement and Concept Design 

In this part coverage includes bridge design process procurement models, specification requirements, design and delivery management processes, design checking and review concepts, the use of standardised components, aesthetics/architectural requirements, standard presentation of drawings and reports, designing for constructability and maintenance. The service life of the structure and components, mining and subsidence, flood plains, bridge loadings, and geotechnical and environmental considerations are also discussed.

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Part 5: Structural Drafting 

This part covers the detailed drawing aspects required to clearly convey to the consultant/construction contractor the specifics of the project. It discusses the various standards including details required for cost estimating and material quantities. Coverage also includes reinforcement identification details.

Part 6: Bridge Construction 

This part provides guidance to the bridge owner's representative on site and focuses on bridge technology, high-risk construction processes e.g. piling, pre-stressing, and the relevant technical surveillance requirements during the construction phase. Bridge geometry, the management of existing road traffic and temporary works are also discussed in this part.

Part 7: Maintenance and Management of Existing Bridges 

Maintenance issues for timber, reinforced and pre-stressed concrete, steel, wrought and cast iron bridges are discussed in this part. Other bridge components including bridge bearings and deck joints are also referred to. This part also covers the monitoring, inspection and management of bridge conditions.

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2

CONCRETE MATERIALS

2.1 Cement 2.1.1

Source

To meet today’s demands for concrete supply, Portland cement is currently manufactured locally and imported into Australia and New Zealand from many sources. Major concrete suppliers usually purchase cement from the same suppliers to ensure they produce concrete with consistent properties. Some producers, however, may vary their cement source, and cement suppliers themselves may source their cement or raw materials from different sources. This situation means that the consistency of cement, and therefore concrete, cannot be guaranteed. In Australia a government-based cement quality assurance scheme is being recreated. However, until it is implemented state authorities will need to have their own quality assurance schemes to cover cement supply. This problem should be managed centrally by a concrete expert in each road authority. In New Zealand, cement and concrete quality is managed though the NZRMCA plant certification scheme. The concrete purchaser manages quality by specifying concrete supply in accordance with NZS 3104 (2003) and concrete construction in accordance with NZS 3109 (1997). These standards provide a minimum level of quality assurance. Extra quality control processes can be specified to address particular concerns for individual structures, such as durability requirements in aggressive environments. The issue is one of having confidence in the quality of cement being used to construct bridges. Once bulk cement is placed in silos the traceability of the origin of the cement becomes problematic. For complex bridges the consistency of the cement properties and concrete mix characteristics becomes more critical. 2.1.2

Cement Reactivity – Setting Process

Historic perspective The reactivity of cement is related to the fineness. The finer the cement, the more rapid the rate of hydration and the rate of strength gain. Cements produced pre the 1960s were of a coarser grind compared to current materials. The changes in the cement manufacturing process, which began in the 1960s, resulted in increasing fineness. As a result, comparatively higher concrete strengths were achieved in shorter periods of time. However, the increase in the heat of hydration of the finer cement causes comparatively higher thermal shrinkage as the concrete cools. In addition, the high early strength results in high early elastic modulus, lower creep and higher drying shrinkage compared with lower strength concrete unless water-reducing admixtures are used. As a consequence researchers contend that the high strength concrete could be more crack prone unless the mix is carefully designed and the concrete properly compacted and cured. The prevalence of premature bridge deck cracking supports the need for careful control. The criticality of process control from batching to curing cannot be over-emphasised as modern high strength concretes may be more crack sensitive. Protecting new concrete from adverse atmospheric conditions and the early application of curing regimes are of particular importance to control early cracking. Austroads 2009 — 3—

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Numerous papers and articles exist in the literature on concrete cracking and durability and provide an historic perspective on the issue. Some examples include Mehta and Burrow (2001) and Purvis et al (1995). For an exhaustive list of references search the Internet under ‘concrete bridge deck cracking’. The scale of the problem of bridge deck crack is indicated by the number of research projects carried out, or currently in progress, into the problem worldwide. 2.1.3

Cement and Durability

The issue of concrete durability arose in the late 1970s when it was noted that bridges constructed in marine environments in the 1960s and 1970s were exhibiting premature deterioration because the cement content had been reduced due to the use of finer and therefore more reactive cements. The strength requirements could still be met even though less cement was used. Investigations carried out by researchers showed that chloride ions had passed through the cover concrete at a comparatively fast rate in the newer bridges, resulting in the loss of alkalinity of the concrete surrounding the reinforcement, thereby leading to its corrosion. The resulting corrosion product being of greater volume than the steel caused the spalling of concrete. Bridges in marine environments constructed prior to the 1960s were in many cases performing satisfactorily. Investigations carried out showed that in many of these bridges the probability of corrosion of the reinforcement was low due to the higher durability of the concrete, because the mixes were designed by proportions of cement, sand and coarse aggregate. In addition, extra cement was added in bridge concretes, which enhanced the durability. As discussed above, the change in cement properties to achieve high early strengths is considered a major contributor to the durability issue. Numerous papers and articles in concrete journals give a background to the concrete durability issue and these can be found listed separately at the end of the References section of this Part. Blended cements As an outcome of the durability issue blended cements consisting of OPC (ordinary Portland cements) from different sources and fly ash were developed to enhance the resistance to chloride and sulphate attack in marine environments. Products referred to as ‘marine blends’ emerged to address the durability deficiencies of the existing cements. More flexibility can be gained if cement suppliers can store cement etc. in separate silos so that blends can be varied to suit particular requirements. Durability measures Numerous research projects were implemented worldwide to address concrete durability. Outcomes of the research pointed to a number of contributing factors including the following: 

Specifications that had previously specified a minimum cement content for various concrete grades had been amended to only require that the required 28-day strength be met.



The introduction of fine grind cements had resulted in the situation where the specified 28day strength could be achieved with reduced cement content compared to the past because of the higher reactivity of the cement. As a result, the water cement ratios had increased for the same strength resulting in less durable concrete (Hawkins 1987).



Poor construction practices in terms of compaction and curing.

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As an outcome of the research, either minimum cement contents or significantly higher concrete strengths were specified e.g. basic concrete 20  40 MPa, prestressed concrete 45  55 MPa. In addition, supplementary siliceous materials such as fly ash and silica fume were used.

2.2 Supplementary Cementitious Materials (SCMs) SCMs are often incorporated into modern concretes to improve durability. The SCMs currently used include fly ash, granulated blast-furnace slag and silica fume which are industrial by-products. In New Zealand a proprietary natural geothermal silica is currently used instead of silica fume. The blending of ordinary Portland cement with fly ashes with specific properties results in concrete with increased durability by enhancing its chemical resistance in terms of chloride ion ingress and alkali aggregate reaction (AAR). It should be noted that not all fly ashes have the chemical properties that result in the enhancement of concrete durability. Hence the need for testing of proposed materials for compliance to specifications. An additional advantage of fly ash is that it reduces the heat of hydration, strength and elastic modulus at an early age resulting in a reduction of concrete prone to cracking. In marine environments the increase in chemical resistance from the use of fly ash results from the fact that the concrete is able to chemically bind free chloride ions that have the potential to cause corrosion of the reinforcement in time. The use of SCMs in the appropriate quantities enhances the resistance to alkali aggregate reaction (AAR) by reducing the alkalis in the concrete and preventing the reaction with the aggregates.

2.3 Aggregates The requirements for aggregates for concrete are set out in AS 2758.1 (1998) and NZS 3121 (1986). 2.3.1

Coarse Aggregate

Issues associated with coarse aggregates include: 

source



shape



degradation



strength.

Source The need for ongoing testing of aggregates from quarries needs to be highlighted. Within any quarry the possibility exists for changes in the petrology of the rock as different areas are mined as a result of encountering dykes, intrusions, etc. The fact that aggregate from a particular quarry was tested and found to be acceptable does not mean that all material in the future will be of the same quality. Strength and shape Requirements for strength and shape are set out in standard specifications. The shape of the aggregate is important, as concrete with flaky aggregate is more difficult to compact than concrete with cubic shaped aggregate.

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Large aggregate provides better interlock with resulting higher shear strength. Aggregates larger than 20 mm may be used in special circumstances with wider spaces. Smaller aggregates of 10–14 mm may be used in congested areas such as prestressing anchorages. However, the concrete will have a comparatively lower shear strength compared to concrete with larger aggregate. 2.3.2

Fine Aggregates

The requirements for fine aggregates for concrete are also set out in AS 2758.1 (1998) and NZS 3121 (1986). The type of fine aggregates used in concrete has traditionally been river sand and crushed sandstone. However, there is an increasing use of manufactured sand as the availability of natural sands diminishes. The use of manufactured sand (quarrying by-product – crusher dust) introduces a number of potential problems for the placement, compaction and finishing of concrete particularly in bridge decks with a large surface area per volume compared to other members. Manufactured sands have a comparatively high surface porosity and surface absorption and as a result have a high and sometimes variable water demand. Therefore the control of the moisture content of the manufactured sand in the batching process is critical. The workability of the concrete is extremely sensitive to variations in the moisture content of the fine aggregate. Instances have occurred where concrete placed at 150 mm slump has become unworkable before it has been fully compacted and finished. In one bridge project the plastic cracking that occurred due to poor compaction as a result of loss of workability, required the complete removal of the deck. The cracks were the full depth of the slab and 1 mm wide. An investigation revealed that in that instance half of the free water was assumed to be in the fine aggregate when in fact that was not the case. The moisture content of the manufactured sand was an assumed figure rather than that determined by testing (Figure 2.1).

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Source: RTA NSW

Figure 2.1: Plastic shrinkage cracking in deck

Source: RTA NSW

Figure 2.2: Pier headstock cracking caused by AAR

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2.3.3

Alkali Aggregate Reaction (AAR)

The reactivity of both coarse and fine aggregates in the concrete matrix has become an increasingly critical issue in concrete bridge construction. AAR is a chemical reaction that occurs between the aggregates and the alkali hydroxides in the pore solution of concrete forming an expansive gel. The gel expands on absorbing water and this can lead to extensive cracking of the concrete with potentially significant effects on the serviceability and capacity of a structure (Figure 2.2, Figure 2.3 and Figure 2.4). The alkali hydroxides most commonly associated with AAR are sodium hydroxides and potassium hydroxides. These may be present initially in the cement, admixtures or the mixing water. For deleterious AAR to occur in a structure the concrete must contain sufficient amounts of reactive aggregates, alkali and moisture. The absence of one or more of these will inhibit the reaction. Using low-alkali cements that limit sodium and potassium content is one approach to reducing the incidence of AAR damage that may result with some potentially reactive aggregates. On the other hand, additional moisture entering the concrete via cracks caused by AAR can accelerate the process. The period for significant AAR damage to occur can be as short as five years and as long as 30 years or more. Three types of AAR have been identified: 

alkali-silica reaction



alkali-silicate reaction



alkali-carbonate reaction.

Alkali-silica reaction (ASR) occurs between the alkali hydroxides and various forms of silica with a more disordered crystalline structure including chalcedony, flint, chert, opal, strained quartz and quartz cement. Alkali-silicate reaction has not been well defined and is considered to occur with aggregates of complex mineralogy such as greywacke, phyllite and argillite. Silicate minerals such as micas and clays have also been reported as AAR susceptible. It appears that alkali-silicate reaction is basically similar to alkali-silica reaction as far as the reaction products are concerned, but the rate of reaction is lower. In general, no distinction is made between these two types of reaction. In both, an expansive gel is formed which produces large swelling pressures on absorbing water, and this may crack the affected concrete. After cracking, the gel penetrates some of the cracks and some of the pressure is relieved. Alkali-carbonate reaction occurs between the alkali hydroxides of the pore solution of concrete and certain dolomitic carbonate rocks, but this is far less common than ASR, and has not been reported in Australia. Specifications for the supply of concrete for bridge works now include requirements for all proposed aggregates to be assessed for AAR reactivity. Typically, a petrographic examination is carried out according to ASTM C295 (2008). Aggregates containing opaline material, unstable silica materials or sheared rock containing moderate amounts of strained quartz and microcrystalline quartz may be eliminated without further testing.

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Aggregates not eliminated by petrographic assessment are then assessed for potential AAR using an accelerated mortar bar or a concrete prism test method. Aggregates classified reactive using the accelerated mortar bar test may be deemed satisfactory for use up to a specified limit of reactivity, subject to: 

the use of a blended cement in the concrete containing supplementary cementitious materials (SCM) such as fly ash, slags and silica fume



retesting using a concrete prism test.

Aggregates classified as reactive according to a concrete prism test must not be used.

Source: RTA NSW

Figure 2.3: Cross-section of concrete core showing expansive gel around aggregate

Source: RTA NSW

Figure 2.4: Vertical cracks below water in octagonal prestressed concrete pile

Fine aggregate can also cause AAR (Section 2.3.1). Rock for which the coarse aggregate has low AAR reactivity may be more highly reactive in the manufactured sand state. This fact needs to be taken into account when determining the percentage of fly ash to be used in a mix.

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2.3.4

Lightweight Aggregates

Where lightweight concrete is required lightweight aggregates are used. Applications may include drop in spans where craneage requirements may limit the mass of members. The current type of lightweight aggregate available is a volcanic material called scoria. In New Zealand pumice is also used. In the past coke breeze was used as a lightweight aggregate. Man-made lightweight aggregates are also available. It should be noted that excess vibration during placement may cause lightweight aggregates to ‘float’. Lightweight aggregates should be used with caution, as they are susceptible to excessive creep and shrinkage. Additional testing is required to determine the concrete characteristics.

2.4 Admixtures The Cement and Concrete Association of New Zealand, www.cca.org.nz, and Cement and Concrete and Aggregates Australia, www.concrete.net.au, websites provide information on concrete admixtures and their applications. Concrete admixtures provide a means to enhance the characteristics of concrete in the fluid, plastic and solid states. The admixtures available for use include: 

Water reducing agents to produce high slump, flowable concrete, while lowering the water/cement ratio to increase strength and improve durability. Available as high range (superplasticers) for high slumps – 150 to 200 mm and normal water reducers to produce the specified slump with 10-15% less water. Superplasticers are usually used together with normal water reducers for maximum efficiency. The superplasticers work by coating the cement particles, which reduces friction, increases the slump, and retards the hydration. Hence when the superplasticer evaporates rapid stiffening (reversion) occurs.



Air entraining agents must be used in concrete subject to freeze-thaw to reduce the risk of damage as a result of the freezing. The air entraining agent produces micro air bubbles in the concrete matrix, which results in discontinuous pores. They are also used in warm climates to enhance durability. Air entrainment is also used to improve workability by making the concrete flow due to the presence of the air bubbles. The presence of the discrete air bubbles reduces the ingress of moisture that under freezing conditions will expand and damage the concrete. The addition of air entraining agents to a mix results in some loss of compressive strength. Consequently, excessive use of air entraining agents by incorrect dosing may be detrimental to the concrete.



Accelerators used to promote early setting, particularly in cold weather conditions. The use of accelerators needs to be treated with caution as a number of the products contain calcium chloride that are a source of harmful chloride ions that can lead to corrosion of reinforcing steel and metal fitments. Only chloride free accelerators should be used in reinforced concrete.



Retarders to delay initial set of the concrete to allow time for placement, compaction and finishing particularly in hot weather.



Shrinkage reducers to reduce the drying shrinkage and consequential cracking.



Corrosion inhibitors for use in concrete in marine environments to maintain the passive environment of reinforcing steel and thereby prevent corrosion. These materials require specialised knowledge and should be thoroughly tested and used with great care.

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There are three classes of corrosion inhibitors – anodic, cathodic and mixed. Anodic inhibiters act to suppress the anodic reaction. The most common of these is calcium nitrite. Cathodic inhibitors act to suppress the cathodic reaction. Mixed inhibitors act to suppress both the anodic and the cathodic reactions. In simple terms, by suppressing the anodic reaction the onset of corrosion will be delayed, but the rate of corrosion will be unaffected or, under certain circumstances, increased. Suppressing the cathodic reaction, the onset of corrosion will not change, but as the reaction rate is governed by the cathode and the availability of oxygen, the reaction rate will be reduced. Mixed inhibitors will both delay the onset of corrosion and reduce the reaction rate. Where inhibitors are used in a concrete element then the inhibitor must be used in all concrete elements electrochemically connected to that element. Failure to do so may result in increased corrosion. The dosage should not be varied within the concrete or unusual and unsafe corrosion conditions may occur. Inhibitors are quite expensive, costing up to $100/cubic metre of concrete, (at the time of writing) and may cause changes in the plastic properties of the concrete. Cautionary Note: It should be noted that while the use of admixtures can improve the characteristics of concrete there are a number of issues to consider in their use: 

There is potential for the use of admixtures to mask clues to problems with the mix design.



Admixtures are not designed to correct deficiencies in the mix design.



As more admixtures are used the interaction between them may produce adverse affects. Multiple admixtures should not be combined in one concrete mix without the approval of the admixture manufacturer and after thorough testing.



As more admixtures are added the level of control in the batching process becomes more critical as the mix may become more sensitive to minor changes in the constituents.



Mix designs that are highly refined in terms of cement content and maximum packing density may also be very sensitive to changes in the constituents including admixtures.



The possibility exists for concrete with superplasticers to undergo premature ‘reversion’ or ‘slump loss’ i.e. revert from 150 mm to 75 mm slump before the time expected. Placement and compaction of the concrete in the event of this occurring becomes problematic. Some superplasticers are more prone to reversion than others. During the construction process planning, there needs to be a nominated site person who will be responsible to determine if reversion has occurred. The available working time needs to be known for specific site conditions for the actual mix.



Incorrect dosages of admixtures may be detrimental to the performance of the concrete. Dosage rate should be determined by trial mix in consultation with the admixture manufacturer.



The lack of adequate mixing will result in uneven distribution of admixtures resulting in differential characteristics of the concrete. This will offset any potential benefits of using the admixtures and may make the situation worse. In the case of corrosion inhibitors, nonuniform distribution may lead to developing differential electrical potentials and as a result accelerate, not inhibit corrosion.

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

The order in which constituents, including admixtures, are added to the mixer will have a significant effect on the efficiency and therefore the amount of admixture needed and its effectiveness. Admixture manufacturers will be able to advise on the appropriate order for particular combinations of admixtures. For example, it may be necessary to add water reducer before superplasticer to ensure the superplasticer is uniformly distributed through the concrete.



Admixtures can add significant amounts of alkali to the concrete. Where AAR is managed by controlling concrete alkali content the alkalis contributed by admixtures must be included in calculations of concrete alkali content.

2.5 Grouts and Mortars 2.5.1

Grouts

Grouts are used in bridgeworks in a number of applications including: 

The grouting of ducts in post-tensioned prestressed concrete members after the stressing operation to provide corrosion protection for the prestressing steel. The prestressing steel will consist of either bar, strand or wire.



The grouting of permanent rock anchors to provide bond to develop the anchor capacity and provide corrosion protection for the tendon.



The grouting of the ducts of tie-backs that are used to provide stability to retaining walls and abutments. The grout provides corrosion protection for the steel tendon resisting the forces involved.



As a surface primer on the hardened concrete at construction joints.

Grout consists of neat cement and water mixed to the specified water/cement ratio to provide the required performance requirements. In some instances fine aggregate may be used but its nominal maximum aggregate size is limited to 1 mm. The performance requirements are: 

Strength – which is controlled by cement properties and water/cement ratio.



Fluidity (ability of a batch of grout to be pumped and to flow into voids for the duration of the grouting operation), which is controlled by the cement particle characteristics and water/cement ratio.



Early expansion (to counter shrinkage), which is controlled by the addition of expansive admixtures to counter early expansion and prevent segmentation of the grout.



Bleed characteristics – to ensure excess water does not remain after hydration is completed. Any excess water will collect in high points of ducts in prestressed concrete girders and in the ducts of vertically prestressed concrete columns and this is unacceptable from a corrosion protection point of view.

The specification requirements for cement and fine aggregate for grout are the same as those required for concrete. Standard specifications for grout specify standard test methods and acceptance criteria for performance requirements. The use of iron or aluminium powders as expanding admixtures is precluded by standard specifications.

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The grout mix design for a project may be developed through a testing program. Alternatively, a proprietary grout may be used provided testing is carried out to confirm the product complies with all requirements of the specification. This includes performance requirements and material properties and the use of approved admixtures. Some state road authorities preclude the use of premixed grouts for mortar pads (Section 2.5.2). Grouting is a critical activity and needs to be strictly controlled. The fact that it is extremely difficult to assess the quality of grouting after completion makes the need for strict supervision of the process imperative. 2.5.2

Mortars

Cement mortar Cement mortar is used in bridgeworks in a number of applications, including: 

support of bearings



support of traffic barrier posts



support of lighting standards



support of fixtures including noise barriers



infill at prestressing anchorage recesses



as a bedding layer for bridge deck joints



minor concrete repairs.

Mortar consists of cement, fine aggregate and water mixed to a specified water/cement ratio to provide the required performance requirements. The mix proportions of cement/sand ratio will vary depending of the performance requirements but will generally range from 1:1 to 1:3. The performance requirements will be determined by: 

specified strength – determined by cement properties and water/cement ratio



control of shrinkage characteristics



friction requirements (particularly for elastomeric bearings)



time to initial set



accessibility of area



specified thickness



plan area



method of installation of the member - e.g. mortar pad installed prior to or after member is temporarily supported in position.

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Dry pack mortar In some instances it may be advantageous to use what is referred to as ‘dry pack mortar’. For a dry pack cement mortar the amount of water used in the mix is only that sufficient to ensure hydration of the cement. The cement mortar is compacted in place by hammering in vertical layers under a horizontal base plate of a component. This results in a high strength, high density mortar which does not slump ensuring uniform bearing over the base plate being supported. The use of dry pack mortar will require the use of packing, usually in the form of steel wedges, to support the component until the cement mortar has cured. When the cement mortar has cured the wedges are removed and additional dry pack mortar installed in the gap remaining. Curing It should be noted that cement mortar requires the same curing regime as concrete to ensure the required strength is obtained and to prevent drying shrinkage cracking. The general tendency on construction projects is for a lack of attention to the curing of mortar. Surface preparation The concrete surface on which the mortar is to be placed should be scabbled to remove surface laitance. It should also be saturated to prevent the hardened concrete absorbing moisture from the wet mortar resulting in a loss of strength and increased drying shrinkage. The surface should be primed immediately prior to installation of the mortar with a cement grout to enhance adhesion. Polymer mortar In some instances, bridge designers may specify a polymer mortar rather than a cement mortar. In a polymer mortar the cement binder is replaced with a polymer such as epoxy resin. Polymer binders are available in a range of types and characteristics depending on the application. For example, thixotropic binder polymers are available that will produce a mortar than will not flow under gravity. For detailed information on polymer binders and mortars consult the manufacturers such as Epirez, Sika and Vivacity Engineering. Cautionary note: (1) The use of proprietary premixed grouting compounds for bridge bearing mortar pads is precluded by some state road authority standard specifications. (2) The use of polymer mortar for elastomeric bearings is problematic because of the lack of friction on the top surface of the mortar. The potential exists for the bearings to ‘walk out’ if the friction allows the bearing to slide rather than shear under horizontal loads. 2.5.3

Mortar Pad Set Up

It is imperative that mortar pads conform to the drawing requirements in terms of position, dimensions and reduced level. Instances have occurred where the top surface of mortar pads for bearings has cast out of level and as a consequence has used part of the rotational capacity of the bearing in compensating for the error. 2.5.4

Mortar Pad Problems

Mortar pad problems include: 

cracking – lack of curing, lack of thickness, lack of strength



drumming, edge lifting – lack of curing, lack of bond, concrete substrate dry when mortar placed, expansive mortar used with no constraints.

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3

CONCRETE REINFORCING MATERIALS

The material characteristics of the steels (reinforcing, prestressing and structural) depend on a number of variables, including: 

manufacturer



country of origin



strength grade



method of manufacture



year of manufacture (of particular importance for assessment of existing bridges)



standard to which it was produced



chemical composition.

3.1 Material Certification In view of the situation regarding identification of reinforcing, prestressing and structural steel it is imperative that a certificate be obtained from the supplier/contractor that identifies the material and certifies its mechanical properties. Random testing of samples of imported steel is required to verify the veracity of the certification provided. Instances have occurred where test results did not correlate with the information provided by the certification. In addition to obtaining a genuine material certificate, it is also important that the heat and batch number can be traced to the material delivered to site. For a report on the performance of seismic grade steel from various suppliers refer to the New Zealand Department of Building and Housing website: http://www.dbh.govt.nz/blc-product-certification

3.2 Heat Numbers The heat number of a piece of steel is the identifier that relates the product to a particular batch of steel produced in the steel making process. The manufacturer of the steel carries out metallurgical testing from each batch of steel to determine its properties to ensure compliance with the required standard. If the heat number of the steel is known it can be related back to the manufacturer’s records at any time.

3.3 Country of Origin The global economy has resulted in large quantities of reinforcing, prestressing and structural steel being traded all over the world. For example, overseas produced reinforcing steel is being imported in large quantities into Australia and New Zealand. In Australia it is estimated that up to 50% of the reinforcing steel sold in Australia comes from overseas. The overseas product is imported by steel merchants and also by local steel producers to supplement supplies during periods of high demand.

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One of the issues with imported reinforcing, prestressing and structural steel is the traceability once it is put into storage. It is imperative that the country of origin of the steel used in bridge construction is determined. For example reinforcing and structural steel cannot be reliably welded without knowing the country of origin and hence the chemical composition. The standards under which steels are produced overseas do not necessarily comply with ANZ standards. For example, USA and EU requirements for elongation for reinforcing steel do not comply with ANZ standards. For further information refer to the Pacific Steel New Zealand website www.pacificsteel.co.nz particularly the paper by Allington, C and Bull, D Influences of Locally Produced and Imported Reinforcing Steel on the Behaviour of Reinforced Concrete Members. Reinforcing steel is required to have raised marks that identify the manufacturer and steel grade. However, caution should be exercised with overseas products as there have been instances of fraudulent identification marks being used. In Australia, a non-profit organisation ACRS (Australian Certification Authority for Reinforcing Steels Ltd) has been set up to administer a third party product certification scheme for steel reinforcement and prestressing strand. The organisation is supported by key construction industry bodies, including Austroads. The reinforcing standards AS/NZS 4671 and the prestressing standard AS/NZS 4672 allow for voluntary third party product certification as one of the methods to prove compliance.

3.4 Carbon Steel Reinforcement 3.4.1

Material Characteristics

Information on the characteristics and issues of reinforcing steel is available on the websites of steel manufacturers in Australia and New Zealand. The OneSteel website www.reinforcing.com under Publications and Design Tools provides information on material issues including: 

frequently asked questions on reinforcement



ductility of reinforcing steel



history of reinforcing steels



Australian standards update for reinforced concrete



application of 500 plus reinforcing bars



technical references on steel reinforcement.

The Pacific Steel website www.pacificsteel.co.nz under Product Information and Product Technical Data and Essential Information about Seismic Reinforcing Steel also provides information on material issues, including: 

reinforcing bar and coil



reinforcing bar welding and bending guidelines



seismic QT and MA bars



standards update

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

seismic reinforcing bar welding and bending guidelines



technical references on steel reinforcement.

3.4.2

Method of Manufacture

Australian and New Zealand steel reinforcing manufacturers produce a range of reinforcing bars. Australia OneSteel produces three types of reinforcing steel, namely: 

500PLUS Tempcore – straight bar 12-40 mm diameter



500PLUS Microalloy – straightened from coil 10, 12, and 16 mm diameter



500PLUS Reidbar – continuously threaded Tempcore bar.

New Zealand Pacific Steel produces a range of reinforcing steel products, including: 

500 grade QT (Quench and Temper)



500 grade MA (Micro-alloyed)



500 grade Reidbar.

The Tempcore and QT bars are quenched, when red hot, in water. The resulting bar has a hard strong casing and a softer more ductile inner core. Microalloying is the more expensive process as it involves the addition of alloys, such as vanadium, at the steel making stage, therefore there is no quenching required. The MA bar has the same hardness and strength and ductility across the full cross-section of the bar. It is important that the Tempcore and QT bars are not heated above the tempering temperature as this will cause normalising of the outer casing to the properties of the core of the bar resulting in a loss of strength. The processes that cause normalising are welding and hot bending. Specific requirements/issues apply to the bending, rebending, welding, and temperature effects of these materials. Refer to specifications, standards and manufacturer’s recommendations. Tempered and quenched bars should not be welded, re-bent or threaded. 3.4.3

Old Reinforcing Steels

Over time various types of reinforcing steel have been used in bridge construction. The material properties, anchorage development lengths and weldability have changed. When assessing the load capacity or rehabilitating an existing bridge it is important to know details of the material used in the bridge. The older types of reinforcing steel include: 

plain bar – round and square



square twist



deformed bar



cold worked bar – CW 60.

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The assessment of the load capacity of an existing bridge will require confirmation of the material properties of the reinforcing steel. This may necessitate the removal of samples of the reinforcing steel for metallurgical investigation. For further information on the history of Australian and New Zealand reinforcing steel refer to the OneSteel website www.reinforcing.com (Publications and design tools – articles and papers) and the Pacific Steel website www.pacificsteel.co.nz. 3.4.4

Packaging and Handling

The microalloyed reinforcing steel with diameters of 12 mm and 16 mm is produced in coils. An issue arises when the ends of the coils are straightened. This straightening results in cold working of the bars which results in decreased ductility. The straightened material should be discarded. 3.4.5

Ductility

The ductility of reinforcing steel has come under focus following the introduction of low and normal ductility classes in AS 4671-2001. Ductility class Ductility classes comprise: 

low ductility, Class L, applies to cold drawn wire used in reinforcing mesh. Elongation < 5%



normal ductility, Class N, applies to reinforcing steel. Elongation > 5%



earthquake ductility, Class E, applies to reinforcing steel. Elongation > 10%.

In New Zealand a specific ductility Class E was developed to use in seismic design. The introduction of ductility classes raises issues in terms of the need for awareness of construction staff and of the identification of the different types of material. It is important that construction staff be trained in the identification of the grades of steel used on a specific site. It is recommended that the use of different grades of reinforcing steel on any one site be avoided to mitigate the risk of the incorrect grade of reinforcing steel being placed in a member (Table 3.1). Information on the research history of the ductility of reinforcing steel is available on the OneSteel website under ‘Ductility of Reinforcement – Research History’. Table 3.1: Strength and ductility of reinforcement Type

Designation grade

Yield strength MPa

Ductility class Clause 6.2.1 AS5100

Bar: plain(fitments only)

R250N

250

N

Bar: deformed

D500N

500

N

Bar: plain deformed and indented (fitments only)

500L

500

L

Bar: deformed

500E

500

E

Welded mesh: plain, deformed and indented

500L 500N

500 500

L N

Note: Refer to AS/NZS 4671 for explanation of designations.

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3.4.6

Weldability

The weldability of reinforcing steel cannot be determined without knowing the material properties and chemical composition. If the source of the material is unknown then welding should not be carried out without material testing. The properties and chemical composition of the weld metal used needs to be compatible with the parent material. In addition, the welding procedure, which includes preparation, consumables (stick electrodes or continuous wire), preheat requirements, temperature limits, weld runs and weld machine settings can only be determined once the material properties and chemical composition are known. Standard welding procedures for reinforcing steel manufactured in Australia are published by the respective companies. ACRS certification of the material will ensure that the weldability issue is addressed. 3.4.7

Tack Welding

Tack welding of reinforcing steel is widely used to enable the prefabrication of cages or to fix reinforcing steel placed in situ. However, the higher strength steels currently being used require a greater degree of control and expertise to ensure a satisfactory result compared to older materials. There is greater propensity for tack welding to reduce the strength of the steel. Tack welding must be viewed as a welding process and therefore needs to be done in a controlled manner. The heat input has the potential to adversely affect the properties of the steel in terms of strength and fatigue. In addition, inappropriate weld settings can result in loss of section, resulting in a loss of strength (Figure 3.1). Tack welding should be carried out by qualified welders. AS/NZS 1554.3 (2008) includes procedures to be adopted for the tack welding of reinforcing steel. The standard also includes a procedure to have a non-standard weld procedure tested for compliance.

Source: RTA NSW

Figure 3.1: Damage to reinforcement due to poor quality tack welding

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3.4.8

Welded Splices

Butt splice A butt splice involves welding two pieces of reinforcing steel end-to-end. The process requires an approved weld procedure (Figure 3.2). For bar diameters > 20 mm the weld procedure becomes more restrictive as the heat input has to be controlled to ensure the properties of the steel are not changed. The weld procedure may require partially welding a series of bars to reduce the heat input to avoid normalising the steel. The butt welding of all bars in column on the one plane is considered poor practice and should be avoided. Welding of half the bars at two levels is better practice (Figure 3.3).

Source: D Carter

Figure 3.2: Butt splice

Alternate column bars spliced at different levels

Source: D Carter

Figure 3.3: Welded butt splices in column

Welded lap splice A welded lap splice involves welding two pieces of reinforcing steel by overlapping them and then running a weld down one or both sides of the splice. The main problem with this type of splice is the difficulty in obtaining good fusion of the weld at the point of contact of the two bars (Figure 3.4 and Figure 3.5).

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Source: RTA.NSW

Figure 3.4: Welded lap splice

Macro test A macro test involves setting up the proposed weld on sample material and carrying out the weld to the approved weld procedure. The welded sample is then cut into sections, polished and visually checked to assess the quality of the weld. This process can be carried out at any time in the process of the work to assess the quality of the work. It should also be used when a new welder is proposed to check competency of the person to carry out the weld to the approved procedure.

Source: RTA NSW

Figure 3.5: Macros of welded lap splice to check the penetration of weld

3.4.9

Mechanical Splices

Mechanical splices are an alternative method to welding to join lengths of reinforcing steel. The use of mechanical splices obviates the need for strict control of the welding process which if not complied with will affect the strength of the bars. In congested areas, particularly in splices in columns, the use of mechanical splices is a more practical alternative and avoids potential problems of congested reinforcement. 3.4.10

Mechanical Couplers

Reinforcing steel can be spliced using proprietary threaded mechanical couplers supplied by a number of companies. It imperative that suppliers be required to provide documentary evidence of testing carried out to demonstrate that the coupler has a tensile capacity equal to or greater than the type of reinforcing steel it is being used with. Random testing to verify the consistency of the strength of mechanical spices is recommended. It is imperative that there be confirmation that the required thread engagement length into both sides of a coupler is achieved. This should be part of the quality procedures.

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3.4.11

Rebending

It is not permissible to re-bend 500 grade bars that are manufactured by tempering and quenching as the ductility of the bar is reduced by cold working and cracking is likely to occur. 3.4.12

Protective Treatments

In an attempt to improve concrete durability two protective treatments have been used on reinforcing steel. Galvanising Hot dip galvanising of reinforcing steel has been used in a number of bridges in aggressive environments to extend their service life. Opinions vary on the cost/benefit of galvanising reinforcing steel. It is considered by some authorities that the thickness of zinc achievable on a bar does not give long-term protection in the situation where chloride ions diffuse through the concrete to the reinforcement. Galvanised reinforcing steel is not used by all Australian state road authorities or in New Zealand. It has been more popular in pre-cast concrete building panels. Epoxy coated bar (ECB) Epoxy coated reinforcing steel has been adopted by a number jurisdictions in the USA, particularly in bridge decks, to attempt to mitigate the effects of chloride ion diffusion from de-icing salts. There are a number of issues that arise in using ECB: 

the risk of damage to the coating during construction



the risk of pin holes (holidays) in the surface of the coating which would allow moisture penetration and result in possible loss of adhesion



the presence of pinholes may also be the location of potential corrosion cells when chloride ions reached the bar



epoxies are not waterproof and moisture may permeate through the coating



cost.

A report by the US Department of Transport (Lee & Krauss, 2004) generally concluded that there was benefit in using ECB. The testing program was carried out on a simulated deck section. However, Pyc et al. (2000) gives a less favourable report of the effectiveness of ECB. The report states that loss of adhesion of the coating occurred before the chloride ions reached the bars. The report does not recommend the use of ECB. The recommendations of the latter report were based on field trials and are considered more indicative of in-service performance. ECBs are not used by Australian and New Zealand road authorities. They are not recommended by Austroads for use in bridges. 3.4.13

Fire Damage to Steel

The implication of a fire on reinforcing steel is potentially serious if the cover concrete is lost. A loss of structural capacity will occur as the temperature increases and the material begins to flow plastically. In the case of high tensile steel annealing of the steel will occur at temperatures in excess of 400 °C.

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3.5 Stainless Steel Reinforcement The selected use of stainless steel reinforcing in substructures of bridges in marine environments offers a means of ensuring that the 100-year design service life of a bridge can be achieved without expensive concrete repairs after 30-50 years. It also obviates the need for other measures to protect the reinforcing steel, such as the use of corrosion inhibitors in the concrete, surface treatments or cathodic protection. A number of Australian state road authorities have recently constructed bridges with stainless steel reinforcing used selectively in the substructures. The stainless steel reinforcing has been used in the outer reinforcement of piles, pile caps and in columns. In columns it is recommended that the stainless steel reinforcing be used at least within the splash zone. However, in relatively short columns and high exposure sites consideration should be given to using stainless steel reinforcing over the full height of the columns. Based on current costs, the selective use of stainless reinforcing steel in the substructure of a bridge increases the cost by approximately 8% compared to using 100% carbon steel reinforcing (at the time of writing). Net present worth calculations indicate the economic benefits of its use taking into account projected maintenance costs over 50 years. Economic considerations indicate that the use of stainless steel reinforcement will be limited to selected use in bridge piers in marine conditions with a maximum bar diameter of approximately 30 mm and small tonnages. For major bridges with high piers the use of cathodic prevention (CP) is considered more economical. The suitable grades of stainless steel reinforcing are: 

304



316



duplex.

For additional information on material properties see the websites provided in Section 3.5.1. Some producers are attempting to reduce the cost of stainless steel reinforcement by the use of cladding. Whilst the cladding is very tough, the method of producing a clad bar introduces some potential weaknesses into the system. Specific areas of concern are: 

cracking under bending – especially for stirrup bends



the need to cap the ends of the bar in the factory – so no field cutting is permissible



welding will damage the cladding, so no welding is permissible



there is no satisfactory repair for a damaged bar



corrosion is anoxic, so there will be no expansive rust and the bar may be lost with no visible deterioration of the member.

Therefore, Austroads does not support the use of stainless steel clad bars in bridges. 3.5.1

Construction Issues

A number of construction issues need to be addressed for the successful use of the material, for example:

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Storage Stainless steel reinforcing must be separated from carbon steel reinforcing. Bending and cutting Dedicated equipment must be used for the material. Equipment used to process carbon steel will result in pitting of the stainless steel. Handling To avoid pitting stainless steel reinforcing must not be dragged over carbon steel reinforcing. Re-bending Re-bending of stainless steel reinforcing is not permitted. The passive layer on the bars is only 1-2 mm thick. Re-bending may result in cracking of the passive layer. Welding Welding of stainless steel reinforcing is possible but is not recommended, as there is a risk of affecting the mechanical material properties of the material. Splicing Splicing of bars is to be achieved by laps or mechanical couplers. Splicing with carbon steel The splicing of stainless steel reinforcing with carbon steel reinforcing has been investigated by a number of researchers. Research has shown that there is no issue in terms of galvanic corrosion. Highways Agency UK (2002) provides detailed information on the use of stainless steel reinforcement in bridges. Ontario Ministry of Transport (2002) provides useful information for site staff. The Australian Stainless Steel Development Association at its website www.assda.asn.au and websites www.arminox.co.au, www.ssina.com and www.stainles-rebar.org, also provide information on stainless steel reinforcement. 3.5.2

Material Characteristics

Refer to 0 for discussion of this subject. 3.5.3

Supply

The stainless steel bar should originate from a UK Certification Authority for Reinforcing Steels (CARES) registered manufacturer. CARES is the British equivalent of Australian Certification Authority for Reinforcing Steels Ltd (ACRS). A special audit of the supplier should be carried out prior to commencing supply and the manufacturer should be required to guarantee the supply of the reinforcement for the project.

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3.6 Prestressing Steel General Prestressing steel is used in prestressed concrete members, stay cables and ground anchors. Prestressing steel is available in the following forms: 

strand – made up of seven separate wires. Sizes 9.3 mm, 12.7 mm and 15.2 mm diameter. The material is supplied in 3 t coils



wire – individual 7 mm diameter wires assembled to produce the required tendon force



bar – threaded bar available in a range of diameters.

3.6.1

Material Characteristics

The OneSteel website www.onesteel.com provides information on the characteristics of prestressing strand. Information on prestressing systems using strand, wire or bars can be found on manufacturers websites and include: 

VSL – www.vsl.com



BBRV – www.bbr.com



Dywidag – www.dywidag-systems.com.

3.6.2

Material Certification

Refer to Section 3.1 for discussion of this subject. 3.6.3

Material Properties

There are a number of material properties of prestressing steel that affect the design, construction and long-term performance of prestressed concrete. 3.6.4

Modulus of Elasticity/Tangent Modulus

Unlike mild steel, prestressing steels do not have a linear stress/strain curve. Therefore, the modulus of elasticity varies with the level of stress in the material. Tangent modulus The tangent modulus is defined as the slope of a line tangent to the stress/strain curve at a point of interest. The tangent modulus can have different values depending on the point at which it is determined. For example, the tangent modulus is equal to the Young’s Modulus when the point of tangency falls within the linear range of the stress/strain curve. Outside the linear elastic region at point A as shown in Figure 3.6 for example, the tangent modulus is always less than the Young’s Modulus and describes the stiffness of the material in the plastic range.

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Et

A

Stress

Strain Source: D Carter

Figure 3.6: Tangent modulus

Stress Es

Strain Source: D Carter

Figure 3.7: Secant modulus

3.6.5

Secant Modulus

The secant modulus is defined as the slope of a line drawn from the origin to the point of interest on the curve section of the stress/strain diagram, as shown in Figure 3.7. Therefore, the secant modulus will have different values depending on the location of the intersect on the curve. It is used to describe the stiffness of the material in the inelastic range.

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The secant modulus is used to calculate the extension of a prestressing strand or tendon at the stress produced by the maximum jacking load. The stress at the maximum jacking load is not to exceed 85% of the minimum ultimate tensile stress. Note that for a draped tendon the variation in jacking force, and therefore secant modulus, along its length must be taken into account when calculating the expected extension of the tendon at the prestressing jack. The calculated elongation for a tendon shown on bridge drawings should state the secant modulus used. The elongation calculations will need to be adjusted using the actual secant modulus for the stressing steel coil/s used in the member. 3.6.6

Relaxation

Relaxation is the loss of stress that occurs in steel when it is stressed and maintained at a constant strain for a period of time. Relaxation is also termed creep. The relaxation that occurs in prestressing strand is reduced by tempering and quenching under load. Prestressing strand that undergoes this process is classified as ‘low relaxation strand’. Current specifications require low relaxation strand be used in bridge members to minimise the prestress losses. Bridge designers take the relaxation losses into account when determining the required prestress in a member. Test certificates provided by suppliers should include relaxation test results. 3.6.7

Creep

Prestress losses also occur in members as a result of concrete creep. The loss of prestress in this instance is a result of strains in the concrete at constant stress. 3.6.8

Anchorages

Proprietary prestressing systems have developed their own anchorages consisting of the anchorages, anchorage reinforcing steel, anchorage head and anchor plate. Each proprietary anchorage system and the range of tendons available must undergo anchorage efficiency testing to AS 4672 (2007) before being approved for use in bridgeworks. The test piece comprises a pair of concrete blocks with the anchorage system and tendon size being assessed. The test piece is subjected to 95% of ultimate load for a specified period. The conducting of these anchorage efficiency tests requires extreme caution as the forces and therefore the health and safety risks must be addressed. The design of the reinforcement to resist the bursting stresses at the end of members behind the anchorages is the responsibility of the designer. The designer must ensure that the reinforcing cage in these areas will allow concrete to be properly placed and compacted. For strands the anchorages consist of barrel and wedges Figure 3.8.

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Source: RTA NSW

Figure 3.8: Barrel and wedges and seven wire strand

For bars the anchorage is achieved via machined or rolled threads and nuts. For wire systems the anchorage is achieved via cold-formed button heads. 3.6.9

Construction Issues

Specifications set out the requirements for storage and handling of prestressing steel. The three high-risk issues are: Weld spatter It is imperative that no welding activities take place near bare stressing steel. Minor weld spatters may cause imperfections in the surface of the material causing high stress concentrations at changes in surface profile. In such situations the weld spatters may go unnoticed until a premature failure occurs. Pitting corrosion If prestressing steel is left exposed to the atmosphere pitting corrosion may occur causing stress raisers leading to premature failure under load. Handling damage When handling strand it is very important to avoid abrasion damage, kinks and nicks all of which may lead to premature failure under stressing loads. The OneSteel website has a data sheet ‘Low Relaxation Strand’ which provides material properties and handling recommendations.

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3.7 Steel and Polymer Fibres Steel and polymer fibres are added to concrete and shotcrete to increase the ductility, control cracking and durability. The steel fibres are made from high tensile steel. The polymer fibres are generally made from polypropylene. The dispersion of the fibres promotes the formation of a large number of micro cracks throughout the concrete as opposed to a smaller number of larger cracks. The shape of the fibres provides anchorage for them to bridge cracks and enable the shrinkage strains to be resisted. As a result the ductility of the concrete is enhanced. The promotion of micro cracks enhances the durability of the concrete by reducing the potential for penetration of aggressive agents. Fibre reinforced concrete and shotcrete have been used extensively in tunnels and road pavements. They have had limited use in bridge decks. Steel fibre reinforced concrete is used in reactive powder concrete to produce compressive strengths of 140 to 160 MPa and tensile strengths of about 40 Mpa (Section 4.9.3).

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4

CONCRETE CHARACTERISTICS

It has been said that good concrete consists of cement, fine aggregates, coarse aggregates, additives and water. It has also been said that bad concrete consists of cement, fine aggregates, coarse aggregates, additives and water. The process control will determine the result. The difference between the two materials is a consequence of the degree of control exercised over the whole concreting process from the mix design, testing, batching, mixing, transporting, placing, compacting to the curing. Deficiencies in one or more parts of the process will compromise the quality of the concrete, which will impact on its service life. The degree of control exercised over each step in the process will determine if a batch of concrete results in good or bad concrete. The trial mix and testing carried out gives an indication of the potential of a concrete to comply with the specification. Whether the in situ concrete achieves that potential is determined by process control. For more information please refer to CCANZ (Cement and Concrete Association of New Zealand) website, www.cca.org.nz and CCAA (Cement and Concrete Association of Australia) website, www.concrete.net.au.

4.1 Design Issues Bridge designers need to be aware of the constraints and limitations of particular materials. In addition, design can have a significant impact on serviceability of particular materials. 4.1.1

Cover

The minimum cover specified must take into account construction tolerances acknowledging that variations in cover will inevitably occur. The quality and thickness of the cover concrete has the highest potential to adversely affect the service life of the concrete. The thinner the cover concrete is, the less time for chloride ions to reach the reinforcement. In general, the life of a bridge in adverse conditions is proportional to the thickness and quality of the cover concrete. Thick members especially columns and pilecaps should be designed with as much cover as possible, especially in aggressive environments. It should be noted that in thinner flexural members such as deck cantilevers and headstocks exceeding the specified cover, results in a reduction in the effective depth of the reinforcement. This reduces the flexural capacity of the section and may lead to cracking (Figure 4.1).

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Excess cover

Effective depth reduced

Source: D Carter

Figure 4.1: Effect of excess cover on cantilever

4.1.2

Congested Reinforcement

Designers need to be aware of the effects of congestion of reinforcement, particularly in splices in columns, in anchorage zones in prestressed concrete members and where reinforcement in different planes intersects. Drawings do not always reveal a congestion problem. Particular areas need to be drawn-up in detail at a large scale to ensure intersecting reinforcement does not clash or cause congestion in terms of being able to place and compact the concrete. The reinforcing layout should include access spaces 100 mm x 100 mm for installation of 50 mm vibrators at regular intervals. In columns with large diameter bars the use of mechanical splices is recommended rather than using lap splices. Varying the length of starter bars to ensure splices occur at two locations is also recommended. The use of concrete with 10-14 mm coarse aggregate size should be considered in areas with the potential for congestion problems. Use of finer aggregates destroys aggregate interlock and reduces shear capacity. 4.1.3

Construction Joints – New Concrete Placed Against Old

Construction joints are a potential risk in terms of concrete deterioration. Preparation of the surface of the set concrete by water or grit blasting, the use of set retarders or by formed indents is imperative. The aggregate in the set concrete must be exposed to ensure adequate bonding. The surface of the old concrete must be in a saturated state to prevent free water in the new concrete being lost by absorption that would result in a loss of strength and durability. In marine environments, or where other aggressive agents are present, construction joints should be located above permanent water levels.

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4.1.4

Match Cast Joints

Match casting joints pose a different problem. It is important to seal the joint at deck level against moisture. This requires the application of a proven sealing material at each joint plus the application of a secondary sealing compound that is used over the entire deck area. 4.1.5

Re-entrant Corners

The intersection of two members other than in the same line has the potential to cause cracking as a result of the stress raiser that occurs at the change in angle. To minimise the potential for cracking it is important to provide a transition across this angle change. In addition, extra reinforcement should be provided along the transition and right angles to it to resist the tensile stresses and to provide anchorage to the main reinforcement. See CCAA and CCANZ websites for handbooks on detailing reinforcing steel. 4.1.6

Over-specifying Concrete Strength

Many designers consider that the higher the strength of the concrete, the better the performance. This is only partially true for concretes above 50 MPa. Higher strength concrete can carry higher loads (in extreme events) and should be more durable, if it is placed, compacted and cured properly. Sometimes very high 28 day strengths (up to 80 MPa) are achieved because of high transfer strengths (40 MPa) in prestressed concrete. Designers should carefully consider the effects of specifying high transfer strengths. The adverse factors are: 

Increased material cost.



Increased labour costs to place and compact.



Possible increase in shrinkage if the mix has a higher cement paste content.



Possibility of placement and compaction problems with high cementitious, low W/C concretes, particularly in hot weather.



Higher alkali content from the cement with an increased risk of alkali – silica reactivity (this can be solved by adding (low alkali) fly ash, etc.).



Unless these ‘sticky’ mixes are properly vibrated, liquefied and compacted, plastic shrinkage and high early drying shrinkage is likely.



High range water reducers and superplasticisers are useful in improving workability especially in congested anchorage zones in beams but result in increased costs. Notwithstanding their use the concrete still requires effective vibration for thorough compaction.



In thicker members, heat of hydration can raise concrete sections to 80 °C in the first 12 hours, with problems of thermal shrinkage cracking if there is sufficient restraint of movement as elements cool.

Work by Altoubat and Lange (2001) and Aitcin (2001) shows that for mixes with high cementitious and low water content and where the concrete is restrained, it is difficult to prevent early shrinkage related cracking with other than water curing.

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In many situations it is not practical to use water curing and it is impossible to prevent restraint. Therefore, designers must consider the practical construction limitations when specifying concrete strength and, in some cases, limit the strength and the minimum water/cementitious ratio. 4.1.7

Restricted Areas

In restricted areas where access is limited to pour concrete from the top of a member e.g. for the roof of a tunnel, consideration should be given to the use of self-compacting concrete (Section 4.9.1). 4.1.8

Thin Elements

In the pursuit of economy, there is always a tendency to reduce member thickness. For concrete decks thicknesses below 180 mm can cause difficulties in placing to layers of reinforcement and maintaining adequate cover and separation between the bars. For girder webs with one layer of reinforcement a thickness of 100 mm requires great control of reinforcement bending tolerances, straightness etc. Reinforcing bars are never perfectly straight. A thickness of 120 mm eliminates most tolerance conflicts, and leads to more durable and robust girders.

4.2 Construction Issues It should be pointed out that trial mix test results will have no relation to the in situ concrete unless the whole construction process is carried out to meet all requirements of the specification. The trial mix results indicate the potential material properties that may or may not be realised depending on the field control. If trial mix test results are above specified requirements many suppliers/contractors will attempt to change the mix to be more economical. Any significant changes in the mix will invalidate the trial mix results. 4.2.1

Monitoring of Test Results

It is important that concrete test results from a batch plant for a particular mix be monitored by the contractor and project engineer over time, for three reasons: 

To pick up trends in the compressive strength results that may not be apparent when individual results are assessed in isolation. Individual tests may result in satisfactory results. However, statistical analysis of all tests should be ongoing.



To determine if the standard deviation used to determine the target for the plant is being achieved.



To detect any changes in cement properties. In large jobs where a significant number of test results are produced or for critical operations or members such as pile caps there is great advantage in doing early strength testing - e.g. at 3, 7 and 14 days to ensure trends in concrete strength are picked up as early as possible.

In the event of particular strength trends being observed action can be taken to ensure the issue is addressed before it becomes a problem. Other issues to be considered in the event of a problem with the concrete test results from a batch plant include: 

Audit of the batching process (calibration of scales, operation of bin gates).



Control of water demand, especially in manufactured sands.



Tolerances of batching.



Records and how they are generated.

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

Mixing time – how long, how verified, uniformity, when last tested?



Consideration should be given to have an inspector present at the batching plant to observe the process and record relevant data to confirm compliance with the specification requirements for the supply of concrete.

4.2.2

Placement

The method and procedure used for the placement of concrete has the potential to affect the quality of the in-place material. The extensive use of pumps to place concrete, while improving the efficiency of the construction process, results in a large volume of concrete being placed in a short time compared to conventional methods such as the use of a kibble (large bucket) and a crane. This situation can lead to the loss of control of the systematic compaction of the concrete. The resources provided to compact the concrete should reflect the rate of placement. e.g. not less than one vibrator for each 10 cubic metres placed per hour with a minimum of two. Standby vibrators should be not less than one quarter of the number in use with a minimum of one. In formed members, the concrete should be placed so as to form a distinct toe (in the form of a thin wedge typically at a 1 in 14 slope, as opposed to a more vertical front) to prevent the possibility of concrete already compacted from collapsing over under-compacted concrete and entrapping pockets of air that may go undetected. When void formers are used, the concrete should be placed from one side of the void former to ensure the concrete flows fully around it. If this procedure is not followed and the concrete is poured on both sides of the void former, there is a high risk that air voids will occur under the void former that may not be readily detectable (Figure 4.2). This requires that the void be firmly fixed to prevent lateral movement as well as vertical movement due to buoyancy.

air void Source: D Carter

Figure 4.2: Air void as a result of pouring concrete both sides of void former

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4.2.3

Compaction

The importance of achieving optimum density of in situ concrete cannot be overemphasised. The failure to obtain the required density influences a number of properties of concrete which in turn affect its long term serviceability Figure 4.3 and Figure 4.4. These include: 

propensity for early and delayed cracking



strength – 1% air voids theoretically cause a 6% reduction in compressive strength and a 4% reduction in flexural strength



fatigue life – a 1% reduction in density ratio (field density/trial mix density) theoretically cause a 10 year loss of fatigue life



structural strength – lower bond strength to reinforcement



durability – the higher sorptivity of inadequately compacted concrete will result in a reduction in durability of the concrete as a result of the increased susceptibility to chemical attack and physical abrasion.

Roads and Traffic Authority and VicRoads specifications have methods for measuring concrete sorptivity to assess concrete quality. The reliability of chloride diffusion models to predict the time for chloride induced corrosion to occur is dependent on the adequacy of the in situ compaction as well as the effect on the mass of the concrete as a result of voids exposing reinforcement to aggressive agents. Compaction is the most important part of the construction process in concrete construction. However, it is not always given the close attention it deserves. It is important that the concrete placing crew be organised to ensure a systematic approach to the process. This requires clear demarcations and responsibilities for the different facets, including designated compaction/vibrator controllers, finishers and curing applications rather than an ad hoc approach. The most efficient way to ensure the process is systematic is to have a supervisor observing and controlling the overall process. There is a need to ensure the compaction of concrete is carried out in a systematic manner by staff trained in the importance of compaction. The compaction of concrete should not be left to the least experienced construction staff. The number of vibrators being used must match the rate of placement as detailed in the specification. Instances have occurred when only one vibrator has been used when the placement rates required more. The lack of adequate compaction affects a wide range of concrete properties and will have a major influence on the long-term serviceability. While the short-term structural integrity of a bridge may not be compromised the long-term durability will be. In his paper Ayton, G P (2001) includes references to relevant statements made by Adam Neville, a world-renowned concrete expert. Neville states ‘…full compaction is more important than a low w/c ratio coupled with poorly compacted concrete’ (Neville 2000). Neville states again, ‘In engineering practice the strength of concrete ... is assumed to depend primarily on two factors only; water cement ratio and the degree of compaction’ (Neville 1986). Removal of cores from a constructed bridge deck to determine in situ densities provides a method of determining the adequacy of the construction process in terms of compaction. However, the best time to address the issue is before and during the construction phase.

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The construction industry needs to provide training in the placement and compaction of concrete to address the issue. In complex pours or to assess the capability of a construction crew a trial pour is a sound investment. Top of deck

Plastic shrinkage crack

Void under reinforcing steel - settlement

Voids due to lack of compaction

Source: RTA NSW

Figure 4.3: Vertical core through deck showing plastic cracking and voids – poor compaction

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Source: RTA NSW

Figure 4.4: Top surface of the cored deck showing severe plastic shrinkage cracking

4.2.4

Finishing

The finishing of concrete is subject to differing opinions on the most appropriate method. The aim of the finishing process is to provide a dense surface layer of concrete to the surface profile requirements in the specification. The finishing of concrete includes both level control and surface compaction and surface finish. One of the issues in bridge deck construction is the lack of adequate compaction of the top 50 mm of the deck. The initial compaction of the top layer of concrete is often lost when it is walked on during the finishing process because of the flexing of the reinforcement. Protection from drying out needs to be included, with special consideration to the rates of moisture loss and bleed, and the total water loss and total available bleed water. One potential problem is completing final finishing before bleeding has ceased. Guidelines for finishing of floor slabs are applicable to bridge decks. See Cement and Concrete Association of New Zealand (c1999). 4.2.5

Bridge Decks

Level control Methods of level control include: 

formwork she-bolts fixed to reinforcement on a grid pattern with the cone screwed and fixed at the required level. This requires the reinforcement to be well supported.



Temporary water pipe screed rails supported by the reinforcement at 2-3 m centres across the deck provided the reinforcement is fixed at the correct level.

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

Screed rails at the edge of the deck to support a vibrating screed set to the required crossfall.



Other spot height control methods.

Finishing The consistency of the slump of the concrete is important to the finishing process to ensure that the finishing is carried out on a steady front. Concrete with high slump will delay the process in one area, which may adversely affect the finishing in other areas. Methods of finishing include: 

Hand screeding This relies on the skill of the worker to obtain the required profile. There are numerous examples of poor deck finish in terms of ride quality and abrasion resistance caused by poor finishing. The finishing quality tends to deteriorate as the deck area increases. Skilled tradesmen are necessary for a good result.



Vibrating screeds Some road authorities in response to the inconsistent results from hand screeding methods have mandated the use of vibrating screeds for deck finishing. Starting and stopping vibrating screeds can cause ridges across the deck. Again, skilled operators are required. A trial pour to assess the capability of the vibrating screed operator would be a sound investment.



Bridge paving machines In some overseas countries hand screeding methods on bridge decks are not permitted. Some road authorities have specified the use of bridge paving machines, which compact, screed and float the deck. No bridge paving machines exist in Australia at the date of publication.

Float decks The most common methods to float decks include: 

Bull float A bull float consists of a wide aluminium float mounted on a long handle to enable a wide area of the deck to be floated off. The disadvantage of this method of floating off the surface is the lack of downward force that can be applied to both finish the surface and compact the top layer to provide a durable surface layer. Bull floating is a finishing operation not a primary method of compacting and finishing.



Power float Opinions vary on the use of a power float known as a ‘helicopter’. The critical issue with its use is the timing as to when to begin the process. Too early and the machine causes depressions in the surface. Too late and the effectiveness of the machine is questionable because initial set has already occurred. There is wide opinion that if used properly the power float results in a dense surface layer of concrete. The process has the added benefit of closing up plastic shrinkage cracks.

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Surface texture The surface of concrete decks requires texturing for two reasons: 

to provide skid resistance where the concrete is the final running surface



to provide a key for the application of sealing compounds and subsequent bonded aggregate seals or asphaltic concrete.

Surface texturing is usually achieved using a broom with a stiffness that produces the texturing depth specified. Cautionary note: The use of an excessively stiff broom may damage the surface profile and may disturb the aggregate near the finished surface. 4.2.6

Deck Sealing

The long-term durability of a bridge deck can be considerably enhanced by the application of waterproof sealing compounds at the time of construction prior to the application of a bonded aggregate bitumen seal or asphaltic concrete. The durability of existing bridge decks will also be enhanced by the application of sealing compounds. The application of a sealing compound will protect the deck from the ingress of water and other deleterious agents such as chlorides, carbon dioxide, chemicals and atmospheric pollutants. In cold weather climates the application of a waterproof seal will reduce the effects of the freeze/thaw cycle. The types of sealants include: 

polymer modified bitumen



rubberised bitumen with 20% crumbed rubber



polyurethane elastomer.

4.2.7

Cold Weather Concreting

It is considered that the amount of damage to concrete caused by low temperatures is underestimated in that the extent of frost-affected areas is not generally appreciated. The cold conditions can cause damage to both immature and hardened concrete. It should be noted that frost damage can occur in what generally would not be classified as cold weather. One of the main issues to appreciate is that water begins to expand at 5 °C and therefore the damage to immature concrete with free water begins before freezing occurs. The RTA has produced a guideline for cold-weather concreting. The guideline contains information from the Bureau of Meteorology on the number of frost days in various regions in NSW. Similar information can be obtained for other states and New Zealand. In mature concrete the presence of cracking can lead to ingress of water into the concrete that under freezing conditions expands, exacerbating the cracking problem. In such cases sealing of the bridge deck with a waterproof membrane is imperative.

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4.2.8

Hot Weather Concreting

The risks of concreting in hot weather are high and include: 

loss of workability due to reduced time to initial set, caused by accelerated hydration at elevated concrete temperatures



inadequate compaction



unforseen transport delays become more critical



high evaporation rates in circumstances of low relative humidity and high wind speeds



changes in atmospheric conditions over the duration of the pour



lack of resources to compete the finishing in the time available.

Standard specifications include specific constraints on concreting in hot weather including: 

maximum ambient temperature



maximum concrete temperature



maximum evaporation rates.

The use of chilled water in the batching process, pre-cooling forms, providing shade and spray cooling of the forms prior to pouring concrete and continuing for a 24 hour period should be considered as contingencies to reduce the potential risks of pouring in hot weather. The application of aliphatic alcohol sprayed on the surface of the concrete after compaction and prior to final finishing is essential to prevent excessive water loss and subsequent plastic shrinkage cracking. Moist curing or curing compound as specified is to be applied after floating.

4.3 Compaction and Density The one part of the concreting process that has the largest potential to adversely affect the properties and performance of the concrete is compaction. Put simply, air in the concrete must be expelled and the aggregate particles brought into close contact to achieve optimum concrete properties. The consequences of failing to expel the air in the concrete include: Reduced tensile strength The strength of the cement matrix and the degree of aggregate interlock are reduced. Increased propensity to crack Research by Queensland Department of Main Roads (QDMR, 2006) found a direct relationship between reduced concrete density and degree of early plastic cracking. The air voids left in the concrete are filled with bleed water as the concrete stiffens. This excess water affects the aggregate bond and hence the tensile strength. The very best methods to increase the durability of a concrete by using sufficient cement plus supplementary cementitious materials, such as fly ash and silica fume, will fail if compaction and curing are inadequate.

4.4 Curing The process of curing concrete is another critical issue that requires close attention to ensure the required strength is attained in situ. The long-term serviceability of the concrete is also dependent on the curing.

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It should be noted that the curing times for concrete containing supplementary cementitious materials (SCMs) will need to be extended to account for the slower hydration, and therefore strength gain, compared to concrete with only ordinary Portland cement. This issue should be addressed as part of the trial mix evaluation. Curing ensures that the water required for full hydration of the cement is available. If premature drying occurs the hydration processes cease prematurely with the result that drying shrinkage occurs. Curing is essential to ensure the discontinuity of the capillaries by the growth of hydration crystals to ensure optimum durability. In addition the strength, durability and abrasion resistance are all reduced. The likelihood of drying shrinkage occurs in the first seven days. However, the susceptibility to plastic cracking occurs in the first hours after placing. In conditions of high temperature and low humidity the risk of cracking becomes higher. Special measures, such as fog curing or the application of aliphatic alcohol, need to be considered at the early stages of the pour. Most specifications include provisions to prevent pouring of concrete in extreme atmospheric conditions of temperature, humidity and wind speed where excessive moisture loss would occur. Different concrete elements are cured by different means depending on the shape, size and method of construction. For more information see CCANZ, CCAA and VicRoads for documents relating to concrete curing www.cca.org.nz, www.concrete.net.au, www.vicroads.vic.gov.au. 4.4.1

Moist Curing

Moist curing is curing by the use of additional water, such that the relative humidity of the air at the concrete surface is never below 98%. Moist curing is the optimum method of curing concrete and it is recommended it be used unless site circumstances such as problems with collection of run-off prevent it being used. The moist curing of bridge decks is recommended because of the large surface area with the potential for drying shrinkage. On horizontal surfaces the concrete is covered by wet hessian or wet sand. However, the hessian or sand must to be kept damp to be effective. This is not always achieved. On vertical surfaces, hessian is often used. However, the effectiveness of the method is reduced unless the hessian is not in direct contact with the concrete surface. The most effective methods to moist cure vertical surfaces are by completely jacketing the concrete and filling the jacket with water or by the use of a series of pipes fitted with micro water sprays. The pipes are bent to follow the shape of the member and spaced to ensure the spray covers the entire surface. A very effective method of moist curing of a bridge deck is to cover the area with 40-50 mm of sand, saturate and then cover the sand with plastic. This method is particularly useful in situations where running water is not permitted or is in short supply. It is also useful in situations where there are no people on site after the pour to ensure the moisture is applied to the material covering the concrete. The disadvantage is that the sand has to be subsequently removed. 4.4.2

Sealed Curing

Sealed curing provides an alternative method of curing concrete. However, there are some issues to be considered when using it, including: 

where self-desiccation occurs, the concrete will dry out and the concrete may crack

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

even where self-desiccation does not occur, curing is only about 80% as effective as proper moist curing.

Sealed curing may be achieved by initially moistening the surface and the sealing by plastic, by the use of curing compounds or by the use of ‘self-curing’ additives. 4.4.3

Curing Compounds

There are a number of issues with the use of curing compounds that have the potential to limit their suitability, including: 

their effectiveness is governed by the application rate. Variations in application rate are likely to occur



the timing of their application is critical. In some situations a method of preventing cracking prior to the curing compound being applied must be considered. The damage to the concrete may have already occurred before the curing compound is applied



testing to ensure the effectiveness of the proposed curing compound needs to be carried out on each project. Instances have occurred where the formulation of a product has been changed but the product name remained the same



their use on bridge decks may result in adhesion problems with sealing materials



the fugitive dyes may not lose colour if not exposed to UV light



if the compound is contaminated by dust before the compound is set, the resulting surface may be stained.

4.4.4

Plastic Sheeting

Plastic sheeting if used properly is very effective. However, there are a number of practical issues that need to be addressed to prevent loss of moisture due to: 

incomplete sealing



excessive air space beneath the sheeting



damaged sheeting



poor choice of colour – e.g. black plastic will increase the temperature variation within the concrete.

4.4.5

Self-curing Additives

The use of self-curing additives raises two issues that need to be addressed: 

they may react oddly with other additives – especially in coloured concretes



they are not tested with the full range of binders available in Australia.

4.4.6

Steam Curing

Steam curing is mainly used in the pre-cast industry where the production process is based on a daily production cycle. The steam curing accelerates the hydration process with the result that eight hours steaming produces the equivalent of seven days moist curing. The specification requirements must be strictly complied with to prevent short and long term damage to the concrete.

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The requirements for steam curing include: 

a minimum preset time before steam is introduced to prevent plastic cracking



a maximum rate of rise of the concrete temperature to minimise the temperature differential between the outer and inner concrete. The limit on rate of rise of temperature also enables the tensile strength of the concrete to be developed to resist thermal strains induced by the increase in temperature



a maximum temperature to ensure the rate of hydration allows ettringite to precipitate. The consequences of curing at too high a temperature may not be realised for many years



the minimum moisture content of the steam to ensure that the concrete is in a moist environment and will not be dried by dry steam



covering of the member to ensure no loss of steam and ensure the steam curing is effective



covers to remain in place until the concrete temperature falls to within the specified temperature above ambient. Failure to comply with this requirement will result in cracking caused thermal shock when the hot concrete comes in contact with cold air



compliance of the steam cycle with the specification needs to be verified by examination of the thermograph record.

4.4.7

Heat Curing

Heat curing of concrete has had limited use. It is regarded as heat accelerated curing using dry heat. The main issue relates to the potential for drying shrinkage to occur. The top surface of the concrete has to be covered with a sufficient depth of water to prevent the top surface drying out. This is not a generally recommended method of curing because of the risk of desiccating the surface concrete which can permanently stop the crystal growth.

4.5 Creep and Shrinkage When concrete is loaded two types of deformations occur, elastic strains and creep strains. Creep strains are time-dependent and begin immediately, diminishing over time. The amount of creep is a function of: 

magnitude of the stress



concrete age and strength



duration of loading



type, size and quantity of coarse aggregate



type and quantity of cement



size and shape of element



volume to surface ratio



amount of reinforcing steel



distribution of prestress across the section



curing regime



temperature and humidity



amount and rate of drying shrinkage and associated shrinkage.

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In prestressed bridge members creep results in loss of prestress with resulting creep deflections of either hog or sag. In prestressed members, excessive creep deflections are associated with triangular stress blocks that result in non-uniform creep across the section. Prediction and monitoring of creep deflections on major prestressed concrete bridges form a large part of the construction engineering. Software packages are available to model creep to predict the effects on the bridge behaviour. 4.5.1

Shrinkage

As discussed above the reactivity, heat of hydration and shrinkage of cement are linked. The shrinkage test in AS 2350 (List) consists of a mortar bar test with limits placed on the maximum allowable shrinkage at 21 days. This is a measure of the shrinkage of the cement under laboratory conditions. However, the in situ shrinkage performance of the concrete is controlled by a number of factors that are discussed in the following sections. A research project was undertaken by the RTA to investigate the premature cracking of bridge decks following numerous instances of the phenomena. Details of the outcomes of the investigation are set out in Sections 4.7.1 and 4.7.2. 4.5.2

Shrinkage Classification

Shrinkage is usually classified into early and late age shrinkage. Early shrinkage occurs in the first seven days under normal ambient conditions. Early shrinkage can be further subdivided into autogenous shrinkage, drying shrinkage and plastic shrinkage. Late age shrinkage is largely due to drying of the concrete after the cessation of curing. Autogenous shrinkage Autogenous shrinkage is caused by reduction of pore moisture due to curing without any change in the total water within the concrete. Autogenous shrinkage is most likely with concretes where the water/binder ratio is low. Concretes with water/binder ratios below 0.36 are susceptible to autogenous shrinkage except where water curing is practised. Concretes with water/binder ratios below 0.30 are susceptible to autogenous shrinkage even where water curing is practised. Early drying shrinkage Early drying shrinkage is caused by the loss of moisture from the concrete as it cures and can result from one of the following: 

where the temperature of the concrete is high (heat of hydration and solar heating)



where the membrane or other sealing method is not sufficiently effective



where the curing is stopped prior to seven days and the concrete dries out



where the curing is not applied soon enough.

Plastic shrinkage Plastic shrinkage, as its name implies, occurs before the concrete has taken initial set. As concrete sets, the hydrated cement occupies less volume than the mixture of cement and water, shrinking the concrete.

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Plastic shrinkage also occurs due to loss of water from the surface of the concrete during placement, compaction and finishing. This loss of water from the concrete will cause a reduction in the concrete volume. Poor compaction and poor protection of the concrete will increase the loss of water during the plastic and early setting phases. Where the available water at the concrete surface is less than the water lost, the plastic shrinkage strains at the surface will be greater than the strains in the mass of the concrete. Most concrete specifications include charts to determine the loss of water from the free surface of plastic concrete under given environmental conditions of temperature, humidity and wind speed. However, the amount of available water is determined by the mix proportions. The use of very fine materials, such as amorphous silica and fine fly ash will decrease the available water for a given water/binder ratio. Similarly, the use of coarse ground granulated iron blast furnace slag will increase the available water for a given water/binder ratio.

4.6 Cracking Shrinkage cracking will occur whenever the tensile strain of the concrete exceeds the capacity of the concrete to accept that strain. In the plastic state, the concrete may be able to accept the strain by flowing. As the concrete stiffens due to setting and/or loss of moisture, the concrete’s capacity to accept tensile strain is reduced. The stiffness of the concrete increases at a faster rate than the tensile strength of the concrete; so a situation may develop where the concrete is unable to accept the strain and also is unable to resist the strain. Reported instances by a number of state road authorities of early cracking of bridge decks have been on the increase in the last five to ten years (Figure 4.5). This increased frequency of cracking has occurred notwithstanding the fact that the results of shrinkage tests carried out have been under the specification limits.

4.7 Crack Control

Source: RTA NSW

Figure 4.5: Block cracking in bridge deck

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This situation above points to deficiencies in the structural design, the mix design and the construction process. The problem of premature cracking, particularly in bridge decks, is a multifactorial problem. The potential exists for a number of factors in the construction process to cause the problem including: Mix design 

Use of high cementitious/low water content in concretes leading to high autogenous shrinkage (Altoubat and Lange, 2001).



The tendency to use lower than required water contents rather than properly control the water in the mix.



Moisture sensitive fine aggregates e.g. manufactured sands.

Batching and mixing 

The increasing use of manufactured sands, without increased controls to ensure that the moisture demand of the sand is properly controlled.



Lack of control of moisture in the aggregates resulting in poor workability and increased risk of high shrinkage.



Errors in batching.

Construction 

Reliance on minimum steel to control cracking.



Poorly designed formwork that deflects excessively under the head of concrete.



Long haul distance reducing the workability time available.



Lack of compaction resulting in reduced density and propensity to crack both in the plastic and solid phases. The RTA specifications now include a requirement for the in situ density of concrete to be within specific limits of the density of the trial mix cylinders. Provisions to remove cores from decks to demonstrate compliance with density are also included in the specifications.



Lack of compaction resulting in reduced compressive and early tensile strength. The longterm fatigue strength is also reduced. This influence of compaction on the propensity for concrete to crack is set out in the following references



Reversion of superplasticers resulting in low slump concrete that becomes unworkable before placement or completion of compaction.



Lack of compaction resulting in high plastic cracking evidenced by cracking coinciding with the reinforcing steel pattern.



Shrinkage and thermal cracking caused by the restraint of the member that does not allow shrinkage and/or thermal strains to take place. For example, a bridge deck cast on planks or girders will be restrained by the bonding that occurs at the interface.



Excessive cover to the reinforcing steel.



Inadequate pre-wetting of the concrete surface on which the deck is poured which results in the dry surface wicking moisture out of the wet concrete.



Early removal of formwork.

Figure 4.6, Figure 4.7 and Figure 4.8 illustrate the critical importance of compaction and its affect on the properties of the concrete.

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Source: Neville (1986)

Figure 4.6: Relationship between strength ratio and density ratio

Source: Ayton (2001)

Figure 4.7: Influence of density on concrete strength

Source: Ayton (2001)

Figure 4.8: Influence of density ratio on fatigue life Note: These curves are provided merely to show indicative relationships between compaction and fatigue life. The x-axis values have been derived by applying the typical density-strength relationships. The curves have been derived by inputting the flexural strength values into the appropriate design model.

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Atmospheric conditions 

Pouring in adverse weather conditions that results in excessive evaporation rates causing premature surface drying and plastic cracking.



Delays in application of curing regime.



Use of sealed curing with very low water/high cementitious concretes.



Thermal cracking caused by variations in temperature between the upper and lower parts of a slab. The variations can be caused by thermal shock where the concrete is suddenly exposed to ambient temperature that is well below the temperature of the concrete.



Freezing of free water at an early age by frost or extreme cold weather conditions. For information on cold weather concreting see RTA (2004).



Freeze/thaw effects. This situation is exacerbated if the deck is already cracked allowing water to penetrate into the deck that freezes and expands causing more cracking. The process continues each cycle with the result that the cracking in the deck gets progressively worse.

The severity of cracking in some bridges has required the replacement of the decks after only an eight-year service life. Cores removed from the decks revealed high void content. 4.7.1

RTA Research Project

The frequency of deck cracking led the RTA to initiate a research project (RTA, 2006) to instrument one deck in a new bridge to monitor the early strains and the temperatures in the upper and lower areas of the deck. The results of the monitoring showed that: 

Tensile strains were low in the first six hours after completion of the pour as the temperature of the concrete increased due to the heat of hydration.



Tensile strains increased as the ambient temperature dropped the first night and the top surface of the deck cooled. The maximum tensile strains were sufficient to crack the concrete.



The temperature differential of the top surface of the deck over a 24-hour period was 29 °C.



As the deck cooled with the diurnal temperature change, the deck was subjected to tensile strains, sufficient to cause cracking of the concrete.



The results of the research highlighted the potential benefit in terms of crack control of maintaining the deck at a constant elevated temperature for at least seven days. This would ensure that the deck would be subjected to compressive strains until such time as the tensile strength of the concrete was developed sufficiently to resist subsequent tensile strains.

4.7.2

Field Trial

The outcomes of the research were subsequently applied to the construction of a new deck. Specific requirements in regard to the deck construction were included in the tender documents, including: 

The contractor’s staff involved in the deck pour to undergo training on the requirements for adequate compaction.

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

The contractor to complete a trial slab to demonstrate the capability of staff to pour the deck. Cores to be removed from the trial slab to determine if the required density has been achieved.



Close attention to be given to compacting equipment and methodology.



Moist curing of the deck using wet hessian.



Insulation of the deck for seven days using mineral wool between two layers of plastic.

The outcome of the trial was that after four months under traffic there were no cracks in the deck. The only cracks in the deck were in the continuity slab over the pier, poured subsequently to provide a link between the adjacent decks for ride quality. These cracks were 0.15 mm wide. A further trial in which a portion of the deck was water cured without the insulation and another portion was sealed cured, again without insulation, showed that for a relatively benign thermal variation (about 12 °C) water curing alone prevented the cracking, but that sealed curing was not effective. The research indicates that close attention to compaction, curing, and maintaining the deck concrete at a constant temperature for seven days, can offset the effects of drying shrinkage and differential thermal effects. However, in cases where water curing is not practical, the concrete needs to contain enough free water at the start of curing to reduce autogenous shrinkage to a level that the concrete can accept. Therefore, designers and specifiers must consider using lower strengths for deck concrete, and even placing upper strength limits on deck concretes. The strategies developed by the RTA research project to address the cracking of bridge decks were found to be similar to those developed by the Ontario Ministry of Transport as set out by Schell and Konecny (2003).

4.8 Investigation of Concrete Construction Quality In the instance of poor concrete construction quality, a decision will have to be made as to what action is required. Options include: 

keep as is



repair



replace



develop a repair method if required.

Immediate repair may be the most cost-effective. Cautionary note: The limitations of investigation and inspection techniques used for the assessment of existing bridges are highlighted in Middleton and Lea (2004). 4.8.1

Concrete Repair Techniques for Construction Defects

Certain repair techniques are very effective if applied soon after construction and may be nominated as a cost-effective alternative to removal and replacement of non-conforming concrete components.

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The repair method must address the fundamental causes of the defect, or isolate the component from future adverse environmental exposures. For example options may include: 

Sealing – silane treatments, paint systems, waterproof membranes, encasement in additional concrete; extensive testing should be carried out to verify the effectiveness of proprietary materials and systems (Sections 5.1 to 5.3).



Cathodic protection (Section 5.2.8).



Patch repairs – the patch repair method may include the use of the same concrete mix used in the member or the use of a proprietary repair material. The efficacy of a proprietary product needs to be established. The application of a hydrophobic impregnating material is recommended on completion of the patch repair.

4.9 Special Concretes 4.9.1

Self-compacting Concrete

Self-compacting concrete is achieved by the addition of a superplasticer and a stabiliser to the mix to significantly increase the ease and rate of flow such that no vibration is required to achieve compaction. However, self-compacting concrete does have a higher void content than a wellcompacted concrete. The pouring of concrete under water in cast-in piles using a tremie pipe, without the need for vibration, has been used successfully in bridge construction for many years. The modern application of self-compacting concrete has focused on improving the performance of concrete in regard to consistency, high strength, durability and speed of construction. Self-compacting concrete was developed in Japan in the late 1980s and has been used widely in bridges, tunnels and buildings. It has also been used in bridge construction in Europe and the USA. The use of self-compacting concrete is particularly beneficial in areas of highly congested reinforcement and in members with no top access e.g. tunnel roofs. However, the use of self-compacting concrete without internal vibration can still result in large voids in congested areas e.g. ends of large precast beams. The use of self-compacting concrete requires thorough testing of the mix design. In addition, other issues must be addressed, including: 

training of concrete plant and construction staff on the nuances of the material



transport



formwork designed for full hydrostatic pressure



carrying out trial pours



the limiting effects of the bridge gradient and crossfall in regard to the stability of wet concrete.

4.9.2

Fibre Reinforced Concrete

The addition of fibres to concrete has the benefit of increasing its toughness, tensile strength, abrasion resistance, and post-cracking behaviour. The fibres also have the benefit of controlling thermal and plastic shrinkage in the immature concrete. Fibres commercially available include steel, glass, polyester, and polypropylene.

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Fibre reinforced concrete has been used in road construction for nosings and slabs in roundabouts for their abrasion resistance. Its application in bridges has been limited because of the additional cost compared with conventional reinforcement. Research into its use in bridge decks is being carried out in Canada and the USA. Fibres are often used in shotcrete to line tunnels. 4.9.3

Reactive Powder Concrete

Normal concrete is limited in its maximum strength and minimum penetrability by the presence of the coarse aggregate. Between about 80 MPa and 140 MPa the strength of the concrete undergoes a transition from being controlled by the strength of the paste, to being controlled by the strength of the coarse aggregate. Similarly, the restraints imposed by the coarse aggregate limit the minimum permeability of the concrete paste fraction due to micro-cracking of the transition zone around the coarse aggregate particles. In the late 1980s and early 1990s, some researchers began to postulate that extremely high strength concretes were possible if the coarse aggregate fraction was omitted from the concrete. The strength would be further enhanced if the remaining fines were somewhat reactive. Theoretical strengths of up to 800 MPa were postulated (Richard and Cheyrezy1994). In Europe, some construction companies saw that such a concrete might have practical advantages, especially in pre-cast work. Studies into the likely durability of such concretes by Andrade and others showed that these concretes possessed superior durability characteristics (Andrade et al. 1997) and (Roux et al. 1996). Bouygues, Lafarge and Rhodia originally developed reactive powder concrete to the point of being a commercial product. They have registered and patented their version of fibre reinforced reactive powder concrete as ‘Ductal®’. VSL Australia is the exclusive licensee in Australia and New Zealand for the product, and, in conjunction with the University of New South Wales, has produced a structural design manual for the material (Gowripalan and Gilbert, 2000). The current material produces a steel fibre reinforced concrete with a compressive strength of about 140 MPa to 160 MPa, and a flexural strength (modulus of rupture) of about 40 MPa. The material is highly impenetrable, thus having high durability. It is extremely tough under impact and blast. It also has very high bond strength to prestressing strand and to deformed reinforcing bars. It is flowable and highly cohesive, cannot be compacted by immersion vibrators and is difficult to finish due to the high volume of fibres. These features make it best suited to pre-cast work.

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5

CONCRETE DURABILITY

5.1 Concrete Distress Mechanisms The deterioration processes that may occur represent complex interactions between the structure and its surrounding environment and sometimes between the components within the concrete matrix. In general, reinforced concrete structures may suffer a reduction in durability performance by any of the following primary mechanisms: 

reinforcement corrosion



loss of bond between the concrete and the reinforcement



disintegration of the concrete matrix.

Disintegration of the surface matrix by chemical attack allows chemicals to penetrate the member, and, for example, cause steel corrosion that can lead to complete disintegration. This can include: 

physical damage to the concrete caused by impact of vehicles or vessels, debris or abrasion by wave action



volumetric change e.g. expansion due to AAR and shrinkage causing cracking which can hasten the corrosion of steel reinforcement.

The loss of cross-sectional area of the reinforcement and the loss of bond will reduce the load carrying capacity of the member. Causes of the above primary distress mechanisms are reviewed in Transit New Zealand (2001). 5.1.1

Reinforcement Corrosion

In general, steel within concrete is held at a pH between 11 and 13 (pH of saturated calcium hydroxide in the pore water). Within this range a tightly bound oxide layer forms to prevent corrosion of the steel. When this layer is disrupted, corrosion may commence. Corrosion product will form at the site where the protective oxide layer is disrupted. Anything that reduces calcium hydroxide starts corrosion. 5.1.2

Damage Caused by Alkali Aggregate Reaction (AAR)

The gel formed by AAR (Section 2.3.3) is expansive and results in cracking of the concrete. For high strength concrete the problem is exacerbated by higher cement contents with potentially greater total alkali content. High strength concrete, being denser, also has less internal voids for the expansive material to move into. Lower strength concretes are protected against cracking by the high void volume that allows the expansive material to disperse. The cracking of the concrete results in penetration of other aggressive agents, including: 

oxygen to sustain corrosion



water



carbon dioxide that reacts with calcium hydroxide reducing the alkalinity



sulphates



chlorides.

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The process requires water to sustain the reaction. Measures taken to inhibit or slow down the penetration of water will be beneficial in stopping or reducing the rate of deterioration (Figure 5.1). The measures available include: 

Sealing of cracks. It is important to seal cracks to ensure water is kept out – not kept in. Partial sealing of the base and sides of a member can make the situation worse.



Application of coatings or membranes.



Encasement of the affected areas using concrete, stainless steel or carbon fibre composite material. It is important that testing be carried out prior to encasing the member to determine if the expansion is nearing completion otherwise the encasement itself may also fracture.

Instances have occurred where cathodic protection (CP) has been used to protect bridges with AAR. However, the CP system must be properly designed and controlled in service to ensure it does not exacerbate the problem.

Source: RTA NSW

Figure 5.1: Severe pile deterioration caused initially by AAR then DEF

5.1.3

Delayed Ettringite Formation (DEF)

This mechanism refers to the delayed formation of ettringite (tricalcium aluminate trisulphate hydrate) usually due to the excessive heating of the concrete during its early hardening stage. As a result of the high temperature the formation of the stable ettringite is suppressed and the metastable monosulphate compound, tricalcium aluminate sulphate hydrate, is formed. When the concrete at a later stage reduces in temperature, the formation of the stable ettringite occurs. However, due to the amount of crystalline water contained in the formed ettringite, large expansive forces are generated in the hardened concrete.

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Ettringite may form in cracks in concrete that occur due to other primary mechanisms e.g. alkalisilica reaction. This type of ettringite formation is a secondary effect and generally harmless. An instance of DEF-related deterioration occurred in prestressed concrete piles in New South Wales (NSW). The severe deterioration of the piles was believed to be the result of initial cracking caused by AAR allowing penetration of seawater into the piles with chlorides, sulphates and magnesium ions. The increased moisture in the piles resulted in the delayed precipitation of ettringite generating destructive expansive forces. Subsequent investigations pointed to steam curing at excessive temperatures > 80 °C as the primary cause of the problem. The excessively high temperatures prevented the formation of ettringite during the early hydration. The problem surfaced 40 years later. The deterioration of the piles highlights the importance of ensuring that steam curing of concrete is carried out in strict accordance with the specification requirements (Section 4.4.6). 5.1.4

Chloroaluminate Formation

Recent work by Shayan (2006) indicates that under certain circumstances the formation of chloroaluminates may occur in a manner analogous to delayed ettringite formation (DEF) and this form may be expansive. Pre-requisite conditions appear to be a ready supply of chloride ions, water and concrete that is cracked by some other mechanism. It appears that, similar to the case of DEF, this mode of occurrence may aggravate any damage caused by AAR. 5.1.5

Carbonation

Carbonation of the cover concrete occurs when carbon dioxide from the atmosphere reacts with calcium hydroxide produced from the cement hydration reactions. As a result the pH of the pore water reduces to the level represented by a saturated calcium carbonate solution of pH 8.3. As the carbonated front approaches the reinforcing, the protective passive film on the steel surface may break down and the corrosion process, in the presence of water and oxygen, may take place. Carbonation is more intense in an environment where the relative humidity is between 60% and 80% compared to drier or more humid environments. In this range of humidity there is sufficient moisture available to form carbonic acid but not so much as to prevent the diffusion of the carbon dioxide into the concrete. Most bridges over permanent water provide the conditions for carbonation to occur. Carbonation is also very common in culverts as the cool environment increases the relative humidity within the culvert for the same absolute moisture content in the air. There is evidence that carbonation ‘pushes’ any chlorides deeper into the concrete, creating a synergistic effect on corrosion of steel. 5.1.6

Chloride Attack

Chloride ions act to disrupt the passive layer of oxides on the surface of the steel. Chloride ions may be present in three forms within hardened concrete: 

chemically bound



physically adsorbed



free chlorides.

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Only the free chloride ions are available for transport to an anode for the corrosion process to begin. Corrosion of steel reinforcement in chloride contaminated concrete is an electro-chemical process and requires oxygen and moisture for the reaction to continue. The oxygen is reduced at the cathode and moisture is necessary for the electrolytic process. Hence, corrosion activity will be greatest where oxygen, moisture and chloride ion concentrations are high e.g. in the splash zone of a marine environment. The chlorides are not bound in the corrosion process, and so act to sustain the corrosion. Chloride ions may enter a concrete structure by the process of diffusion for structures in saltwater or by capillary absorption for structures above water. Chlorides may also enter through cracks and other defects in the concrete. Chlorides may also be present either from contamination of the concrete materials or as a component of concrete additives. Standard specifications place limits on the amount of chlorides that may be present in fresh concrete. Despite the fact that specifications preclude the use of calcium chloride in concrete for bridge works, there are still instances where it has been used to achieve a rapid set to speed up production. Calcium chloride in reinforced concrete will cause disintegration of reinforcement in 10 to 30 years. Calcium chloride must be banned from all precast factories (Figure 5.2 and Figure 5.3).

Source: RTA NSW

Figure 5.2: Corrosion in precast culvert due to the use of calcium chloride

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Source: RTA NSW

Figure 5.3: Chloride attack in tidal channel

5.1.7

Sulphate Attack

Sulphates are found in fresh and sea water, industrial or domestic sewage and in soils that contain iron sulphides and have been exposed to air (acid sulphate soils). In sulphate attack, damage to concrete is caused by an expansive chemical reaction between tricalcium aluminate in the cement and sulphates in solution which produces both gypsum and calcium sulphoaluminate (ettringite). The crystals of ettringite occupy a larger volume than the original compounds. The larger volume leads to concrete expansion, cracking, and disintegration. The primary requirements of sulphate attack are: 

the availability of soluble sulphates



a relatively permeable concrete matrix that allows sulphate solution to penetrate



the availability of tricalcium aluminate component.

In contrast to the usual increase in corrosion with increase in temperature, sulphate attack diminishes with increasing temperature in the range 0-80oC (RTA 2005). 5.1.8

Acid Attack

In contrast to sulphate attack where only certain compounds in the cement system react, acid attack destroys the complete system. Acids in concentrations common in natural waters and soils tend to dissolve the carbonate layer on the surface of concrete, preventing further carbonation. Concrete will deteriorate because the calcium hydroxide in the concrete and the acids attacking it form water-soluble salts which are subsequently leached (Figure 5.4). The resistance to acid attack is independent of the permeability of the concrete and dependent upon the amount of acid available to attack the structure.

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The rate of acid attack of any concrete is controlled by the nature of the acid, the concentration of free hydrogen ions (the pH), the availability of the acid and the solubility of the calcium salts formed by exchange reactions with the salts dissolved in the water and the rate at which insoluble salts are removed by mechanical action.

Source: RTA NSW

Figure 5.4: Acid attack

An instance of acid attack of concrete occurred near Lithgow in NSW. The bridge was constructed over a cutting in volcanic tuff that has high sulphur content. The first sign that a problem existed was the observation of the disintegration of a concrete kerb at the base of the cutting. Testing of the ground water resulted in a pH of 2.2. The bridge piers consisted of precast, prestressed segmental columns supported on spread footings. The material was removed around the piers and footings and revealed that deterioration had occurred to a depth of 15 mm over 20 years. Remedial measures included restoration of the piers by constructing a concrete collar around them and isolating the pier from the volcanic tuff by backfilling with calcium carbonate. It is interesting to note that the acid had not attacked the mortar between the precast column segments. It is considered that this was due to the high strength and low permeability of the sand and cement mortar. 5.1.9

Physical Damage

Physical damage is defined as the damage caused to a concrete structure due to an external force or loading pattern as distinct from the chemical attack of the concrete matrix. The following types and causes of physical damage are noted: 

cracking due to overloading of structural elements



impact damage and abrasion of surfaces due to vehicles



fire on or below a bridge



abrasion of surfaces due to water-borne debris and suspended sediments in high velocity streams.

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5.1.10

Freeze/Thaw

The transition of water to ice produces an increase in volume of 9%. For saturated concrete this volume increase will cause spalling of the affected concrete. The limiting value of the water content causing damage to occur depends on: 

the age of the concrete



pore size distribution and the pore shape



the rate of cooling and frequency of freeze/thaw cycles



any drying out which may occur between freeze/thaw cycles.

5.1.11

Fire Damage

Concrete Concrete has high thermal insulating properties and provides a high level of fire protection. However, high intensity fires will cause damage to concrete and possibly reinforcing and prestressing steel, depending on the temperature and duration of the fire (Figure 5.5). Concrete begins to lose strength when heated to 100 °C. A permanent loss of 25% of strength occurs when concrete is heated to 250 °C. Fire damaged concrete will exhibit a change in colour. The assessment of the structural implications of fire damage to a concrete bridge will require input from concrete specialists with experience in fire damage. The assessment will require the extraction of cores from the affected areas to establish the extent of damage. The need for the imposition of a load limit or a temporary closure should be carefully considered in the first instance until a detailed inspection is carried out. It is important to establish the depth/temperature profile in the concrete. The depth of fire damage can be measured by detecting the depth of the 250 °C isotherm, as this is the lowest temperature with a clear indication of fire effect in the form of a colour change. The depth of the 100 °C isotherm can then be determined by extrapolating from the 250 °C isotherm. Mild steel Reinforcing steel is unaffected by fire. However, higher reinforcement grades that are cold worked steel will be affected if the steel temperature exceeds 400 °C. In the case of prestressing steel, the material is affected by temperatures >100 °C. The affects of the fire also include the loss of: 

protection to the reinforcement from the effect on the cover concrete, and hence the long term durability



composite action of decks due to effects on the reinforcing steel



anchorage for reinforcing steel



effective section which affects concrete compression and shear.

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Source: RTA NSW

Figure 5.5: Fire damage to Yowaka River Bridge

5.2 Protection of Concrete in Adverse Environments 5.2.1

Curing

Proper curing of concrete after placement is the first and most important step for concrete protection and should not be overlooked. Specification requirements must be carefully complied with. Curing is the name given to procedures used for promoting the hydration of cement, and consists of control of temperature and of the moisture movement from and into the concrete. The objective of curing is to keep concrete saturated, until the originally water-filled space in the fresh cement paste has been filled to the desired extent by the products of hydration of cement and hence enhancing its durability. Curing methods that are based on moisture control include wet curing and sealed curing (e.g. curing compounds). Curing methods that are based on elevated temperature to promote hydration are referred to as accelerated curing (e.g. steam curing).

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The choice of a curing method depends on a number of factors including the type of structure, the orientation of the surface to be cured, and the type of cementitious materials in the concrete mix. To ensure the long-term durability enhancement of supplementary cementitious materials (SCM) concretes (see the next section), moist curing is highly recommended. Curing compounds are less effective for SCM concretes with high cement replacement levels in marine and other aggressive environments. However, they are better than no curing. 5.2.2

Chemical Composition of Supplementary Cementitious Materials (SCMs)

SCM materials are often incorporated in modern concretes to supplement Portland cement for various purposes. Fly ash, ground granulated iron blast-furnace slag and silica fume, which are industrial by-products, are the main SCM in Australia. In New Zealand a proprietary natural geothermal product is currently used instead of silica fume. There are two main groups of reactions responsible for strength development in modern concretes that incorporate SCM. The first and main group is the hydration of hydraulic cement compounds. The second reaction is known as the pozzolanic reaction. Pozzolanic reaction occurs between an added pozzolan, as a mineral admixture, and the lime produced during hydration of the hydraulic cement. Fly ash and silica fume have pozzolanic properties, while slag has primarily the properties of hydraulic cement. Fly ash is a by-product of the combustion of pulverised coal in thermal power plants. By oxide analysis, it is predominantly composed of silica, with alumina, oxides of iron and minor amounts of other oxides and organic matter. Blast furnace slag is a mixture of lime, silica and alumina with small quantities of other materials (magnesia, alkalis, etc.). It is developed in a molten form simultaneously with iron in a blast furnace. Rapid chilling with high pressure, high volume water sprays, forms a glassy (amorphous) granular, coarse sand-like material known as granulated iron blast-furnace slag. In Australia fly ash and blast-furnace slag are readily available from the power generation industry. In New Zealand most SCM materials need to be imported and therefore come at a premium as materials used to improve concrete quality. It should be noted that not all fly ash and blast furnace slag materials have the chemical properties that result in the enhancement of concrete durability. Hence the need for testing of proposed materials for compliance to specifications. Silica fume is produced in submerged arc furnaces used to manufacture ferro-silicon and silicon metal alloys. Cements that comprise a Portland cement and one or more SCM are known as ‘blended’ cements. 5.2.3

SCM and Concrete Protection

Most of the desirable attributes of SCM concretes depend on the combinations of paste enhancement, water reduction and pozzolanic activity, resulting in refinement of the pore distribution in the paste fraction of the concrete, and hence enhancing physical resistance. Also SCM concretes have high chemical resistance in different aggressive environments. SCMs often tend to be finer than cement and hence provide a smaller pore structure in the cement, which enables hydration products to more effectively fill the voids within the matrix.

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For marine exposure, SCM concretes in general perform better than would be predicted from physical resistance enhancement alone. This is due to a high chemical resistance. The chemical resistance, in respect to this environment, is the ability of SCM concretes to chemically bind the harmful chloride ions in concrete. The use of SCM enhances the concrete resistance to the deleterious expansion of AAR, where aggregate of potential AAR is incorporated in the concrete mix. However, the use of silica fume in concretes for the purpose of suppressing AAR is contentious. One issue is the fact that the silica fume tends to aggregate in lumps, which can actually cause AAR. The natural silica product used in New Zealand is not prone to this problem. When properly cured, concretes containing SCM have superior sulphate and acid resistance than concretes without SCM. 5.2.4

Waterproofing Membranes

Waterproofing membranes are increasingly being used particularly on bridge decks. The main materials used at this stage (based on economics) are rubberised or polymer modified bituminous surface treatments, slurry seals and some bituminous proprietary products. Some sheet membranes have been used (Section 4.2.6). 5.2.5

Protective Coatings

Protective coatings play a key role in the overall repair process. They are characterised by their ability to prevent the ingress into the concrete of water, oxygen, deleterious materials such as chlorides, carbon dioxide, and other aggressive liquids or vapour. Materials which are commonly used to formulate proprietary coating systems for the protection of concrete include organic and inorganic generic types of materials such as polyurethane resins, polymer modified Portland and blended cements, epoxy resins, chlorinated rubber, acrylic resins, vinyl ester, bituminous and silane/siloxane. Protective coatings and treatments are required to have a range of properties to ensure an effective and durable protection of concrete. These include adherence to the concrete surface, alkali resistance, ability to accommodate movements in the concrete, resistance to chalking, UV radiation, water, chloride and carbon dioxide ingress and penetration capability for pore lining. In terms of carbonation, film-forming coatings such as acrylics, epoxies and polyurethanes are effective barriers. Pigmented coatings give much better protection and decorative finish and are more durable than unpigmented coatings. It should be noted that where deterioration has already begun, but is not yet visible, the application of a protective coating will prevent further ingress of corrosive agents, but will not halt the deterioration process. Therefore, protective coatings are best applied to the fresh concrete before exposure to adverse environments. In order to ensure the quality of application, tests should be specified before and after the application of the coatings. Prior to application of the coatings these include surface moisture condition of the concrete substrate and moisture content to demonstrate that it is free of water back -pressure. After application tests may include bond strength and wet and dry film thickness to confirm compliance with the materials manufacturer’s specification and depth of penetration of the pore lining penetrant.

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Coatings used as part of a repair system must be compatible with the other components of the repair system. 5.2.6

Hydrophobic Impregnating Materials

Silane/siloxane materials are hydrophobic impregnating materials that do not form a thin film on the concrete surface. These materials impregnate the concrete and react with the moisture and silicates present in the cement, thus modifying the concrete surface to form a water repellent but vapour permeable (breathable) barrier. Silanes/siloxanes prevent contamination by preventing water containing salts and other deleterious substances from entering the concrete. However, being vapour permeable they have low resistance to carbon dioxide and are not effective against carbonation. Re-application of the material at the time interval recommended by the manufacturer is required for the ongoing protection of the concrete. There is an extensive range of commercial products available. RTA has conducted extensive testing of the efficacy of the currently available silanes/siloxanes. Information on the test results can be obtained from the RTA. 5.2.7

Corrosion Inhibitors

Corrosion inhibitors based on calcium nitrites have been extensively used overseas in reinforced concrete structures. Such corrosion inhibitors are now available in the Australian market. From a study by the RTA, calcium nitrite, when appropriately used, can extend the service life of reinforced structures in marine environments. However, calcium nitrite inhibitors retard the curing of concrete and this has to be properly managed. The user of corrosion inhibitors requires specialised advice as they have the potential to reduce the durability of concrete as well as enhancing it. 5.2.8

Cathodic Protection

Cathodic protection (CP) of reinforced concrete is a practical long-term solution for the protection of new bridges and for the rehabilitation of bridges suffering from chloride-induced corrosion. The principle of the operation of a CP system is based on reversing the flow of electrical currents that sustain the corrosion process. This is achieved by imposing a direct external current source into the system via an external anode. The impressed current is transferred through the concrete matrix to the embedded reinforcement. New bridges In the case of new bridges the electrical connectivity of the reinforcement has to be verified prior to the pouring of concrete. Provision for CP at construction (electrical connectivity and installation of cabling and junction boxes) is a very economical method of ensuring future protection of the reinforcement. Existing bridges The use of CP on existing bridges is a very cost-effective alternative to the difficult and extensive removal of chloride-contaminated concrete surrounding the steel reinforcement and the subsequent ongoing repair operations required to achieve the life expectancy of a structure. However, it may be difficult to obtain full electrical connectivity for the system to operate.

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CP systems There are a number of CP systems available. Each system or combination of systems can differ in terms of design life, ease of installation, aesthetic effect on the structure, imposition of dead loads, cost, degree of access requirements and suitability for particular structural components. Expert advice from corrosion engineers is essential. Some of the systems used are: 

internal anode/water-soil anode combination



titanium mesh/cementitious overlay



slotted/water-soil anode combination



mesh or slotted ribbon anodes with overwrap systems



impressed current soil anode



spray zinc/sacrificial anodes combination.

Overwrap systems are only considered appropriate for small to medium size columns/bridges where aesthetic appearance is not considered a problem. Sacrificial and sprayed zinc systems are uncomplicated, require very little maintenance, are self-regulating and can be installed at low initial cost. They are also only considered appropriate for small to medium size columns/bridges. Side effects of CP include possible acid attack at the anode/concrete interface and hydrogen embrittlement of prestressing steels due to over-protection and potential for AAR due to the increase in alkalinity at the steel. Other problems could include electrical short circuits and galvanising. Such potential problems can be overcome with careful diagnostic assessment, design, installation and subsequent monitoring and maintenance. All cathodic protection systems should be installed with a remote monitoring capability to provide readily available information on their performance. The systems should be controlled centrally by people with expertise in CP systems. The use of CP impressed current systems is preferred to galvanic CP on bridges. Galvanic CP systems cannot be remote monitored.

5.3 Durability Assessment Techniques Cautionary note: The limitations of investigation and inspection techniques used for the assessment of existing bridges are highlighted in Middleton and Lea (2004). 5.3.1

Desk Top Investigation

An integral part of the investigation of deteriorated structures is the collation and review of background information, including drawings, specifications, construction records, details of previous investigations and repairs, and other historical data. 5.3.2

Visual Inspection

A visual inspection is one of the most important steps in a detailed site investigation and will basically detect obvious indications of concrete deterioration such as cracks, stains, spalls and physical damage. Visual inspection will not, however, give any information on the possible contamination or deterioration of apparently sound concrete.

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5.3.3

Defect Mapping

A comprehensive investigation of a deteriorated concrete structure will include the mapping of identifiable defects and the locations where other test procedures are performed. Defects will include: 

cracking



scaling – local flaking or peeling away of the surface portion of hardened concrete or mortar



spalling – concrete fragments, usually detached from the parent concrete



efflorescence – deposition of white salts or lime mortar on the concrete surface



honeycombing – clear evidence of voids or spaces between the coarse aggregate particles



dampness – wet or moist areas of concrete



joint leakage – water and contaminated fluids originating from the road surface.

5.3.4

Measurement of Crack Development

Crack development over a period of time can be measured using various methods including optical equipment and electrical strain gauges. The Demec gauge, a mechanical strain gauge, is another simple way to measure crack movements with studs fixed permanently on either side of appropriate cracks. This is only necessary for critical cracks. Visual estimates are sufficient for most situations. Pen sized optical crack measurers and comparator cards with lines of varying thicknesses are suitable for most requirements. The extent, width and date of observation should be recorded in records and on the component. The measurement of crack movement is important in establishing whether a crack is live or not, so that the appropriate repair method can be adopted. 5.3.5

Cover Surveys

The depth of cover, size and location of reinforcement can be measured with electromagnetic cover meters. Note that that the instrument must be calibrated to the size of bar and anticipated depth. Cover readings should be analysed statistically. Cover surveys are correlated with other elements of the investigation, including defect mapping, carbonation depths and chloride profiles. 5.3.6

In Situ Compression Testing

A number of destructive and non-destructive tests are available to measure the surface hardness of concrete and hence predict strength. The most commonly used device is the non-destructive rebound hammer called the Schmidt hammer. Other tests include the Windsor probe test, pull-off test, internal fracture and surface hardness. Significant skill and training are required to use all types of equipment. 5.3.7

Ultrasonic Pulse Velocity

The ultrasonic pulse velocity technique measures the speed of travel of a pulse through concrete and is used for relative strength assessment and the detection of voids, delamination, under-compaction and honeycombing. The presence of steel reinforcement can affect pulse velocity values and needs to be considered when interpreting test results.

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5.3.8

Ground Penetrating Radar

Ground penetrating radar has been used successfully to assess the extent of defects in concrete members. Applications have included the assessment of flotation of voids in prestressed planks and to determine the extent of voids in the web of a prestressed box girder. 5.3.9

Permeability and Water Absorption

Determination of the permeability, water absorption and volume of permeable voids (interconnected void space) can give a good indication of the quality of the concrete microstructure and its ability to limit the rate of ingress of aggressive agents such as chlorides and carbon dioxide. 5.3.10

Concrete Sampling

Concrete sampling is undertaken by taking cores from the member in question. The requirements for the minimum diameter, aspect ratio, number, specimen conditioning, testing and age correction factor are set out in specifications and the relevant standards. A number of tests can be undertaken on the cores to assess the quality and quantity of the main physical and chemical characteristics of the concrete, including: 

visual assessment of integrity



petrographic description of aggregates



chloride content



concrete density



compressive strength



cement content



water cement ratio



concrete permeability to water and air



volume of permeable voids.

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6

METALLIC MATERIALS

6.1 Historic Development 6.1.1

Structural Iron and Steel

The modern steels that are used in bridges today are the result of the long development of metallic materials over many centuries. Each step in the process resulted in the refinement of material properties to meet the need for different bridge forms. 6.1.2

Iron to Cast Iron to Wrought Iron to Steel

Iron Iron is the most important of the industrial metals. Its basic alloys, cast iron, wrought iron and steel, are the world’s cheapest and most useful metals. They have made a significant contribution to the development of modern civilisation, particularly since the start of the industrial revolution over 200 years ago. From bridges to railways, ships, motor vehicles, machinery, canned foods, knives and forks, even reinforced and prestressed concrete, iron and steel have played a fundamental role. Iron is a generic term that can be applied to the pure element, iron, or to its alloys, particularly cast iron and wrought iron, but not generally to steel because it has proved to be ‘something different’ and is by far the more important and dominant metal. Steel bridges are usually referred to as metal bridges, not iron bridges, such is the important distinction between the two. The three basic iron alloys consist almost entirely of two elements, iron and carbon, with iron usually in excess of 95% and carbon at a maximum of 4%. Special alloys have other elements added, usually at the expense of the iron, in order to achieve particular characteristics. For example, non-corrodible stainless steel has 12-30% chromium and some nickel, whereas manganese imparts hardness and long wearing qualities. When viewed through a microscope, iron appears as a collection of grains. Pure iron has a useful strength (equal in tension and compression), is easily worked into shapes by rolling or forging (it is malleable) and is weldable. When overstressed, it deforms by a large amount before breaking (it is ductile), but it is relatively soft and so is easily abraded. Wrought iron is almost pure iron but it is the result of an expensive manufacturing process. Any impurities in wrought iron are as inclusions between grains of iron. Carbon The introduction of carbon changes all these basic characteristics, initially for the better then gradually for the worse. As little as 0.25% creates mild steel, which is a much stronger metal than wrought iron because the carbon is held as an alloy with the iron atoms in the grains causing a locking action that resists deformations. However, malleability, ductility and weldability remain good. Being readily rolled into plates, bars, wire and a large range of structural shapes, it is the most widely used steel. Its manufacturing process, incorporating large, open-hearth furnaces, allows huge quantities to be made much more cheaply than wrought iron.

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As the amount of carbon increases, it continues its locking action but begins to push the iron atoms apart. The introduction of 0.45% carbon creates high strength steel with a doubling of strength but at the expense of a significant loss of malleability, ductility and weldability. Loss of ductility means an increase in brittleness, hence, high strength steels are more susceptible to brittle failure. The higher the carbon content the more brittle the steel, and hence the more susceptible the steel is to brittle failure. The higher strength has little effect on the susceptibility of the steel to fatigue under cyclic loading, so the combined effect of the more brittle behaviour and the fatigue behaviour renders very high strength steel unsuitable for general structural use. By the time the percentage of carbon reaches 1%, strength is still high but the other characteristics are unsuitable for structural use, such as bridges. However, the steel is very hard and is therefore widely used for machine parts and tools. At 2%, the carbon can no longer be held within the grains, and so precipitates out, with the iron and carbon being separate phases and cast iron is the result. It has useful compression but negligible tensile strength. It cannot be rolled or worked but has improved fluidity suitable for casting into moulds, hence the name cast iron. Under load, particularly in tension, it fractures without signs of distress as it is now brittle (a dangerous condition) and so it cannot be welded. It has become a niche product suitable for particular applications such as columns in buildings and trestles for bridges. At 4% carbon the metal is useful only where sheer mass is desirable, such as engine blocks for motor vehicles. Historically, steel and high strength steels have been used for two thousand years but mainly in weaponry, particularly for swords. The high costs of labour intensive production and the small quantities produced meant that steel was not available for general use, such as for bridges. But cast iron was well known and widely used. It was cheaper to make and large quantities could be produced. The first metal bridge, the 1776 Coalbrookdale Bridge on the Severn River in the United Kingdom, was an open, lightweight arch, a basic compression structure, for which cast iron was ideal and affordable (Figure 6.1).

Source: Austroads (2001)

Figure 6.1: Coalbrookdale cast iron bridge

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Concurrently, ironmakers were experimenting with methods to refine cast iron, as had been done by blacksmiths for centuries, but in economic quantities. The most successful method was developed by Henry Cort in England in 1783. His ‘puddling’ process raised a quantity of cast iron to a spongy white-hot mass that was beaten under a forge hammer (it was wrought or worked or shaped) such that the impurities and the carbon were oxidised and squeezed out as a slag. The process was repeated a number of times until a uniform mass of near pure iron was obtained. It was malleable and was able to be worked into many forms, merchant bars or structural sections. This was wrought iron. Wrought iron Wrought iron was an outstanding contribution to iron technology and increased production from one ton per day to 15 tons per day. This process provided the means for making pure iron of reasonably uniform quality and in quantities needed for the great industrial expansion of the early 1800s. Wrought iron became the major civil engineering material for railways and bridges where the brittleness of cast iron made it unacceptably dangerous. However, when appropriately used, a mix of cast iron and wrought iron elements could create useful bridges in which compression members are cast iron and tension members are wrought iron. There are some 150-year-old examples still in use in England and Europe. The quest to further improve wrought iron continued. Experience had shown that small amounts of carbon could increase strength, but the manufacturing process beat it all out again. A new process was required. Steel The breakthrough came in 1856 when Henry Bessemer invented his converter, which was basically an iron pot with holes at the bottom by which air could be blown through a molten mass of cast iron to oxidise the impurities in only about 20 minutes. The resulting pure iron could then be transferred to another furnace where pre-determined amounts of carbon, or any other alloying material, could be added. An economical process for the production of steel had been invented. It led to the mass production of uniformly reliable, low cost steel which was stronger than wrought iron, was equally strong in tension and compression, was malleable, ductile and tough. 6.1.3

Cast Iron

Material properties Cast iron is one of the oldest ferrous metals used in construction. It is primarily composed of iron, carbon and silicon but may also contain traces of sulphur, manganese and phosphorous. It has a relative high carbon content of 2-5%. It is hard, brittle and non-malleable, i.e. cannot be bent, elongated or hammered into shape. Its structure is crystalline and relatively weak in tension with very low ductility. Typical properties of cast iron are outlined in Table 6.1. The most common form is grey cast iron, which can be easily cast but cannot be forged or worked mechanically. In grey cast iron the carbon content is in the form of flakes distributed throughout the metal. In the other form of cast iron, white cast iron, the carbon content is chemically combined as carbide of iron. White cast iron has superior tensile strength and malleability. It is known as malleable cast iron.

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Table 6.1: Typical properties of cast iron Cast Iron Period

BC-1920s (Still used in many applications, but in few bridges)

Yield strength – tension (Mpa)

No yield point - (brittle failure)

Ultimate strength – tension (MPa)

140

Yield strength – compression (MPa)

240

Brittleness

Very

Fatigue resistance

Poor

Weldability

No

Main uses in bridges

Pier caissons, bearings

Main fastening method

Bolts/rivets

Composition – carbon

Above 1.7% (Generally 2-4%)

Other chemical features

-

Heat treatable?

No

Deterioration mechanisms Corrosion – Cast iron is highly susceptible to corrosion when the humidity is above 65%. The rate of corrosion depends on the type of material, protective coatings used and the severity of exposure. Corrosion may be accelerated by atmospheric pollutants such as sulphur dioxide. Graphitisation –Cast iron contains carbon, in the form of graphite, in its molecular structure. It is composed of a crystalline structure, as are all metals. One condition that can occur in the presence of acid rain and/or salt or brackish water is graphitisation. Under this condition the stable graphite crystals remain in place but the less stable iron is converted into somewhat soluble iron oxide and is leached out. As a result the cast iron retains its shape but has no mechanical strength due to the loss of the iron (Figure 6.2). The corrosion process is galvanic. Instances have occurred where the loss of section has affected the structural capacity of members to the point where strengthening was required. Testing members for the effects of graphitisation involves the coring of the material to ascertain the extent of the loss of section. Cores should be removed from the member both above and below water level. The tidal range should also be taken into account when assessing the material.

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Source: RTA NSW

Figure 6.2: Graphitisation of cast iron

Grain growth – Over time the crystalline structure changes with the result the grains become larger and the material becomes more brittle. It is the practice in NSW to replace all cast iron shoes in timber trusses with fabricated steel shoes in conjunction with other maintenance work. Cast iron applications Pier caissons and columns – Bridge pier caissons were constructed using cast iron in the lower sections below the point of fixity where they were subjected to axial loads only. In the sections of the caissons subjected to combined bending and compressive loads wrought iron was used (Figure 6.3).

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Source: RTA NSW

Figure 6.3: Wrought iron caissons above ground – cast iron caissons below ground

The level of the connection of the cast iron to wrought iron is important as bank or stream bed scour may result in the cast iron being subjected to bending for which it was not designed. Instances have occurred where a combination of bank scour and the freezing of expansion bearings resulted in additional bending moments to caissons resulting in fracturing of the cast iron.

Source: RTA NSW

Figure 6.4: Cast iron columns on timber bridge pier

In some shorter span bridges cast iron was used as pier columns (Figure 6.4). They were usually filled with stone rubble and grouted.

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Cast iron in timber trusses – Cast iron was used on timber truss bridges at the member connections (Figure 6.5). However, due to the brittleness of the material, cracking of the castings often occurred. The material was also used in tension rod anchorages, column heads and bearings.

Source: RTA NSW

Figure 6.5: Cast iron shoe at lower end of timber truss member

Cast Iron Repairs – Cast iron cannot be satisfactorily repaired by welding. 6.1.4

Wrought Iron

General Wrought iron was used in a number of road bridges in Australia and New Zealand in the 19th century. Many of the bridges survive today. The material was used in trusses and plate web girders (Figure 6.6). In NSW 25 wrought iron lattice truss bridges were built. Many of them are still in service.

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Source: RTA NSW

Figure 6.6: Wrought iron lattice truss

Material properties Wrought iron is a two component metal consisting of high purity iron and iron silicate, an inert glass-like slag. The materials are merely mixed and not chemically joined as in an alloy. Slag accounts for 1-3% in the form of small fibres up to 20,000 per 25 mm of cross-section. The material has a laminated structure and as a result the transverse strength is significantly lower than the strength in direction along the laminations. Manufacturing process The first metal bridge, the 1776 Colebrookdale bridge in the United Kingdom, was an open, lightweight arch; a basic compression structure, for which cast iron was ideal and affordable. The manufacturing process was labour intensive with the result that the mechanical properties were highly variable even from the same iron works. The process was complex, time consuming and required a highly skilled work force. Molten iron was cast into ingots then stacked in a puddling hearth. It was heated and puddled to remove impurities. The iron was then transferred to a shingling hammer, which was a giant power hammer capable of earth shattering blows that formed the iron into a billet. This was then rolled, cut into lengths, restacked, and the whole process repeated over and over again. The more times the process was repeated, the better quality of the finished wrought iron. The hammering process squeezed most of the slag out of the iron.

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Close examination of the edge of some wrought iron members will reveal a laminated structure (Figure 6.7). This is indicative of the repeated process of heating, folding over, and hammering to improve the material properties.

Figure 6.7: Wrought iron plate showing laminar structure

Wrought iron was made in batches or charges of about 200 kg and therefore in 1889 when the British production was 2.2 million tonnes there would have been about 11 million individual charges in some 5400 iron works. Hence, the highly variable mechanical properties of wrought iron.

6.2 Structural Assessment of Existing Bridges The structural assessment of a wrought iron bridge requires a rigorous approach in terms of: 

material properties



connections and second order effects



inspection



measure up



an understanding of the nature of the material and how it relates to the method of manufacture



load effects both globally and at member level



dynamic effects of current heavy loads.

A number of road authorities have carried out structural assessments of wrought iron bridges, which have included: 

structural modelling



inspection



fatigue assessment



load testing to calibrate structural models



tensile testing of samples removed from non-critical members



in situ and laboratory hardness testing



removing 25 mm cores from selected members for micrographic examination and chemical analysis for comparison with those of samples used for tensile testing.

The test results confirm the high variability of the material. 6.2.1

Yield Strength

Yield strengths of samples removed from three bridges in one state varied markedly. Yield strengths varied from 237 to 310 MPa. The tensile test also indicated that the thinner plates have higher yield strengths than thicker plates. Hardness tests carried out indicated that some members could have yield strengths as low as 200 MPa.

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6.2.2

Ultimate Tensile Strength

The ultimate tensile strength (UTS) of samples removed from three bridges also varied markedly. UTS varied from 338-427 MPa. 6.2.3

Ductility

The percentage elongation at failure varied from 9% to 18% that is indicative of a ductile failure. However, subsequent testing of another bridge resulted in elongation as low as 5%. These results highlight the variable material properties of wrought iron that result from the differing degrees of refining and working used by different foundries and at different times within a foundry. AS 5100.6 (2004) Clause 2.8 states that the minimum characteristic elongation at failure must be 5% for the design rules for steel to be used to assess the capacity of a wrought iron structure. 6.2.4

Members and Connections

Wrought iron rivets were used to construct wrought iron truss members and built-up beams. Member-to-member connections were also achieved using wrought iron rivets. Assessment of the rivet capacity may necessitate the removal of a section of non-critical member with the rivets still intact. Actual member dimensions and thicknesses should be verified on site. Actual edge distances and rivet layout should also be verified on site. Any section losses due to corrosion should be accounted for in any structural assessment. Second order effects at connections should also be investigated. 6.2.5

Weldability

Welding of wrought iron is possible but is not recommended because of the possibility of secondary effects. These include the heat input that can cause cracking on cooling in situations where the member or area of welding is restrained. There is also the risk of lamellar tearing if welding is carried out on the surface of a member because of the low strength of wrought iron normal to the direction of laminations. Welding of tension connections subjected to fatigue in particular is to be avoided. Welding may result in the loss of the laminar structures and may cause the formation of irregular lumps of slag within the heat-affected zone. These may result in major reductions in fatigue life. Bolting should be used in preference to welding. 6.2.6

Fatigue

Wrought iron has good fatigue strength. The laminations prevent the propagation of cracks, thus any cracking has to be re-established as it passes through each lamination. Any structural assessment should include a fatigue analysis. Fatigue cracks are often initiated at connections and at pitted and corroded surfaces. However, there is a need to distinguish between cracking that occurred at fabrication and in service.

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6.3 Structural Steel 6.3.1

Modern Steel Properties

The typical properties of the materials described are summarised in Table 6.2. Table 6.2: Typical bridge material properties Period Properties and applications

Cast iron

Wrought iron

Mild steel

BC-1920s (Still used in many applications, but few in bridges)

1850-1900s

1890s-today (Possibly some in 1860-1890 period)

Yield strength – tension (MPa)

(Brittle, no yield point)

210

220-450

Ultimate strength – tension (MPa)

140

345

400-600

Yield Strength – Compression (MPa)

140

210

220-450

Brittleness

Very

Good

Good

Fatigue resistance

Poor

Best

Good

Weldability

No

Was done by fusion Welding is problematic - avoid

Progressively improved

Main uses in bridges

Pier caissons, bearings

Truss members; caissons

Trusses, girders, cables

Main fastening method

Bolts/rivets

Rivets

Rivets then bolting and welding

Composition – carbon

Above 1.7% (Generally 2-4%)

Low 0.02-0.04% typical

Low 0.15-0.25%

1 - 2% ferrous silicate slag in long fibres

Manganese to 1% Silicon to 0.2%

No

No

Other chemical features Heat treatable?

No

Source: RTA NSW

The material properties information shown in Table 6.2 is indicative only. The properties of materials in existing bridges will vary from one to another. In addition descriptions of metals have evolved over time as properties have improved. For example, modern iron castings can in fact have a wide range of properties. Steels have varying strength, depending on their composition – most notably carbon. Additionally, properties of steels are affected by the rate of cooling, work hardening or cold working. 6.3.2

Hardness

The hardness of steel can be used as an indicator of the tensile strength. The testing of hardness is generally determined by testing its resistance to deformation. A number of test methods are available including Brinell, Vickers, and Rockwell. Hardness testing can be used in a number of bridge applications including: 

Assessment of the variation in strength of older wrought iron and steel where it is not possible to obtain samples for destructive testing. A statistically significant number of tests are required to enable the testing to identify variations in strength. It should also be noted that in the case of wrought iron the inherently highly variable microstructure may give widely scattered results, which must be treated with caution.



Assessing the properties of the heat affected zone of welds.

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

To determine variations in quality of steels.

Significant variations in hardness are an indicator of either poor quality control in the original material, or possibly that different types of steel were used in different parts of the bridge – perhaps to suit required strength. Hardness of steel can be changed by straining it beyond its yield point.

Source: Davis, Troxell & Wiskocil (1964)

Figure 6.8: Increase in yield point by repetitive straining

Figure 6.8 shows how repeated extensions actually increase the yield point of the metal. This was used to increase the yield point of reinforcing steel for many years with the designation CW16, standing for a 16 mm bar that had been cold worked. In the case of older reinforcing bars this was done by twisting the bars, with the result that the outside of the bar was harder than the centre – which had been strained less.

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Source: Davis, Troxell & Wiskocil (1964)

Figure 6.9: Effect of hardening and tempering

Figure 6.9 shows the stress-strain curve of steel before and after hardening. Whilst the hardened material will take a higher load before getting to yield, its failure is then brittle. This situation can be rectified by heat treating, or ‘tempering’ the steel after it has been hardened. The current class of N bars uses a different process to achieve a similar strength, but with better ductility. Summary The properties of steel and wrought iron have undergone major changes over the years. In assessing or repairing any existing steel bridge it is imperative that the particular steel be identified by carrying out one or more of the following: 

examination of bridge drawings for the nominated steel classification



hardness testing



destructive testing to determine tensile strength; the removal of samples from an existing member should only be undertaken after a structural assessment of the bridge has been carried out to identify suitable locations; the designer should provide detailed information on the location, sample dimensions, method of removal and method to restore the member and protective treatment to exposed surfaces



chemical composition to determine weldability and welding consumables if repairs or welding to the bridge are required.

Steel manufacturers and National Association of Testing Authorities (NATA) certified laboratories offer a service in metallurgical identification of steels. It is only when the steel is identified and the mechanical properties and chemical composition are known that a metallurgist can advise on what can and cannot be done with a particular steel.

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6.3.3

Ductility

Ductility is an important property of a metal as it indicates its behaviour at high stress levels. A ductile material can undergo large plastic deformations (yielding) before failure occurs. This is an important property for the bridge engineer. It means that if the material is being subjected to excessively high stresses warning signs will be evident in the form of permanent beam deflections, as opposed to elastic beam deflections that recover when the load is removed. Figure 6.10 shows the plastic deformation that occurs in a tensile test of a reinforcing bar. The plastic deformation has caused ‘knecking’ at the point of failure and an indicator of a ductile material.

Source: D Carter

Figure 6.10: Tensile test of reinforcing bar – ductile failure

6.3.4

High Strength Steel

Steel strengths have increased over time. The yield stress of ordinary carbon steel has increased from about 180 MPa to over 400 MPa. Modern high strength steels offer economic advantages in steel bridge construction as less steel mass is required compared to that required when using lower strength steel. It should be noted that the high strength properties may be lost when the steel is heated, depending on the process that was used to increase the yield strength. The reduction in strength is a consequence of metallurgical changes that occur as a result of direct heating or incorrect welding procedures including tack welding. High strength steels usually have a lower ductility than lower strength steels. In the past the low ductility caused brittle fracture, particularly at low temperatures. Modern high strength steels have addressed this issue. Very high strength steels are prone to hydrogen embrittlement, which occurs when hydrogen diffuses into the steel as a result of exposure to it. For this reason high strength steel produced by cold working should not be galvanised as hydrogen embrittlement may occur. The welding of high strength steel requires an even stricter control of the welding process, compared to normal grade steels, as there is an increased risk of detrimental effects on the material if the specified welding procedures are not complied with.

6.4 Aluminium 6.4.1

Material Properties

Pure aluminium has an ultimate tensile strength of about 90 MPa. However, by cold-working the tensile strength is approximately doubled. The alloying of aluminium with small percentages of other metals such as manganese, silicon, magnesium or zinc results in much higher tensile strengths. The alloys are classified into two categories, non-heat treatable and heat treatable.

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6.4.2

Non-heat Treatable Alloys

The initial strength of alloys in this group depends on the effects of elements such as manganese, silicon, iron and magnesium in isolation or in combination. The non-heat treatable alloys are designated in the 1000, 3000, 4000 or 5000 series. These alloys can be strengthened by work hardening denoted by the H series tempers. 6.4.3

Heat Treatable Alloys

The initial strength of alloys used in this group is enhanced by the addition of alloying elements, which show increasing strength with increasing temperature. Using the proper combination of heat treatment, quenching, artificial ageing and cold-working the highest strengths are obtained. Temper designation The basic temper designations and subdivisions are as follows: 

F – as fabricated



O – annealed, recrystallised



H – strain hardened — H1 – strain hardened only — H2 – strain hardened and then partially annealed — H3 – strain hardened and then stabilised



T – thermally treated to produce stable tempers other than F, O or H — T1 – cooled from elevated temperature and naturally aged at room temperature — T3 – solution heat treated, cold worked and naturally aged — T4 – solution heat treated and naturally aged — T5 – cooled from elevated temperature and artificially aged — T6 – solution heat treated and artificially aged — T8 – solution heat treated, cold worked and artificially aged — T9 – solution heat treated, artificially aged and then cold worked.

It should be noted that specifications for material used in extruded sections in traffic barrier railing and pedestrian railing are designated as Alloy 6061-T6 and Alloy 6063-T6 respectively. Typical properties are set out in Table 6.3. Table 6.3: Typical properties of aluminium used in bridge applications Alloy

6061 – T6

6063 – T6

UTS ( MPa)

310

241

0.2% yield ( MPa)

276

214

Elongation (%) in 50 mm

12

12

6.4.4

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6.4.5

Material Certification

The material used in the works is to be tested and certified by a NATA registered laboratory. As set out above, there is a wide range of materials available. Therefore, identification of the material to be used in the works is essential. The test certificates are to be related to the aluminium by trade marks and heat number, which are marked on each piece. 6.4.6

Welding

Welding of aluminium is to be carried out in accordance with AS 1665 (2004) Welding of Aluminium Structures. The requirements for welding of steel in regard to weld procedure prequalification and welders’ pre-qualification also apply to aluminium. Certification is also required that the welding consumables (rods and continuous wire) are compatible with the material being welded. 6.4.7

Fatigue

The fatigue strength of aluminium is an issue for members subjected to repeated loading. Unlike steel the fatigue life of aluminium has no lower limit of stress required to cause fatigue damage. As a consequence a high number of cycles at low stress levels can cause a fatigue failure. An instance has occurred where the welded connection between the balusters and the bottom rail have cracked after ten years in service. It is suspected that a sympathetic vertical vibration in the rail induced by the vertical vibrations in the bridge under traffic loads along with the effects of wind induced vibrations have caused the failure, as shown in (Figure 6.11).

Source: RTA NSW

Figure 6.11: Fracture in aluminium weld

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7

STEEL DISTRESS MECHANISMS

7.1 Corrosion The corrosion of steel is its tendency to return to a lower energy state. The process of corrosion is for a metal atom to be oxidised by losing one or more electrons and leave the metallurgical structure and hence reducing the volume of the metal. Therefore, steel is inherently unstable and on this basis one of the primary considerations in designing a steel bridge and managing it in service is to counter this instability. This can be achieved by various measures that prolong its service life including: 

good design details that reduce the potential for corrosion



maintenance of the protective coating



good maintenance practices such as the removal of accumulated debris, dirt and contaminants from the steel surfaces.

The main types of corrosion are: 

Metallic corrosion – the reaction between the steel and its environment to form chemical compounds. The process is an electrochemical reaction and involves the formation of a cell similar to a battery. The cell drives the removal of electrons from the metal and results in the reduction of the bulk of the steel.



Galvanic corrosion – the corrosion that occurs when two different metals are in electrical contact when an electrolyte is present. The more active metal corrodes faster. The Galvanic Series lists metals from the least active platinum to the most active magnesium.

The two primary factors that influence the corrosion of steel are the metal alloy itself and the environment (Figure 7.1 and Figure 7.2). In terms of a steel bridge other factors also have an influence including: 

Contact with other metals – in the presence of an electrolyte the more reactive metal corrodes e.g. galvanised steel bolts used with stainless steel plate will corrode faster as the zinc is more reactive than stainless steel. The stainless steel will corrode slowly or not at all.



The location within a bridge – the level of contaminants e.g. chlorides, acids etc. in areas not subject to rainfall will be higher than in those areas where they are regularly washed off.



Proximity to other surfaces – if a steel section is in contact with a concrete surface a type of corrosion termed ‘crevice corrosion’ will occur as a result of the corrosion cell that is driven by the presence of moisture at the interface and the contaminants present (Figure 7.3).



The type of protective coating system used.



The amount of moisture present.



The presence of contaminants.



The accumulation of debris in or on members.



The size of the component.

For detailed information on metal corrosion go to the Australian Corrosion Association – www.corrosion.com.au and the Corrosion Doctors - www.corrosiondoctors.org.

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Source: RTA,NSW

Figure 7.1: Corrosion due to accumulation of dirt in member

Source: RTA NSW

Figure 7.2: Crevice corrosion at steel/timber interface

Source: RTA NSW

Figure 7.3: Crevice corrosion at steel/steel interface

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7.2 Fatigue Fatigue in metal is related to the stress range the material is subjected to under cyclic loading. Fatigue cracks initiate at stress raisers that can be notches in the material caused by welded or bolted connections, geometric shape and defects in the material or weld metal. The stress range is critical as the cyclic loading may subject the member to both tensile and compressive stresses. In such instances the peak-to-peak stress range becomes the fatigue design criteria. As a result of extensive testing the range of stress which numerous connections and geometrical details can be subjected to without causing a fatigue failure has been established. In addition the number of load cycles to failure for a specific detail was also determined. See Part 6 Steel and Composite Construction of AS 5100 for additional information on fatigue.

7.3 Brittle Fracture As well as strength, toughness is a critical property for bridge steels. This is a measure of their ability to carry loads applied suddenly, such as the impact of a truck bumping over a brick sitting on a bridge deck. Early cast irons and steels did not perform well under these circumstances particularly on cold days, as temperature affects how the steel behaves. Tests to measure this property use an impact testing machine, which applies an impact load from a pendulum to a specimen with a standard notch machined in it. The height that the pendulum swings up after hitting the test piece indicates the amount of energy the specimen absorbs – a high swing indicates that the specimen did not absorb much energy, and vice versa (Figure 7.4).

Source: Davis, Troxell & Wiskocil (1964)

Figure 7.4: Schematic drawing of a standard impact testing apparatus

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The failure of the King Street Bridge in Melbourne in 1962 from brittle fracture on a cold morning under the impact of a passing truck is shown in (Figure 7.5). The cracks in the webs of the girders propagated from welding defects and the brittleness of the steel failed to stop the crack from spreading up to 1500 mm through the webs. Designers now address the issue of brittle failure by specifying steel properties according to the extremes of low temperature the steel will be subjected to in service. Designation of L0, L10 and L15 relates to steels that have good ductility down to 0 °C, -10 °C and -15 °C respectively.

Source: Royal Commission on the failure of King Street Bridge, Melbourne, Victoria, 1964

Figure 7.5: Brittle failure of King Street Bridge girder

7.4 Protective Coatings 7.4.1

History of Protective Coatings

Up to the mid-1920s pigmented oil paints were used which consisted of natural oils and metal oxide pigments. Lead oxides (red lead and white lead) were the best pigments for protection. Lead oxides work by saponification of the pigment, improving the performance of the paint. Red lead oxide is commonly used as the pigment in primers. In the early 20th century alkyd base and micaceous iron oxide (MIO) were developed. Alkyds are a thermoplastic polymer, and are the reaction product of oils or fatty acids, alcohols and polybasic acids. They are oil soluble and are a single pack paint and are compatible with lead oxides. Lamellar pigments, such as MIO or aluminium flake, were introduced in the 1920s and resist the entry of oxygen and moisture to a greater degree. In NSW, red lead/MIO alkyd systems were first used on Tom Uglys bridge and then on the Sydney Harbour bridge.

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Chlorinated rubber Chlorinated rubber was invented in the 1940s. A solvent carried chlorinated natural or synthetic rubber, for example, neoprene. It is a single pack thermoplastic paint with a high solvent content – up to 30% v-v. Polychlorinated biphenyls (PCB) were sometimes used as a plasticiser. Chlorinated rubber cures by evaporation of the solvent and can be applied in medium build coats. It has an infinite overcoat life but solvents can soften the cured resin. It is prone to solvent bubbling if applied in too thick a layer and to solvent entrapment if applied to relatively porous substrates. Care is required when over coating an existing coating. 7.4.2

Present Protective Coatings

Epoxy coatings Epoxy coatings are the mainstay of coatings and were invented in the early 20th century and comprise high molecular weight polyamines. They have excellent resistance to atmospheric and submerged exposure, and also to many chemicals. The recoatability of epoxies varies from being poor to excellent. Epoxies are formulated from low to ultra-high build coating thicknesses and can accept a wide range of pigments. Epoxy primers Epoxy primers can be formulated to coat poor to excellent surface preparation, including rust penetrating primers, but suffer from very high chalking when exposed to UV light. Polyurethane Polyurethane coatings are based on isocyanate reactions with amine and urea co-reactants. They were invented by Otto Bayer in 1937, and developed as a replacement for rubber in WW II and as paint for aircraft. Polyurethanes can be tailored for specific properties. Solvent and water-borne formulations exist and high temperature and ‘room’ temperature cure formulations are available. Aliphatic diisocyanites are used for resistance to UV degradation, improving gloss and colour retention. Siloxanes Siloxanes are based on silicon-oxygen groups to form long chain molecules. The family of coatings includes inorganic zinc silicates. There are two types of siloxane coatings – pure inorganic siloxanes and hybrid inorganic/organic siloxanes: 

inorganic polysiloxanes have a long chain Si-O backbone with hydrogen attached to the silicon atoms



hybrid polysiloxanes have a long chain Si-O backbone with organic side chains attached to the silicon atoms.

Bridge and industrial top coatings are usually based on hybrid inorganic/organic chemistry. Two formulations of hybrid polysiloxanes are used – epoxy polysiloxanes and acrylic polysiloxanes:

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

Epoxy polysiloxanes contain epoxy side chains and have a similar workability and chemical resistance as epoxies but have better corrosion resistance than epoxies. They also have similar or better weathering characteristics when compared with polyurethanes



Acrylic polysiloxanes contain acrylic side chains and have a similar corrosion resistance as polyurethanes, but not quite as good as epoxies or epoxy-polysiloxanes. They have better weathering characteristics when compared with polyurethanes.

Fluoropolymers Fluoropolymers are based on fluorinated hydrocarbon chemistry. There are two basic families – vinylidene fluoride (VDF) and fluorethylene vinyl ether (FEVE): 

The FEVE polymers are of a lower molecular weight solution polymer. These coatings offer excellent weathering and corrosion resistance.



VDF and FEVE have a high chemical resistance and high gloss retention, which make these coatings very resistant to graffiti. They require special treatment to overcoat.

Qualities of the paints Lead based primers can be both red lead and white lead based. Red lead reacts with linseed oil or alkyds to form soaps. This acts as a barrier against moisture and salts, protecting the steel. It does not require a blast cleaned surface before application and is compatible with alkyd top coats. White lead was also used as a primer. Cautionary note: Lead based paints are toxic and therefore touching, scaping or blasting of the material is a potential health hazard. The management of lead based paints is a specialised area and requires the input of experts in the field. Zinc rich paints have several properties: 

Zinc rich paints have both galvanic and barrier qualities provided the zinc content is high enough. With exposure, the zinc corrodes, producing a barrier effect. This galvanic behaviour tends to restrict under-film corrosion.



Zinc silicate was invented in the late 1930s by Victor Nightingall in Australia. Its earliest extant usage was on the Whyalla pipeline in South Australia. There are two formulations, one water borne and the other solvent borne.



Zinc silicates do not need top coating. Old zinc silicate coatings can be recoated with zinc silicate but requires careful treatment. The intercoat adhesion (zinc silicate over zinc silicate) is poor initially, but improves with time. Application of top coats on a repaired zinc silicate coating is problematic. Zinc silicate coatings are very sensitive to surface preparation.



Zinc epoxy (primer). There are two classes of zinc epoxy (primer) – very high zinc and high zinc. — —

Very high zinc (over 90% by weight) primers have some galvanic action to start with. These primers are sensitive to application, curing and the thickness of top coats. High zinc (80% to 90% by weight) primers have only a barrier action. These coatings are much less sensitive to application, but still require good curing. Thickness of top coats is also less of an issue.



Zinc phosphate epoxy (primer). This primer has corrosion inhibiting properties and is more tolerant of surface preparation then zinc epoxy. It is a very good maintenance primer.



Zinc phosphate alkyd is used where compatibility is required with an existing alkyd or chlorinated rubber coating.

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Typical current systems 

Inorganic zinc silicate is used on its own and its build is about 75 microns, although some formulations permit higher build. It is satisfactory for up to moderately high corrosion exposure. As it is a matt coating, mid grey colour, it may not be suitable if aesthetics are important. It is also not suitable for immersion.



Epoxy is comprised of a zinc rich primer, preferably inorganic zinc silicate, an MIO epoxy build coat and an epoxy finish coat. It is used where colour and gloss retention is not important. e.g. inside hollow members, and can be used in severe environments.



Polyurethane is comprised of a zinc rich primer, preferably inorganic zinc silicate, an MIO epoxy build coat and a polyurethane finish coat. It is used where colour and gloss retention is important and can also be used in severe environments.



Polysiloxane is comprised of a zinc rich primer, preferably inorganic zinc silicate, possibly with an MIO epoxy build coat, and a polyurethane finish coat. It is used where colour and gloss retention is very important. It can also be used in severe environments. The RTA specifies a three-coat system, but there are arguments for a two-coat system.



Metallic zinc – galvanising produces metallic zinc and zinc iron alloys bonded metallurgically to the steel substrate.

Galvanising is used on bridges for the following components: 

bearings



traffic barriers



pedestrian railing



anchor bolts



fitments



steel members.

Design issues 

The thickness of the galvanising is a function of the mass of the component being galvanised. Therefore, light items such as bolts have a much thinner layer of zinc than heavy steel plate.



The use of galvanised components and members in marine environments is not recommended.



Vent holes must be provided in closed sections to prevent the build up of high pressure when the member is immersed in the molten zinc bath.



The process includes the immersion of the member in acid in the pickling process to clean the steel of impurities and scale. This situation presents a problem if partial penetration wells are used as the acid accumulates in the gap causing long-term corrosion. This occurs where hollow steel sections are welded to base plates using a fillet weld and not a full penetration

Hot zinc metal spray 

Hot metal zinc spray is the process of spraying a metallic zinc coating to steel using equipment that melts the zinc at the spraying head.



It has been used on rolled steel bridge members as a primer and over coated with conventional paints. It has not been used extensively but its in service performance has been good. Austroads 2009 — 88 —

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7.5 Interior of Steel Members The corrosion protection requirements for the interior of steel members depend on the physical details and the corrosion protection system adopted. 7.5.1

Hermetically Sealed

If the interior of the member is hermitically sealed and provided no excessive amount of moisture remains then no further corrosion protection measures are required. 7.5.2

Hot Dip Galvanising

If a member is to be hot dipped galvanised vent holes must be provided to prevent the build up of dangerous internal pressures during the galvanising process. The size and location of the vent holes need to be critically determined to ensure the efficiency of the galvanising process and to ensure the pickling (acid bath) and neutralising liquids and excess zinc are drained from the member. In the case where members with steel hollow sections welded to steel base plates are to be hot dipped galvanised the weld detail is of critical importance (Figure 7.6 and Figure 7.7). Full penetration welds are to be used. If fillet welds are specified the gap between the steel section and the base plate traps acid from the pickling process resulting in corrosion that will lead to failure. Steel hollow section Fillet weld – partial penetration leaves length where acid remains from galvanising

Full penetration weld required

Base plate

Source: D Carter

Figure 7.6: Welding of hollow steel to base plate - full penetration weld compared to fillet weld

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Source: QDMR

Figure 7.7: Base plate showing corrosion of fillet weld

7.5.3

Steel Box Girders

The interior of steel box girders is required to be accessible for inspection and therefore require that a protective coating system be applied and be maintained. 7.5.4

Steel Trough Girders

Steel trough girders do not have a continuous top flange and therefore there is a risk that cracking in the composite concrete deck slab above will allow the ingress of water into the interior of the girder. Drain holes need to be provided in the bottom flange of the girder to ensure that water does not accumulate in the interior. The inspection of the interior of all steel girders should be included in routine maintenance inspections.

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8

METALLIC MATERIALS – CONNECTIONS AND FABRICATION

8.1 Rivets Prior to the development and acceptance of welding, riveting was the main means of connecting steelwork. Rivets are typically made from steel or wrought iron with good malleability properties. This means that the material can be made to change shape without developing cracks or otherwise weakening. Initially rivets were installed manually. The blank rivet, essentially looking like a cuphead bolt without a thread, was heated in a small furnace to a cherry red colour. It was removed by tongs and placed in the hole. With one person leaning against the head, another person with a special shaped hammer beat the other end into a matching cup shape. As the rivet cooled, its contraction would add to the tightness of the joint already achieved by the hammering process. This procedure was improved by the use of pneumatically driven tools, which hammered the rivet head more efficiently – but essentially achieved the same result. Whilst rivets tend to hold a joint tight, they are not designed to use friction between the connected surfaces. Rather, they are sized to carry the load in shear across the rivet. Overloaded joints will exhibit enough movement to indicate that the tolerance in the rivet holes has been used up by movement, plus some more indicating that the rivets have begun to shear. The ability of rivets to expand and fill holes during installation made them the perfect fasteners for bridges that required tight fitting connections. The level of skill required to install rivets is considerably higher than that to install bolts. The rivets consisted of a solid cylindrical shank with a manufactured head on one end. The rivets were heated in the field to a cherry red (approximately 1800-2300°F which is 980-1260 °C), inserted into the hole and a head formed on the blunt end with a pneumatic rivet gun. The gun was fitted with the proper die to form the head by rapid, successive blows to the rivet (Figure 8.1). Shop riveting was done in a similar fashion except that the rivet was driven in one stroke with a pressure type riveter. As the rivet cools it shrinks creating a clamping force between the parts it connects. Measurements have shown this force to approach the yield load of the rivet. This residual force contributes to the frictional resistance of the connection, but unlike high strength bolts, which are tightened by a specific procedure, this force is unpredictable and is not included in the design or utilised in calculating the load capacity of the bridge. Inspection and assessment of rivets – existing bridges The inspection and load capacity assessment of existing bridges presents three issues: 

the shear capacity of the rivets



the effect of the degree of corrosion of the rivet head on its capacity



the effects of long term shear loads on the rivet material.

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Source: State Library of NSW

Figure 8.1: Power riveting

A report Reichle (1999) provides useful information on rivet behaviour and the effects of rivet head corrosion.

8.2 Bolts Bolting has always been used for connections which could not be riveted, or which might need to be undone. Examples include bolts used for securing bearings. Particularly with the introduction of welding, bolts began to be used for joints connected on site – so-called ‘field joints’. Field joints are common in truss bridges, but also in girder bridges where they are used to increase length of the girders plus connect the cross bracing. Initially bolts were no stronger than the parent metal, but this evolved to the more recent situation where ordinary bolts (or ‘black’ bolts) are still used for non-structural connections, but all structural joints use high strength bolts. These have markings to indicate their properties (Figure 8.2). Whereas structural steel has yield strength of 250, 350 or 450 MPa, high strength bolts go much higher. Table 8.1 indicates current grades.

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Table 8.1: Bolting classification Grade

General description

Minimum tensile strength MPa

Minimum yield strength MPa

Bolting category

Mode of action

4.6

Commercial

400

240

4.6/S

Snug tight

8.8

High strength

830

660

10.9

Precision

1000

900

12.9

Precision

1200

1080

8.8/S

Snug tight

8.8/TB

Fully tensionedbearing

8.8/TF

Fully tensionedfriction grip Used for specific applications such as machine anchors etc.

Figure 8.2: Markings for high strength bolts

For bolts to achieve the TF (tension-friction) or TB (tension-bearing) category, they must be tightened to beyond their yield stress. For TF joints to work, they rely on a known value of friction co-efficient between the joining surfaces. Care must be taken with these to match the design assumptions for friction. The designer should specify the condition of the mating surfaces for TF joints, as different coatings will have different friction characteristics. The most common coating conditions specified are galvanised, hot zinc metal spray, inorganic zinc primed and organic zinc (zinc epoxy) primed. Top coats are not normally specified as these are more likely to render the connection ineffective. Full tensioning is achieved by one of three methods: 

Part-turn method – This utilises the known extension coming from the slope of the thread. The bolt is first brought to snug tightness, then the nut turned, say 120° to achieve the specified elongation. The degree of turn required is a function of the length of the bolt.



Torque control method – Torque control typically utilises a torque wrench or mechanical gun with a clutch, which slips when the required torque is attained. Particular care is required here with calibration based on bolts with the same thread friction – putting grease on the threads may result in the gun failing the bolt in tension before the torque resistance is reached.



Load indicator method – This uses washers with upstands. The required torque is indicated by the upstands being closed to a defined gap.

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These methods all require sound judgement and field experience to achieve a reliable, repeatable outcome. It is worth noting that bolts that have been fully tightened cannot be reused as a result of work hardening. The material has been taken beyond the yield point. Consequently, if a joint is undone for repair etc. it should be re-made using new bolts. Material certification As with reinforcing and prestressing, steel bolts are now imported into Australia and New Zealand from a number of countries. This situation raises issues in terms of the need for verification of the compliance of these materials with local standards. Instances have occurred of bolts failing during tightening despite being designated by the markings as high strength bolts. Subsequent testing revealed the bolts did not comply with local standards. Instances have also occurred where bolts with a particular manufacturer’s identification marking were, in fact, manufactured by others. Material certification together with random sampling and testing are recommended.

8.3 Proprietary Mechanical Fasteners In addition to standard nuts and bolts there are a number of proprietary fasteners and anchors available for specific applications. The types include: 

expanding anchors



anchors for fastening to blind holes



screw in anchors



U bolts



Stud shear connectors (welded to steel girders to develop composite action with concrete decks (Figure 8.3 and Figure 8.4).

Information on specific applications and design information can be obtained from manufacturers. Concrete deck

Shear connectors

Steel girder

Concrete deck

Shear connectors

The shear connectors make the slab and girder act as one Source: RTA NSW

Figure 8.3: Stud shear connectors used for composite action girder/slab

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Source: RTA NSW

Figure 8.4: Stud Shear Connectors on top flange of a steel girder

Information on specific applications and design information can be obtained from the relevant manufacturers.

8.4 Proprietary Chemical Fasteners Chemical fasteners and anchors use polymers as the adhesive to anchor bolts, bars and threaded rod to provide the required capacity as opposed to mechanical devices. One advantage of chemical anchors is that they do not impart expansive stress on the surrounding material to develop their capacity. Proprietary chemical fasteners and anchors are available for a range of applications including: 

hold down bolts



anchorages into vertical and horizontal faces



anchoring reinforcing steel into concrete



anchoring threaded bar into concrete.

The products are available in various forms: 

adhesive resin plus anchor bar or bolt



adhesive resin only.

The adhesives used include a range of resins including epoxies and polyesters. Some resins are supplied in vial with the resin and the hardener inside. Insertion of the anchor bar breaks the vial and brings the hardener and the resin together. Mixing is required to produce a homogeneous material. Alternatively, the hardener and resin are supplied and proportioned and mixed on site. The advantage of the vial concept is that no proportioning is required on site and only the required amount of resin is used reducing waste material. Additional information can be obtained from manufacturers.

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8.5 Welding Welding involves connection by melting both the parent metal and the filler metal so that the end product is a continuous path of steel. Welding in bridges gradually replaced riveting – mainly between the 1930s and the 1950s. However, welding did develop before that, with fully coated electrodes being developed in 1911. An important aspect of the welding process is the use of fluxes. The main role of the flux is to generate a gas around the weld metal that prevents harmful gases (particularly hydrogen and oxygen) from becoming absorbed into the molten metal and then being retained on solidification – potentially causing embrittlement and other defects. The residue of the flux is called slag, and the weld process needs to keep this from being incorporated in the weld. Fluxes may be coated on the outside of the welding rod – as is typically the case for stick welding, or deposited from a hopper onto the weld area for other types of welding, particularly those that use raw wire fed from a spool. 8.5.1

Welding Methods

Welding on bridges is carried out using a number of methods including: 

shielded manual metal arc (Figure 8.5)



submerged arc (Figure 8.5)



MIG (metal inert gas) (Figure 8.6)



flux cored arc welding (Figure 8.6)



stud welding.

For details of typical weld defects see Section 16.7.3 of Austroads (2009).

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Source: NAASRA (1980)

Figure 8.5: Shielded manual metal-arc welding and submerged-arc welding

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Source: NAASRA (1980)

Figure 8.6: Metal inert gas welding and flux-cored arc welding

The electrodes used in welding must be compatible with the material being welded. The mechanical properties of the weld metal depend on the wire and flux combination. Particular attention should be paid to storage of electrodes as some flux coatings absorb moisture, which will adversely affect the weld. Flux for submerged-arc welding should be dry and free of contamination from dirt and other foreign material. 8.5.2

Type of Welds

Fillet welds - Fillet welds are the most common welds, typically joining two members at right angles (Figure 8.7 and Figure 8.10). They typically do not require any edge preparation.

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Source: :NAASRA (1980)

Figure 8.7: Fillet weld terminology and dimensions

Butt welds - Butt welds provide a full strength method of connecting plates in one plane and similar connections where the weld metal is laid into a slot created by preparing the edges of the plates to be joined (Figure 8.8 and Figure 8.11). Partial penetration butt welds can be specified if the full depth of the plate is not required for strength, but this is uncommon as the gap left can start corrosion and it also looks like a defect when inspected (Figure 8.9).

Source: NAASRA (1980)

Figure 8.8: Butt weld terminology and dimensions

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Source: NAASRA (1980)

Figure 8.9: Partial penetration butt welds

Whilst welders strive to achieve perfect welds, for any given application there are a range of acceptable defects, and this needs to be considered when deciding whether an identified defect should be repaired. Defects are identified by a range of destructive and non-destructive methods, the details of which are outside this guide. However, they include: 

Visual inspection – surface inspection, looking for size, shape and visible cracks.



Dye penetrant inspection – an aid to visual inspection, where the penetrant seeps into cracks and is then visible when the surface is cleaned off.



Radiographic (X-ray) inspection – works like X-rays of bones to detect cracks and flaws.



Magnetic particle inspection – a sprinkle of magnetised filings can form patterns that indicate surface or subsurface flaws.



Ultrasonic inspection – using ultrasonic waves to detect defects by the wave reflection.

8.5.3

Effects of Welding

When steel is heated to high temperatures in the welding process metallurgical changes occur in the weld metal and the heat affected zone of the parent material. This produces significant changes in the grain structure that results in changes in strength, hardness and ductility. The heating and cooling process also results in stresses being locked in the material. Post-weld heating is designed to relieve these stresses. The welding process may also cause distortion of the component, which must be addressed by the fabricator in the weld procedure. 8.5.4

Construction Issues

The steel fabrication of a major member or smaller component on any bridge project requires advanced planning by the project manager to ensure that all the pre-work activities are completed so that the required degree of control is achieved to produce the specified quality. 8.5.5

Weld Categories

The drawings should be examined to determine what weld categories are included in the works. The weld category will determine the type of inspection that is required. Weld categories include: 

GP (general purpose weld) – Inspection requires 100% visual scanning for defects and omissions and 100% examination of the quality of the specified extent of welding.



SP (special purpose weld) – SP welds are the minimum requirement for all bridge works. Inspection requirements will vary depending on the location and type of member. Inspection will include visual scanning and examination plus any one or more of these requirements: —

magnetic particle testing

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— —

dye penetrant testing ultrasonic or radiographic inspection.

The percentage of weld to be tested by magnetic particle and dye penetrant testing will vary from 10-100% depending on the component. 

8.5.6

FP (fatigue purpose weld) – FP welds are specified where the fatigue is a primary design consideration. The inspection requirements are similar requirements to those for SP welds or to the requirements of AS/NZS 1554.4 (2004) category FP whichever has the highest inspection requirement. Weld Procedure Qualification

All weld procedures are to be qualified and approved by a welding inspector. The weld procedure will include: 

edge preparation and set up



welding consumables/electrodes



welding machine settings



pre and post heat treatment



interpass temperature – the temperature in the steel immediately before the second and subsequent passes of a multiple pass weld. If the temperature of the steel drops below the interpass temperature before subsequent welding passes additional preheating is required



weld speed



the range of materials and material thicknesses on which the procedure may be used.

Obtaining weld procedure qualification may include one or more of the following: 

Using a previously approved procedure.



Carrying out weld test pieces and subsequently testing them to destruction to confirm the adequacy of the weld; the test pieces are to replicate the conditions under which the weld will be carried out.



Preparation of a macro test piece cut from the test piece and polished to show the cross-section of the weld to assess its adequacy.



Impact testing to verify that the heat affected zone of the parent metal still retains its impact properties.

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Source: D Carter

Figure 8.10: Macro – full penetration fillet weld flange to web

Source: D Carter

Figure 8.11: Macro – butt weld (double sided)

8.5.7

Welders

Each welder to be used on the works is to be pre-qualified by carrying out weld tests to the approved welding procedure unless documentary evidence is produced that demonstrates the capability to produce SP welds of the type involved. Pre-qualification is to be by macro testing to demonstrate the ability to produce sound welds using the approved procedures. The weld designation will designate what pre-work is maintained on the welding processes. 8.5.8

Welding of High Strength Steels

Welding of high strength steel is generally in accordance with category SP unless stated otherwise on the drawings. However, there are additional requirements in regard to preheat, treatment of consumables, and heat straightening. 8.5.9

Weld Defects

In the welding process defects can occur as a result of deficiencies in the process caused by the welder or the welding process including: 

incomplete penetration



lack of fusion

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

loss of edge of the weld



undercut



overlap



slag inclusions



porosity



cracks



incorrect profile and size of weld.

The size and extent of any weld defects need to be established to determine if remedial measures are required or if the defects are within prescribed limits. The implications of a defect will depend on the particular member and the location and size of the defect. For details of typical weld defects see Section 16.7.3 of Austroads (2009). 8.5.10

Stud Welding

Stud welding is the process used in bridges to connect shear connectors to the top of steel members or at other locations as specified. The process is semi–automated using a dedicated welding machine with the surface being free of any deposits. However, unlike conventional welding there is no added weld metal. No drilling of holes or weld edge preparation is required. The studs are supplied as proprietary products and come complete with a ceramic ferrule to shield the weld in the in the molten state. The process is as follows: 

place the stud against the steel surface and then switch the current on



lift off the stud while the current is flowing creating an arc which melts the end of the stud and the steel surface



plunge the stud into the weld pool fusing the two surfaces together.

For additional information on welding see: 

Welding Technology Institute of Australia website www.wtia.com.au.



The ESDEP (European Steel Design Education Program) course which provides a range of lecture notes on steel bridges including: — —

WG2 – Applied Metallurgy WG3 – Fabrication and Erection.

See the following website for full details of the course: http://www.kuleuven.ac.be/bwk/materials/Teaching/master/toc.htm

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9

NON-METALLIC MATERIALS

9.1 Elastomers Elastomers are a range of polymers that include natural rubber and synthetic materials. The most common elastomers used in bridge bearings are natural rubber and neoprene. Various additives are used to achieve the required physical characteristics and to improve the resistance of the material to oxidation, ozone and sunlight. 9.1.1

Bridge Bearings

The manufacturer can adjust the stiffness of the material depending on the design requirements. The four design parameters of an elastomeric bearing are: 

shape factor



compressive stiffness to resist vertical loads



rotational stiffness to resist rotation at the supports caused by member deflection



shear stiffness to resist horizontal loads from thermal and creep movements and braking loads.

Elastomeric bridge bearings come in the form of: 

Plain strip bearings – usually up to 125 mm x 25 mm and available in various lengths. These bearings are usually used in short span bridges.



Plain pad bearings – square, rectangular or circular. Dimensions vary according to the design requirements.



Laminated pad bearings – square, rectangular or circular. Dimensions vary according to the design requirements. Laminated bearings are reinforced by steel plates that are internally bonded to the elastomer during the manufacturing process. The dimensions of the bearing and number and size of steel plates depend on the design requirements.

Under load laminated bearings will exhibit ripples on the vertical face of the bearing. This is normal behaviour. However, if the bearing is unevenly loaded the ripples on one side of the bearing may be deeper. This situation may require further investigation. For specific details of bridge bearings see Austroads Guide to Bridge Technology – Part 3: Typical Superstructures, Substructures and Components. 9.1.2

Construction Issues

It is imperative that the bearing seating is planar and to the specified level to ensure the bearing is uniformly loaded. The seating of members on bearings should be checked at the time of erection of the member. The position of the bearing seating must be accurately set out to ensure that the required distance from the edge of the seating to the bearing face is achieved. Failure to provide the required edge distance may lead to a lack of friction to confine the lower edge of the bearing and result in lifting of the edge of the bearing and possible tearing of the material.

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Neat epoxies (i.e. no filler included) on bearing seats do not provide sufficient friction between the surface and the bearing and can lead to bearings ‘walking out’. If repairs are carried out to seatings using epoxies coarse sand should be broadcast on the surface to ensure sufficient friction is developed. Poured cementitious grouts may also lack sufficient friction to retain the bearings, and should not be used. Bearing plates and other devices attached to precast members should be checked for accuracy of position and level to ensure uniform loading of bearings. Vertical hog of prestressed members can often result in a gap on the front edge of the bearing resulting in non-uniform loading. Similarly, girders placed on a cross-fall or on a slope may cause non-uniform loading unless precautions are taken. 9.1.3

Serviceability Issues

Shear strains Over time the shear strain of a bearing will often increase to excessive amounts due to the combined effects of: 

movements in the substructure



movements in the superstructure due to creep and thermal movements.

It is generally accepted that the maximum acceptable shear strain of an elastomeric bearing is equal to its depth. However, lower threshold values may apply in some authorities. Once the maximum shear strain of a bearing is reached the bridge should be jacked-up to allow the bearing to reset. Failure modes Failure modes include: 

horizontal splits due to excessive strains or the debonding of the steel plates in laminated bearings



lifting of the edges of the bearing due to lack of friction causing excessive strains



crushing of the bearing following the development of a number of horizontal splits that extend into the bearing



deterioration due to the effects of ozone resulting in the breakdown of the material



bearings ‘walking out’ due to lack of friction or the bearing being too lightly loaded



instability where the bearing is too tall for its base dimension.

9.2 Fibre Reinforced Polymers (FRP) 9.2.1

History

FRP composites have been used for some time in the aerospace industry. The first applications in concrete structures occurred in the 1980s in Europe and Japan where they were retrofitted to existing structures to increase load capacity and seismic resistance. Since that time the use of FRP systems has progressively increased in both the area of bridge strengthening and new FRP bridges. By 1997 more than 1500 concrete structures had been strengthened using externally bonded FRP materials.

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9.2.2

Types of Materials

Currently, most FRP materials are made from one of the following: 

Glass FRP (GFRP).



Carbon FRP (CFRP).



Aramid (Kevlar) FRP (AFRP).

The materials can be manufactured in different forms: 

Fabric with parallel or cross-ply fibres



Pre-cured strips in which the fibres are bound into a resin matrix ready to be bonded to members



Prepreg tape, which consists of fibres pre-coated with resin. It is a semi-hardened product and comes in a variety of forms – rolls, cut sheets or pre-cut forms. The material is applied to the concrete member and then heat cured in one to three hours



Tendons for prestressing applications



Reinforcing bars



Poltruded sections (resin matrix and fibres extruded through a die of the required cross-section).

9.2.3

Material Characteristics

FRP materials are four to six times stronger than steel and concrete but are only a fraction of the weight. However, under load their behaviour is different to conventional bridge materials. They behave linear elastically to failure i.e. there is no yield point. This results in a lack of ductility at failure compared to other materials and consequently there is no indication of imminent failure. This issue is addressed in design codes and design guides by limiting the stresses at the serviceability limit state to a lower enough level to ensure there is no possibility of lack of ductility being an issue. In FRP member stiffness will generally control the design rather than the strength. A comparison of different materials is provided in Table 9.1. Table 9.1: Comparison of material properties of FRP to steel, concrete and timber Material

Stiffness E (Gpa)

Tensile (MPa)

Compressive strength (MPa)

Steel

200

300

300

7650

Concrete

28

5

50

2500

Glass composite

30

500

400

1800

Carbon composite

90

900

800

1300

Hardwood timber F27

18

50

60

1100

Softwood timber F11

10

40

25

800

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Density (kg/m3)

GUIDE TO BRIDGE TECHNOLOGY PART 2: MATERIALS

Glass fibre reinforced polymer (GFRP) GFRP was the first FRP material used in bridge applications. However, some of the shortcomings listed below have meant it has a limited use: 

susceptible to creep rupture at low sustained stress levels; in California GFRP shells applied to concrete columns failed instantaneously due to creep rupture



susceptible to degradation in alkaline environments



not suitable where exposed to marine environments



low fatigue resistance



high impact tolerance



not tolerant of alkalinity/acidity.

Carbon fibre reinforced polymer (CFRP) The development of CFRP technology initially occurred in the aerospace industry. Its applications in bridges have been steadily growing. Its comparatively high cost has limited its broader use. However, as the material cost decrease applications in bridges will increase. Properties include: 

high fatigue resistance



not affected by UV degradation



creep rupture not an issue



low impact tolerance



high resistance to alkalinity/acidity.

9.2.4

Glass Transition Temperature of Polymers

The stability of FRP at elevated temperatures is a function of the temperature at which it was cured, defined as the glass transition point (melting point). A differential scanning calorimeter is used to determine the glass transition temperature and can also be used to determine the degree of cure. If a resin is uncured heating above the glass transition temperature will result in the glass transition temperature continuing to increase. The glass transition temperature of a resin must be in excess of the maximum temperature the material will be subjected to in service. 9.2.5

Resins and Moisture

There is a wide range of resins available for use in FRP materials. These include epoxies, vinylesthers and polyesters. However, some resins are susceptible to deterioration by moisture. The type of resin used must be suitable for the exposure conditions and the likelihood of moisture coming in contact with the resin. Some resins can also be affected by atmospheric moisture at the time of curing.

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9.2.6

FRP Bridge Applications

New bridges – design issues The main design issue to address in an FRP road bridge is robustness. Generally strength is not an issue as the material is designed to operate at low stress levels given the non-ductile failure mode. However, over its service life the bridge will be subjected to repetitive live loading with the possibility of high impact loading as a result of adverse conditions, such as uneven approaches. A combination of FRP materials, such as CFRP and GFRP, in flexural members allows the designer to engineer the yield and ultimate behaviour (Figure 9.2). Examples exist overseas where FRP bridge decks with thin multiple webs have failed under in-service traffic. The construction of bridge superstructures from FRP (Figure 9.1 and Figure 9.2) has a number of advantages over conventional materials, including the following: 

high strength



low mass compared to conventional materials with resulting cost savings in substructures and foundations



reduced transport costs particularly to remote sites



high durability



non-corrosive



high fatigue resistance of CFRP.

Source: RTA NSW

Figure 9.1: FRP span – bridge over Orara River at Coutts Crossing

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Source: RTA NSW

Figure 9.2: Proof loading of FRP span for Coutts Crossing

9.2.7

FRP Timber Member Replacements

There are still some 20,000 timber bridges in use on the road and rail networks. Many of these bridges will be required to remain in service for the foreseeable future. The sourcing of timber to maintain these bridges is becoming increasingly difficult as the availability of suitable timber diminishes. In addition the cost of structural timber has increased significantly over the last 10-15 years. This situation led to the development of FRP girders as an alternative for timber girder replacements. To date FRP girders have been developed based on design criteria required by QDMR and RTA for their respective applications. Prototype girders have been tested to destruction to confirm compliance with the design criteria. Trial installations of FRP girders in timber beam bridges have been carried out by QDMR. In NSW a trial installation of an FRP cross girder in a timber truss bridge has also been carried out (Figure 9.3).

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Source: RTA NSW

Figure 9.3: Trial FRP cross girder for timber truss bridge

9.2.8

Bridge Strengthening

FRP strips or fabric are used to increase the flexural and shear strength of concrete members. The material is either applied in the form of pre-cured strips bonded to the concrete or impregnating fabric with resin and then applied to the concrete surface. Carbon fibre wraps are used to strengthen concrete columns by providing confinement to the member. Glass fibre shells were used to strengthen bridge columns in California. However, the material failed prematurely due to creep rupture. GRFP is not suitable for use in applications where it is under constant strain. The most important characteristics of FRP in repair and strengthening applications are the speed and ease of installation. Cost savings in labour, road closures, the need for handling equipment and overcoming site constraints offset the costs of FRP compared with other methods such as bonded steel plates and external prestressing. The use of FRP strips removes the safety issue associated with lifting and support of heavy steel plates into restricted areas. FRP materials can also be used to strengthen walls. The use of CFRP to strengthen steel members is an issue due to possible corrosion caused by galvanic action between the two materials. Examples of FRP usage include: 

Westgate Bridge, Melbourne – CFRP strips used to increase shear and flexural strength of prestressed concrete box girder.



Oyster Channel Bridge, Yamba, NSW. – CFRP strips used to increase the shear capacity of a reinforced tee beam bridge.



Hornibrook Highway, Brisbane – CFRP wrap used to confine concrete columns.

Flexural strengthening Where CFRP strips are used to increase the flexural strength of a member the anchorage of the strips to develop their tensile capacity is a critical consideration. The strips must extend sufficiently past the point at which they are required flexurally to develop their full tensile capacity.

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The propensity for laminates to peel from the concrete substrate for beams under flexure is an issue that needs to be addressed in the design by providing a sufficient anchorage length. In some instances secondary fixtures have been used to prevent peeling. Ongoing monitoring of the integrity of the bonding of the laminates should be included in the maintenance procedures. Limitations on over-reinforcing the member Flexural strengthening of reinforced concrete beams using CFRP strips should be limited to the point where in the event of the loss of integrity of the strips the stresses in the original reinforcing steel do not exceed the yield stress. In addition, the design must be such that non-ductile failure occurs due to an over-reinforced member i.e. compression failure in the concrete. Shear strengthening Anchorage of laminates is the critical design issue. The development length may require protrusion into the deck slab or additional horizontal laminates bonded to the vertical leg. Where vertical laminates on each face of a member are lapped on the soffit, the laminates should be staggered so each one is bonded direct to the concrete and not to another laminate. Corners of members must be modified to provide a curved surface to prevent high contact pressures (Figure 9.4).

Source: RTA NSW

Figure 9.4: Shear strengthening of a reinforced concrete T- beam bridge with CFRP strips

Columns and piles Attention to the detail at the extremities of the member is critically important, as a lack of adequate confinement will compromise the strengthening measures. On square and rectangular columns the corners must be rounded to the radius required for the thickness of the laminate used. The aspect ratio of a column may result in the FRP wrap not being effective in providing confinement. In such instances the FRP wrap will need to be fixed to the concrete column by bolting or ties to be effective. The pre-strain of CFRP systems should be limited to 50% of ultimate strain due to damage tolerance concerns with fibres in the one direction.

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Options for wrapping systems include: 

Wet lay-up using fabric, tape or individual tow (a bundle of continuous untwisted fibres).



Pre-impregnated (prepreg) in the form of tow, tape or fabric.



Prefabricated shells – split tube or in two halves. The integrity of the system relies on the strength of the connections.

The moisture content of the concrete may be an issue where heat curing is required. Water vapour may cause blistering of the wrap. Automated wrapping equipment is available to apply the particular system used. Design standards for column strengthening have been published by road authorities in the USA, Europe and Japan. 9.2.9

Design Issues – Strengthening

The design rationale must be supported by research and testing. Design guidelines are available from a number of authorities and industry bodies. The strength and quality of the concrete to which the laminates will be bonded is of critical importance. Pull-off testing is required to assess if the strength of the concrete is sufficient to resist the shear stresses at the FRP/concrete interface. The evenness of the concrete and the quality of the surface preparation are critical. Any unevenness in the surface will impose high point loads on the laminate leading to possible failure. Poor surface preparation will compromise the bond strength. Blow holes or areas where slurry loss has occurred during construction will require repair prior to installing the laminates. In some situations the risk of fire damage must be addressed. This may require the application of a fire retardant coating to protect the FRP material. Non-destructive testing (NDT) methods should be used to confirm that full bonding of the FRP materials to the substrate has been achieved. This may include the use of thermal imaging techniques or similar test methods. To provide long term monitoring of the in-service performance of CFRP strips additional non-load bearing strips should be installed at the time of construction to provide pull-off test specimens of laminates to assess the long-term performance of the materials. Over-reinforcing concrete members In concrete members the increase in load capacity should be limited to the point where in the event of the FRP laminate de-bonding or failing for other reasons, the stress in the original reinforcing steel under the increased loading does not exceed yield. This is considered appropriate given the relatively short time the technology has been used and provides redundancy in the member.

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9.2.10

Strengthening Materials

Pre-cured FRP laminates Pre-cured FRP laminates are supplied by various manufacturers in varying thicknesses and widths as part of their system. A requirement should be included in the specification to provide confirmation of the material properties of pre-cured laminates to be used on a project. It is recommended that third party testing be carried out on materials to ensure compliance with the specification including: 

material properties of the fibres in the laminate



material properties of the resin matrix



material properties of the laminate (note: it is the strength of the fibre/resin combination that is important, not the fibre strength in isolation)



the fibre/resin ratio; a high fibre/resin ratio results in a thinner layer of resin which optimises the fibre strength; a low fibre/resin ratio will result in a thicker layer of resin which will result in the capacity of the laminate being limited to the strength of the resin



material properties of the resin used to bond the laminate to the concrete.

Wet lay-up The fibre fabric is applied to the concrete surface and the resin applied by roller to infuse into the fibre. Alternatively, the resin is infused into the fabric and then layed-up on the concrete surface. Dry lay-up The fibre fabric is applied with the resin partly cured, (prepreg) and then heat cured, or a dry fabric with the resin is infused by roller, or by a vacuum process. 9.2.11

Construction Issues

The quality of the construction process is critical to the success of the strengthening of members using FRP laminates. Some construction issues include: 

material storage and handling



health and safety issues



sampling of the mixed resin on site and testing to confirm material properties and adequacy of mixing



a laminate support system that ensures a thin layer of bonding material to optimise strength of the FRP. This will require a support system that exerts sustained pressure to the laminate over the curing period



surface preparation and surface profile quality



treatment of surface voids



curing times



inspection methods



acceptance criteria.

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9.2.12

Monitoring

In view of the lack of information on the long-term performance of FRP materials it is recommended that an ongoing inspection program be put in place. This may include visual and NDT methods such as thermal imaging to detect delaminations. In addition, the installation of bonded laminate test pieces adjacent to the strengthening should be carried out to allow future destructive testing over time to check bond performance.

9.3 Polytetrafluoroethylene (PTFE) PTFE is the most commonly used fluorocarbon polymer. 9.3.1

Chemical Resistance

PTFE resins are resistant to attack to practically all chemicals except molten alkali metals – such as sodium and potassium. No known solvents will swell or dissolve PTFE below 300 °C. 9.3.2

Working Temperature Range

PTFE has a wider working range than any other plastics material. The lowest temperature is about -196 °C and the upper limit is 250 °C. 9.3.3

Coefficient of Friction

Frictional qualities are exceptional and better than any dry, solid material. The dynamic friction is higher than static friction, which eliminates any ‘stick/slip’ performance. The friction of lubricated PTFE is lower than dry PTFE, hence the manufacturing requirements in specifications in regard to lubrication. In bridge bearings the sliding surface in contact with the PTFE must be polished stainless steel to a required surface finish. 9.3.4

Extrusion of PTFE

Instances have occurred in service where the layer of PTFE in contact with the sliding surface has extruded out. This has generally been a result of the lack of a recess in which the PTFE can be confined. The later specification requirements to provide lubrication of the PTFE via silicon grease in dimples in its surface appear to have overcome this problem. The extrusion issue can also be related to excessive contact stresses as a result of overloading of the bearing due to design errors or structural movements. 9.3.5

Cleanliness of the PTFE Surface

It is imperative that the sliding surface in contact with the PTFE be kept clean. Any accumulation of dirt or dust will shorten the service life of the PTFE. Bearings with the facility to wipe the sliding surface clean, as movement occurs, will have a longer service life compared to other types of bearings.

9.4 Polystyrene Polystyrene is a lightweight cellular plastic material and is used extensively in bridge construction in the following applications: 

void formers in prestressed and precast concrete members

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

block-outs in formwork to form recesses



as a separator between adjacent concrete faces when new concrete is placed adjacent to hardened concrete.

The material suitable for use in bridge applications must be rigid cellular polystyrene complying to Class S of AS 1366.3 (1992). In the case of void formers the polystyrene is to be coated with a minimum 2 mm thick rapid curing solventless aromatic urethane coating or equivalent.

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10

TIMBER

10.1 Botanical Classification Timber is classified botanically as either hardwood or softwood. It has nothing to do with hardness but relates to the growth system and cell structure of the timber. For example, balsa is botanically a hardwood. Within a hardwood or softwood tree there are two kinds of wood – sapwood and heartwood. 10.1.1

Softwood

The wood of conifers (e.g. pines) and a few other trees is commonly known as softwood, or sometimes as ‘non-pored’ wood. The bulk of the wood is made up of cells that are all much the same, long narrow cells fitting closely together called tracheids (Figure 10.1). The walls of these tracheids are made of carbohydrates and cellulose. The tracheids are held together by lignin.

Source: RTA NSW (1983)

Figure 10.1: Softwood cell structure

10.1.2

Hardwood

In most hardwood trees (e.g. eucalypt, red cedar) two distinct types of cells occur – vessels and fibres. Vessels or pores are the large ducts that carry the sap from the roots to the leaves. Wood possessing vessels is sometimes called ‘pored wood’. The more common term for pored wood is hardwood. Fibres impart strength in hardwood trees and make up the bulk of the wood. As in softwoods, the walls of these cells are of cellulose, cemented together by lignin.

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The tensile strength of timber along the fibres (along the grain) is significantly higher than across the fibres (perpendicular to grain) (Figure 10.2). This fact is important in engineering applications to ensure the loads are applied in the strongest directions. Tension perpendicular to the grain will have the tendency to split the timber. The cell structure also explains why it is important to seal the ends of timber members as moisture travels much faster along the grain compared to across the grain. This has implications for durability.

Source: RTA NSW (1983)

Figure 10.2: Hardwood cell structure

Most Australian native trees are hardwoods. In most species there is no significant difference in the strength of sapwood and heartwood. However, sapwood has low durability compared to heartwood. Durable native hardwoods are preferred for bridge timbers because of their high strength and durability.

10.2 Moisture Content of Timber The condition of timber is classified by its moisture content. Timber freshly cut from a tree is classified as ‘green’. Timber that has dried in air or has undergone accelerated drying by heating (kiln drying) and has a moisture content of 10-15% is classified as ‘seasoned’. It should be noted that large hardwood sections e.g. 175 x 350 x 8000 may take in excess of 30 -40 years to season in air. Kiln drying of large sections is not practical. As the timber dries the moisture content reduces and as a result there is a reduction in density.

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10.3 Shrinkage As timber loses moisture it undergoes a volumetric change that results in shrinkage of the member. The shrinkage occurs in two directions, parallel to the growth rings (maximum) and at right angles to the growth rings. The degree of shrinkage varies with the species. Splits and checks seen on the surface of a piece of timber are caused by the difference in shrinkage between the timber on the outer face and inner timber.

10.4 Engineering Classification 10.4.1

Hardwood

Extensive full scale testing of round girders and sawn sections cut from hardwood trees was carried out in Australia in the 19th and 20th centuries. Numerous tests were carried out on each of the hardwood species to determine their individual engineering properties including: 

tensile strength along the grain



tensile strength perpendicular to grain



bearing strength



shear strength



compressive strength



strength for bolted connections – minimum edge distances, bearing strength



Modulus of Elasticity (MOE) – stiffness



Modulus of Rupture (MOR) – ultimate bending stress.

The tests were carried out on green as well as seasoned timber. Using the test data each hardwood species was allocated a ranking for both green and seasoned timber referred to as strength group. 10.4.2

Softwood

The strength groups for softwoods are generally lower than structural hardwoods. 10.4.3

Strength Properties

The classification for structural timber is set out in AS/NZS 2878 (2000). For green timber the strength groups are S1 (strongest) to S7 (weakest) (Table 10.1). For seasoned timber the strength groups are SD1 (strongest) to SD8 (weakest) (Table 10.2).

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Table 10.1: Strength properties of green timber Strength group Strength property

S1

S2

S3

S4

S5

S6

S7

Modulus of rupture MOR (MPa)

103

86

73

62

52

43

36

Modulus of elasticity MOE (MPa)

16300

14200

12400

10700

9100

7900

6900

Compressive strength (MPa)

52

43

36

31

26

22

19

Table 10.2: Strength properties of seasoned timber Strength group Strength property

SD1

SD2

SD3

SD4

SD5

SD6

SD7

SD8

Modulus of rupture MOR (MPa)

150

130

110

94

78

65

55

45

Modulus of elasticity MOE (MPa)

21500

18500

16000

14000

12500

10500

9100

7900

Compressive strength (MPa)

80

70

61

54

47

41

36

30

10.5 Structural Grading – Australia 10.5.1

Visual Grading

The strength group for each species indicates the potential structural strength of the timber. However, structural defects such as knots, slope of grain, splits etc. in individual logs or sawn pieces of timber result in some reduction in structural strength and as a result the log or piece of timber is downgraded. This is referred to as visual grading and is carried out by qualified timber inspectors. Hardwood timber is generally visually graded. The grading of timber provides the designer with information on the stress that the timber is able to carry under in-service loads. 10.5.2

Mechanical Grading

Mechanical grading is carried out by subjecting each piece of sawn timber to a standard bend test. The deflection of the timber under the applied load is measured and the modulus of elasticity (MoE) calculated and correlated to strength. As a result each piece of timber is stress graded rather than structurally graded purely on the basis of species and the existence of defects as is done in visual grading. In the case of softwood species, such as Radiata Pine and Douglas Fir, each piece of timber is stamped with a stress grade e.g. F7-7 MPa bending strength, F11-11 MPa bending strength. Machine stress grading is based on the results of testing a large number of pieces of timber species to failure to provide a statistical basis for the process.

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10.5.3

Australian Standards

There are a number of Australian Standards for timber and timber products, which relate to timber bridge components: AS 2082 (2007) Timber – Hardwood – Visually stress-graded for structural purposes These visual grading rules for structural timbers set out limits for the size and/or frequency of a number of strength reducing characteristics depending on the strength grouping of the species. AS 2878 (2000) Timber – Classification into strength groups This standard sets out the procedure for the strength grouping of a species and gives the strength groups of an extensive list of species. It is important to note that the strength group is a rating applied to the species and not to an individual piece of timber of the species. AS 3818.1 (1998) Timber – Heavy structural products – Visually graded – General requirements This standard sets out the general requirements for grade, including definitions and methods of measurement. AS 3818.3 (2001) Timber – Heavy structural products – Visually graded – Piles These grading rules set out limits for the size and/or frequency of a number of strength reducing characteristics as well as the requirements for the shape of a pile. AS 3818.6 (2003) Timber – Heavy structural products – Visually graded – Decking for wharves and bridges AS 3818.7 (2006) Timber – Heavy structural products – Visually graded – Large section sawn hardwood engineering timbers AS 3818.8 (2005) Timber - Heavy structural products - Visually graded - Stumps and sole plates AS 5604 (2005) Timber – Natural durability ratings Until the publication of AS 5604, the available durability ratings for timber were for in-ground ratings only. AS 5604 provides: 

a durability rating for in-ground use



a durability rating for above-ground use when exposed to the weather



a lyctid susceptibility rating



a termite-resistant rating (when used as timber framing not in-ground contact).

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10.6 Structural Grading – New Zealand 10.6.1

Visual Grading

There are minimum target values for visually graded timber and these are shown in Table 10.3. Table 10.3: Minimum target values for visually graded timber Grade

Bending strength fb (MPa)

Compression strength Fc (MPa)

Tension strength ft (MPa)

Modulus of elasticity E (GPa)

Fifth percentile modulus of elasticity (GPa)

Radiata Pine & Douglas Fir

VSG10

20.0

8.0

8.0

10.0

6.7

Radiata Pine & Douglas Fir

VSG8

14.0

18.0

6.0

8.0

5.4

G8*

11.7

12.0

4.0

6.5

4.4

Species and moisture condition

Moisture condition – dry (m/c = 16%)

Moisture condition – green (m/c = 25%) Radiata Pine & Douglas Fir

*G8 is a visual grade, which has been verified, in the green condition for when timber will be used in a service situation where the moisture condition may be 25% or over.

10.6.2

Machine Stress Graded Timber

Table 10.4 provides minimum target values for machine stress graded timber. Table 10.4: Minimum target values for machine stress-graded timber Grade

Bending strength fb (MPa)

Compression strength Fc (MPa)

Tension strength ft (MPa)

Modulus of elasticity E (GPa)

Fifth percentile modulus of elasticity (GPa)

Radiata Pine & Douglas Fir

MGS15

41.0

35.0

23.0

15.2

11.5

Radiata Pine & Douglas Fir

MGS12

28.0

25.0

14.0

12.0

9.0

Radiata Pine & Douglas Fir

MSG10

20.0

20.0

8.0

10.0

7.5

Radiata Pine & Douglas Fir

MSG8

14.0

18.0

6.0

8.0

5.4

Radiata Pine & Douglas Fir

MSG6

10.0

15.0

4.0

6.0

4.0

Species and moisture condition

Moisture condition – dry (m/c = 16%)

Note: A producer can offer other species or grades with different stresses and moduli subject to verification in accordance with the standard NZS 3622 (2004).

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10.6.3

New Zealand Standards

There are a number of New Zealand standards for timber and timber products, which relate to timber bridge components (listed below). Verified Timber grades can be produced by two types of grading system, machine stress grading and visual grading. Timber graded through both systems is tested in the same way through random sampling and testing on a calibrated bending test machine. Rigid pass requirements must be adhered to by all Verified Timber licensees to ensure that the standards of strength and stiffness are maintained. For information relating to New Zealand standards refer to www.vereifiedtimber.co.nz. NZS 3622:2004 Verification of Timber Properties This standard describes procedures for the initial evaluation and daily quality control requirements necessary to ensure that timber has the structural properties claimed for it. The procedures apply to timber that is visually and machine stress graded. NZS 3603:1993 Timber Structures Standard Sets out in limit state design format the requirements for methods of design of timber elements of buildings and applies specifically to sawn timber, glue laminated timber, natural round timber and construction plywood. AS/NZS 4063:1992 Timber – Stress-graded – In-grade strength and stiffness evaluation Describes procedures for evaluating structural properties of graded timber and verifying accuracy of specific grading techniques. Requirements for resolving doubts concerning specified design properties of particular populations of graded timber are specified. AS/NZS 1748:2006 Timber – Mechanically stress-graded for structural purposes Specifies structural property, marking and physical requirements for mechanically stress-graded timber. Physical requirements are given for strength considerations as well as for utility considerations.

10.7 Deterioration Mechanisms The common faults occurring in timber bridge components are splits, shakes, checks, pipes, and knots. 10.7.1

Splits, Shakes and Checks

These faults affect both the strength and durability of the piece (Figure 10.3). Deep checks tend to reduce durability as they provide a convenient entry for fungal spores and insects to the central part of a piece. Both splits and checks are a result of longitudinal separation of the wood fibres. In sawn timber: 

A surface check extends from the surface towards the centre of the piece of timber but with a depth of only about 10% of the thickness of the piece.



A check is deeper but does not go all the way through the piece.



A split goes right through the piece.

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Barrel Check

Pipe

Star Shake

Ring Shakes

Source: RTA NSW (1983)

Figure 10.3: Common timber faults

10.7.2

Pipe

A pipe is simply a hollow up the centre of a log commencing at the butt. It is often associated with the development of heart rot. 10.7.3

Knots

Knots are usually round or oval in shape on the surface of the timber. In the living tree they were the limbs or branches. The effect of knots on durability depends on their occurrence, type and size. 10.7.4

Wood Decay

Wood decay only occurs as a result of fungal spores in the air. When three conditions co-exist the fungal spores grow, and in so doing, break down the timber cell structure. The three conditions are: 

moisture content > 20%



the presence of oxygen



temperature range 24-32 °C.

On the basis of the above it can be stated ‘no water, no decay’. 10.7.5

Types of Fungal Decay

Wood decay in timber varies in pattern depending on the environmental conditions and the species of fungi colonising the wood. The main types of fungal decay affecting timber are: 

brown rot



white rot.

Brown rot 

Attacks the cellulose leaving chemically degraded lignin (the brown colour).



Can delete up to 65% of weight.

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

Spreads rapidly so that decay is still occurring in the original location while continuing to spread.



Affected wood seems better than it is.



Causes less reduction in toughness than white rot but other strength properties decrease more quickly.

White rot 

Attacks lignin and cellulose.



Can delete up to 100% of weight.



Often decomposes the cell from the centre out and completes the destruction of each cell before moving to the next cell.



Reduces toughness more rapidly than does brown rot.

Fungi will attack both sapwood and heartwood under favourable moisture and temperature conditions, causing breakdown of the wood substance, which is then said to be decayed or rotted. Where pieces of timber are used externally and are against another piece of timber or other material and there is a moisture trap, there is a greater potential for decay. 10.7.6

Effects and Indications of Fungal Decay

Generally speaking the effects of decay are loss of strength, density, and structure. Brown rot causes the infected timber to cube or crumble into a powder. White rot feeds on the cellulose and lignin, which results in a bleached appearance. The decayed area is usually stringy and, when dry, the wood is easily crumbled between the fingers. 10.7.7

Indicators of Decay

Indicators of decay are as follows: 

colour changes



softening



change in density



change in odour



loss of cross-sectional area due to crushing.

10.8 Durability Timber species are classified by durability class of 1 to 4 to indicate their resistance to fungal and termite attack. The durability class of each species was determined by long term in-ground exposure tests. The durability class is an indication of the expected life of a particular species in contact with soil. Species such as ironbark and turpentine are durability class 1, whereas species such as spotted gum and yellow stringy bark are durability class 2.

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Table 10.5: Durability class and in-service life

10.8.1

Durability class

Life in years

1

> 25

2

15-25

3

8-15

4