Durable Post-tensioned Concrete Structures

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A cement and concreteindustrypublication

Technical Report No. 72

Durable Post-tensioned Concrete Structures

0

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Acknowledgements The Concrete Society is grateful to the following for the provision of photographs: Strongforce/Laing O'Rourke: Figures 18, 20 and 21 Sir Robert McAlpine: Figures 19 and 22.

Published by The Concrete Society CCIP-047 Published September 2010 ISBN 978-1-904482-62-8 © The Concrete Society The Concrete Society Riverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 (0)1276 607140 Fax: +44 (0)1276 607141 www.concrete.org.uk

CCIP publications are produced by The Concrete Society (www.concrete.org.uk) on behalf of the Cement and Concrete Industry Publications Forum - an industry initiative to publish technical guidance in support of concrete design and construction.

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Durable post-tensioned concrete structures Contents Preface

VI

List of figures

vii

List of tables

vii

1.

Introduction

1

1.1

1

General background

1.2 Technical background

2.

2

1.2.1

Post-tensioned bridges

2

1.2.2

Post-tensioned buildings

4

1.3 Summary of progress

4

1.4 Summary of key provisions

5

1.4.1

Design and detailing

6

1.4.2

Duct and grouting systems

6

1.4.3

Grout materials

7

1.4.4

Certification of post-tensioning operations and training

8

1.4.5

Testing

9

Recommendations for durable post-tensioned concrete bridges

11

Factors affecting durability

13

2.1

13

Corrosion of prestressing steel

2.2 Materials and components

14

2.3 Construction quality

14

2.4 Expansion joints

14

2.5 Construction joints

14

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3.

2.6 Cracking

15

2.7

15

Duct and anchorage layout

2.8 Precast segmental construction and joint type

16

2.9 Proximity to sea water

16

2.10 Road salts, waterproofing and drainage

16

2.11 Access for inspection and maintenance

16

Available protective measures

18

3.1

18

Design strategy - multi-layer protection

3.2 The structure as a whole

4.

5.

19

3.2.1

General

19

3.2.2

Bridge deck waterproofing systems

19

3.2.3

Coatings

20

3.2.4

Drainage

21

3.3 Individual structural elements

21

3.4 Prestressing components

22

3.4.1

Prestressing tendons

22

3.4.2

Ducts

23

3.4.3

Anchorage location

25

3.4.4

Anchorage details

29

Grouted bonded post-tensioned construction for bridges

30

4.1

Grouts and grouting

30

4.2 Vents and grout injection

31

4.3 Recommended protection systems

31

4.3.1

Prestressing system

33

4.3.2

The deck and its elements

34

4.3.3

Possible additional measures for exceptional structures

34

External unbonded post-tensioned construction for bridges

35

5.1

Advantages and disadvantages

35

5.2

Background

35

5.3 Structural design and basic performance requirements

36

5.4 Available protective measures

37

5.5

37

Detailing

5.6 Tendon systems

38

5.7

41

De-tensioning and replacement of external tendons

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6.

7.

8.

Segmental construction

44

6.1 Multi-layer protection

44

6.2 Anchorage location and detailing

45

Void grouting

47

7.1

47

Overview

7.2 Aims of void grouting

48

7.3 Condition of bridge stock and potential demand

49

7.4 Inspection records

49

7.5 Grouting materials

50

7.6 Grouting equipment and methods

50

7.7 Determining the void characteristics

51

7.8 Flushing with water

52

7.9 Effect of existing defects

52

7.10 Specification for grouting

53

7.11 Trials

54

7.12 Quality control

54

Test methods for grouted post-tensioned concrete bridges

56

8.1

56

Introduction

8.2 Range of tests considered

56

8.3 The need for testing

56

8.4 Test methods appropriate in particular circumstances

58

8.4.1

Type approval at pre-contract stage (duct systems, grout materials and procedures)

58

8.4.2 Trial grouting within a contract (geometry, materials and procedures)

59

8.4.3

Duct assembly verification before main grouting

59

8.4.4

Duct integrity after concreting or assembly of precast units, but before main grouting

60

8.4.5 Grout stiffness test of main grouting

60

8.4.6 Automated quality control testing of main grouting

61

8.4.7 Survey of existing grout conditions before regrouting

61

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9.

Recommendations for durable post-tensioned concrete buildings

63

Durable post-tensioned concrete buildings

65

9.1 Factors affecting durability

65

9.2 Materials and components

66

9.3 Construction quality

66

9.4 Expansion joints

66

9.5 Construction joints

66

9.6 Cracking

67

9.7 Ducts and anchorage layouts

67

9.8 Proximity to seawater

69

9.9 Road salts

69

9.10 Access for inspection and maintenance

69

10. Available protective measures

70

10.1 The structure as a whole

70

10.2 Individual structural elements

70

10.3 Prestressing components

70

10.3.1 Prestressing tendons

70

10.3.2 Ducts

70

10.3.3 Anchorages

71

11. Grouted bonded post-tensioned construction for buildings

iv

73

11.1 Grouts and grouting

73

11.2 Vents and grout injection

74

11.3 Recommended protection systems for buildings

74

11.3.1 General

74

11.3.2 Prestressing system

75

11.3.3 The slab

75

11.3.4 Possible additional measures

75

11.4 Void grouting

76

11.5 Test methods for grouted post-tensioned buildings

76

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12. Unbonded post-tensioned construction for buildings

77

12.1 Introduction

77

12.2 Recommended protection systems for buildings

77

12.2.1 General

77

12.2.2 Prestressing system

78

12.2.3 The slab

79

12.2.4 Possible additional measures

79

Recommendations for specifications for durable post-tensioned concrete

81

13. Recommendations for specifications for duct and grouting systems for post-tensioned tendons

83

13.1 Introduction

83

13.2 Guidance on the project specification

83

13.2.1 Trials

84

13.2.2 Grout materials

85

13.2.3 Ducting for bridges and other aggressive environments

85

13.2.4 Ducting for internal elements of buildings

86

13.2.5 Vents

86

13.2.6 Testing

87

13.2.7 Grouting

88

14. Contractor's quality scheme requirements

90

14.1 Introduction

90

14.2 Basic quality system elements

90

14.3 Product requirements

92

14.4 Certification

94

References

95

Appendix A. Test methods

99

A1 Leaktightness tests for duct systems

99

A2 Grout stiffness tests

100

A3 Void sensors

101

A4 Duct pressure sensors

102

A5

102

Automated quality control systems

A6 Volume of voids before regrouting

104

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Preface This Report is a revision of the second edition of Technical Report 47, Durable post-tensioned concrete bridges®, which was published by The Concrete Society in 2002. The recommendations in the second edition have been reviewed and extended, and where new international and European standards now exist, now make reference to them. This has enabled some simplification in the text. The most significant extension to this Report is to include recommendations for post-tensioning in buildings as well as in bridges, where significant experience has been gained in recent years. The measures described are aimed at improving design, detailing, specifications, materials, construction methods and testing for grouted post-tensioned concrete with either internal or external tendons. Producing this revised and updated Technical Report has been undertaken by a small group of people fully aware of the current state of the art and I am particularly grateful to Tony Jones of Arup and AndyTruby of Gifford for their assistance in expanding the scope to include buildings and I am grateful to all who have contributed, entirely on a voluntary basis. At a time when the Eurocodes are upon us, the post-tensioning industry is preparing to follow new procedures and Standards and the relevant documents for design and construction of post-tensioned concrete are largely in place. However, it should be remembered that practices continually develop and evolve and while these new standards will improve performance significantly, there will always be scope for further development. I am indebted to Mark Raiss and George Somerville who masterminded the production of the first edition ofTR47 in 1996 which formed the original basis for this Report. G.M.Clark

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List of figures Figure 1

Buried anchorage at end of deck with an abutment gallery.

Figure 2

Exposed anchorage at end of deck with an abutment gallery.

Figure 3

Exposed external anchorage at end of deck with an abutment gallery.

Figure 4

Restressable external anchorage at end of deck with an abutment gallery.

Figure 5

Anchorage at top blister using exposed anchor.

Figure 6

External blister and bonded face anchorages for in-situ segmentai construction.

Figure 7

Anchorage at bottom blister using buried anchor (internal tendon).

Figure 8

Top pocket anchorage. (This is NOT recommended,unless external protective layers are used.)

Figure 9

Buried anchorage for stressed or dead end.

Figure 10 Exposed anchorage for stressed or dead end. Figure 11 Face anchor details in in-situ segmentai construction. Figure 12 Grout vent details at deck surface. Figure 13 Exposed anchorage for restressing the end of an unbonded external tendon. Figure 14 Exposed anchorage for the dead end of an unbonded external tendon. The detail is also applicable for the live end where restressing is not required. Figure 15 Top deviatorfor external tendon. Figure 16 Face anchor details for precast segmentai construction. Precast segmentai construction using internal grouted tendons is NOT recommended, unless continuity of the duct is assured. Figure 17 Combined face anchor and shear key details for precast segmentai construction. Precast segmentai construction using internal grouted tendons is NOT recommended, unless continuity of the duct is assured. Figure 18 Live end anchor at construction joint adjacent to unstressed pour strip. Figure 19 Pour strip after tendon stressing and prior to fixing reinforcement and casting the concrete. Figure 20 Dead end anchorage at construction joint. Figure 21 Top pocket before (top) and after (bottom) casting the concrete. Figure 22 Edges of post-tensioned slabs. Figure A1 Location of spongeometer within the grouting system. Figure A2 Instrumentation within the Oxford grout quality control system.

List of tables Table 1

Test methods applicable during construction.

Table 2

Test methods applicable during service life.

vii

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Introduction 1

1. Introduction The first edition of Concrete Society Technical Report 47, Durable post-tensioned concrete bridges^, published in 1996, recommended new standards and practices for the design and construction of durable bonded post-tensioned concrete bridges. It covered the key elements of design, detailing, materials, grouting and certification for installation. This resulted in the lifting of the moratorium for in-situ post-tensioned construction that had been imposed by the Department of Transport in 1992.

1.1 General background

The Concrete Society Working Party continued working to improve and update its recommendations, particularly on test methods, while developing solutions for grouted precast segmental construction, which was not covered in the first edition. Account was taken of international developments, especially those involving specifications, and close contact maintained with similar groups in other countries and with the International Federation for Structural Concrete (fib). The Working Party incorporated the best of these new developments into the second edition ofTR47, published in Z002, while ensuring that the basic principles and performance requirements were met. Although relatively few bridges of this type have been built in the UK in recent years, there has been significant feedback from the use of the recommendations in practice, both nationally and internationally. The scope of the second edition was extended to include: external unbonded prestressing • remedial (void) grouting of existing bridges • updated information on new test methods. The second edition included a revised Specification for duct and grouting systems, together with notes for guidance. That Specification, coupled with the CARES certification scheme for the supply and installation of post-tensioning systems in concrete structures, has represented the state of the art for about 10 years. More recently, developments of international standards have taken place and the publication in 2007 of revised versions of BS EN 445(2), BS EN 446'3' and BS EN 447'4', which embody many of the proposals in the second edition of TR47, have led to the need for an updated Technical Report. Experience of both grouted and unbonded post-tensioning in buildings, especially flat slabs, has grown significantly in recent years and this Report has now been extended to include recommendations for this type of use. In addition to its use in bridges and in buildings, post-tensioning is also used in a variety of other types of structures, such as storage silos, tanks and other containment structures. The principles described in this Report will be equally applicable to such structures, but detailed guidance (e.g. on the layout of tendons and the provision of vents) is not given because of the variety of such structures.

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introduction

While this Technical Report is primarily concerned with sound principles supported by good practice and procedures, the importance of attitude and awareness is also stressed. Since 1996 awareness has increased significantly. Grouting is an installation-sensitive operation, requiring skill and care on the part of all concerned.

1.2 Technical background

Surveys of bridge durability have been undertaken throughout the world but it is impossible to estimate accurately the number of post-tensioned bridges that have suffered tendon corrosion.

1.2.1 Post-tensioned bridges

The first serious problem in the UK was the collapse of Bickton Meadows footbridge in Hampshire in 1967, since when appreciation of the problem has slowly grown. In 1981 the Transport Research Laboratory published the results of an investigation into the grouting of 12 post-tensioned concrete bridges constructed between 1958 and 1977'5'. Voids were found in the ducts of ten of the bridges. The results were passed to the Standing Committee on Structural Safety*6' which concluded that, in structures containing a large number of tendons, "the risk of sufficient tendons failing by corrosion at any time to cause sudden collapse is considered to be small". In 1980 Angel Road Bridge, North London was found to have wires broken due to corrosion behind some of the anchorages. The deck was propped and has since been replaced. An inspection of Taf Fawr Bridge, MerthyrTydfil, South Wales in 1982(32). In this context, a site-specific duct assembly verification test is also recommended and possible additional tests are described which may be considered in certain circumstances. Both of these relate to measurements of the degree of sealing provided by the duct system but, at this time, pending further development and experience of use, neither is included in the Standards.

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Technical Report No. 72

Durable post-tensioned concrete structures

Recommendations for durable post-tensioned concrete bridges

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Factors affecting durability 2

2. Factors affecting durability In broad terms, deterioration mechanisms that can affect structural concrete may be classified as those that directly attack the concrete and those that directly or indirectly attack the reinforcement or prestressing components. Attack of the concrete is not covered in this Report. However, sulfate attack and alkali-silica reaction are now well understood and guidance is available - see for example BRE Special Digest 1(37) and Concrete Society Technical Report 30, Alkali-silica reaction: minimising the risk of damage to concrete^. Proven solutions are established to deal with different intensities of the various mechanisms, mostly in material specification terms. A major hazard for bridges is corrosion of the prestressing steel, and this is the prime concern of this chapter.

2.1 Corrosion of prestressing steel

Corrosion may result from: • chlorides in the ingredients in the concrete (or grout) • carbonation of the concrete, resulting in reduced alkalinity in the concrete • external chlorides penetrating to the steel, from sources such as de-icing salts or seawater. Of these, strict limits have been placed on chlorides in the concrete (or grout) in codes and standards for more than 20 years. Carbonation can be a hazard for buildings, but the dominant factor for bridges and car parks is undoubtedly external chlorides. It follows that, in developing a design strategy, the nature and intensity of the aggressive actions - and how they might penetrate to the steel - is of fundamental importance. This applies both to conceptual design and to the evolution of design details. The transport mechanisms for chlorides are much influenced by the combined effects of wind, water and temperature, in both ambient and micro-climate terms. Resisting these influences requires an integrated approach, involving design concept, detailing, construction quality and material selection. The importance of integrating these aspects cannot be overemphasised. The purpose of this chapter is to identify the key factors that affect durability, based on feedback from performance in service. The main focus is on the performance of bridges and buildings as a whole. The factors considered are: • materials and components • expansion joints • construction quality • construction joints • cracking • duct and anchorage layout • precast segmental construction and joint details • proximity to seawater • road salts, waterproofing and drainage • access for inspection and maintenance.

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Factors affecting durability

2.2 Materials and components

The quality of materials and components is of great importance, and therefore the derivation of good specifications is crucial. This should be done with a clear idea of performance requirements, and of a methodology that will ensure that the chosen items do in fact comply.

2.3 Construction quality

Poor workmanship and construction defects are major issues, which strongly influence the level of durability actually achieved. A good example of this, in the past, was ineffective grouting for post-tensioned work. However, the issue is wider than that, ranging from poor compaction and failure to achieve specified covers to cases where joints are poorly made (either in the structural elements themselves or in fitting together the various pieces of hardware involved in prestressing operations). Substantial loads and forces are involved in casting and stressing prestressed concrete structures - often involving large pressures and strains. There is therefore a design element involved in ensuring that temporary conditions during construction are properly considered, and in deriving details that enable materials and components to be fitted together on site.

2.4 Expansion joints

A high proportion of expansion joints leak and their effectiveness and life span are very dependent on the quality of installation and maintenance. The Highways Agency has produced a Departmental Standard on the requirements for expansion joints, BD 33/94, and a Departmental Standard and Advice Note on Design for durability, BD 57/1 and BA 57/01(40'. These documents encourage the use of continuous bridge decks and integral abutments wherever possible, to eliminate expansion joints and hence reduce the risk of contaminants reaching sensitive parts of the structure. Where expansion joints are used, provision should be made for inspecting them and the structure underneath, and the details should be based on the assumption that joints will leak and will not provide protection against ingress of water and road salts. Appropriate drainage paths for the leakage should be provided which ensure that it cannot get access to the prestress anchorages or bearings and that the water is not allowed to pond. This is especially important if intermediate joints have to be located over piers, in ensuring that drainage paths are kept clear of anchorages, because here it is often difficult to provide an inspection gallery.

2.5 Construction joints

Well-made construction joints should not leak, particularly when protected by waterproofing membranes. However, waterproofing membranes often do not provide a complete seal, and do not last indefinitely, and joints leak. It is therefore advisable to keep construction joints in deck slabs away from anchorages and prevent, by means of drips, any access for the leakage to reach the anchorages. If possible, joints in ducts should also be kept away from construction joints. In sequential or segmental construction, where the prestressing anchorages are inevitably located at construction joints, care should be taken in detailing.

14

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Factors affecting durability

Emphasis should be given to not creating planes of weakness, which permit easy access to water (in the form of spray, runoff or ponding) that acts as a transport mechanism for contaminants, and to detailing protection for the anchorages and preventing ingress of water. Provision for ease of inspection is also important.

2.6 Cracking

Cracking in concrete can occur for a number of reasons - see Concrete Society Technical Report 22, Non-structural cracks in concrete^. Its relevance to durability is largely related to corrosion, and depends on the type and magnitude of the cracks - see Concrete Society Technical Report 44, The relevance of cracking in concrete to corrosion ofreinforcement^. Care is required, when considering the layout and sequencing of concrete pours and prestressing, to minimise the risks of cracking, particularly near anchorages. Applying a low initial prestress at an early age can help counteract early-age cracking. The reinforcement provided in the direction of the prestress is usually much less than that used in reinforced concrete bridges and should be checked for adequate distribution of cracking in accordance with BD 28/87. Greased strands are enclosed in a PE sheath, and pass through pipe assemblies both at the anchorages and the deviators. One advantage is that the strands can be stressed (and de-stressed) using a monostrand jack, requiring less working space behind the anchorages. The strands can also be extracted individually and replaced. A system is also available of sheathed and greased strands within a PE pipe, which is grouted prior to stressing. The grout is used to fix the strands in position, reducing the risk of displacement and of tears in the sheaths. In this case the duct must be supported to avoid displacement by the weight of grout. Deviators are generally of steel or reinforced concrete; in the latter case, they may be lined with a pipe, or cushioned in some other way. Usually, they are designed to accommodate an unintentional angle change of 0.02 radians. The angle change has a major influence on their design, in terms of the forces to be resisted, and minimum radii are usually given in specifications (e.g. BD 58/94