Concrete Society-Durable Bonded Post-Tensioned Concrete Bridges (Concrete Society Technical Report) (1996)

Second Editioh

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Durable post-tensioned concrete bridges

Report of a Concrete Society Working Party in collaboration with the Concrete Bridge Development Group

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Climb form and formworks 0

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Heavy lifting

Superstructure erection

Special construction methods 0

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Grouting

Ground anchors

Repair 81strengthening 0

Monitoring

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Durable post-tensioned concrete bridges

Cover photograph: Braidley Road Bridge, Bournemouth. Courtesy of Gifford and Partners

Durable post-tensioned concrete bridges Concrete Society Technical Report No. 47 Second Edition

ISBN 0 94669 1 96 7 0 The Concrete Society 1996, 2002

Further copies of this publication and information about other Concrete Society publications may be obtained from: The Concrete Society Century House, Telford Avenue Crowthorne, Berkshire RG45 6YS, UK Tel: +44 (0)1344 466007, Fax: +44 (0)1344 466008 Email: [email protected],www.concrete.org.uk All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to The Concrete Society. Although The Concrete Society (limited by guarantee) does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Society, its servants or agents. Readers should note that all Concrete Society publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Concrete Society Technical Report No. 47 Second Edition

Durable post-tensioned concrete bridges

Report of a Concrete Society Working Party in collaboration with the Concrete Bridge Development Group

The Concrete Society

MEMBERS OF THE WORKING PARTY

Since the inception of the Working Party in 1992 there have been many changes to the representation on the various Groups, too numerous to list periods of service for individuals. The members of the Working Party for this revision were: Mr G. M. Clark (Convenor from November 1999) Professor G. Somerville (Convenor to November 1999) Mr G. Bell Professor J. Clarke Mr D. Collings Mr J. Darby Mr R. Digman Mr M. Haynes Mr K. P. Houlden Mr N. Loudon Mr A. Low Ms L. J. Smith Mr P. Stanley Mr M. Walker Dr R. J. Woodward

Gifford and Partners British Cement Association VSL (UK) Limited The Concrete Society Robert Benaim & Associates Consultant CARES Balvac Whitley Moran Ltd Balvac Whitley Moran Ltd Highways Agency h P Hyder Consulting Mott MacDonald The Concrete Society TRL

Valuable assistance has been given by the following: Mr G. Bowring Mr B. Bowsher Professor J. Bungey Mrs A. Croft Dr D. W. Cullington Mr B. Hill Mr D. Jones Mr D. R. Moffett Dr M. Raiss Mr J. D. N. Shaw Mr D. Storrar Dr H. P. J. Taylor Dr G. P. Tilly

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Balvac Whitley Moran Ltd CARES University of Liverpool The Concrete Society TIU Highways Agency Freyssinet Ltd Balvac Whitley Moran Ltd Robert Benaim & Associates Weber SBD Highways Agency Tarmac Precast Concrete Ltd Gifford and Partners

CONTENTS

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Members of the Working Party Preface

vi

4.1 4.2 4.3 4.4

PART ONE RECOMMENDATIONS FOR DURABLE POST-TENSIONED CONCRETE BRIDGES 1 1.I 1.2 1.3 1.4

INTRODUCTION. . . . . . . . . . . . . . . 3 3 General background 3 Technical background 4 Summary of progress 5 Summary of key provisions 1.4.1 Design and detailing 5 5 1.4.2 Duct and grouting systems 1.4.3 Grout materials I .4.4 Certification of post-tensioning operations and training 6 6 1.4.5 Testing

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11

FACTORS AFFECTING DURABILITY. . . . 8 Genera1 8 Materials and components 8 Construction quality 8 Expansion joints 8 Construction joints 9 Cracking Duct and anchorage layout 9 Precast segmental construction and joint type Proximity to seawater 9 Road salts, waterproofing and drainage Access for inspection and maintenance

3 3.1 3.2

AVAILABLE PROTECTIVE MEASURES. . . I I Design strategy - multi-layer protection The bridge as a whole 3.2.1 General 3.2.2 Bridge deck waterproofing systems l2 3.2.3 Coatings l2 3.2.4 Drainage l2 Individual structural elements l2 3.3.1 General l2 3.3.2 Concrete quality and cover l2 13 Prestressing components 3.4.1 Introduction 13 13 3.4.2 Prestressing tendons 13 3.4.3 Ducts 15 3.4.4 Anchorage location 17 3.4.5 Anchorage details

3.3

3.4

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GROUTED BONDED POST-TENSIONED CONSTRUCTION. . . . I 8 Introduction ' 18 Grouts and grouting 18 Vents and grout injection 18 Recommended protection systems 19 4.4.1 General 19 4.4.2 Prestressing system 19 4.4.3 The deck and its elements 20 4.4.4 Possible additional measures for exceptional structures 21 I

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5.1 5.2 5.3 5.4 5.5 5.6 5.7

6 6.1 6.2

EXTERNAL UNBONDED POST-TENSIONED CONSTRUCTION . . . . 2 2 Introduction 22 Background 22 Structural design and basic performance requirements 22 Available protective measures . 23 Detailing 23 Tendon systems 23 Detensioning and replacement of external tendons 25 SEGMENTAL CONSTRUCTION . . . . . . . 2 7 General 27 Anchorage location and detailing 28

VOID GROUTING 7.1 Overview Aims of void grouting 7.2 Condition of bridge stock and potential 7.3 demand 7.4 Inspection records 7.5 Grouting materials 7.6 Grouting equipment and methods Determining the void characteristics 7.7 Flushing with water 7.8 Effect of existing defects 7.9 7.10 Specification for grouting 7.11 Trials 7.12 Quality control

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8 8.1 8.2 8.3

29 29 29 30 30 30 31 31 31 32 32 32 33

TEST METHODS FOR GROUTED POST-TENSIONED CONCRETE. . . . . . . 3 4 Introduction 34 Range of tests considered 34 The need for testing 34

Durable post-tensioned concrete bridges

8.4

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Test methods appropriate in particular circumstances 8.4.1 Type-approval at pre-contract stage (duct systems, grout materials and procedures) 8.4.2 Trial grouting within a contract (geometry, materials and procedures) 8.4.3 Duct assembly verification before main grouting 8.4.4 Duct integrity after concreting or assembly of precast units, but before main grouting 8.4.5 Grout stiffness test of main grouting 8.4.6 Automated quality control testing of main grouting 8.4.7 Survey of existing grout conditions before re-grouting

REFERENCES.. . . . . . . . . .

35 35 35 36

36 36 36 37

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PART TWO REQUIREMENTS FOR DURABLE POST-TENSIONED CONCRETE BRIDGES NOTES FOR GUIDANCE ON SPECIFICATION FOR DUCT AND GROUTING SYSTEMS FOR POST-TENSIONED TENDONS . . . . . . . . 4 3 10.1 Introduction 43 10.2 Notes for guidance on the specification 43 10.2.1 Trials 43 10.2.2 Grout materials 44 10.2.3 Ducting 44 10.2.4 Testing 45 10.2.5 Grouting 45

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CONTRACTORS QUALITY SYSTEM REQUIREMENTS. . . . . . . . . . . . . . . 4 6 11.1 Introduction 46 11.2 Basic quality system elements 46 11.3 Product requirements 47 11.4 Certification 48

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SPECIFICATION FOR DUCT ANDGROUTING SYSTEMS FOR POST-TENSIONED TENDONS . . . . . . . . 4 9 Clause 1 Planning, trials and basic requirements 49 Clause 2 Grout materials 50

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Clause 3 Clause 4 Clause 5 Clause 6 Clause 7 Clause 8

Duct systems Grouting equipment Batching and mixing of grout Injecting grout Grouting during cold weather Properties of grout Clause 8.1 Fluidity Clause 8.2 Bleeding Clause 8.3 Volume change Clause 8.4 Strength Clause 8.5 Sieve test Clause 8.6 Sedimentation test Clause 9 Testing of grout Clause 9.1 General conditions Clause 9.2 Fluidity test Clause 9.3 Bleeding test Clause 9.4 Volume change test Clause 9.5 Sedimentation'test Clause 10 Admixtures Clause 10.1 General Clause 10.2 Types Clause 10.3 Chemical composition Clause 10.4 Material requirements Clause 10.5 Dosage Annex 1 (Appendix 1 7 K to the Specification for Highway Works) - Concrete - Duct and Grouting Systems for Post-Tensioned Tendons Annex 2 Suggested Amendments to the Method of Measurement for Highway Works

APPENDIX A TEST METHODS. . . . . . . . . . . . . . . . A1 Leak tightness tests for duct systems A2 Grout stiffness tests A3 Void sensors A4 Duct pressure sensors Automated quality control systems A5 A6 Volume of voids before re-grouting A7 Stability bleeding test (inclined tube test) A8 Alternative bleeding test '

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50 51 51 52 53 53 53 53 54 54 54 54 54 54 54 55 56 56 57 57 57 58 58 58

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61 61 62 62 62 63 64 65

APPENDIX B UNITS . . . . . . . . . . . . . . . . . . . . . . . . . 6 6 APPENDIX C SOURCES OF FURTHER INFORMATION

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SUBJECT INDEX. . . . . . . . . . . . . . . . . . . 6 8

This Report is a revision of the first edition of Technical Report 47, which was published by The Concrete Society in 1996. The recommendations in the first edition have been extended and improved, based on experience of use and on much of the similar work that has been carried out internationally. The measures described are aimed at improving design, detailing, specifications, materials, construction methods and testing for grouted post-tensioned concrete bridges with either internal or external tendons. Producing this extensively revised and updated second edition of the Report has been a success thanks to the cooperation from all parts of the industry - clients, designers, contractors and specialist post-tensioning f i s - and I am grateful to all who have contributed, entirely on a voluntary basis.

At a time when the International Federation for Structural Concrete (fib) is preparing revised guidelines for grouting, and several other countries are improving their specifications, it is appropriate that the UK should be fully up-todate with state-of-the-art recommendations for grouting practices. 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. My particular thanks go to all of the current Working Party members and to the Highways Agency for their support and cooperation. 1 am also indebted to Mark Raiss and George Somerville who masterminded the production of the first edition which formed the basis for this new edition. G. M. Clark

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PART ONE

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RECOMMENDATIONS FOR DURABLE POST-TENSIONED CONCRETE BRIDGES

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Introduction

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Factors affecting durability

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Available protective measures

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Grouted bonded post-tensioned construction

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External unbonded post-tensioned construction

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Test methods for grouted post-tensioned concrete References

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Segmental construction Void grouting

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1 INTRODUCTION

1.1

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GENERAL BACKGROUND

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1.2 TECHNICAL BACKGROUND

The first edition of Concrete Society Technical Report 47, published in 1996, recommended new standards and practices for the design and construction of durable bonded posttensioned 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. Since 1996, the Concrete Society Working Party has 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 has been 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)).Wherever possible, the Working Party has incorporated the best of these new developments into this second edition, while ensuring that the basic principles and performance requirements are 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 this second edition has been extended to include: external unbonded prestressing remedial (void) grouting of existing bridges updated information on new test methods. Part Two of this Report includes a revised Specification for duct and grouting systems, together with notes for guidance. This Specification, coupled with the CARES certification scheme for the supply and installation of post-tensioning systems in concrete structures, represents the primary defensive layer of the recommended multi-layer design approach. Some coverage is given to all relevant test methods, but the Report focusses on techniques of value in the early stages of prestressing and grouting. 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. \

Surveys of bridge durability have been u n d e r t a a throughout the world but it is impossible to estimate accurately the number of post-tensioned bridges that have suffered tendon corrosion. 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 posttensioned concrete bridges constructed between 1958 and 1977 (1). Voids were found in the ducts of 10 of the bridges. The results were passed to the Standing Committee on Structural Safety 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” (2). 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, Merthyr Tydfil, South Wales in 1982 (3) revealed severe corrosion of the prestress that led to the deck being replaced in 1986. In 1985 the road bridge at Ynys-y-Gwas, West Glamorgan, South Wales collapsed due to corrosion of the prestress at the segmental joints (4). Prestress corrosion was discovered at Folly New Bridge, Bladon, Oxfordshire in 1988, the M1 Blackburn Road Bridge, Sheffield in 1990 and Botley Road Flyover, Oxfordshire in 1992, all of which have been replaced. At Folly New Bridge more than half the tendons had corroded right through, behind the anchorages. Interest and concern grew in other countries throughout the 1990s, as more cases of corrosion became known. In 1992, the bridge across the River Schelde in Belgium collapsed without warning as a result of corrosion of the posttensioning through the hinged joint of the end tie-down member. Of particular interest is the Niles Channel Bridge in Florida; built in 1983, this 1390 m viaduct is of precast segmental box construction, with external tendons in grouted polyethylene tubes. Investigations in 1999 showed that one 19-strand tendon had failed close to the anchorage, which itself was heavily corroded, with no effective protection. Failure was attributed to corrosion caused by corrosive bleeding water, and there was general evidence of inadequate grouting. As a result, the State of Florida proposed significant changes to the specification and to grouting operations. New recommendations for grout, grouting and installation ( 5 ) have been introduced in the USA. The collapse in May 2000

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Durable post-tensioned concrete bridges

of a bridge in North Carolina was attributed to use of calcium chloride in the grout used to plug temporary tubes through which pre-tensioning deflector struts were positioned. In 2000/01 the Mid Bay Bridge in Florida had a major regrouting repair contract (6). Elsewhere, presentations of UK and French experience of corrosion at a 1999 conference (7) led to a review of specification and operations. In Germany, the Federal Ministry of Transport, Construction and Housing has introduced a Guideline for concrete bridges with external tendons (8). While this focuses on avoiding the use of couplers at the same cross-section, it also bans the use of grouted tendons within the webs of box beams, but not in the top and bottom slabs. The reason for this appears to be concern over reinforcement congestion, which may inhibit proper compaction of the concrete and the achievement of adequate cover. In Japan, experience of voids and corrosion in post-tensioned bridges has led the Japan Highways Public Corporation to ban cementitious grouting of post-tensioned structures; the focus is therefore on the use of unbonded external tendons, and on the development of pre-formed tendons pre-grouted with epoxies. The Japanese ban does not appear to be nationwide for bridges, and does not apply to buildings. The UK bridges that failed had internal prestress, but previous corrosion problems with external prestress had led to this method of post-tensioning not being used for a number of years; that situation has since been reversed, and design standards now exist (9). The Highways Agency’s series of special inspections of posttensioned bridges had the purpose of determining the condition of the prestressing and the efficacy of the grouting (IG11). Other bridge owners have been slower to respond and it is a matter of concern that problems are often found by accident, either during demolition of redundant bridges or when other work is being carried out (12). For example, the problems at Blackburn Road Bridge were only discovered during routine deck resurfacing. In 1992 the British Cement Association commissioned a desk study to collate the available information (13). The general impression is that there have been few cases of serious corrosion and that the performance in service of post-tensioned concrete bridges is generally good. However, it should be remembered that inspection of tendons is difficult and in some locations almost impossible, so past statements such as, for example, in the United States, “there is visual evidence of corrosion in less than about 0.1% of bridges”, must be treated with caution. It is especially important to recognise that the only sure way to find voids is to drill inspection holes into the ducts. Summary information, collated by TRL, of special inspections of over 200 post-tensioned structures on motorways and trunk roads was available to the Working Party, and it appears that: the incidence of severe or heavy corrosion was small (approximately 2%)

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roughly 92% of bridges were classed as good or as having minor problems

4.3%required attention and 3.5% had significant defects. There was evidence of voids in grouted ducts but most were fairly small and had not led to any significant deterioration in the structures. None of the bridges were considered unsafe but several had significant defects. Statistics from inspections have to be treated with caution, because the development of prestressing and grouting technology and types of structure have evolved and the likelihood of poor quality may be very different for each ‘family’ of structure from a different era. It was also apparent that detailed inspection of post-tensioned structures was difficult. However, the need for improved design and construction practices remained strong. Given this general background of uncertainty it was not surprising that the Department of Transport issued a temporary ban (14) in 1992 and later developments have vindicated this action. On a positive note, the lifting of the ban in 1996 for all forms of post-tensioned construction (other than precast segmental construction with internal grouted tendons) has given motivation to the further research and development reported in this report. This was confirmed by the issuing of Interim Advice Note 16 in 1999 (15).

1.3 SUMMARY OF PROGRESS Since the 1996 edition of this Report the activities of the Working Party have continued, and the need for awareness of ongoing and new activities elsewhere became more acute. These included: publication of CEN Standards on various aspects of prestressing technology conversion of ENV 1992 (Eurocode 2) into an EN, in which, for prestressing systems, a strong dependence is likely to be placed on Technical Approval methods developed by European Organisation for Technical Approvals (EOTA) formation of anfib Working Party on grouting in 1998 development of the Oxford grout quality control system seminar at Cambridge, September 1999, to present the output from a BRITE Euram project on QA in grouts and grouting (16) seminar at Croydon, November 1999, to present findings of an AngloFrench group on post-tensioned bridges (7) publication offib guidance specifications for plastic ducts in 2000 (17) research and development of new specifications and materials in the USA in response to discoveries of incidences of poor quality fib workshop at Ghent, Belgium, November 2001, on durability of prestressing tendons (18).

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Introduction

In their different ways, these activities affected the Working Party, in terms of the input to this second edition of TR47. Since compatibility is important, the approach adopted has been to refer to new specifications and guidance whenever possible, rather than to revise TR47 in isolation. The principal aim of the Working Party has been to generate confidence in the industry’s ability, with revised procedures, to design and build durable post-tensioned concrete structures. In pursuit of this aim the Working Party considered the following areas: design and detailing duct and grouting systems grout materials certification of post-tensioning operations and training testing

then it may be used. This is an area where product development is continuing, as well .as long-term research (20). External unbonded tendons are now covered in this second edition. This method may be used for any form of prestressed construction, including both in situ and precast segmental construction, provided the recommendations are followed.

1.4.2 Duct and grouting systems An interim specification and commentary were published by the Working Party in 1993 (2I) and the lessons learnt from their use discussed at a seminar in 1994 (22). The 1996 specification was based on drafts of European Standards, which were published subsequently (23), and other international documents (24). It has been updated as Chapter 12 in Part Two. The main differences between the specifications in this ‘ Report and in the first edition are as follows: requirement for full-scale grouting trials on each project relaxed 0

and has now added: external and unbonded construction remedial grouting of existing bridges

revised specification of the properties of the grout with a new bleeding test

new test methods.

clear recommendations for plastic ducts.

As the Highways Agency had published design guidance on external and unbonded prestressing (19), the 1996 edition of this Report concentrated on internal, bonded prestressing, but in this second edition, design details for external prestressing are also given. The Post-Tensioning Association and CARES co-operated with the Working Party to produce the Certification Scheme requirements in Chapter 11 of this Report.

These build on the innovations introduced in the first edition: ducts to be of electrically non-conductive, corrosionresistant durable material forming a double corrosion protection system in combination with the grout duct systems pressure tested additional vents additional testing.

1.4

SUMMARY OF KEY PROVISIONS

1.4.1 Design and detailing Those aspects of design and detailing that affect the durability of post-tensioned concrete bridges are discussed. Various factors affecting durability are considered and the concept of multi-layer protection is introduced. This requires the provision of a number of protective measures on the basis that any individual layer of protection may become ineffective but that the multi-layer approach gives adequate assurance of protection against corrosion. The effectiveness of the various possible layers of protection is discussed and recommendations are made for a protection system for a typical road bridge in the UK. In’ particular, recommended details are given for the layout and protection of anchorages. Segmental construction is a common and economic method for prestressed concrete bridges. The recommendations herein are considered valid for in situ segmental construction, since duct continuity through the joint - a key performance parameter in these recommendations - can be assured. For precast segmental construction, this is more difficult. The situation is reviewed in Chapter 6, and a number of possibilities put forward; if, in a particular case, any one of these can be shown to be equivalent to duct continuity,

The use of plastic ducts is intended to ensure that the duct itself provides additional protection against corrosion by preventing contact between the contaminants and the tendon. Protection is thereby given by the concrete’cover to the duct, the duct itself and the alkaline environment of the grout. Pressure-testing before concreting will check the integrity of the duct and is a useful check on how carefully the duct has been assembled. Recommendations on the use of plastic ducts, and on the required properties of the materials and components, are given in an fib Technical Report (17). A further advantage of non-metallic ducts is that some test methods reviewed by the Working Party can ‘see’ through plastic-type ducts but not through metal ducts. The Working Party has considered the use of’ vacuum grouting which, at first sight, appears to offer a complete solution to any problems of filling ducts with grout. Simply, the technique creates a vacuum in a duct and makes grout available with some added pressure to get it into the duct. Assisted by the vacuum, the duct will be completely filled. Providing a vacuum pump (or pumps) and the associated valves etc. and operating the system is more expensive than straightforward pressure grouting. The method has had very limited use in the UK (see, for example, Reference 25) although it is more widely used elsewhere in Europe. However, it has a very relevant application for re-grouting as will be discussed. While the Working Party has undertaken

5

Durable post-tensioned concrete bridges

some development work on vacuum grouting, the emphasis in this Report is on getting the standards and procedures right for pressure grouting, which will be used in the majority of cases.

1.4.3 Grout materials Before 1992, it was common practice in the UK and elsewhere to use general purpose cement for grout in combination with admixtures and water, mixed on site, and described as ‘common grout’. The properties of such cement are variable, particularly from one plant to another, resulting in variability in the properties of the grout. In addition, difficulties can arise due to variations in the weight of bagged cement; tolerances of 2 kg in 50 kg bags are not uncommon, and outside the desired tolerance of 2%. However, it is understood that tolerance on the weight of new 25 kg bags will in the future in the UK be reduced to f 1% which will improve one variable.

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The Specification in Chapter 12 in this Report calls for stringent testing of the grout and, during site application, it became apparent that it was difficult to maintain consistency and reliability of common grout under all circumstances, such as variable temperature ‘conditions. Consequently a Working Party sub-group has developed a pre-packed ‘special grout’ with more reliable and consistent properties, though it is still subject to quality control and testing. A research project was initiated, supported by LINK funding and overseen by the sub-group, to: e develop an improved grout with properties that consistently meet the revised specification *

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demonstrate that the grout has adequate performance under site conditions investigate methods of grouting and monitoring, including trials of the ‘grout stiffness test’ provide data that satisfy the Highways Agency that the grout can be used in bridge construction.

The results from this project were available in a draft final report in May 1996 (26), and were considered when formulating the recommendations in the first edition of this Report. Since then, feedback has been obtained on the use of special grouts in practice. In addition, results have become available from a major BRITE Euram project on grouts and grouting, including the development of improved test methods (16). All of this information has been used in re-formulating the notes for guidance, and the specification. Grouts meeting the performance requirements are now available, some as a combination of packaged products. The first edition of this Report included both common and special grouts within its scope. The Working Party is of the opinion that special grouts should generally receive first consideration because of their better and more consistent properties. Feedback indicates that grouts mixed on site using specially controlled materials can be used successfully, but are applicable mainly to large projects, where more trials are feasible, and safeguards can be built in, to ensure dedicated

6

and consistent sources of compatible cement and additives, for the entire job. Prepacked special grouts should be the first choice for quality, to minimise variables and attendant risks but this does not exclude combinations of controlled materials on the basis that the quality of the end product is the important factor.

1.4.4 Certification of post-tensioning operations and training It has been recognised that good quality workmanship is fundamental to the production of durable post-tensioned concrete bridges. This requires good procedures and appropriate training. In the past, grouting of ducts has sometimes been undertaken by inadequately trained staff and the importance of good grouting has not been properly recognised on site. There are even instances of ducts being left totally un-grouted. Consequently CARES together with the Post-Tensioning Association developed a Certification Scheme in consultation with the Highways Agency, which is referred to in Chapter 11. Similar schemes are used elsewhere in Europe but this is the first time such a scheme has been developed for use in the UK. Since 1996, CARES has made regular reports to the Working Party, as more companies have become certified, and on problems experienced in practice. One bridge was closely monitored. In general, the specification has worked well. Most problems have been of a practical nature, involving connections, vents, gaskets and taps, and a lack of data on the friction characteristics of the ducts. All of these points have been considered, in producing the revised recommendations of this second edition. The CARES scheme is now coming of age and is a positive contributor to improved durability.

1.4.5 Testing The fact that internal tendons cannot be inspected visually means that reliance has to be placed on indirect methods to confirm the adequacy of the corrosion protection system employed. The Working Party has considered numerous tests suggested for this purpose. These are all listed, although some remain at the development stage, and are unlikely to prove appropriate for routine use. The Report concentrates on tests that are unique to the grouting process, are of practical application, and provide information relating to quality at a stage when remedial action remains possible. Innovative methods that are not likely to be widely known are fully described. These include a method based upon the stiffness of grout developed by the Working Party before the first edition of TR47. Pressure is applied to the grout before it has hardened, and analysis of the ‘spongy’ response enables accurate calculation of the total volume of trapped gas. The technique was first investigated within a number of research projects, including site trial, and was known as the ‘Belmec Spongeometer’ (27). It has now been further developed by J. Darby, and is incorporated in a device that provides immediate results together with computer records of variables influencing grouting quality.

Introduction

For external tendons, inspection and testing are somewhat easier because the tendons are normally accessible. This does of c o m e require a regular programme of inspection to be followed after construction and in service, in order to reap the benefit. In the context of this Report, the Specification states a minimum mandatory level of testing. This may be associated with the required properties of the grout, where specific tests are given: it may also be associated with the duct system,

,

'

,where strong reliance is now placed on the standardised approval system developed by fib (17). In this context, a sitespecific duct assembly verification test is also required. 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 on use, neither are included in the specification.

2 FACTORS AFFECTING DURABILITY _

2.1

GENERAL

In broad terms, deterioration mechanisms that can affect structural concrete may be classified as:

(a) those that directly attack the concrete

0

precast segmental construction and joint details

e

proximity to seawater

0

road salts, waterproofing and drainage

0

access for inspection and maintenance.

(b) those that, directly or indirectly, attack the reinforcement or prestressing components.

2.2

Class (a) is not covered in this Report. Mechanisms such as sulfate attack and alkali-silica reaction are now well understood and guidance is available (28.29). Proven solutions are established to deal with different intensities of the various mechanisms, mostly in material specification terms.

The quality of materials and components are 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.

For class (b), the major hazard is corrosion, and this is the prime concern of this Report. Corrosion may result from: chlorides in the ingredients in the concrete (or grout) mix 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) mix in codes and standards for more than 20 years. Carbonation can be a hazard for bridges, but the dominant factor 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 over-emphasised. 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 as a whole. The factors considered are: materials and components expansion joints

MATERIALS AND COMPONENTS

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 bridges - 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 le+ and their effectiveness and life span are very dependent on the quality of installation and maintenance. The Highways Agency has produced Departmental Standard BD 33/94 on the^ requirements for expansion joints (30) and a Departmental Standard and Advice Note on design for durability (31). 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.

construction quality construction joints cracking duct and anchorage layout

8

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

Factors affecting durability

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 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. Emphasis should be given to not creating planes of weakness, which permit easy access to water (spray, run-off, driven or ponding) 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.

anchorages. For example, the layout of tendons .for span-byspan construction will'be' different to that for'structures cast in one pour. The significance for durability of tendon profiles and anchor locations should also be considered at an early stage. The tendon profile and duct size affect the ease of grouting. Anchorage location influences the ease of stressing and subsequent inspection, as well as susceptibility to water ingress. For example, anchorages in top pockets in the deck have often been used in span-by-span construction. They are easy to construct, stress and subsequently fill, but there is a concern that, by their shape and location, they may provide a path for contaminants to the prestress cables. Anchorage layouts are especially important for external prestressing systems, as is the detailing of ducts where they pass through deflectors and diaphragms.

2.8

PRECAST SEGMENTAL CONSTRUCTION AND JOIN? TYPE

'

2.6

CRACKING

Cracking in concrete can occur for a number of reasons (32); its relevance to durability is largely related to corrosion, and depends on the type and magnitude of the cracks (33). 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 accord$nce with BD 28/87 (34). Cracks parallel to and aligned with the ducts can occur - due, for instance, to transverse bending in reinforced sections, or to thermal effects at significant changes in cross-section - and may require consideration, as potential planes of weakness similar to the joints referred to in Sections 2.4 and 2.5. Such cracks may be limited either by design of reinforcement or by the introduction of an extra layer of protection into the multilayer protection system. Cracks at right angles to ducts are less likely to be critical, in terms of affecting the integrity and durability of the ducts, provided that their widths are limited in accordance with normal design practice.

2.7

DUCT AND ANCHORAGE LAYOUT

The method and form of construction should be considered at the preliminary design stage. They will often significantly affect the layout of prestressing tendons and the location of

Segmental construction is considered separately in Chapter 6, since particular care is needed. Only general aspects are considered here. The only two bridge collapses in the UK due to tendon corrosion have been in segmental structures. Both had precast segments with thin mortar joints and incompletely grouted ducts. When considering the risks associated with segmental construction, it is important to understand the significance of the different types of joint. Joint types in previous use may be sub-divided as follows: 0

thin mortar joints

0

wider, in situ concrete joints

0

match-cast joints - epoxy or dry.

Sufficiently wide in situ concrete joints, and match-cast joints properly sealed with epoxy resin, can be satisfactory in durability terms. The main durability problems have been with thin mortar joints. Difficulties in forming these have led to the joint material being highly permeable, and they should not be used. Special consideration has also to be given to the continuity of the ducts ,across the joints. The Working Party believes that duct continuity across the joints is vital when grouted tendons are used unless some other protective systems are proven. Ongoing research may help to form definitive advice on this in the future (20).

2.9

PROXIMITY TO SEAWATER

Bridges in coastal areas and over the sea are at risk due to corrosion induced by splash or spray of wind-borne chlorides. This is true of all forms of construction: in such situations both steel and concrete bridges need greater corrosion protection.

2.10 ROAD SALTS, WATERPROOFING AND DRAINAGE Road salts are applied to most UK road bridges in the winter. On some structures in the UK and many in other countries

9

Durable post-tensioned concrete bridges

warning. One of the advantages, however, of internal prestress is that the concrete itself forms one of the layers of protection. Although it is not possible to see the tendons, the use of non-metallic ducts can facilitate inspection using radar and other non-destructive techniques. Radiography can be used to inspect tendons within metal ducts, but it is less convenient than radar and has safety implications.

concern is that, if the tendons cannot be inspected, corrosion may proceed undetected and lead to collapse without

I0

Access for inspection and maintenance should be regarded as an essential element in the multi-layer protection strategy, and should always be provided. The use of and guidance on integral bridges (BD 57/01 and BA 57/01 Designfor durubility (31)) has not yet really addressed the application of posttensioning but design details should still apply in principle. In particular, inspection galleries should be provided, so that anchorages (and their protective systems) can be inspected; provision is also required at or near key locations such as deviators and joints. Further research is necessary for posttensioned .integral bridges. These key elements should feature strongly in any inspection checklist, together with checks on changes in moisture conditions, caused, say, by failed expansion joints or blocked drains. Maintaining the exposure condition assumed in design is an important element in management and maintenance.

3 AVAILABLE PR

3.1

DESIGN STRATEGY PRQTECTION

CTIWE MEASURES

- MULTI-LAYER

For the purpose of defining the standards and practices in this Report, the concept of multi-layer protection has been introduced. This has been used for ground anchorages and requires the provision of a number of protective measures, on the basis that the total integrity of one layer will maintain the integrity of the whole even if another of the layers of protection becomes partially ineffective.

For the bridge as a whole, the factors that might require consideration include (31): location of the bridge, and the associated general and local exposure conditions provision of continuity, possibly in the form of integral bridges

concept of multi-layer protection is the right approach, but it is important to maintain a reasonable perspective. Experience and judgement are needed to suit each set of circumstances and it would not be appropriate to recommend a fixed number of layers of protection. As an example, if any one of the protective measures could be guaranteed to totally exclude all contaminants, no other layers would be necessary. Conversely, if any one of the proposed layers were found to be ineffective, it should not be considered a suitable layer of protection. The easy design option is to use every conceivable protective measure available: the skill in durability design’is to choose the most cost-effective measures to suit the particular situation, while ensuring integrity and durability. The designer should consider the risk of corrosion, the life span of the various layers of protection, the opportunities for inspection and the possibility of maintenance, together with the integrity and life span of the structure.

access for inspection, testing, maintenance, and possible replacement of short-life elements

3.2 THE BRIDGE AS A WHOLE

type of cross-section, and its shape, particularly at its boundaries

3.2.1 General

method of construction, with its associated buildability and workmanship factors deck waterproofing system provision of effective drainage and avoiding ingress of water. Procedures for the design of individual elements are available These control other important durability issues, such as the quality of the concrete and the thickness of the cover to the ducts and reinforcement; they also draw attention to important features such as time-dependent movement and deformation at different times, both during construction and in service. Associated with this are other relevant matters, not normally covered in design codes, such as the avoidance of poor details that are known not to work well in practice. (35-38).

Finally, there is the protection of the prestressing hardware itself this involves consideration of filling the ducts with cement grout corrosion-resistant duct material ducts designed to exclude contaminants location, detailing and protection of anchorages. A full treatment of all the above factors is beyond the scope of this Report but those most directly related to post-tensioned construction are reviewed prior to developing the core quality recommendations. The Working Party believes that the

There is much that can be done, both quantitatively and qualitatively, to tackle the major threat of corrosion due to chlorides. In ‘load’ terms, the source of chlorides can be either de-icing salts or seawater. To reach the bridge, chloride transport mechanisms are required. In general climatic terms, this involves a combination of water and wind. In local exposure terms, water in the form of vapour, spray, driven rain, run off or ponding can interact with the outer surfaces. The effect of this interaction can be exacerbated by the influence of temperature, causing joints to open or cracks to form. Designers therefore need to carefully consider the location of the bridge, what local conditions can form, and how these interact with the outer surfaces. A prime concern is to minimise the uptake of water, and to get rid of any water that does reach the bridge as quickly as possible. This involves a combination of conceptual design, structural detailing and attention to bridge ‘finishings’, such as drainage, waterproofing and surfacing.

In extreme situations, there may be a case for controlling local conditions with external barriers. There is certainly a case for looking carefully at both the profile and texture of the outer surfaces. Movement, particularly longitudinal, should be considered. Continuous or integral bridges can prevent moisture reaching sensitive areas such as anchorage zones. If an articulation system is used, then joints have to be carefully designed and detailed, with provision made to quickly remove water that which will inevitably leak through.

Durable post-tensioned concrete bridges

While estimates can be made of the likely climatic conditions, and the effects of temperature assessed in terms of stress and deformation, effective design and detailing are largely qualitative, based on experience and feedback. A further essential element, at the conceptual stage, is to make positive plans for inspection, maintenance and the replacement of elements with a short service life.

3.2.2 Bridge deck waterproofing systems The waterproofing system is the first line of defence against ingress of road salts applied from the bridge road surface. Unfortunately there are no systems available that can be guaranteed to remain waterproof throughout the life of a bridge. It is understood that the range of modem high-quality liquid-applied membranes is likely to be more effective than earlier systems. These membranes can be applied in either one or ‘two coats; however applied, they should be proved using ‘pin-hole’ detection equipment which will give reasonable assurance of the integrity of the membrane. Careful preparation of the concrete surface and application of the membrane are important, and checks should be carried out for adhesion and thickness. Current standards are given in BD 47/99 and BA 47/99 (39). The Highways Agency’s Specificationfor Highway Works (37) requires all proprietary materials for waterproofing systems to have a current British Board of Agrkment (BBA) Road and Bridges Certificate and for the permitted waterproofing system (PWS) to be registered.

3.2.3 Coatings The use of surface treatments on concrete can provide a protective barrier against aggressive agents. Detailed guidance is given in Concrete Society Technical Report 50, Guide to su@ace treatmentsfor protection and enhancement of concrete (40). In selecting a surface treatment, whole-life performance should be taken into account as the costs of application, maintenance, expected life and possible reapplication can be significant.

Surface coatings There are many surface coating materials available including polymer-modified cementitious coatings, synthetic rubbers and bituminous materials.

Pore-lining penetrants Pore-lining penetrants are low-viscosity materials that impregnate the pore structure of the concrete and interact, sometimes chemically, with the internal concrete surfaces. They confer water-repellency to concrete. As the pores and capillaries within the concrete remain open they do not act as effective barriers against the diffusion of gases (e.g. oxygen and carbon dioxide) or the transmission of water vapour.

Pore-blocking sealants Pore-blocking systems consist of materials that either react with concrete to form pore-blocking products or physically block the pores without reacting with concrete. These materials do not prevent water penetration and chemical

12

attack but the rate at which they occur is reduced. They do not provide an effective barrier against very aggressive salt solutions. They may sometimes be used in combination with inorganic coatings but this should be checked with the suppliers of both materials. Non-reactive pore-blocking materials rely on sufficient solids being camed into the concrete to effectively block the pores and capillaries. Depending upon the porosity of the concrete and the number of applications that are acceptable, a balance is required between the viscosity of the treatment and its related solids content. Solvented systems usually contain enough solids for a two- or three-coat application to be used to seal average quality concrete. Some low-solids waterborne products may also be used as sealers. The solids are dispersed as fine particles rather than in solution and the effectiveness of even very low viscosity products may be limited if the particles are large in relation to the pore size. In-surface sealing can be achieved with solvent-free systems. However, even with the lowest viscosities available, the depth of penetration is likely to be very limited unless a vacuum-assisted application technique is used.

3.2.4 Drainage It is essential that the drainage system should work efficiently to remove water from the road surface as well as the water that passes through the surfacing down to the bridge deck waterproofing system. The details of the drainage paths should be such that if items of the equipment fail, leak or become blocked, then the water does not find access to the prestressing system. BD 57/01 and BA 57/01 (31) give advice on drainage systems, and on dealing with the passage of water at boundaries and at supports.

3.3 ONDOVBBUAL SURUCTUBBAL ELEMENTS 3.3.1 General Many of the comments in Section 3.2 apply equally to individual structural elements in terms of concrete profiling, texture, and articulation. However, additional features arise from the structural design of the elements themselves, as contained in Codes and other authoritative guidance documents. Mostly, these relate to stress levels, and the control of cracking, both at early ages and in service due to the influence of loads, creep and temperature. In material specification terms, there is also the basic protective layer of an adequate cover to the steel in a good-quality concrete.

3.3.2 Concrete quality and cover BS 5400: Part 4 (35) gives recommendations for minimum concrete strength and cover for post-tensioned concrete bridges, and the Specificationfor Highway Works (37)gives a concrete specification which, together with the specified cover and good-quality construction, will give a reasonably dense, impermeable concrete protection to the ducts. This guidance is augmented by BD 57/01 and BA 57/01 (31),

Available protective measures

which include requirements for increasing cover by. 10 mm. In normal circumstances there is no reason to believe that concrete designed and constructed in accordance with current standards and specifications does not provide adequate protection to the tendon. However, feedback from service (41) has demonstrated that the specified cover is not always achieved in practice; good quality control is essential. As with all concrete structures, it is possible in special circumstances to improve the concrete protection by increasing the cover or reducing the permeability of the concrete (42). However, increasing cover often requires increased section thickness and increased prestress adding overall weight and cost to the structure. Reducing the permeability of the concrete is possible by reducing the waterkement ratio or by cement replacement with pulverised-fuel ash (pfa) or ground granulated blastfurnace slag (ggbs). Cement replacement can also have other benefits such as reducing the heat of hydration (and consequent cracking) and improving the workability and finish - although some concerns have been reported in Belgium and France about the interaction of high slag cements and high tensile steel. However, the Working Party is not aware of any specific cases of such problems.

Strand can be made with a built-in protection layer. This may be a physical layer such as galvanising or epoxy coating or an additional means of inspection such as ‘intelligent strand’: in this a fibre-optic sensor is passed through the centre wire of the strand and can be used to monitor strains and breakages in the strand. The effectiveness of these special inspection and monitoring facilities needs to be carefully considered. Of particular concern in the past has been the susceptibility of some types of strand to hydrogen embrittlement. It is understood that strand manufactured to BS 5896: 1980 (431, prEN 10138 (4) or ASTM A416-99 (45) is not likely to be subject to hydrogen embrittlement, and any of these strands may be used (46). There have also been concerns about the effectiveness of epoxy coating particularly for reinforcement. Any small defect in the coating can increaie the likelihood of local corrosion and the use of epoxy-coated reinforcement in the USA is being reviewed. It is not clear whether the problems associated with epoxy-coated bars also apply to epoxy-coated strand.

3.4 PWESBRESSUMG QOMPPONENBS

‘Intelligent strand’ may prove useful and is currently being tried on a bridge in the UK. However, as it is still being investigated, it cannot be recommended yet for widespread use until satisfactory results are confirmed.

3.4.1 Introduction

3.4.3 Ducts

In simple terms, the essential elements of prestressing components are:

During the development of post-tensioning systems, several methods have been used to form ducts. For straight tendons formers have sometimes been used which are subsequently removed leaving unlined ducts. Lined ducts have normally been formed using welded or spirally wound steel tube although cardboard tube has occasionally been used (4). Unlined and biodegradable ducts such as cardboard tubes should not be used. The advantages of spirally wound tubes are that they are flexible and can be bent on site to the required profile.

0

prestressing tendons

0

ducts, containing the tendons

0

anchorage system

0

protective system overall.

It is possible to consider the tendons and ducts in a general way. Ducts will differ depending on whether bonded or unbonded construction is used. The primary protective system for bonded construction is the cement-based grout. For external unbonded construction, a wider range of protective systems is available; the anchorage methods may also be different, and there is the further need to consider special features such as deviators and couplers.

More recently, non-metallic ducts have been used. These are made of high-density polyethylene (HDPE), now known as PESO, or polypropylene, and have a number of advantages: 0

corrosion-resistance

0

better sealing against ingress of contaminants

0

3.4.2 Prestressing tendons ’

Prestressing tendons have developed significantly since the 1950s with major improvements in technology and increasing ability to provide larger, more concentrated forces, while adjusting to the variety of construction methods that have been introduced. To some extent, the type of tendon still relates to the individual prestressing system, but the designer can rely on the characteristics and mechanical properties specified in national and international Standards: this is fundamental to structural design, and not especially the concern of this Report. A number of steel strand types are available; these normally consist of seven wires. The diameter of the strand, its compactness, strength and metallurgical properties may all vary.

0

can be pressure-tested during construction to demonstrate integrity more potential to be ‘seen’ through by some nondestructive testing techniques.

If the duct is to be used as one of the protective layers in the system, it should not itself be subject to corrosion, which would make the protection ineffective. The main advantage of non-metallic ducts is that they can form a sealed system around the tendon and minimise the risk of contaminants reaching the tendon. The advantage of being able to penetrate non-metallic ducts with testing techniques remains to be proved. The methods currently available have limitations and only provide partial information about conditions within the duct. However, with further development, more detailed detection of voids and corrosion may be possible.

13

Durable post-tensioned concrete bridges

The revised grouting specification in Chapter 12 requires the use of corrosion-resistant ducts which for internal tendons are bonded to the surrounding concrete. The ducts should be pressure-tested before concreting to verify the assembly. Walls thicker than a specified minimum are used to allow for the effect of the stressed tendon ‘biting’ into the duct wall.

a gap around the duct. This has not been observed in grouting trials, presumably because the early gain in strength of the concrete can partly restrain the expansion of the duct: the effects have been shown to be minimal and this is thought to be purely a theoretical problem. This observation has been confirmed elsewhere (47).

The use of non-metallic ducts requires a re-assessment of some of the properties of ducts and their effects on the whole protection system. The main properties of the materials currently used for non-metallic ducts may be compared with those for steel ducts. Non-metallic ducts:

It is also true that, under grouting pressure, the duct would expand again into any gap. Full-scale trials have shown the importance of the surrounding concrete in providing restraint to plastic ducts, especially in maintaining the integrity of the joints between duct lengths; this integrity has, of course, to be maintained while concrete is being placed, so the ducts themselves and their support system should be robust during casting.

do not corrode effectively resist the passage of chloride ions do not conduct electricity have a high coefficient of thermal expansion (typically 140 x 1 V P C ) have a low Young’s modulus (typically 800 N/mm2). Although there are few examples of ducts corroding from the outside the fact that a non-metallic duct material cannot corrode is obviously an advantage. Ducts that corrode do not provide a physical barrier between any contaminants and the tendon. Ingress of chloride ions through the duct material is theoretically possible, but is not likely given the thickness of the material. It is interesting to note that HDPE is used as the outer skin of marine electricity cables, which have to be protected against chloride ingress. Another issue to be considered is the risk of stray current corrosion. Overhead alternating-current power systems can induce voltages and currents in nearby metal objects, and, if not controlled, can generate harmful potentials on exposed conductors. Third-rail direct-current power systems can leak currents into the ground, which attempt to return to the power supply via the lowest impedance route; if not controlled, the resulting stray currents can cause accelerated corrosion of prestressing tendons at the point where the current exits the tendons. Adjacent to AC power lines, conductive paths should not exceed 500 m - well beyond the longest post-tensioning system tendon. Normally the principal control measure is to ensure that the tendons and anchorages are electrically isolated. This needs particular care and is probably only totally reliable with plastic encapsulation of anchorages. The use of plastic ducts, together with a nominal concrete cover of 45 mm round all metal parts of the anchorages may be sufficient to achieve this. If it is necessary to expose metal p&ts at either or both ends of the tendons, then the tendon system should be earthed at one end only. The coefficient of thermal expansion of steel and concrete are similar and consequently changes in temperature do not cause significant relative strains. However, this is not true for nonmetallic ducts, which have a high coefficient of thermal expansion. Concerns have been raised that an increase in temperature, say during hydration of the concrete, could cause a non-metallic duct to expand more than the concrete. A situation may arise where the duct expands while the concrete is plastic and contracts after the concrete has hardened, leaving

14

One claimed advantage for non-metallic ducts is their ability to form a sealed system to the tendon that will exclude contaminants from the duct. While it would be possible to design such a duct system, currently available ducts cannot be assumed to be guaranteed sealed and fully watertight. The pressure-test acceptance criteria in the Specification are related to what is possible using the currently available nonmetallic ducts and demonstrate that the duct has been properly assembled. Tests by the Working Party show that the acceptance criteria specified will not guarantee a full barrier to the ingress of contaminants at the joints in the duct. Difficulties were experienced with joints between lengths of duct, and with seals between the ducts and anchorages, and also with venting tubes. It is for this reason that multi-layer protection is required. It is hoped that fully sealed ducting systems will be developed in due course, in which case acceptance criteria would be much tighter than currently specified: on the other hand, less dependence on multi-layer protection would then be required. The development of prestressing systems based on nonmetallic ducts is still at a comparatively early stage, especially for bonded construction. Much work has been done on the properties of the basic materials, but the design and manufacture of plastic ducts varies between suppliers. Based on all available information, the fib Technical Report Corrugated plastic ducts for internal bonded post-tensioning (17) distinguishes between ducts for external unbonded construction and bonded construction, but the emphasis is on defining performance requirements for bonded tendons and on test methods to ensure these are met. This is the basis of this Report and for a systems approval approach, which should become the norm in the future. The duct should be large enough both to allow threading of the prestress tendon and to facilitate grouting. The diameter depends on the size of the tendon, and the overall length and curvature of the duct. Normally a maximum tendon to duct area ratio of 0.40 to 0.45 should be used, increasing with tendon size. The tendon to duct area ratio is defined as the area of the strand, based on its actual steel area, divided by the internal cross-sectional area of the duct. For short tendons with little change in direction, the tendon to duct area ratio can be increased provided that grouting trials show that the duct can be satisfactorily grouted.

Available protective measures

3.4.4 Anchorage location

unbonded systems. In both cases, the Working Party considered the subject to be so important that illustrations are given of preferred solutions.

Feedback from inspections clearly demonstrates that the ingress of contaminants is most common at anchorages, with corrosion being initiated in the tendon immediately behind the anchorage (often in the presence of imperfect grouting). It is very important, therefore, that attention is given to the location of anchorages, and to detailing to prevent the access of water to the ducts. Anchorage systems vary with tendon type and supplier. The guidance in this chapter relates to anchorages for internal grouted tendons and external

The layout of prestress and location of anchorages are dependent on the method of construction. In the case of a simply supported beam cast in situ the anchorages would normally be at the ends of the beam (Figures la), but there are many forms of construction that require anchorages at different locations. These may be broadly sub-divided as follows:

The deck overhang must be free from construction joints as they could leak, allowing water to reach the anchorage

The deck overhang must be free from construction joints as they could leak, allowing water to reach the anchorage

\

- - -

\

J

I

I

1

I I

t

waterproofing

1-

Min. ROO mn

Min. 1500 mm preferably 2 1800 mm

Min. 800 mm

Min. 1500 mm preferably 5 1800 mm

I

v

\

\

\Abutment drainage channel

Abutment drainage channel

Figure Buried anchorage at end of deck with an abutment gallery.

Figure 3: Exposed external anchorage at end of deck with an abutment gallery.

The deck overhang must be free from construction joints as they could leak, allowing water to reach the anchorage

The deck overhang must be free from construction joints as they could leak, allowing water to reach the anchorage

,Expansion

joint

\

waterproofing Liquid applied membrane

JJ] Min. 800 mm 4

.

Min. 1500 mm preferably 5 1800 mm

'-

I

Space sufficient to attach jack

Min. 1500 mm preferably 3 1800 mm

+ \

'Abutment drainage channel Abutment drainage channel

Figure 2: Exposed anchorage at end of deck with an abutment gallery.

Figure 4: Restressable external anchorage at end of deck with an abutment gallery.

Durable post-tensioned concrete bridges

Liquid applied waterproofing

Construction joint

I

Min.500mm

Note: Anchorage end cap filled with grout during tendon grouting operation L

.

’

.

.

‘-.

\ waterproofing membrane

Drip to prevent seepage through construction joint from reaching anchors

\ Anchorage end cap Grout injection holehent in anchorage

Figure 5: Anchorage at top blister using exposed anchor.

Liquid applied waterproofing membrane

Liquid applied waterproofing membrane (Note: Membrane may be omitted if not in a vulnerable position)

Construction joint

Anchorage end cap

Min 500mm Concrete cast after stressing and grouting

/

“.I 1

Figure 7: Anchorage at bottom blister using buried anchor (internal tendon).

I

Figure 6: External blister and bonded face anchorages for in situ segmental construction

,

dead-end anchorages within the body of the concrete anchorages in blisters within the span, either inside a box girder or below the slab in a beam and slab deck (see Figures 5-7) anchorages in pockets in the top surface of the deck (see Figure 8, Chapter 4, page 18.) face anchorages on the joint between segments in spanby-span, in situ or precast segmental construction. Wherever anchorages are located the designer should ensure that an adequate multi-layer protection system is provided. There has been much discussion within the Working Party and elsewhere on the merits of anchorages in pockets in the top surface of the deck. There have been examples of severe

corrosion of tendons with top pocket anchorages but none have occurred, to the knowledge of the Working Party, where the tendon has been properly grouted. It is not clear whether or not top pockets constitute an additional level of risk. However, there is a strong feeling amongst many engineers that the top pocket provides an unnecessarily convenient route for contaminants into the anchorage and tendon. There are few situations in which an alternative to the use of top pockets cannot be found, and it is recommended that they should not normally be used. If there is no alternative (for example in some bridge refurbishment schemes) then the design and construction should ensure that contaminants are excluded both during construction and in service by taking additional protective measures (see Figure 8). The location and detailing of anchorages depend on whether the anchor head is left exposed. One of the ‘preliminary design recommendations’ in Reference 48 is that, “All anchorages, apart from dead-end anchorages deep within a concrete mass, be designed so that they are inspectable”. Exposed anchorages go some way to allaying fears of lack of inspectability of prestressing systems, but they do increase the risk of corrosion by reducing the number of layers of protection. The end cap of the anchorage has to be removable to inspect the ends of the strand and, once removed, it may be difficult to replace and ensure an airtight seal. The

Available protective measures

advantages of burying the anchorage in a concreted pocket are that the end cap can be filled with grout rather than grease and is itself surrounded in concrete. Inspectable or buried anchorages are equally acceptable and the designer should choose the type most appropriate to the location under consideration. Dead-end anchorages deep within the concrete are also acceptable but have the practical disadvantage of requiring the tendon to be in place before concreting. At expansion joints, BD 57/01 and BA 57/01 (31) recommend the use of an abutment gallery for inspections of the joint and the concrete below the joint. This gallery can also be used to inspect exposed anchorages and the concrete and waterproofing covering buried anchorages (see Figures 1 4 ) . Anchorages that are required to be inspectable should also be replaceable. Wherever possible, BD 57/01 and BA 57/01 (31) recommend the use of integral bridges, avoiding the use of expansion joints. The Working Party sub-group has not been able to agree on the most appropriate measures to protect anchorages at the ends of the deck. One possibility is that an abutment gallery is provided, similar to that required at expansion joints, so that the end of the deck can be inspected. It may also be possible to have a smaller gallery for inspection using a TV camera, although such a gallery would not facilitate maintenance should this be required. Alternatively, it may be possible to dispense with visual inspection by placing a corrosion probe adjacent to the anchorage and using a buried anchor detail. It is anticipated that, with greater use of integral bridges, new details will be forthcoming.

3.4.5 Anchorage details

Figures 9-17, in Chapters 4, 5 and 6, give typical anchorage details for a number of situations. In developing the details the following points have been taken into account: (a) The anchorage should have multi-layer protection. No individual layer can be assumed to be effective for the life of the structure. (b) The detailing of expansion joints should be such that, when the joint leaks, the water is directed away from the anchorages and into a properly detailed drainage system. (c) The anchorage end cap should be filled with grout during grouting of the tendon. The end cap is shown bolted to the anchor plate in the figures, but in some systems the cap is bolted to the anchor head. Provided that the anchor plate and head are machined for a close fit, this is an acceptable detail. (d) Any concrete used to infill pockets should have flowable non-shrink characteristics in accordance with BD 27/86 (49), and should be held in by reinforcement. (e) The anchorage or capping concrete should be covered with the bridge waterproofing membrane. (f) Exposed anchorages should be protected from seepage

through construction joints in thin slabs by drips. It should be noted that deck overhangs without construction joints necessitate special devices to enable stressing jacks to access the anchorages, Normally a crane with a special lifting device or a jacking trolley would be required. For very large tendons this can be prohibitive and consideration could be given to use of a construction joint with a waterbar and sealant in a surface groove.

,

.

.

17

4 GROUTEDBONDED -POST-TENSIONED CONSTRUCTION

4.1

INTRODUCTION

The overall design strategy recommended in this Report is that of multi-layer protection. Available protective measures are reviewed in Chapter 3 and in this chapter the focus is on grouted bonded construction. It begins with a review of grouts and grouting, and is followed by details of a recommended protection system for this form of construction.

4.2

GROUTS AND GROUTING

One of the main advantages of a properly grouted duct is the alkaline environment created within the duct. A major concern with existing bridges has been the quality of the grouting and the protection provided to the tendons. The grouting operation has often been undertaken by inadequately trained personnel who have not understood its importance. There are even instances of ducts being left completely ungrouted (50). The grouting specification in Chapter 12 gives details of improved grouting methods, vent layout and materials. Site operations should be carried out by companies and operatives satisfying the CARES certification scheme - see Chapter 11. The use of plasticisers in grouts is common practice, and the resulting improved workability and reduced waterkement ratios can only be beneficial. There has been discussion of the safety of using expanding agents in grout. The expansion is normally achieved by incorporating aluminium particles and it has been suggested that the hydrogen given off can cause embrittlement of the prestressing steel. The Working Party has not been able to find any evidence to support this suggestion, but it has been found that aluminium particles

I(

Min. 500 mm

can generate air bubbles. To meet the requirements of the revised Specification it is likely that plasticisers and expanding agents will be required. The measures proposed, together with the certification scheme, are a substantial improvement on previous grouting operations and give assurance that the ducts are adequately filled with grout. They cannot guarantee complete filling of all ducts and some small voids may still occur. It must, however, be remembered that voids themselves do not cause corrosion of the tendons. One of the main concerns with bonded tendons and anchorages is the difficulty of inspection. It may be possible to partially overcome this difficulty by installing sensors or probes that allow remote monitoring of the potential for corrosion at the probe location. This may be particularly useful at anchorages for integral bridges, which may be difficult to inspect. The Working Party has not adequately investigated the usefulness and reliability of such probes and cannot yet recommend their use. However, it is anticipated that with some development they may be used as an additional assurance, if required, by warning of potential corrosion.

4.3

VENTS AND GROUT INJECTION

Trials undertaken on behalf of the Transport Research Laboratory (51. 52) showed that an additional grout vent just beyond the crest in the tendon profile, in the direction of grouting, can be beneficial as that area is particularly prone to trapping air. Any air or bleed water camed past the vent at the crest during grouting should migrate back up the duct and

Liquid applied waterproofing membrane, double thickness required over pocket. (Note: Second complete layer covers parch first layer)

*I

J

construction einforcement lapped with arters from main member

Pocket filled with non-shrink flowabl capping concrete, to ED 27/06 item 4

Drain pipe or other detail to ensure pocket kept free from water and contaminants during construction

Figure 8: l o p pocket anchorage. (This is not recommended, unless external protective layers are used.)

18

Grouted bonded uost-tensioned construction

Sealant at construction

Note: Anchorage end cap filled with grout during tendon grouting operation. In some systems the end cap is bolted to the anchor

1 Liquid applied waterproofing membrane end cap vent

. Anchorage end cap cover 50 mm Grout injection holelvent in anchorage

I '

II

Pocket filled with capping concrete, to 0D 27/06 item 4 Reinforcementlmesh lapped with starters from main member

Note: Anchorage end cap filled with grout during tendon grouting operation. In some systems the end.cap Min. 500 mm is bolted to the anchor head

Figure 10: Exposed anchorage for stressed or dead end.

Figure 9: Buried anchorage for stressed or dead end.

be removed at this additional vent. The additional vent should be closed before the crest vent. The trials also showed that larger vents are desirable. The intention of raising vents, where possible, at least 500 mm above the duct profile, is to provide an effective head of grout. The specification gives guidance on spacing, location and size of duct vents based on these trials. If the direction of grouting is not known at the time of casting, additional vents will be required on both sides of the crest vent. All anchorages are supplied with a tapped hole for use either for injecting grout or as a vent. The end cap to the anchorage is also supplied with an air vent and the cap is to be filled with grout during grouting of the cable. For grout injection, the anchorage hole must be positioned so that grout is injected from the bottom. At the far end of the cable the anchorage hole should be positioned at the top to act as a vent during grouting. The final vent is the tubed vent on the end cap, which must always be placed at the highest point. Anchorages with two holes avoid the possibility of the grout hole being placed incorrectly. In the design of the vents it must be remembered that the vent is part of the protective system and needs to be sealed to the same level of air-tightness as the duct. This requires an engineered connection between the vent and the duct and a sealable stopper where the vent exits the concrete. If this is the top surface of the deck and the vent is extended above deck level for grouting purposes, the extension will have to be removed before stopping up the duct. Vents should be sealed on completion of grouting and also on removal of any extension tubes. An example of such a detail is given in Figure 12. Care is necessary here to ensure that the former for the pocket is held in position, and to avoid the top reinforcement in the deck.

4.4

RECOMMENDED PROTECTION SYSTEMS

4.4.1 General In proposing the multi-layer protection concept, an outline has been given in general terms of all the factors that could come into the design equation, in consciously designing for durability. This section is focussed on protection of the prestressing system for grouted post-tensioned construction. In making the recommendations below, the Working Party has concentrated on the minimum protective measures required for a typical road bridge in the UK. The environmental conditions may be less onerous for rail bridges, in the absence of de-icing salts, and for bridges in other countries .where the environment is less aggressive. To set the scene, the standards and practices outlined herein are seen as a compatible package of design, material and construction measures for UK applications. For the prestressing system itself, the basis of the core recommendations is one of quality, linking the Specification to the supporting certifcation scheme and underpinned by the design recommendations. All of this is summarised in the sub-sections which follow, together with the review of available test methods in Chapter 8, and including the development work undertaken on tests for checking the sealing of duct systems and for detecting and measuring voids in fluid grout. These have shown great potential for the future, but at present, are seen as potential additional protective measures, in support of the core requirements.

4.4.2 Prestressing system (a) Anchorage and vent locations and detailing should follow the logic outlined in Figures 8 to 12.

19

Durable post-tensioned concrete bridges

Reinforcement from 1st segment projects into 2nd segment

Liquid applied waterproofing

Note: Anchorage end cap filled with grout during tendon groutingoperation

Ld

--.

Anchorage end cap vent

I Grout injection holehent in anchorage

(a) Grout vent cast into concrete with a recess at deck level

Anchorage end cap surrounded in in situ concrete from 2nd segment

C

._ 1

2

c U)

8 1st segment

2nd segment (b) Vent extension pipe and valve fitted during grouting. Note: Extension pipe must be rigid if used with grout stiffness test

(a) Elevation

Note: Anchorage end cap filled with grout during tendon grouting operation

Liquid applied waterproofing membrane, double thickness over vent

\

Anchorage end cap

Ld (c) Vent capped and recess filled with non-shrink mortar Grout injection hotelvent and anchorage end cap vent

(b) Plan

Figure 12: Grout vent details at deck surface.

Figure 11: Face anchor details in in situ segmental construction.

(b) Ducts and vents should be in accordance with the specification, corrosion-resistant and pressure-testable. The ducts and vents should be pressure-tested.

(b) Specification should be in accordance with the Specification f o r Highway Works (37) as amended by the Specification in this Report.

(c) Full-scale representative grouting trials should be used to prove the grouting method, materials and personnel, where there is no previous history. (d) Method statements should be prepared in advance for all prestressing operations and should be approved by an appropriately 'experienced chartered engineer. (e) All operations associated with the installation, stressing and grouting of tendons should be undertaken under the certification scheme.

(c) A waterproofing system complying with BD 47/99 (39), and having a current Road and Bridges Certificate issued by the Highways Agency, should be used on the deck surface and in the other locations recommended in BD 57/01 (31). It should be checked for integrity using appropriate non-destructive test equipment, including pin-hole detection equipment for liquid-applied waterproofing systems. Where a double thickness is used, the first layer should be proved before the second layer is applied. In addition, the membrane should be used to protect any anchorages left exposed in abutment galleries, inside box girders and on bridge deck soffits.

4.4.3 The deck and its elements (a) Concrete strength and cover requirements should be in accordance with BS 5400: Part 4 (35336) modified as required by BD 57/01 (31) where appropriate.

20

(d) The expansion joints and drainage system should be detailed to ensure that, in the event of equipment failure

Grouted bonded post-tensioned construction

or leakage, water cannot find access to the prestressing system.

protective measures from the list below could be considered. Each structure will have to be considered on its own'merits.'

(e) Strand should be in accordance with BS 5896: 1980 (43) or similar.

increased cover

4.4.4 Possible additional measures for exceptional structures

perfectly sealed ducts

For exceptional structures in unusually aggressive environments (for example, bridges over the open sea), additional

I

reduced concrete permeability corrosion-monitoringdevices special strands.

-

,

.

21

5 EXTERNAL UNBONDED 1 POST-TENSIONED CONSTRUCTION

In external unbonded construction ...e prestressing tendons are unbonded and laterally outside the concrete cross-section, and the forces are transferred to the concrete through end anchorages and deviators; this includes such tendons located inside a box section. It follows that the design and detailing of these elements, and the development of protection systems for the prestressing hardware as a whole, are particularly important. The advantages and disadvantages of external unbonded construction, compared with grouted bonded construction, may be summarised as follows.

Advantages reduces self weight placing concrete in the webs is easier permits simpler tendon layouts reduces prestressing losses, especially due to friction facilitates inspection, re-stressing and replacement gives early warning of failure reduces importance of cracking in the concrete in terms of corrosion protection for the tendons can permit more rapid construction, with bigger spans due to reduced weight.

The first example in the UK was the Braidley Road Bridge in Bournemouth designed by Gifford and Partners in 1967. Experience is therefore relatively recent, and limited, in the UK. Attitudes to the method have varied. After Braidley Road, there were few examples until the 1990s. It is looked on more favourably, as the practical difficulties of inspecting grouted bonded construction have become clear. The ability to inspect, and, if necessary, replace external unbonded tendons, is perceived as a major advantage, and it is the only method currently allowed in the UK for precast segmental construction. In durability terms, the track record of external unbonded construction is good. There have been instances of corrosion, but no collapses; the ease of inspection and remedial action has been a plus - indeed, external prestressing has been used in strengthening a number of existing bridges, such as Kingston Bridge, Glasgow (53) and Medway Bridge in Kent (54). Reviews have been conducted on performance in service and examples of distress have occurred, frequently due to lack of appreciation of the local forces and strains induced by the prestressing, or to neglecting temperature effects.

the structural role of anchorages and deviators is more critical

Over the years, the technology of prestressing has developed significantly in response to new construction methods and the demand for bigger spans. Cable-stayed bridges are an example, and the protection systems developed for them have been of benefit to external unbonded construction generally. Many such developments relate to the evolution of particular prestressing systems, and generalisation in this Report is rather difficult. However, some basic principles are set down in this Chapter, and references given to cover detailed aspects.

the safety and security of the prime structural members is more at risk, for example, in relation to vandalism

5.3

Disadvantages the eccentricity of prestress is generally less the tendons do not necessarily reach their ultimate strength at failure of the structure, i.e. the structure is over-reinforced

can be more expensive. In general, the advantages become more significant for bigger spans (greater than 40 m) or for long viaducts.

5.2

BACKGROUND

The use of external unbonded tendons is not new and their origin can be traced back to the work of Dischinger in Germany in 1928. The Magnel prestressing system, developed in Belgium in the 1940s, used unbonded construction, and Freyssinet progressed the technique further in the 1950s. French

22

and German engineers were responsible for developing the technology in other countries, particularly in North America, using in-situ and precast segmental methods, and there is now a flourishing American Segmental Bridge Institute.

STRUCTURAL DESIGN AND BASIC PERFORMANCE REQUIREMENTS

The structural performance of external unbonded construction is well understood, and design methods are given in relevant codes BS 5400 (35) and ENV 1992-2 (55). In applying these in the UK, additional specific requirements are contained in Highways Agency Standards and Advice Notes: BD 57/01 and BA 57/01 (31) are generally relevant in durability terms, but BD 58/94 (19) and BD 55/94 (56) focus on external unbonded prestressing. Going beyond basic design, BA 58/94 (19) gives a good deal of information at the detailing level, in terms of

External unbonded uost-tensioned construction

loads to be carried by anchorages and deviators 0

coefficients of friction

0

radii of curvature of tendons protection systems.

Clearly, these are key documents. In addition, they contain two important requirements, which can have a strong influence on the prestressing system adopted, and on the level or prestress needed. These requirements are as follows: 0

The prestressing and protection system should permit easy inspection, and tendons should be capable of being re-stressed or replaced, if necessary. The failure of either two tendons or 25% of the tendons at one section should not lead to collapse.

Some recent UK practice with external tendons has involved locating intermediate blocks or bonding points in the span, to ensure some increase in strain in the tendons under the design ultimate loads. The choice of re-stressable and replaceable tendons or only replaceable tendons has a significant impact on the design, and whether provision is made to detension by jacks and whether the consequent projecting lengths of strand are provided at permanent anchorages. The decision should be taken with reference to the number of tendons, the spans and the consequential costs.

5.4

AVAILABLE PROTECTIVE MEASURES

Details of protective measures available internationally are given in the 1996 FIP state-of-the-art report Materials and systems for external pre-stressing (57). Most experience in the UK has been with the use of grease, wax or cement grout contained in PE tubes. It is recommended that only plastic ducts should be used, which should be of polyethylene material, at least of strength class PE80. Plastic ducts for external systems are relatively thick and smooth (in comparison to those for bonded construction which are corrugated). The systems now available will no doubt continue to evolve and improve over time. Their success will depend not only on the system hardware and on the chosen protective material, but also on detailing, and on ensuring that the protective barriers are properly sealed and continuous, and that the protective materials can be placed effectively under site conditions. This is the main focus of the general guidance that follows, which does not concentrate on any particular system.

5.5

from the tendons should be transferred to the structure in a controlled manner. Duct layout is influenced by the construction method and the form of the structure. Simply supported structures (or determinate bridges made continuous for durability reasons) require different layouts and patterns to cantilever methods or span-by-span construction. Doubly inclined profiles, up to 200 m or more, may be required, or short straight lengths horizontal or inclined - dictated by the construction method, while still ensuring that the final stress conditions are satisfactory at all sections, under service conditions. Ducts should be tested for watertightness. They should also be located well clear of water, preferably in a non-aggressive environment. Particularly vulnerable areas are where ducts are connected to the anchorages or pass over deviators: detailing in these areas is especially important. Where it is necessary to join lengths of duct, electrofusion or butt-fusion jointing can normally be used for PE ducts. Again, detailing is important and such joints should preferably be kept clear of construction joints (up to 1 m is suggested), to reduce the risk of penetration. For longer tendons, consideration needs to be given to duct movements during stressing and the provision of a joint with a sleeve coupler that allows the duct to shorten. With external tendons inside a box it is not feasible to extend vents at high points to 500 mm above the duct, as is required for internal tendons, as this would need holes through the slab. However, the ability to check the duct for voids after grouting by impact-echo techniques gives added confidence that heavily outweighs this disadvantage. Use of tendon couplers should be given careful consideration. Cast-in couplers have the security of preventing progressive collapse but unrestrained couplers in the deck void can be vulnerable in this respect. An alternative to the use of couplers is to cast an intermediate diaphragm, and to anchor a tendon on its far side, with a new tendon being started on the other side of the same diaphragm: this also facilitates replacement. It is important to detail for access. At its simplest, this is required for ease of inspection, but access is also needed behind anchorages, for re-stressing or replacement. In these cases, a substantial length of tendon is required behind anchorages to permit the jacks to be re-attached, with an allowance for extension. This length should be specified, since it will depend on whether single strand jacks or cable jacks are required. In service, this extra tendon length is usually protected by wax or grease within a tube, irrespective of the protective material used elsewhere in the duct. A typical arrangement is shown in Figure 13.

DETAILING 5.6 TENDON SYSTEMS

In very simple terms, the objective in detailing external prestressing systems is to get the tendons in place from one anchor to the other, to the correct design profile, and with the protection measures intact, both before and after the stressing operations. The profile is adjusted by suitably placed deviators, and the tendons are essentially straight between deviators, and between anchorages and deviators. The force

Anchorages and deviators are considered together, because detailing solutions for each are inter-related, and are dependent on the type of tendon and the protective system adopted. There are also differences between the technologies offered by the prestressing suppliers. General guidance is available (581, but most detail is contained in manufacturers’ literature.

23

Durable post-tensioned concrete bridges

500 mm PE duct filled,with grout

Figure 13: Exposed anchorage for re-stressing the end of an unbonded external tendon.

P

Note: Anchorage end cap filled with grout during tendon grouting operation. In some systems the end cap is bolted to the anchor head

L

There are three basic tendon systems:

Unbonded tendons protected by a cement grout

*

In this case, the duct is a continuous tube, e.g. of PE. At anchorages it passes through the concrete, usually, but not necessarily, via an outer pipe and trumpet to the anchorage - the so-called double envelope system. This is illustrated in Figure 14. At deviators, it again passes through an outer tube, often with a bell mouth to prevent damage to the plastic. For easy removal,

unbonded tendons protected by a cement grout unbonded tendons protected by a flexible product (e.g. wax) tendons made with sheathed and greased monostrandst within a grouted system.

Length of strand sufficient to,allow re-stressing

.

/

Anchorage end cap

It

Grout injection holehent in anchorage

I

Note: Anchorage end cap filled Min.?,,,,

with petroleum wax or similar

PE duct filled with grout (debonded from guide tube)

Figure 14: Exposed anchorage for the dead end of an unbonded external tendon. The detail is also applicable for the live end where re-stressing is not required.

24

External unbonded post-tensioned construction

in Figure 15. The location of such vents (for either grouts or waxes) should be considered at the design stage, and will depend, in part, on the prestressing and protection systems to be used. Such vents by necessity have to be of smaller diameter than the specification minimum of 15 mm. It may also be necessary to locate drains at low-point deviators, to remove water that may have accumulated in the duct.

it is important that the tendon can move freely in these zones; in this regard, it is important to have a good seal between the anchor head and the duct. Protection of the anchorage itself is also important, and may be done using a long cap to accept the extra tendon length required for re-stressing. These tendons are not normally suited to re-stressing unless they are straight and a shimmed or screwed anchor head is used.

Unbonded tendons protected by a flexible product

5.7

The most common filler material is petroleum wax, which is heated to about 90°C and poured while still liquid. In general, only a single duct is used (no double envelope), and care is necessary to prevent the wax running out of the duct. There has been limited feedback on some waxes being brittle.

DETENSIONINC AND REPLACEMENT OF EXTERNAL TENDONS

Detensioning and replacement of external tendons requires special consideration with particular emphasis on safety. The operation should be camed out only by trained and experienced personnel directed by a competent supervisor, from firms accredited by CARES. Exclusion of personnel from the area immediately adjacent to the detensioning operation as well as the entire tendon length is of paramount importance. ' Investigations before detensioning should include:

A number of instances have been reported of wax products becoming unstable and leaking out, especially at high temperatures, and care should be used when considering these products.

Tendons made with sheathed and greased monostrands 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.

removing anchorage caps to inspect the condition and length of strand protruding tapping the duct to check the adequacy of grouting. The five most common arrangements of external tendons are as follows: I . Tendons are fully grouted within a PE duct and the tendons are cropped.

2. Tendons are grouted within a PE duct and a jacking length is left that will allow detensioning. 3. PE ducts are filled with a flexible filler (wax or grease) and the strands are free to move individually.

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 (19)).

4. Strands are individually greased and sleeved in plastic inside the PE duct. The tendon is normally grouted prior to stressing to prevent trapping of the strands. This tendon can be single strand stressed.

5. Strands are individually greased and sleeved and are exposed inside the bridge deck. The strands are deflected over specially made saddles.

One particular problem is the location of vents at high points, where deviators are positioned. One solution used is illustrated

I ~

Deviator tube Direction of grouting

Figure 15: Top deviator for external tendon.

25

Durable post-tensioned concrete bridges

Each will behave differently during detensioning, and five possible corresponding systems are described below. In all cases, particular attention must be paid to health and safety issues.

Detensioning - System 1 Typically, in this system the tendons are cropped close to the bearing plate. Detensioning has to be done by exposing the strands and severing them, preferably at a point close to one of the anchorages. The adequacy of the grouting must be established before beginning the operation. This can normally be checked simply by tapping the duct. Detensioning this type of tendon requires special consideration. If one of the strands is cut the force in this strand will be transmitted to the others via bond to the grout. For example, in a 19 strand tendon with a force of 60% UTS, after cutting seven strands the force in the remaining 12 uncut strands will be 95% UTS of the strand (ultimate capacity). This situation is potentially dangerous as the strands are approaching their yield point. For safety reasons it is advisable to remove the grout over a substantial length of the tendon and clamp the bare strands together. This procedure was used on 11 tendons on the Mid Bay Bridge in Florida in 2000 ( 6 ) . The following procedure should be adopted: Mark the position on the duct where the tendon will be cut. Carefully remove a section of duct exposing the grouted tendon. Remove the grout by a method that will not damage the strands. Protect the tendon either side of the cut position by a suitable protective screen. Cut the strands remotely, if possible. Use of a flame cutter is preferred, which softens the steel and releases strain before the cut is complete. If a heating method is chosen for detensioning the tendon either side of the exposed area should be screened from the other tendons to prevent heat transmission. If personnel cannot be excluded from the area, provide substantial screens so flying debris cannot cause injury. This system should incorporate a bond breaker inside the anchor guide which allows the tendon to be extracted for replacement. After detensioning the tendon can be cut into short lengths and removed from the bridge. It should be noted that the deviators should follow a circular radius to facilitate removal and very short cropping should be avoided (enough strand should be left to accommodate strand couplers).

Detensioning - System 2 The adequacy of the grouting needs to be checked as for System 1 . The ends of the tendons are not cropped and normal practice is to leave enough strand projecting to accommodate a jack for detensioning. The projecting length is protected by a grease- or wax-filled cap. This cap is removed and the strands and anchorages are cleaned and inspected before detensioning.

26

As the ungrouted length of strand is very short in this system, the stroke capacity of the jack should be sufficient to release the force at the anchorage. It may be necessary to overstress the tendon to release the wedges. Detensioning is therefore a hazardous procedure and it is recommended that this is carried out by a specialist company. Single strand detensioning should not be used from a safety point of view as the force will transfer through the grout into the other strands and lead to failure of the remaining strands at the anchorage. On detensioning at the anchorage the tendon force will be transferred into the grout and will normally crush it. Care is needed to ensure the tendon is free to move along its length. After detensioning the complete tendon it can be cut into short lengths and removed.

Detensioning - Systems 3 and 4 As each strand within the tendon of both these systems is completely free the force can normally be released strand by strand. However, there is a risk of the strands being trapped by others at the deviators (in System 3) so particular care is necessary to measure and monitor the release of extension andor load. Prestressing jacks normally have a useful stroke of 200 mm so this will be the maximum extension that can be released from the tendon in one operation. Should greater release be anticipated the detensioning system should have the capability of reseating the wedges as the tendon has to be re-anchored at an intermediate stage. The tendon may be detensioned by other means. It is always advisable to seek the assistance of a specialist company.

Detensioning - System 5

\

The detensioning will depend on whether or not the strands have been cropped. If they have not been cropped they can be detensioned one at a time with a single strand jack. If the strands have been cropped detensioning can be camed out by cutting the strands one by one after removing the plastic sleeve. The cutting can be done using a disc cutter or a cutting torch and it is recommended that this is only carried out under the control of an experienced supervisor and preferably remotely. Before cutting, a bound timber packing system should be put in place, to prevent the whipping movement of the strand when cut. Alternatively, special devices can be used to clamp the strands one at a time, transfer the load through a parallel bar system prior to cutting and then gradually enable controlled release. This system was used at Braidley Road Bridge, Bournemouth when the strands were replaced in the early 1980s.

Replacement With all replaceable systems it is advisable to replace the ducting as the tendon will have cut into the wall of the duct at the deviation points. The anchorages should be designed so that the tendon can be extracted after detensioning. In the case of a grouted tendon a liner is normally in place to ensure that grout does not bond to the guide tubes. On replacing the duct the tendons can be installed, stressed and grouted or greased as in the original installation.

I

6 SEGMENTAL CONSTRUCTION

6.1

,

GENERAL

Segmental construction is a common form of construction particularly for major spans and viaducts, built by cantilevering or span-by-span methods. For in situ segmental construction, concrete sections are cast against previously constructed sections, and it is possible to have duct continuity through the joint. This is satisfactory as far as the recommendations in this Report are concerned and either grouted bonded or external unbonded methods may be used, provided that the guidance given in this Report is followed.

I-

Liquid applied waterproofing membrane, double thickness required over joint. (Note: Second complete layer covers T t c h first layer)

Min.500 mm *

Epoxy resin sealant

Note: Anchorage end cap filled with grout during tendon grouting operation

Recess vent Anchorage end cap vent

Anchorage end cap

For precast segmental construction, modern methods generally involve match-cast segments with thin epoxy joints. In some countries, dry joints have been used, but these are not recommended for conditions in the UK. One of the principles of the multi-layer protection strategy is that a continuous sealed duct is a key layer, in preventing contaminants reaching the prestressing steel. When the first edition of this Report was published in 1996, the Working Party was unaware of any detail that could guarantee duct continuity, directly or indirectly, and the recommendation was that only external unbonded prestressing should be used for precast segmental construction. Since 1996 the Working Party has sought reliable solutions to this problem. There appear to be three possibilities: The development of proprietary splicing sleeves for the duct at the joints. It is believed that one such solution has been developed, but full details have not yet been launched onto the market. Any such solution would be proven via a Technical Approvals system, which would require the development of a suitable test and acceptance criteria. Pending further experience with this system, it may be prudent to assume that some percentage of the internal tendons is lost (say 5-10%).

Grout injection holehent in anchorage

\

2nd segment

(a) Elevation

Note: Anchorage end cap filled with grour during tendon grouting operation

.

'

'

-

-

Further research on the use of epoxy resins in thin matchcast joints, both in the laboratory and in the field. The Working Party has kept in close contact with the research at the University of Texas in Austin into corrosion protection at segmental joints (20). Precast segmental bridges have been in service in the UK and especially in the USA for up to 30 years. The development of a design-based solution, based on the multi-layer protection strategy, possibly involving a combination of external unbonded and internal bonded tendons (enough, say, to carry all dead loads), and additional protective layers, to give an overall reliability comparable to that of continuous ducts, as part of the system. Current

c1

1st segment

Inlet for grouting recess

Epoxy resin sealant

c Anchorage end cap

,Inlet

in anchorage

and vent for grouting recess

(b) Plan Figure 16: Face anchor details for precast segmental construction. Precast segmental construction using internal grouted tendons is, not recommended, unless continuity of the duct is assured.

27

Durable post-tensioned concrete bridges

Liquid applied waterproofing membrane, double thickness required over joint. (Note: Second complete layer covers patch first layer)

. .

Note: Anchorage end cap filled with grout during tendon grouting operation

Anchorage end cap vent

,

.-

Epoxy resin sealant

practice with this approach is to have a minimum of 75% ., of the tendons as external unbonded. I.

What is being described here is a developing situation. However, the Working Party believes that internal prestressing should be reconsidered for precast segmental construction.

6.2 ANCHORAGE LOCATION AND DETA1LING

3I 1

Giout injection hole/vent in anchorage

Inlet for grouting recess

c

._ 0 ._ c

8 5 c

s

There are some special considerations for segmental construction, which mainly relate to face anchorages at joints, both for in situ and match-cast precast segmental construction. Typical layouts for precast segmental construction are shown in Figures 16 and 17,and for in situ segmental construction in Figure 11. The particular concern is to protect the anchorage, including the cap, from any possible water leakage in the joints. Special care needs to be paid to the following details: 0

2nd segment

1st segment

Sealing of the anchorage recess or box-out, particularly if the distance from the edge of the recess to the edge of the segment is small (this concrete is easily damaged). '

(a) Elevation 0

sealant

Location of inlet/outlet/vent pipes. For precast segmental construction Figures 16(a) and 16(b) require the anchorage pipes to be threaded through the second segment as it is erected. Special attention is required to prevent these pipes becoming restricted during the erection process. Figures 17(a) and 17(b) show a detail that does not require pipes to pass from the first to the second segment.

Anchorage end cap

-

Grout injection holehent and anchorage end cap vent

-

Inlet and vent for grouting recess 1

(b) Plan Figure 17: Combined face anchor and shear key details for precast segmental construction. Precast segmental construction using internal grouted tendons is not recommended, unless continuity of the duct is assured.

.. 28

7 VOID GROUTING

7.1

-_.

OVERVIEW

The discovery of deficie :ies in the groutin 3f x t -tensioned concrete bridges has persuaded bridge managers of the need to consider remedial work. This Chapter provides guidance on where it is appropriate and how it might be done. Void grouting is the injection of grout into voids left in tendon ducts after the original grout has hardened. The term “void grouting” is used in this Report to describe this process to distinguish it from “re-grouting’’which means the re-injection of grout into ducts while the original grout is still fluid. Void grouting has much in common with grouting in new construction - referred to here as “new grouting”. In many respects, therefore, the recommendations for new grouting in this Report may be applied to void grouting. Attention is drawn to differences between new and void grouting in this Chapter and ways are suggested for accommodating them. Void grouting has two objectives -to improve the protection of the tendons in order to extend the life of the structure and to bond them to the structure as originally intended in order to take advantage of the structural superiority of bonded tendons over un-bonded tendons. Void grouting presents more challenges ‘than new grouting. The current view is that the quality achievable in void grouting will therefore often be lower than in new grouting. This does not imply that a lower standard of workmanship should be set. On the contrary, greater care and ingenuity will often be required to adapt grouting methods to the conditions as found. The possibility that a lower quality will be obtained in void grouting should not deter the engineer from recommending it. Even a partially successful void-grouting operation may provide sufficient benefits to justify the works.

To grout all the voids in a bridge is a major task not to be undertaken lightly. Partly for this reason, some engineers take the view that void grouting should be done in exceptional cases only. The view taken in this Report is that voids should be grouted as part of a bridge management plan. Alternative or complementary strategies include monitoring, strengthening and re-inspection at intervals. It is acknowledged that, for reasons of access, void grouting is not practical in all cases and it should not be undertaken if the potential benefits are considered to be insufficient. Many of the issues have been discussed by Tilly (59). Although void grouting is dealt with as a separate topic in this Report, in terms of selecting structures, it should be viewed in the wider context of managing the bridge over its expected lifetime. Management strategies may include:

do nothing at present plan for a re-inspection of the stressing system after a few years monitoring only grout the voids strengthen the structure by other methods combinations of monitoring, void grouting and strengthening. Detailed advice on these matters is outside the scope of this Report. The reliability of void grouting depends on the nature of the voids, the filling that can be achieved and the risk factors influencing ongoing corrosion. The “do nothing” option applies to the least vulnerable bridges where the risk of corrosion is low. Where the risk is higher but void grouting is difficult, “monitoring only” may be the preferred option. In such cases, the rate of deterioration is likely to be important. The main uncertainties affecting the management of these structures are the limited knowledge of the deterioration rates and the structural capacity in the presence of defects. In a structure or member containing relatively few tendons, each one is likely to be structurally crucial. This may not be the case when there are a great many tendons and a degree of redundancy. In addition, void grouting of structures with many tendons may be impractical because of the scale of the work. The inspection may only have covered a small proportion of the tendons and relatively little may be achieved by grouting just these. As the reliability of the void grouting decreases, the justification for additional measures such as monitoring or strengthening increases. However, even a combination of measures is likely to be significantly cheaper than replacing the structure, especially if disruption costs are considered.

7.2

AIMS OF VOID GROUTING

Void grouting can improve the protective environment and structural behaviour of the tendons. Tendon protection is improved because the new grout helps provide a better barrier to water ingress by filling the voids that would otherwise allow water to migrate freely through the structure, possibly bearing chlorides. In addition, providing the grouting material is cementitious, protection will be improved by an increase in the alkalinity of the environment surrounding the tendon and the passivity arising

29

Durable Dost-tensioned concrete bridaes

from this. These are both important reasons for grouting in new construction.

contract. Further inspection will have to take place duct by duct before grouting commences.

Structurally beneficial effects arise from the improved bond between the tendons and the structure. When there is no bond, the flexural capacity of the member is reduced because the force in an unbonded tendon does not reach yield at failure. Shear may also be affected. Providing bond over all or most of the tendon length will also reduce the loss of effective tendon section if a wire fractures by allowing reanchoring to take place.

The continuity between voids in a particular duct is rarely recorded in inspection reports because the inspection strategy adopted in most Special Inspections is invasive inspection at critical points. The presence of grout cover or a grout wash over the tendon is not always formally recorded although it can sometimes be seen in photographs of exposed tendons. Such factors have to be established for assessing the need for and practicality of void grouting, and potential deficiencies in the data have to be addressed when its merits are being considered.

At present, it is not possible to quantify the improvement achieved when partially grouted tendons are grouted. However, it is reasonable to assume that, if successfully accomplished, the new condition will be similar to that achieved in new construction. It should be noted, however, that when tendons re-anchor, transverse forces are created locally in the structure. If several tendons attempt to re-anchor in one region badly affected by corrosion, longitudinal cracking may occur.

7.3

CONDITION OF BRIDGE STOCK AND POTENTIAL DEMAND

A review of data from the programme of Special Inspections of post-tensioned bridges in England (7311) revealed that about 20% of bridges inspected contained significant voids in the ducts. This means that, at the inspection point, the duct contains no grout or a void of significant size such that a substantial part of the tendon is exposed. In about half these bridges (10%of the total) voids of this size occur at at least 5% of the inspection points and in some cases the proportion rises to more than 25%. In a further 10% of the total bridges, the voids are large enough to leave the tendon partially exposed. These figures can be compared with the number of recommendations relating to the grouting of voids in the Special Inspection reports (specifically, the Phase 3 reports, see References 7 and 11). Void grouting is mentioned in about 10% of the reports, and recommended in about half of these (5% of the total). Of the remaining 5%, most reports give a qualified recommendation for void grouting subject to further assessment or investigation, or the dissemination of advice on the practice.

7.4

INSPECTION RECORDS

Records of inspections carried out in accordance with BA 50193 (7,11) provide a good indication of the condition of a bridge as found at sample locations. They are the starting point for assessing the need for void grouting, although supplementary information from further inspections may be needed, before embarking on void grouting. Existing inspection records may show that void grouting is not needed or characterise the bridge sufficiently to suggest there is a need. However, it may be necessary to remedy deficiencies in the inspection data before making a firm proposal for void grouting or specifying and planning a void-grouting

30

Where the volume of the void has been measured, it is sometimes clear that whole ducts are completely un-grouted. In other cases, where the void volume is only a proportion of the duct volume, the situation along the duct is not clear. Void grouting is more difficult where duct continuity is interrupted by fully grouted sections, or compromised by sections containing small voids that inhibit grout flow. Information like this has not normally been obtained in inspections or recorded in the reports.

a.5

GROUTING MATERIALS

It is recommended that the same materials are used for void grouting as are currently used for new grouting - namely cementitious special grouts with properties as defined in this Report. This type of material has been used in recent voidgrouting contracts, but it has been shown that blockages are likely when these grouts are required to pass though narrow passages. Flow is reduced and a grout plug may form in the void as the pressure rises. To avoid this, the minimum suitable void cross-section should be established. When voids with small cross-sections are to be grouted, there may be an inclination to seek grouts with suitable flow characteristics, such as low viscosity, even if other properties are compromised slightly. The current recommendations for viscosity using the flow cone test are considered valid for void grouting except when narrow passages present particular difficulties. Particle size is not thought to be a critical influence on the flow of grout through typical voids. However, if a significant amount of grout must flow through small voids (say less than 5 mm) the use of grout designed for this purpose may be desirable (16), and specialist advice may be sought on the use of fine-grain cements. One possible difficulty in using fine-grain materials is that the grout may remain fluid enough for injection for a shorter time. However, experiments have shown (60) that grouts of this type can be produced that will pass though gaps as small as 1 mm. The property of resistance to plug formation has been described as ‘differential pressure microstability’ (61). Chemical reactive resins can flow through smaller voids and fissures than cementitious grouts because they are wholly liquid and not suspensions of particles in a fluid, but their

Void grouting

bond and tendon protection characteristics are considered inferior to cementitious materials at this time.

7.6

GROUTING EQUIPMENT AND METHODS

The equipment for pressure grouting in new construction described elsewhere in Part Two is satisfactory for grouting voids. Where the volume of grout is small, the equipment and method of operation should be selected with this in mind. For example, pressure pots can be used for injecting grout into a small void such as at an anchorage or a high point in a duct. The grouting specification and contractor's method statement, including the equipment, must take account of site conditions. Requirements may include the continued operation of the structure and any routes beneath it. In most instances, access to the tendon duct for grout injection and venting will be through holes drilled through the concrete. The equipment must provide an adequate seal on the drilled hole. This is not particularly difficult and can be accomplished by use of an expanding nozzle for injection and resin adhesive for sealing vent pipes. Recent void-grouting trials have shown that, provided the process is well managed, cementitious grout can be made to pass though small voids (about 5 mm measured radially) by pressure grouting alone. Nevertheless, blockages can occur where voids of small cross-section are very long or particularly small, or restrictions such as spacers are present. Observation of recent void-grouting 'contracts on site suggests that, if a 'blockage occurs during grouting, it can sometimes be cleared by the application of vacuum (62). Although this appears logical, at present the evidence for this is considered inconclusive. There is little published data on the use of vacuum-assisted grouting (but see Lapsley ( 6 3 ) ) . A LINK project included a limited evaluation of vacuum grouting (I6, 26), but the technique cannot be recommended on the results of that trial alone. Users of vacuum-assisted grouting report that removing air from small discrete voids, particularly those with small and tapering cross-sections, improves penetration of the grout. Evidence is needed to confirm what is a reasonable view. Such voids may occur at the raised ends of ducts with parabolic profiles, which are likely to contain voids that are not accessible at the top. Where these are sealed at the time of grouting, pressure grouting will compress the trapped air and leave a void, albeit smaller than before. In the mean time, vacuum assistance has been specified in recent void-grouting contracts on bridge sites, and specifications have been prepared on a bridge-specific basis for such work.

7.7

DETERMINING THE VOID CHARACTERISTICS

Normal practice for determining the characteristics of a void is to extend the invasive inspection carried out under Phase 3

of the Special Inspection ( 1 1 ) or in other inspections that have been the source of information used to determine the need for grouting voids. The aim should be to establish as far as possible the characteristics of each void to be grouted, including its length, cross-section at suitable intervals, volume, presence and position of constrictions, continuity and extent and position of leakage. The ability to do this will depend on the available access and the acceptability of drilling closely spaced holes. The extent of drilling should be decided with the agreement of the engineer in order to avoid unacceptable damage to the fabric of the bridge. Some useful tests are described in Appendix A. Establishing the void characteristics is important for assessing the suitability of ducts for grouting, planning the operation in detail, including the injection and venting points, and estimating the quantities of grout required. Existing and recently developed methods for determining void characteristics in Special Inspections are, in principle, applicable to void-grouting operations. Their use can be explored in trials either as a separate exercise, before being specified for use throughout the works, or as part of proving suitability within a contract. The difficulty of establishing accurately the size and nature of voids should not be underestimated. However, provided that during grouting the grout is seen to flow adequately out of all the vent points, the precise details need not be known. When a blockage occurs between vent points that have previously been shown to be continuous for the passage of air, it may be possible to inject in the reverse direction. This may leave a void ungrouted somewhere between the two points, but at least the barrier to the movement of water should have been improved. Use of a grout flow meter is recommended to compare the volume of grout injected with the previously determined volume of void.

7.8

FLUSHING WITH WATER

Unlike for new grouting flushing with water is recommended where the duct is partially filled with hardened grout because it improves the flow of grout during void grouting. However, if grouting can be accomplished without introducing flushing water, this is preferable. For example, when grouting a wholly voided metal duct, there is no existing grout surface to absorb water from the fresh grout and the large crosssection to be grouted presents no additional problems over new grouting. Water can help to identify continuity between duct entry (injectiodvent) points and may indicate the presence of narrow passages when the flow is unexpectedly slow between two consecutive access points. It may also remove or dilute water that contains chlorides, which had previously entered the duct during service, or flush out a proportion of any chloride deposits left behind when water has 'dried out. After flushing, the water should be removed (e.g. by draining, use of compressed air or vacuum) but pockets of

'31

Durable post-tensioned concrete bridges

water are likely remain in the duct. Some of this water will mix with or be displaced by the grout during void grouting and should be expelled from the duct. Grout should be passed out of the vent points until its fluidity matches that at the injection point. Some pockets may remain trapped (although this has not been demonstrated). Nevertheless, it is suggested that the practice of flushing with water be left to the judgement of the engineer.

example, relating to drilling into the ducts for investigation purposes and subsequently for venting or injection. Some provisions may apply only after modification, for which examples are given in the paragraphs below. A convenient way of defining variations to the Specification is to use Appendix 17/X to the Specification for Highway Works, which is reproduced as Annex 1 to the Specification in Chapter 12.

Attempts have been made to estimate the void volume by collecting flushing water as it is drained off. From the remarks above, it is should be clear that only a rough estimate is likely to be obtained in this way. It is better to find the void volume with an air pressure method, provided that the equipment used can cater for the large leakage rates that may be encountered.

There must sometimes be enough flexibility for the engineer to accept methods proposed by the contractor and adapted to the conditions on site. However, significant factors must not be overlooked by default, and the quality of work must be maintained.

Flushing to remove grout during a failed grouting operation is not recommended because cells of grout are left in the duct: the flushing water passes over the grout even when it remains fluid.

7.9

EFFECT OF EXISTING DEFECTS

The ducts and tendons targeted for void grouting will sometimes contain defects as well as voids. Even if defects have not been found at the inspection points, they may be present at other locations. If the existing grout is of poor quality or in poor condition, this should not deter the use of void grouting. However, the possibility of blockages occurring during the grouting operation may need to be taken into account. If exposed tendons occur in combination with conditions suitable for corrosion, particularly high chloride concentrations, and the presence of existing corrosion, including wire fractures, consideration should be given to alternative management strategies, as well as to void grouting. Grouting cannot be relied on to prevent further corrosion at these locations. Moreover, in certain situations, void grouting could result in additional and possibly accelerated corrosion. The benefits of re-establishing bond and improving the reanchoring properties are still relevant. If the defects as stated are present but not widespread, grouting is still recommended.

7.10 SPECIFICATION FOR GROUTING The Specification in Chapter 12 is recommended as the basis for the specification of void-grouting works. This Specification has been successfully used for void-grouting projects after appropriate modifications. Several provisions in the Specification relate to new grouting only, and should be excluded from the void-grouting specification or amended accordingly. For example, pressure testing ducts before concreting is, not appropriate although the benefits of measuring duct leakage before grouting remain. In some cases, entirely new provisions are needed, for

32

Maximum pressures during void grouting. Where small passages have to be grouted, it may be desirable to increase the maximum pressure to ensure that grout flows through to the next larger void. Pressures higher than 8 bar (800 Wa) require the approval of the engineer. Sudden application of high pressure to drive grout along small passages may cause a blockage. Distance between vent points. The distance between vent points may be shorter or longer than normal. Reasons are the need to establish void characteristics at sufficient intervals this may require closely spaced holes - and access difficulties that may restrict hole drilling anywhere but at the ends of the span. Height of vents above the duct. Restrictions on height will sometimes prevent the standard height being achieved. The specification will provide for this, allowing'the maximum practical height to be used in all locations. This possibility is already recognised in the specification. Amount of grout removed from each vent. When vents are very closely spaced because of the spacing of holes used for invasive drilling, and all holes are used as vents, it is not necessary to collect a full volume of grout from all vents. An exception might be where flushing with water has been used. The procedure. should be covered in the specification and agreed with the engineer. Holding pressure after grouting. It may be desirable for the normal requirements, e.g. for holding pressure, length of time, re-injection, to be reconsidered. This may apply where it proves impossible to fully seal the void being grouted. Rate of progression of grouting along duct. It is accepted that this cannot be controlled for void grouting to the same degree as for new grouting. Because the void-cross section changes as the grout progresses along the duct, the volume required to fill a given length may change rapidly. This can be anticipated to some extent if the characteristics of the void are established before grouting.

7.11 TRIALS Trials are essential for all void-grouting contracts. The ducts, tendons and void characteristics should be faithfully reproduced in the trials.

Void grouting

The trials should employ the methods used to characterise the voids and establish the extent of continuity, and the proposed grouting method. If the ducts in the structure leak significantly, the ability of the proposed method to counteract this should be demonstrated with matching leakage rates. If water is to be used to flush the void before grouting and provide lubrication, this should form part of the trials. Coloured grout can be used, either for the ‘original’ grout used to partially grout the duct to prepare the specimens for the trials or for the void-grouting material.

7.12

QUALITY CONTROL

Quality control requires the following elements: 0

0

0

e

demonstration in advance of equipment, materials and techniques in a representative situation detailed method statements, including a planned reaction to possible scenarios highlighted in risk assessments

a flexible approach to enable the best grouting to be achieved according to the circumstances found meticulous record taking that includes an agreed statement of the likely duct conditions on completion of the remedial grouting operation.

The control of quality should begin with thorough documentation of the void characteristics. A schedule of ducts, voids, vent pipes and injection points, etc. should be submitted before work starts, and records should be kept during grouting, as for new grouting. It will be impossible to know exactly what each void is like but a full record of site information made during grouting will help in resolving, without delay, questions that arise during and after the work. Experienced site personnel must be used, to operate the pump, at the injection points and to manage the works. The quality of the grout is important, particularly at the vent pipes, and this can be monitored. The significance of the different operations and material properties must be recognised, for

instance, acceptable limits for grout fluidity, ’ the correct amount of water, preliminary testing. and the need to have everything to hand so there are no delays during the work. Careful notes should be taken during grouting to monitor fluidity, pumping pressure, flow time and any problems encountered, for each voided length attempted. Any locations where it is not clear if grouting has been successful, or where flow times are shorter or longer than expected, can then be inspected carefully later. After grouting, the quality achieved should be established by invasive drilling at points other than the injection and vent positions. Suitable places include vulnerable locations, for example, at the top of a profile, or where something unexpected happened during the process. It is reasonable to assume that, in a well-controlled grouting operation in which the flow of grout through all vent pipes has been achieved in compliance with all procedures, the voids have been satisfactorily filled. Sample drilling may be used to confirm this. If coloured grout is used for void grouting in the works, the success of the grouting process can be established more easily. Where invasive drilling has been carried out, the characteristics of the duct after the voids have been grouted may be established by measurements of void volume, continuity and leakage.

If the most vulnerable positions are inspected, and are seen to be fully grouted, this demonstrates that there is grout in the duct that was not present before. The tendons are at least better protected than before the voids were grouted. The automated quality control system described in Appendix AS may be appropriate for the control of void grouting, as the principles are the same as in new grouting. However, the equipment has yet to be tested on remedial grouting work, for instance, when there are constrictions between larger voids. As with other grouting operations, the quality requirements in Chapter 11 should be followed.

33

ETHODSFQR P

8.1

UNUWODUCBION

-

Table 1 : Test methods applicable during construction ~~

This Chapter covers tests relating to the durability of prestressing tendons, the efficiency of the grouting process, and also outlines acceptable alternative tests. The Testing subgroup of the Working Party considered a wide variety of testing methods for durability and grouting efficiency before publication of the first edition of this Report in 1996. The general. approach to testing taken at that time remains unchanged in this edition. However, the subject has continued to develop .and the latest developments are reported here.

Feature under test

The Chapter summarises the complete range of test methods considered, but then concentrates on those that have most to offer. Further details can be found in Guide to testing and monitoring the durability of concrete structures (64). An update on the most appropriate test methods is given in Appendix B. Emphasis is given to tests specifically designed for grouted construction.

Grouting pressure Overall quality control

Routine testing of the quality of completed grouting was not common in the past, although such testing is common for many other construction activities. This may have been because suitable test methods were not available, but it may have allowed defective workmanship to go undetected on occasions.

Voids above grout

Test method

Before grouting

Duct leakage under pressure (see Appendix AI) Bleeding I .5 m vertical tube test

Sealing of duct Suitability Of grout During grouting

I Voids in grout

I

Feature under test

1. The test should be taken at an early stage when remedial action is possible.

Degree of corrosion risk

2. The test should interrupt the production process as little as possible.

8.2

Tendon integrity

RANGE OF TESTS CONSUDEWED Prestress loss

Table 1 shows test methods applicable during construction and Table 2 shows test methods applicable during service life.

8.3 THE NEED FOR BESTUNG Selection of tests and the amount of testing required are matter of judgment. At one extreme, testing may be seen as a needless expense if reliability is already guaranteed in some

34

Radiography Impulse radar Impact echo (sonic) Ultrasonic transmission Ultrasonic reflection Thermography Radiometry/tomography Pressure/volume/leakagetesting

Table 2: Test methods applicable during service life

In searching for appropriate tests, the Testing sub-group set the following criteria against which to judge tests to assess the quality of grouting during construction:

3. The test should be simple, so that ambiguous interpretation is unlikely.

Grout stiffness test (see Appendix A2) Void sensors (see Appendix A2) Duct pressure sensor (see Appendix A3) Automated quality control system (see Appendix A5)

Survey of existing ducts before regrouting

Test method

Half-cell potential Resistivity Electrical continuity Corrosion Fibre optic integrity Acoustic monitoring Ultrasonic electronic pulse RlMT electronic pulse Maenetic flux exclusion Strain by fibre optic sensors Strain by vibrating wire gauge or Demec gauge Vibration monitoring Duct leakage under pressure (see Appendix AI) Volume of voids (see Appendix A6) Automated quality control system (see Amendix AS)

Note: Some of these methods are still under development, and inclusion does not imply that they are appropriate for routine application.

Test methods-forgrouted post-tensioned concrete

other way, and requirements that are too stringent cause needless expenditure and delay. On the other hand, even extensive testing is likely to form a very small proportion of the total expenditure, and it is essential that all concerned have full confidence in the form of construction and the workmanship within each application. Only then can longterm durability be assured.

Examples of such testing procedures are given in the fib Technical Repoit Corrugated plastic ducts for internal, bonded post-tensioning (17). This describes testing requirements for duct materials (such as flexural behaviour, load and wear resistance), leak tightness and system approval testing (which includes grouting a 30 m-long duct enclosing tendons and encased in concrete).

A single test usually evaluates the effectiveness of only one layer of protection. Clients, designers and contractors should therefore reflect upon the level of assurance that they require, and commission a combination of tests that they judge will meet their needs.

The tests described in Appendices A 1 and A2 are also appropriate for use at the ‘type-approval’ stage. These tests are designed to check the basic product, but can also provide performance data against which workmanship on site can later be judged. The relevant tests in Appendix A are:

An insight into the problem of grouting defects can be obtained from the extensive programme of special inspections of grouted bridge structures that has taken place in the UK over recent years. This programme showed that only small voids were present in 40% of structures. On the other hand, over 35% of structures contained, in varying degrees, either large voids or ungrouted tendons. There is no doubt from this survey that bridges can be grouted extremely well, even those early bridges that were grouted with simple equipment. The problem is therefore one of achieving good grouting every time. Quality assurance certification schemes have an important role in regulating consistent performance, but testing may be seen as the definitive proof that a quality product has been produced. The problems discovered in post-tensioned grouted construction have stimulated a number of research activities. These have led to a greater understanding of the behaviour of materials, but have also indicated the subject to be more complex than had been previously assumed. Testing also has a role in recording the actual behaviour of materials under construction site conditions. Only in this way can the technology of grouting be advanced still further.

8.4

TEST MIEETIHIODS APPROPRIATE IN PART0CULAW CU RCUMSBANC ES

It has been emphasised that the most useful tests are those undertaken at a sufficiently early stage for defects to be corrected. The stages at which the tests might be undertaken are therefore considered below. Under each heading, the most appropriate tests are discussed, with reference to Appendix A, where further details of the tests themselves may be found.

8.4.1 Type-approval at pre-contract stage (duct systems, grout materials and procedures) One of the most efficient means of achieving a quality end product is to test certain aspects that influence quality in advance of potential contractual complications. This testing may include the following, separately or in combination: 0

testing of’duct systems

0

testing of grout materials

e

testing of combinations of duct systems, grout materials, and duct geometry.

0

0

Test A 1, leak tightness tests for duct systems. These give values of air leakage from the duct that should be expected for the particular system. Test A2, grout stiffness tests. These give the amount of gas that may be expected to be trapped within a grouted duct for the tested system, given that the grout material, mixing plant, duct geometry and procedures remain unchanged.

Pre-contract type-approval testing should be undertaken under the scrutiny of an independent quality assurance certification scheme, such as that provided by CARES. This will result in confidence in the data produced, and its documentation, and provide a link to the procedures adopted for use of the products on site. It is necessary to record all details of the grouting material, mixing plant, duct geometry and procedures in place during pre-contract testing, because significant departure from them can influence the quality of the finished product. The tests only demonstrate satisfactory quality with given materials providing all other factors influencing the grouting remain unchanged. Type-approval tests could be extended to cover variations in grouting material, mixing plant, duct geometry and procedures. In that case, providing documentation and independent certification are also.present, they may provide sufficient confidence to remove the need for the trial grouting during individual contracts. The need to ensure that conditions are as anticipated will remain, with a consequent need for quality control measures. If type-approval testing is to be adopted for a project, it should be considered as part of the multi-layer protection strategy discussed in Section 3.1.

8.4.2 Trial grouting within a contract (geometry, materials and procedures) Trial grouting is taken here to describe the grouting of a trial duct within a contract, before approval of the contractor’s proposals, the duct being cut up after grouting to provide definitive evidence that the ducts are adequately grouted. This is somewhat similar to type approval at pre-contract stage, except that the proposed materials, geometry and procedures will be identical to those proposed for the main works.

Durable post-tensioned concrete bridges

As with type-approval testing, the duct assembly verification test A1 and grout stiffness test A2 may be used to provide evidence the duct has been correctly assembled and to measure the volume of trapped gas in the trial grouted duct(s). These tests will provide values of these parameters that are both achievable with the selected materials and procedures, and also result in acceptable grouting quality. If tests A1 and A2 are used during the trial grouting, they also provide achievable target values for use during the main works.

8.4.3 Duct assembly verification before main grouting

I

Despite the fact that duct systems themselves are satisfactory, a number of aspects may go wrong during assembly. These include local damage to ducts and seals, misalignment, and use of incompatible components. It is therefore important to check that the system has been correctly assembled. The tests for this are described in Appendix Al.

8.4.4 Duct integrity after concreting or assembly of precast units, but before main grouting Damage to ducting sometimes occurs during concreting. The system should therefore be tested after concreting. If this is undertaken prior to stressing, specially extended end caps are required, but if undertaken after stressing the permanent caps should be used. The test will also check their effectiveness. The tests described in Appendix A1 can be used for this purpose. Testing after concreting should not be avoided because of fear of discovering damage that is difficult to remedy. It is important that levels of leakage discovered following concreting should not be regarded as ‘failure’. The concept of multi-layer protection is such that if each layer is as good as practicable, then the construction as a whole is satisfactory. Some, but not all, of the causes of leakage may be difficult to remedy at this stage. Easily corrected leakages are those that are accessible, e.g. at vents or at end caps. In many cases, simple measures such as an extra sealant between precast units may be considered an appropriate solution.

8.4.5 Grout stiffness test of main grouting The grout stiffness test was developed after reviewing the available test methods, and fills a need for a test that met the criteria in Section 8.1. The test is described in Appendix A2. The alkalinity of the grout provides an effective protection against tendon corrosion, particularly in the absence of chloride contamination. Measurement of the voids within grout thus indicates directly the likely effectiveness of grout as a protective layer. The major advantage of the grout stiffness test is that the existence of significant voids within any duct is detected at a stage when the void can be removed by further grouting and bleeding at vent pipes. If the test has not been calibrated during use on trial ducts, the results may be interpreted by reference to other contracts, or more particularly by reference

36

to other ducts within the same structure. Variability in trapped gas due to changes in mixing, admixtures, temperature, incomplete filling or inadequate venting will be detectable. In addition, the equipment will indicate whether the duct is well sealed, or leakage is occumng at any end caps or vent pipes. If the stiffness test has been calibrated during a similar precontract type-approval test or against a trial duct within the contract, it can be demonstrated that the volume of trapped gas is within limits known to be acceptable. This will further increase confidence in the finished product. An alternative method of detecting the presence of voids during grouting is the use of void sensors, as described in Appendix A3. These have been shown to detect voids adjacent to the sensors. If used to monitor grouting during a contract, the sensors should be placed at vulnerable locations and be wired to an accessible location before placing of concrete. This would be an expensive and time-consuming operation.

8.4.6 Automated quality control testing of main grouting Equipment developed for the grout stiffness test has been further developed with the support of the Highways Agency and the British Cement Association. This development was envisaged in Reference 27. The equipment is described in Appendix A5. Automated quality control equipment is designed to monitor all aspects of grouting that may influence the final quality, and to record them automatically throughout the grouting operation. The equipment is placed within the grout flow line between the grout pump and the duct. Measurements that are stored include temperature and grout flowrate. The equipment automatically tests the volume of gas trapped either within the ducts or within samples of grout in the test chamber, with the results of the analysis displayed for immediate use as well as for later reporting. Quality assurance certification schemes will ensure that the equipment and training of operatives is such that a high quality of grouting can be achieved. Automated quality control testing demonstrates that the quality has been achieved on every duct. The benefits of an automated quality control system are as follows: The presence of excessive trapped gas within the grout, and hence probable voids, is detected at a time when further grouting or venting can remove the voids. Leakage from the duct can be detected, and defects causing the problem such as loose end caps can be rectified. If pre-contract type approval tests or trial ducts have established satisfactory gas contents, it can be demonstrated that the same quality has been achieved in the works. Without such tests and trials, the equipment will still detect voids in excess of what might normally be expected.

Test methods for grouted post-tensioned concrete

0

The equipment produces a report of all events and their timing. If any unforeseen events occur, the full circumstances are recorded for analysis.

To force grout to move, pressure is applied at one end of the system, which decreases along the length of grout within the system. The grout pressure is generally indicated by a gauge adjacent to the injection pump. The automated quality control system described in Appendix A5 continuously measures the grout pressure closer to the point of injection into the duct. If there is a particular need to monitor pressure within the duct itself, duct pressure sensors described in Appendix A4 may be used.

8.4.7 Survey of existing grout conditions before re-grouting Surveys of existing structures have shown that voids are often present within ducts, and in some cases remedial grouting is required. Remedial grouting is a more difficult operation than grouting of new ducts. Knowledge of the extent and nature of the voids to be filled can assist in the regrouting operation, and increase the probability of providing the tendon protection and bonding required. The test methods for voids above grout carried out after grouting but before acceptance in Table 1 have all been used to assess voids within existing structures, with varying degrees of success. The ideal test for this purpose would be completely non-destructive, and would not disturb the environment within the duct. Non-destructive tests that meet this ideal are not yet sufficiently precise, although this may improve with development. Currently the most effective methods still

involve intrusive examination of the duct, generally through a small drilled hole. Tests applicable to existing ducts are described in Appendix A6. These should be aimed at providing answers to the following questions: 0

0

What is the corrosion risk arising from an identified defect? i.e. is there a need to re-grout? What is the most appropriate procedure for re-grouting?

The answers to both questions depend primarily upon (a) the size and distribution of voids and (b) the connectionpf these voids to the atmosphere. The size of voids is certainly an indication of the likelihood of exposed tendons, as well as the presence of passages of sufficient size for successful re-grouting. The location and interconnection of these voids is also of great importance. However, the connection to the atmosphere may be considered to be of even greater importance. If a void is well sealed and tendons have only a wash of grout, they will retain their alkaline protection over a long period. If the void is only small, but there is air leakage that enables carbonation of the grout, the durability will be significantly reduced. If water can gain entry, particularly if it is contaminated with chlorides, the durability will be reduced further. Re-grouting of sealed voids is also more difficult, particularly if access to the end of the void is denied by the geometry of the structure. The testing described in Appendix A4 is therefore aimed at reducing these uncertainties to enable re-grouting operations to be planned effectively.

37

9

FERENCES

1. WOODWARD R.J. Conditions within ducts in posttensioned prestressed concrete bridges. TRRL, Crowthorne, 1981. Laboratory Report 980. 22pp. 2. STANDING COMMITTEE ON STRUCTURAL SAFETY. Third Report of the Committee for the Year Ending 31 March 1979. London. pp.15-18. 3. PORTER M.G. Repair of post-tensioned concrete structures. Concrete bridges: investigation, maintenance and repair, Proceedings of Symposium, September ' 1985. The Concrete Society, Crowthorne. pp. 1-27. 4. WOODWARD R.J. and WILLIAMS F.W. Collapse of Ynys-y-Gwas Bridge, West Glamorgan. Proceedings, Institution of Civil Engineers. Part 1, Vol. 84, August 1988. pp.635-669.

5. POST-TENSIONING INSTITUTE. Guide specification for grouting of post-tensioned structures. Phoenix, Arizona, 2 0 0 1 . 6 9 ~ ~ . 6. FLORIDA DEPARTMENT OF TRANSPORTATION. Mid-Bay Bridge post-tensioning evaluation. 6 November 2001. 7Opp. www 11.myflorida.com/structures/memos/memos.htm '

7. HIGHWAYS AGENCY, T U , SETRA and LCPC. Posttensioned concrete bridges. Thomas Telford, London, 1999. 164pp. 8. FEDERAL MINISTRY OF TRANSPORT, CONSTRUCTION AND HOUSING. Guidelines for concrete bridges with external tendons. Verkehrsblatt-Verlag, Dortmund, No. 17/1999. 9. RAISS M.E. Durable post-tensioned concrete bridges. Concrete. Vol. 27, No. 3, May/June 1993. pp.15-18. 10. HIGHWAYS AGENCY. Post-tensioned concrete bridges, prioritisation of special inspections. Departmental Standard BD 54/93. 14pp. 11. HIGHWAYS AGENCY. Post-tensioned concrete bridges, planning, organisation and methods for carrying out special inspections. Advice Note BA50/93. 40pp.

12. WOODWARD R.J. Evidence of problems. TRL Seminars on Inspection of post-tensioned concrete bridges, held on various dates 1992-1994.

15. HIGHWAYS AGENCY. Post-tensioned grouted duct concrete bridges. Interim Advice Note 16. 1999. 16. GIFFORD AND PARTNERS. QA of grouting post-tensioned concrete structures. Proceedings of Seminar, Cambridge, 23-24 September 1999. Gifford and Partners, Southampton. (Workshop organised under Brite Euram Contract BRPR-(395-0099, BE95-1675. Improved quality assurance and methods of grouting post-tensioned tendons.) 17. INTERNATIONAL FEDERATION FOR STRUCTURAL CONCRETE. (FCdCration Internationale du Beton). Corrugated plastic ducts for internal bonded post-tensioning. Lausanne, 2000. fib Technical Report, Bulletin No. 7. 46PP. 18. INTERNATIONAL FEDERATION FOR STRUCTURAL CONCRETE. (FCdCration Internationale du Beton). Durability of post-tensioning tendons. Taerwe L. (Ed.) Lausanne, 2001. Bulletin 15. 284pp. 19. HIGHWAYS AGENCY. The design of concrete highway bridges and structures with external and unbonded prestressing, Departmental Standard BD 58/94 and Advice Note BA 58/94. lOpp and 12pp. 20. WEST J.S., VIGNOS R.P., BREEN J.E. and KREGER M.E. Corrosion protection for bonded internal tendons in precast segmental construction. Center for Transportation Research, The University of Texas at Austin, October 1999. Research Report 1405-4. 21. THE CONCRETE SOCIETY. Grouting specifications. Concrete. Vol. 27, No. 4, July/August 1993. pp.23-28. 22. THE CONCRETE SOCIETY/CONCRETE BRIDGE DEVELOPMENT GROUP. Durable post-tensioned concrete bridges. Proceedings of Seminar, 18 May 1994. The Concrete Society, Crowthorne, 1994. 120pp. 23. BRITISH STANDARDS INSTITUTION, London. BS EN 445: 1997 Groutforprestressing tendons. Test methods. 18pp. BS EN 446: 1997 Grout for prestressing tendons. Grouting procedures. 12pp. BS EN 447: 1997 Grout for prestressing tendons. Specification for common grout. 10pp.

24. FEDERATION INTERNATIONALE DE LA P R ~ C O N 13. CLARK L.A. Per$ormance in service of post-tensioned TRAINTE. Grouting of tendons in prestressed concrete. bridges.British Cement Association, Cmwthome, 1 9 9 2 . 6 6 ~ ~ . Thomas Telford, London, 1990. FIP Guide to Good 14. DEPARTMENT OF TRANSPORT. Standards for postPractice. 16pp. tensioned prestressed bridges to be reviewed. London, 25 September 1992. Press Notice No. 260.

38

References

25. BALVAC WHITLEY M O W . Felton Bypass, Vacuum assisted pressure grouting of the post-tensioned Macalloy bar ducts on the River Coquet Bridge. 1981. 20pp.

41. WALLBANK E.J. The performance of concrete in bridges. A survey of 200 highway bridges. HMSO, London, 1 9 8 9 . 9 6 ~ ~ .

26. TILLY G.P. and WOODWARD R.J. Development of improved grouting for post-tcnsioned bridges. Posttensioned concrete structures 1996. Proceedings, FIP Symposium, London, September 1996. The Concrete Society, Crowthorne, 1996. Vol. 1, pp.55-64.

42. HOBBS D.W. (Editor). Minimum requirements for durable concrete. Carbonation and chloride-induced corrosion, freeze-thaw attack and chemical attack. British Cement Association, Crowthorne, 1998. Publication 45.043. 174pp.

27. DARBY, J.J. Control of grouting quality by the measurement of total gas content within fresh grout. Posttensioned concrete structures 1996. Proceedings, FIP Symposium, London, September 1996. The Concrete Society, Crowthorne, 1996. Vol. 2, pp.669-676.

43. BRITISH STANDARDS INSTITUTION. BS 5896: 1980 Specification for high tensile steel wire and strand for the prestressing of concrete. 16pp.

28. BRE. Concrete in aggressive ground. CRC Ltd, Garston, 2001. Special Digest 1. (Four parts.) 29. THE CONCRETE SOCIETY. Alkali-silica reaction: minimising the risk of damage to concrete. Crowthorne, 1999. Technical Report 30 (third edition). 72pp. 30. HIGHWAYS AGENCY. Expansion joints for use in highway bridge decks. Departmental Standard BD 33/94. 18PP. 3 1. HIGHWAYS AGENCY. Design for durability. Departmental Standard BD 57/01 and Advice Note BA 57/01. 12pp and 15pp. 32. THE CONCRETE SOCIETY. Non-structural cracks in concrete. Crowthorne, 1992. Technical Report 22 (third edition). 48pp. 33. THE CONCRETE SOCIETY. The relevance of cracking in concrete to corrosion of reinforcement. Crowthorne, 1995. Technical Report 44. 32pp. 34. HIGHWAYS AGENCY. Early thermal cracking of concrete. Departmental Standard BD 28/87 and Advice Note BA 24/87. 13pp and 19pp. 35. BRITISH STANDARDS INSTITUTION. BS 5400: Part 4: 1990 Steel, concrete and composite bridges. Code of practice for design of concrete bridges. 66pp. 36. HIGHWAYS AGENCY. The design of concrete highway bridges and structures. Use of BS 5400: Part 4: 1990. Departmental Standard BD 24/92. 10pp. 37. DEPARTMENT OF TRANSPORT. Manual of contract documents for highway works, Volume I : Specification for Highway Works. The Stationery Office, London, 1992. 38. CIRIA. Bridge detailing guide. London, 2002. Publication C543. 272pp.

44. BRITISH STANDARDS INSTITUTION. prEN 10138. Prestressing steels. Part 3. Strand. (In course of preparation.)

45. AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM A 41 6/A416M-99. Standard speci$cation for steel strand, uncoated seven-wire for prestressed concrete. West Consohocken, Philadelphia. 46. HAMPEJS G., JUNGWIRTH D., MORF U. and TIMINEY P. Prestressing materials and systems: galvanisation of prestressing steels. Commission Reports. FIP Notes. 1991/4. pp.34. 47. KOLLEGGER J. Investigations on a plastic duct for bonded post-tensioning. Bauingenieur Vol. 69, 1994. pp.1-10. 48. RICKETTS N.J. Post-tensioned concrete bridges: Improved design methods, details and monitoring. Transport Research Laboratory, Crowthorne, 1993. Interim Summary Report, Project Report PR/BR/2/93. 49. HIGHWAYS AGENCY. Materials for the repair of concrete highway structures. Departmental Standard BD 27/86. 16pp. 50. TRANSPORT RESEARCH LABORATORY. Papers presented to a one-day seminar on Inspection of post-tensioned concrete bridges, held on a number of occasions between 1992 and 1994.

5 1. TRANSPORT RESEARCH LABORATORY. Grouting of ducts in post-tensioned prestressed concrete. Crowthorne, 1986. Contractor Report 24. 52. WOODWARD R.J. and MILLER E. Grouting posttensioned concrete bridges: the prevention of voids. Highways and Transportation. Vol: 37, No. 6, June 1990. pp.9-17. 53. COLLINGS C.M. and TELFORD I. Kingston Bridge Phase I strengthening. Current and future trends in bridge design, construction and maintenance 2. Das, P.C. et al. (Eds) Thomas Telford, London, 2001.

39. HIGHWAYS AGENCY. Waterproojing and surfacing of concrete bridge decks. Departmental Standard BD 47/99 and Advice Note BA 47/99. 39pp and 18pp.

54. CLARKE N. Threading the needle at Medway Bridge. Concrete. Vol. 35, No. I , January 2001. pp. 19-22.

40. THE CONCRETE SOCIETY. Guide to surface treatments for protection and enhancement of concrete. Crowthorne, 1997. Technical Report 50. 88pp.

55. BRITISH STANDARDS INSTITUTION. ENV 1992-2: 2001. Eurocode 2 Design of concrete structures. Concrete bridges.

39

Durable post-tensioned concrete bridges 56.* HIGHWAYS AGENCY. The inspection of bridge super. struciures and foundat;ons, retaining wall and buried structures. Advice Note BA 55/94. 13pp. 57. FEDERATION INTERNATIONALE DE LA PRECONTRAINTE. Materials and systems for external prestressing. Lausanne, 1996. FIP State of the art report. 15PP. 58. SERVICE D'ETUDES TECHNIQUES DES ROUTES ET AUTOROUTES (SETRA). Prkontrainte extcfrieure - Guide technique. (External prestressing). Bagneaux, France, 1990. 120pp. 59. TILLY G.P. Performance and management of posttensioned structures. Structures and Buildings. Vol. 152, No. 1 , 2002. 60. MATHEY B., DEMARS P., ROISIN E and WOUTERS M. Investigation and strengthening study of twenty damaged bridges: Belgian case history. Bridge Management 3, Inspection, maintenance, assessment and repair. University of Surrey, April 1996. Harding J, Parke G, and Ryall M. (Eds). E&FN Spon, London, 1996. 61..HENRICHSEN A. and STANG H. Materials design of high performance grouts - Part 1. Proceedings of Joint Cluster 2 and 6 Seminar QA of grouting post-tensioned concrete structures, Cambridge, 23-24 September 1999. Gifford and Partners, Southampton. (Workshop organised under Brite Euram Contract BRPR-CT95-0099, BE95-

40

1675. Improved quality assurance a@ methods of grouting . . , ' post-tensioned tendons.) 62. MILNER A. and HAYNES M.D. Re-grouting post-tensioned concrete bridge tendon ducts. Structural faults and repair conference. London, 1999. Engineering Technics Press, Edinburgh, 1999. 63. LAPSLEY R.D. Grouting of post-tensioned structures recent advances in vacuum grouting techniques and grout mix design. Post-tensioned concrete structures 1996. Proceedings, FIP Symposium, London, September 1996. The Concrete Society, Crowthorne, 1996. Vol. 2, pp.659-668. 64. CONCRETE BRIDGE DEVELOPMENT GROUP. Guide to testing and monitoring the .durability of concrete structures. The Concrete Society, Crowthorne, '2002. CBDG Technical Guide No. 2. 120pp. '

65. HIGHWAYS AGENCY. Manual of contract documents for highway works, Volume 4: Bills of quantities. Section 1. Method of measurement for highway,works. The Stationery Office, London, 1998. 66. BRITISH STANDARDS INSTITUTION. BS EN IS0 9002: 1994 Quality systems. Model for quality assurance in production, installation and servicing. 20pp. 67. BRITISH STANDARDS INSTITUTION. BS 4447: 1973 (1990) Specification for the performance of prestressing anchorages for post-tensioned construction. 12PP.

PART TWO

-

REQUIREMENTS FOR DURABLE POST-TENSION ED CONCRETE BRIDGES Notes for guidance on specification for duct and grouting systems for post-tensioned tendons . . . Contractors quality systems requirements

. . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . .

Specification for duct and grouting systems for post-tensioned tendons . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

. . . . page43 ,

. :.

page46

. . . . page49

10 NOTES FOR CUI

SPECIFICATION FO SYSTEMS FOR POS

I CT AND GROUTING SIONED TENDONS

10.1 INTRODUCTION

12 and should be added to the Method of Measurement for Highway Works (65).

The edition of the Concrete Society Specification in Chapter 12 was drafted in 2001 and updates the version published in 1996. It takes account of further experience gained on testing of grout, grouting trials and systems development in the intervening period and various research projects as well as wider international experience drawn upon through the International Federation for Structural Concrete @).

The Specification sets out requirements for special grout, and material properties that are considered desirable for grout for post-tensioning. This special category :of grout will be supplied either as a pre-bagged formulated product or as a mixture of controlled products matched to the performance requirements.

The changes to the Specification are in the following areas: single grout performance specified 0

air testing requirements amended

0

minimum pumping rate removed

0

downstream crest vent positions clarified

*

re-venting requirements expanded

0

rigidity of ducts addressed

0

sedimentation test introduced

*

fluidity limits changed

0

notes on the need for full-scale trials expanded specification for use of special grout defined scope for external tendons extended

*

notes on segmental construction expanded

e

new bleeding and volume change suitability test clarification of terms ‘Engineer’, ‘Specifier’, ‘Designer’.

The following definitions are used: The Specifier is the Authority or responsible body defining the specification that will apply for prestressing operations. The Engineer is the person or organisation responsible for administering the contract, supervising where required and accepting or considering proposals from the Contractor. The Engineer may be the Designer’s Representative and may be responsible for accepting or otherwise compliance with the Specification. The Designer is the person or organisation responsible for designing the prestressing layout and setting the geometrical parameters. The Designer may also be responsible for defining the detailed Specification requirements. In this context he may also be the Specifier. The Specification should be read in conjunction with the Specification for Highway Works (37) but takes precedence over the equivalent clauses therein. Amendments to the method of measurement are included,as Annex 2 to Chapter

Users of this Specification should consider carefully whether to adopt the specification for special grout or to specify common grout in accordance with BS EN 445, 446 and 447 (23). The Working Party strongly recommends the use of special grout meeting this Specification because the performance of grout made with ordinary cement is often variable. It is noted that the Highways Agency has signalled its intention to use only special grout in the future. Furthermore, CEN are to revise these Standards in the near future to incorporate stateof-the-art international recommendations. Users should note that the Specification requires suppliers and installers of post-tensioning systems to be certified under the CARES Scheme outlined in Chapter 11 or an equivalent scheme. This improved Specification is for UK applications ‘and is intended to be used by Specifiers in conjunction with the recommendations for design details given in Part One of this Report. Users in other countries should consider whether environmental conditions and design requirements are compatible with the UK.

80.2 NOVES FOR GUIDANCE ON T HE S PECIF I CAT10 N 10.2.1 Trials The Specification in Chapter 12 allows for the Engineer to call for full-scale trials if considered necessary to demonstrate that the grouting will provide adequate protection to the tendons. This requirement should be specified in Annex 1 to the Specification (Appendix 1 7 R to the Specification for Highway Works) and fully detailed on a Contract drawing, including trial beam size, concrete grade, cover to reinforcement and tendons, reinforcement and tendon details, together with requirements for testing and investigation. The Designer should recognise that the purpose of the trial is to test the contractor’s proposed systems, methods, materials and personnel that are to be used in the permanent works. The trial should also incorporate any special requirements of

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43

Durable post-tensioned concrete bridges

construction sequence and configurations. Requirements for disposal of the trial beam should be specified. The trials should be carried out well in advance of the planned use of post-tensioning in the permanent works (56 days is the default period in the Specification). In particular, any proposals for untried systems should be given due time for acceptance. The installation of the permanent works may commence immediately after completion of successful grout trials. Where a supplier/installer has evidence of satisfactory grouting from a significant number of previous trials using the same procedures, equipment and materials, the Specifier may consider whether the cost of a job-specific trial is justified. This would particularly apply to use of special grout where repeatability is more assured, or to small projects. Nevertheless, full-scale trials &e very effective in establishing suitable materials and procedures. EOTA ‘type approval’ for prestressing systems is being developed in Europe and this may include tests for ‘groutability’. Such standards may enable the Specifier to dispense with job-specific trials for routine applications. Grouting techniques such as vacuum grouting and postinjection re-grouting (as carried out in Germany) are available from some suppliers and can be considered to be demonstrated as suitable either for new works or for remedial works as appropriate. Such applications should also be subject to trials.

10.2.2 Grout materials Composition of the grout is classified by default as special grout (see Clause 2 of the Specification). The Designer should specify the grout type required. Performance of the grout will in all cases be assured by suitability trials, irres,pective of whether full-scale grouting trials have been called UP. If bagged cement is used, variations in age, chemical composition, fineness and temperature can have significant effects on the performance of the grout. Additionally, the weight of bagged cement is permitted under current British Standards to vary by up to 6% from the nominal weight, which could also significantly affect performance of the grout. The watedcement ratio should normally be in the range 0.35 to 0.40 in order to achieve the performance requirements of the Specification.

10.2.3 Ducting Ducting should generally comply with, as a minimum, the requirements of jib Corrugated plastic ducts for internal bonded tendons (17) but where this Specification is more onerous it over-rides the jib recommendations. Clause 3 requires the ducting to form an air- and waterresistant protective barrier as an additional defence against corrosive contaminants. This follows the philosophy of multi-layer corrosion protection. The intention is that, if the duct is inadvertently not completely filled with grout, the risk

44

to the tendons is significantly reduced as the ,protective ducting should maintain a corrosion-free environment. Polyethylene and polypropylene are suitable materials for ducting but other materials may also be suitable. Debate continues over the minimum wall thickness of ducting, and over the air-pressure test requirements. It is generally agreed internationally that, for polyethylene and polypropylene ducting, a wall thickness of 1.0-1.5 mm is adequate to protect against ingress of chlorides. For internal tendons, the Specification requires a minimum thickness for internal tendons of 2 mm (as manufactured) and 1.5 mm after tensioning. The purpose of air testing is to demonstrate, first, that the system provides an adequate degree of resistance to contaminants and, second, that the system is correctly assembled and has no significant leaks. ‘The pressure testing requirements make reference to compliance testing before installation and duct assembly verification testing. It is expected that all currently available systems can pass the latter test but designers should seek the prestressing supplier’s guidance before completing Appendix 17/X (Annex l), particularly with respect to compliance testing for more onerous criteria as suggested later. It is recommended that supplier’s guidance is sought on availability and compliance before completing Appendix 17/X. There are circumstances where the requirement for a sealed ducting system may be difficult to meet, e.g. in segmental construction. The Designer should consider the options. Sealing ducts at joints in segmental construction is an issue that remains to be satisfactorily proven, though if full duct continuity can be achieved and demonstrated by tests the duct can be considered as one of the layers of multi-layer protection. At the time of writing, precast segmental construction with internal grouted tendons is not permitted by the Highways Agency. However, it is believed that satisfactory evidence of proving iffective duct continuity could enable this to be reconsidered on a project by project basis. Minimum wall thickness of ducting after tensioning should be considered by the Designer and appropriate requirements specified in Appendix 17/X, taking account of minimum radii of curvature of the tendons which will tend to bite into the duct wall. Type and spacing of duct supports need careful attention to avoid this. Reference should be made to manufacturers’ and suppliers’ data. Excessive grouting pressures may have adverse effects on duct integrity. The internal diameter of vents should be as large as possible but designers should bear in mind the sizes currently available. The vents, connections and taps should be sufficiently robust to withstand full grouting pressure. Duct invert drains at low points should not normally be necessary, can be a practical difficulty and are not recommended. For most applications a vent height of at least 500 mm above the adjacent concrete surface is recommended to help entrapped air and water to escape. For some configurations of

Notes for guidance on specijication

tendons this may not be appropriate and the Designer should specify an alternative in Appendix 17/X. Some designers and some applications may require closer or wider vent spacing than the 15 m in the general Specification. Any other requirement should be given in the Appendix. For external tendons it can be important to anticipate any sagging of the duct due to the weight of grout, particularly for tendons stressed after grouting, and appropriate temporary duct support should be provided during the grouting operation.

10.2.4 Testing The bond length for internal tendons is required to be achieved in less than 40 diameters of the duct. If the Designer has different requirements, they should be specified in the Appendix. This is taken to mean the bond length to transfer the increase in load from SLS to ULS. The SpecifierDesigner should consider the total volume of grout in relation to the rate of sampling and set sensible limits for the number of acceptance tests to be carried out. The mandatory duct assembly verification test (Clause 3) is intended to demonstrate that the system has been correctly assembled. If the system fails to meet the test criteria, it should be dismantled, any damaged items replaced, and the system reassembled and retested. If it still fails to comply, sealing joints with a suitable sealant can improve matters. Acceptance would then be subject to the Engineer’s decision on the results of retesting. Consideration should be given to performance of the duct system under full grouting pressure. Appendix A describes additional tests for measuring the effectiveness of seal of the duct system, which the Designer may consider adopting in appropriate circumstances. These methods require further experience and development before adoption as a specification requirement. The fluidity of the grout during injection should be high enough to be pumped effectively and to fill the duct adequately, but low enough to expel the air and any water in the duct. The time during which fluidity is maintained will often need to be more than the minimum of 30 minutes given in the Specification and a target of 90 minutes is a sensible upper limit. The grout should be sufficiently stable to bleed very little and so the materials segregate and settle to a minimal extent. It has been observed that the type’of bleed test currently specified in most national codes, and in the first edition of this Report, fails to identify potentially unstable grouts. The important feature missing from these tests is the destabilising effect of the ‘wick action’ caused by the strands. This shortfall has been addressed by LCPC in France by the development of an inclined tube test, which is included in Appendix A7 as an acceptable alternative test. It has also

been addressed in the BRITE Euram Project ‘QA of grouting’ ( ‘ 6 ) , by the development of a 1.5 m vertical test. The latter test is simpler, quicker and more economical, and is incorporated in the Specification. It has also become evident that specifications that require bleed water to be reabsorbed in 24 hours have no logical basis. If bleed water develops, reabsorption will merely create an air void. Likewise, requirements to measure the bleed water after three hours are not necessarily relevant to modem thixotropic grouts. a . The Engineer should adopt a pragmatic approach to the size of acceptable voids in ducts. The 5% limit in the Specification would normally be acceptable at a crest in a duct in which the steel tendons are embedded in grout in the lower part of the duct and the vents are properly filled and sealed, and the surface is waterproofed. The requirements for testing in the Specification (see Table S2) should be included in Appendix 1/5 to the Specification for Highway Works (37). It is recommended that, where the system includes end caps at anchorages intended to be left in place, these are left undisturbed and completeness of grouting is tested by sounding and visual examination of vent holes in order to avoid disturbing the seals.

10.2.5 Grouting Normally, grout injection should not exceed the rate of 10 m of duct per minute. For certain applications, where ducts are outside the normal range of size (i.e. not multi-strand tendons in 80-125 mm ducts), this may be increased to 15 m of duct per minute. To minimise the risk of blockages of pumping equipment or delivery hoses or of lumps forming in the grout, it is advisable to wash out equipment with water at least every three hours. This is especially recommended before grouting very long tendons and in warm weather. In cold weather it is necessary to measure the temperature of the concrete structure (for internal tendons) or the air void around the ducts (for external tendons) to comply with specifications to avoid freezing the grout. Air temperature measurement is straightforward but measuring the temperature of the structure can be more difficult. Recommended procedures are to seal the ducts, say, 12 hours before grouting and measure the air temperature inside the ducts, or to form a small pocket in the concrete, fill it with water, again, say 12 hours before grouting and measure the temperature of this water. Grouting plant should be located as close as practicable to the point of injection to keep supply lines short.

4s

and verification activities as defined in Clause 4.1.2.2 of BS EN I S 0 9002 (66). The details in this Chapter are intended to give guidance to post-tensioning contractors to help develop their quality system procedures in line with the requirements of Concrete Society Technical Report 47, Second edition. CARES and its certification scheme PT2(a)i for The supply and installation of post-tensioning systems in concrete structures are UKAS-accredited and were key elements in the lifting of the moratorium on post-tensioned bridge construction in the UK in 1996. In order to give the customer assurance that the technical requirements and the spirit of TR47 are applied in practice it is essential that the post-tensioning contractor has an appropriate quality system and certification to CARES Scheme PT2(a)i or equivalent. The requirements of the CARES certification scheme have been developed through agreement by relevant interested parties (clients, contractors, specifiers and technical experts) and represent a consensus of opinion. The scheme is kept under review to ensure that it meets industry requirements. The scheme covers both relevant office activities and site practice. The scheme is based on the premise that the post-tensioning contractor will supply all of the post-tensioning system, materials and equipment, the components of which have been proved to be suitable and compatible, and which are correctly installed. I

11.2 BASIC QUALUTY SYSTEM ELEMENTS The CARES scheme relates to the quality system and product requirements for the supply and installation of posttensioning systems in concrete structures using bonded or unbonded tendons in accordance with the relevant product standard andor contract specification. The post-tensioning contractor’s quality system shall be based on the following essential elements covering both office and site activities: Quality system. The post-tensioning contractor shall have a quality system that complies with the requirements of BS EN IS0 9002 (66) and CARES Scheme PT2(a)i.

0

*

46

Resources. The post-tensioning contractor shall identify the resource requirements in a quality plan and provide adequate resources, including trained personnel for the management, supervision and performance of the work

The post-tensioning contractor shall have a documented procedure that details the attendances required for the post-tensioning operations. The provision of attendances shall be agreed between the customer and the posttensioning contractor.

Quality planning. The post-tensioning contractor shall produce a quality plan for each structure, identifying structure-specific details, on which it is contracted to operate. The quality plan shall include method statements for the relevant key post-tensioning activities, e.g. duct installation, tendon installation, tendon tensioning, tendon anchorages, tendon protection and grouting. The quality plan shall also identify the human resources, responsibilities, hold points (and release authorities), processes, materials, equipment, controls, measuring and test equipment, standards and levels of acceptability required to meet the contract requirements.

Contract review. The contract review procedure shall ensure that the responsibilities of all relevant parties are identified and all relevant design details, e.g. posttensioning system, tendon configuration, tensioning requirements, tension increments, grout, grout mixing, grout placing, grout testing, resource requirements, attendances, are clearly, adequately and unambiguously defined. Records of contract review shall be maintained.

Quality records. The post-tensioning contractor shall keep quality records relating to the technical details of posttensioning contracts including site installation records. Traceability. The post-tensioning contractor shall ensure that materials and components are traceable from source to their location within the structure. Purchasing. The post-tensioning contractor shall have a system for purchasing materials and services from subcontractors that includes all aspects of the material or service specification that are important in ensuring satisfactory product quality and identification. The post-tensioning contractor shall be responsible for the provision of post-tensioning system components, grout components and post-tensioning equipment.

Storage. The post-tensioning contractor shall ensure that materials are stored and segregated in a manner that prevents corrosion, damage, deterioration or contamination. Handling. The post-tensioning contractor shall handle materials and equipment in a way that preserves their

Contractors quality scheme requirements

quality and prevents them from becoming damaged, contaminated or corroded.

Inspection and testing. The post-tensioning contractor shall ensure that inspection and testing are conducted in accordance with the quality plan, appropriate standards and contract specifications. Control of non-conforming product. The posttensioning contractor shall ensure that non-conforming work and materials are not used in the works and that they are adequately segregated and identified. Internal quality audits. The post-tensioning contractor shall undertake internal quality audits in order to verify the effectiveness of the quality system, including site activities. Training. It is essential that all post-tensioning operations are carried out by operatives with appropriate knowledge, training and proven experience. The post-tensioning contractor shall: (a) define the categories of on-site personnel, e.g. trainee, operative, supervisor, engineer (b) define the knowledge, skills and experience required for each personnel category (c) evaluate and endorse experience of personnel based on objective evidence such as verifiable training records (d) provide relevant theoretical and practical training (e) determine the level of knowledge and skill attained during training (f) issue statements of achievement that identify the level

of training achieved, the date of issue and the date of expiry.

11.3 PRODUCT REQUIREMENTS Ducts. The post-tensioning contractor shall ensure that: (a) ducts comply with the contract specification (b) ducts are correctly assembled, sealed, installed and adequately fixed to resist movement and floatation during concrete placement

The tensioning procedure shall include the direct measurement of tendon force and load vs. extension measurements for verification purposes.

Anchoring of post-tensioning tendons. The posttensioning contractor shall ensure that tendons &e adequately anchored and that the anchorages and tendons are protected from corrosion and mechanical damage. Anchorages shall comply with the performance requirements of BS 4447 (67) or equivalent. Protection of post-tensioning system components. The post-tensioning contractor shall provide protection for the post-tensioning system components against corrosion, contamination and mechanical damage during and after installation, prior to grouting. The post-tensioning contractor shall give due consideration to the duration and type of exposure to which the post-tensioning system components are likely to be subjected when selecting the method and type of protection. Grout. The properties of the grout shall comply with the requirements of this Specification andor the contract specification. Grout shall be produced using fresh materials only. Before use, the post-tensioning contractor shall assess the grout properties in accordance with the methods specified in this Specification, using the materials, material sources, plant and personnel proposed for use on site. Grout preparation shall be undertaken under the temperature conditions expected on site. The assessment shall be made sufficiently in advance of the grouting operations to allow adjustments to the materials, procedures or equipment.

Grouting trials. The post-tensioning contractor shall undertake full-scale grouting trials, where required by the contract specification, to verify the proposed grouting methods and procedures. Grouting. The post-tensioning contractor shall have a procedure to control the grouting process. The procedure shall include: (a) the planning of resources, supervision, material and the attendances required (b) preparation of the duct

(c) ducts are free from standing water and contamination at all times and are thoroughly clean before grouting

(c) production of grout including the grouting equipment, admixtures and grout components

(d) duct vents are identified and protected from damage.

(e) an assessment of the amount of cemenvgrout

Tendon installation. The post-tensioning contractor shall ensure that tendons are installed safely and without degradation, contamination or damage to the tendon or the duct. Records of any problems encountered during the tendon installation process, e.g. blockages or the use of excessive force, shall be kept. Tensioning. The post-tensioning contractor shall ensure that tendons are tensioned to the correct force in the correct sequence.

(d) the injection of grout and venting of the duct (f) corrective action in the event of blockage or break-

down (g) grouting in cold weather and grouting in hot weather (h) tests to measure the grout properties including: fluidity (immediately after mixing and at the end of the injection period), volume change, bleeding, segregation and compressive strength (i) sealing of ducts and vents.

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Durable post-tensioned concrete bridges

11.4 CERTIFICATION The details in this Chapter are only intended to give guidance to post-tensioning contractors to help develop their quality system procedures in line with the requirements of TR47. In order to give assurance of full compliance with TR47, posttensioning contractors shall have an appropriate quality system certified by CARES to CARES Scheme PT2(a)i or equivalent.

Further informatjon is available from: I

,

UK CARES Pembroke House, 21 Pembroke Road, Sevenoaks, Kent, TN13 IXR, UK. Tel: +44(0)1732 450000, Fax: +44(0)1732 455917 E-mail: [email protected], www.ukcares.com

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12 SPECIFICATION FOR DUCT AND GROUTING SYSTEMS FOR POST-TENSIONED TENDONS This specification should be read in conjunction with the Notes for guidance in Chapter 10. It is recommended for reproduction with specifications for post-tensioned concrete bridges; and reference should be made to Concrete Society Technical Report 47, Second edition, 2002. An electronic version is available on request.

CLAUSE 1

PLANNING, TRIALS AND BASIC REQUIREMENTS Site operations that form part of this Specification shall be carried out by organisations certificated by CARES in accordance with the requirements of the CARES scheme for The supply and instullation of post-tensioning systems in concrete structures, outlined in Chapter 11 of Concrete Society Technical Report 47, Second edition, or equivalent scheme. Grout for protection of prestressing tendons shall be special grout as required in Appendix 17/X of the Specification for Highway Works (reproduced as Annex 1 to this Specification) and defined in Clause 2. The contractor shall provide verification of the grout mix and of the grouting operations, for duct installation, testing, concreting, grouting, and any other requirements, in accordance with the details described in Appendix 17/X, by full-scale trials or by appropriate relevant previous experience, as required in the Contract. The trials are required to demonstrate that the grouting methods and procedures proposed by the Contractor will ensure that grout fills the ducts and surrounds the prestressing steel. The contractor shall submit a detailed method statement, at least 4 weeks prior to use in any trials or in the works, covering proposed materials, ducts, anchorage and vent arrangements, personnel, equipment, grouting procedures and quality control, for the approval of the Engineer. Where full-scale trials are required, these shall be started at least 56 days before the planned commencement of fixing ducts for prestressing for the permanent works, unless specified otherwise in Appendix 17K. The trials shall incorporate all relevant details of ducts, vents, duct supports, deviators, prestressing anchorages and couplers, prestressing strands, grout inlets and outlets. The tendons shall be sufficiently tensioned that the strands within the duct take up a representative alignment. All systems, methods and materials are to be those proposed for the permanent works and shall have been submitted to the Engineer as part of the detailed method statement required. Grouting and testing shall be carried out in accordance with the Specification. After 3 days the contractor shall carefully cut or core the trial section to expose cross-sections and longitudinal sections of the duct, anchorages and any other locations where required or as further directed by the Engineer to demonstrate that the duct is satisfactorily grouted. The contractor shall prepare a report, giving full details of the trial, testing results and photographs of the exposed sections. Grouting of the ducts shall be shown to leave no void that either has a radial dimension greater than 5% of the diameter (or appropriate dimension in the case of oval ducts, anchorages, etc) or poses a risk to the protective system. The location of the voids with respect to grout vents and their adequate grouting and subsequent sealing, and the disposition of the steel strands within the body of the grout shall be reported. Prestressing for the permanent works shall not be permitted without the written approval of the Engineer to the grouting procedures and formal acceptance of the results of the grouting trial. Irrespective of whether the Contract requires full-scale grouting trials, the contractor shall carry out a materials suitability assessment in accordance with Clause 2. The Contractor may propose alternative procedures for grouting, in which case these shall be demonstrated by full-scale trials to the satisfaction of the Engineer.

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Durable post-tensioned concrete bridges

CLAUSE 2

GROUT MATERIALS The properties of the grout, made with the materials, and using the plant and personnel proposed for use on site, shall be assessed for suitability for the intended purpose sufficiently in advance of grouting operations to enable adjustments to be made in the use of materials, plant or personnel. All retrials shall be at the contractor’s expense. Grout shall comply with the requirements in Clause 8. The materials’ assessment shall consist of the preparation of the grout, made with the materials, and using the plant and personnel proposed for use on site, and the testing of it in accordance with Clause 9. The preparation shall be carried out under the conditions of temperature expected on site. If grouting operations are likely to cover different seasons, the assessment shall include the range of expected temperatures. The sources of materials and procedures approved as a result of satisfactory trials shall not be departed from without the written approval of the Engineer. Grout shall consist of pre-bagged products complying with the above but only requiring a measured amount of water together with controlled admixture to be added on site. Where the Contract allows grout to BS EN 447 (Common Grout), unless otherwise agreed as a result of trials, the grout shall consist only of Portland cement complying with BS 12 Class 42.5N, admixtures complying with Clause 10 and water complying with BS 3148. Cement type CEMl to BS EN 197-1 is also acceptable. The materials used shall be such that the chloride ion content of the grout shall not exceed 0.1 % by mass of the cement.

CLAUSE 3

UCT SYSTEMS The system of ducts, duct connectors, grouting connections, vents, vent connections, drains, transitions to anchorages and deviators and caps for anchorages shall form a complete encapsulation for the tendons which is resistant to the passage of air and water. Ducts shall be of proven corrosionresistant durable material such as high-density polyethylene or polypropylene. Ducting that may degrade or corrode during the expected life of the structure in the presence of contaminants permeating the surrounding concrete is not permitted. The system shall be fully compatible with the prestressing anchorages, couplers and other details. Where ducts are non-conductive, metal parts of anchorages shall be electrically bondedato the adjacent reinforcement at each end of the tendon and the electrical continuity of the structure over the length of the tendon shall be tested. The following air-pressure tests shall be carried out on site unless specified otherwise in Appendix 17K:

Duct assembly verification tests Each complete duct system (including vents, anchorages, anchorage caps, and where appropriate couplers and their connections), shall be air-pressure tested before concreting (or after concreting in the case of segmental construction), to a pressure of 0.1 bar (10 kPa) unless otherwise specified in Appendix 17/X, to demonstrate that the system is undamaged and has been correctly assembled. The testing shall demonstrate that a loss of pressure no greater than 10% occurs after 5 minutes, unless specified otherwise in Appendix 17K. The minimum manufactured wall thickness of ducting for internal tendons shall be 2 mm. The duct rigidity and the type and spacing of fixings and supports shall be such as to maintain line, position and cross-section shape during concreting. Local deformation of the duct at supports shall be avoided. For external tendons the minimum wall thickness shall be 4 mm for durability or such thicker wall as required to withstand the grouting pressures (normally 6 bar (600 kPa)) or the particular duct configuration. The contractor shall provide evidence of testing to demonstrate the following requirements: (a) The wall thickness of ducts after tensioning the tendons shall be not less than 1.5 mm, unless specified otherwise in Appendix 17K.

50

Specification for duct and grouting systems for post-tensioned tensions

(b) For internal tendons the duct shall transmit full bond strength from the tendons to the surrounding concrete over a length no greater than 40 duct diameters or such other requirement as given in Appendix 1 7 K . This bond length requirement applies to the increase in load from SLS (after losses) up to ULS. (c) The duct system shall comply, as a minimum, with the fib recommendations for Plastic ducts for internal bonded prestressing tendons, and with any more onerous physical requirements of this Specification. Vents providing an air passage of at least 15 mm internal diameter shall be provided at the anchorages and in the ducts at troughs and crests and beyond each intermediate crest in the direction of grout flow at the point where the duct is one half diameter lower than the crest (but not further than 1 m) unless described otherwise in Appendix 1 7 K , and elsewhere, as required by the Engineer. The maximum spacing of vents shall be 15 m unless specified otherwise in Appendix 17K. The vent diameter and spacing may be varied in full-scale trials demonstrating the suitability of alternatives. The vents shall be rigidly connected to the ducts and shall be capable of being closed and re-opened. Holes in the ducts shall be at least the internal diameter of the vents and shall be formed before pressure testing. All ducts shall be kept free from standing water at all times and shall be thoroughly clean before grouting. For external tendons the arrangement and detailing of vents at positions within deflectors/ diaphragms shall have been proven by detailed testing.

All anchorages shall be sealed by caps and fitted with grouting connections and vents. Sealing of anchorages shall permit the flow of grout through the anchor head. Vents on each duct shall be identified by labelling and shall be protected against damage at all times. Vents at high points shall extend to a minimum of 500 mm above the highest point on the duct profile unless specified otherwise in Appendix 1 7 K . The Contractor shall comply with any additional requirements given in Appendix 1 7 K .

CLAUSE 4

GROUT1NG EQUlBRA ENT Grouting equipment shall consist of a mixer plus a storage reservoir (or two mixers in parallel) and a pump with all the necessary connection hoses, valves, measuring devices for water, dry materials, admixtures and testing and weighing equipment. The mixing equipment shall be capable of producing a grout of homogeneous consistency and shall be capable of providing a continuous supply to the injection equipment. The capacity of the equipment shall be such that the duct can be filled and vented without interruption and at the required rate of injection. The injection equipment shall be capable of continuous operation with little variation of pressure and shall include a system for recirculating the grout while actual grouting is not in progress. The equipment shall have a constant delivery pressure; it shall be equipped witha two pressure gauges. and a pressure relief valve to prevent pressures above 1 N/mm2. All piping to the grout pump shall have as few bends, valves and changes in diameter as possible and shall incorporate a sampling Tee with locking-off valve. The equipment shall be capable of maintaining pressure on completely grouted ducts and shall be fitted with a valve that can be locked off without loss of pressure in the duct. During the grouting operation the contractor shall provide adequate flushing-out plant to facilitate complete removal of the grout in the event of a breakdown of the grouting equipment or other disruption before the grouting operation has been completed and shall demonstrate that this equipment is in full working order. All equipment shall be kept free from build-up of adhering materials by washing as required.

CLAUSE 5

BATCHING AND MIXING OF GROUT ,

I

All materials shall be batched by mass except the mixing water and liquid admixtures, which may

51

Durable post-tensioned concrete bridges

be batched by mass or volume. Bagged materials shall either be weighed before opening or shall be clearly weight-marked with stated tolerance. The accuracy of batching shall be, or have been (in the case of pre-bagged materials): . f

2% for dry materials, cement and admixtures

* 1% for mixing water of the quantities specified. The mixing water shall include the water content of liquid admixtures. Depending upon environmental or material influence (e.g. temperature, configuration of the tendon and properties of the materials used), the waterkement ratio shall be kept as low as possible having regard to the required plastic properties of the grout. The waterkement ratio shall be recorded. The material shall be mixed to produce a homogeneous grout and kept in slow continuous agitation until pumped into the duct. Unless manufacturers specify otherwise, water shall be added to the mixer first, followed by the dry materials which may be added as a whole or in part in sequence until the total quantities are added. The minimum mixing time determined from grouting trials shall be adhered to. The temperature of freshly mixed grout shall be between 5°C minimum and 30°C maximum. The maximum temperature may be increased provided trials demonstrate that the grout meets the requirements of Clause 8.1.

CLAUSE 6

INJECTING GROUT A check shall be made to ensure that the ducts, vents, inlets and outlets are capable of accepting injection of the grout. This shall be done by blowing through the system with dry, oil-free air and testing each vent in turn. Any water in the ducts shall be removed before grouting starts. Grouting of the ducts shall be carried out within 28 days of installation of the tendon or as soon as is practicable thereafter, in which case additional measures shall be taken to avoid corrosion of the prestressing steel. The Engineer’s written agreement to start shall be obtained. Injection shall be continuous, and it shall be slow enough to avoid causing the grout to segregate. Except in exceptional circumstances, grout shall only be injected from one end of a tendon. The method of injecting grout shall ensure filling of the ducts and complete surrounding of the steel. Grout shall be allowed to flow from each vent and the remote end of the duct until its fluidity is equivalent to that of the grout injected, by visual acceptance. In the event of disagreement, testing may be carried out in accordance with Clause 8. Following this, a further 5 litres at each vent, or such other requirement of Appendix 17/X, shall be vented into a clean receptacle and then discarded. The opening shall be firmly closed. All vents shall be closed in a similar manner one after another in the direction of the flow except that at intermediate crests the vents immediately downstream shall be closed before their associated crest vent. The injection tubes shall then be sealed off under pressure. A pressure of 5 bar (500 kPa) shall be maintained for at least one minute. Grout vents at high points shall be reopened immediately after, while the grout is still fluid, and any escape of air, water or grout recorded and reported immediately to the Engineer. A further pumping of grout shall then be carried out to expel bleed water and/or entrapped air. This shall be carried out with the vents open one at a time sequentially in the direction of grouting and a further 5 litres shall be vented at each open vent. A visual inspection of the vented grout shall be carried out and, if there is any doubt about its quality, bleed and fluidity testing shall be carried out immediately. The injection tubes shall then be sealed off under pressure. A pressure of 5 bar (500 kPa) shall be maintained for at least one minute. The filled ducts shall not be subjected to shock or vibration within 24 hours of grouting.

52

Specifcation for duct und grouting systems for post-tensioned tendons

When the grout has set, the grout vents shall be temporarily reopened. If voids are apparent on inspecting vents at end caps, the end caps shall be removed to demonstrate that they are satisfactorily filled with grout. End caps, which have been removed, shall then be replaced by end caps permanently sealed against ingress of contaminants, such sealing to be proved to the satisfaction of the Engineer. If the anchorage caps are removed a photographic record shall be taken, clearly identifying the individual anchorages, and included in a report to the Engineer. If, in the opinion of the Engineer, there is cause for doubt that the ducts or any part of the system are not satisfactorily filled with grout, the Engineer may require investigations to be carried out. The contractor shall keep full records of grouting for each duct in accordance with the certification scheme requirements for installation of post-tensioning. Copies of these records shall be supplied to the Engineer within 24 hours of grouting. Grout vents shall be positively sealed to be waterproof on completion of grouting by a means separate from the concrete waterproofing.

CLAUSE 7

GROUTING DURING COLD WEATHER When the ambient temperature is expected to fall below 5"C, accurate temperature records shall be kept of the maximum and minimum air temperatures, and the temperatures of the structures adjacent to the ducts to be grouted. No materials in which frost or ice is present shall be used, and the ducts and equipment shall be completely free of frost and ice.

No grout shall be placed when the temperature of the structure adjacent to the ducts is below 4 ° C or is likely to fall below 4°C during the following 48 hours, unless the member is heated so as to maintain the temperature of the placed grout above 5°C for at least 48 hours. Methods of heating shall be to the approval of the Engineer. Ducts shall not be warmed with steam.

CLAUSE 8

PROPERTIES OF GROUT

Clause 8.1

Fluidity When tested by the method given in Clause 9, the fluidity of the grout shall meet the criteria given in Table S1. Additionally, the fluidity (flow cone passage time) at outlets shall not vary from that of the injected grout by more than 20%. Table S I : Test requirements for fluidity of grout. ~

Test method

Fluidity immediately after mixing

I

Cone 65%

Variations in temperature and humidity on site are likely to cause variations in the test results and shall be reported. The temperature of the freshly mixed grout and the ambient temperature and humidity shall be given in all test reports.

Clause 9.2

Fluidity test Principle The fluidity of the grout, expressed in seconds, is a measure of time necessary for a stated quantity of grout to pass through the orifice of the cone, under stated conditions.

Apparatus The following apparatus is required for the test: (a) Cone

A cone of the dimensions given in Figure S1. It shall be robust and manufactured from materials not reactive with any materials specified.

(b) Sieve

The sieve aperture shall be 1.5 mm and the sieve shall be fitted as shown in Figure S 1, and shall be removable.

(c) Stopwatch The stopwatch shall be accurate to 0.2 s in 60 s and shall show time to 0.1 s. (d) Cylinder

A graduated cylinder of minimum I litre capacity and diameter in the range 60 mm to 150 mm.

Test procedure Mount the cone with its axis vertical and its largest diameter uppermost. Fix the sieving medium at the position indicated in Figure S1. During the test prevent the cone from vibrating. Place the cylinder under the cone outlet. All surfaces of the cone shall be clean and shall be dampened so that the surfaces are moist but without free water. Close the lower cone orifice. Pour the grout through the sieve to fill the conical section of the cone. Pouring shall be sufficiently slow to prevent the build-up of air in the grout in the cone. Open the lower cone orifice and at the

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~

Specification for duct and grouting systems for post-tensioned tendons

280? 10

All dimensions in millimetres Dimensions are internal

Figure S1: Cone for fluidity test

same time, start the stopwatch. Measure the time taken to the nearest 0.5 s to fill the cylinder to I litre. The presence of lumps on the sieve shall be reported. For suitability testing, three tests shall be carried out, the first immediately after the grout is mixed and the remaining two tests at the estimated time to grout a duct or a minimum of 30 minutes after the grout is mixed. The grout shall be kept agitated while awaiting testing.

Reporting of results Report the time to the nearest 0.5 s. Report also the presence of lumps. Report the result as the average of the times determined in the second and third tests, separately from the result of the first test. Report the average time to the nearest 0.5 s. For acceptance testing, the test shall be performed on grout from each anchorage outlet and from the mixer at the start and end of grouting of each duct.

Clause 9.3

Bleeding test

Principle The test consists of measuring the quantity of water remaining on the surface of the grout which has been allowed to stand protected from evaporation.

Apparatus (a) Vertical duct of transparent material with one end sealed: Internal diameter,

di

=

Height,

h

=

60 to 100 mm about 1.5 m

Effective diameter, d

=

about 16 mm

1

=

1.4 m

Number of strands, N

=

sufficient to fill about 30% of cross-sectional area of the duct

(b) Prestressing strands: Length,

i.e. 3Nd2 =

di2

55

Durable post-tensioned concrete bridges ~

~~

~

Test procedure Set up the rigid transparent duct in a vertical position with its open end at the top. Provide rigid fixing preferably on a concrete floor so that no movement or vibration can occur. Install the strands in the duct, ensuring that they are all firmly located on the base. Strands shall be wiped clean before placement to remove oil or rust. Immediately after mixing pour the grout into the top of the duct at a steady rate to ensure there is no trapped air. Fill the duct to a height, h,, about 10 mm above the tops of the strands. Put a cap on to the top of the duct to minimise evaporation. Record the starting time and height of the grout; with an external vernier height gauge or using a scale fitted to the duct. Measurements should ignore the meniscus on the grout. Record the height of'grout, h,, at 15-minute intervals for the first hour and subsequently at 30minute intervals until three consecutive results are similar. Take further readings at 24 hours. NB: The purpose of recording at intervals instead of simply taking the final value is to be able to follow the behaviour of expansive grouts. If the grout has been left for a time before being poured into the duct, expansion may be completed and measured behaviour will be affected. Record the height of bleed water, hw, at the same times as the grout. Record any inhomogeneities that may develop in the appearance of the grout as seen through the transparent duct. Examples of inhomogeneities are:'. '

- formation of lenses of bleed water below the top of the grout - segregation leading to areas of different coloured grout. Reportingof results h-w- ~ 1 0 0 % h.." The report shall state the maximum and minimum air temperatures and the grout temperature at the time of test, the bleeding at 3 hours and at 24 hours. Bleeding is expressed as:

For acceptance testing one test per 1.5 m3 of grout or a minimum of one test per day shall be performed.

Clause 9.4

Volume change test Principle The volume change is measured as a percentage of the volume of grout between the start and the end of the test. The test measures mainly the volume change caused by segregation, contraction or expansion. The volume change test may shall be a continuation of the bleeding test.

Apparatus As for the bleeding test in Clause 9.3.

Test procedure As for the bleeding test in Clause 9.3.

Reporting of results h -h, x 100% ho The report shall state the range of air temperatures during the test and the volume change. Volume change is:

-g-

For acceptance testing one test per 1.5 m3 of grout or a minimum of one test per day shall be performed,

Clause 9.5

Sedimentation test Principle The sedimentation is measured as a percentage difference in density of the grout between the samples taken from the top and bottom of the test cylinder,

56

--

Specifcation for duct and grouting systems for post-tensioned tendons

Apparatus A transparent graduated cylinder 40 mm to 60 mm internal diameter and greater than 175 mm in height.

Test procedure Place the cylinder on a horizontal surface free from shocks or vibration. Fill it with grout to the top and seal the cylinder to prevent evaporation. At least 24 hours after filling the cylinder remove the grout column intact from the cylinder. Cut the grout column into four approximately equal segments. Keep the segments immersed in water. Measure the density of each segment by an approved method.

Reporting of results Sedimentation is given by the ratio of the density of the segment from the top divided by the density of the segment from the bottom. The ratio of the density of the mid-top segment divided by the density of the mid-bottom segment shall also be reported. For acceptance testing the test shall be carried out twice for each different supplier’s batch of dry materials subject to a minimum of one test for each grouting operation. The testing requirements are summarised in Table S2. Table S2: Minimum test requirements for grout.

1. Suitability testing Sampled immediately after mixing, one test. After estimated time to grout duct or minimum of 90 min from initial mixing. Two tests averaged in both cases.

Fluidity

Bleed Volume change Sedimentation Strength

I Each sampled immediately after mixing, 3 tests averaged.

I

2. Acceptance testing

*

*

Fluidity

Sampled immediately after mixing, one test from mixer. After flow through duct, one test from each anchorage outlet. On completion, one test from mixer.

Bleed Volume change Strength

One test per day or one per 1.5 m3 of grout, unless specified otherwise.

Sedimentation

One test per day for site-mixed grout, or one test per pre-bagged supplied batch (by manufacturer’s reference numberi subject to a minimum of one test per continuous grouting operation.

For large projects with extensive grouting requirements these sampling rates may be reduced.

CLAUSE 10

ADMIXTURES

Clause 10.1

General Admixtures shall be used where required to achieve a low waterlcement ratio and impart good fluidity, minimum bleed and volume stability or expansion to the grout to comply with Clause 8. For site-mixed grout, admixtures shall be added on site during the mixing process and used in accordance with the manufacturer’s recommendations. For pre-bagged grout they shall form a preblended component.

Clause 10.2

TYPes Admixtures are divided into two types - expanding and non-expanding. Expanding grout admixtures are supplied as powders that expand to ensure that there is no overall decrease in the volume of grout at the end of the hardening period.

57

Durable post-tensioned concrete bridges

Non-expanding grout admixtures are supplied in liquid or powder form. Both types of grout admixture may also permit a reduction in waterkement ratio, improve fluidity, reduce bleeding and retard the set of the grout. Admixtures may be used to obtain the required grout performance. Admixtures used in combination shall be checked for compatibility.

Clause 10.3

Chemical composition Admixtures shall not contain substances in quantities that will adversely affect the grout or cause the grout to promote corrosion of the prestressing steel by rusting, pitting, stress corrosion or hydrogen embrittlement.

Clause 10.4

Material requirements The admixture shall not segregate and shall be uniform in colour. The composition shall not change and the supplier shall operate a quality system complying with BS EN I S 0 9001 or BS EN IS0 9002. The quality system shall be certified by a third party accredited by the United Kingdom Accreditation Service (UKAS). Where appropriate, admixtures shall comply with BS EN 934-4. Other admixtures shall be permitted provided they satisfy Sections 6, 7.1 and 7.3 of BS 5075: Part 1 and full account is taken of their effects on the finished product and their fitness for purpose. Data on their suitability, including previous experience with such materials, shall be made available and records of the details and performance of such materials shall be maintained.

Clause 10.5

Dosage The optimum dosage of any admixture shall be determined by trial mixes with the cement to be used in the grout. This dosage shall be expressed as percent by mass of the cement. It shall be within the range recommended by the supplier and shall not exceed 5% by mass of the cement. The method of measuring dosage and checking weights of pre-packed dry materials shall comply with Clause 5 or as otherwise agreed with the Engineer.

PUBLICATIONS REFERRED B

IN THE SPECIFICATION

BRITISH STANDARDS INSTITUTION. BS 12: 1996 Specification for Portland cement. 20pp. BS 1881. Testing concrete. (Various parts) BS 3 148: 1980 Methods of test for water for making concrete (including notes on the suitability of the water). 4pp. BS 5075: Part 1: I982 Concrete admixtures. Specification f o r accelerating admixtures, retarding admixtures and water reducing admixtures. 22pp. BS EN 197-1: 2000 Cement. Composition, specijkations and conformity criteria for common cements. 50pp. BS EN 447: 1997 Grout for prestressing tendons. Specification for common grout. 1Opp. BS EN 934-4: 2001 Admixtures for concrete, mortar and grout. Admixtures for pre-stressing tendons. 20pp. BS EN IS0 9001: 2000 Quality management systems. Requirements. 40pp. BS EN I S 0 9002: 1994 Quality systems. Model for quality assurance in production, installation and servicing. 20pp. F6DERATION INTERNATIONALE DU BETON. Corrugated plastic ducts for internal bonded post-tensioning. Lausanne, 2000. Technical Report, Bulletin No. 7. 46pp.

I

I

HIGHWAYS AGENCY. Manual of contract documents for highway works, Volume I : Speczfication f o r Highway Works. The Stationery Office, London, 1992. Manual of contract documents for highway works, Volume 4: Bills of quantities. Section 1. Method of measurement for highway works. The Stationery Office, London, 1998, amended May 2001.,

58

Specification for duct and grouting systems for post-tensioned tendons

ANNEXES TO THE SPECIFICATION

ANNEX 1 (APPENDIX 17/X FOR THE SPECIFICATION FOR HIGHWAY WORKS) CONCRETE - DUCT AND GROUTING SYSTEMS FOR POST-TENSIONED TENDONS TENDON REFERENCE: (complete this for each different group or type of tendons) GROUT DEFINITION: Grout type:

[Special]

REQUIREMENTS FOR TRIALS/TESTS: Full-scale grouting trials required: [NoNes] Drawing Reference: ooo/oo/oo (Full details including location of cuts to be defined on drawing) Time at which trials are to be carried out (days before planned use in the permanent works): [56 days] (Optional additional testing requirements to prove protection against ingress of contaminants are given in Chapter 10 of Concrete Society Technical Report 47 (2002)). Availability of compliant duct components should be discussed with manufacturers. Required duct assembly testing pressure: Permitted pressure loss in 5 minutes: Minimum duct wall thickness as manufactured: Minimum duct wall thickness after tensioning: Maximum duct to concrete ultimate bond length:

[O.l bar] [10 kPa] [ 10%] [2.0 mm internal] [4.0 mm external] [ 1.5 mm] f i b requirements]

REQUIREMENTS FOR DUCT SYSTEM: Distance beyond crests to next vent:

Maximum vent spacing: Minimum vent height above surface: Other requirements:

[At the point where the duct is one half diameter lower than at the crest, but not more than I m] [I5 ml [500 mm]

[-I

REQUIREMENTS FOR GROUTING: Minimum volume of grout expelled after visual test:

[5 litres]

[NOTE: Default values shown in brackets]

59

Durable post-tensioned concrete bridges

ANNEX 2 SUGGESTED AMENDMENTS TO THE METHOD OF MEASUREMENT FOR HIGHWAY WORKS

'

In-Situ Post-Tensioned Prestressing for Structures Units

Itemisation

33

Delete (i) and add

(i)

tendons, stressing and grouting, protective covering to external tendons, grouting trials . . . . . number.

36

Add the following feature to Group

Group

Feature

I

5 Grouting Trials

Stressingand 38 Grouting Internal Tendons, StresSingExternal Tendons and Final Stressing and Grouting Tendons of Members supplied Partially Prestressed

Amend item (e) to read as follows:

Item Coverage (e)

in the case of internal tendons, grouting, sealing vent holes and end anchorages, treating ends of tendons and filling anchorages and jack seating recesses with in 'situ concrete (as this Series paragraph 5);

Grouting Trials 40

The items for grouting trials shall be in accordance with the Preambles to Bill of Quantities General Directions include for: Excavation in any material (as series 600 paragraphs 17, 18, 19 and 23); Concrete (as this series paragraphs 4 and 9); Formwork (as this series paragraph 14); Reinforcement (as this series paragraph 25); Tendons (as this series paragraph 36); Stressing and grouting (as this series paragraph 37); Protective covering to external tendons (as this series paragraph 38); Pressure testing of system; Cutting and sectioning of member; Reporting of results to Engineer; Retrials as instructed by Engineer where unsatisfactory; Breaking up and disposal.

60

I -

APPENDIX A -----TEST METHODS

This Appendix gives further information on some of the tests referred to in the main part of the Report. The tests described are those that have been developed for the quality control of grouted post-tensioned concrete, as information on these is less likely to be readily available than on test methods of more general application. In general the tests required have been referred to in the Specification and these additional tests should only be considered for special situations.

Pal

LEAK 81GHTNESS TESTS R DUCT SYSTEMS

Tests for the leak tightness of duct systems are appropriate in the following situations: *

type-approval of duct systems

a

trial ducts and duct assembly verification of works ducts, both immediately after assembly and after concreting

e

surveys of grouted ducts prior to re-grouting.

The tests comprise the application of air or water pressure to the duct,'and measurement of the leakage by either loss of pressure or leakage flowrate. Both of these tests are described below.

Air test for loss of pressure The normal test for duct assembly verification is the application of a pressure of 0.1 bar (10 E a ) , with a requirement that the loss of pressure should be no greater than 10%after 5 minutes. This test has the merit that it has been used on many contracts, and the criteria can be met by the duct systems currently on the market. It has been reported that the test is sensitive to a very small hole in the duct, but the size of the hole, or whether contamination can gain ingress though such a hole are not known. The test effectively measures the ratio of the total leakage to the duct volume, and does not provide any measure of the absolute value of leakage, which may be concentrated at one location. The test is generally conducted at a pressure that is just 2% of the pressure applied .to the duct during grouting. Research is required to establish whether or not this pressure is sufficient to demonstrate sealing of the duct against ingress of contaminants. The method is not suitable for testing of ducts prior to re-grouting because the void volume within these ducts is so variable.

This test has the merit that an absolute measure of total leakage is obtained, and the duct can be characterised for leakage over a range of pressures. The effect of movement of the duct, or sealing of a known leak, can also be correlated with the immediate effect on leakage rate. The method is suitable for the survey of existing ducts prior to re-grouting. . The air leakage at a point that still provides sufficient encapsulation has not been established at the present time, but can be more easily investigated if the leakage flowrate and pressure combinations are known. The limitation of the method is that it has not been used on a range of duct types to establish achievable standards.

Water test for leak tightness Thefib proposal for testing of plastic ducts (17) includes a water test on a 1100 mm-long sample of duct including a coupler. The specimen is subjected to specified bending before testing with water, internally and then externally, at a pressure of 0.5 bar (50 Wa). Leakage and its location under each test are noted. This test has the advantage that it tests directly one of the most important properties of the duct system, its ability to withstand ingress of water that may contain chlorides. The higher test pressure of 0.5 bar (50 E a ) and testing of a duct that has been bent also give greater reassurance than a lower pressure air test. Unfortunately the test at present does not appear to have any criteria for acceptance. Neither is there any accompanying air testing that can be used to correlate with site testing, which will of necessity be with air. If the test were to be repeated on a variety of duct systems, together with air tests for leakage flowrate, the data thus gathered would enable this test to become of greater value.

A2

GROUT STIFFNESS TESTS

Air test for leakage flowrate

One of the greatest risks to the durability of tendons arises from lack of protection by grout due to the presence of voids. These voids are either gas trapped within the fluid grout at the time of grouting or formed after bleed water has been reabsorbed. A method of measuring the total volume of void has been developed which exploits its compressibility within an otherwise incompressible fluid. The method was developed during a number of research projects, and the background is more fully described in Reference 27.

An alternative method of measuring the leak tightness of ducts is to apply a variable air pressure, and measure the rate at which air leaks from the duct over a wide range of pressures. This requires a sensitive electronic flowmeter to provide accurate results at the low air flows involved.

Research using this apparatus has led to a greater understanding of the sources of voids within grouted ducts. The mixing process first entrains air, and this gradually rises and escapes from the fluid. Subsequent venting will remove most of this air, but air reaching the surface after venting will

~

61

Durable post-tensioned concrete bridges

remain as a void. If raised pressure is maintained until grout stiffens, bubbles will be compressed with a reduced tendency to rise. Gas-entraining agents incorporated to counteract shnnkage are a further source of trapped gas, and the rate of production of this gas will depend upon the temperature of the mix. Whether this remains as a void will again depend upon the time of production relative to the time of venting, and the maintenance of pressure. Use of the grout stiffness test has demonstrated the presence of a small volume of trapped gas within apparently well-grouted ducts. This is believed to be due to air trapped within strand crevices andor the ribs of ducting. Air within such strand crevices has been found by pressure transmission along a 5 m length of grouted duct. These minor distributed voids are not believed to pose a significant threat to durability. The technique was first developed to measure the volume of trapped gas using apparatus illustrated in Figure Al. (This was called the ‘Spongeometer’ in the first edition of TR47 and in early published papers.) Hydraulic pistons intrude into the main chamber, compressing any trapped gas. The volume of this gas may be computed from the change in pressure. This void measurement technique has now been incorporated within the ‘Oxford grouting quality control system’ as more fully described in Appendix A5. The equipment has been demonstrated to detect deliberate voids at a distance of 60 m, and to provide information on the effect of gas-entraining agents and mixer types. Samples of grout may also be tested at any stage in the grouting process to measure the gas entrained in the material delivered by the mixer. In order to achieve sufficient accuracy allowance should be made for the small movements of piston seals and duct walls. When incorporated within the equipment equipment described in Appendix A5 there are facilities to include these corrections, and to store the results electronically. When the corrections are made, the equipment is also suitable for use on external tendons. An automatic washout facility minimises delay during use. Interpretation of results is enhanced if the system is calibrated when the results of preliminary tests are known from type-approval testing or on trial ducts.

r-

Supply line to grouted duct

Pressure transducers

\

Pressure chamber

tSupply

line from grout pump

Figure A I : Location of grout stiffness pressure chamber within the grouting system.

62

A3

WOiD SENSORS

Sensors that can be installed within or around ducts have been developed for measuring the passage of grout and the height or quantity of grout within a section of duct (16). Two main types of sensor were tested, capacitance and resistance. The capacitance sensors were contained within a sleeve around the duct, and capacitance was calibrated as a function of the height of grout within the duct. Resistance sensors comprised a short section of duct connected to the rest of the duct with heat-shrinkage sleeves. When the duct is grouted, the resistance between two opposing electrodes drops significantly (by a factor of 1000). Void sensors may have a role in quality control of grouting if information is required on the filling of a duct only at a particular location. They do however require installation of specialist equipment and wiring in advance of concreting, and are therefore more likely to be appropriate in a research situation.

UCT PRESSURE SENSORS Injection pressure has conventionally been measured at the grout pump, which is the location on the grout line that experiences maximum variation. The automated quality control equipment described in Appendix A5 measures the pressure with two sensors closer to the point of injection. It is also possible to install pressure sensors along the line of the duct (16). Electrical and pneumatic pressure sensors were used. They were separated from the fluid grout by a rubber membrane, and fitted at the base of a PVC access tube. In this way the sensors could be recovered, although if this procedure were to be adopted for works ducts the access hole would then need to be made good. Duct pressure sensors can be used to provide an additional record for the grouting and information on the pressures in the duct during the first 24 hours after grouting. They are practical and can be withdrawn before the grout sets so that the vents can be filled and sealed in the normal way.

A5 AUTOMATED QUALITY CONTROL SYSTEMS Effective quality control processes require all relevant information to be recorded. This can be used to demonstrate that all procedures have been followed and completed, but also enables problems to be diagnosed and where possible rectified. Grouting of post-tensioned ducts is a process which requires operatives to perform a number of different operations, such as bleeding specified quantities of grout at vents, sealing vents and applying specified pressures. Final quality is sensitive to any departure from the specified requirements. Good grouting is so important for long-term durability that automatic recording and testing of workmanship to demonstrate compliance are justified.

~

-~

Test methods

The equipment described in Section A2, and originally called the ‘Spongeometer’, has now been further developed, with the support of the Highways Agency and British Cement Association. The device is now known as the ‘Oxford grout quality control system’. The method of measuring the volume of trapped gas remains the same as the equipment described in Appendix A2, but other parameters are also measured, and all information is automatically recorded. The following features are now included: 0

0

0

0

0

0

0

0

0

0

selection of automated test sequences to output the volume of trapped gas within either grout samples or grouted ducts analysis of test results by statistical means with immediate display of results to the operator automatic correction for movement of seals at joints of ducts, enabling use on external tendons

a facility to signal to the grout mixing operative at a remote location the requirement for further grout to be pumped to the duct dual measurement of grout pressure to provide warning of any discrepancies in measurement of this vital parameter recording grout pressures, grout temperature, ambient temperature, and flowrate of grout at any specified time intervals automatic washout of the pressure cylinder without opening to minimise interruption to the grouting process electronic control of all valves to speed use automatic sequencing of operations by preset combinations, operated from a display screen storage of all results for the production of a report when grouting is finished.

An outline of the instrumentation for the system is shown in Figure A2. Use of equipment of this type can demonstrate that the required quality has been achieved, as well as provide an incentive to operatives who will be aware that their work is under constant surveillance. The final report produced from the stored information provides a permanent record of the grouting operation. As the equipment is used on a variety of contracts and duct systems, technical data on grouting will be accumulated. This will advance knowledge of the technicalities of grouting, and enable further improvements to be made.

Chamber wash unit

1

Grouted

ki,,, i Pressure chamber

Grout mixer and pump

Drain valves

Computer control unit

I I

///

!

, /

Recorded continuous

o Time o Pressures e Flowrates o Grout temperature o Ambient temperature

test data

Volume of grout trapped within ducts or grout samples

,, \

\

\

x

Operation sequences controlled at remote panel

e Fill chamber e Test grout sample e Fill duct o Test duct o Washout chamber

Figure A2: Instrumentation within the Oxford grout quality control system.

The following methods have been used:

Boyle’s law vacuum method The duct is evacuated to the maximum vacuum that can be achieved, and the pressure measured. A valve is then operated to connect the evacuated duct to the top of a Perspex tube, which in turn is connected to a water reservoir. The water is drawn into the Perspex tube by the vacuum, and the volume of void within the duct is equal to the volume of raised water after correction for pressure by Boyle’s law. (PV = constant)

Boyle’s law pressure method

One of the most important parameters influencing successful re-grouting of ducts is the volume and disposition of the void to be filled. There are several methods by which the volume can be measured, each of them involving air pressures. They all require a hole to be drilled to intersect the duct, preferably at the top. A pressure-tight connection to the drilled hole is then required, such as by a resin seal or expansion device within the hole.

A steel container is first pressurised to a known value. (A fire extinguisher casing has been used for this purpose.) A valve is then operated to connect this pressurised container to the duct void, and the new pressure is noted. On the assumption that the duct void is sealed, the volume of void may be calculated by Boyle’s law. ( P V = constant)

The ‘Oxford void volume measurement equipment’ This proprietary equipment determines the void volume by the electronic timing of pressure changes resulting from a

63

Durable post-tensioned concrete bridges

particular leakage rate. The leakage can thus include any leakage'from the duct arising from cracks or orifices. Selection of the most appropriate method depends upon the particular circumstances of the test. If the void is well sealed either the Boyle's law vacuum method or the pressure method can give accurate results. However, the vacuum method is more cumbersome, and the application of vacuum has the potential danger of drawing moisture into the duct. The pressure method is relatively cheap, although the result will not be accurate for voids that are very significantly larger or smaller than the pressurised container. The Oxford void volume measurement equipment produces more accurate results, and remains effective for ducts with moderate leakage. The benefits are greatest if the voids are much smaller or larger than the pressurised container. Voids that leak through cracks, and are thus most vulnerable to tendon corrosion, can still be measured. The method is particularly appropriate for surveys in advance of re-grouting, where detailed knowledge of small voids and leaks is essential if grouting is to be undertaken with confidence that the voids will be filled.

A7

STABILITY BLEEDING TEST (INCLINED TUBE TEST)

Objective This .test determines the bleed properties and stability of grout that has successfully passed preliminary testing, at full scale and including the filtering effect of strands. It also allows the' proposed grouting procedures to be confirmed; in particular the effect of time between ending an initial grouting and starting of relgrouting on site, if'specified, and the equipment used on site. The purpose of the test is to confirm that a duct on site can be completely filled using the proposed grout, equipment and procedure without excessive .. . bleeding. '

Test method In a first test phase, the bleed water and air accumulated on top of a tube filled with grout should be determined. The grout is injected under pressure and is setting such that water losses due to evaporation are prevented. In a second phase, the effect of re-grouting of a tube on bleed water and air accumulated can be determined, if such a procedure is envisaged.

Test equipment and set-up Two transparent PVC tubes, of approximately 80 mm diameter and 5 m long, equipped with caps at each end including a grout inlet at the lower end, and a grout vent at the top. The tubes should be able to sustain a grout pressure of at least 10 bar (1000 kPa). 12 prestressing strands per tube, each of approximately 16 mm diameter, i.e. a total of 24. Grouting equipment as per the grouting method statement. A thermometer with automatic recording.

64

Test procedure The two tubes are fixed on their supports such as to avoid noticeable deflections, at an inclination of 30 f 2" against a horizontal reference line. Twelve strands should be installed in each tube. The caps are subsequently installed on the tube ends (fixed with glue), see Figure A3. The grout is prepared as per the grouting method statement. Specimens should be taken from the grout mix to confirm flow time. In case of a thixotropic grout, other methods may be used.

Grouting offrst tube Grout is injected into the first tube (Tube 1) from the bottom end. When the grout exits from the vent at the top with the same consistency as it enters at the bottom, the valve should be closed, and the grout pressure should be maintained for the time specified in the method statement. Subsequently, the valve at the bottom is closed, and grouting of Tube 1 is considered complete. The level of air, water, and any other residual liquid on top of the grout should be measured, see Figure A3. Such residual liquid on top of the grout can be distinguished from the grout by its whitish to yellowish colour, usually clearer than the grout. A minimum of four measurements of levels should be taken between 0 and 24 hours after completion of grouting, with one measurement just before re-grouting of Tube 2 is started. Four measuring intervals are suggested: 30 minutes, 1 hour, 2 hours and 24 hours after grouting.

Grouting of second tube Grouting of Tube 2 should follow the same procedure as Tube 1, and should be done at roughly the same time. At a time specified in the method statement for re-grouting, the mixing of grout in the equipment is started again, and the flow time of the grout is determined again. Subsequently, the inlet and vent valves on Tube 2 are opened again, and grouting is ktarted again. This will allow any liquid accumulated on top to be replaced by grout. When grout exits from the vent on top, the valve is closed, and the grout pressure is maintained for the time specified in the method statement. Subsequently, the valve at the bottom is closed, and re-grouting of Tube 2 is considered complete. The time between initial grouting and re-grouting, and the duration of the second mixing activity, should comply with the grouting method statement. Typically, this time will be between 30 minutes and 2 hours. Similarly to Tube 1, the levels are measured between 0 and 24 hours after completion of the initial grouting. One of the measurements should be taken just prior to re-grouting Tube 2 followed by measurements at 30 minutes, 1 hour and 2 hours after completion of re-grouting.

Measurements and observations The following measurements and observations should be made and recorded: 0

description of test set-up

Test methods

Injection of grout

30'+ 2 '

//

w/ V

Figure A3: Inclined tube test set up.

grout mix design, origin and certificates of all constituents grout mixing procedure flow time of grout mix before initial grouting, and before re-grouting (or viscosity of a thixotropic grout) method statement for grouting measurements of level of air, water and residual liquid on top of the grout any observations and comments on the formation of bleed or liquid, or on difficulties encountered during the test development of temperature during the entire test period photos illustrating test set-up, and details of top end of tube with air, water zind residual liquid.

A8

ALTERNATIVE BLEEDING TEST

At the time of publication of this second edition, alternative tests for bleeding and stability of grouts to those described

previously are being developed. The desire is to create the simplest possible test that can reliably distinguish between good and bad grouts. The inclined tube test described in A7 is not a simple test and is quite expensive and time-consuming to carry out. The test in Clause 9.3 of the Specification simulates a vertical duct 1.5 m tall and was developed for more routine use. In the USA a 1 m vertical tube test with a single strand to act as a wick is generally specified. It is not yet known whether this is as discerning and it is thought that the shorter tests should perhaps be inclined to simulate conditions in the field. The major international prestressing specialists are carrying out trials to compare the various test methods and it may be that a simpler alternative bleedinglstability test can be introduced in the near future. However, until such a test is fully correlated and proven to detect poor quality grouts, the tests in this report are recommended.

65

APPENDIX B UNITS Preferred units

Conversion factors

Thefib technical report on grouting quotes pressure as ‘x’ bar (‘y’ Wa). This convention has been adopted in this report when quoting air and grout pressures.

The following conversion factors are used: 1 bar = 100 kPa = 0.1 MPa = 0.1 N/mm* = 14.504 p.s.i.

Table B 1 : Commonly used pressures (Units and values in the original documents are shown in bold)

TR47 - after grouting, pressure maintained for 1 minute

TR47 Pressure relief valve on delivery equipment TR47 Grout cube strength at 7 days ~

~

~

~

bar

1

kPa

[

5.0

[

500

1 I

TR47 Duct assembly test pressure

~

1

~

I

1 I

0.1 10.0

~

~

I

1000

~

0.5

50.0

fib grouting pressure for bond test

1-5

100-500

fib duct assembly test

0.1

66

1 [ I

10

fib report testing of plastic ducts, water test

fib grout strength before cutting sample to test voids

MPa

I

Nlmd

0.5

,

0.01

I [ I

1

p.s.i.

72.5 1.45

q-+ 1.0

145.0

* 0.1-0.5

25.0 (min)

0.1-0.5

14.5-72.5

APPENDIX C SOURCES OF FURTHER INFORMATION

American Segmental Bridge Institute 9201 N. 25th Avenue, Suite 15OB Phoenix, A 2 85021, USA Tel: +00(602) 997 9964; Fax: +00(602) 997 9965 E-mail: [email protected] www.asbi-assoc.org Bridge Deck Waterproofing Association Century House, Telford Avenue Crowthome, Berkshire RG45 6YS Tel: 01344 725 727; Fax: 01344 772 426 E-mail: enquiries@cbdg,org.uk Bridge Joint Association Century House, Telford Avenue Crowthome, Berkshire RG45 6YS Tel: 01344 725 727; Fax: 01344 772 426 E-mail: enquiries@cbdg,Org.uk www.bridgejoints,org.uk British Board of Agrement (BBA) PO Box 195, Bucknalls Lane, Garston, Watford, Herts WD25 9BA Tel: 01923 665 300 Fax: 01923 665 301 E-mail: [email protected] www.bbacerts.co.uk British Cement Association Century House, Telford Avenue Crowthorne, Berkshire RG45 6YS Tel: 01344 762 676; Fax: 01344 761 214 E-mail: Iibrary@bca,org.uk www.bca,org.uk

CSS (formerly County Surveyors’ Society) Richard Wills (Hon. Secretary) Director of Highways & Planning Lincolnshire County Council 4th Floor, Orchard Street City Hall, Lincoln LNI IDB Tel: 01522 553 000; Fax: 01522 512 335 E-mail: [email protected] www.cssnet.org.uk

International Federation for Structural Concrete FCdCration Internationale du Btton (fib) Case Postale 88, CH-1015 Lausanne, Switzerland Tel: (+41.21) 693 2747; Fax (+41.21) 693 5884 E-mail: [email protected] http://fib.epfl.ch The Highways Agency St Christopher’s House Southwark Street, London SE1 OTE Tel: 0207 921 4936; Fax: 0207 92 I 463 I E-mail: [email protected],gov.uk www.highways,gov.uk (Note: Highways Agency Departmental Standards and Advice Notes are available at www.archive.offcial-documents.co.uk) Oxford Grout Quality Control System John Darby Associates Limited 9 Holland Road Abingdon, Oxfordshire OX14 IPH Tel: 01235 525 693; Fax: 01235 200 876 E-mail: jdarby @netcomuk.co.uk

Concrete Bridge Development Group Century House, Telford Avenue

Newark Laboratories Ltd

Crowthorne, Berkshire RG45 6YS Tel: 01344 725 727; Fax: 01344 772 426 E-mail: enquiries@cbdg,org.uk www.cbdg.0rg.uk

Quarry Farm View, Bowbridge Lane Newark, Notts NG24 3BZ Tel: 01636 705100; Fax: 01636 640640 E-mail: [email protected]

The Concrete Society Century House, Telford Avenue Crowthorne, Berkshire RG45 6YS Tel: 0 1344 466 007; Fax: 0 I344 466 008 E-mail: enquiries@concrete,org.uk www.concrete,org.uk

TRL Ltd. (formerly Transport Research Laboratory) Old Wokingham Road, Crowthorne Berkshire RG45 6AU Tel: 01344 773131; Fax: 01344 770686 E-mail: [email protected]

Post-TensioningAssociation clo Freyssinet Limited 7 Hollinswood Court, Stafford Park 1, Telford TF3 3DE Tel: 01952 201 901; Fax: 01952 201 753 E-mail: [email protected] www,freyssinet.co.uk Post-Tensioning Institute 1717 W. Northern Ave., Suite 114 Phoenix, A 2 85021 USA Tel: +00 (602) 870 7540; Fax: +00 (602) 870 7541 E-mail: [email protected] www.post-tensioning.org PrecastPrestressed Concrete Institute 209 W. Jackson Blvd Chicago, IL 60606-6938, USA Tel: +00 (3 12) 786 0300 Fax: +00 (3 12) 786 0353 E-mail: [email protected] www.pci.org

UK Certification Authority for Reinforcing Steels (UK CARES) Pembroke House, 2 I Pembroke Road Sevenoaks, Kent TNI 3 1 XR Tel: 01732 450 000; Fax: 01732 455 917 E-mail: [email protected] www.ukcares.com United Kingdom Accreditation Service

(UKAS) 21-47 High Street Feltham, Middlesex TW13 4UN. Tel: 020 8917 8400; Fax: 020 8917 8500 E-mail: info @ ukas.com www.ukas.com

67

Durable post-tensioned concrete bridges

INDEX

quality 8

abutment galleries 15-17

inspection 10

continuous bridge decks 8 corrosion

abutments 9 access anchorages 15 inspection and maintenance 10, 11, 23,24 admixtures for grouts 57-58 anchorages dead-end 16, 19 details 5, 15-17, 18-21, 23-25, 27-28 end caps 16-17, 19,24,27-28 in pockets 9, 16, 18 in blisters 16 layout 9 automated quality control testing, grouting 34,36-37,62-63

replacement 25-26 testing 7

anchorages 3 risk 34

fluidity of grout 53, 54-56

tendons 3-5, 14,21

flushing ducts with water 31-32

couplers 23

I

cover 12-13,21 cracking 9 dead-end anchorages 16, 19

equipment 51

defects, ducts 32

quality control 4

de-icing salts 9-10, 11-12 design recommendations 3-7, 16,22-23

specification 32,45, 51-53

strategy, multi-layer protection 11 details

ban on post-tensioned construction 3 4 '

bleeding test, grout 5, 34, 53-54, 55-56 bonded post-tensioned construction 18-2 1 bridge decks, continuous 8

.

BlUTE Euram project, grouts and grouting 4, 6,45 carbonation 8 CARES certification scheme 3, 5, 6, 18, 25, 35,43,46-48,49 certification, post-tensioning operations 6, 18, 20, 25, 35-36,43,46-48, 49 chemical reactive resin grouts 30-3 1 chlorides 8-10, 11-12 coatings 12 codes and standards 4

anchorages 16-17 external unbonded construction 23, 24 detensioning, of external tendons 25-26 deviators, unbonded construction 23, 24 drainage 8-9, 11-12, 20-21 duct systems 5-6, 13-14 continuity, segmental construction 9, 27-28 defects 32 flushing with water 3 1-32 joining 23 layout 9

concrete materials 13 quality 8, 11, 12-13, 21 construction

68

trials 5, 14, 20, 32-33, 35-36,4344,47, 49, 59, 60 grout materials 6, 18, 24-24, 30-31, 32, 44-45,50 common 5 mixing and batching 51-52 properties 53-54 special 6, 30,43 stiffness test 6, 34, 35, 36, 6 1 4 2 hydrogen embrittlement 13, 18

access 8-9, 10, 11, 23, 24 anchorages 16-17 prestressing systems 10, 18 records 30 integral abutments 9

specification 4, 43-45, 50-5 1

integral bridges 11, 17 intelligent strand 13

testing 34-36, 45, 50-51, 61 durability, post-tensioned construction 3-4.8-10, 11

:'

inspection

pressure sensors 34, 62

cold weather grouting 53 common grout 5

grouted bonded post-tensioned construction 18-21 grouting 18-19, 29-33,45,47, 51-53

joints construction 9

embrittlement, prestressing steel 13, 18

expansion 8-9, 17

expanding agents in grout 18,57-58

precast segmental construction 9

expansion joints 8-9, 20-21 epoxy coating, tendons 13

leak tightness tests, duct systems 61

external unbonded tendons 5,22-26

joints 9

anchorage layout 9

methods 11

detensioning 25-26

maintenance, access 10, 11, 23, 24 management, post-tensioned structures 29

-_ , Index

materials selection 13, 57-58

quality system

duct systems 7, 13, 35, 61, 62 r:.

.,

, ’

,

measurement, post-tensioning 60

post-tensioning 4 U 8 , 62-63

fluidity of grout 53, 54-55

monitoring

radar testing 10

grouting operations 6-7, 35, 62-63

radiography 10, 34

grouts 6 7 , 35, 45, 53-57

grouting 33 tendons 13, 18,21

Oxford grout quality control system 62-64 permeability, concrete 13 pin-hole detection equipment 12

reinforcement, crack control 9 replacement, external tendons 25-26 road salts 9-10, 11-12

training 6, 47 trials, grouting 5, 14, 20, 32-33, 35-36,

segmental construction 5, 9, 20, 27-28 special grout 6, 30,43

type-approval testing 35, 44

I

pore-blocking sealants 12

specification, duct and grouting systems

pore-lining penetrants 12

4,5,32,4345,49-58

4, 30, 31

unbonded post-tensioned construction 22-26

units 66

standards 4

vacuum grouting 5-6, 3 1,44

stray current corrosion 14

variability of grouts 6 venting systems 5, 18-20, 27-28, 31-32,

strengthening existing bridges 22

preferred units, pressure 66

strength, grout 54

pressure-testing, ducts 5, 14

43-44,47,49,59,60

spongeometer 6, 62, 63

precast segmental construction 5, 9, 27-28

prestressing tendons 13, 21

type-approval testing 35,44 volume change, grout 54, 56 tolerances, grout 6

seawater 9

special inspections, post-tensioned bridges

6, 18, 20, 25, 35-36, 43, 4648, 49

sedimentation 54, 56-57 tendons 6

sedimentation testing, grouts 54, 56-57

planning 49 plastic ducts 5, 10, 13-14, 23

post-tensioning operations, certification

structural design, unbonded construction 22-23

44-45

void characteristics 3 1, 63-64 void grouting 29-33

durability problems 3 - 4

surface coatings 12

void sensors 34, 62

operations 20

surveys of bridge durability 3 4

voids in grouted ducts 3 4 , 29-33, 34

systems 19-20 product requirements 47 protection systems 19-21, 23

volume change, grout 54, 56 tendons 13, 14,23-25 testing 34-37 bleeding 5, 45, 53, 55-56, 64-65

quality control 8, 13 grouting 33, 3 6 3 7

a

grout stiffness 36, 61 re-grouting 5, 37, 63-64

non-destructive testing 10, 13, 34 non-metallic ducts 5, 10, 13-14, 23 notes for guidance on specification 4 3 4 5

,

corrosion protection systems 6

water tightness, ducts 23 waterproofing 9-10, 12,20 workmanship 8, 11

difficulties of inspecting tendons 6

69

This report is a non-technical introduction and guide to the selection of admixtures for concrete. It is written for site engineers, consulting engineers, architects and others working with concrete who are not necessarily materials specialists. This new edition has been fully updated in line with current practice. The objective of the guide is to encourage the use of admixtures in an informed and responsible way. It recognises the many applications and types of concrete, and the properties of concrete both in the plastic and hardened state. Admixtures are extensively used for many large and prestigious structures but the benefits are equally applicable to smaller projects. Case studies illustrate their wide application.

I , c DESIGN GUIDANCE:FOR STRENCTHENINC;_CONCRETE STRUCTURES USING F I B R 9 M P O S I T E MATERIALS Fibre composites have beenhsed for many years '\ in the aerospace and automotive indusmes They are used in construction, for exakple as structural !\ elements and cladding, and a recent development \' is the use of externally bonded fibre composiFs for strengthening concrete structures The Report .. sets out to address the problem of the lack of independent guidance on how the design of strengthening work should be camed out and the lack of national standards and codes of practice The guidance is not specific to any particular type of material or technique, and covers the use both of manufactured composites bonded to the concrete surface and composites formed in situ on the surface

Re$ TR 18 (2nd edition). 2002, 68 pages.

Ref TR 55 2001, 72 pages

DIAGNOSIS OF DETERIORATION IN CONCRETE STRUCTURES This Concrete Society Report gives a thorough and comprehensive overview of concrete deterioration, and can be used as a reference document in its own right or as the basis for more specialised investigation. This publication is ideal for engineers or surveyors responsible for concrete structures, or for advising clients and owners. It will also be useful for those planning or undertaking inspections and testing programmes and those responsible for interpreting the results.

GUIDE TO SURFACE TREATMENT FOR PROTECTION AND ENHANCEMENT OF CONCRETE Surface treatments can protect concrete against most aggressive agents, and also provide the opportunity to enhance the appearance of the structure. This comprehensive Report will help engineers, architects, building owners and managers, and consultants to specify and use surface treatments on new and existing concrete for both these purposes. It will also be of interest to contractors, applicators and suppliers.

Ref: TR 54. 2000. 72 pages.

R e j TR 50. 1997, 88 pages.

GUIDE TO TESTING AND MONITORING THE DURABILITY OF CONCRETE STRUCTURES

THE AESTHETICS OF CONCRETE BRIDGES

A GUIDE TO THE SELECTION OF ADMIXTURES FOR CONCRETE

Assessing durability is an essential function of engineers responsible for maintaining concrete structures, and those who supervise inspection and testing do not always have detailed knowledge and understanding of the appropnate tests and interpreting the results This Concrete Bridge Development Group Technical Guide provides a comprehensive but straightforward guide to test selection, procurement and measures to reduce nsks of inaccuracy

Ref: CBDG/OOS. 2002, 120 pages.

\\

In this Guide, prepared by a Concrete Bridge Development Group Task Group, the appearance of bridges when viewed from a range of distances is considered, and the aspects most relevant to each viewpoint (distant, middle, close-up) are discussed. Lavishly illustrated with photographs of features that are crucial to the long-term appearance of bridges, this report also focuses on special bridges designed to form a focal point for a particular location. Includes a section on the critical interaction between durability and aesthetics. R e j CBDG/OII. 2000, 36 pages.

r

For more information on these, and other Concrete Society publications please contact: The Concrete Bookshop,Century House, Telford Avenue, Crowthorne, Berkshire, RG45 6YS, UK Tel: +44(0)1344 725704; Fax: +44(0)1344725202; Email: [email protected];www.concretebookshop.com I

I

-:

70

experience across all disciplines.

Concrete Society Foundation The Society acknowledges with special thanks the support for the Concrete Society Foundation from the following .organisations: . Alan Stone Associates b P Beighton Construction Ltd British Cement Association Charles Scott & Partners CONSTRUCT Costain Group Edmund Nuttall Ltd Fitzpatrick Contractors Gifford & Partners Halcrow Group Ltd John Doyle Monofloor Technology Ltd North East Slag Cement Ltd Pick Everard Ready-mixed Concrete Bureau Tarmac Recast Try Construction

state-of-the-art reports,,'\:ecommendations and practical guides, and development.

COWCRIEUIE

COWIFIERENCIES, Society's regions and clubs arrange a comprehensive - -. .--

.

AWARDS

_ I _ -

.

.

-

, \

\ '----->-.---.-.--

Awards for excellence are made annually for buildings, CiviFengineering structuSs--and-mafre structures. The Awards are featured in CONCRETE magazine each year.

COWCRIETIE ON UWE WEB

Special Category Members

www.concrete.org.uk and www.concreteon-line.corn

The , Society is supported by its membership subscriptions. In addition: it receives special support from:

Concrete at your Fingertips provides quality information to members and the construction industry 24 hours a day. Concrete Directory: Members' contact information, categorised under business interests, products and services. Discussion Forum: The opportunity to share information and experience on-line. Vacancies: Employment opportunities in the concrete industry.

b P British Cement Association Byme Bros (Formwork) Ltd Castle Cement Duffy Construction John Doyle Construction Lafarge Cement UK Faber Maunsell North East Slag Cement Ltd O'Rourke Group P C Harrington Contractors Ltd RMC Readymix Ltd Rugby Cement Sir Robert McAlpine

BUIBLUCATIONS Authoritative, independent guidance on concrete materials, design and construction is one of the key outputs of The Society. Publications are sold through the Concrete Bookshop (www.concretebookshop.com),with substantial discounts available to all members.

WOW TO JOIN Group Membership is for firms, partnerships, government departments, educational establishments etc. Personal Membership gives Concrete Society benefits to individuals. Special Category Membership gives special recognition to main sponsors of the Society.

CONCRETE B R I D G E DEVELOPMENT G R O U P

? f i g C O N C R E T E BRLDGE D E V E L O P M E N T G R O U P

T

he Concrete Bridge Development Group aims to promote excellence in the design, construction and management of concrete bridges.

With a membership that includes all the sectors, bridge-owners and managers, contractors, designers and suppliers, involved in the concrete bridge industry, the Group acts as a forum for debate and the exchange of new ideas. A major programme of bridge assessment,

strengthening and widening is underway to accommodate European standards and the increasing pressure of the UK road network. The Group provides an excellent vehicle for the industry to coordinate an effective approach and to enhance the use of concrete.

Second Edition

Durable post-tensioned concrete bridges Report of a Concrete Society Working Party in collaboration with the Concrete Bridge Development Group This fully revised and updated publication provides bridge engineers with authoritative, practical guidance on designing, specifying and constructing durable post-tensioned concrete bridges. A multi-layer protection approach to durability of post-tensioned construction is presented which has been developed by a Working Party of leading bridge engineers, materials specialists, contractors and bridge owners. The standards and practices outlined in the core of the Report are a package of design, materials and construction measures. For the prestressing system itself, the basis of the recommendations is one of quality, linking together recommended design details, specifications for duct and grouting systems, and a quality assurance scheme for supply and installation of post-tensioning systems. Test methods for grout materials and duct systems are reviewed, and results of research and site trials undertaken in conjunction with the Working Party are summarised with up-to-date information and techniques. The scope of this second edition has also been extended to include: 0

external unbonded prestressing

0

remedial (void) grouting of existing bridges.

Duruble Post-tensioned Concrete Bridges provides bridge owners and specifiers with the technical basis for continuing successful use of post-tensioned concrete for durable bridge construction.

ISBN 0 946691 96 7

THE CONCRETE SOCIETY Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK Tel: +44 (0) 1344 466007, Fax: +44 (0) 1344 466008 Email: [email protected]; www.concrete.org.uk