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GLAMORGAN
University of South Wales
Technical Report No. 72
Durable Post-tensioned
Concrete Structures
Published by The Concrete Society
CCIP-047
Published September 2010 ISBN 978-1-904482-62-8 © The Concrete Society
The Concrete Society
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Contents
Preface
List of figures
List of tables
1. Introduction
1.1 General background
1.2 Technical background
1.2.1 Post-tensioned bridges
1.2.2 Post-tensioned buildings
1.3 Summary of progress
1.4 Summary of key provisions
1.4.1 Design and detailing
1.4.2 Duct and grouting systems
1.4.3 Grout materials
1.4.4 Certification of post-tensioning operations and training
1.4.5 Testing
Recommendations for durable post-tensioned
concrete bridges
2. Factors affecting durability
2.1 Corrosion of prestressing steel
2.2 Materials and components
2.3 Construction quality
2.4 Expansion joints
2.5 Construction joints
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2.11 Access for inspection and maintenance
3. Available protective measures
3.1 Design strategy - multi-layer protection
3.2 The structure as a whole
3.2.1 General
3.2.2 Bridge deck waterproofing systems
3.2.3 Coatings
3.2.4 Drainage
3.3 Individual structural elements
3.4 Prestressing components
3.4.1 Prestressing tendons
3.4.2 Ducts
3.4.3 Anchorage location
3.4.4 Anchorage details
4. Grouted bonded post-tensioned construction for bridges
4.1 Grouts and grouting
4.2 Vents and grout injection
4.3 Recommended protection systems
4.3.1 Prestressing system
4.3.2 The deck and its elements
4.3.3 Possible additional measures for exceptional structures
5. External unbonded post-tensioned construction for bridges
5.1 Advantages and disadvantages
5.2 Background
5.3 Structural design and basic performance requirements
5.4 Available protective measures
5.5 Detailing
5.6 Tendon systems
5.7 De-tensioning and replacement of external tendons
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7.1 Overview
7.2 Aims of void grouting
7.3 Condition of bridge stock and potential demand
7.4 Inspection records
7.5 Grouting materials
7.6 Grouting equipment and methods
7.7 Determining the void characteristics
7.8 Flushing with water
7.9 Effect of existing defects
7.10 Specification for grouting
7.11 Trials
7.12 Quality control
8. Test methods for grouted post-tensioned concrete bridges
8.1 Introduction
8.2 Range of tests considered
8.3 The need for testing
8.4 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 regrouting
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9.2 Materials and components
9.3 Construction quality
9.4 Expansion joints
9.5 Construction joints
9.6 Cracking
9.7 Ducts and anchorage layouts
9.8 Proximity to seawater
9.9 Road salts
9.10 Access for inspection and maintenance
10. Available protective measures
10.1 The structure as a whole
10.2 Individual structural elements
10.3 Prestressing components
10.3.1 Prestressing tendons
10.3.2 Ducts
10.3.3 Anchorages
11. Grouted bonded post-tensioned construction for buildings
11.1
Grouts and grouting
11.2 Vents and grout injection
11.3 Recommended protection systems for buildings
11.3.1 General
11.3.2 Prestressing system
11.3.3 The slab
11.3.4 Possible additional measures
11.4 Void grouting
11.5 Test methods for grouted post-tensioned buildings
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12.2.3 The slab
12.2.4 Possible additional measures
Recommendations for specifications for
durable post-tensioned concrete
13. Recommendations for specifications for duct and grouting systems for
post-tensioned tendons
13.1 Introduction
13.2 Guidance on the project specification
13.2.1 Trials
13.2.2 Grout materials
13.2.3 Ducting for bridges and other aggressive environments
13.2.4 Ducting for internal elements of buildings
13.2.5 Vents
13.2.6 Testing
13.2.7 Grouting
14. Contractor's quality scheme requirements
14.1 Introduction
14.2 Basic quality system elements
14.3 Product requirements
14.4 Certification
References
Appendix A. Test methods
A1 Leaktightness tests for duct systems
A2 Grout stiffness tests
A3 Void sensors
A4 Duct pressure sensors
A5 Automated quality control systems
A6 Volume of voids before regrouting
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and European standards now exist, now make reference to them. This has enabled some simplification in the text. The most significant extension to this Report is to include recommendations for post-tensioning in buildings as well as in bridges, where significant experience has been gained in recent years.
The measures described are aimed at improving design, detailing, specifications, materials, construction methods and testing for grouted post-tensioned concrete with either internal or external tendons.
Producing this revised and updated Technical Report has been undertaken by a small group of people fully aware of the current state of the art and I am particularly grateful to Tony Jones of Arup and AndyTruby of Gifford for their assistance in expanding the scope to include buildings and I am grateful to all who have contributed, entirely on a voluntary basis.
At a time when the Eurocodes are upon us, the post-tensioning industry is preparing to follow new procedures and Standards and the relevant documents for design and construction of post-tensioned concrete are largely in place. However, it should be remembered that practices continually develop and evolve and while these new standards will improve performance significantly, there will always be scope for further development.
I am indebted to Mark Raiss and George Somerville who masterminded the production of the first edition ofTR47 in 1996 which formed the original basis for this Report.
Figure 4 Restressable external anchorage at end of deck with an abutment gallery. Figure 5 Anchorage at top blister using exposed anchor.
Figure 6 External blister and bonded face anchorages for in-situ segmentai construction. Figure 7 Anchorage at bottom blister using buried anchor (internal tendon).
Figure 8 Top pocket anchorage. (This is NOT recommended,unless external protective layers are used.)
Figure 9 Buried anchorage for stressed or dead end. Figure 10 Exposed anchorage for stressed or dead end.
Figure 11 Face anchor details in in-situ segmentai construction. Figure 12 Grout vent details at deck surface.
Figure 13 Exposed anchorage for restressing the end of an unbonded external tendon. Figure 14 Exposed anchorage for the dead end of an unbonded external tendon. The
detail is also applicable for the live end where restressing is not required. Figure 15 Top deviatorfor external tendon.
Figure 16 Face anchor details for precast segmentai construction. Precast segmentai construction using internal grouted tendons is NOT recommended, unless continuity of the duct is assured.
Figure 17 Combined face anchor and shear key details for precast segmentai construction. Precast segmentai construction using internal grouted tendons is NOT recommended, unless continuity of the duct is assured.
Figure 18 Live end anchor at construction joint adjacent to unstressed pour strip. Figure 19 Pour strip after tendon stressing and prior to fixing reinforcement and casting
the concrete.
Figure 20 Dead end anchorage at construction joint.
Figure 21 Top pocket before (top) and after (bottom) casting the concrete. Figure 22 Edges of post-tensioned slabs.
Figure A1 Location of spongeometer within the grouting system.
Figure A2 Instrumentation within the Oxford grout quality control system.
List of tables
Table 1 Test methods applicable during construction. Table 2 Test methods applicable during service life.
1. Introduction
The first edition of Concrete Society Technical Report 47, Durable post-tensioned concrete
bridges^, published in 1996, recommended new standards and practices for the design
and construction of durable bonded post-tensioned concrete bridges. It covered the key elements of design, detailing, materials, grouting and certification for installation. This resulted in the lifting of the moratorium for in-situ post-tensioned construction that had been imposed by the Department of Transport in 1992.
1.1
General background
The Concrete Society Working Party continued working to improve and update its recommendations, particularly on test methods, while developing solutions for grouted precast segmental construction, which was not covered in the first edition. Account was taken of international developments, especially those involving specifications, and close contact maintained with similar groups in other countries and with the International Federation for Structural Concrete (fib). The Working Party incorporated the best of these new developments into the second edition ofTR47, published in Z002, while ensuring that the basic principles and performance requirements were met. Although relatively few bridges of this type have been built in the UK in recent years, there has been significant feedback from the use of the recommendations in practice, both nationally and internationally.The scope of the second edition was extended to include: external unbonded prestressing
• remedial (void) grouting of existing bridges • updated information on new test methods.
The second edition included a revised Specification for duct and grouting systems, together with notes for guidance. That Specification, coupled with the CARES certification scheme for the supply and installation of post-tensioning systems in concrete structures, has represented the state of the art for about 10 years. More recently, developments of international standards have taken place and the publication in 2007 of revised versions of BS EN 445(2),
BS EN 446'3' and BS EN 447'4', which embody many of the proposals in the second edition
of TR47, have led to the need for an updated Technical Report.
Experience of both grouted and unbonded post-tensioning in buildings, especially flat slabs, has grown significantly in recent years and this Report has now been extended to include recommendations for this type of use.
In addition to its use in bridges and in buildings, post-tensioning is also used in a variety of other types of structures, such as storage silos, tanks and other containment structures. The principles described in this Report will be equally applicable to such structures, but detailed guidance (e.g. on the layout of tendons and the provision of vents) is not given because of the variety of such structures.
While this Technical Report is primarily concerned with sound principles supported by good practice and procedures, the importance of attitude and awareness is also stressed. Since 1996 awareness has increased significantly. Grouting is an installation-sensitive operation, requiring skill and care on the part of all concerned.
1.2
Technical background
Surveys of bridge durability have been undertaken throughout the world but it is impossible to estimate accurately the number of post-tensioned bridges that have suffered tendon corrosion.1.2.1 Post-tensioned bridges
The first serious problem in the UK was the collapse of Bickton Meadows footbridge in Hampshire in 1967, since when appreciation of the problem has slowly grown. In 1981 the Transport Research Laboratory published the results of an investigation into the grouting of 12 post-tensioned concrete bridges constructed between 1958 and 1977'5'. Voids werefound in the ducts of ten of the bridges. The results were passed to the Standing Committee on Structural Safety*6' which concluded that, in structures containing a large number of
tendons, "the risk of sufficient tendons failing by corrosion at any time to cause sudden collapse is considered to be small".
In 1980 Angel Road Bridge, North London was found to have wires broken due to corrosion behind some of the anchorages. The deck was propped and has since been replaced. An inspection of Taf Fawr Bridge, MerthyrTydfil, South Wales in 1982<7' 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'8'. 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 post-tensioning through the hinged joint of the end tie-down member. Of particular interest is the Niles Channel Bridge in Florida. Built in 1983, this 1390m 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 have been introduced in the USA by the Post-tensioning Institute19'.
The collapse in May 2000 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'10'.
Elsewhere, presentations of UK and French experience of corrosion at a 1999 conference'11'
led to a review of specification and operations. In Germany, the Federal Ministry of Transport, Construction and Housing has introduced Guidelines for concrete bridges with external
tendons^. While this focused on avoiding the use of couplers at the same cross-section,
it also banned 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 led the Japan Highways Public Corporation to ban grouted internal tendons in post-tensioned structures; the focus tended towards the use of unbonded external tendons, and on the development of preformed tendons pre-grouted with epoxies. The Japanese developed and introduced transparent ducts.
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; see for example Raiss'13'.
The Highways Agency's series of special inspections of post-tensioned bridges, under BD 54/93'14' and BA 50/93'15', had the purpose of determining the condition of the
prestressing and the efficacy of the grouting. 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, as reported by Woodward'16'. 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'17'. The general impression was that there had been few cases of serious
corrosion and that the performance in service of post-tensioned concrete bridges was 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 in internal tendons is to drill inspection holes into the ducts.
Summary information of special inspections of over 200 post-tensioned structures on motorways and trunk roads was collated by the Transport Research Laboratory (TRL) and made available to the Working Party. It appeared that:
• the incidence of severe or heavy corrosion was small (approximately 2%) I 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 was 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 the 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 on post-tensioning for bridges in 1992'18' 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 Note16<19' in
1999. This was superseded in 2002 by Interim Advice Note 47(20) which referred to the
second edition of TR47. In 2003 the Highways Agency revised their Specification for Highway Works to include many of the recommendations of the second edition ofTR47.
1.2.2 Post-tensioned buildings
The use of bonded prestressing in buildings has grown significantly over the last 10 years. The work tends to be carried out by a specialist contractor who often also takes on design responsibility for the slab. Recently concerns over the adequacy of grouting in buildings led to the grouting in a number of recently completed post-tensioned slabs being investigated. Significant numbers of either completely or partially ungrouted ducts were found. This led to CARES revising its post-tensioning certification scheme for buildings'21'. Much ofthe guidance on the grouting of post-tensioned buildings refers to the second edition of TR47. However, as the guidance was predominantly related to bridges and some clearly not relevant to buildings, this could lead to confusion and differences in interpretation of what was best practice for buildings.
1.3 Summary of progress
Since the 2002 edition of TR47 there have been several important international developments. These include:I publication of BS EN 445<2), BS EN 446<3' and BS EN 447(4', fully revised to incorporate
current best practice for grouting
• publication of BS EN 13670'22', the concrete execution standard for Europe
• publication of fib recommendations for grouting in 2002<23'
• publication of fib recommendation Durability of post-tensioning tendons in 2006'24'
• publication of ETAC 013<25> giving Technical Approval Guidelines for post-tensioning
• publication of CWA14646'26' giving requirements for installation of post-tensioning
systems in Europe
• publication of the second edition of the CARES model specification for post-tensioning of slabs in buildings'27'
development of several pre-bagged factory-produced grouts
' preparation of a new edition of the National structural concrete specification^.
In their different ways, these activities affected the Working Party, in terms of the input to this Technical Report. Since compatibility is important, the approach adopted has been to refer to new specifications and guidance whenever possible, rather than to revise the Report in isolation. This has enabled omission of some of the text which was in the second edition ofTR47.
The principal aim of the Working Party was 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 I grout materials
• certification of post-tensioning operations and training • testing
: external and unbonded construction
• remedial grouting of existing bridges • new test methods
and has now added:
1 grouting in buildings
; unbonded tendons in buildings.
1.4
Summary of key
provisions
This section discusses those aspects of design and detailing that affect the durability of post-tensioned concrete bridges and buildings. 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.
1.4.1 Design and detailing
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 j o i n t - a key performance parameter in these recommen-dations - 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, then it may be used. This is an area where product development is continuing.External unbonded tendons were covered in the second edition of TR47. This method may be used for any form of prestressed construction, including both in-situ and precast segmental construction, provided the recommendations are followed.
The recommendations for building structures follow the same general principle of a multi-layer protection system but acknowledge that in most enclosed structures the building fabric provides one of these layers. Emphasis is placed on ensuring that the grouting is carried out effectively to provide the second layer of protection. For external structures, and in particular car parks, the approach is similar to bridges albeit recognising differences in the number of tendons and their drape.
1.4.2 Duct and grouting
systems
An interim specification and commentary were published by the Working Party in 1993'29'
and the lessons learnt from their use discussed at a Concrete Society/Concrete Bridge Development Group seminar in 1994'30'. The 1996 specification was based on drafts of
European Standards and other international documents, for example the FIP Guide to good practice Grouting oftendons in prestressed concrete^ .The specification which was in the second edition of TR47 is now deleted as it has been superseded by the publication of British Standards BS EN 445«, BS EN 446<3>, BS EN 447(4> and BS EN 13670<22). However,
there are recommendations in this Technical Report for design aspects and for procedures in the installation process which go beyond the British Standards.
The main differences between the specifications in the second edition of this Report and in the first edition were as follows:
• requirement for full-scale grouting trials on each project relaxed
I revised specification of the properties of the grout with a new bleeding test I clear recommendations for plastic ducts.
These built on the innovations introduced in the first edition:
I ducts to be of electrically non-conductive, corrosion-resistant durable material forming a double corrosion protection system in combination with the grout
• duct systems pressure tested • additional vents
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 a fib Technical Report'32'. 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.
For enclosed building structures it is recognised that metal ducts can still provide an adequate and economic solution. The nature of the typical metal ducts used in building structures means that pressure testing is not possible. For this reason, in enclosed buildings the duct should not be considered to give any protection.
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, Balvac Whitley Moran'33') although it is more widely used
elsewhere in Europe. However, it has a very relevant application for regrouting as will be discussed. While the Working Party has undertaken 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
Prior to 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 arose due to variations in the weight of bagged cement; tolerances of ±2kg in 50kg bags were not uncommon, and outside the desired tolerance of 2%. However, tolerance on the weight of new 25kg bags in the UK is now ±1% which will improve one variable.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 developed a prepacked '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:
• develop an improved grout with properties that consistently meet the revised specification • 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
The results from this project were available in a draft final report in May 1996'34', and were
considered when formulating the recommendations in the first edition of TR47. Since then, feedback has been obtained on the use of special grouts in practice. In addition, results became available from a major BRITE EURAM project on grouts and grouting, including the development of improved test methods'35!.
Grouts meeting the performance requirements are now commercially available, some as a combination of packaged products.
The first edition of TR47 included both common and special grouts within its scope. This was the terminology used in the previous editions of the European Standards. The revised Standards simply use the term 'grout' although ETAG 013<25' still includes for special grout.
The Working Party is of the opinion that special grouts (meaning in this context a pre-bagged product simply requiring the addition of water) 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 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.
The CARES post-tensioning certification scheme now requires the use of a pre-bagged grout requiring only the addition of water on site. This is the approach recommended for all grouting both in bridges and in buildings.
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 and buildings. 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 ungrouted. Consequently CARES, together with the Post-Tensioning Association, developed a Certification Scheme in consultation with the Highways Agency, which is referred to in Chapter 14. Similar schemes are used elsewhere in Europe but this was the first time such a scheme had 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 in particular was closely monitored and in general the specification 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 revised specifications. The CARES scheme is now coming of age and is a positive contributor to improved durability. A revision of the scheme was made in 2007/8 to reflect experience of use in the intervening years and to align it with European requirements.
In response to the increase in post-tensioned building construction, CARES has broadened its scheme to include specific post-tensioning requirements for buildings which in some aspects are distinctly different to highways structures. Additionally CARES has introduced certification of pre-bagged grouts as this eliminates or reduces the potential problems relating to the variability of cement and workmanship issues during grout production.
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 considered numerous tests suggested for this purpose. These are discussed in Appendix A, although some remain at the development stage and are unlikely to prove appropriate for routine use.This 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' - see Darby'36'.
It was further developed and incorporated in a device that provided immediate results together with computer records of variables influencing grouting quality. Unfortunately at the time of writing it is understood that despite considerable efforts to promote its use, the equipment has not been taken up and has now been scrapped.
For external tendons, inspection and testing are somewhat easier because the tendons are normally accessible. This does of course require a regular programme of inspection to be followed after construction and in service, in order to reap the benefit.
BS EN 445<2>, BS EN 446<3> and BS EN 447<4' state a minimum mandatory level of testing.
This may be associated with the required properties of the grout, where specific tests are given. There may also be testing associated with the duct system, where strong reliance is now placed on the standardised approval system developed by //£>(32). In this context, a
site-specific duct assembly verification test is also recommended and possible additional tests are described which may be considered in certain circumstances. Both of these relate to measurements of the degree of sealing provided by the duct system but, at this time, pending further development and experience of use, neither is included in the Standards.
Recommendations for durable
post-tensioned concrete bridges
2. Factors affecting durability
In broad terms, deterioration mechanisms that can affect structural concrete may be classified as those that directly attack the concrete and those that directly or indirectly attack the reinforcement or prestressing components.
Attack of the concrete is not covered in this Report. However, sulfate attack and alkali-silica reaction are now well understood and guidance is available - see for example BRE Special Digest 1(37) and Concrete Society Technical Report 30, Alkali-silica reaction: minimising the risk of damage to concrete^. Proven solutions are established to deal with different intensities
of the various mechanisms, mostly in material specification terms.
A major hazard for bridges is corrosion of the prestressing steel, and this is the prime concern of this chapter.
2.1
Corrosion of
prestressing steel
Corrosion may result from:
• chlorides in the ingredients in the concrete (or grout)
• carbonation of the concrete, resulting in reduced alkalinity in the concrete
• external chlorides penetrating to the steel, from sources such as de-icing salts or seawater. Of these, strict limits have been placed on chlorides in the concrete (or grout) in codes and standards for more than 20 years. Carbonation can be a hazard for buildings, but the dominant factor for bridges and car parks is undoubtedly external chlorides.
It follows that, in developing a design strategy, the nature and intensity of the aggressive actions - and how they might penetrate to the steel - is of fundamental importance. This applies both to conceptual design and to the evolution of design details. The transport mechanisms for chlorides are much influenced by the combined effects of wind, water and temperature, in both ambient and micro-climate terms. Resisting these influences requires an integrated approach, involving design concept, detailing, construction quality and material selection. The importance of integrating these aspects cannot be overemphasised. The purpose of this chapter is to identify the key factors that affect durability, based on feedback from performance in service. The main focus is on the performance of bridges and buildings as a whole. The factors considered are:
• materials and components • expansion joints
• construction quality • construction joints • cracking
• duct and anchorage layout
• precast segmental construction and joint details • proximity to seawater
• road salts, waterproofing and drainage • access for inspection and maintenance.
2.2
Materials and
components
The quality of materials and components is of great importance, and therefore the derivation of good specifications is crucial. This should be done with a clear idea of performance requirements, and of a methodology that will ensure that the chosen items do in fact comply.
2.3
Construction quality
Poor workmanship and construction defects are major issues, which strongly influence the level of durability actually achieved. A good example of this, in the past, was ineffective grouting for post-tensioned work. However, the issue is wider than that, ranging from poor compaction and failure to achieve specified covers to cases where joints are poorly made (either in the structural elements themselves or in fitting together the various pieces of hardware involved in prestressing operations). Substantial loads and forces are involved in casting and stressing prestressed concrete structures - often involving large pressures and strains. There is therefore a design element involved in ensuring that temporary conditions during construction are properly considered, and in deriving details that enable materials and components to be fitted together on site.2.4
Expansion joints
A high proportion of expansion joints leak and their effectiveness and life span are very dependent on the quality of installation and maintenance. The Highways Agency has produced a Departmental Standard on the requirements for expansion joints, BD 33/94<39>,and a Departmental Standard and Advice Note on Design for durability, BD 57/1 and BA 57/01(40'. These documents encourage the use of continuous bridge decks and integral
abutments wherever possible, to eliminate expansion joints and hence reduce the risk of contaminants reaching sensitive parts of the structure.
Where expansion joints are used, provision should be made for inspecting them and the structure underneath, and the details should be based on the assumption that joints will leak and will not provide protection against ingress of water and road salts. Appropriate drainage paths for the leakage should be provided which ensure that it cannot get access to the prestress anchorages or bearings and that the water is not allowed to pond. This is especially important if intermediate joints have to be located over piers, in ensuring that drainage paths are kept clear of anchorages, because here it is often difficult to provide an inspection gallery.
2.5
Construction joints
Well-made construction joints should not leak, particularly when protected by water-proofing membranes. However, waterwater-proofing membranes often do not provide a complete seal, and do not last indefinitely, and joints leak. It is therefore advisable to keep construction joints in deck slabs away from anchorages and prevent, by means of drips, any access for the leakage to reach the anchorages. If possible, joints in ducts should also be kept away from construction joints. In sequential or segmental construction, where the prestressing anchorages are inevitably located at construction joints, care should be taken in detailing.Emphasis should be given to not creating planes of weakness, which permit easy access to water (in the form of spray, runoff or ponding) that acts as a transport mechanism for contaminants, and to detailing protection for the anchorages and preventing ingress of water. Provision for ease of inspection is also important.
2.6
Cracking
Cracking in concrete can occur for a number of reasons - see Concrete Society Technical Report 22, Non-structural cracks in concrete^. Its relevance to durability is largely related to corrosion, and depends on the type and magnitude of the cracks - see Concrete Society Technical Report 44, The relevance of cracking in concrete to corrosion ofreinforcement^. Care is required, when considering the layout and sequencing of concrete pours and prestressing, to minimise the risks of cracking, particularly near anchorages. Applying a low initial prestress at an early age can help counteract early-age cracking. The reinforcement provided in the direction of the prestress is usually much less than that used in reinforced concrete bridges and should be checked for adequate distribution of cracking in accordance with BD 28/87<43) or the relevant part of BS EN 1992<44>.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 multi-layer 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 anchorages. For example, the layout of tendons for span-by-span 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, due to their shape and location, they may provide a path for contaminants to the prestressing tendons. 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 joint type
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. Those in previous use may be subdivided as follows:
• thin mortar joints
• wider, in-situ concrete joints
u 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. Research at the University of Texas at Austin on behalf of the Texas Department of Transportation supports this in concluding that epoxy joints can still be subject to penetration and allow corrosion if not formed perfectly; see Salas eta/.'45'.
2.9
Proximity to seawater
Structures 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 structures 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, road salts are not used. Chloride-induced corrosion is one of the major concerns in concrete bridges, but where the structure is not subject to road salts or wind-borne chlorides, and it is clear that road salts will not be applied in future, it may be possible to reduce some of the layers of protection described in the following chapters.
Attention to detail in the design and application of bridge deck waterproofing systems and drainage systems is vital. This applies to car parks and to bridges of all types.
2.11
Access for inspection
and maintenance
One of the main concerns about internal prestressing systems is the inability to inspect the tendons visually. However, it should be remembered that external prestress can usually only be inspected in the straight sections between anchorages and deviators and then only if it is not enclosed in a grouted duct, although tapping of external ducts can be helpful in detecting voids. Enclosure of external prestress within a duct with holes for inspection using an endoscope is possible. The concern is that, if the tendons cannot be inspected, corrosion may proceed undetected and lead to collapse without warning.
However, one of the advantages 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.
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, Design for durability^) has not yet really addressed the application of post-tensioning 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 post-tensioned 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 protective measures
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.
3.1
Design strategy -
multi-layer protection
The factors that might require consideration (see BD 57/01 and BA 57/01, Design for
durability1'40'!) will include:
: the location of the bridge, and the associated general and local exposure conditions the provision of continuity, possibly in the form of integral bridges
• access for inspection, testing, maintenance, and possible replacement of short-life elements
the type of cross-section, and its shape, particularly at its boundaries
i the method of construction, with its associated buildability and workmanship factors. I the deck waterproofing system
I the provision of effective drainage and avoiding ingress of water.
Procedures for the design of individual elements are available, for example BS 5400'46' (which
was effectively withdrawn in April 2010), BD 24/92<47>, BS EN 1992-2<44> (implemented by
IAN 123/10'48') and CIRIA C543<49>. 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.
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 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 was 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.
3.2
The structure as a
whole
There is much that can be done, both quantitatively and qualitatively, to tackle the major threat of corrosion due to chlorides.
3.2.1 General
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, runoff 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 which will inevitably leak through.
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 modern high-quality liquid-applied membranes are 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 ofthe membrane. Careful preparation ofthe 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 47/99'50'. The Highways Agency's Specification for Highway Works^ requires all proprietary materials for waterproofing
systems to have a current British Board of Agrement (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 surface treatments for protection and enhancement of concrete^. 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 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 carried 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 waterbome 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 57/01'40' give advice on drainage systems, and ondealing with the passage of water at boundaries and at supports.
3.3
Individual structural
elements
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.
BS 5400: Part 4<46' (which was effectively withdrawn in April 2010), BS EN 1992-2<44> and
BS 8500<53' give recommendations for minimum concrete strength and cover for
post-tensioned concrete bridges, and the Specification for Highway Works^ 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 57/01'40', which include requirements for increasing
cover by 10mm. 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 (see for example Wallbank'54') 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 (see for example Hobbs'55'). 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 water/cement ratio or by cement replacement with fly ash (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.
3.4
Prestressing
components
In simple terms, the essential elements of prestressing components are: : I the prestressing tendons
• the ducts, containing the tendons • the anchorage system
I the overall protective system.
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.
3.4.1 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.
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/stress corrosion cracking (HE/SCC). It is understood that strand manufactured to BS 5896<56), prEN 10138<57> or ASTM A416/A416M<58) and adequately
protected with grout/grease is not likely to be subject to HE/SCC. Where there is a risk that such protection cannot be provided orgreater confidence that HE/SCC will not occur then the specification for strand should require that it is tested in accordance with the procedure given in BS EN ISO 15630-3'59'. Acceptance criteria are given in the fib report Stress corrosion cracking resistance test for prestressing tendons^.
There have also been concerns about the effectiveness of epoxy coating, particularly for reinforcement. Any small defect in the coating can increase the likelihood of local corrosion. Pinhole detection techniques have improved with new manufacturing processes. However, it is not clear whether the risk can be completely discounted as damage to coating can also occur during installation.
'Intelligent strand' may prove useful and has been 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 . 2 DlJCtS 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 - see Woodward and Williams'8'. 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.
More recently, non-metallic ducts have been used. These are made of high-density poly-ethylene (HDPE), now known as PE80, or polypropylene, and have a number of advantages:
corrosion-resistance
11 better sealing against ingress of contaminants
• can be pressure-tested during construction to demonstrate integrity
I more potential to be 'seen through' by some non-destructive 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.
The Highways Agency revised grouting specification for bridges 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. A suitable test method is described in the fib Technical Report Corrugated plastic ducts for internal bonded
post-tensioningW and in Section A1 in Appendix A. Walls thicker than a minimum are used
to allow for the effect of the stressed tendon 'biting' into the duct wall.
The use of non-metallic ducts requires a reassessment 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:
• do not corrode
• effectively resist the passage of chloride ions
• I do not conduct electricity
3 have a high coefficient of thermal expansion (typically 140 x 10~6/°C)
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 alternating-currents in nearby metal objects, and, if not controlled, can generate harmful potentials on exposed conductors. Third-rail DC 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 500m - 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 45mm round all metal parts of the anchorages, may be sufficient to achieve this. If it is necessary to expose metal parts 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 of concrete are similar and consequently changes in temperature do not cause significant relative strains. However, this is not true for non-metallic 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 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 by Kollegger'61'.
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.
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 are related to what is possible using the currently available non-metallic 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.
