Durable Post-tensioned Concrete Structures

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GLAMORGAN

University of South Wales

19/11/2015

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Acknowledgements

The Concrete Society is grate ful to the fo llow ing for the provision of photographs:

Strongforce/Laing O'Rourke: Figures 18, 20 and 21

Sir Robert M cAlpine: Figures 19 and 22.

Published by The Con crete S ociety

CCIP-047

Published September 2010

ISBN 978-1-904482-62-8

© The Concrete Society

The Concrete Society

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guidance in support of concrete design and cons truction.

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Printed by Ruscombe Printing Ltd, Reading, UK.

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Durable post-tensioned concrete

structures

Contents

Preface

List of figures

List of tables

1 .

Introduction

1.1

General backgrou nd

1.2

Technical backgrou nd

1.2.1 Pos t-tens ione d bridges

1.2.2 Po st-ten sion ed buildings

1.3

Summary

of

progress

1.4

Summary

of key

provisions

1.4.1 Design

and

detai l ing

1.4.2 Duct

and

grou ting systems

1.4.3 Gro ut ma terials

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 Con struction qua lity

2.4 Expansion joi nt s

2.5 Con struction join ts

V I

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2.6 Cracking

2.7 Duct and anchorage layout

2.8 Precast seg me ntal con struc tion and join t typ e

2.9 Proximity to sea water

2.10 Road salts, wa terp roo fing and drainage

2.11 Access for inspection and maintenance

3. Available protec tive measures

3.1 Design strategy - multi-la ye r prote ction

3.2 The struc ture as a wh ole

3.2.1 General

3.2.2 Bridge deck wa terp roo fing systems

3.2.3 Coatings

3.2.4 Drainage

3.3 Individual structu ral elements

3.4 Prestressing com po nen ts

3.4.1 Prestressing ten do ns

3.4.2 Ducts

3.4.3 Anchorage location

3.4.4 Ancho rage details

4. Grou ted bonded post-tensioned construction fo r bridges

4.1 Grou ts and gro utin g

4.2 Vents and grout injec tion

4.3 Recomm ended protec tion systems

4.3.1 Prestressing syste m

4.3.2 The deck and its elemen ts

4.3.3 Possible ad ditio na l measures for exc ep tiona l structures

5. Externa l unbonded post-tensioned cons truction for bridges

5.1 Advantages and disadvantages

5.2 Background

5.3 Struc tural design and basic performa nce re quirem ents

5.4 Available prote ctive measures

5.5 Detailing

5.6 Tendon systems

5.7 De-tension ing and replacement of external tend ons

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6. Seg me ntal construction

6.1 Multi-layer protection

6.2 Anchorage location and detailing

7. Void grouting

7.1 Overview

7.2 Aims of void grouting

7.3 Condition of bridge stock and po tentia l demand

7.4 Inspection records

7.5 Grouting materials

7.6 Grouting equipment and methods

7.7 Determ ining the void characteristics

7.8 Flushing w ith water

7.9 Effect of existing defects

7.10 Specification for grouting

7.11 Trials

7.12 Quality control

8. Test me thods 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-contrac t stage (duct systems, grout materials

and procedures)

8.4.2 Trial grou ting w ithin a con tract (geometry, materials and procedures)

8.4.3 Duct assembly verifica tion before main grou ting

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 Autom ated q uality c ontrol testing of main grouting

8.4.7 Survey of existing grout conditions before regrouting

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Recommendations fo r durable post-tensioned

concrete buildings

63

9 . Dura ble post-tensione d concrete buildings

9.1 Factors affecting durab ility

9.2 Materials and components

9.3 Con struction qua lity

9.4 Expansion joints

9.5 Construction joints

9.6 Cracking

9.7 Ducts and anchorage layouts

9.8 Proxim ity to seawater

9.9 Road salts

9.10 Access for inspection and maintenance

10 . Available protec tive 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 p rotection systems for buildings

11.3.1 General

11.3.2 Prestressing system

11.3.3 The slab

11.3.4 Possible addition al measures

11.4 Void grouting

11.5 Test methods for grouted post-tensioned buildings

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12. Unbon ded post-tensioned construction for buildings

12.1 Introduction

12.2 Recommended pro tection systems for buildings

12.2.1 General


12.2.3 The slab

12.2.4 Possible additiona l measures

Recommendations for specifications for

durable post-tensioned concrete

13. Recom menda tions 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 interna l 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 m ethods

A1 Leaktightness tests for duct systems

A2 Grout stiffness tests

A3 Void sensors

A4 Duct pressure sensors

A5 Autom ated quality control systems

A6 Volume of voids before regrouting

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Preface

This Report is a revision of the second e dition of Technical Report 47, Durable post tensioned

concrete bridges®,

which was published by The Concrete Society in 2 002. The recomm endations

in the second edition have been reviewed and extended, and where new international

and European standards no w exist, now make reference to t he m . This has enabled some

simplification in the text. The most significant extension to this Report is to include

recomm endations for post-ten sioning in buildings as we ll as in bridges, where significant

experience has been gained in recent years.

The measures described are aimed at improving design, detailing, spec ifications, materials,

construction m ethods 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 sm all group of

people fully aware of the current state of the art and I am particularly gra teful to Tony Jones

of Arup and AndyTruby of G ifford for th eir assistance in expanding the scope to include

buildings and I am grateful to all who have con tribute d, entirely on a volu ntary 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 docum ents for design and cons truction

of post-tensioned concrete are largely in place. However, it should be remembered that

practices con tinually develop and evolve and while these new standards will improve

performance significantly, there will always be scope for further development.

I am indebted to Mark Raiss and George Somerville who masterminded the production of

the first edition ofTR47 in 1996 which formed the original basis for this Report.

G.M.Clark

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List of figures

Figure 1 Buried anchorage at end of deck wit h an abu tme nt gallery.

Figure 2 Exposed anchorage at end of deck wit h an abu tme nt gallery.

Figure 3 Exposed external anchorage at end of deck w ith an abutm ent gallery.

Figure 4 Restressable external anchorage at end of deck w ith an abutm en t gallery.

Figure 5 Anchorage at to p blister using exposed anchor.

Figure 6 External blister and bonded face anchorages for in-situ segmentai construction .

Figure 7 Anchorage at bott om blister using buried anchor (internal tend on).

Figure 8 Top pocket anchorage. (This is NOT recommended,unless externa l protective

layers are used.)

Figure 9 Buried anchorage for stressed or dead end.

Figure 10 Exposed anchorage fo r stressed or dead end.

Figure 11 Face anchor details in in-situ segmentai construction .

Figure 12 Grout ven t 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 tend on. The

deta il is also app licable for the live end where restressing is not required.

Figure 15 Top de viat orf or externa l tend on.

Figure 16 Face anchor deta ils for precast segm entai const ruc tion . Precast segmentai

construction using internal grouted tendons is NOT recommended, unless

con tinuity of the d uct is assured.

Figure 17 Comb ined face anchor and shear key details for precast segmentai constru ction.

Precast segmentai construction using internal grouted tendons is NOT

recomm ended, unless continuity of the duct is assured.

Figure 18 Live end anchor at cons truction join t adjacent to unstressed pour strip.

Figure 19 Pour strip after tend on stressing and prior to fixing reinforceme nt and casting

the concrete.

Figure 20 Dead end anchorage at construction join t.

Figure 21 Top pocket before (top) and after (bottom ) casting the concrete.

Figure 22 Edges of post- tens ione d slabs.

Figure A1 Location of spongeo meter with in the grou ting system.

Figure A2 Instrumentation w ithin the Oxford grout quality contro l system.

List of tables

Table

Test methods applicable during construction.

Table 2 Test me thods app licable during service life.

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Introduction 1

1.

Introduction

The first e dition of Concrete Society Technical Report 47, Durable

post-tensioned concrete

bridges^,

published in 1996, recomm ended new standards and practices for the design

and constru ction of durable bonded post-tens ioned concrete bridges. It covered the key

elements of design, detailing, ma terials, grout ing and c ertification for in stallation. 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 Gene ral 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 intern ationa l developments, especially those involving specifications, and close

contact maintained with similar groups in other countries and with the International

Federation for Structural C oncrete

(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 typ e have been b uilt 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

• reme dial (void) grou ting of existing bridges

• updated information on new test methods.

The second edition included a revised Specification for duct and grouting systems, together

w ith notes for guidance. That Specification, coupled w ith 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 pub lication in 2007 of revised versions of BS EN 445

(2)

,

BS EN 44 6'

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 u pdated Technical Report.

Experience of bo th grouted and unbonded post-te nsioning 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-ten sioning is also used in a variety of

other types of structures, such as storage silos, tanks and other containm ent structures.

The principles described in this Report will be equally applicable to such structures, but

detailed g uidance (e.g. on the layout of tend ons and the provision of vents) is not given

because of the variety of such structures.

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introduction

Wh ile this Technical Report is primarily concerned wit h sound principles supported by

good practice and procedures, the importance of a ttitud e 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 pro blem in the UK was the collapse of B ickton Meadows footbridge in

Hampshire in 1967, since when appreciation of the problem has slowly gro wn . In 1981 the

Transport Research La boratory published the results of an investigation into th e g rou ting

of 12 post-tensioned concrete bridges constructed between 1958 and 1977'

5

'. Voids were

found in the ducts of te n o f the bridges. The results were passed to the Standing Co mm ittee

on S tructural Safety*

6

' which concluded that, in structures containing a large number of

tendons, "th e risk of sufficient tendons failing by corrosion at any time to cause sudden

collapse is considered to be sma ll".

In 1980 Angel Road Bridge, No rth 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, M erthy rTyd fil, South Wales in 1982<

7

' revealed severe

corrosion o f 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 th e

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 thro ugh out the 1990s, as more cases of

corrosion became known.

In 1992, the bridge across the River Schelde in Belgium collapsed w itho ut warning as a

result of corrosion of the post-tensioning through the hinged joint of the end tie-do wn

member. O f particular interest is the Niles Channel Bridge in Florida. Built in 1983, this

1390m viaduct is of precast segmental box construction, wi th 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 attr ibut ed 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 recommend ations for grout,

grou ting and installation have been introduced in the USA by the Post-tensioning Institute

19

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Introduction 1

The collapse in May 2000 of a bridge in North Carolina was attributed to use of calcium

chloride in the grout used to plug tempo rary tubes through wh ich pre-tensioning de flector

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 spec ification and operations. In Germany, the Federal Ministry of Transport,

Construction and Housing has introduced Guidelines

for concrete

bridges

w ith external

tendons^.

Wh ile 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 bo tto m slabs. The reason for this appears to be concern over reinforcem ent

congestion, which may inhibit proper comp action of the concrete and the achievement

of adequate cover. In Japan, experience of voids and corrosion in post-tens ioned bridges

led the Japan Highways Public Corporation t o ban grouted internal tendons in post-

tensioned structures; the focus tended towards the use of unbonded external tendons,

and on the de velopmen t of preformed tendons pre-grouted w ith epoxies. The Japanese

developed and introduced transparent ducts.

The UK bridges that failed had internal prestress, but previous corrosion p roblems 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 cond ition of th e

prestressing and the efficacy of the g rou ting. Oth er bridge owners have been slower to

respond and it is a matter of concern that problems are often found by accident, either

during dem olition of redundant bridges or when othe r work is being carried out, as reported

by Woodward'

16

'. For example, the p roblems at Blackburn Road Bridge were only discovered

during routine deck resurfacing.

In 1992 the British Cem ent Association com missioned 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 a lmost impossible, so past statements such as, for exam ple, in the

United States, "there is visual evidence of corrosion in less than abou t

0.1%

of bridges",

must be treated with caution. It is especially imp ortan t to recognise that the o nly 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

mo torwa ys 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 9 2% of bridges were classed as good or as having minor problems

• 4.3% required attention and 3.5% had significant defects.

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1 Introduction

There was evidence of voids in grouted ducts but most were fairly small and had not led to

any significant d eteriora tion in the structure s. No ne 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 te chnolog y 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 p ost-tensioned structures was difficult.

However, the need for improved design and constru ction practices rem ained strong.

Given this general background of uncertainty it was not surprising that the Department

of Transport issued a temp orary ban on post-ten sioning for bridges in 1992'

18

' and later

developments have vindicated this a ction. 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 mo tivation to the further research and development

reported in this Report. This was confirmed by the issuing of Inte rim Advice Note16<

19

' in

1999. This was superseded in 20 02 by Interim Advice N ote 47

(20)

which referred to the

second e dition 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 th e last 10 years.

The work tends to be carried out by a specialist con tractor who often also takes on design

responsibility for the slab. Recently concerns over the adequacy of grouting in buildings led

to the gro uting in a number of recently com pleted post-tensioned slabs being investigated.

Significant numbers of either com pletely or partially u ngrouted ducts were found. This

led to CARES revising its po st-tensioning certifica tion scheme for buildings'

21

'. Much of

the guidance on the grouting of post-tensioned buildings refers to the second edition of

TR47.

However, as the guidance was pred om inantly related to bridges and some clearly

not relevant to buildings, this could lead to confusion and differences in interpretation of

wh at was best practice for buildings.

1.3 Summary of progress

Since the 2 002 edition of TR47 there have been several impo rtant intern ation al

developm ents. These include:

I pub lication 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 e xecution standard for Europe

• publication

of fib

recommendations for grou ting in 2002<

23

'

• publication

of fib

recommendation

Durability of

post tensioning

endons

in 2006 '

24

'

• public ation of ETAC 013<

25

> giving Technical Approval Guidelines for post-te nsionin g

systems in Europe

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Introduction 1

• publication of CW A14 646'

26

' giving requirements for installation of post-tensioning

systems in Europe

• publication of the second edition of the CARES mo del specification for post-tension ing

of slabs in buildings'

27

'

development of several pre-bagged factory-produced grouts

' preparation of a new edition of the

Na tional structural concrete specification^.

In their different ways, these activities affected the Working Party, in terms of the input to

this Technical Report. Since co mp atib ility is impo rtant, the approach ado pted 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 Wo rking Party was to generate confidence in the industry's

ability, with revised procedures, to design and build durable post-tensione d concrete

structures. In pursuit of this aim the W orking Party considered the fo llow ing areas:

• design and deta iling

• duct and grou ting systems

I grout materials

• certification of post-ten sioning operations and training

• testing

:

external and unbonded co nstruc tion

• remedial grou ting of existing bridges

• new test methods

and has now added:

1

grouting in buildings

;

unbonded tendons in buildings.

1.4 Sum mary 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 dur ability are

considered and the conce pt of m ulti-laye r protection is introduced. This requires the

provision of a number of protec tive measures on the basis that any individ ual layer of

protection may become ineffective but that the multi-layer approach gives adequate

assurance of protection against corrosion.

The effectiveness o f the various possible layers of protection is discussed and recomm endations

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.

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1 Introduction

1.4.1 Design and detailing

Segmental construction is a com mo n and economic m ethod 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-

dation s - can be assured. For precast segm ental construc tion, this is more difficult. The

situation is reviewed in Chapter 6, and a number of possibilities pu t forw ard; if, in a particular

case, any one of these can be shown to be equivalent to duct con tinuity, the n it may be

used. This is an area where product developm ent is continuing.

External unbonded tendons were covered in the second e dition of TR47. This method may

be used for any form of prestressed co nstruction, 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 inter im specification and comm entary were published by the Wo rking 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 othe r intern ationa l docum ents, 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 44 5« , 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 e dition were as follows:

• requirement for full-scale groutin g trials on each project relaxed

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

I clear recommen dations for plastic ducts.

These built on the innovations introduced in the first edition:

I ducts to be of electrically non-co nductive, corrosion-resistant durable material forming

a double corrosion protection system in comb ination w ith the grout

• duc t systems pressure tested

• add itional vents

• additional testing.

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Introduction 1

The use of plastic ducts is intended to ensure that the duc t itself provides a ddition al

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 e nvironm ent of the gro ut. Pressure-testing before conc reting will check the

integrity of the d uct 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 co mpo nents, are given in a

fib

Technical Report'

32

'. A further advantage of

non-m etallic ducts is tha t some test m ethods reviewed by the W orking 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 me tal ducts used in b uilding

structures means th at 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 w hich, 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 ge t it into the duct. Assisted by the vacuum, the duct w ill be completely

filled.

Providing

a vacuum pump (or pumps) and the associated valves etc. and operating the system is

more expensive than straightforwa rd pressure grouting . The me thod has had very lim ited

use in the UK (see, for example, Balvac W hitley Moran'

33

') althoug h it is more widely used

elsewhere in Europe. However, it has a very relevant application fo r regrouting as wi ll be

discussed. Wh ile the W orking Party has undertaken some development work on vacuum

grouting, the emphasis in this Report is on gettin g the standards and procedures right for

pressure gro uting , which w ill be used in the ma jority of cases.

1.4.3 Grout materials

Prior to 1992 it was com mo n 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 'comm on grout'. The properties of such cement are variable, particularly fro m one plant

to a nother, resulting in variab ility in the properties of the grout. In add ition, difficulties

arose due to variations in the weight of bagged cem ent; tolerances of ±2kg in 50kg bags

were not unco mm on, and outside the desired tolerance of 2% . However, tolerance on

the w eight 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 com mon g rout under all circumstances, such as variable temperature conditions.

Consequently a Working Party sub-group developed a prepacked 'special grout' w ith more

reliable and consistent properties, though it is still subject to q uality con trol and testing. A

research project was in itiated , supported by LINK fund ing and overseen by the sub-group, to:

• develop an improved grou t with properties that consistently meet the revised specification

• dem onstrate that the grou t has adequate performance under site conditions

• investigate methods of grouting and mon itoring , including trials of the 'grout stiffness test'

• provide data that satisfy the Highways Agency that the gro ut can be used in bridge

construction.

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1 Introduction

The results from this project were available in a draft final rep ort in May 19 96'

34

', and were

considered when for mu lating the recommend ations in the first edition of TR47. Since then,

feedback has been obtained on the use of special grouts in practice. In ad dition , results

became available fro m a major BRITE EURAM project on grouts and gro uting, 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 ed ition of TR47 included both com mon and special grouts w ith in its scope. This

was the term inolog y used in th e previous editions of the European Standards. The revised

Standards simply use the te rm 'grou t' although ETAG 013<

25

' still includes fo r special grout.

The Working Party is of the opinion tha t special grouts (mean ing in this con text a

pre-bagged product simply requiring the addition of water) should generally receive first

consideration because of the ir 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 co mp atible cement and

additives, for the entire job . Prepacked special grouts shou ld be the first choice for qua lity,

to minimise variables and attendant risks but this does not exclude combinations of

controlled m aterials on the basis that the quality of the end product is the important factor.

The CARES po st-tensioning certificatio n 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 trainin g. In the past, grouting of ducts has sometime s 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 t otall y ungrouted . Consequently

CARES, togethe r w ith the Post-Tensioning Association, developed a Ce rtification Scheme

in consultation wi th th e 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 th e W orking Party, as more companies have

become certified, and on problems experienced in pra ctice. (One bridge in particular was

closely monitored and in general the specification worked well.) Most problems have been

of a practical nature, invo lving connections, vents, gaskets and taps, and a lack of data on th e

friction characteristics of the ducts. All of these p oints have been considered, in producing

revised specifications. The CARES scheme is now com ing of age and is a positive con tributor

to improved durab ility. A revision of the scheme was made in 200 7/8 to reflect experience

of use in the interve ning years and to align it w ith European requirem ents.

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Introduction 1

In response to the increase in post-tensioned building con struc tion, CARES has broadened

its scheme to include specific post-tensioning requirements for buildings which in some

aspects are distinctly different to highways structures . Additiona lly 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 interna l tendons cannot be inspected visually m eans that reliance has to be

placed on indirect methods to confirm the adequacy of the corrosion protection system

emp loyed. The Working Party considered num erous 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 rou tine use.

This Report concen trates on tests that are unique t o th e gr outing process, are of pra ctical

application, and provide information relating to quality at a stage when remedial action

remains possible. Innovative methods th at are no t likely to be widely known are fully

described.

These include a me thod based upon the stiffness of gro ut developed by the

Wo rking Party before the first edition of TR47. Pressure is applied to the g rout before it has

hardened, and analysis of the 'spongy' response enables accurate c alculation of th e to ta l

volume of trappe d gas. The technique was first investigated with in a num ber of research

projects, inc luding 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 e quipm ent has not been taken up and has now been scrapped.

For external tend ons, inspection and testing are som ewh at easier because the tendons

are norm ally accessible. This does of course require a regular programme of inspection t o

be follow ed 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 minim um mandatory level of testing.

This may be associated with the required p roperties of th e gro ut, where specific tests are

given. There may also be testing associated wit h the duct system, where stron g reliance is

now placed on the standardised approval system developed by //£>

(32)

. In this conte xt, a

site-specific d uct assembly verification tes t is also recom mend ed and possible add itional

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 furth er developm ent and experience of use, neither is included in the Standards.

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Technical Report No. 72

Durable post-tensioned concrete structures

Recommendations for durable

post-tensioned concrete bridges

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Factors affecting durab ility 2

2. Factors affecting durability

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

classified as those that directly attack the concrete and those that directly or indirectly

attack the reinforcement or prestressing components.

Attack of the concrete is not covered in this Report. However, sulfate attack and alkali-silica

reaction are now well un derstood and guidance is available - see for examp le 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 w ith different intensities

of the various m echanisms, mo stly in ma terial 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 s teel

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 pene trating 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

dom inant factor for bridges and car parks is undoubtedly external chlorides.

It follows tha t, in developing a design strategy, the nature and intensity of the aggressive

actions - and how they m ight penetrate to the steel - is of fundam ental im portan ce. This

applies b oth to concep tual 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

ma terial selection. The impor tance of integrating these aspects canno t be overemphasised.

The purpose of this chapter is to identify the key factors tha t affect dura bility, based on

feedback from performance in service. The main focus is on the performance of bridges

and buildings as a who le. The factors considered are:

• materials and comp onents

• expansion joints

• construction quality

• construction joints

• cracking

• duct and anchorage layout

• precast segmen tal con struction and jo in t details

• proxim ity to seawater

• road salts, wa terpro ofing and drainage

• access for inspection and maintenance .

13

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2.2 M aterials 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 w ith a clear idea of

performance requirements, and of a methodology that will ensure that the chosen items

do in fact comply.

2.3 Co nstruction q ual i ty

Poor workmanship and construction defects are m ajor issues, which strongly influence the

level of du rability a ctually achieved. A good exam ple of this, in the past, was ineffective

grouting for post-tensioned work. However, the issue is wider than that, ranging from

poor com paction 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 co nstruction are properly considered, and in deriving details tha t e nable m aterials

and components to be fitted toge ther on site.

2.4 Expansion join ts

A high propo rtion 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 Departme ntal Standard on the requirements fo r expansion join ts, BD 33/94<

39

>,

and a Departmental Standard and Advice Note on

Design for

durability,

BD 57/1 and

BA 57/01

(40

'. These docum ents encourage the use of continuou s bridge decks and integral

abu tmen ts wherever possible, to elimina te expansion joints and hence reduce the risk of

contam inants reaching sensitive parts of the structure .

Where expansion joints are used, provision should be made for inspecting the m and the

structure underneath, and the details should be based on the assumption that joints will

leak and will n ot provide prote ction against ingress of water and road salts. Appropriate

drainage paths for the leakage should be provided w hich ensure that it can not get access

to the prestress anchorages or bearings and that the w ater is not allowed to pond. This is

especially imp ortan t if intermed iate 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 C onstruct ion joints

Well-made construction joints should not leak, particularly when protected by water-

proofing membranes. However, waterproofing membranes often do no t provide a complete

seal, and do no t last indefinitely, and join ts 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 constru ction joint s, care should be taken in detailing.

14

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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 imp orta nt.

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 durab ility is largely related to

corrosion,

and depends on the type and m agnitude of the cracks - see Concrete Society

Technical Report 44 ,

The relevance o f cracking in concrete to corrosion ofreinforcement^.

Care is required, when considering the layout and sequencing of concrete pours and

prestressing, to minim ise the risks of cracking, particula rly near anchorages. App lying a low

initial prestress at an early age can help counteract early-age cracking. The reinforcem ent

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

wit h BD 28/87<

43

) or the relevant part of BS EN 1992<

44

>.

Cracks parallel to and aligned w ith th e ducts can occur - due, for instance, to transverse

bending in reinforced sections, or to thermal effects at significant changes in cross-section

- a n d 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 introd uction of an extra layer of protection into the m ulti-layer

prote ction system. Cracks at righ t 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 wit h norm al design practice.

2.7 Duct and anchorage

layout

The method and form of co nstruction should be considered at the preliminary design stage.

They will often significantly affect the layout of prestressing tendons and the location of

anchorages. For examp le, the layout of tendons for span-by-span construction w ill 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 subsequen t inspection , as we ll as suscep tibility to wa ter 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 the re is a concern tha t, due to

their shape and location, they may provide a path for contaminants to the prestressing

tendons. Anchorage layouts are especially imp orta nt for externa l prestressing systems, as

is the d etailing of ducts w here they pass throu gh deflectors and diaphragms.

15

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2.8 Precast segmental

construct ion and joi nt type

Segmental constru ction is considered separately in Chapter 6, since p articular care is

needed; only genera l aspects are considered here. The only tw o bridge collapses in the UK

due to tendon corrosion have been in segmental structures. Both had precast segments w ith

thin mortar joints and incompletely grouted ducts. When considering the risks associated

with segmental construction, it is impo rtant 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 join ts

u

match-cast join ts - epoxy or dry.

Sufficiently wide in-situ concrete joints, and match-cast joints properly sealed with epoxy

resin,

can be satisfactory in durab ility term s. The m ain durab ility 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

con tinuity of the ducts across the joints. The Wo rking Party believes that duct con tinuity

across the joints is vital when grouted tendons are used unless some o ther 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 pene tration and allow corrosion if not form ed 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 constru ction: in such situations

structures need greater corrosion protection.

2.10 Road sa lts,

waterproofing and drainage

Road salts are applied to most UK road bridges in the w inter; on some structures in the UK,

and many in other c ountries, 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-bor ne chlorides, and it is clear that road salts wil l not be applied in future, it may

be possible to reduce some of the layers of pro tection described in the fo llow ing 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 inspec tion

and maintenance

One of the main concerns abo ut internal prestressing systems is the inab ility to inspect

the tendons visually. However, it should be remembered tha t e xternal prestress can

usually only be inspected in the stra ight sections between anchorages and deviators and

then on ly if it is not enclosed in a grouted duc t, although tap ping of externa l ducts can be

helpful in detec ting voids. Enclosure of extern al prestress within a duct w ith holes for

inspection using an endoscope is possible. The concern is tha t, if the tend ons cannot be

inspected, corrosion may proceed undetected and lead to collapse without warning.

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Factors affecting durab ility

However, one of the advantages of internal prestress is tha t the concrete itself forms one

of the layers of p rote ction . Althoug h 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 with in me tal ducts bu t it is less

convenient than radar and has safety imp lications.

Access for inspection and maintenance should be regarded as an essential element in the

mu lti-layer prote ction 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 ap plication 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 strong ly in any inspection checklist, togeth er

with checks on changes in moisture conditions, caused, say, by failed expansion joints or

blocked drains. M aintaining the exposure con dition assumed in design is an im porta nt

element in management and maintenance.

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Available protective measures

3. Available protective measures

For the purpose of d efining the standards and practices in this Report, the con cept of

mu lti-layer prote ction has been introdu ced. This has been used for ground anchorages

and requires the provision of a number of protective measures, on the basis that the to ta l

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 - m u lt i-

layer protection

The factors that mig ht require consideration (see BD 57/01 and BA 57/01,

Design for

durability

1

'

40

' ) w i l l inc lude:

: the location o f the bridge, and the associated general and local exposure cond itions

the provision of continuity, possibly in the form of integral bridges

• access for insp ection, testing, maintenance, and possible replacement of shor t-life

elements

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

i the me thod of constru ction, wit h its associated buildability and workm anship factors.

I the deck waterp roofing system

I the p rovision of e ffective drainage and avoiding ingress of water.

Procedures for the design of individual elements are available, for example BS 5400'

46

' (which

was effectively with dra wn in April 201 0), BD 24/92<

47

>, BS EN 1992-2<

44

> (implemented by

IAN 123/10'

48

') and CIRIA C543<

49

>. These con trol o ther imp orta nt dura bility issues, such as

the quality of the concrete and the thickness of the cover to the ducts and reinforcement;

they also draw atten tion to important features such as time-dependent movem ent and

deform ation at different times, both during cons truction and in service. Associated w ith th is

are other relevant m atters, n ot norma lly covered in design codes, such as the avoidance

of poor details that are known not to work well in practice.

Finally, there is the pro tection o f the prestressing hardware itself. This involves consideration of:

• fi l ling the ducts wit h cement grout

• corrosion-resistant duct mate rial

..] ducts designed to exclude con tamina nts

• location , deta iling and protec tion of anchorages.

A full treatme nt 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 judg em ent are needed to suit each set of circumstances and it w ould not

be appropriate to recommend a fixed number of layers of p rotection. As an example, if any

one of the protective measures could be guaranteed to totally exclude all contaminants,

no other layers would be necessary.

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Conversely, if any one of the proposed layers was found to be ineffective, it should not be

considered a suitable layer of pro tectio n. The easy design option is to use every conceivable

protective measure available: the skill in dur ability design is to choose the mo st 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 struc ture 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

transpo rt mechanisms are required. In general climatic terms, this involves a co mb ination

of water and wind. In local exposure terms , water in the for m of vapour, spray, driven rain,

runoff or ponding can interact w ith th e o uter surfaces. The effect of this inte ractio n can

be exacerbated by the influence of tem pera ture, causing joints t o open or cracks to for m.

Designers therefore need to carefully consider the location of the bridge, wh at local

conditions can form, and how these interact with the outer surfaces. A prime concern is

to minimise the uptake of water, and to g et rid of any water th at does reach the bridge as

quickly as possible. This involves a com bina tion of conceptual design, struc tural detailing

and atte ntio n to bridge 'finishings', such as drainage, wate rproofing 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 b oth the profile and tex ture 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 jo ints have to be carefully designed and de tailed, with

provision made to quickly remove water which will inevitably leak through.

Wh ile estimates can be made of the likely climatic conditions, and the effects of temperatu re

assessed in terms of stress and deformation, effective design and detailing are largely

qualitative, based on experience and feedback. A further essential element, at th e conceptua l

stage, is to make positive plans for inspection , maintenance and the replacem ent o f

elements w ith a short service life.

3.2.2 Bridge deck

waterproofing systems

The waterp roofing system is the first line of defence against ingress of road salts app lied from

the bridge road surface. Un fortun ately the re are no systems available that can be guaranteed

to remain waterproof through out th e life of a bridge. It is understood that modern high-

quality liquid-applied membranes are likely to be more effective than earlier systems.

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These membranes can be applied in either one or tw o coats; however applied, they should

be proved using 'pin- hole' detection equipm ent, w hich w ill give reasonable assurance of

the integrity ofthe membrane. Careful preparation ofthe concrete surface and application

of the membrane are im portan t, 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 treatm ents on concrete can provide a protective barrier against

aggressive agents. Detailed guidance is given in Concrete Society Technical Report 50,

Guide o surface treatments for protection a nd enhancement of concrete^.

In selecting a

surface treatment, whole-life performance should be taken into account as the costs of

app lication, m aintenance, 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 chem ically, w ith 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 produ cts or physically block the pores wit ho ut reacting with concrete. These

materials do not prevent water penetration and chemical attack but the rate at which

they occur is reduced. They do no t prov ide an effec tive 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 n umbe r of a pplications th at 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 tw o- or three-c oat application to be used to seal average-qua lity

concrete. Some low-solids wa terbom e products may also be used as sealers. The solids

are dispersed as fine particles rather tha n in solu tion and the effectiveness of even very

low viscosity produ cts may be limited if the particles are large in relation to the pore size.

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In-surface sealing can be achieved w ith solvent-free systems. How ever, even with the

lowest viscosities available, the depth of penetration is likely to be very lim ited unless a

vacuum-assisted application tec hnique is used.

3.2.4 Drainage

It is essential tha t the drainage system sho uld work efficiently to remove water from the

road surface as we ll as the w ater th at passes through the surfacing dow n to the bridge

deck waterpro ofing system . The details of the drainage paths should be such that if items

of the equipment fail, leak or becom e blocked, then th e wa ter does no t find access to th e

prestressing system. BD 57/01 and BA 57/01'

40

' give advice on drainage systems, and on

dealing with th e passage of water at boundaries and at supports.

3.3 Individual structural

elements

Many of the com ments in Section 3.2 apply equally to individual structural elements in terms

of concrete profiling, texture , and articulation . However, additiona l features arise from the

structural design of the elements themselves, as contained in Codes and other authoritative

guidance docume nts. M ostly, these relate to stress levels, and the con trol of cracking,

both at early ages and in service due to the influence of loads, creep and tem perature . In

ma terial specification terms, there is also the basic pro tective layer of an adequate cover

to the steel in a good-qu ality concrete.

BS 540 0: Part 4<

46

' (which was effectively w ithdraw n in April 201 0), 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 H ighway Works^

gives a concrete

specification wh ich, together with the specified cover and good -quality construction, will

give a reasonably dense, imperm eable concrete protec tion 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 norma l circumstances there is no reason to believe that concrete

designed and con structed in accordance with current standards and specifications does

not provide adequate p rotec tion to the ten don . However, feedback from service (see for

example Wallbank'

54

') has demon strated th at the specified cover is not always achieved

in practice; good q uality con trol is essential.

As w ith all concrete structures, it is possible in special circumstances to improve th e

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 ove rall weight and cost to the structure. Reducing

the permeability of the concrete is possible by reducing the wa ter/cem ent ratio or by cement

replacement w ith fly ash (pulverised fu el ash, pfa) or grou nd 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-ten sile steel. However, the W orking Party is not aware of any

specific cases of such problems.

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3.4 Prestressing

components

In simple terms, the essential elements of prestressing comp onents 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 wil l differ depending

on whether bonded or unbonded construc tion is used. The primary p rotective system for

bonded constru ction is the cem ent-based grou t. For external unbonded constru ction, a

wider range of protective systems is available; the anchorage m ethods may also be different,

and there is the furthe r need to consider special features such as deviators and couplers.

3.4.1 Prestressing tendons

Prestressing tendons have developed significantly since the 1950s with ma jor improvements

in technolog y and increasing ab ility to provide larger, more con centrate d 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 intern ationa l Standards; this is funda me ntal to structu ral design, and not especially

the concern of this Report.

A nu mber of steel strand types are available; these norm ally consist o f seven wires. The

diameter of the strand, its compactness, strength and metallurgical properties may all vary.

Strand can be made w ith a bu ilt-in prote ction layer. This may be a physical layer such as

galvanising or epoxy coa ting or an add itional means of inspection such as 'intelligen t strand':

in this a fibre-op tic sensor is passed throu gh th e centre wire of the strand and can be

used to m on itor strains and breakages in the strand . The effectiveness of these special

inspection and mo nitor ing facilities needs to be carefully considered.

Of particular concern in the past has been the suscep tibility of some types of strand to

hydrogen embrittlement/stress corrosion cracking (HE/SCC). It is understood that strand

manu factured 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 1 563 0-3'

59

'. Acceptance criteria are given in the

fib

report

Stress corrosion cracking resistance test or prestressing tendons^.

There have also been concerns abou t the effectiveness of epoxy coa ting, 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 whe ther the risk can be completely discounted as damage to coating can

also occur during insta llation.

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'Intelligent s trand ' may prove useful and has been tried on a bridge in the UK. However,

as it is still being investigated, it cannot be recommended ye t for w idespread use u ntil

satisfactory results are confirm ed.

3 . 4 . 2 D l J C t S During the development of post-tensioning systems, several me thods have been used to form

ducts. For straight tend ons, formers have sometimes been used which are subsequently

removed, leaving un lined ducts. Lined ducts have norm ally been forme d using welded or

spirally wou nd steel tube a lthoug h cardboard tube has occasionally been used - see

Woodward and Williams'

8

'. Unlined and biodegradable ducts such as cardboard tubes

should no t be used. The advantages of spirally wound tubes are tha t the y are flexible and

can be bent on site to the required pro file.

More recently, non-me tallic ducts have been used. These are made of high-density p oly-

ethylene (HDPE), now known as PE80, or po lypropylene, and have a number of advantages:

corrosion-resistance

11 better sealing against ingress of contaminants

• can be pressure-tested during construction to dem onstrate integ rity

I more po tentia l to be 'seen thro ugh ' by some non-de structive testin g techniques.

If the du ct 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 m ain advantage

of non-metallic ducts is that they can form a sealed system around the tendon and

minimise the risk of conta mina nts reaching the tendo n. The advantage of being able to

penetrate non -me tallic ducts w ith testing techniques rema ins 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 m ay be possible.

The Highways Agency revised g routing specification for bridges requires the use of co rrosion-

resistant ducts which for interna l tendons are bonded to the surround ing concrete. The

ducts should be pressure-tested before concreting to verify the assembly. A suitable test

metho d 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 min imum are used

to allow for the effect of the stressed tendon 'biting' into the duct wall.

The use of n on-m etallic ducts requires a reassessment of som e of the properties of ducts

and their effects on the w hole protection system. The main properties of the m aterials

currently used for non-m etallic ducts may be compared with those for steel ducts. Non -

metallic ducts:

• do not corrode

• effective ly resist the passage of chloride ions

•

I do not conduct electricity

3 have a high coefficient of ther ma l expansion (typically 140 x 10~

6

/°C)

• have a low Young's mod ulus (typically 800M Pa).

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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 contam inants and the tend on. Ingress of

chloride ions through the duct ma terial is theore tically possible, but is not likely given the

thickness of the mate rial. It is interesting to note tha t HDPE is used as the o uter skin of

marine electricity cables, wh ich 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 me tal objects, and, if

not co ntrolled, can generate harm ful 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 n ot co ntrolled, the resu lting stray currents can cause

accelerated corrosion of prestressing tendons at the point w here the current exits the

tendons. Adjacent to AC power lines, conductive paths should not exceed 500m - we ll

beyond the longest post-tensioning system tendon. N orma lly the principal control measure

is to ensure that the tendon s and anchorages are electrically isolated. This needs particular

care and is probably only tot ally reliable with plastic encapsulation of anchorages. The use

of plastic ducts, together with a nom inal concrete cover of 45m m 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 ther ma l expansion o f steel and of concrete are similar and consequently

changes in temp erature do not cause significant relative strains. However, this is not true

for no n-m etallic ducts, which have a high coefficient of the rma l expansion. Concerns

have been raised that an increase in tempera ture, say during hydration of the concrete,

could cause a non-m etallic duct to expand m ore than the concrete. A situatio n may arise

where the duct expands while the concrete is plastic and contracts after the concrete has

hardened, leaving a gap arou nd the duct. This has no t been observed in g routing trials,

presumably because the early gain in strength of the concrete can p artly restrain the

expansion of the d uct; the effects have been shown to be minim al and this is thoug ht t o

be purely a theo retical prob lem. This observation has been confirme d by Kollegger'

61

'.

It is also true tha t, under gro uting pressure, the duct wo uld expand again into any gap.

Full-scale trials have shown the importance o f 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 the ir support system should be robust during casting.

One claimed advantage for non-m etallic ducts is their a bility to fo rm a sealed system to the

tendon tha t w ill exclude contaminants from the duct. While it would be possible to design

such a duct system, currently available ducts cann ot be assumed to be guaranteed sealed

and fully w ater tight. The pressure-test acceptance criteria are related to wh at is possible

using the currently available non-metallic ducts and demonstrate that the duct has been

properly assembled. Tests by the W orking Party show th at the acceptance criteria specified

will not guarantee a full barrier to the ingress of contaminants at the joints in the duct.

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Difficulties were experienced wit h joints between lengths of duct, and w ith seals between

the ducts and anchorages, and also with ven ting tubes. It is for this reason that mu lti-layer

prote ction is required. It is hoped tha t fully sealed ductin g systems wil l be developed in due

course, in which case acceptance criteria wo uld be much tighter than currently specified;

on the other hand, less dependence on multi-layer protection would then be required.

The developme nt of prestressing systems based on n on-m etallic ducts is still at a

comp aratively early stage, especially for bonded co nstruction . Much w ork has been done

on the properties of the basic m aterials, but the design and ma nufacture of plastic ducts

varies between suppliers. Based on all available inform ation ,

the fib

Technical Report

Corrugated plastic ducts

for internal

bonded

post-tensioning

{32)

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

is becoming the norm.

The duct should be large enough both to allow threading of the prestressing tendon and

to fa cilitate gro uting. The diameter depends on the size of the tend on, and the overall

length and curvature of the duct. Norm ally a maximum tendon -to-duct area ratio of

0.40 to 0.45 should be used, increasing with te ndo n size. The ten do n-to -du ct area ratio

is defined as the area of the strand, based on its a ctual steel area, divided by the interna l

cross-sectional area of the d uct. For short tendons w ith 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.

3.4.3 Anchorage location

Feedback from inspections clearly demonstrates tha t the ingress of con tamina nts is most

common at anchorages, with corrosion being initiated in the tendon immediately behind

the anchorage (often in the presence of imperfect grouting). Therefore, it is very important

tha t a tten tion is given to the location of anchorages and to detailing to prevent access of

water t o the d ucts. Anchorage systems vary with tendo n type and supplier. The guidance

in this chapter relates to anchorages for interna l grouted tendons and external unbonded

systems. In both cases, the W orking Party considered the subject to be so imp orta nt tha t

illustrations are given of preferred solutions (see below).

The layout of the prestressing tendons and the location of anchorages are dependent on the

method of construction. In the case of a simply supported beam cast in situ the anchorages

wo uld norm ally be at the ends of the beam (Figures 1-4), but there are many form s of

construction that require anchorages at different locations. These may be broadly sub-

divided as follow s:

• dead-end anchorages with in the body of the concrete

I anchorages in blisters with in 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)

• face anchorages on the join t betwe en segments in span-by-span, in-situ or precast

segmental construction.

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Figure 1

Buried anchorage at end of deck with an

abutment gallery.

F igure 2

Exposed anchorage at end of deck with an

abutment gallery.

The deck overhang must be free from

cons truct ion joints as they co uld leak,

al lowing water to reach the anchorage

L iqu id ap p l i ed— '

wa te rp roo f i ng

membrane

Expansion joint

•

n

Min.

8 0 0 m m

Min 1500mm

preferab ly > 1800m m

^Abutment dra inage

channel

The deck overhang must be free fr om

constru ct ion jo in ts as they co u ld leak,

a l lowing water to reach the anchorage

Liquid applied

waterproofing

membrane

Expansion join t

Min 1500mm

preferably > 1800m m

Abutment drainage

channel

Figure 3

Exposed external anchorage at end of deck

with an abutment gallery.

Figure 4

Restressable external anchorage at end of

deck with an abutm ent gallery.

The deck overhang mu st be free fro m

construction joints as they could leak,

allowing water to reach the anchorage

Expansion join t

Space for stressing

M i n 8 0 0 m m

Min 1500mm

preferably > 1800mm

v

Abutment drainage

channel

The deck overhang must be free fr om

construction joints as they could leak,

a llowing water to reach the anchorage Expansion jo in t

Min 1500mm

preferably

> 1800mm

Abutm ent drainage

channel

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Liquid applied

waterproofing

membrane

Min.500mm

-

Construction join t

Note: Anchorage end cap

fitted with grout during

tendon grouting operation

Drip

to

prevent seepage

through construction joint

from reaching anchors

Liquid applied

waterproofing

membrane

Grout injection

hole/vent

in

anchorage

Figure

5

Anchorage

at

top blister using exposed

anchor.

Figure

6

External blister an d bonded face anchorages

for in-situ segmental construction.

Liquid applied

waterproofing

membrane

Construction Joint .

Min.

500mm

Restressable

anchor

Figure 7

Anchorage

at

bo ttom blister using buried

anchor (internal tendon).

Liquid applied waterproofing membrane

(Note:

Membrane

may be omitted

if not in a vulnerable position)

Anchorage

end cap

Concrete cast

after stressing

and grouting

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Min 500mm

Liquid applied w aterproofing membrane

double thickness required over pocket

(Note: Second complete layer covers

patch first a yer)

Sealant at

construction

joint

Pocket filled w ith

non-shrink flow able

capping concrete

to BD 27/86 item 4

Reinforcement lapped with

starters from main member

Drain pipe or other de tail to ensure

pocket kept free from water and

contaminants during construction

Figure 8

Top pocket anchorage.

(This is NOT recomm ended, unless external

protec tive layers are used.)

Wherever anchorages are located the designer should ensure that an adequate multi-

layer pro tection system is provided.

There has been much discussion within the W orking 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 w ith to p pocket anchorages but none have occurred, to the knowledge

of the W orking Party, where the tendo n has been properly grou ted. It is no t clear whe ther

or no t top pockets con stitute an addition al level of risk. However, there is a strong feeling

among many engineers that the top pocket provides an unnecessarily convenient route

for conta minan ts into the anchorage and tendo n. There are few situations in which an

alternative to the use of top pockets cannot be found, and it is recommended that they

should n ot norm ally be used. If there is no alternative (for example in some bridge

refurbishment schemes) then th e design and con struction 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 whethe r the anchor head is left

exposed.

One of the 'preliminary design recommendations' in a report by Ricketts'

62

' is

tha t "All anchorages, apart from dead end anchorages deep within a concrete mass, be

designed so tha t th ey 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 numb er of layers of

prote ction. The end cap of the anchorage has to be removable to inspect the ends of the

strand and, once rem oved, it may be difficu lt to replace and ensure an airtig ht

seal.

The

advantages of burying the anchorage in a concreted pocket are tha t the end cap can be

filled wit h g rout 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 with in the concrete are also acceptable but have the practical

disadvantage of requiring the tendo n to be in place before concreting.

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At expansion joints, BD 57/01 and BA 57/01'

40

' recommend the use of an abu tme nt 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 wa terproofing covering buried anchorages

(see Figures 1-4). Anchorages tha t are required to be inspectable should also be replaceable.

Wherever possible, BD 57/01 and BA 57/01 recommend the use of integral bridges, avoiding

the use of expansion joints . The Work ing Party sub-group has not been able to agree on the

most appropriate measures to prote ct anchorages at the ends of the deck. One possibility

is that an abutm ent gallery is provided, similar to th at 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 fa cilitate maintenance

should this be required. Alternatively, it may be possible to dispense w ith visual inspection

by placing a corrosion probe adjacent to the anchorage and using a buried anchor detail.

It is anticipated tha t, with greater use of integ ral bridges, new details w ill be forthco ming .

3.4.4 Anchorage details Figures 9-17 give typical anchorage details for a number o f situations. In developing the

details the following points have been taken into account:

I The anchorage should have m ulti-layer protection . No individual layer can be assumed

to be effective for the life of the structure.

I The detailing of expansion joints sh ould be such that, when the jo in t leaks, the water

is directed away fro m th e anchorages and in to a properly detailed drainage system.

I The anchorage end cap should be filled with grout d uring groutin g of the ten don . The

end cap is shown bolted to the anchor p late in the figures, but in some systems the cap

is bolted to th e anchor head. Provided tha t the anchor plate and head are m achined

for a close fit, this is an acceptable deta il.

I Any concrete used to infill pockets should have flowable non-shrink cha racteristics in

accordance with BD 27/86

(63)

, and should be held in by reinforcement.

B The anchorage or capping concrete should be covered with the bridge waterp roofing

membrane.

I Exposed anchorages should be protected fro m seepage throu gh c ons tructio n joints in

thin slabs.

It should be noted that deck overhangs without construction joints necessitate special

devices to enable stressing jacks to access the anchorages. N orm ally a crane w ith a special

lifting device or a acking trolley wo uld be required. For very large tendons this can be

prohibitive and consideration could be given to use of a construc tion join t w ith a waterbar

and sealant in a surface groove .

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Grouted bonded post-tensioned construction for bridges

4 . Grouted bonded post-tensioned

construction for bridges

The overall design strategy recommended in this Report is that of multi-layer protection.

Available protective measures are reviewed in Chapter 3 and in Chapter 4 the focus is on

grouted bonded construction. The chapter begins with a review of grouts and g routing and

is followed by details of a recommended protection system for this form of construction.

4.1 Grouts and g rou ting

One of the main advantages of a properly grouted duct is the alkaline environm ent created

within the du ct. A major concern w ith existing bridges has been the qu ality of th e grouting

and the protection provided to the tendo ns. The grouting operation has often been under-

taken by inadequately trained personnel who have not understood its importance. There

are even instances of ducts being left com pletely ungrou ted'

64

'. The gro uting Standards

give details of improved grouting methods, vent layout and materials. Site operations

should be carried ou t by companies and operatives satisfying the CARES certificatio n

scheme - see Chapter 15.

The use of plasticisers in grouts is common practice, and the resulting improved workability

and reduced water/ce me nt ratios can on ly be beneficial. There has been discussion of the

safety of using expanding agents in g rou t. The expansion is norm ally achieved by incorporating

alum inium particles and it has been suggested that the hydrogen given off can cause

em brittlem en t 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 can

generate air bubbles. To meet th e requireme nts of the revised Standards it is likely th at

plasticisers and expanding agents w ill 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 canno t guarantee complete filling of a ll ducts and some

sma ll voids may still occur. However, it m ust be remembered tha t voids themselves do

not cause corrosion of th e tendon s.

One of the main concerns with bonded tend ons and anchorages is the difficu lty o f inspection.

It may be possible to partially overcom e this difficulty by installing sensors or probes that

allow remote mon itoring of the p oten tial 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 a dequately investigated the usefulness and reliab ility of such

probes and cannot yet recommend their use. However, it is anticipated that with some

developm ent they may be used as an add itional assurance, if required, by war ning of

potential corrosion.

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4.2 Vents and gro ut

injection

Trials undertaken on behalf of the Transport Research Laboratory

65

'

66

' showed tha t an

additional grout vent just beyond the crest in the tendon profile, in the direction of grouting,

can be beneficial as tha t area is particularly prone to trap ping air. Any air or bleed water

carried past the vent at the crest during g routing should migrate back up the duct and be

removed at this additional vent. The additional vent should be closed before the crest vent.

The trials also showed th at larger vents are desirable. The inte ntion of raising vents, where

possible, at least 500 m m above the d uct p rofile, is to provide an effective head of grout.

This Report gives guidance on spacing, location and size of du ct vents based on these trials.

If the direction of gro uting is not known a t the time o f casting, add itional vents wil l be

required on both sides of the crest vent.

All anchorages are supplied w ith a tapped hole for use either for injecting gro ut or as a

vent. The end cap to the anchorage is also supplied w ith an air vent and the cap is to be

filled with g rout d uring grouting of th e 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 pos itioned at the top to act as a vent du ring grou ting. The final

vent is the tubed ven t on the end cap, which must always be placed at the highest p oint.

Anchorages with tw o holes avoid the possibility of the grout hole being placed incorrectly.

In the design of the vents it mu st be remembered th at the ve nt is part of the protective

system and needs to be sealed to the same level of airtightness as the duct. This requires an

engineered connection between th e vent and the duct and a sealable stopper where the

vent exits the concrete. If this is the to p surface of the deck and the ve nt is extended above

deck level for gro uting purposes, the extension w ill have to be removed before stopp ing

up the duct. Vents should be sealed on c om pletion of grou ting and also on remo val of

any extension tubes. An example of such a deta il is given in Figure 12. Care is necessary

here to ensure tha t the form er for th e pocket is held in po sition, and to avoid the top

reinforcement in the deck.

4.3 Recommended

protection systems

In proposing the multi-layer protec tion concept, an outline has been given in general terms

of a ll the factors that could com e into th e design equ ation, in consciously designing for

durab ility. This section is focused 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 en vironme ntal cond itions may be less onerous for rail bridges, in the absence

of de-icing salts, and for bridges in othe r countries where the environment is less aggressive.

To set the scene, the standards and practices outlined herein are seen as a comp atible package

of design, material and c onstru ction measures for UK applications. For the prestressing

system itself, the basis of the core recom mendations is one of quality, linking the Standards

to the supporting certification scheme and underpinned by the design recommendations.

All of this is summarised in the subsections which follow, togethe r w ith 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 de tecting and measuring voids in

fluid gro ut. These have shown great pote ntial for the fu ture but at present are seen as

potential additional protective measures, in support of the core requirements.

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Sealant at construction

jo i n t

Anchorage

end cap vent

Note: Anchorage end

cap filled with grout

during tendon grouting

operation. In some

systems the end cap

is bolted to the anchor

head

Anchorage end

cap minimum

cover 50mm

Grout injection,

hole/vent in

anchorage

Pocket fi l led w ith

non-shrink flowable

capping concrete, to

BD 27/86 i tem 4

Reinforcement/mesl

lapped with starters

from m ain member

Min 500mm

Liquid applied

waterproofing

membrane

Figure 9

Buried anchorage fo r stressed or dead end.

Grout injection,

hole/vent in

anchorage

Liquid applied

waterproofing

membrane

Anchorage

end cap vent

Anchorage end cap

Note: Anchorage end

cap fil led with grout

during tendon grouting

operation. In some

systems he end cap

is bolted to the anchor

head.

Reinforcement from 1st

segment projects into

2nd segment

Grout injection

hole/vent in

anchorage

1st Segment

(a) Elevation

.Liquid applied

waterproof ing

membrane


filled with grout during


Anchorage end cap vent

Anchorage end cap

surrounded in in situ

concrete from 2nd

segment

2nd Segment

Note:Anchorage end cap



Anchorage end cap

Grout injection hole/vent and

anchorage end cap vent

(b) Plan

Figure 1 0

Exposed anchorage fo r stressed or dead end.

Figure 11

Face anchor details in in-situ segmental

construction.

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Grouted bonded post-tensioned construction

for

bridges

Figure 12

Grout vent details

at

deck surface.

(a) Grout vent cast into concrete with

a

recess

at

deck level.

.

o

(b) Vent extension pipe and valve fitted during gro uting.

Note: Extension pipe must be rigid

f

used with grout stiffness

test.

Liquid applied waterproofing membrane,

double thickness over vent

<t • o \

• • • . . " . • . . • •

• t>

(c) Vent capped and recess filled w ith n on-shrink m ortar

4.3.1 PrestreSSing System

-

Ducts and vents should

be

corrosion-resistant and pressure-testable. The ducts

and

vents should

be

pressure-tested.

• Where there is no previous history, full-scale representative g routing trials should be

used to prove the grou ting m etho d, materials and personnel.

I Method statements should be prepared in advance for all prestressing op erations and

should

be

approved

by

an appropriately experienced C hartered Engineer.

All operations associated with the installation, stressing and gro uting of tendons

should be undertaken under the CARES cer tification scheme.

Anchorage and vent locations and detailing should fo llow the logic outlined in Figures 8 -12.

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4.3.2 The deck and its

elements

Concrete strength and cover requirements should be in accordance w ith BS 5400: Part 4'

46

'

(effectively w ithdrawn in April 2010) and BD 24/921

47

' m odified as required by BD 57/01<

40

>

and BS 8500(

53)

where app ropria te. For design to the Eurocodes, BS EN 1992-2<

44

>

(implemented by IAN 123/10<

48

') should be used.

Specification should be in accordance w ith the Specification

for H ighway Works^

as

amended to adop t BS EN 445<

2

), BS EN 446<

3

>, BS EN 447<

4

> and BS EN 13670'

22

'.

A waterproofing system complying with BD 47/99'

50

', and having a current Road and

Bridges Certificate issued by the Highways Agency, should be used on the deck surface

and in the o ther locations recomm ended in BD 57/01<

40

. It should be checked for integ rity

using appropriate non-destructive test equipment, including pin-hole detection equipment

for liquid-applied w aterpr oofing systems. Where a double thickness is used, the first

layer should be proved before the second layer is applied. In ad dition , the membrane

should be used to prote ct any anchorages left exposed in abutm ent galleries, inside

box girders and on bridge deck soffits.

The expansion joints and drainage system should be detailed to ensure that, in the event

of equ ipmen t failure or leakage, water ca nnot find access to the prestressing system.

Strand should be in accordance with BS 5896<

56

> or similar.

4.3.3 Possible additional

measures for exceptional

structures

Additional protective measures from the list below could be considered for exceptional

structures in unusually aggressive environments (e.g. bridges over the open sea), with

each structure being considered on its own merits:

I increased cover

• reduced concrete perm eab ility

• perfectly sealed ducts

corrosion-m onitoring devices

I special strands.

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External unbonded post-tensioned construction for bridges

5. External unbonded post-tensioned

construction for bridges

In external unbonded construction the prestressing tendons are unbonded and laterally

outside the co ncrete cross-section, and the forces are transferred to the concrete thro ugh

end anchorages and deviators; this includes such tendons located inside a box section. It

follows that the design and d etailing of these elements, and the development of protection

systems for the prestressing hardware as a who le, are pa rticularly impo rtant.

5.1 Advantages and

disadvantages

The advantages and disadvantages of external unbonded construction, compared with

grouted bonded construction, may be summarised as follows.

Advantages

Reduces self-weight.

• Makes placing concre te in the webs easier.

• Permits simpler tend on layouts.

• Reduces prestressing losses, especially due to fric tio n.

Facilitates inspection, restressing and replacement.

Gives early w arning of failure.

Reduces importance of cracking in the concrete in terms of corrosion p rote ction for

the tendons.

• Can perm it more rapid construction, wit h bigger spans due to reduced we ight.

Disadvantages

:

' The eccentric ity 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.

I The structural role of anchorages and deviators is more critical.

:

The safety and security of the prime struc tural mem bers is more at risk, for exam ple in

relation to vandalism.

• Can be more expensive.

In general, the advantages become m ore significant for bigger spans (greater than 40m )

or for long viaducts.

5.2 Background

The use of extern al unbonde d 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 cons truction, and Freyssinet progressed the technique

further in the 1950s. French 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.

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The first example in the UK was the Braidley Road Bridge in Bournemouth designed by

Gifford in 1967. Therefore experience in the UK is relatively recent and lim ited, although in

the 2 000s some notable externally prestressed bridges have been constructed. Attitudes

to the me thod have varied. After Braidley Road, there were few examples u ntil 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 tendon s, is perceived as a major advantage, and it is the only me thod curren tly

allowed in the UK for precast segmental construction.

In terms of du rability, the track record of extern al 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'

67

' and Medway Bridge in

Kent'

68

'. Reviews have been conducted on performance in service and examples of distress

have occurred, frequ ently due to lack of apprec iation of th e local forces and strains induced

by the prestressing, or to th e neglecting of tempe rature effects.

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.

5.3 S tructura l design and

basic performance

requirements

The structural performance of external unbonded construction is well understood, and

design m ethods are given in relevant codes BS 54 00 '

46

' (effectively w ithdrawn in April 2010)

and BS EN 199 2-2'

44

'. In applying these in the UK, add itional specific requirements are

contained in Highways Agency Standards and Advice Notes: BD 57/01 and BA 57/01'

40

'

are generally relevant in durability terms, but BD 58/ 94 and BA 58/9 4'

69

' focus on externa l

unbonded prestressing.

Going beyond basic design, BA 58 /94 gives a good d eal of inform ation at the detailing

level, in terms of:

i loads to be carried by anchorages and deviators

• coefficients of frictio n

• radii of curvature of tendons

• protec tion systems.

Clearly, these are key documents. In addition, they contain two important requirements,

which can have a strong influence on the prestressing system ad opted, and on the level

of prestress needed. These requirements are as follows:

• The prestressing and prote ction system should perm it easy inspection, and tendons

should be capable of being restressed or replaced, if necessary.

I The failure of either two tendon s or 25% of the tendons at one section should no t lead

to collapse.

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Some recent UK practice wit h e xternal tendons has involved locating interme diate blocks

or bonding points in the span, to ensure some increase in strain in the tendons under the

design u ltima te loads. The choice of tendons tha t can be restressed and replaced or tendons

tha t are only replaceable has a significant impact on the design, and whether provision is

made to de-tension by jacks and whether the consequent projecting lengths of strand are

provided at pe rmanent anchorages.

The decision should be taken wit h reference to the number of tendo ns, the spans and the

consequen tial costs.

5.4 Available protective

measures

Details of protective measures available internationally are given in the 1996 FIP state-

of-the-art report

M aterials and systems for external prestressing

1

-

70

1

.

Most experience in the

UK has been w ith the use of grease, wax or ceme nt g rout contained in polyethylene (PE)

tubes. It is recommended tha t o nly plastic ducts should be used, which should be of PE

material, at least of strength class PE80. Plastic ducts for external systems are relatively

thick and smooth (in comparison to those for bonded construction w hich are corrugated).

The systems now available w ill no d oub t continue to evolve and im prove. Their success

will depend not only on the system hardware and on the chosen protective material but

also on de tailing, and on ensuring that the protective barriers are properly sealed and

continuous, and tha t the pro tective materials can be placed effectively under site conditions.

This is the main focus of the general guidance tha t follows, which does not conce ntrate

on any pa rticular system.

5.5 Detail ing

In very simple terms, the objective in detailing e xternal prestressing systems is to get th e

tendons in place from one anchor to the other, to the correct design p rofile, and with the

prote ction measures intact, both before and after the stressing operations. The pro file is

adjusted by suitably placed deviators, and the tend ons are essentially straigh t between

deviators, and between anchorages and deviators. The force from the tend ons should be

transferred to the structure in a controlled manner.

Duct layout is influenced by the construction metho d and the form of the structure. Simply

supported structures (or determinate bridges made continuous for durability reasons)

require different layouts and patterns to cantilever m ethods or span-by-span c onstru ction.

Doubly inclined profiles, up to 200m or more, may be required, or short straight lengths

- horizontal or inclined - dictated by the co nstruction me thod, while still ensuring that

the final stress conditions are satisfactory at a ll sections, under service co nditions.

Ducts should be tested for wate rtightness. They should also be located we ll clear of water,

preferably in a non-aggressive env ironm ent. 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, electro-fusion or butt-fusion

join tin g can no rmally be used for PE ducts.

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Again, detailing is imp ortant and such joints shou ld preferably be kept clear of construction

joints (up to 1m is suggested), to reduce the risk of pe netration . For longer tendons,

consideration needs to be given to duct move men ts durin g stressing and the provision of

a join t w ith a sleeve coupler that allows the du ct to shorten.

Temporary access holes into box sections from the deck above, which are often provided

by the contractor during construction, must be carefully positioned and detailed to ensure

they remain watertight after reinstatement and so that if any leakage occurs it cannot

drip onto the tendo ns.

Any drainage system inside box section decks should similarly be positioned and

coordinated with the tendon layout to avoid this risk as well.

Permanent access holes from the road surface should be avoided if at all possible.

W ith externa l tendon s inside a box it is not feasible to extend vents at high points to

50 0m m above the du ct, as is required for inte rnal tendo ns, as this wou ld need holes

through the slab. However, the a bility to check the duc t for voids after grouting by impact-

echo techniques gives added confidence th at heavily outweighs this disadvantage.

Use of tendo n couplers should be given careful co nsideration . Cast-in couplers have the

security of p reventing 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 interm ediate

diaphragm , and to anchor a tendo n on its far side, w ith a new tendon being started on

the oth er 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 restressing or replacement. In these cases, a

substantial length of tendon is required behind anchorages to permit the jacks to be

reattached, wit h an allowance for extension. This length should be specified, since it w ill

depend on whe ther single-strand jacks or cable jacks are required. In service, this extra

tendo n length is usually protected by wax or grease with in a tube, irrespective of the

protective material used elsewhere in the duct. A typical arrangement is shown in Figure 13.

5.6 Tendon system s

Anchorages and deviators are considered toge ther, because detailing solutions for each are

interrelated, and are dependent on the type o f tendo n and the protective system adopted.

There are also differences between the technologies offered by the prestressing suppliers.

General guidance is available'

71

', but most detail is contained in manufacturers' literature.

There are three basic tendo n systems:

• unbonded tendons protected by a cement grout

• unbonded tendon s protected by a flexible product (e.g. wax)

• tendons made w ith sheathed and greased mono strands, within a grouted system.

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Guide tube

Grout injection hole/vent

in anchorage

PE duct fil led with grout

(debonedfrom guide tube)

Length of strand sufficient to al low re-stressing

Liquid applied

waterproof ing

membrane

Min 500mm

Anchorage

end capvent""

Anchorage end cap

Note: Anchorage end cap fil led

with pertroleun wax or similar

Figure 13

Exposed anchorage for restressing the end of

an unbonded external tendon.

PE duct fil led w ith grout

Liquid applied waterproo fing

membrane


Anchorage end cap

Grout injection hole/vent in

anchorage

Note: Anchorage en d cap filled

with grout during tendon grouting

operation. In some

systems

the end

cap s bolted to the anchor head

Figure 14

Exposed anchorag e for the dead end of an

unbonded external tendon .

The detail is also applicable for the live end where

restressing is not req uired.

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Unbonded tendons protected by a cement grout

In this case, the duc t is a continu ous tube, for example of PE. At anchorages it passes

throu gh the concrete, usually, but not necessarily, via an outer pipe and trum pe t to the

anchorage - the so-called double envelope system. This is illustrated in Figure 14. At

deviators, it again passes throug h an outer tube, often w ith a bell mouth to prevent damage

to the plastic. For easy removal, it is imp orta nt th at the ten don can move freely in these

zones; in this regard, it is impo rtan t 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 restressing. These tendons are no t normally

suited to restressing unless they are straight and a shimm ed or screwed anchor head is used.

Unbonded tendons protected by a flexible product

The most common filler material is petroleum wax, which is heated to about 90°C and

poured wh ile still liquid. In general, only a single duct is used (no doub le envelope), and care

is necessary to prevent the wax running ou t o f the d uct. There has been lim ited feedback

on some waxes being br ittle.

There have been a number of reported instances of wax products becom ing unstable and

leaking out, especially at high tempe ratures, and care should be used when considering

these products.

Tendons made w ith sheathed and greased monostrands

Such monostrands are described

in fib

Bulletin No. 11

(72

>. Greased strands are enclosed in

a PE sheath, and pass throu gh pipe assemblies both at the anchorages and the deviators.

One advantage is tha t the strands can be stressed (and de-stressed) using a m onostrand

jack, requiring less wo rking space beh ind the anchorages. The strands can also be extracted

individually and replaced. A system is also available of sheathed and greased strands

with in a PE pipe, which is grouted prior to stressing. The grout is used to fix the strands in

position, reducing the risk of d isplacement and of tears in the sheaths. In this case the

duct must be supported to avoid displacement by the weight of grout.

Deviators are generally of steel or reinforced concrete; in the latter case, they may be lined

wit h a pipe, or cushioned in some other w ay. Usually, they are designed to accom mod ate

an un intentio nal 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 m inimu m radii are usually given

in specifications (e.g. BD 58/94<

69

').

One p articular problem is the location of vents at high points, where deviators are

positioned. This is illustrated in Figure 15. The loca tion 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 m inim um of 15m m. It may also be necessary to locate drains at

low-po int deviators, to remove water that may have accumulated in the du ct.

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PEduct

Vent

Vent

Direction of gro uting

Deviatortube

Figure 15

Top deviator for external tendo n.

5.7 D e-tensioning and

replacement of external

tendons

De-tensioning and replacement of external tendons requires special consideration with

particular emphasis on safety. The opera tion should be carried o ut o nly by trained and

experienced personnel directed by a com petent supervisor, from firms accredited by

CARES. Exclusion of personnel from the area imm ediately adjacent to the de-tensioning

operation as well as the en tire tendo n length is of param ount impor tance. Investigations

before de-tensioning should include:

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 w ithin a PE duct and the tendons are cropped .

Tendons are grouted w ithin a PE duct and a a cking length is left tha t w ill allow

de-tensioning.

' PE ducts are filled w ith a flexible filler (wax or grease) and the strands are free to move

individually.

• Strands are individua lly greased and sleeved in plastic inside the PE duct. The tendon is

norma lly grouted prior to stressing to prevent trapping of the strands. This tendon can

be single strand stressed.

• Strands are ind ividua lly greased and sleeved and are exposed inside the bridge deck.

The strands are deflected over specially made saddles.

Each of these will behave differe ntly during the de-tensioning process, and five possible

corresponding systems are briefly described below. In all cases, particular atte ntio n must

be paid to health and safety issues.

De-tensioning-System 1

Typically, in this system the tendons are cropped close to the bearing plate. De-tensioning

has to be done by exposing the strands and severing them , preferably at a poin t close to one

of the anchorages. The adequacy of the gr outing must be established before beginning

the op eration. This can norma lly be checked simply by tapping the d uct.

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De-tensioning this type of tend on 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 wit h a force of 60% ultimate tensile strength (UTS), after

cu tting seven strands the force in the rema ining 12 uncut strands will be 9 5% UTS. This

situation is poten tially dangerous as the strands are approaching their yield point.

For safety reasons it is advisable to remove the grout over a substa ntial leng th of the

tendo n and clamp the bare strands togethe r. This procedure was used on 11 tendons on

the Mid-Bay Bridge in Florida in 2000<

10

>.

The following procedure should be adopted:

• Mark the position on the duct where the tendon w ill be cut.

I Carefully remove a section of duct exposing the grouted tend on.

Remove the grout by a method that will not damage the strands.

Protect the tendon either side of the cut position by a suitable prote ctive screen.

Cut the strands rem otely, if possible. Use of a flame cutter is preferred, which softens

the steel and releases strain before th e c ut is complete. If a heating m etho d is chosen

for de -tensioning, the tendo n either side of the exposed area should be screened from

the other tendons to prevent heat transmission.

If personnel cannot be excluded fr om the area, provide su bstantial screens to ensure

tha t flying debris canno t cause injury.

This system should incorpo rate a bond breaker inside the anchor guide wh ich allows the

tendon to be extracted for replacement. After de-tensioning, 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 fac ilitate remo val, and very short cropping shou ld be avoided

(enough strand should be left to accommodate strand couplers).

Detens ion ing-System 2

The adequacy of the grou ting needs to be checked as for System 1.

The ends of the tendons are no t croppe d and norm al practice is to leave enough strand

projecting to accom mod ate a ack for de-tensioning . The projecting length is protected

by a grease- or wa x-filled cap. This cap is removed and the strands and anchorages are

cleaned and inspected before de-tensioning.

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 tend on to release the wedges. De-tensioning is therefore a hazardous

procedure and it is recommend ed t ha t th is is carried out by a specialist company. Single-

strand de-tensioning 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 de -tensioning at the anchorage the tendon force will be transferred into the grout

and will norma lly crush it. Care needs to be taken to ensure the te ndo n is free to move

along its length.

After de-tensioning the complete tendon it can be cut into short lengths and removed.

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De-tensioning - Systems 3 and 4

As each strand with in the tendo n of both these systems is comp letely free, the force can

norm ally be released strand by strand . However, there is a risk of the strands be ing trapped

by others at the deviators (in System 3) so particular care is necessary to measure and

monitor the release of extension and/or

load.

Prestressing jacks no rmally have a useful

stroke of 200mm so this will be the maximum extension that can be released from the

tendon in one operation. Should greater release be anticipated , the d e-tensioning system

should have the capability of reseating the wedges as the tendo n has to be re-anchored

at an intermediate stage.

The tendon may be de-tensioned by other means. It is always advisable to seek the

assistance of a specialist company.

De-tensioning - System 5

The de-tensioning will depend on whether or not the strands have been cropped. If they

have not been cropped they can be de-tensioned one at a time w ith a single-strand jack.

If the strands have been cropped, de-tensioning can be carried o ut by cuttin g the strands

one by one after remo ving the plastic sleeve. The cut ting 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 wh ipping m oveme nt of the strand when cut.

Alternatively, special devices can be em ployed to clamp the strands one at a tim e, transfer

the load through a parallel bar system prior to c utting and then gradually enable co ntrolled

release. This system was used at Braidley Road Bridge, Bournemouth when the strands

were replaced in the early 1980s.

Replacement

W ith all replaceable systems it is advisable to replace the d ucting as the te ndon w ill have

cut into the w all of the du ct at th e dev iation p oints. The anchorages should be designed

so tha t the tend on can be extracted after d e-tensioning. In the case of a grouted tend on

a liner is norma lly in place to ensure tha t gro ut does not bond to th e guide tubes.

On replacing the duct th e tendo ns can be installed, stressed and gro uted or greased as in

the original installation.

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6 Segmental construction

6. Segmental construction

Segmental construction is a comm on form of construction, particularly fo r 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 co ntinu ity thr oug h the join t. This is satisfactory as far as the recomm endations

in this Report are concerned and either grouted bonded or external unbonded methods

may be used, provided tha t the guidance given in this Report is follow ed.

For precast segmental construction, modern methods generally involve match-cast

segments with t hin epoxy joints. In some countries, dry joints have been used, but these

are not recommended for conditions in the UK.

6.1 Multi-layer protection One of the principles of the multi-layer protection strategy is that a continuous sealed

duct is a key layer, in preventing contam inants reaching the prestressing steel. When the

first edition of TR47 was published in 1996, the Working Party was unaware of any detail

that could guarantee duct continuity, directly or indirectly, and the recommendation was

tha t only externa l unbonded prestressing should be used for precast segm ental construction.

Since 1996 the W orking Party has sought reliable solutions to this problem . There appear

to be three possibilities:

I The development of prop rietary splicing sleeves for the duct at the joints . At least one

such solution has been developed, and has been trialled on a bridge in the USA but f ull

details of its pe rformance have no t been revealed to the marke t. Any such solution

wou ld be proven via a Technical Approvals system, which wo uld require the development

of a suitable test and acceptance criteria. Pending further experience wit h this system

it may be prude nt to assume tha t some percentage of the internal tendons is lost (say

5-10 % ). Key to th is is the positioning of the splicing sleeve in the m atch-casting process

to ensure the correct alignment.

I Further research on the use of epoxy resins in thin ma tch-cast jo ints , both in the

laboratory and in the field. The Working Party has kept in close con tact wi th the research

at the U niversity of Texas at Austin into corrosion prote ction at segm ental joints

45

'

73

'.

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 ad dition al protective layers, to give

an overall reliability comparable to that of continuous ducts, as part of the system.

Current practice with this approach is to have a minim um of 75 % o f the tendons as

external unbonded.

Wh at is being described here is a developing situation . However, a t present it is believed that

use of internal groute d tendon s for precast segmental cons truction wo uld be treated as a

Departure from Standard if an acceptable duct joint device or other protection strategy

was proposed.

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Segmental construction 6

6.2 Anchorage loc ation and

detail ing

There are some special considerations for segme ntal con struction, and these m ainly

relate to face anchorages at joints, both for in-situ and match-cast precast segm ental

cons truction. Typical layouts for precast segm ental con struction are shown in Figures 16

and 17 and for in-situ segm ental construction in Figure 11. The p articular concern is to

protect the anchorage, including the cap, from any possible water leakage in the joints.

Figure 16

Face anchor details for precast segmental

construction.

Precast segmental construction using internal

grouted tendons is NOT recommended, unless

continu ity of the duct is assured.

Special care needs to be paid to th e follo wing details:

I Sealing of the anchorage recess or box-out, pa rticula rly if the distance from the edge

of the recess to the edge of the segment is small (this concrete is easily damaged).

• Location of inlet /ou tlet/v en t pipes. For precast segmen tal construction Figures 16(a)

and 16(b) require the anchorage pipes to be threaded throug h the second segment as

it is erected. Special atten tion 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.

Min.

500mm

Liquid appl ied waterproofing mem brane,

double thickness required over joint.

(Note:

Second

complete layer

covers

patch first ayer)

Note:

Anchorage end cap

filled with grout

during


(a) Elevation

1st Segment

Epoxy resin sealant

Recess ven t


Anchorage end cap

Grout injection ho le/vent

in anchorage

Inlet for gro uting recess

2nd Segment




(b) Plan

Epoxy resin

sealant

Anchorage end cap


in anchorage

Inlet and vent for

grou ting recess

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6 Seemental construction

Figure 17

Combined face anchor and shear key details

for precast segmental construct ion.

Precast segm ental cons truction using internal

grouted tendons is NOT recommended, unless

con tinuity of the du ct is assured.

L i q u id a p p l i e d wa t e r p r o o f i n g m e m b r a n e ,

d o u b l e t h ic k n e s s r e q u i r e d o v e r j o i n t .

(Note: Second complete layer covers

patch first layer)




Anchorage end

cap vent

_Epoxy resin sealant

-Recess vent

Anchorage end cap

Shear key

Inlet for grou ting

recess


in anchorage

1st segment

2nd segmen t

(a ) E leva t i on

Note:

Anchorage end cap

filled with grout during -

tendon

grouting

operation

r-^\\\\\

-< -

Grout injection hole/vent »-\ \

and anchorage end cap vent ^

s

Epoxy resin

sealant

Anchorage

end cap

-«—

Inlet and vent for

grouting recess

(b)Plan

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Void grouting 7

7. Void grouting

The discovery of deficiencies in the g routing of post-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 m ight be done.

7.1 Overview

Void grouting is the injection of grout into voids left in tendon ducts after the original

grou t has hardened. The term 'void gr ou ting ' is used in this Report to describe this process

to distinguish it from 'regrouting' which means the reinjection of grout into ducts while

the original grou t is still fluid.

Void grou ting has much in commo n with grou ting in new construction - referred to here

as 'new g routin g'. In many respects, therefore, the recommen dations for new gro uting in

this Report may be applied to void gro uting. Atten tion is drawn to differences between

new and void grouting in this chapter and ways are suggested for accom mod ating th em .

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 unbonded tendons.

Void gro uting presents more challenges than new grou ting. The current view is that the

quality achievable in void grouting will therefore often be lower than in new grouting.

This does not imply tha t a lower standard of workmanship should be set. On th e contrary,

greater care and ingenuity will often be required to adapt grouting methods to the conditions

as found . The possibility that a lower quality w ill be obtained in void grouting shou ld not deter

the engineer from recomm ending it. Even a partially successful void-g routin g op eration

may provide sufficient benefits to justify th e 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 gro uting should be done in exce ptional

cases only. The view taken in this Report is tha t voids should be grou ted as part of a

bridge management

plan.

Alternative or complementary strategies include monitoring,

strengthening and reinspection at intervals. It is acknowledged that, for reasons of access,

void grou ting is not practical in all cases and it should not be undertaken if the pote ntial

benefits are considered to be insufficient.

Although void grouting is dealt with as a separate topic in this Report, in terms of selecting

structures for void grouting it should be viewed in the wider context of m anaging the bridge

over its expected lifetim e. M anagement strategies m ay include:

• do noth ing at present

• plan for a reinspection of the prestressing system after a few years

• mon itoring only

• grout the voids

• strengthen the structure by other methods

• combinations of monitoring, void grouting and strengthening.

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7 Void groutinj

Detailed advice on these matters is outside the scope of this Report.

The reliability of void g routing depends on the nature of the voids, the filling th at 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 gro uting is difficult, 'm on itorin g on ly' m ay be the preferred opt ion . 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 structur al capacity in the presence of defects.

In a structure or m ember containing relatively few tendons, each one is likely to be

structura lly crucial. This may not be the case when there are a great m any tendons and a

degree of redundancy. In addition, void grouting of structures with many tendons may be

impractical because of th e 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 g routing decreases, the justification for a dditiona l measures

such as mo nito ring or strengthen ing increases. However, even a comb ination of measures is

likely to be significantly cheaper than replacing the struc ture, especially if disruption costs

are considered.

7.2 Aims of void gro uting

Void grouting can improve the protective environment and structural behaviour of the

tendons.

Tendon pro tection is improved because the new gro ut helps provide a better barrier to

water ingress by filling the voids that w ould otherwise a llow water to migrate freely through

the structure, possibly bearing chlorides. In addition, providing the grouting material is

ceme ntitious, protection w ill be improved by an increase in the alkalinity of the environment

surrounding the te ndon and the passivity arising from th is. These are both imp orta nt

reasons for grouting in new construction.

Structurally beneficial effects arise from the improved bond between the tendons and the

structure. When there is no bond, the flexural capa city of the m ember 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 m ost of the te ndo n length w ill also reduce the loss of effective

tendon section if a wire fractures by allowing re-anchoring to take place.

At present, it is not possible to quantify the improvement achieved when partially

grouted tendons are grouted. H owever, 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, tha t whe n tendon s 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.

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Void groutin

7.3 Condition of bridge

stock and poten t ial

demand

A review of data from the programm e of S pecial Inspections of pos t-tensioned bridges in

England'

1115

' revealed that about 20 % of bridges inspected contained sign ificant voids in

the ducts. This means that, a t the inspection point, the duct contains no grout or a void of

significant size such that a substantial part o f the tendon is exposed. In abou t half these

bridges (10% of the tota l), voids of this size occur at 5% , at least, of the inspection points

and in some cases the proportion rises to more than 25% . In a further 10% of the tota l

bridges, the voids are large enough to leave the ten don 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). 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 recommen dation

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 w ith BA 50/93 '

15

' provide a go od

indication of the co ndition of a bridge as foun d 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 th at v oid gro uting is not needed or they m ay

characterise the bridge sufficiently t o 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 contract. Further inspection will have

to take place duct by duct before grouting commences.

The con tinuity between voids in a particular du ct is rarely recorded in inspection reports

because the inspection strategy adopted in m ost Special Inspections is invasive inspection

at critica l points. The presence of grout cover or a grout wash over the tend on is no t

always formally recorded althoug h it can sometim es be seen in photographs of exposed

tendon s. Such factors have to be established for assessing the need for and pra cticality of

void g rou ting, and po tential deficiencies in the data have to be addressed when its merits

are being considered.

Where th e volum e of the void has been m easured, it is sometimes clear that w hole ducts

are complete ly ungrou ted. In other cases, where the void volume is only a pro portio n 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.

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Void groutin;

7.5 Grouting materials

It is recomm ended tha t the same m aterials are used for void grouting as are curren tly

used for new gro uting , namely cemen titious special grouts with properties as defined in

Standards. This type of m aterial has been used in recent void-gro uting contracts, but it

has been shown t ha t blockages are likely when these grouts are required to pass thoug h

narrow passages. Flow is reduced and a grout plug m ay form in the void as the pressure

rises. To avoid this, the min imum suitable void cross-section should be established.

Whe n voids with sma ll cross-sections are to be grou ted, there may be an inclination to

seek grouts wi th suitable flow characteristics, such as low viscosity, even if other properties

are comp romised slightly. The current recomm endations 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.

How ever, if a significant a mo unt of grout must flow through sma ll voids (say less

than 5mm ) th e use of grout designed for this purpose may be desirable'

35

', and specialist

advice may be sough t on the use of fine-grain ce ments.

One possible difficulty in using fine-grain materials is that the time frame during which

the grout remains sufficiently fluid for injection may be curtailed. However, experiments

have shown (see Mathey

etal.

(74)

) that grouts of this type can be produced that will pass

thoug h gaps as sma ll as 1mm. The property of resistance to plug forma tion has been

described as 'differentia l pressure m icrosta bility' - se e Henrichsen and Stang'

75

'.

Chemically reactive resins can flow through smaller voids and fissures than can cem entitious

grouts because they are wholly liquid and n ot suspensions of particles in a fluid, bu t

currently (2010) their bond and tendon protection characteristics are considered inferior

to cemen titious materials.

7.6 Gro ut ing equipment

and methods

The equipment for pressure grouting in new construction is normally satisfactory for

grouting voids. Where the volume of grout is small, the equipment and meth od of operation

should be selected wit h this in m ind. For example, pressure p ots can be used for injecting

grou t into a small void such as at an anchorage or a high point in a duc t. The gr outing

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 m ost instances, access to the ten don duct for g rout injection and venting will be through

holes drilled th rou gh the concrete. The equipm ent must provide an adequate seal on the

drilled h ole. This is not particularly difficu lt and can be accomplished by use of an expanding

nozzle for injection and resin adhesive for sealing vent pipes.

Void-grouting trials have shown that, provided the process is well managed, cementitious

grout can be made to pass though small voids (about 5mm measured radially) by pressure

gro uting alone. N evertheless, blockages can occur where voids of sma ll cross-section are

very long or particularly small, or restrictions such as spacers are present.

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Observation o f void-g rou ting contracts on site suggests tha t, if a blockage occurs during

grouting, it can sometimes be cleared by the application of vacuum-see for example

Milner and Haynes'

76

'. Altho ugh this appears logical, at present the evidence fo r this is

considered inconclusive. There is little published data on the use of vacuum-assisted

grou ting (but see Lapsley'

77

'). A LINK p roject included a limited evaluation of vacuum

grouting'

3435

', but the technique cannot be recommended on the results of that trial alone.

Users of vacuum-assisted gro uting report tha t removing air from small discrete voids,

particularly those with small and tapering cross-sections, improves penetration of the grout.

Evidence is needed to con firm wh at is a reasonable view. Such voids m ay occur at the

raised ends of ducts w ith parabolic profiles, which are likely to conta in voids that are not

accessible at the top . Where these are sealed at the tim e o f gro uting , pressure g routing

wi ll compress the trapped air and leave a

void,

albeit smaller than before.

In the meantime , vacuum assistance has been specified in recent void-g rou ting contracts on

bridge sites, and specifications have been prepared on a bridge-specific basis for such work.

7.7 Determining the void

characteristics

No rm al practice for determ ining the characteristics of a void is to extend the invasive

inspection carried out under Phase 3 of th e Special Inspection'

15

' 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 len gth, cross-section at suitable intervals, volume, presence and

position of con strictions, con tinu ity and ex tent and position of leakage. The ability to do

this w ill depend on the available access and the accep tability 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 im por tant for assessing the s uitab ility o f 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 method s for d eterm ining void characteristics in Special

Inspections are, in p rinciple, applicable to void -gro uting operations. Their use can be

explored in trials e ither as a separate exercise, before being specified for use th roug hou t

the works, or as part of proving suitability within a contract.

The difficulty of establishing a ccurately the size and na ture of voids should not be under-

estimated. 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 . Wh en a blockage occurs

between vent p oints that have previously been shown to be continuous for th e 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 movem ent 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.

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7 Void groutin

7.8 Flushing w ith wa ter

Unlike for new grouting , flushing with water is recommended where the duc t is partially

filled with hardened grout because it improves the flow of grout during void grouting.

However, if grou ting can be accomplished w itho ut introducing flushing water, this is

preferable. For example, when grou ting a wholly voided me tal duct, there is no existing

grou t surface to absorb water fro m th e fresh grout and the large cross-section to be

grouted presents no additional problems over new grouting.

Water can help to identify continuity between duct entry (injection/vent) 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 w ater th at contains chlorides,

which had previously entered the duct during service, or flush out a proportion of any

chloride deposits left behind when w ater has dried out. After flushing, the wa ter sho uld

be removed (e.g. by draining, use of compressed air or vacuum) but pockets of water are

likely to remain in the duct. Some of this water w ill mix with or be displaced by the grout

during void gro uting and should be expelled from the d uct. Grout sho uld 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.

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 w ith an air pressure

method,

provided th at th e e quipm ent used can cater for the large leakage rates tha t may

be encountered.

Flushing to remove grou t during a failed g routing op eration is not reco mm ended because

cells of grout are left in the duc t: the flushing water passes over the grou t even when it

remains fluid.

7.9 Effect of existing

defects

The ducts and tendons targeted fo r void gro uting w ill sometimes co ntain defects as we ll as

voids. Even if defects have no t been found at the inspection points, they may be present

at other locations.

If the e xisting grout is of po or qua lity or in poor c ond ition, this should n ot de ter the use

of void grouting. H owever, the possibility of blockages occurring during the grou ting

operation may need to be taken into account.

If exposed tendons occur in com bination wi th conditions suitable for co rrosion, particularly

high chloride concentrations, and the presence of existing corrosion, including wire fractures,

consideration should be given to alternative managem ent strategies, as we ll as to vo id

grouting. Grouting cannot be relied on to prevent further corrosion at these locations.

Moreover, in certain situations , void grou ting could result in addition al and possibly

accelerated corrosion.

The benefits of re-establishing bond and improving the re-anchoring properties are still

relevant. If the defects as stated are present but not widespread, grouting is still recommended.

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Void groutin,

7.10 Spe cification f or

grout ing

BS EN 4 45'

2

', BS EN 446<

3

> and BS EN 447<

4

> are recomm ended as the basis for the

specification of void-grouting works with appropriate modifications.

Several provisions in the Standards relate to new grouting only, and should be excluded from

the void-gro uting specification or amended accordingly. For example, the recomme ndation

in this Report for pressure testin g ducts before concreting is not appropriate although th e

benefits of measuring duct leakage before gro uting remain. In some cases, entirely new

provisions are needed, for example relating to drilling in to the ducts for investigation

purposes and subsequently for ve nting or injection. Some provisions may apply only after

mo dification , for w hich exam ples are given in the paragraphs below. A convenient way of

defining variations to the Specification is to use Appendix 17/6 to the Specification

for

Highway Works^.

There must some times be enough flexibility for the engineer to accept m ethods proposed

by the co ntractor and adapted to the conditions on site. However, significant factors m ust

not be overlooked by default, and the quality of work must be maintained.

Maximum pressures during void grouting

Where sm all passages have to be grouted, it may be desirable to increase the maxim um

pressure to ensure that grout flows through to the next larger void. Pressures higher than

8 bar (800kPa) require the approva l of the engineer. Sudden app lication of high pressure

to drive grout along small passages may cause a blockage.

Distance between vent points

The distance between ven t points may be shorter or longer than norm al. Reasons are the

need to establish void characteristics at sufficient intervals - this may require closely

spaced holes - and access difficulties th at may restrict hole drilling anywhere but at the

ends of the span.

Height of vents above the duct

Restrictions on height w ill som etimes prevent the standard he ight being achieved. The

specification should provide for this, allowing the maximum practical height to be used in

all locations.

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 gro ut

from all vents. An exce ption mig ht be where flushing with water has been used. The

procedure should be covered in the specification and agreed with the engineer.

Holding pressure after g rou ting

It may be desirable for th e norm al requirements, e.g. for holding pressure, length of time ,

reinjection, to be reconsidered. This m ay apply where it proves impossible to fully seal the

void being grouted.

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7 Void sroutirr

7.11 Trials

7.12 Qua l ity co ntrol

Rate of progression of g routing a long duct

It is accepted that this cannot be controlled for void g routing to th e 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 m ay change rapidly. This can be anticipated

to some extent if the characteristics of the void are established before grou ting.

Trials are essential for all void-g rou ting contracts. The ducts, tendons and void characteristics

should be faithfu lly reproduced in the trials.

The trials should e mp loy the method s 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 ab ility of the proposed metho d to counteract this should be demonstrated

with match ing leakage rates. If water is to be used to flush the vo id before gro uting 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.

Quality control requires the following elements:

. dem onstration in advance of equipmen t, materials and techniques in a representative

situation

• detailed me thod sta teme nts, including a planned reaction to possible scenarios

highlighted in risk assessments

a flexible approach to enable the best grou ting to be achieved according to the

circumstances foun d

I meticulous record taking that includes an agreed statem ent of the likely duct conditions

on completion of the remedial grouting operation.

The control of qua lity should begin with thorough docum entation of the void characteristics.

A schedule of du cts, voids, vent pipes and injection points, etc. should be submitted

before work starts, and records should be kept during gr outin g, as for new grouting . It wil l

be impossible to know exa ctly what each void is like but a full record of site inform ation

made during grou ting wi ll help in resolving, w itho ut delay, questions tha t arise during and

after the work.

5 4

Experienced site personnel m ust be used, to operate the pum p, 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 mon itored . The significance of the different operations and m aterial

properties m ust be recognised - for instance, acceptable limits for grou t fluidity, the

correct amount of water, preliminary testing and the need to have everything to hand so

there are no delays during the work.

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Void groutin

Careful notes should betaken 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 g routin g has been successful, or where flow times are shorter or

longer than expected, can then be inspected carefully later.

After grou ting, the qu ality achieved should be established by invasive drilling at points

other tha n 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 th roug h a ll vent pipes has been achieved in com pliance with all procedures,

the voids have been satisfactorily filled. Sample drilling may be used to confirm this. If

coloured grou t is used for vo id gro uting in the works, the success of the g routin g process

can be established more easily.

Where invasive d rilling has been carried out, the characteristics of the duct after the voids

have been grouted may be established by measurements of void volume, co ntin uity and

leakage.

If the most vulnerable positions are inspected, and are seen to be fully gro uted, this

demonstrates th at there is grout in the d uct tha t was no t present before. The tendons are

at least better protected than before the voids were grouted.

As wi th o ther g rou ting operations, the quality requirements in Chapter 14 should be

followed.

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Test methods for grouted post-tensioned concrete bridges

8. Test methods for grouted post-tensioned

concrete bridges

This chapter covers tests relating to the durab ility o f prestressing tendon s, the efficiency

of the g routing process, and also outlines acceptable altern ative tests.

8.1 Introduct ion

8.2 Range of tests

considered

8.3 The need for testin g

The testing sub-group of the W orking Party considered a wide variety of testing m ethods for

durability and grou ting efficiency before publication of the first ed ition ofTR47 in 1996. The

general approach to testing taken at th at time remains unchanged in this Report. However,

the subject has continued to develop and the latest developments are reported here.

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 du rability of

concrete

structures

1

-

7

^.

Emphasis is given to tests

specifically designed for grouted construction.

Routine testing of the qu ality of com pleted grouting was not com mo n in the past, although

such testing is com mo n for many other co nstruction activities. This may have been

because suitable test m ethods were not available, but it may have allowed defective

workmanship to go undetected on occasions.

In searching for app ropriate tests, the testing sub-group set the fo llow ing criteria against

which to judge tests to assess the quality of grouting during construction:

i The test should be taken at an early stage when rem edial action is possible.

: The test should inte rrup t the production process as little as possible.

i The test should be simple, so tha t ambiguous inter pre tation is unlikely.

Table

shows test me thod s applicable during co nstruction and Table 2 shows test

methods applicable durin g service life.

The selection of tests and the a mou nt of testing required are a ma tter of judgemen t. Ato ne

extreme, testing may be seen as a needless expense if reliability is already guaranteed in

some other way, and requirem ents tha t are too stringent cause needless expenditure and

delay. On the othe r hand, even extensive testing is likely to form a very small prop ortion

of the tot al ex pend iture, and it is essential tha t all concerned have full confidence in the

form of co nstruction and the workmanship with in each application. Only then can

long-

term durability be assured.

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Test methods for grou ted post-tensioned concrete bridges 8

Table 1

Test methods applicable during construction.

Table 2

Test m ethods applicable during service Life.

Note

Some of these

methods are only

part ial ly developed,

and inclusion does not

imply that they are

appropriate for rout ine

appl icat ion.

Feature under tes t

Test method

Before grouting

Sealing of duct

Suitability of grout

Duct leakage under pressure (see Appendix A1)

As BS EN 445, BS EN 446 and BS EN 447

During grouting

Voids in grout

Grouting pressure

Overall quality control

Voids above grout

Grout stiffness test (see Appendix A2)

Void sensors (see Appendix A2)

Duct pressure sensor (see Appendix A3)

Autom ated quality co ntrol system (see Appendix A5)

Radiography

Impulse radar

Impact echo (sonic)

Ultrasonic transmission

Ultrasonic reflection

Thermography

Radiometry/tomography

Pressure/volume/leakage testing

Feature under test

Degree o f corrosion risk

Tendon integr ity

Prestress loss

Survey of existing ducts

before regrouting

Test method

Half-cell potential

Resistivity

Electrical c ontinu ity

Corrosion

Fibre-optic integrity

Acoustic monitoring

Ultrasonic electronic pulse

RIMT (Reflectometric Impulse Measurement Technique) electronic pulse

Magnetic flux exclusion

Strain by fibre-optic sensors

Strain by vibrating wire gauge or Demec gauge

Vibration monitoring

Duc t leakage under pressure (see App endix A1)

Volume of voids (see Appendix A6)

Autom ated quality control system (see Appendix A5)

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 tha t they

require, and commission a combination of tests that they judge will meet their needs.

An insight into the problem of grouting defects can be obtained from the extensive

programm e of Special Inspections of grouted bridge structures that has taken place in the

UK over recent years. This programm e showed tha t o nly sm all voids were present in 40%

of structures. On th e othe r hand, over 35% of structures co ntained, in varying degrees,

either large voids or ungrouted tendons. There is no dou bt fro m this survey tha t bridges

can be grouted extremely well, even those early bridges tha t were grouted w ith simple

equipm ent. The problem is therefore one of achieving good g routing every tim e.

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8.4 Test methods

appropriate in particular

circumstances

8.4.1 Type approvai at pre-

contract stage (duct systems,

grout materials and

procedures)

Qu ality assurance certification schemes have an impo rtant role in regulating con sistent

performance, bu t testing may be seen as the de finitive proof tha t a quality product has

been produced.

The problems discovered in post-tensioned grouted construction 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 constru ction

site cond itions. Only in this way can the technology of gro uting be advanced still further.

It has been emphasised tha t the most useful tests are those undertaken at a sufficiently

early stage for defects to be corrected. The stages at which the tests mig ht be undertaken

are therefore considered below. Under each heading, the most appropriate tests are

discussed, wi th reference to A ppendix A, where further details of the tests themselves

may be found.

One of the m ost efficient means of achieving a qua lity end produ ct is to test certain

aspects that influence quality in advance of potential contractual complications. This

testing may include the following, separately or in combination:

• testing of duct systems

• testing of grout materials

• testing of com binations of duct systems, grout m aterials, and duct geom etry.

Examples of such testing procedures are given in the

fib

Technical Report

Corrugated

plastic

ducts

fo r internal,

bonded

post-tensioningW .

This describes testing requirements

for duct materials (such as flexural behaviour, load and wear resistance), leaktightness

and system approval testing (which includes grouting a 30m-long duct enclosing tendons

and encased in concre te). Most of these are also published in ETAG 013<

25

'.

ETAG 013 and C WA 1464 6'

26

' contain the European Approval requirements for post-

tensioning kits and for the installation and will apply to the UK on adoption of European

Standards throug h the forth com ing revision to BS EN 13670

(22

>. Further approvals are also

given in BS EN 447 for initial type tes ting of grout fa brication.

The tests described in Appendices A1 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 Append ix A are:

I Test

A1,

leaktightness tests for duct systems. These give values of air leakage from the

duct tha t should be expected for the particular system.

•

Test A2, grout stiffness tests. These give the am oun t of gas tha t may be expected to be

trapped within a grouted duct for the tested system, given that the g rout material,

mixing plant, duct geometry and procedures remain unchanged.

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"est methods for grouted post-tensioned concrete bridges

Pre-contract type-approval testing should be undertaken under the scrutiny of an

independen t qu ality assurance ce rtifica tion scheme, such as tha t p rovided by CARES. This

w ill result in confidence in the data produced, and its docu men tation, and provide a link

to the procedures adopted for use of the products on site.

It is necessary to record all details of the gro uting m aterial, mixing plant, duct geometry and

procedures in place during p re-contract testing , because significant de parture fro m them

can influence the quality of the finished product. The tests only demonstrate satisfactory

quality w ith given materials so long as all other factors influencing the gro uting remain

unchanged.

Type-approval tests could be extended to cover variations in grouting ma terial, m ixing

plant, duct geom etry and procedures. In tha t case, provided that do cum enta tion 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 w ill remain on going, with a consequent need for qua lity

con trol measures.

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 w ithin a contract, before

approval of the contractor's proposals, the d uct 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, equipment,

personnel and procedures will be identical to those proposed for the m ain works.

As with type-ap proval testing, the d uct assembly verification test A1 and grout stiffness

test A2 m ay be used to provide evidence tha t th e duct has been correctly assembled and

to measure the volum e of trapped gas in the tria l grouted duct(s). These tests wi ll provide

values of these parameters tha t 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 tria l gro uting , they wou ld also provide achievable

target values for use during the m ain works.

8.4.3 Duct assembly

verification before main

grouting

Despite the fact th at duct systems themselves are satisfactory, a number of aspects may

go wro ng during assembly. These include local damage to ducts and seals, misalign men t,

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 A1 .

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8.4.4 Duct integrity after

concreting or assembly of

precast units, but before

main grouting

Damage to d ucting sometimes occurs during concre ting. To determine th is, the system

should 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 wi ll also check the ir effectiveness.

The tests described in Appendix

A

can be used for this purpose.

It is illogical not t o test a fter concreting for fear of discovering damage that is difficult t o

remedy.

It is imp orta nt th at levels of leakage discovered follow ing concreting should

n ot

be regarded

as 'failure'. The concept of mu lti-layer prote ction 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, for example 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 G rout stiffness test of

main grouting

The grout stiffness test was developed after reviewing the available test methods, and fills

a need fora test that met the criteria in Section

8.1.

The test is described in Appendix A2.

Unfortunately, however, the equipment which was developed to carry out the test has

since been scrapped, but the principles on w hich the te st is described are sound.

The alkalinity of the grout provides an effective protection against tendon corrosion,

particularly in the absence of chloride contamination. Measurement of the voids within

grou t thus indicates d irectly the likely effectiveness of g rout as a protective layer.

The major advantage of the grou t stiffness test is that th e 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 d uring use on trial ducts,

the results may be interpreted by reference to other contracts, or more particularly by

reference to other ducts within the same structure. Variability in trapped gas due to changes

in m ixing, admixtures, temperature, incomplete fil l ing or inadequate venting will be

detectable. In addition, the equipment will indicate whether the duct is well sealed, or

leakage is occurring at any end caps or vent pipes.

If the stiffness test has been calibrated during a similar pre-contract typ e-app roval test or

against a tria l duct within the contract, it can be demo nstrated that the vo lume of trapped

gas is w ithin limits known to be acceptable. This w ill further increase confidence in the

finished product.

An alternative me thod o f 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 mo nitor grou ting 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.

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8.4.6 Automated quality

con trol testing of main

grouting

Equipment developed for the grout stiffness test was developed wit h the suppo rt of the

Highways Agency and the British Cement Association (now the Mineral Products Association).

This development was envisaged in Reference 34 . The equipment is described in Appendix A5.

Unfortu nately, however, the equipme nt was no t taken up by industry and has now been

scrapped. The description is included in this Report as the principles remain sound.

Autom ated quality control equipment is designed to monitor all aspects of gro uting that

may influence the final quality, and to record them automatically througho ut th e gro uting

operation. The equipment is placed within the grout flow line between the grout pump

and the duct. M easurements that are stored include temperature and grout flo w rate. The

equipment automatically tests the volume of gas trapped either within the ducts or within

samples of gro ut in the test chamber, with the results of the analysis displayed fo r immediate

use as we ll as for later reporting.

Qu ality assurance certification schemes will ensure that the equipm ent and training of

operatives is such that a high quality of gro uting can be achieved. Au tom ated qua lity

control testing demonstrates that the qu ality

has

been achieved on every duct.

The benefits of an automa ted q uality c on trol system are as follows:

I The presence of excessive trapped gas with in the grout, and hence prob able voids, is

detected at a time when further grou ting or venting can remove the voids.

I Leakage from the duct can be dete cted , and defects causing the p roblem such as loose

end caps can be rectified.

I If pre-contract type approval tests or trial ducts have established satisfactory gas

conten ts, it can be demo nstrated th at th e same quality has been achieved in the

works. With ou t such tests and trials, the equipm ent will still detect voids in excess of

what might normally be expected.

I The equipme nt produces a report of all events and their timin g. 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 gro ut w ithin the system. The grout pressure is generally indicated by a

gauge adjacent to the injection pu mp. The autom ated quality con trol system described in

Appendix A5 continuously measures the grout pressure closer to the p oint of injection

into the duct. If there is a particular need to m on itor pressure within the duct itself, duct

pressure sensors described in Appendix A4 may be used.

8.4.7 Survey of existing grout

conditions before regrouting

Surveys of existing structures have shown tha t voids are often present w ithin ducts, and

in some cases remedial groutin g is required. Remedial grou ting is a more diff icult opera tion

than g rout ing of new ducts. Knowledge of the extent and nature of the voids to be filled

can assist in the regrouting operation, and increases the probability of providing the tendon

protection and bonding required.

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The test methods for voids above grout carried out after grouting but before acceptance,

as given in Table 1, have all been used to assess voids with in existing structures, w ith varying

degrees of success. The ideal test for this purpose w ould be completely non-d estructive ,

and would not disturb the environment w ithin 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 throu gh 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:

• Wh at is the corrosion risk arising from an ide ntified defect? Is there a need to regrout?

• Wh at is the most appropriate procedure for regrouting?

The answers to both questions depend p rimarily upon (a) the size and distribu tion of

voids and (b) the connection of these voids to the atmosphere.

The size of voids is certainly an indication of the likelihood of exposed tendon s, as we ll as

the presence of passages of sufficient size for successful regrouting. 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 impo rtance. If a void is well sealed

and tendon s have only a wash of grout, they w ill retain their alkaline pr ote ction over a

long period. If the void is only sm all, but there is air leakage that enables ca rbona tion 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. Regrouting of

sealed voids is also more difficult, particularly if access to the end of the void is denied by

the geom etry of the structure.

The testing described in Appendix A6 is therefore aimed at reducing these uncertainties

to enable regrouting operations to be planned effectively.

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Recommendations for durable

post-tensioned concrete buildings

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Durable post-tensioned concrete buildin;

9. Durable post-tensioned concrete buildings

The basic requirements for durable p ost-tensioned buildings are no different to those for

bridges. However, there is often a significant difference in:

• the environm ental exposure

• the number of ducts

• the drape of these ducts.

For these reasons it has been common practice to relax many of the established require-

ments fo r bridges when constru cting buildings. This may be appropriate in man y cases

but the consequences need to be considered. For example a car park has a very similar

exposure to a bridge and therefore the dur ability strategy should no t be significantly

differen t. Transfer beams in buildings w ill have large drapes and the con trol of materials,

workm anship and vent position should be the same as for a bridge beam to ensure that

the tendo n is fully grouted.

The aim of this chapter is to highlight w here procedures for bridges may need to be

mo dified for buildings. The aim is to refer to the main section of this Report as far as is

practical, and hence the subsections fo llow the same sequence as the sections dealing

with bridges.

9.1 Factors affe cting

durabil i ty

Chapter 2 discusses the main factors a ffecting th e durability of bridges. A key factor is the

penetra tion of external chlorides, which is also relevant to exposed buildings such as car parks.

However fo r most buildings the m ajority of the structure w ill be in a much less aggressive

environment and the principal requirement is to ensure that tendons are protected from

water and other harmful materials from the tim e of construction.

Compared w ith bridges, buildings, including car parks, have low cover and in many cases no

links enclosing the ducts. This means that the ducts are far less confined than in bridges.

As a result, excessive pressure in the ducts can lead to damage of the cover concrete; this

may be due to overpressure during gro uting but spa lling due to wa ter freezing in voids in

ducts has occurred on a car park structure. Care wit h expanding agents in grout must also

be taken as the expansion can cause h igh interna l pressure w ithin the ducts. Such spa lling

can lead to a safety issue due to falling concre te. Where com plete detac hm ent does not

occur, durability may still be compromised and the possibility of detachment occurring at

a later date can not be ruled out. It wou ld also appear possible that where wa ter has

entered a duct during con struction, freezing and associated spalling could occur p rior to

the enclosure of the building.

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Durable post-tensioned concrete buildings

9.2 Materials and

components

As for bridges, the q uality of materials and compon ents used in post-tensioned buildings

is of great impo rtance, and therefore th e derivation of good specifications is crucia l 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.

9.3 Co nstruct ion qual i ty

The importance of construction quality in relation to the durability of the structure has

been discussed in Section 2.3.

9.4 Expansion join ts

For enclosed buildings the environm ent is relatively benign and no spe cial precautions are

required.

How ever, for car parks and other e xternal structures the guidance in Section 2.4

should be followed.

9.5 C onstruct ion joints

Figure 18

Live end anchor at c onstru ction join t adjacent

to unstressed pour strip.

Figure 19

Pour strip after tend on stressing and prior to

fixing reinforce ment and casting the concrete.

Wh ile it is possible to have con tinuity of prestress across construction join ts, it is more

typical to have an unstressed pour strip. Figure 18 shows a live end anchor prior to

concre ting the slab and Figure 19 shows the ends of the tendons after stressing and

cutting, prior to the reinforcement being fixed and the concrete cast in the pour strip.

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Durable post-tensioned concrete buildinj

Figure 20

Dead end anchorage at construct ion joint .

In many case it w ill be beneficial to have a dead end anchor cast into the concrete and

therefore avoid the use of an anchorage at the construction join t altogether; see Figure 20.

However, this is not possible where double live ends are required and for structures

exposed to water th e guidance in Section 2.5 should be follow ed.

Where only dead end anchors are used, and so no space is required for the jacks, several

bespoke details have been developed to allow th e slabs to move apa rt during stressing with

con tinuity being provided later by locking the join t. The behaviour of such details should

be checked from first principles taking into consideration the effects of workmanship.

9.6 Cracking

Cracking lim ited t o th e values recom mende d in the various Codes of Practice is unlikely

to have any detrim enta l effect on enclosed structures. Indeed it is likely that wide r cracks

w ill have little e ffect p roviding the strand is protected inside a fully grou ted o r greased

duct. For external structures exposed to chlorides, BS EN 1992-1-1<

44

' requires the tendon

to be in a zone of compression and so cracking perpendicular to the te ndon should be

avoided. Section 2.6 gives further advice for ch loride environm ents.

9.7 Ducts and anchorage

layouts

General advice on duct and anchorage layouts is given in Section 2.7. For enclosed building

structures the use of top pockets is less problem atic due to the lack of water and they

provide a convenient solution when abutting existing construction. In addition the edge

of the slab is often the only pa rt of a structure tha t is either fully exposed or exposed

within an uncontrolled environment within the cladding make-up, and the use of a top

pocket (see Figure 21) may be beneficial.

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Durable post-tensioned concrete buildin;

Figure 21

Top pocket before ( top) and after (botto m)

casting the concrete.

However, care should be taken that damage is not allowed to occur during construction

while any pockets can allow wa ter in to ducts. One way of avoiding this is to provide

tem pora ry drains similar to those in Figure 8; these would need to pass thro ugh the

formwork.

Where anchorages are exposed at the edge of a structure, either perm anently or due to

the phased nature of construction, the corrosion protection to the anchorage should be

designed to reflect this; see Section 10.3.3 and Figure 22.

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Durable post-tensioned concrete buiidin)

9.8 Proximity to sea water

As indicated in Section 2 .9, structures in coastal areas and over the sea are at risk due to

corrosion induced by splash or spray of wind-borne chlorides and need greater corrosion

protection.

9.9 Road salts

As noted in

Design recommendations for multi-storey and underground carparks^,

published

by the Institution of Structural Engineers (IStructE), the exposure conditions for car parks

in the UK are much more severe than for con ventional buildings. During the w inter,

de-icing salts from roads are carried in by vehicles and are deposited on the concrete slabs,

particularly on access ramps and in well-used parking bays. A frequent factor is the retention

of water contam inated w ith salt, either on rough textured surfaces or in areas where

ponding occurs. The absence of go od drainage to carry these salts away can lead to very high

salt concentrations, perhaps ten times tha t of seawater. The IStructE recomm endations

concentrate on the concrete specification and general aspects for improving durability such

as wa terpro ofing and providing adequate drainage. No specific guidance for post-tensioned

concrete is given apart from noting that "Problems with corrosion of post-tensioned

tendons, particularly in bridges, have highlighted the importance of rigorous grouting

methods and checks".

9.10 Access fo r inspec tion

and m aintenance

Inspection and maintenance is required for all buildings including post-tensioned structures;

some guidance on procedures for car parks is given in the IStructE recommendations. For

buildings, access to the general structure will be relatively easy although as discussed

above inspection of anchorages may only be possible with removal of cladding panels.

For grouted tendons the use of non -destructive methods to check for voids within ducts

has me t w ith little success, principally due to the use of m etallic ducts. However, where

gross defects are expected it has been found relatively convenient to drill in to the duct to

inspect. Precautions should be taken to avoid damaging tendons. D rills are available which

cut out on contact with the metal duct; the final penetration into the duct can then be

carried out manually. Even if it is possible to gain access to anchorage positions, for

bonded structures, the anchorage w ill be hidden by a grout pack and only significant

deteriora tion w ill be obvious, and the effectiveness of grou ting m ay need to be inferred

from the condition of the adjacent duct.

The use of non -meta llic ducts to fac ilitate inspection has not been validated and the

relatively small size of ducts and any voids within may make this less reliable than bridge

ducts.

Research in this area wou ld be useful.

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10 Available protective measures

10.


The principles o f mu lti-layer prote ction, as discussed in Section 3.1, are equally applicable

to building structures. For internal buildings the envelope forms another significant barrier

wh ich helps pro tect the tendons. For external structures, such as car parks, such add itional

protection may not be available and the design strategy for multi-layer protection should

follow that of bridges in Chapter 3.

10.1 The structure as a

whole

As discussed above, for buildings, the envelope provides the first line of defence and for

most w ell-ma intaine d structures this reduces the reliance on other barriers. Wh ilst this is

generally true , care should be taken to highlight any parts of the stru cture t ha t are not

within a controlled environment and consider the need for additional measures.

For externa l structures, and in particular car parks, the discussion in Section 3.2 for bridges is

equally valid. In addition it should be noted that waterproofing systems in car parks are often

less robust than in bridges and can suffer fro m wear. Furthermore, wate rpro ofing to car parks

is often only carried ou t to the roof slab and, as discussed above, chloride-laden water

carried in on cars is potentia lly the main risk. Therefore, before relying on a car park water-

proofing system, the nature of the system and its likely maintenance should be considered.

10.2 Individual structural

elements

BS EN 1992-1-1'

44

' gives recomm endations for the design of building elem ents; these include

limitatio ns on stresses and crack widths and, via BS 850 0'

53

', guidance o n appropriate

covers and concrete quality. It should be noted that, for buildings, the geometry of the

duct often dictates its cover, with the minimum cover generally required to be at least

half the larger duct dimension for rectangular ducts. This is partly to lim it th e possibility

of spalling during the g routin g process.

10.3 Prestressing

components

10.3.1 Prestressing tend ons

Inform ation on prestressing tendons has been given in Section 3.4.1.

10.3.2 Ducts

The most comm on ducts in bonded post-tensioned building structures are oval galvanised

steel ducts with a folded seam. The galvanising is intended to avoid corros ion of the du ct

surface p rior to and during cons truction. In addition, the ga lvanising also serves as

lubrication and thus reduces the fr iction losses. However, the ducts can no t be considered

to provide a long-term protective layer. For car park construction it may be appropriate

to use non -me tallic ducts as a protective layer, the benefits of which are discussed in

Section 3.4.2.

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Available protective measures 10

There are currently at least four European suppliers of oval non-metallic ducts to the UK

along with suppliers of plastic m onostrand ducts for grou ting. The systems typically include

all components required to make a sealed duct system. Thus it is possible to have a plastic

duct system that provides a com plete protective barrier for th e tendon s. Whilst several of

the systems are tested to the requirements of

fib

Bulletin No. 7,

Corrugated

plastic d ucts

for internal bonded post-tensioning^,

there is some practical concern about testing full

assemblies of oval ducts wit ho ut the concrete cast around the m . Pressure change may

occur due to the oval duct changing shape and this change in shape may cause additional

leakage at join ts. For this reason, where pressure testin g is used, it w ill probably on ly be

appropriate after casting of the concrete.

Typical oval ducts have tendon area-to-duct ratios similar to those in bridges and a

maximum of 0.5 is normally specified (see CARES Model Specification'

27

'). However, both

the size of the ducts and the strand layout with in mean that the size of void left w ithin

the d uct is significantly smaller than for typical bridge ducts. Nonetheless, numerous

grout trials have shown tha t w ith correct materials and workman ship the grou t is able to

penetrate these sm all voids.

10.3.3 Ancho rages

For typical intern al structures w here the anchorage is inside the building envelope, the

normal solution is to protect the anchorage with a mortar pad. Whilst the pad will only

provide nominal durability protection, in an internal environment this has been found to

be acceptable. For oval metallic ducts it is not normal to have vent points within the

anchorage cap and the vent comes off the duct or the back of the anchorage. It has been

argued that the mo rtar prote ction applied to the anchorages allows air to escape from

the anchorage and therefore allows complete grouting of the anchorage. This appears to

be demo nstrated by limited trials. However, the fact tha t the anchorage mo rtar pack is

not a irtight means tha t the protec tion for the anchorage is questionable. If voids exist,

the anchorage provides a path for moisture and oxygen to th e tendons. For this reason

this form of anchorage should not be used in exposed locations with ou t further protection.

Figure 22 shows the edges of post-tensioned slabs after the areas around the end

anchorages have been concreted.

Due to the visual imp lications it is not no rma l to continue anchorages to the outside face

of the building bu t where this is unavoidable the end of the anchorage should be covered

by sufficient reinforced concrete to provide the required dur ability.

As an alternative, the principles in Section 3.4.4 should be followed; in particular, systems

are available for oval ducts which incorporate plastic anchorage caps with the vent/injection

pipe coming o ut of the cap. These tend to be more comm on with the plastic duct systems.

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10 Available protective measures

Figure 22

Edges of post-tensioned slabs.

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Grouted bonded post-tensioned construction for buildings 11

11.

Grouted bonded post-tensioned

construction for buildings

As discussed previously, the strategy of this Report is tha t of mu lti-layer prote ction. For

an internal environme nt th e first line of defence is the building envelope; indeed for m ost

buildings, provided the tendons can be protected from corrosion during construction, the

building envelope should prevent furthe r ingress of the elements required for corrosion.

For most internal buildings the second layer of p rotection is the concrete cover to the

duct. However, for the protection to be effective, adequate grouting of the tendon ducts,

to expel any detr ime ntal ma terial, is essential. For externally exposed eleme nts, where

the envelop offers no or limited protection, further layers of protection are required and

these are discussed tow ards the end of this chapter.

11.1 Grouts and grou t ing

A properly grouted duct provides an alkaline environment which protects the strand from

corrosion. There has been concern tha t tendons are not always adequately grouted. Reasons

for this appear to be related to poor quality con trol procedures and /or inadequately trained

personnel. This has led to occurrences of co mpletely ungrouted ducts and to partially

grouted ducts. In this con text partially grouted means that part lengths of tendons were

not grou ted, presumably due to inappropriate action being taken when a blockage was

encountered. To address these shortfalls the CARES cer tification scheme has been updated

(see Chapter 14) and a ll site operations should be carried out by companies and operatives

satisfying the CARES Certification Scheme.

Wh ilst grouts for buildings were traditionally made from site-mixed cement and adm ixtures,

the CARES certification scheme now requires approved pre-bagged grouts to be used.

This change was driven by the va riability in cemen t bag we ight and properties, com bined

with the desire to minimise the influence of w orkmanship on gro ut properties. Such grouts

still need both suitability testing and inspection testing, carried out on a site-by-site basis,

to con firm that the methods and equipment used are appropriate.

W hilst the use of approved installers and pre-bagged grouts w ill maximise the chances of

complete filling of voids, one of the drawbacks of bonded post-tensioned construction is

tha t it is very difficult to inspect the adequacy of grouting achieved. Wh ilst there is the

possibility of installing sensors and probes for monitoring the potential for corrosion, the

large number of d ucts and less rigorous inspection regime, for b uildings when compared

with bridges, makes this impractical. Alternative methods, including the placement of an

easily traceable indicator in the g rout, are still under developme nt but most w ill require

non-metallic ducts to be used.

Experience shows that a significant number of defects in grouting occur due to whole ducts

being missed. Therefore, it is impo rtan t tha t the grou ting records include duc t-by-du ct

sign-off. It has also been proposed th at the volume of grou t used should also be recorded;

this is of value in co nfirm ing th at ducts have been grou ted, and possibly high lighting any

significant defect due to blockage of a large length of tendons. As the g rout used may be

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11 Grouted bonded post-tensioned construction for buildin;

11.2 Vents and gro ut

injection

11.3 Recommended protec-

tion systems for buildings

11.3.1 General

less than a litre per metre and significant amou nts are drawn o ff at vent locations, it is

unlikely tha t such procedures w ill be able to accurately highlight ducts with smaller defects.

It is also recommended that checking of grout levels invents prior to cutting off vent tubes

be recorded.

For elements with shallow drapes - less than 5 00 m m - the requirements for vents can be

relaxed compared to those for bridges. However, for deeper elements the requirements

for grouting are no different and reference should be made to Section 4.2.

For shallow slabs, vents should be provided at in jection and exit points a nd, where the

length of the tendon exceeds 2 0m , at intermediate high-point vents such that the distance

between high-point vents is no more than 20m. The requirement for high points may be

relaxed if grout trials are carried ou t to a similar geom etry using the w orkforce and equipmen t

to be used on the project, to d emonstrate th at adequate gro uting is achieved.

A prestressing strand w ithin a dry duct is unlikely to corrode significantly during the life of

the structure, regardless of the presence of voids with in th e g rout. Once com pleted, the

building envelope shou ld prevent ingress of water, but w ater th at enters the duct during

construction will s till be available for corrosion. The first way in which wa ter may enter is

during the 'blowing through' of ducts with water prior to grouting. For this reason oil-free

compressed air should be used instead of water. The second way is due to water ponding

on the slab and entering the duct thro ugh vent tubes. The chances of this can be minimised

by ensuring that th e vent tub e is adequately filled before it is cut down .

Adequate grouting is required for durable post-tensioned structures. Whilst recommendations

are made for the type of grouts to be used, the principal factor in the qu ality of grou ting

achieved is the wo rkman ship. As such, the contra ctor should operate a quality system

tha t maximises the chances of a successful grou ting o pera tion. R ecommendations for

such a system are given in Chapter 14.

The two extremes of environment are fully enclosed buildings and buildings exposed to the

environment such as a car park. The rest of this section looks at the systems necessary for

these two situations. These are given by way of example and represent reasonable extremes.

However, it should be noted that some enclosed buildings may have more severe environ-

ments due to the ir use, e.g. swim ming pools, wh ilst m any exte rnal structures will not face

environm ents as aggressive as car parks. In these cases judge me nt w ill be required based

on the background above.

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Grouted bonded post-tensioned construction for buildings 11


The requirements for a ll buildings are as follow s:

• Ducts and vents should be in accordance with the specification, corrosion-resistant

and pressure-testable. The ducts and vents should be pressure-tested.

Full-scale representative grou ting trials should be used to prove the grou ting m etho d,

materials and personnel, unless there is evidence fr om a previous tria l using similar

geometry, and the same personnel, equipment and materials.

I Meth od s tatemen ts should be prepared in advance for all prestressing operations and

should be approved by an app ropriately experienced C hartered Engineer.

I All operations associated w ith the installation, stressing and grouting of tendons should

be undertaken under the certification scheme.

• Anchorage and vent locations and detailing should follow the logic outlined in

Sections 10.3.3 and 11.2 respectively.

In ad dition , for car parks:

Ducts should be corrosion-resistant and p ressure-testable. Sufficient ducts and vents

should be pressure-tested to demonstrate that the system and workmanship produce a

sufficiently airtight solution. It is proposed that initial pressure tests are carried out prior

to th e g routing tria l to dem onstrate the adequacy of the system. Such tests should be in

accordance with Appendix

A

and carried out after the casting of concrete. Thereafter it

is proposed th at con trol tests are carried ou t on 1 in 10 ducts with in the structure and

tha t in the event of a failure, in addition to proposals to improve procedures, the testing

regime is increased un til such time tha t the requirements of Appendix

A

are achieved.

11.3.3 The slab

The concrete strength and cover requirements shou ld be in accordance with BS 850 0

(53)

and crack width requirements in accordance w ith BS EN 1992-1 ~1<

44

'.

If a wa terpro ofing system is used it should be checked for integrity using appropriate

non-destructive test e quipment, including pin-hole d etection equipment for liquid-

applied w aterp roofin g systems. Where a double thickness is used, the first layer should

be proved before the second layer is applied. In add ition, the membrane should be

used to protect any anchorages left exposed.

For car parks the expansion joints and drainage system sho uld be detailed to ensure

tha t, in the event of equipm ent failure or leakage, water cannot find access to the

prestressing system.

Strand should be in accordance with BS 5896<

56

' or similar; see Section 3.4.1 for further

details.


measures

For buildings in unusually aggressive environm ents th e approaches recomm ended for car

parks above, or for bridges as discussed in Chapter 4, could be considered. However, for

most buildings the critical activity is the grouting and so where greater certainty is required

consideration should be given to increasing the level of checking during grouting . This could

be by ensuring tha t there is a third party responsible for checking grou ting activities. An

alternative would be to instruct the contractor installing the tendons to request an audit

under their certification scheme. Such audits are valuable in ensuring that the contractor is

work ing to the requirements of the scheme but is clearly less effective than physical checking.

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11 Grouted bonded post-tensioned construction for building

11.4 Void g rout ing

The guidance given in Chapter 7 should generally be follow ed. It should be noted th at

vacuum gro uting has been used with apparent success on a num ber of building projects

using oval ducts.

11.5 Test m ethods for

grouted post-tensioned

buildings

The guidance given in Chapter 8 is generally appropriate. It sho uld be noted that oval

me tal ducts are no t a irtigh t so pressure-testing of these systems, in buildings, is not

applicable. In ad ditio n, even if a system th at is pressure-testable is used, as discussed in

Section 11.3.2, it may not be appropriate to test every duct. As stated in Section 8.3, an

element of judge me nt is required when considering the ap propriate tests to be carried

out. However, it is often appropriate to carry out a grouting tr ial as discussed below.

Grouting trials

The purpose of gro uting trials is to dem onstrate that the personnel, equipment, ma terials

and procedures can achieve adequate filling of the ducts. For this reason the trials should be

representative of the g eom etry of the structural e lement to be grouted, the ducts should

include the same num ber of tendons and these should be partially prestressed to take up

the correct profile. Grouting procedures are generally more critical for longer tendons with

higher profiles and more strands per duct. The trials should therefore consider a sensible

compromise between the worst case on the project and a trial tha t is representative of the

ma jority of tendo ns. On ly in special cases wou ld there be a need for tw o grouting trials

on the same project.

It will often be acceptable to use grout trials from previous p rojects. However, as noted

above,

these projects should have the same personnel, equipm ent, materials, procedures

and similar geometry to the structure under consideration. Therefore, it is important to

compare the details of the historic trials undertaken in addition to confirming that they

were successful. Given the natural changes in personnel, renewal of equipment, and the

recent change to pre-bagged grouts, it is unlikely that trials m ore than a year or so old

can be considered relevant.

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Unbonded post-tensioned construction for buildings 12

12.

Unbonded post-tensioned construction

for buildings

As discussed

for

grouted tendons,

the

strategy

of

unbonded tendons

is

also th at

of

multi-

layer protection. For an internal environment the first line of defence is the building

envelope; indeed for most buildings, provided the tendons can be protected from corrosion

during construction, the building envelope should prevent further ingress of the elements

required

for

corrosion.

12.1 Introduction

Unlike grouted tendons,

the

unbonded strands generally used

in

building projects

are

coated with either high-density polyethylene or polypropylene. The interstices between

wires are also n orma lly filled w ith grease to repel m oisture. These elements to geth er

provide an essential additional layer of protection.

As unbonded tendons formed of conventional coated monostrand never achieve a bond

between the strand and the surrounding concrete, the anchorage com ponents remain

active throughout the life of the structure. Therefore the successful pr otec tion of the

anchorage components,

and in

pa rticular

the

uncoated sections

of

strand w ithin

the

anchorage, are essential.

A so-called 'after-bond' 19-wire coated strand has been developed in Japan, wh ich acts as

unbonded during stressing

but has a

ribbed sheath

and

contains

a

delayed a ction epoxy

filling, rather th an grease, effectively turn ing it into a bonded tendon after a short period.

It has yet to be used in the UK at the t ime of writ ing but it has an obvious advantage of

being much less heavily reliant on the permanent effectiveness of the anchorages.

For externally exposed structures where the envelope offers no or l imited, protection,

further layers of protection are required and these are discussed toward s the end of the

section.

12.2 Recommended

protect ion systems

for

buildings

For enclosed buildings the two recommended primary protection layers are the envelope

of

the

bu ilding, which

is

outside

the

scope

of

this R eport,

and the

high-density

polyethylene

or

polypropylene sheath.

12.2.1 General It is essential tha t the tendon and the protective sheath remain undamaged during

installation and the contractor should therefore operate a quality system th at maximises

the chances of a successful installation. Method statements should be prepared in advance

for

all

prestressing operations

and

should

be

approved

by an

appropriately experienced

Chartered Engineer.

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12 Unbonded post-tensioned construction for buildinj

The buried 'dead end ' anchorages for unbonded tendons are afforded protec tion via the

passive environm ent provided by the surrounding concrete. Such anchorages should be

provided with cover equivalent to the normal reinforcement for the appropriate exposure

condition. A minimum of 25mm should be provided in all instances.

At the stressed end, i.e. the 'live end ', the anchorages shou ld generally be placed inboard from

the slab edge and a recessed pocket formed to give access for the stressing jack. At the

point w here the jack and wedges grip the strand, the outer sheath and the accompan ying

grease is removed during the installation process. It is recommend that anchorage compo-

nents are coated wi th grease of similar specification t o th at used in the tendo n and tha t a

wa tertigh t cap is applied over the coated area. The minim um concrete end cover to th e

cap and any other anchorage components should be 25mm.

The pockets should be sealed with mortar/re nder. The deta iling of this area requires some

careful atten tion , as mo rtar/render can be permeable and subject to shrinkage. It is

recommended therefore that a non-shrink additive is used for any infill mortar/render.

The use of pockets in the top surface of the slab should be given careful consideration; see

also Section 3.4.3. It is considered tha t the top pocket provides an unnecessarily conven ient

route for contaminants into the anchorage and tendon. If there is no alternative then the

design and construction should ensure that contaminants are excluded both during con-

struction and in service by taking ad ditional protective measures

The use of surfaced-fixed, exposed anchorages is not recommended in bu ilding structures

unless special attent ion is given to their de tailing and their protection from the ingress of

the elements required for corrosion. In such instance the requirements for fire protection

may provide a m ore onerous requirement for protection.

For some buildings the concrete structure can be exposed to the en vironm ent and one of

the more onerous conditions is that of a car park. The rest of this section looks at the

systems necessary for a typical enclosed building and an exposed car park. These are given

by way of exam ple and represent reasonable extremes. H owever, it should be noted th at

some enclosed bu ildings may have more severe environm ents due to their use, e.g. swim ming

pools, wh ilst many externa l structures wi ll not face environments as aggressive as car parks.

In these cases judg em ent w ill be required based on the background above.


The requirements for a ll buildings are as follows :

I The coating to unbonded strand should be either high-density polyethylene or poly-

propylene. The coating thickness should be at least 1.0mm unless otherwise stated in

the project specification.

• All operations associated with the installation and stressing of tendons should be

undertaken under the CARES certificatio n scheme.

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Unbonded post-tensioned construction for buildings 12

In addition for car parks:

The use of surfaced-fixed, exposed anchorages is not recommended.

I The use of pockets in the top surface of the slab is not recomm ended.

J Consideration should be given to using an increased coating thickness to the strand

from 1.0mm to 2.0mm depending on exposure conditions.

12.2.3 The slab

The concrete strength and cover req uirements should be in accordance with BS 8500

(53

>

and crack width requirements in accordance with BS EN 1992-1-1'

44

). In addition:

I If a wate rproo fing system is used it should be checked for integrity using approp riate

non-destructive test equipment, including pin-hole detection equipment for liquid-

applied waterp roofing systems. Where a dou ble thickness is used, the first layer should

be proved before the second layer is applied. In add ition, the membrane should be

used to protect any anchorages left exposed.

For car parks the expansion joints and drainage system should be detailed to ensure

tha t, in the event of e quipment failure or leakage, water cannot find access to th e

prestressing system.

I Strand should be in accordance wi th BS 589 6'

56 )

or similar. Where a dditional protection

from hydrogen emb rittlement/stress corrosion cracking is required, further testin g in

accordance w ith BS EN ISO 15630-3(

59

' and the FIP

Report on prestressing steel^

should be specified un til similar requirem ents are included in BS 5896<

56

>.


measures

For buildings in unusually aggressive environments the approaches recommended for car

parks above, or for bridges as discussed in Chap ter 4, could be considered.

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Recommendations for specifications

for durable post-tensioned concrete

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Recommendations for specifications for post-tensioned tendons 13

13. Recommendations for specifications for

duct and grouting systems for post-tensioned

tendons

UK specifiers should adop t the recomm endations for design details given in this Report.

Users in other countries should consider whether environme ntal conditions and design

requirements are compatible with the UK.

13.1 Introduction

The version of The Concrete Society Specification published in the second edition of TR47

was drafted in 20 00 /20 01 and updated the version published in 1996. It took account of

further experience gained on tes ting of g rout, grou ting trials and systems developm ent in

the intervening period and in the LINK research project as we ll as wider intern ation al

experience drawn upon throu gh the Internationa l Federation for Structural Concrete

(fib).

In 200 7 new versions of BS EN 445<

2

), BS EN 446<

3

> and BS EN 447<

4

' were published and

the UK experience was input into the com mitte e d rafting of the revisions. Consequently

there is considered no need to reproduce the previous specification in this Report, although

there remain a number of important recommendations which should be included in

Project Specifications. These are outlined in this chapter to assist those writing a Project

Specification, and should be considered fo r use in standard industry specifications which w ill

form part of the overall Execution Specification as envisaged in BS EN 13670,

Execution

of concrete structures

1

-

2

^.

In 200 5

fib

published the important Bulletin 33,

Du rability of

post tensioning

tendons^

which adopts many of the recommendations of the second edition of TR47 and is

recommended to the reader.

Under the auspices of European Standards, every post-tensioned concrete s tructure w ill be

governed by BS EN 13670. This is turn requires a Project Specification and this w ill norma lly

be a suitable Na tional Specification such as the Highways Agency

Specification fo r Highway

Works^

or the

National Structural Concrete Specification^.

These Specifications should be

drafted to embody the principles in BS EN 13670 which calls up the grouting specifications

BS EN 445(

2

), BS EN 446<

3

) and BS EN 447<

4

>.

13.2 Guidance on th e

project specification

Site operations should only be pe rm itted to be carried out by organisations c ertificated

by CARES in accordance wi th the requirem ents of the CARES scheme for

The supply and

installation of post tensioning systems in concrete structures^

outlined in this Report, or

equivalent scheme.

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13 Recommendations for specifications for post-tensioned tendons

EOTA 'type approval' for prestressing systems has been developed in Europe and this

includes tests for 'gro utab ility'. There is a requirement in BS EN 13670 fo r post-tensioning

systems to hold a European Technical Approval as outlined in ETAG O13

(25)

, but there is a

need for the Project Specification to state requiremen ts for insta llation and in this respect

BS EN 13670 refers to C WA 1464 6,

Requirements for the nstallation o f post tensioning kits

fo r

prestressing

o f

structures

and qualification of

the specialist company

and its personnel -

26

1 3 . 2 . 1 T r i a l s The Project Specification should call for full-scale trials if considered necessary to demonstra te

that th e gro uting will provide adequate pro tection to the tendons. This requirement should

be specified in the Project Specification and fully detailed on a contract drawing, including

tria l beam size, concrete grade, cover to reinforcem ent and tendons, reinforcem ent and

tendon details, together with requirements for testing and investigation. The purpose of

the trial is to test the contractor's proposed systems, methods, materials and personnel

tha t are to be used in the perm anent w orks. The trial should also incorporate any special

requirements of construction sequence and configurations. Requirements for disposal of

the tri al beam should be specified.

The trials should be carried ou t we ll in advance of the planned use of po st-tensioning in

the perma nent w orks (eight weeks is recommen ded). In particular, any proposals for

untried systems should be given due time for acceptance. The installation of the permanent

works may no rmally be allowed to commence imme diately after com pletion of successful

grout trials.

Where a supplier/installer has evidence of satisfactory grou ting from a significant number of

previous trials using the same procedures, equipm ent and m aterials, the specifier may

consider whether the cost of a ob-specific tr ial is justified. This would particularly apply to

use of a pre-bagged grout where repeatab ility is more assured, or to small projects. Never-

theless, full-scale trials are very e ffective in establishing suitable materials and procedures.

The trials are required to demonstrate that the g routing methods and procedures proposed

by the contractor will ensure that grout fills the ducts and surrounds the prestressing steel.

The contractor should be required to submit a detailed method statement, at least four

weeks prior to use in any trials or in the works, covering proposed m aterials, ducts,

anchorage and vent arrangements, personnel, equipment, grouting procedures and

quality control, for the approval of the engineer.

The trials should incorpo rate all relevant details of ducts, vents, duct supports, deviators,

prestressing anchorages and couplers, prestressing strands, grout inlets and outlets. The

tendons should be sufficiently tensioned tha t the strands with in the d uct take up a

representative a lignmen t. All systems, metho ds and materials should be those proposed

for the permanent works and should have been submitted as part of the detailed method

statement required. Grouting and testing should be carried out in accordance with the

Standards.

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After three days the c ontractor should carefully cut or core the tria l section to expose

cross-sections and longitud inal sections of the duct, anchorages and any o ther locations

where required or as further directed to demonstrate that the duct is satisfactorily grouted.

The contractor should prepare a repo rt, giving full details of the

trial,

testing results and

photographs of the exposed sections.

Grou ting of the ducts should be shown to leave no void that either has a radial dimension

greater than 5% of the diame ter (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 gro ut vents and their adequate gro uting and subsequent sealing, and the

disposition of the steel strands within the body of the gro ut should be reported.

Prestressing for the permanent works should not be permitted without written approval

to th e gro uting procedures and form al acceptance of the results of the gr outing

trial.

Irrespective of whether the contrac t requires full-scale groutin g trials, the contra ctor

should carry out a materials su itab ility assessment in accordance with BS EN 446<

3

>.

Grouting techniques such as vacuum g rou ting and po st-injection regrouting (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.

13.2.2 Grout materials Composition of the grout is vitally important and it is recommended to specify pre-

bagged factory-form ulated products simply needing the add ition of a specified am ount

of water. The designer should specify the grout type required. Performance o f the grout

w ill in all cases be assured by suitab ility trials, irrespective o f wh ether 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 g rout. Additionally, the w eight of

bagged cement is perm itted under current British Standards to vary by up to 6% from the

nominal weight, which could also significantly affect performance of the grout.

The water/cement ratio will normally be in the range 0.30-0.40 in order to achieve the

performance requirements of the Standards.

13.2.3 Ducting for bridges

and other aggressive

environments

Ducting for bridges and other aggressive environmen ts should generally com ply with, as

a minim um , the requirements of

fib

Bulletin 7,

Corrugated plastic ducts for internal bonded

tendons

1

-

32

).

It is generally agreed internationally tha t, for polyethylene and polypropylene d ucting , a wall

thickness of

1.0-1.5mm

is adequate to protect against ingress of chlorides. To allow for

damage during tensioning a minimum manufactured thickness of 2mm is recommended.

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The system of ducts, duct connectors, g routing connections, vents, vent connections, drains,

transitions to anchorages and deviators and caps for anchorages shou ld for m a complete

encapsulation for the ten dons which is resistant to the passage of air and water. D ucts

should be of proven corrosion-resistant 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 contam inants permeating the surrounding concrete should not

be perm itted. The system should be fully co mpa tible with the prestressing anchorages,

couplers and other d etails. Where ducts are non-con ductive, m etal parts of anchorages

should be electrically bonded to th e adjacent reinforcement at each end of th e tendon and

the electrical continu ity of the structure over the length of the tendon should be tested .

The duct assembly ve rification test described in Appendix

A

should be carried out on site

for plastic du ct systems.

The duct rigidity and the type and spacing of fixings and supports shou ld be such as to

maintain line, position and cross-section shape during concreting. Local deformation of

the duct at supports should be avoided.

For external tendons the minimum wall thickness should be 4mm for durability or such

thicker wall as required to withstand the grouting pressures (normally about 6 bar (600kPa))

or the particular duct configura tion. For external tendons it can be impo rtan t to anticipate

any sagging of the duct due to the weight of grout, particularly for tendons stressed after

grouting,

and appropriate temp orary duct support should be provided during the grou ting

operation.

13.2.4 Ducting for internal

elements of buildings

For internal elements where it is considered acceptable to use non -encap sulating duct

systems the ducts should be robust enough to resist damage during co nstru ction, for

example smooth galvanised steel of minimum wall thickness 0.35mm or corrug

galvanised steel with a minim um wa ll thickness of 0.3mm .

1 3 . 2 . 5 V e n t S

Vents providing an air passage of at least 15mm internal diame ter shou ld be provided at

the anchorages and , for bridges, in the ducts at troughs and crests and beyond each

intermediate crest in the direction of g rout flow at the point where th e duct is one-half

diameter lower than the crest (but not further than 1m) and elsewhere, as required by the

system. For buildings where the drape is less than 5 00 m m , vents are gene rally placed at

duct crests as required to com ply w ith th e m aximum spacing. The m axim um spacing of

vents should be 15m for bridges and 20 m for buildings unless required otherwise by the

system or demonstrated by trials. Some configurations and some applications may

require closer or wider ve nt spacing.

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The vent diameter and spacing may be varied in full-scale trials demon strating the suitability

of alternatives. The vents should be rigidly connected to the ducts and should be capable

of being closed and reopened. Holes in the ducts should be at least the internal diameter

of the vents and should be formed before any pressure testing. All ducts should be kept

free from stand ing water at all times and be thorou ghly clean before grouting .

For external tendons the arrangement and detailing of vents at positions w ithin deflectors/

diaphragms shou ld have been proven by detailed testing.

All anchorages for bridges and special applications should ge nerally be sealed by caps and

fitted with grou ting conn ections and vents. Sealing of anchorages should perm it the flow

of grout through the anchor head.

For most applications a vent height of at least 500mm above the adjacent concrete surface

is recommended t o help entrapped air and water to escape. For some con figurations of

tendons this may not be appropriate and the designer should consider an a lternative

13.2.6 Testing

The duct assembly verification test is intended to demo nstrate tha t plastic duct systems

have 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 comp ly, sealing o int s w ith 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 grou ting pressure.

Appendix A describes additional tests for measuring the effectiveness of seal of the duct

system,

which the designer m ay consider adop ting in ap propriate circumstances. These

methods require furthe r experience and development before a dop tion as a specification

requirement.

The fluidity of the grou t dur ing injection should be high enough to be pumped effectively

and to f ill the duct a dequately, but low enough to expel the air and any water in the duct.

The time during which fluidity is maintained may need to be more than the minimum of

30 minutes given in the Standards and a target o f 90 minutes is a sensible upper lim it.

The grout should be sufficiently stable to bleed very litt le, so the materials segregate and

settle to a minimal extent. It is recommended that the allowable volume change limits in

BS EN 445P', BS EN 446<

3

> and BS EN 447<

4

> are amended in the Project Specification

such that -0 .5% < 8V < 2.5% , where

5 V

is the volum e change.

The requirement within BS EN 447 for density tests has been questioned in practice. There

is difference of op inion as to the value of this test over and above the fluid ity test. It is

understood that a correlation between fluidity and density is possible but that the

correlation can vary significantly at different temperatures. Further research is required

on this. In the mea ntime the density test should be carried ou t and reported as required.

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13.2.7 Grouting

It was observed that th e type of bleed test previously specified in m ost nation al Codes,

and in the first edition of TR47, fails to identify poten tially unstable grou ts. The imp ortan t

feature missing from the tests is the destabilising effect of the 'wick ac tion ' caused by the

strands. This sh ortfall has been addressed by the developm ent of new tests which are

now covered in BS EN 445 . Further research developme nt o f new bleed tests is still being

carried out in the USA.

It is recommended tha t, where the system includes end caps at anchorages intended to

be left in place, these are left undisturbed and completeness of g rou ting is tested by

sounding and visual exam ination of ven t holes in order to avoid d isturbing the seals.

It is recommended that all bridges and structures in aggressive environments are classed

as Inspection Class 3 in BS EN 13670'

22

' and BS EN 446(

3

>. Inspection Class 2 could be

considered for less sensitive applications.

Norm ally, grout injection should not exceed the rate of about 10m of duct per minute. For

certain applications, where ducts are outside the norm al range of size (i.e. not m ulti-

strand tendons in 80-125mm ducts) this may be increased to 15m of duct per minute.

It is recommended that a record is made of the grout volume injected into each tendon,

preferably by use of a flow meter.

To minimise the risk of blockages of p um ping eq uipment 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 recom mend ed before gro uting very lon g tendons and in

warm weather.

In cold weather it is necessary to measure the temperature of the co ncrete structure (for

internal tendons) or the air void around the ducts (for external tendons) to comply with

specifications t o avoid freezing the grou t. 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 g rou ting , 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 gro utin g, and measure the tem pera ture of this water.

The filled ducts should not be subjected to shock or vibration within 24 hours of grouting.

Whe n the gr out has set, the grou t vents should be tem pora rily reopened. If voids are

apparent on inspecting vents at end caps, the end caps should be removed to demonstrate

tha t they are satisfactorily filled w ith grou t. End caps, which have been rem oved, should

then be replaced by end caps perma nently sealed against ingress of con tamina nts.

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If the anchorage caps are removed, a photographic record should be taken, clearly

identifying the individual anchorages, and included in a report.

If there is cause for doubt that the ducts or any part of the system are not satisfactorily

filled with g rout, investigations should be carried out.

The contractor should keep full records of grou ting for each duct in accordance with the

certification scheme requirements for installation of pos t-tensioning and in accordance

wit h Standards.

Grout vents should be positively sealed to be waterp roof on com pletion of grou ting by a

means separate from the concrete waterproofing.

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14 Contractor's quality scheme requirements

14. Contractor's quality scheme requirements

The details in this chapter are intended to give guidance to post-tensioning contractors

to help develop their qu ality system procedures in line w ith the requirements of this

Technical Report.

14.1 Introduction

The CARES certification scheme PT1, The

supp ly and nstallation

of post tensioning systems

in concrete structures^,

is UKAS-accredited and was a key element in the lifting of the

moratorium on post-tensioned bridge construction in the UK n 1996. In response to the rapid

increase in post-tensioned concrete frame constru ction, CARES has developed a certification

scheme specifically

for

this method

of

construction, namely PT2,

The supply and installation

of

post tensioning systems in

concrete

structures

(excluding

highways

structures)

1

?

0

1

.

In order to give the customer assurance that the technical requirements and the spirit of

this Report

are

applied

in

practice

it is

essential tha t

the

post-tensioning contractor

has

an appropriate quality system and product certification to CARES schemes PT1 or PT2 as

appropriate.

The requirements of the CARES certification scheme have been developed through

agreement by relevant interested parties (clients, contra ctors, 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 o ffice a ctivities

and site practice.

The schemes are based on the premise tha t 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

comp atible,

and

which

are

correctly installed.

14.2 Basic qu ality system

elements

The CARES scheme relates

to the

quality system

and

product requirements

for the

supply

and installation of post-tensioning systems in concrete structures using bonded or unbonded

tendons

in

accordance wit h

the

relevant product standard and/or contract specification.

The post-tensioning contractor's quality management system shall be based on the

following essential elements covering both office and site activities.

Qua lity management system

The post-tensioning contractor shall have a quality system th at complies w ith the

requirements

of BS EN ISO

9001(

81

'

and

CARES Appendix PT1

or PT2 as

appropriate.

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 post-tensioning contractor.

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Contractor's quality scheme requirements 14

Quali ty management system planning

The post-tensioning contractor shall ensure that the planning of the quality management

system is carried out in order to meet the qu ality objectives.

The post-tensioning contracto r shall produce a quality plan for each structure, iden tifying

structure-specific details, on which it is contracted to operate. The quality plan shall include

meth od statemen ts for the relevant key post-tension ing activities, e.g. duct insta llation,

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.

Provision of resources

The post-tensioning contrac tor 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 and verification activities as defined in Clause

6.1

of BS EN ISO

9001'

81

'.

Contract review

The contrac t review procedure shall ensure tha t the responsibilities of all relevant parties are

identified and all relevant design details, e.g. post-te nsion ing system, tendon configuration,

tensioning requirements, tension increments, grout, grou t mixing, grout placing, grout

testing,

resource requirements, attendances, are clearly, adequately and unambiguously

defined.

Records of contract review shall be maintained.

Quali ty records

The post-tens ioning contracto r shall keep quality records relating to the techn ical details

of post-tensioning contracts including site installation records.

Traceability

The post-ten sioning contrac tor shall ensure tha t materials and com ponents are traceable

from source to their location within the structure.

Purchasing

The post-ten sioning contrac tor shall have a system for purchasing materials and services

from subcontractors tha t includes all aspects of the ma terial or service specification th at

are important in ensuring satisfactory product quality and identification.

The post-tensioning contrac tor shall be responsible for the provision of po st-tensioning

system components, grout components and post-tensioning equipment.

Storage

The post-ten sioning contra ctor shall ensure tha t materials are stored and segregated in a

manner that prevents corrosion, damage, deterioration or contamination.

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Handling

The post-tensioning co ntractor shall handle m aterials and equipment in a way tha t preserves

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

Inspection and tes ting

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 post-tensioning contractor shall ensure that non-conforming work and materials are

not used in the works and tha t they are adequately segregated and ide ntified.

Internal quality audits

The post-tensioning contractor shall undertake internal quality audits in order to verify

the effectiveness of the qua lity system, including site ac tivities.

Training

It is essential tha t a ll post-tension ing operations are carried out by operatives with

appropriate knowledge, training and proven experience.

The post-tensioning contractor shall:

• Define the categories o f on-s ite personnel, e.g. trainee, opera tive, supervisor, engineer.

• Define the kn ow ledge , skills and experience required for each personnel category.

• Evaluate and endorse experience of personnel based on objective evidence such as

verifiable training records.

Provide relevant theoretical and practical training.

• Determine the level of knowledge and skill attained du ring training .

• Issue stateme nts of achieveme nt that identify the level of training achieved and

submit them to CARES in accordance with CARES PT9<

82

'.

CARES shall issue post-tensioning personnel with ID cards as appropriate in accordance

with PT9.

14.3 Product requirements

Ducts

The post-tensioning contractor shall ensure that:

• Ducts comply with the contract specification.

- Ducts are correc tly assembled, sealed, installed and adequately fixed to resist movem ent

and floatation during concrete placement.

• Ducts are free from standing water and con tam ination at all times, and are thoroug hly

clean before grouting.

• Duct vents are iden tified and protected from damage.

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Contractor's qua lity scheme requirements 14

Tendon installation

The post-tensioning contractor shall ensure that tendons are installed safely and without

degradation, contam ination or damage to the tendon or the duct. Records shall be kept

of any problems encountered d uring the tendon installation process, e.g. blockages or the

use of excessive force.

Tensioning

The post-tensioning c ontractor shall ensure that tendons are tensioned to the correct

force in the correct sequence.

The tensioning procedure shall include the direct measurement o f tendon force and load

versus extension m easurements for verification purposes.

Anchoring of post-tensioning tendons

The pos t-tensioning contrac tor shall ensure that tendons are adequately anchored and

that the anchorages and tendons are protected from corrosion and mechanical damage.

Anchorages shall comply wit h the performance requirements of BS EN 13391'

83

' or equivalent,

e.g.

a European Technical Appro val t o ETAG 013<

25

>.

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 me thod and type of p rotec tion.

Grout

The properties of the g rout s hall comply w ith th e requirements of BS EN 447

(4

>. In the

case of non-highways structures, grout shall be pre-bagged m aterial requiring only the

addition of water. Grou t shall be produced using fresh materials only.

Before use, the post-tens ioning contrac tor 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

tempe rature 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 c ontractor shall have a procedure to contro l the grouting process. The

procedure shall com ply w ith BS EN 446<

3

'. The properties of grou t shall be determine d in

accordance w ith BS EN 445(

2

>.

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14.4 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 this

Technical Report. In order to give assurance of full comp liance w ith the report, post-

tensioning c ontracto rs s hall have an appropriate qua lity system certified by CARES to

CARES Schemes PT1, PT2 or equivalent.

Further in formatio n is available fro m: UK CARES,

Pembroke House,

21 Pembroke Road,

Sevenoaks,

Kent

TN13 1XR,

UK.

Tel:+44(0)1732 450000,

Fax:+44(0)1732 455917,

E-mail: [email protected], www.ukcares.com

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References

References

1. THE CONCRE TE SOCIETY.

Durable post-tensioned concrete bridges

The Concrete Society, Camberley,

First Edit ion 1996, Second Edit ion Z002.

2.

BRITISH STANDARDS INSTITUTIO N, BS EN 445 .

Grout for prestressing tendons -Test methods,

BSI,

London, 2007.

3. BRITISH STANDARDS INSTITU TION, BS EN 44 6.

G rout for prestressing tendons-Grouting procedures,

BSI,

London, 2007.

4 .

BRITISH STANDARDS INSTITU TION , BS EN 447.

Grout for prestressing tendons-Basic requirements,

BSI,

London, 2007.

5 . WO ODW ARD, RJ.

Conditions within ducts in post-tensioned prestressed concrete bridges,

Laboratory

Repor t 98 0, TRRL, Crowthorne, 1981.

6. STAND ING COMMITTEE O N STRUCTURAL SAFETY.

Third Report of the Committee for the Year

Ending 31 March

7979, SCOSS, London, pp. 15-18.

7. PORTER, MG . Repair of post-tens ioned conc rete structures ,

Proceedings of Symposium on Concrete

Bridges: investigation, maintenance and

repair,

September 1985,

The Concrete Society, Camberley,

1985,

pp. 1-27.

8. W OO DW AR D, RJ and WILLIAMS, FW. Collapse of Ynys-y-Gwas Bridge, West Glam organ,

Proceedings,

Institution of Civil

Engineers, Part 1, Vol . 84, August 1988, pp. 635 -669 .

9. POST-TENS IONING INSTITUTE.

Guide specification for grouting of post-tension ed structures,

Phoenix, Ar izona, 200 1.

10 .

FLORIDA DEPARTMENT OF TRANSPO RTATION.

Mid-Bay Bridgepost-tensioning evaluation,

10 October 20 01 . h t tp : //www11.myf lo r ida .com/s t ruc tu res /memos.

11 . HIGHW AYS AGENCY, TRL, SETRAand LCPC.

Post-tensioned concrete bridges,

Thomas Tel ford,

London, 1999.

12 .

FEDERAL MINISTRY OFTRANSPORT, CONS TRUCTION AN D HOU SING.

Guidelines for concrete

bridges with external tendons,

Verkehrsblat t-Ver lag, Dor tm und , No. 17/1999.

13 .

RAISS, ME. Durable post-tens ioned concre te br idges, CONCRETE, Vol. 27, No. 3, May/June 1993,

pp.

15-18.

14 . HIGHWAYS AGENCY, BD 54/93 and BA 54/ 93.

Design m anual for roads and bridges,

Vo lume 3 :

Highway structures: inspection and maintenance,

Section 1:

Inspection,

Part 2:

Post-tensioned

concrete bridges, prioritisation of special inspections,

H

gh

ways Agency, London, 1993 .

15.

HIGHWAY S AGENCY, BA 50/9 3.

D esign manual for roads and

bridges, Volume 3:

Highway structures:

inspection and maintenance,

Section 1:

Inspection,

Part 3:

Post-tensioned concrete bridges, planning,

organisation and methods for carrying out special inspections,

Highways Agency, London, 1993.

16 .

WO OD WA RD , RJ. Evidence of problems.

TRL Seminars on Inspection of post-tensioned concrete

bridges,

he ld on var ious dates 19 92-1994 .

17.

CLARK, LA.

Performance in service of post-tensioned bridges,

Br i tish Cement Associat ion (now

Mineral Products Association), Camberley, 1992.

18 . DEPARTMENT OF TRANSPORT.

Standards for post-tensioned prestressed bridges to be reviewed.

Press Notice No. 260, London, 25 September 1992.

19 .

HIGHWAYS AGENCY, IAN 16.

Design manual for roads and bridges,

Volume 2:

Highway structures:

design (substructures and special structures) materials,

Section 2:

Special structures, Post-tensioned

grouted duct concrete bridges,

H ighways Agency, London, 1999.

2 0 .

HIGHW AYS AGENCY, IAN 47.

Design manual for roads and bridges,

Volume 2:

Highway structures:

design (substructures and special structures) materials,

Sect ion 3:

Materials and components,

Post-tensioned grouted duct concrete bridges,

H ighways Agency, London, 2002.

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References

2 1 . CARES. The supply and nstallation of post-tensioning systems n concrete structures.

http://wvjw.ukcares.co.uk/listjntro.htmlitposttensioning

22 .

BRITISH STANDARDS INSTITUTION,

BS EN

13670.

Execution

of'concretestructures, BSI, London, 2009 .

23 . FEDERATION INTERNATIONALE DU BETON. Grouting of tendons in prestressed concrete. Guideto

good practice, Bulletin No. 20, fib, Lausanne, 2002.

24.

FEDERATION INTERNATIONALE DU BETON. Durability of post-tensioning tendons, Bulletin No. 33,

fib, Lausanne, 2006.

25 .

EUROPEAN ORG ANIS ATIO N FORTECHNICAL APPROVALS, ETAG 013. Post-tensioning kits for

prestressing of structures, EOTA, Brussels, 2002.

26.

BRITISH STANDARDS INSTITUTION,

CW A

14646.

Requirements

forthe installation of post-tensioning

kits for prestressing of structures and qualification of the specialist company and ts personnel, BSI,

London, 2003.

27. UK CARES. Model specification f or bonded and unbonded post-tensioned floors (Second Edition),

UK CARES, Sevenoaks, 2008.

28.

CONSTRUCT. National structural concrete specification,

The

Concrete Centre, Camberley, Fourth

Edition 2010.

29 . THE CONCRETE SOCIETY. Gro uting specifications, CO NCRETE, Vol. 27, No. 4, July/August 1993,

pp.

23-28.

30. THE CONCRETE SOCIETY/CONCRETE BRIDGE DEVELOPMENT GROUP. Durable post-tensioned

concrete bridges, Proceedings

o f

Seminar,

The

Concrete Society, Camberley,

1994.

3 1 . FEDERATION INTERNATIONALE DE LA PRECONTRAINTE. Grouting o f tendons in prestressed

concrete,

FIP

Guide

t o

Good Practice, Thomas Telford, London, 1990,16pp.

32 . FEDERATION INTERNATIONALE DU BETON. Corrugated plastic ducts for internal bonded

post-tensioning, Bulletin No. 7, Taerwe, L (Ed.), fib, Lausanne, 200 0.

33 . BALVAC WHITLEY MO RAN . Feiton

Bypass,

Vacuum

assisted pressure

grouting of the post-tensioned

Macalloy ba r ducts on the River Coquet Bridge. 1981, 20pp

3 4. TILLY, GP and WOODWARD, RJ. Development of improved grouting fo r post-tensioned bridges,

Proceedings of FIP Symposium Post-tensioned concrete structures 7996,The Concrete Society,

Camberley, 1996, Vol. 1, pp. 55-64 .

35. GIFFORD A ND PARTNERS. QA of grouting post-tensioned concrete structures. Proceedings o f

Seminar, Cambridge, 23-24 September 1999. Gifford and Partners, Southampton. (Workshop

organised under BRITE EURAM Contract BRPR-CT 95-0099, BE95-1675, Improved quality

assurance

and methods of grouting post-tensioned tendons.)

36 . DARBY, JJ. Control o f grouting quality by the measurement of to ta l gas content with in fresh grout,

Proceedings

of FIP

Symposium Post-tensioned concrete structures

1996,

London, September

1996,

The Concrete Society, Camberley, 1996, Vol. 2, pp. 669-676.

37. BUILDING RESEARCH ESTABLISHMENT. Concrete in aggressive ground, Special Digest 1 (Four p arts),

CRC Ltd, Garston, 200 5.

38. THE CONCRETE SOCIETY. Alkali-silica reaction: minimising t he risk of damage to concrete,

Technical Report

30

(Third Edition),

The

Concrete Society, Camberley,

1999.

39. HIGHWAYS AGENCY, BD 33/94. Design manual for roads an d bridges, Volume 2: Highway structures:

design (substructures

and

special structures) materials, Section

3:

Materials

and

components, Part

6:

Expansion

oints for use n highway bridge decks, Highways Agency, London, 1994.

4 0 . HIGHWAYS AGENCY, BD 57/01 and BA 57/01. Design manual for roads an d bridges, Volume 1:

Highway structures: approval

processes

and general design, Section

3:

Generaldesign, Parts

7 and 8:

Design o r durability, Highways Agency, London, 200 1.

4 1 .

CONCRETE SOCIETY. Non-structural cracks in concrete, Technical Report 22 (Third Edition), The

Concrete Society, Camberley, 1992.

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References

4 2 . CONCRETE SOCIETY.

The

relevance of cracking in concrete to corrosion of reinforcement, Technical

Report 44,The Concrete Society, Camberley, 1995.

4 3 .

HIGHWAYS AGENCY, BD 28/ 87 and BA 24/87 . Design manual for roads and bridges, Volume 1:

Highway structures: approval

processes

and general design, Section 3: General design, Part 14: Early

thermal cracking of concrete, Highways Agency, London, 1987.

4 4 . BRITISH STANDARDS INSTITUTION, BS EN 1992. Eurocode 2 Design of concrete structures. Part 1-1:

General rules and rules for buildings, Part 2: Concrete bridges, BSI, London, 2004 and 2005.

4 5 . SALAS, RM, SCHOKKER, AJ, WEST,JS, BREENJE and KREGER, ME. Conclusions, recommendations

and design guidelines for corrosion protection of post-tensioned

bridges,

Research Report 0-1405-9,

Center for Transp ortation Research, The University of Texas at Austin, 20 04.

4 6 .

BRITISH STANDARDS INSTITUTION, BS 540 0. Steel, concrete and composite bridges, Part 4: Code

of practice for design of concrete bridges, BSI, London, 1990.

47 .

HIGHWAYS AGENCY, BD 24/92. Design manual for

roads

and

bridges,

Volume 1: Highway structures:

approval processes and general design, Section 3: G eneral design, Part 1: The design of concrete

highway bridges and structures. U se of BS 5400: Part 4:1990, Highways Agency, London , 1992.

4 8 .

HIGHWAYS AGENCY, IAN 123/10. Use of Eurocodes for the design ofhighway structures, Highways

Agency, London, 2010.

4 9 . CONSTR UCTION INDUSTRY RESEARCH AN D INFORMAT ION ASSOCIATION. Bridge detailing

guide, C543, CIRIA, London, 2002.

5 0 . HIGHWAYS AGENCY, BD 47/9 9 and BA 47/ 99. Design manual for roads and bridges, Volume 2:

Highway structures: design (substructures and special structures) materials, Section 3: Materialsand

components, Parts 4 and 5: W aterproofing and surfacing of concrete bridge

decks,

Highways Agency,

London, 1999.

51 .

HIGHWAYS AGENCY. Manual of contract documents for highway

works,

Volume 1: Specification for

Highway

Works,

Highways Agency, London, 20 05 (plus amendm ents).

52. THE CONCRETE SOCIETY. Guide to surface treatments for protection and enhancement of concrete,

Technical Report 50 , The Concrete Society, Camberley, 1997.

53.

BRITISH STANDARDS INSTITUTIO N, BS 850 0. Concrete-Complementary British Standard to

BS

EN 206-1, Part 1: Method of specifying and guidance for the specifier, Part 2: Specification for

constituent materialsand concrete, BSI, London, 2002.

5 4 . WALLBANK, EJ.

The

performance of concrete in bridges. A survey of 200 highway bridges, HMSO,

London, 1989.

55 . HOBBS, DW. (Ed.) Minimum requirements for durable concrete. Carbonation and chloride-induced

corrosion, freeze-thaw attack and chemical attack, Publication 45.043, British Cement Association

(now Mineral Products Association), Camberley, 1998.

5 6 .

BRITISH STANDARDS INSTITUTIO N, BS 5896 . Specification for high tensile steel wire and strand for

the prestressing of concrete, BSI, London, 1980.

57. BRITISH STANDARDS INSTITUTION, prEN 10138.

Prestressing steels.

Part 3: Strand. (In preparation.)

58 .

AMERICAN SOCIETY FORTESTING AN D MATERIALS, ASTM A416/A 416M. Standard specification

for steel strand, uncoatedseven-wire for prestressed concrete, ASTM, West Conshohocken,

Philadelphia, 2006.

59 .

BRITISH STANDARDS INSTITU TION, BS EN ISO 1563 0-3. Steel for the reinforcement and

prestressing of concrete-Test methods, Part 3:

Prestressing

steel, BSI, London, 2002.

6 0 . FEDERATION INTERNATIONALE DE LA PRECONTRAINTE. Report on prestressing steel, 5:

Stress

corrosion cracking resistance test for prestressing tendons, FIP (now fib), Lausanne, Sw itzerland,

September 1980.

6 1 .

KOLLEGGER, J. Investigations on a plastic duct for bonded post-tensioning , Bauingenieur, Vol. 69,

1994,

pp. 1-10.

62 . RICKETTS, NJ. Post-tensioned concrete bridges: Improved

design

methods, details and monitoring,

Interim Summary Report, Project Report PR/BR/2/93,Transport Research Laboratory, Crowth orne, 1993.

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Appendix A

This test has the m erit tha t an absolute measure of to ta l leakage is obta ined, and the

duct can be characterised fo r leakage over a range of pressures. The effect of mo vem ent

of the duct, or sealing of a known leak, can also be correlated w ith the imm ediate effect

on leakage rate. The me thod is suitable for the survey of existing ducts p rior to regrouting.

The air leakage at a poin t tha t st ill provides sufficient encapsulation has not been established

at the present tim e, but can be more easily investigated if the leakage flow rate and pressure

com binations are know n. The limitation of the m ethod is tha t it has not been used on a

range of duct types to establish achievable standards.

Water test for leaktightness

The

fib

proposal for te sting of plastic ducts'

311

includes a water test on a 110 0mm -long

sample of duc t including a coupler. The specimen is subjected to specified b ending before

testing with water, internally and then externally, at a pressure of 0.5 bar (50kPa). 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, nam ely its ability to withstand ingress of water t hat may contain chlorides.

The higher test pressure of 0.5 bar (50kPa) and testing of a duct that has been bent also

give greater reassurance than a lower pressure air test. U nfortun ately th e test at present

does not appear to have any criteria for acceptance. Neither is there any accompan ying

air testing th at can be used to correlate wit h site testin g, which shall of necessity be with

air. If the test were to be repeated on a variety of du ct systems, togeth er w ith air tests for

leakage flow rate, the data thus gathered w ould enable this test to become of greater value.

A2 Grou t sti ffness tests

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 trapp ed gas within the fluid grou t at

the time of grouting or formed after bleed water has been reabsorbed. A method of

measuring the to ta l volume of void has been developed wh ich e xploits its co mpressibility

within an otherwise incompressible

fluid.

The method was developed during a number of

research projects, and the background is more fully described in Reference 34.

Research using this apparatus has led to a greater understanding of the sources of voids

with in grouted ducts. The m ixing process first entrains air, and this gradually rises and

escapes from the fluid. Subsequent venting wi ll remove most of this air, but air reaching

the surface after ve ntin g w ill remain as a void. If raised pressure is maintained until grout

stiffens, bubbles wi ll be compressed with a reduced tendency to rise. Gas-entraining agents

incorporated to cou ntera ct shrinkage are a further source of trapped gas, and the rate of

production of this gas will depend upon the tem perature of the mix. Whether this remains

as a void will again depend upon the time of prod uction relative to the tim e of venting,

and the maintena nce of pressure. Use of the gro ut stiffness test has dem onstrated the

presence of a small volum e of trapped gas within appare ntly we ll-grou ted ducts. This is

believed to be due to air trapped w ithin strand crevices and/o r the ribs of ducting . Air

with in such strand crevices has been found by pressure transmission along a 5m length of

grouted duct. These mino r distributed voids are no t believed to pose a significant threa t

to durability.

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Appendix A

Figure A1

Location of spongeometer within the grouting

system.

The technique was first developed to measure the volum e of trappe d gas using apparatus

illustrated in Figure

A1.

(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 compute d from the change in pressure.

This void m easurement technique was incorporated w ithin the 'Oxford gro uting qua lity

con trol sy stem' as more fully described in Section A5.

The equipment was demonstrated to detect voids at a distance of 60 m , and to provide

information on the effect of gas-entraining agents and mixer types. Samples of grout

could be tested at any stage in the grou ting process to measure the gas entrained in the

material delivered by the mixer. In order to achieve sufficient accuracy allowance had to be

made for the small movements of piston seals and duct walls. When incorporated within

the equipm ent described in Section A5 there were facilities to include these corrections,

and to store the results electronically. When the corrections were made, the equipm ent

was also suitable for use on external tendons. An au tom atic was hout fac ility minimises

delay during use. Interpretation of results was enhanced if the system was calibrated du ring

type approval testing or on trial ducts.

Pressure transducers

Supply line to grouted duct

Pressure chamber

Controlled Pistons

Supply line

from grout

pump

A3 Vo id sensors

Sensors that can be installed w ithin or around ducts have been developed for measuring

the passage of grout and the height or quantity of grout within a section of duct'

34

'.

Two m ain types of sensor were tested, nam ely capacitance and resistance. The capacitance

sensors were contained wit hin a sleeve around the duct, and capacitance was calibrated

as a func tion of the height of g rout w ithin the d uct. Resistance sensors com prised a short

section of duct connected to the rest of the duct w ith heat-shrinkage sleeves. When the

duct is grouted , the resistance between tw o opposing electrodes drops significantly (by a

factor of 1000).

Void sensors may have a role in quality con trol of gro uting if inform ation is required on

the filling of a duct only at a particular location . They do however require ins tallation of

specialist equ ipme nt and wiring in advance of concreting, and are therefore more likely to

be appropriate in a research situatio n.

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Appendix A

A4 Duct pressure sensors

A5 Autom ated qual i ty

co ntro l systems

Injection pressure has conven tionally been measured at the g rout pump , which is the

location on the grout line that experiences ma ximum variation. The auto ma ted q uality

con trol eq uipme nt described in Section A5 measures the pressure w ith tw o sensors closer

to the point of injection. It is also possible to install pressure sensors along the line of the

duct'

34

*. Electrical and pneum atic pressure sensors were used. They were separated fr om

the fluid grou t by a rubber mem brane, 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 wou ld then need to be made good .

Duct pressure sensors can be used to provide an ad ditiona l record for the g routing and

inform ation on the pressures in the d uct du ring the first 24 hours after grou ting. They are

practical and can be withd raw n before the grou t sets so that the vents can be filled and

sealed in the norm al way.

Effective qu ality con trol processes require a ll relevant info rma tion to be recorded. This

can be used to d em onstrate t ha t a ll procedures have been followe d and also enables

problems to be diagnosed and where possible re ctified.

Grouting of post-tensioned ducts requires operatives to perform a number of different

operations, such as bleeding specified quan tities of gro ut at vents, sealing vents and

applying specified pressures. Final quality is sensitive to any departure from the specified

requirements. Good grouting is so imp ortant for long-term durability that automa tic

recording and testing of workmanship to demo nstrate compliance is ustified.

The equipment described in Section A2 was further developed, with the support of the

Highways Agency and British Cem ent Association. The device became known as the

'Oxford grout quality control system' (see Figure A2). Unfortunately the device was not

taken up by industry and has now been scrapped. However, the principles remain

valid.

The method of measuring the volume of trapped gas was the same as the equipment

described in Section A2, but other parameters were also measured, and all in form ation

was autom atically recorded. The follow ing features were included:

• selection of autom ated test sequences to ou tpu t the volume of trapped gas wi thin

either grout samples or grouted ducts

analysis of test results by statistical means with imm ediate display of results to th e

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 du ct

• dual measurem ent of grou t pressure to provide war ning of any discrepancies in

measurement of this vital parameter

I recording grout pressures, grou t tempera ture, amb ient tempe rature, and flow rate of

grout at any specified time intervals

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Appendix A

i autom atic washout of the pressure cylinder wit ho ut opening to m inimise interru ption

to the g routing process

electronic contro l of a ll valves to speed use

I autom atic sequencing of operations by preset combinations, operated from a display

screen

i storage of all results for the pro duction of a report when grouting is finished.

Use of equipment of this type can demonstrate that the required quality has been achieved,

as well as provide an incentive to operatives who wi ll be aware that their wo rk is under

constant surveillance. The final report produced from the stored information could provide

a permanent record of the grouting op eration . If the equipm ent were used on a variety of

contracts and duct systems, technica l data on gro uting would be accu mula ted. This could

advance knowledge of the technicalities of grou ting, and enable further improv eme nts to

be made.

Figure A2

Instrumentation within the O xford grout

quality co ntrol system.

-

Grouted

duct

rt/V

1 K& -

Drain valves =Q ^

Chamber

Wash unit

A.

Pressure

chamber

•fc^— Bleed valves

i9 )

Grout mixer

and pump

Computer control unit

y

y

y

y

y

Recorded

continuous

date

• Time

• Pressures

• Flowrates

• Grout

temperature

• Ambient

temperature

; \

1 x

i \

1

\

1 s

1 \

1

N

R e c o r d e d

tes t da ta

Volume of

grout trapped

within ducts

or grout

samples

Operation

sequences

controlled at

remote panei

• Fill cham ber

• Test grou t

sample

• Fill duc t

• Test duc t

• Washout

chamber

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Appendix A

A6 Volume of voids before

regrouting

One of the most important parameters influencing successful regrouting of ducts is the

volume and disposition of the void to be filled. There are several methods by which the

volum e can be measured, each of them involving air pressures. They all require a hole to

be drilled to intersect the duct, preferably the top of the duct. A pressure-tight connection to

the drilled hole is then required, such as by a resin seal or expansion device wi thin the hole.

The following methods have been used.

Boyle's law vacuum method

The duct is evacuated to the maxim um vacuum tha t can be achieved, and the pressure

measured. A valve is then operated to connect th e evacuated duct to the top of a Perspex

tube, wh ich in turn is connected to a water reservoir. The water is drawn in to the Perspex

tube by the vacuum , and the volume of void w ithin 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

A steel container is first pressurised to a known value. (A fire extinguisher casing has been

used for this purpose.) A valve is then o perated t o connect this pressurised container to

the duct void, and the new pressure is noted. On the assumption that the duct void is

sealed,

the volum e o f void may be calculated by Boyle's law

(PV=

constant).

The 'Oxford void volume measurement equipment

1

This proprietary equipment determined the void volume by the electronic tim ing of

pressure changes resulting from a 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 tes t. If the void is we ll sealed, either the Boyle's law vacuum me thod or the pressure

method can give accurate results. However, the vacuum method is more cumbersome,

and the application of vacuum has the p oten tial danger of drawing moisture into the duct.

The pressure me thod is relatively cheap, although the result w ill not be accurate for voids

tha t are very significantly larger or smaller than the pressurised co ntainer.

The Oxford void volume measurement equipmen t produced more accurate results, and the

principle remains effective for ducts wi th mod erate leakage. The benefits are greatest if

the voids are much smaller or larger than the pressurised container. Voids tha t leak throu gh

cracks, and are thus most vulnerable to tend on corrosion, could still be measured. The

method would be particularly appropriate for surveys in advance of regrouting, where

detailed knowledge of sma ll voids and leaks is essential if grouting is to be undertaken

wit h confidence tha t the voids will be filled.

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CARES and Post Tensioning

CARES Approved Installation

• Approval of post-tensioning contractor,

method statements, post-tensioning

kit, m aterials.

• Trained installers.

• Verif ied comp liance of post-tensioning

kit and components with standards

and codes.

• Tracea bility of post-tensioning

kit components: anchorages,

strand/bar, duct and grout.

• Ma intenance of testing and

installation records.

or High Risk

Non CARES approved post-tensioning

contractor may result in incorrect

installation of post-tensioning system.

Non-CARES post-tension kits or materials

may not meet standard requirements.

Failure of post tensioned system or

compromised structural performance

and reduced durability due to

incorrect or incompatible components

or unsatisfactory grouting.

N o traceability w ith difficulties to proper

recourse on non conformity or failure.

Why take the RISK?

For further information about CARES and an up-to-date list of manufacturers and suppliers

holding CARES certification please consult the CARES website: ww w.ukcares.com

Alternatively, co ntact the office:-

UK Ce rtification A uthority for Reinforcing Steels,

Pembroke Ho use, 21 Pembroke Road,

Sevenoaks, KenHN13 1XR

Telephone-01732 450000

Fax -01732 455917

E-mail - [email protected]

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POST-TENSIONING

EXPERTISE ACROSS

THE GLOBE

CCL has a world class reputation for providing high quality,

imaginative post-tensioning solutions. Throughout every

stage of t he process, CCL can help you realise your

project's full p otential.

CCL is a CAR6S approved com pany. It's h igh perform ance

post-tens ioning systems are designed, manufactured and

tested to exceed the requirements of European Standard

ETAG013andAASHT0.

www.cclint.com

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CARES is a UK base d, internationally recognised certification

body fully, accredited and operating independently for the

benefit of a ll key sectors of the s upply ch ain for construction

materials and products. The key products covered by CARES are

reinforcing steels, prestressing steels and associated products, for

their manufacture, processing and stocking and distribution

including installation against defined product standards and

design codes. The CARES certification schemes ensure that

products supplied to the construction industry consistently satisfy

the customer's requirements.

Importance of using an app rove d installer.

The installation of post-tensioning systems is a highly specialised

operation requiring a CARES approved post-tensioning

contractor. CARES app roved post-tensioning contractors are

regularly audited at he ad office a nd on site to ensure installation

procedures are adeq uate, trained post-tensioning personnel are

used and that a qualified post-tensioning kit is being installed.

Failure to use a CARES approved post-tensioning contractor could

result dangers to site personnel or compromised structural

performance because of:

a) Incorrectly installed post-tensioning system; under stressed, over

stressed or inaccurate profiles or alignment.

b) Failure of post-tensioning kit components.

c) Use of an unqu alified post tensioning kit of unknown

jerformance or incompatible components.

i) Incorrectly grouted or ungrouted tendons.

Compliance with product standards and codes.

Ml products c overed by CARES schemes are tested against

eleygnt produc t standards O Lspecifications. Strand complies with-

JS5896 (or prEN 10138), high tensile bar complies with B S4486

or prEN10138), post-tensioning anchors comply with

SSEN13391 a nd post-tensioning kits com ply w ith ETAG 013.

Confidence in

Post-Tensioning

Construction

Impo rtance o f correct materials an d products.

Post-tensioning system components carry large forces (over

20tonnes per strand) and any weakness due to substandard or

incorrect components can be catastrophic.

Risks due to poor grout and incorrect grouting .

The grout or filling material protects the tendon from corrosion and

provides a bond with the surrounding concrete. Failure to fully grout

tendons with the correct materials can significantly reduce their

corrosion resistance and compromise the durability of the structure.

Traceability of people and p roducts.

The CARES scheme requires traceability of post-tensioning kit

components to a location within the structure and the use of

personnel whose training has been verified by CARES.

Monitoring of Scheme performance.

The CARES certification schemes require that records of complaints

relating to compliance of the product made against appro ved firms

are pro perly addressed and that details of these complaints are

returned to CARES at regular intervals. Further action may be taken

by CARES against the approved firm if required. CARES acts as a

point of reference when the performance of one of its approved firms

casts doubts on the effectiveness of the relevant certification scheme.

CARES list of app roved firms.

CARES regularly updates a list of approved firms which is

maintained on the CARES website (www.ukcares.com ). Each firm's

entry gives the scope of the CARES certification as well as other

key details of the firm. Oc casionally firms m ake spurious claims

regarding CARES appro val. This may relate to ap prova l of the firm

itself or of the products and services for which they might hold

appro val. If there are any doubts concerning the appro val status of

a firm, then the CARES list of ap proved Firms should be consulted or

alternatively the CARES office should be contacted for verification.

Why take the RISK?

For further information about CARES and an up-to-date list of manufacturers and suppliers

holding CARES ce rtification please consult the CARES website: www.ukcares.com

Alternatively, contact the office:-

UK Certification Authority for Reinforcing Steels,

Pembroke House, 21 Pembroke Road,

Sevenoaks, Kent T N I3 1XR

Te lephone-01732 450000

Fax-01732 455917

E-mail - [email protected]

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THE CONCRETE

BOOKSHOP

For technical publica tio

m ultim ed ia, British standards

and more... _ ^

^ ^ M

If we don't have it,

it

f

s not about concrete

visit The Concrete Bookshop now

Phone: +44(0)700 460 7777

Email: [email protected]

www.concretebookshop.com

The Concrete Bookshop is wholly owned by T he Concrete Society

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B a lv a c P o s t

T e n s i o n i n g

Balvac design, supply and install cost effective

post tensioning systems for:

•

BRIDGES

•

BUiLDiNGS

•

CAR PARKS

• SILOS • TANKS • NUCLEAR

•

RETROFIT STRENGTHENING

Balvac

is

CARES certified for the supply and installation

of

internal and external pos ttensioni ng systems, which have

been tested and approved

in

accordance w it h ETAG 013.

Our fully qualified and experienced supervisors and operatives

are directly employed and a ll hold relevant CSCS cards.

For preliminary budget information for your

next scheme contact us.

B a l v a c

Balvac Ltd

Sherwood House

Gadbrook Business Centre,

R u d h e a t h Northwich,

Cheshire, CW9 7TN

Tel:+44(0) 1606 333036

Fax:+ 44(0) 1606 812497

e-mail :[email protected]

or [email protected]

www.balvac.co.uk

A BaifCll " Beatty Company

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Jhe

•

I n f o r m a t i o n S e r v ic e s

Don't get left in

m f o s e r v i c e s @ c o n c r e t e o r g u

services, please contact Inform ation Services on

+44 (0 )1 276 607 140

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Gifford

Challenge and innovation are constant features at Gifford. One such

example is our detailed design for a 150,000m3 LNG tank in Portugal

where, because of historic activity, the client doubled the seismic risk

threshold to 1 in 10,000; i.e. the tank had to withstand the wo rst earth-

quake likely to be experienced in 10,000 years. We used non-linear finite

element analysis to design an outer containment pre-stressed concrete

tank capable of containing a major spill and operating basis earthquake.

The tank is amon g the first in the world to be designed to the new

applicable Eurocodes.

From pre-stressed to post-tensioning; we're proud of our concrete

heritage, our long association with the Co ncrete Society and fib, and

the legacy of our w ork - reflected not just in the structures w e have

engineered worldw ide but in the improved industry standards we have

helped to create an d deliver.

Rhinefield Bridge

First post-tensioned concrete bridge

design in Britain by EWH Gifford in 1949 .

Deeside Road Link, River Crossing

The first major use of plastic ducts in

the UK.

Finback Bridge, Manchester

Post-tensioned internal and external

tendons.

Capital Gate, Abu Dhabi

Vertical post-tensioned concrete core.

Opening Autumn 2010.

gifford.uk.com

Concrete

Advisory

Service

Help and advice w hen

you need it

For details of this and other membership benefits, please contact our membership

depa rtment on +44 (0) 1276 607146 or visit www.concrete.org.uk/membership

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i a revision of the second edition of Techn ical

Report 47, Durable post-tenshned concrete bridges which

was publ ished by The Concrete Society in 20 02 . The

recomm endations in the second edit ion have been reviewed

and extended, and where new internat ional and European

standards now exist, now make reference to them. This has

enabled some simplification in the text. The most significant

extension to this Repo rt is to include recom me ndations

for pos t-tensioning in buildings as w el l as in bridges, wh ere

significant experience has been gained in recent years.

The measures described are aimed at imp roving design, detailing,

specifications, materials, construction methods and testing for grouted

ensioned concrete with either internal or external tendons.

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Documents

Durable Post-tensioned Concrete Structures