Tendon Grouting- VSL

7/27/2019 Tendon Grouting- VSL

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Page 1Copyright 2002 by VSL International Ltd., Subingen/Switzerland - All rights reserved - Printed in May 2002

GROUTING OF

POST - TENSIONING TENDONS

5VSL ReportSeries

Introduction

The VSL Grouting Package

Cementitious Grout

Grouting on Site

Inspection and Monitoring of Tendons

Repair of Tendons with Defective Grouting

Conclusions

References

Appendices

PUBLISHED BYVSL INTERNATIONAL LTD.LYSSACH / SWITZERLAND


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Page 2Copyright 2002 by VSL International Ltd., Subingen/Switzerland - All rights reserved - Printed in January 2002

PREFACE

The concept of prestressed concrete has been around for one hundred years, althoughhandicapped in the early days by a lack of suitable high strength materials. Its real development

began some sixty years ago and has progressed since then in terms of technology, systems,achievable spans and engineering ingenuity has been remarkable. Without question, it is aneconomic and technically efficient system, in countering the weakness of concrete in tension byexplicitly introducing a precompression to resist imposed loads.

Having said that, it had not been without its traumas in the last two decades. Mostly, these wereconcerned with durability, mainly due to corrosion caused by chlorides emanating from sourcessuch as de-icing salts and seawater. While the vast majority of structures have behavedsatisfactorily, sufficient examples of deterioration were found to cause concern and to questionthe quality of grout and grouting especially.

In the UK in particular, a ban was introduced in 1992 by the Highways Agency on groutedpost-tensioned concrete bridges, until satisfactory new standards and practices were introduced.

This took four years, culminating in reference [1] to the present Report (with a second edition dueto be published shortly reference [17]. Performance requirements for grouts were set, newgrouts were developed and extensive field trials undertaken. The sensitivity of grout and groutingto variability in practice was fully recognized and dealt with. However, it was also found necessaryto consider all aspects of design, detailing, materials and workmanship in a coherentcomprehensive way.

Parallel activity has occurred in other countries, and internationally, work has been coordinatedunder the auspices of FIP (now fib). It is reasonable to claim that, in the last decade, the wholeprocess of prestressing and grouting has been the subject of a rigorous review leading to newtechnology, and a re-statement of good practice and how to achieve it.

In writing the Preface to The Concrete Society TR47 (Reference [1], I gratefully achnowledge the

co-operation of all sections of the industry. This included the prestressing companies, all of whomoperate internationally. Since then, they have been adapting the new general standards andpractices, to suit their individual systems all supported by extensive research and development.This particular Report is a classic example of that. I am especially attracted to the emphasis put onthe following:

The need for a holistic approach, embracing design, detailing, materials and constructionpractice;

Recognition that grouting is a skilled and sensitive operation, requiring specialistexperience and expertise, to carry it out properly;

The questioning attitude to past test methods for grouts and grouting, while putting forwardproposals, which give a better measure of key characteristics and properties;

The obvious desire to adapt and upgrade VSL technology, to give a much better balance

between load capacity and durability performance, than in the past.

Professor George SomervilleConsultantConvener, UK Working Party on Durable Post-tensioned Bridges(June 1995 November 1999).


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Contents

Page1. Introduct ion 4

1.1 Durability of Post-Tensioned Structures 41.2 Past Experience with Post-Tensioning Tendons 41.3 Bonded versus Unbonded Tendons 61.4 Plastic Ducts for Bonded Post-Tensioning Tendons 81.5 Intent of the Report 9

2. The VSL Grout ing Package 9

2.1 General Systems and Services 92.2 The VSL Grouting Package 10

3. Cement i t ious Grout 10

3.1 Common Grout Specifications and Recent Trends 103.2 Grout Constituents 123.3 Grout Characteristics 14

3.4 Recommended Grout Performance Specification and Testing 203.5 Stages of Grout Testing 20

4. Grout ing on Site 21

4.1 General 214.2 Training and Qualification of Personnel 224.3 Grouting Equipment 234.4 PT System Detailing for Grouting 234.5 Grouting Procedures on Site 27

5. Inspect ion and Monitor ing of Tendons 31

5.1 Inspection Methods 315.2 The Engineer's Approach to Tendon Inspection 33

5.3 Monitoring - New Developments 35

6. Repair of Tendons with Defect ive Grout ing 35

6.1 General 356.2 Preparation 356.3 Access to the Tendon 366.4 Grouting of Voids 366.5 Closing of the Tendon 376.6 Repair of External Tendons 38

7. Conclus ions 39

References 41

Appendices 43

- Appendix A: Specific Recent Grout Test Procedures 43

Authors :

Hans Rudolf Ganz, Dr. sc. techn., Civil Engineer ETHStephanie Vildaer, Materials Engineer


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47.0%

No voids

23.0%

Small voids

9.0%

Mediumvoids

9.0%

Large voids

12.0%

Ungrouted

tendon

48.0%No corrosion

1.6

Severe

corrosion

0.7

Heavy

corrosion

7.7%

Moderate

corrosion

42.0%

Minor

corrosion

1. Introdu ct ion

1.1 Durabil i t y of pos t-

tensioned structures

For a long time concretestructures, and in particularprestressed concrete struc-tures, have been consideredinherently durable with little tono need for maintenance.More recently it has been rec-ognized that this is not true,and that concrete structurescan suffer durability problemsunder certain conditions. Inmany cases, such problems

were accompanied with corro-sion of the non-prestressedand prestressed reinforce-ment in the structure. How-ever, the corrosion of the rein-forcement is usually not theroot cause of the durabilityproblem but rather a conse-quence of inadequate consid-eration for durability in theoverall design of the structure.

It has been recognized that a

design for durability relying ona single layer of protectioncannot guarantee reliable

overall protection of the rein-forcement. Therefore, the con-cept of multi-layer protectionhas been created, [1]. In thisconcept the first and perhapsmost important layer of protec-tion is the overall concept anddesign of the structure. A keyelement in this design is tokeep water off the structure andthe reinforcement, and/or to as-sure that it drains quickly fromthe structure. A second layer ofprotection can be provided withwater-proofing membranes inparticular on critical surfacesexposed to water and other ag-

gressive media such as de-icing salts. A third layer of pro-tection in concrete structures isprovided with dense concretedesigned specifically for lowpermeability. A fourth layer ofprotection for the tendons ofpost-tensioned structures hasbeen introduced in the early1990s, and consists of a leaktight encapsulation of the ten-dons with robust, corrosion re-sistant plastic. The last layer of

protection of post-tensionedstructures is provided directlyonto the prestressing steel in

the form of cementitious grout,or by other types of protectionsystems applied in the factorysuch as grease and plasticsheathing for monostrands.

Grouting, as reviewed in detailin this report, is the last, and isonly one, of the layers of pro-tection of tendons in post-tensioned structures. Whilehigh quality grouting is impor-tant for the durability of ten-dons, it alone cannot guaranteethe durability of tendons. It isthe owners and the engineersobligation to select and specify

a suitable combination of inde-pendent layers of protectionadapted to the particular envi-ronment in which the structureis built. Additional layers of pro-tection provided during con-struction have a relatively in-significant cost compared withrepair of durability problems ofa structure in operation.

1.2 Past experience with

post- tensioning tendon s

While the idea of prestressedconcrete is much older, the real

Fig. 1: Results of post-tensioning tendon inspection of 447 bridges in the UK, [4].

a Size of voids in tendons b Tendon corrosion


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19%

30%

19%

30%

2%

0%

0%

No Information 19%

No Corrosion 30%

Hardly Visible Corrosion 19%

Superficial Corrosion(Removable) 30%

Moderate Corrosion (Local) 2%

Severe Corrosion 0%

Pitting Corrosion 0%

zet

b) Tendon corrosion in incompletely filled tendon ducts

Fig. 2: Results of post-tendoning tendon inspection of 10 bridges in Vienna, Austria, [5].

76%

3%

6%

4%

2%

3%

6%

Completely Grouted

Incomplete at Transition Point

Incomplete at Low Point

Incomplete at High Point

Incomplete at End Anchorage

Incomplete at Coupler

Incomplete at Other Points

76%

3%

6%

4%

2%

3%

6%

a) Type of voids in tendon

use of the technology startedin the second half of the1940's with projects by E.Freyssinet, F. Dischinger, G.Magnel, U. Finsterwalder, F.Leonhardt and W. Baur, and

many others, [2]. Hence, onecould say that prestressedconcrete has existed for about50 years. Most of the projectsbuilt in prestressed concretein accordance with the rulesfor good design, detailing, andpractice of execution havedemonstrated the excellentdurability of prestressed con-crete in general, and of post-tensioning tendons in particu-lar. In [3] e.g. it is stated that

"It must be emphasized thatinstances of serious corrosionin prestressed concrete struc-tures are rare when one con-siders the volume ofprestressing material (strand,wires and bars) that have

been consumed worldwide overthe years".

The technology of prestressedconcrete did receive extremelynegative press with the tempo-

rary ban of prestressed con-crete bridges using post-tensioning tendons introducedin 1992 by the HighwaysAgency in the UK. The tempo-rary ban was only lifted fouryears later after a detailed re-view of all aspects of bridgedesign and detailing, of thespecifications for materials andgrouting works, and of thequalification of personnel andcompanies. As a consequence

of this action in the UK, a seriesof systematic investigations intothe durability of prestressedconcrete and post-tensioningtendons were initiated in theUK, France, Switzerland, Aus-tria, and elsewhere, [4,5,6].

While all these investigationsconfirmed that the large major-ity of prestressed structuresand post-tensioning tendonsshow excellent durability withinsignificant corrosion defects

only, if any, they all found someinstances with durability prob-lems and post-tensioning ten-don corrosion.

In [4], e.g., a summary of find-ings of a total of 447 stateowned post-tensioned bridgesinspected in the UK is pre-sented. The following resultswere found: 47% of the post-tensioning tendon ducts werecompletely grouted, i.e. had no

voids; 23% of the ducts hadsmall voids; 18% had mediumto large voids; and 12% werenot grouted at all, see Fig. 1. Inthese 447 bridges, 10% of thepost-tensioning tendonsshowed moderate to severecorrosion.

In [5] information on an investi-gation of 10 bridges built in Vi-enna between 1956 and 1978is provided. A total of more than

10,000 duct locations wereopened locally, and the statusof tendon grouting and tendoncorrosion was recorded. Theresults of this investigation con-firmed that the actual perform-ance and durability of the post-tensioning tendons is excellent,and document the good qualitywith which these projects werebuilt. 76% of all the openedduct locations were completelyfilled. The 24% of duct locationswhich were not completely filledwere essentially found in oneproject with stressbar tendonsfor which a undersized duct di-ameter had been used. Ofthese 24% duct locations withgrouting defects, only 2%showed moderate/local corro-sion, i.e. 48 out of 10,000 loca-


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tions. The remaining locationsshowed either no corrosion,minor or superficial corrosionwhich could be removed bycleaning with a soft cloth, seeFig. 2.

In [6] investigations on 143projects, including 107bridges, 23 ground anchorprojects, and 13 others, arepresented. The majority of thestructures documented gooddurability of the post-tensioning tendons or groundanchor tendons. Of the 14bridge projects with groutingdefects the majority of corro-sion problems in the tendons

was caused by ingress of wa-ter containing chlorides. A du-rable and leak tight encapsu-lation of the tendons, e.g. withrobust plastic ducts, was con-sidered essential to improvethe protection and to assurethe durability of grouted post-tensioning tendons.

More recently, durability prob-lems due to incomplete grout-ing and corrosion have been

reported in the USA. On theMid-Bay project in Florida,one completely and one partlyfailed external tendon werefound during a detailed in-spection, [7]. During inspec-tion many end anchorages ofthe external tendons locatedat the high point of the tendonprofile were found incom-pletely filled with grout.

While each of the above re-ports contains some very spe-cific information, the results ofall investigations show somecommon trends and conclu-sions. These may be summa-rized as follows:

(1) Review the detailing ofpost-tensioned structures

and of the post-tensioningtendons: It is, e.g. not surpris-ing that tendons, anchored at alocation where water from thebridge deck drains over the an-chorage, and where no sealing

of the anchorage is provided,may develop corrosion of theprestressing steel at some time.It is not surprising either thattendons crossing porous mortarjoints without encapsulation in adurable sheath may experiencecorrosion. Relatively small im-provements in the detailing ofpost-tensioned structures andthe post-tensioning tendons willsignificantly enhance the dura-bility of these structures and

tendons, often at only a mar-ginal cost, if any.

(2) Review the specifica-tions for cement grout forpost-tensioning tendons: Ithas been shown that the speci-fications used today are notstringent enough in terms ofacceptance criteria or are usingtest methods which are not ableto detect poor performance of aparticular grout mix. This com-

ment applies in particular to therequirements and tests typicallyused for the bleed of grout.Grouts with excessive bleed orsegregation will almost inevita-bly produce locations inside atendon, such as at high pointsof the profile, which are left par-tially grouted. If such locationsare dry with possibly a film ofalkaline grout on the tendonthere will be no corrosion of theprestressing steel. However, ifwater is available and/or findsaccess there is a risk of corro-sion. Introducing tighter specifi-cations for the quality of groutshas a cost since often excesswater will need to be replacedwith cement and specific admix-tures. However, this extra costis marginal for the project.

(3) Rely on post-tensioning specialist contrac-tors with well-trained and ex-perienced personnel for theexecution of the post-tensioning works and grout-

ing: All grouts whether pre-pared on site from cement andadmixtures or ready-mixedgrouts are eventually mixedwith water on site before injec-tion. Utilizing personnel whounderstand the importance ofthis activity and who have suffi-cient experience with groutingto realize if there is a problem,and react, is an absolute ne-cessity to assure good qualitygrouting. It is important that

owners and their representa-tives only accept specialistcompanies with well-trainedand experienced personnel forpost-tensioning activities.Leonhardt already said: "Theresponsibility involved in thedesign and construction ofprestressed concrete requiresthat only engineers and con-tractors may carry out this spe-cialized work who have col-lected sufficient knowledge and

experience and who can assurean accurate and careful execu-tion", [2].

1.3 Bon ded versus unbon ded

tendons

With the above referencedproblems found in grouted ten-dons, the discussion on thebest option of corrosion protec-tion of tendons has beenlaunched again. This questionis not new and different timesand people have chosen theirpreference in the early years ofpost-tensioned concrete. Forexample, unbonded tendonshave been preferred byDischinger in early post-tensioned structures, [8]. How-


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ever, under the influence ofFreyssinet and other promi-nent engineers, the advan-tages of structures withbonded tendons were empha-sized and this type of tendon

became the common practice.External unbonded tendonswere banned in the UK in the1970's after some problemshave been found. Externaltendons have later beenstrongly promoted by JeanMuller and other French engi-neers in conjunction with pre-cast segmental bridge con-struction in France and in par-ticular in Florida. Under theauspices of SETRA (State de-

sign office of highway author-ity) many bridges have beenbuilt in France using either ex-ternal tendons only or a com-bination of internal bonded,and external unbonded ten-dons. While this constructionpractice was not acceptedpreviously, the German high-way administration recentlydeclared unbonded tendonsas the preferred type of bridgetendons, [9].

The above clearly documentsthat there is not one superiortype of tendon. A particularpreference of one type ratherseems to be the consequenceof a personal choice of engi-neers or of a particular periodof time. As history has shown,these preferences change. Itmay therefore, be appropriateto repeat the strengths andweaknesses of bonded andunbonded tendons again forreference. In our opinion,there is no one type of tendonwhich answers to all require-ments, and it is up to the en-gineer to select the type oftendon best suited to a par-ticular project and construc-tion method. A systematic en-

forced switch from one practiceto another is neither justified bypast experience nor warrantedin terms of risk.

Advantages of grouted bonded

tendons can be summarized asfollows:

Provision of active corro-sion protection: Theprestressing steel is activelyprotected, i.e. passivated,against corrosion throughthe alkaline environmentprovided by the cementi-tious grout. To initiate cor-rosion prestressing steelfirst needs to be depas-sivated.

Provision of bond of thetendon to the structure:Bond allows a significant in-crease of the prestressingforce in a cracked sectionafter decompression, andpermits the pre-stressingsteel to reach the yield oreven ultimate strength. Thishas significant effects onthe strength of the section,on the crack distribution in

the prestressed member,and on the energy dissipa-tion of the member, [8].Bond has also a very bene-ficial effect on the redun-dancy of a prestressedmember. A local defect inthe tendon remains local,i.e. the tendon force is notaffected over the entire ten-don length.

Cost effectiveness: Ce-mentitious grout is a verycost effective injection ma-terial for which long andgood experience exists. Thecompatibility of cementitiousgrouts with prestressingsteel is well proven over along period of time.

Advantages of unbonded ten-dons can be summarized asfollows:

Future adjustment ofprestressing force:Prestressing forces of un-

bonded tendons can theo-retically be adjusted at anytime during the design life ofa structure. However, allnecessary tendon details forlater stressing need initiallybe provided such as accessand clearance for jacks, andsufficient overlength ofprestressing steel to con-nect the jack to the strand.While re-stressing of ten-dons was a justified con-

cern when long term lossesdue to creep and shrinkageof concrete, and relaxationof prestressing steels, werenot yet well understood, thisis no longer the case today.The authors are not awareof any recent case wherere-stressing of a tendonwas necessary due to ex-cessive losses of tendonforce. We would like to givea word of caution because

re-tensioning of a tendon,initially stressed to 70-80%of its strength, at some timeduring the design life of thestructure is certainly not aneasy task. Hence, if an in-crease in prestressing forceis ever required, the bestoption seems to be to pro-vide additional tendons tothe structure. A number ofrecent standards such asAASHTO, [10], actually re-quire new structures to bedetailed for the addition offuture external tendons topotentially increase theprestressing force to ac-commodate potential in-crease of loads or excessloss of tendon force. Ac-cording to these standards,


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anchorages and deviationdetails need to be pro-vided to allow addition of afixed number of tendons,e.g. 2 per section, or of agiven percentage

(AASHTO: 10%) of the ini-tial prestressing force.This procedure keeps theinitial investment to aminimum, and greatly fa-cilitates the future additionof tendons to the struc-ture, if ever needed.

Facilitated inspection oftendon: Since unbondedtendons are placed exter-nally to the structure, ac-

cess to the tendon for in-spection is facilitated overa substantial portion of thetendon length. Such ac-cess is not usually avail-able near the anchoragesand/or at tendon deviationpoints where such ten-dons often are anchoredor deviated in massivediaphragms.

While access to the ten-

don is facilitated, inspec-tion of the prestressingsteel inside the tendon orbundle of prestressingstrands is not necessarilyprovided. Hence, specialinspection or monitoringdevices still need to beused to collect informationon the actual performanceand durability of the steel.

Replaceability of ten-

dons: Unbonded externaltendons may be replacedat any time during the de-sign life of a structure. Re-placement is preceded byeither de-tensioning of thetendon if the necessarytendon details have beeninitially provided, or by

gradual cutting of the ten-don according to specificprocedures adapted to theparticular site and tendontype. The actual removal ofthe tendon is then possible

if appropriate details havebeen provided initially atanchorages and deviationpoints. Installation of a newtendon can then follow. Theauthors are of the opinionthat tendon replacementshould only be considered ifthere is a significant risk ofunexpected tendon failurewith consequential damageor risk to persons. In allother cases, and in particu-

lar if the structure can ac-cept additional prestress,rather addition of new thanreplacement of existing ten-dons should be considered.Such favourable conditionsto avoid replacement existin particular for bonded ten-dons in structures with suf-ficient concrete dimensions.

1.4 Plast ic du cts for b onded

post- tensioning tendon s

Provision of a corrosion resis-tant and leak tight encapsula-tion of the tendon can assure avery effective protection of thetendon. This concept has beenused since many years for theprotection of prestressedground anchors. In the early1990s, VSL introduced the cor-rugated plastic duct system,PT-PLUS, for bonded post-tensioning (PT) tendons whichtogether with suitable accesso-ries such as connection detailsand anchorage caps provides acomplete leak tight encapsula-tion of the post-tensioning ten-dons.

The UK has made the encapsu-lation of tendons in plastic

ducts compulsory in 1996, [1].As a further step forward, theconcept of verifying the leaktightness of the system hasbeen introduced at the time.This verification is done by air

pressure testing of the assem-bled duct and anchorage sys-tem. Pouring of concrete is onlyapproved when the duct systemis confirmed to be sufficientlyair tight.

If the encapsulation of tendonsin plastic is supplemented withspecific details at the anchor-ages, a Electrically IsolatedTendon (EIT) can be provided.In addition to the above men-

tioned advantages, an EIT al-lows monitoring of the providedencapsulation at any time dur-ing the design life of the ten-don. A simple measurement ofthe electrical resistance be-tween the tendon and the struc-ture can be used to confirm theintactness of the encapsulationof the tendon at any time. Itcan, in particular, be used toconfirm the proper installationand the compliance of the ten-

don with the project specifica-tions at the time of construction.Encapsulation of tendons inplastic duct systems combinedwith EIT measurement hasbeen introduced in Switzerlandin 1993. Since that time morethan 20 bridge structures havebeen built with this concept.The positive experience withthe concept has now led to theintroduction of new guidelinesfor the protection of tendons inSwitzerland [11]. While still ac-cepting some application of cor-rugated steel duct in benignenvironment, these guidelinesrequire encapsulation of ten-dons in plastic, in general. EITis specified for a percentage oftendons to verify theencapsulation, and in general,


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lation, and in general, forstructures exposed to straycurrents.

Complete encapsulation ofpost-tensioning tendons in

plastic ducts and EIT are ef-fective protection methods.When combined with highquality grouting, they are con-sidered a major step forwardto achieve reliable long-termdurability of post-tensioningtendons.

1.5 Intent of the repo rt

The intent of this report is toprovide a sound basis for

owners, engineers, and con-tractors to have total confi-dence in the technology ofgrouted, bonded post-tensioning. This is achievedby:

Providing information onservices available from theVSL Group, your specialistcontractor for post-tensioning and related en-gineering, on any aspect

related to grouting andpost-tensioning (Chapter2).

Providing information onrecent progress in the de-sign and testing of cemen-titious grout mixes and im-proving existing knowl-edge on the interactionbetween cement, water,and admixtures (Chapter3).

Providing information onstate-of-the-art groutingprocedures on site to as-sure complete filling ofpost-tensioning tendonswith grout over the entirelength of the tendon, seeFig. 3 (Chapter 4).

Providing information onavailable inspection andmonitoring techniques onexisting grouted post-tensioning tendons. Suchtechniques allow to either

confirm their good health orto detect defects to allowsubsequent repair (Chapter5).

Providing information onavailable repair methods forgrouted post-tensioningtendons which have beensuccessfully used (Chapter6).

This report is specifically written

for grouting of post-tensioningtendons either internal or exter-nal to the structure. The reportdoes not cover grouting ofground anchors or stay cables.

As recognized during recent in-vestigations of post-tensionedbridges, careful detailing of thestructure for tendon layout, an-chorage and coupler locations,etc. is essential for the durabil-ity of post-tensioning tendons

and the structure. However, thisaspect goes beyond the scopeof this report, and the interestedreader is referred to other pub-lications, such as [1,8].

2. The VSL Grou t ing

Package

2.1 General sys tems an d

services

The VSL Group provides acomprehensive range of ser-vices in connection with post-tensioned structures, including:

Assistance to owners, engi-neers and contractors withpreliminary and final designstudies of post-tensionedstructures.

Assistance to contractorswith the selection and de-tails of the constructionmethod of post-tensionedstructures.Detailed design of the post-tensioning system adaptedto a particular project.

Supply and installation in-cluding stressing and grout-ing of the post-tensioningsystem.Supply of post-tensioningmaterials, equipment, andsupervising personnel.Complete erection of post-tensioned bridge deckssuch as precast segmentalsuperstructures working asa subcontractor.Use of other VSL Systems

such as slipforming orclimbforming, rock and soilanchors, stay cables, heavylifting, bearings, expansionjoints, stressbar systems,the retained earth systemVSoL, PT-PLUSTM plasticduct system, [12, 13], etc.Design and execution of re-pair and strengtheningworks for concrete struc-tures.Design and execution ofspecialized foundationworks such as diaphragmwalls, barrets, caissons,piles, soil grouting, etc.Design, supply and execu-tion of members made ofthe ultra-high performanceconcrete, DUCTALTM.

Fig. 3: Properly grouted tendon sec-tion


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The actual extent of the VSLservices will usually be deter-mined in discussions with theowner, engineer, contractor,and the VSL Organisation. Inmany cases the combination

of several VSL Systems andServices is possible on a par-ticular project. This enablesthe use of labour and equip-ment to be rationalized withcorresponding cost savings.

The reader is encouraged tovisit the Internet sites of theVSL Intrafor Group athttp:\\www.vsl-intl.com andhttp:\\www.intrafor.com formore details on our systems

and services, or to consult ourbrochures, e.g. [14].

The VSL Group can offer awide range of specialized pub-lications on the above sys-tems and services. Pleasecontact your nearest VSL Or-ganisation for a copy.

2.2 The VSL gro ut ing

package

Grouting has been consideredby many as a simple activityof mixing cement and water,and pumping it into a duct. Inaddition to this perceived sim-plicity, it is an activity wherepeople's hands can get dirty.It has therefore, attractedmuch less interest and atten-tion than placing and stressingoperations, and has been of-ten considered something"everyone can do" anyway.With the past experience re-ported under Section 1.2, andwith the knowledge collectedin recent research presentedlater in this report, an increas-ing number of clients and en-gineers have started to realisehow complex the grouting of apost-tensioning tendon actu-

ally is. Already the individualgrout constituents, cement andadmixtures, are complex mate-rials. The interested reader isreferred to specialist literaturesuch as [15]. The interaction

between the individual con-stituents is even more complex.However, the properties of thegrout are also affected by theequipment used to mix andpump it. The properties of thegrout inside a post-tensioningtendon are further influenced bythe detailing of the tendon andvents, the ambient conditions,and the grouting proceduresutilized. Last but not least, allthese activities are carried out

by human beings with differentbackgrounds, education, andtraining.

In view of the above complexityand the many interfaces tomanage, only a global ap-proach considering all activitiesas one package will assure op-timum results. In the authorsopinion it is in the owner'sbest interest to consider thepost-tensioning and grouting

as one package, and subcon-tract the entire package toone Single Source post-tensioning specialist contrac-tor such as VSL. If assignedsuch a full package, VSL willuse VSL-HPI GroutTMwhich is ahigh performance cementitiousgrout offering performancecharacteristics which havebeen optimised with VSL pro-prietary procedures. Our tech-nical staff will provide post-tensioning system details whichare fully compatible with thegrout materials, equipment, andprocedures intended to be usedon site. This can be furthercomplemented with the use ofthe VSL PT-PLUSTM plasticduct system. VSL will then as-sure that the grout will be in-

jected by experienced, well-qualified and trained personnel,with VSL optimised groutingequipment. The grouting workswill be carried out in compli-ance with our standard proce-

dures adapted to the particularconditions of the site, and ap-plying state-of-the art testingand QC procedures as pre-sented later in this report.

The overall objective of VSLwith the above full package ap-proach is to enhance the dura-bility of post-tensioned struc-tures by improving the quality ofgrouting. Owners, engineers,and contractors relying on the

above approach will quickly re-alize the advantages provided.

3. Cement i t ious Grou t

3.1 Common g rout

speci f icat ions and

recent trends

Specifications for cementitiousgrouts changed little over along period of time up untilquite recently. The Fdration

Internationale de la Prcon-trainte (FIP) Guide to GoodPractice on "Grouting of ten-dons" can be considered as afairly representative documentfor grouting up to today, [16].Most national standards inEurope and Asia, and recom-mendations such as the onesissued by the Post-TensioningInstitute (PTI), used the sameor similar grout testing proce-dures, and either the same orsimilar acceptance criteria. TheFIP Guide to Good Practicealso has become the basis forthe European Standards, EN445, 446, 447 for Grouting,[17].


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a) Standard bleed test in 100 mm contain-ers

b) 1 m high tubes with and without strand

Optimised Grout

Common Grout

c) 5 m Inclined Tube with 12 strands

Fig. 4: Bleed and volume change behav-iour in different test specimens.

The main properties consid-ered relevant for the perform-ance of grouts in these docu-ments are:

The flowability of grout:

This was considered im-portant to ensure com-plete filling of the tendonduct.Volume change of grout:This was considered im-portant to be maintainedwithin a specified rangearound zero to completelyfill the tendon duct.Bleed of grout: It wasconsidered important tolimit free water inside the

tendon duct, and anybleed water to be reab-sorbed by the grout withina specified time.Strength of grout: Thiswas considered to providean indication of the groutquality with respect to itsbond and shear strength.Resistance of grout tofreezing: This was con-sidered important for ap-plications in cold climates.

Table 1 gives a summary ofthe specified properties in thedocuments of FIP, [16], andEuropean Standards (EN),[17]. The recent revision of thePTI Guide Specification forGrouting, [18], is also shownfor reference. It includes addi-tional tests for setting timeand permeability of grout.

Table 1 also summarizes the

test methods or specimensused to check the properties.These are flow cones with an

efflux opening of 10-12.7 mm;small scale plastic cylinders ofdiameter and height in the or-

der of 100 mm (FIP), or cylin-ders of 50 mm diameter and200 mm height (EN) for volumechange and bleed; and prisms,cubes, or cylinders in the orderof 50 to 100 mm for strength.

Recent investigations and ex-perience on sites have shownthat the specifications [16,17]are either not relevant, or thatthe specimens and test meth-ods are not representative of

the real behaviour of grout in-side a tendon duct. The firstcomment applies in particular tothe strength of grout. A well de-signed grout mix will typicallydevelop a strength much in ex-cess of the specified values.The second comment appliesto the bleed and volumechange of grout. It has beenrealised recently that the bleedbehaviour of grout inside aplastic container of the speci-

fied size is insignificant com-pared to the real bleed behav-iour inside an inclined duct withprestressing strands. Fig. 4shows the bleed and volumechange behaviour of two groutmixes in different test speci-mens. The grout called "Com-mon Grout" used a plasticizingand expansive admixture with awater/cement ratio of 0.38. Thegrout called "Optimised Grout"used another plasticizing, and a

stabilising admixture but with-out expansion with a water /cement ratio of 0.32.The Common and OptimisedGrout mixes had comparable

Property FIP Guide [16] EN 447 [17] PTI [18] Test method/specimen

Flowability / Fluidity

Volume change

Bleed

Strength at 7 days

at 28 days

1)

-2% to + 5%

2%

20 MPa

30 MPa

25 seconds

- 1% to + 5%

2%

27 MPa

30 MPa

11 to 30 seconds

0% to + 0,1 %

0%1)

21 MPa

35 MPa

1)Wick-Induced test at 3h

Flow cone ( 10 or 12.7 mm)

Plastic cylinder (100-200mm high)

Plastic cylinder (100-200 mm high)

Cube or cylinder (50-100 mm)

Table 1: Common specifications for grout

Note: 1) No limit is specified, but water/cement ratio is recommended to not exceed w/c 0.40 to 0.45


12/46


flow times. Four different testspecimens were used, i.e.plastic container of about 100mm height, plastic pipe of 80mm diameter and 1m highwithout strand, plastic pipe of

80 mm diameter and 1 m highwith one strand, and a 5 mlong tube of 80 mm diameterinclined at 30 degrees tohorizontal with 12 strands. Allfour specimens were filledwith the same batch of eachgrout mix.

The results for bleed and ex-pansion were quite differentfrom one test specimen to theother. The 100 mm container,

typically specified in stan-dards, showed only an insig-nificant difference in bleed be-tween the two grouts. Actu-ally, both grout mixes wouldhave satisfied the specifica-tion of Table 1. The 1m highpipe without strand showedboth grout mixes with nobleed. However, the commongrout showed significant ex-pansion due to the expansiveadmixture. This was signifi-

cantly different in the 1m pipewith strand. The CommonGrout now showed significantbleed but no more expansion.The Optimised Grout stillshowed no bleed. Finally, theInclined Tube test confirmedthe poor performance of theCommon Grout with about800 mm bleed water and noapparent grout expansion.The Optimised Grout stillshowed an insignificantamount of bleed in the orderof 5 mm bleed water on thetop of the pipe. It shall bementioned again that all fourspecimens were filled with thesame batch of each grout mix,i.e. there was no variation ofgrout properties between dif-ferent specimens.

The above results were ob-tained in a series of tests doneby VSL. The same phenome-non has been recognized andconfirmed by others. It was in

particular France who devel-oped the Inclined Tube test af-ter a series of grouting prob-lems with excessive grout seg-regation and bleed had beendetected on sites. The InclinedTube test was the only testmethod which was able to real-istically reproduce the phenom-ena found on site. This led theFrench administration to specifythe Inclined Tube test as basisfor the approval of a particular

grout mix before its use on site,[19]. Later on, in order to re-duce the expenses for testingon site, the UK introduced theidea of the 1.5m pipe with anumber of strands such as to fillabout 30% of the pipe sectionas standard bleed test, [20]. AEuropean working group on theapproval of post-tensioning sys-tems introduced the 1m pipewith one single strand underthe name of "Wick-Induced"

Bleed test, [21]. A similar testhas been introduced by PTI,[18].

The above evidence has con-firmed that the grout test meth-ods and acceptance criteriawhich have been used, and arestill being used in most placesaround the world, are unfortu-nately not representative of thereal performance of grout in atendon duct. They are not ableto correctly differentiate be-tween a poor and a good qual-ity grout. These test methodsneed to be replaced quickly bytest procedures which are con-firmed to be representative,with more stringent acceptancecriteria. Only such representa-tive test methods with stringent

acceptance criteria will consis-tently assure that exclusivelygood quality grout mixes areused for the injection of post-tensioning tendons. The In-clined Tube test has been con-

firmed to be the most represen-tative test method. This andother new test procedures areincluded in Appendix A to thisreport.

3.2 Grout co nst i tuents

3.2.1 General

Grout is composed of cement,water and admixtures. Theseconstituents have a complex in-

teractions. This applies, in par-ticular, to the admixtures andcertain reactive components ofthe cement such as tri-calciumaluminate (C3A). Also the parti-cle size of the cement has asignificant effect on the interac-tion between the grout con-stituents. Unfortunately, many ifnot all of these cement andadmixture characteristics orparticles are not part of the ma-terial specifications of national

or international standards.Hence, specifying a cement forgrouting of tendons accordingto a national or internationalstandard is not sufficient to as-sure consistent grout proper-ties. Rather the entire spectrumof chemical and physical prop-erties of the cement must beknown, and must be maintainedwithin acceptable tolerance, incombination with particular ad-mixtures, to assure consistentproperties and quality of a par-ticular grout mix.

It is beyond the scope of thisreport to review all the parame-ters of grout constituents whichaffect the grout properties.However, the following sectionswill briefly review some aspects


13/46


of each constituent consid-ered essential for good groutperformance.

3.2.2 Cement

(1) Type of cement:Port-land cement is recommendedfor grouting of tendons. Othertypes of cement may be con-sidered for grouting of ten-dons but only after detailedtesting for their suitability inparticular in terms of long termcorrosion protection (e.g. forslag cements) or in terms ofdevelopment of hydrogen gas(e.g. silica fume cements).

(2) Specific surface ofcement: The Blaine value isan indirect measure of thespecific surface of cement.Blaine values of cement varywidely across the world (rangeof 250-450 m2/kg) and evenvary between batches fromthe same supplier. Cementswith low Blaine values (i.e. lowspecific surface) tend to easilyflocculate, i.e. form lumps.This will create non-

homogeneous grouts whichhave a tendency to easily seg-regate. Cements with highBlaine values often requirelarger quantities of water andadmixtures to wet their sur-face, and to provide a certainviscosity or flow time of grout.In addition, cements with highBlaine value have an earlierbeginning of setting or stiffen-ing. The above leads to aminimum specified Blainevalue of 300m2/kg to avoideasily flocculating grouts. HighBlaine values do not causeperformance problems. How-ever, a reasonable upper endof Blaine values for cementused in grouts is in the orderof 380m2/kg, mostly for eco-nomical reasons. As men-

tioned above, this value shouldbe maintained within a rea-sonably small range of toler-ance to assure consistentproperties of a particular groutmix.

(3) Chloride content ofcement:The cement shall onlycontain insignificant traces ofchlorides to avoid corrosion ofthe tendon. A typical limit is0.05% of chlorides by weight ofcement.

(4) Tri-Calcium Aluminate(C3A) content of cement:C3Ais strongly reactive with admix-tures. Its content in cement

may vary widely around theworld (range of about 2 to 12%of clinker). A relatively low C3Acontent is desirable but may notbe easy to obtain. Cementswith medium to high C3A con-tent are more delicate in theirinteraction with admixtures, andnecessitate a detailed testingseries to assure compatibility ofthe cement with a particularadmixture.

(5) Age of cement: Ce-ment carbonises with age andwith this reduces its reactivitywith admixtures and water. Onthe other hand, freshly pro-duced cement may still be in-adequately cooled. Hence, theage of cement to be used forgrouting of tendons must becontrolled, and kept within areasonably small range of a fewweeks. Alternatively, the ce-ment may be sealed in air tightcontainers for longer storage.

(6) False set and flash setof cement: These are twophenomena which are relatedto the calcium sulfate in thecement. If calcium sulfate isadded in the form of gypsum,this may, when mixed with wa-

ter, provide a structure insidethe cement having some rigid-ity, i.e. stiffening the grout. Thisis known as "false set". On theother hand, "flash set" results incements which have insufficient

sulfate present effectively tostop the hydration of tri-calciumaluminate (C3A) to the hydraterather than to ettringite. Flashset is accompanied with the re-lease of considerable amountsof heat. Cement which showseither of the two phenomena isnot suitable for grout, and mustbe avoided.

(7) Source of cement:With what was said in Section

3.2.1 it is clear that only ce-ments of one particular sourceor supplier may be used for aparticular grout mix for groutingof tendons.

The interested reader is re-ferred to specialised literaturefor more details on the charac-teristics of cements, [15].

3.2.3 Water

(1) Water quality: Watermust be free of impurities whichcould influence the setting ofthe grout and must not containsubstances which are harmfulto the prestressing steel. Ingeneral, it may be assumedthat drinking water satisfiesthese requirements. In case ofdoubt, or if no drinking water isavailable, the water should beanalyzed in a qualified labora-tory and contents of organicparticles, sulfates, sulfides,carbonates, and chloridesshould be limited to maximumvalues in the order of 100-500mg/l.


14/46


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.28 0.30 0.32 0.34 0.36 0.38 0.40 0.42

w/c

Bleeding(%)

Fig. 5: Effect of water / cement ratio on bleed at 3 hours

3.2.4 Admix tures

(1) Type of admixtures:Types of admixtures differ bythe nature of the moleculesused. They differ in their de-

gree of efficiency, i.e. thequantity needed to achieve acertain performance. Depend-ing on the nature of moleculesused they will interact differ-ently with different sources ofcement. Admixtures are avail-able in liquid form or as pow-der. Admixtures are availableto modify grout properties inmany ways including the fol-lowing: Plasticizing, stabilis-ing, retarding, accelerating,

thixotropic, expansive, etc.Admixtures with combined ef-fects are also available. Theuser is advised to ask the sup-plier of an admixture for adetailed certificate of theproduct, and to perform de-tailed suitability testing incombination with the particularcement intended to be usedfor a grout mix.

(2) Shelf life of admix-

ture:Properties of admixtureschange over time. Therefore,admixtures for which the shelflife recommended by the sup-plier has exceeded, should bediscarded.

(3) Dry extract of admix-ture: Many admixtures comein liquid form, i.e. are mixedwith water. Since it is the drycontent of the admixturewhich is relevant for the inter-action with the cement, itneeds to be declared and con-trolled within an acceptablerange to assure consistentproperties of a particular groutmix.

(4) Corrosiveness ofadmixtures:Admixtures shall

not contain products which areharmful to the prestressingsteel. This applies in particularto chlorides. But also calcium-nitrite has been reported tocause corrosion of prestressing

steel. The supplier therefore,should provide confirmationthat the particular admixturedoes not contain substancespotentially harmful to theprestressing steel, and/or thesuitability of the admixtureshould be confirmed by a quali-fied laboratory.

3.3 Grou t character is t ics

The following is a review of se-

lected grout characteristics andof the effect of certain parame-ters on them. This review pro-vides a better understanding ofthe behaviour of grouts, andassists in defining the relevantcharacteristics for grout specifi-cations, and acceptance crite-ria.

3.3.1 Bleed

Water is needed in grout for the

hydration of cement. However,in practice typically much morewater, than is needed for hydra-tion, is provided to achieve asufficiently low viscosity of groutfor injection. In such a situationof excess water, the cementparticles tend to flocculate

(form lumps), and to settle(sedimentation), with the lighterwater moving upwards, and col-lecting at the top of the grout.This sedimentation leads to anapparent reduction of grout vol-

ume. This movement of watermay wash out certain compo-nents of the cement and admix-tures, and thus may cause seg-regation of the grout.

Bleed and sedimentation ofgrout is probably one of themain reasons, if not the mostimportant, for grouting and du-rability problems with tendons.Excess bleed water will collectat high points of tendon profiles

and leave the prestressing steelin these areas without protec-tion from alkaline grout. Suchunprotected, exposed areashave been found in the investi-gations referenced in Chapter1, see [4,5,6]. In cases wherethe bleed water was reab-sorbed and ingress of addi-tional water and chlorides wasprevented by leak tight con-crete cover or encapsulation bythe sheath, no or only insignifi-

cant corrosion was found inthese exposed areas even afterlong time. However, in less fa-vourable cases, these locationsoften showed tendon corrosion.

The amount of bleed dependson different parameters of


15/46


0

200

400

600

800

1000

0.0 1.0 2.0 3.0

Density of grout (kg/m3)

Height(mm)

Mix 1 w/c 0.28

Mix 2 w/c 0.4

Grout column

Fig. 6: Effect of sedimentation on grout density

which the quantity of wateradded initially is the most im-portant. The quantity of wateradded to a given quantity ofcement, or the water/cementratio (w/c), must be kept as

small as possible to limit theexcess water. Actually, theamount of bleed is not propor-tional to the water/cement ra-tio. There seems to be athreshold at which bleed sud-denly becomes significant.Likely this threshold dependson the actual grout constitu-ents. Fig. 5 shows the effectof the water/cement ratio onthe amount of bleed for oneparticular grout mix used in

the VSL research. Thethreshold in this case is at a

water/cement ratio of w/c0.30 - 0.32.

High pressure to which thegrout is exposed will increasebleed. This is well known forlong vertical tendons for whichspecial grout mixes must bedesigned and special groutingprocedures be applied toavoid problems due to bleed.

For such applications, thebleed properties of grout shallbe verified at elevated pres-sure, see [18].

As demonstrated in the In-clined Tube test, the additionof prestressing steel, and inparticular strand, inside theduct significantly increasesthe amount of bleed water col-lected at the high point. Asmooth duct as typically used

for external tendons will fur-ther facilitate the movement ofbleed water to the high pointcompared to a corrugatedduct as typically used for in-ternal tendons.

For all of the above said,bleed of grout must be strictly

controlled and kept to an insig-nificant quantity under all cir-cumstances. The most effectivemeasure is first of all to reducethe amount of water added tothe cement as much as feasi-

ble. The desired low viscosity ofgrout for injection can be as-sured, even with low wa-ter/cement ratio, if suitableplasticizing admixtures areused.

3.3.2 Segregation and sedi-

mentat ion

The phenomena of segregationand sedimentation were intro-duced in Section 3.3.1. As

mentioned, they are a conse-quence of bleed and possiblyother characteristics of thegrout constituents which favourinstability of the mix. Both ef-fects produce grout which has ahigher density near low pointsof the tendon profile, and alower density at high points. As

documented in Inclined Tubetests, segregation in additionoften goes with a change ofcolour of grout, e.g. dark greyat locations with higher density,and lighter grey and / or whitishor yellowish colour at locationswith lower density. This changeof colour is a consequence of

the washing out of cement oradmixture particles by the bleedwater. This washing out mayfurther cause changes of thegrout properties such as a re-duction of the pH-value at the

high point.

Segregation and sedimentationcan easily be confirmed bymeasurement of the density ofgrout at different locations andby observation of the grout col-our. Fig. 6 shows the effect ofsedimentation due to excesswater for a particular grout mixin 1m grout pipes. The groutdensity of the mix with excesswater drops significantly to-

wards the top of the pipe, leav-ing a low density and porousgrout near the top.

Grout mixes with a tendency tosegregation or sedimentationwill be detected in an InclinedTube test.

3.3.3 Viscosi ty and f low t ime

Freshly prepared grout for post-tensioning tendons must beeasily pumpable for injectioninto the ducts, i.e. it must havea relatively low viscosity. Inpractice, the efflux time (flow


16/46


0

5

10

15

20

0 15 30 45 60 75 90 105 120

Time (min)

Flowtimeincone(

sec)

VSL HPITM

at 40C

Fig. 7: Grout mix optimised for stability of flow time at 40C

time) of a given quantity ofgrout from a cone has beenused as a measure for the vis-cosity. Therefore, the twoterms are considered equiva-lent for the purpose of this re-

port.

The viscosity of grout can bereduced by the addition of wa-ter. To achieve a viscosity ofbasic grout without plasticizingadmixtures which can easilybe pumped, water/cement ra-tios in the order of 0.4 - 0.45are required. This is signifi-cantly more water thanneeded for hydration of thecement, and will produce un-

stable grout mixes likely toshow excess bleed, sedimen-tation and possibly segrega-tion. To avoid these problemssuitable plasticizing admix-tures can be used which allowto reduce the water/cementratio down to the order of 0.30for low viscosity grouts suit-able for injection in tendons.Grout mixes with low wa-ter/cement ratio are inherentlymore stable and less likely to

show excess bleed, sedimen-tation and segregation.

The actual amount of waterneeded for a particular groutmix is often determined by tri-als such as to produce a de-sired viscosity or flow time ofthe grout. When using the flowcone according to the Euro-pean Standard EN 445, [17]flow times should be kept be-low 25 seconds for injection.In practice, values between 13and 18 seconds are often de-sirable. However, the absolutefigure of the flow time de-pends to some degree on theparticular application, equip-ment, and procedures used.

The flow time of a particulargrout mix should remain stableover a sufficiently long period oftime, at a given temperaturerange, to avoid problems duringinjection due to stiffening of the

grout. It is not sufficient for thiscase to give an upper limit ofthe flow time. Rather thechange of flow time over time isimportant. Grout mixes sub-jected to elevated temperaturesare more likely to show rapidchanges of flow time than groutmixes at low temperature. Withsuitable design and eventualuse of specific admixtures, theflow time of grout mixes can bemaintained stable over an ex-

tended duration of time even athigh temperatures. Fig. 7shows an example of the flowtime development over time fora grout mix optimised by VSLfor high temperature. Even at atemperature of 40C, the flowtime changed by less than twoseconds over a period of two

hours. This was achieved witha grout of a water/cement ratioof w/c = 0.28, and without cool-ing the constituents or grout.

In the past, it has been recom-mended at elevated tempera-tures to add some extra waterto the grout to compensate for

the expected more rapid stiffen-ing, see e.g. FIP Guide, [16].However, in view of the unde-sirable effects of excess wateron bleed, sedimentation, andsegregation described above,

this practice should be aban-doned. Instead, the grout mixshould be optimised for the ex-pected range of temperatureswith specific admixtures, and aminimum quantity of water.

3.3.4 Volum e chang e

Volume change of grout is pri-marily due to two effects, i.e.shrinkage of grout and sedi-mentation of grout. Unfortu-

nately, in practice the two ef-fects are often combined.

Sedimentation of grout hasbeen discussed in Section 3.3.1and 3.3.2. It is best controlledby a low water/cement ratio andthus, by controlling bleed. If notcontrolled, sedimentation of

grouts with excess water cancause volume changes in theorder of a few percent of the ini-tial volume, in the first fewhours after injection. This isshown in Figs. 8a and 8b forfour different grout mixes. Thefour mixes were made from thesame cement but differed in the


17/46


0

500

1000

1500

2000

2500

3 7 28

Age of grout (days)

Shrinkage(m/m)

Mix 1

Mix 2

Mix 3

Mix 4

Mix 5

Mix 6

c) Shrinkage values for different grout mixes optimised by VSL (standard test

specimens)

Fig. 8: Bleed, sedimentation, and shrinkage behaviour of different grout mixes.

0

0.5

1

1.5

2

2.5

33.5

4

4.5

0 3 6 9 12 15 18 21 24

Time (hours)

Sedimentation(%

)

Mix 1

Mix 2

Mix 3

Mix 4

b) Sedimentation characteristics of different grout mixes (1m grout column speci-mens)

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30Time (hours)

B

leed(%)

Mix 1

Mix 2

Mix 3

Mix 4

a) Bleed characteristics of different grout mixes (1m grout column specimens)

control of bleed by stabilisingadmixtures. It is evident thathigh bleed leads to high sedi-mentation in the order of sev-eral percent, see Mixes 1 and2. However, if bleed is con-

trolled, sedimentation is alsocontrolled and remains insig-nificant, see Mixes 3 and 4.

Shrinkage of grout, on theother hand, is a completelydifferent phenomenon whichdepends primarily on the typeof the cement and to somedegree on the amount of wa-ter. Fig. 8c shows selected re-sults of shrinkage measure-ments on six different grout

mixes over time up to 28days. At 28 days maximumshrinkage values were below

2000 m/m, and hence, aboutone order of magnitude lowerthan the effect of sedimenta-tion.

In view of the above, sedi-mentation needs to be strictlycontrolled since it can poten-tially cause voids in the orderof a few percent of the original

volume. This is best achievedby controlling the bleed of thegrout. On the other hand, vol-ume change of grout due toshrinkage is about one orderof magnitude lower, andhence, insignificant for thecreation of voids in the crosssectional dimension of ten-dons. Along the tendon axis,shrinkage is completely re-strained by the prestressingsteel in a similar manner tothe restraint of concreteshrinkage in highly reinforcedconcrete sections. Based onthe above, the use of expan-sive admixtures is not neces-sary. When considering inaddition the results of thebleed tests shown in Fig. 4,the use of expansiveadmixtures is not rec-

recommended at all. In fact, thetesting shown in Fig. 4 demon-strated that the effect of the ex-pansive admixture was can-celed by the presence of theprestressing steel. At best, ex-

pansive admixtures create aporous grout.


18/46


Fig. 9: Mud Balance equipment for den-

sity measurement of fluid grout.

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35 40 45

Time (hours)

Heatofh

ydration(mwatt/gof

cement)

20C

8C

End of setting

Grout is stiff

Start of setting

0

1

2

3

4

56

7

8

9

10

0 5 10 15 20 25 30 35 40 45

Time (hours)

Heatofhydration(m

watt/gof

cement)

40C

20C

End o f s e t t in g

Gr o ut is st i f f

S t a r t o f se t t i n g

b) Setting behaviour at 20C and 40C of grout Mix 2

Fig. 10: Setting behaviour of two grout mixes optimised by VSL

3.3.5 Corrosiveness and

toxic i ty

Grouts for post-tensioningtendons shall not cause cor-rosion of the prestressing

steel and shall not be toxic.This can be achieved if thegrout constituents areselected as discussed in Sec-tion 3.2.

3.3.6 Density

The density of grout is an ex-cellent indication of theamount of water used in agrout mix. It can be measuredeasily by comparing theweight and volume of a givenquantity of grout in either itsliquid or hardened state. Foran optimised grout mix with awater / cement ratio in the or-der of w/c = 0.3 the groutdensity is around 2,050 -2,100 kg/m3. For a grout mixwith w/c = 0.4 the density willbe around 1,900 kg/m3or be-low, because a part of thecement is replaced by excesswater. Measurement of den-sity is easily achieved on sitewith the Mud Balance which istypically used for geotechnical

grouting, see Fig. 9. There-fore, density measurement isrecommended as a control ofthe quality of the grout mix onsite, both at the mixer and atgrout vents.

3.3.7 Sett ing tim e

Stiffening and setting of groutshould not commence too earlydue to the risk of clogging dur-ing grouting. Start of setting of

grout must allow sufficient re-serve time to properly finishgrouting including such special

activities as re-grouting etc.The actual time necessary de-pends on many parameters in-cluding type of cement, size oftendon, and in particular theambient temperature. Start ofsetting may be significantly re-duced at high temperatures,and may be extended at low

temperature. Fig. 10 shows thesetting behaviour of two groutsusing two different admixturesbut the same cement, opti-mised by VSL, at 8 and 20,and at 20 and 40C, respec-

tively.

The setting time of a grout mix

can be adjusted within certainlimits to a desired value. Thiscan be achieved by selecting aparticular source of cement,e.g. choosing a low or highBlaine value, and by use ofsuitable accelerating or retard-ing admixtures. In any case, thesetting characteristics of a par-


19/46


Item Grout performance characteristics Test method Acceptance criteria

(1) Bleed and Segregation of groutInclined Tube test, Appendix A1, [21]andWick-Induced Bleed test, Appendix A2, [21]

Bleed water: 0.3%1)

Air void: 0.3%1)

Segregation: No significant segregation visibleto the naked eye

1)of original grout volume

(2) Flow Time of grout Flow Cone according to EN 445, [17]Initial Flow Time: 25 secondsChange of Flow

Time in 45 minutes: 3 seconds

(3) Sedimentation of grout Sedimentation test, Appendix A3, [21] Variation of density: 5 %

(4) Corrosiveness of groutChemical analysis of grout by qualified labora-tory

Chloride content: 0.1 %2)

2)of cement weight

Essential

(5) Toxicity of groutDeclaration of materials or chemical analysis by

qualified laboratoryGrout shall not contain toxic materials

(6) Strength of grout Test according to EN 445, [17] Compressive strength at 7 days: 30 MPa

(7) Volume Change of grout Test according to EN 445, [17]Volume change at 24 hours: -0.5% to +1%

1)

1)of original grout volume

Forrefer-

ence

(8) Setting Time of groutMeasurement of heat of hydration by qualifiedlaboratory

Start of setting: 3 hoursDeclaration of start, peak, end of setting

(9) Water / Cement Ratio of groutWeight measurement of constituents includingliquid in admixtures

Declaration of water/cement ratio of groutFor

re-

cord

(10) Density of grout Volume and weight measurement of grout Declaration of density of grout

3) (11) Frost Resistance of grout Testing by qualified laboratory Declaration of frost resistance of grout

Table 2: Proposed performance specification of groutNote: 3) Essential for cold climate only

0

20

40

60

80

100

120

7 28

Age of grout (days)

Strength(MPa)

Mix 1

Mix 2

Mix 3

Mix 4

Mix 5

Mix 6

I I

Fig. 11: Actual strength of VSL optimised grouts (40x40x160 mm prism halves)

ticular grout mix at the ex-pected temperature should beknown to the user, beforestarting grouting works onsite, and must be compatiblewith the anticipated grouting

procedures and schedule.

3.3.8 Strength

For bonded tendons, the groutmust attain a minimumstrength to assure sufficientbond between the prestress-ing steel and the structure.Most standards specify agrout strength in the order of25-35 MPa at 28 days, meas-

ured on cubes. Sometimes, aminimum strength is also re-quired to transfer compressiveforces across the tendonducts, such as in slabs nearcolumns, or for shear in websof girders.

For an optimised grout mixwith low water/cement ratio

and without expansive admix-tures, typically grout strengthsare achieved which far exceedthe above requirements. Fig. 11gives a summary of strengthsmeasured on prism halves(40x40mm) for VSL optimised

grouts. Compressive strengthsat 7 and 28 days were at least75 MPa and 95 MPa, respec-tively, and hence, far abovemost requirements typicallyspecified in standards.

3.3.9 Frost resis tance

For certain applications in cold

climates where freezing is aconcern, grout for post-tensioning tendons must pos-sess a sufficient frost resis-tance. This can be primarilyachieved with a dense groutwith low water / cement ratio,

with as little excess water aspossible.

Other methods which havebeen proposed to improve thefrost resistance of grout includethe entrainment of air in the or-der of 6 - 10% air pores, or thereplacement of about 10 % ofthe water in the mix with anti-


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Grout Test Methods Approval Testing Suitability TestingAcceptance / QC Testing

and Proposed Test Frequency

(1) Bleed / Segregation:- Inclined Tube- Wick Induced

xx

not requiredx

not requiredx (2 specimens / day)

(2) Flow Time- Initial- Change

xx

xx

x (1 specimen / 3h)not required

(3) Sedimentation x x not required

(4) Corrosiveness x not required not required

(5) Toxicity x not required not required

(6) Strength x x x (1 test with 2 specimens / day)

(7) Volume Change x x x (1 specimen / day)

(8) Setting Time x not required not required

(9) Water / Cement Ratio x x x (record for every mix)

(10) Density x x x (1 specimen / 3h)

(11) Frost Resistance for specific use only for specific use only not required

Table 3: Recommended testing regime and test frequency in different stages

freeze. Detailed testing ofgrouts modified with anti-freeze is recommended toavoid undesirable effects onthe grout performance. En-trainment of air will reduce the

strength of grout. The effec-tiveness of air entrainment isin question and must be veri-fied with representative sizegrout samples including theprestressing steel.

3.4 Recommended grout

per formance speci f icat ion

and test ing

Based on the discussion andresults presented in the previ-

ous section, only a small num-ber of grout performancecharacteristics are consideredessential. Many of the typi-cally specified characteristicsin the past are not consideredessential but may still be used

for the record and as reference.

Table 2 gives a listing of thegrout performance characteris-tics considered essential for ahigh quality grouting of post-

tensioning tendons. These in-clude items (1) to (5), plus (11),if relevant. Table 2 also in-cludes characteristics (Items (6)to (8)) which are considered oflesser importance and whichare typically satisfied by welloptimised grouts, as a matter ofcourse. Finally, characteristics(9) and (10) are listed for whichno requirements are stated butfor which the actual valuesshould be declared for the re-

cord and future reference.

Table 2 also includes proposedtesting methods, and the corre-sponding proposed acceptancecriteria. For the proposed testmethods reference is made to

the draft Guideline for Euro-pean Technical Approval ofPost-Tensioning Systems, [21],and to the European StandardEN 445, [17]. These referencesallow one to specify values for

acceptance criteria. Otherstandards exist with test meth-ods that can be consideredequivalent. When specifyingsuch alternative standards withdifferent test procedures, oneshould be aware that likely thevalues of acceptance criteriawill change also. Some newtest methods from [21] not yetcommonly known are pre-sented in Appendix A.

3.5 Stages of grout test ing

There are different stages ofgrout testing each one with aparticular objective. The follow-ing is a brief review of thesestages of testing.


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(1) Initial testing of grout:These tests typically serve toselect or determine a particu-lar grout mix design in alaboratory. In the past theinitial testing of grout was

often based on trial and errorprocedures with testing until asatisfactory solution wasfound.

VSL has introduced scientifi-cally based proprietary optimi-sation procedures to obtain aspecific grout mix design foroptimised bleed and segrega-tion properties. These optimi-sation procedures also assurethat all the grout constituents

have acceptable properties forgrout for post-tensioning ten-dons, and that they are com-patible between each other.Grout mixes which have beendesigned with this optimisa-tion procedure, and which sat-isfy all performance criteria,i.e. pass the approval testing,

obtain the VSL-HPI Groutlabel.

(2) Approval Testing of

Grout:These tests serve to confirminitially the compliance of aparticular grout mix with thegrout performance specifica-tions listed in Table 2. Typi-cally, Items (1) to (10) will becovered, see Table 3. Item(11) will only be verified forspecific applications in coldclimate. Grout produced fortesting should be preparedwith equipment comparable tothe one intended to be usedon site. Approval testing istypically done in a workshopor on site, under conditionscomparable to the site wherethe grout is intended to beused.

These tests are one part of theapproval procedure for the par-ticular grout mix. However, inaddition, adequate QA proce-dures must be implemented toassure the consistency of the

grout constituents for the par-ticular grout mix. If both partsare satisfied, the mix can beconsidered approved as groutfor post-tensioning tendons,and approval tests do not needto be repeated for future appli-cations.

(3) Suitability Testing ofGrout: These tests serve toconfirm the suitability and cer-tain performance characteristics

of an approved grout mix on aspecific site. This testing shouldbe done under representative,expected climatic conditions,with the grouting equipment in-tended to be used on site, andcarried out by the personnel in-tended to complete the groutingworks.

Table 3 lists the testing recom-mended to confirm the suitabil-ity of an approved grout mix for

use on site. Bleed and segrega-tion is checked with the Wick-Induced test only in comparisonto the corresponding results ob-tained during approval testing.

(4) Acceptance / QC Testingof Grout:These tests serve toconfirm the consistency of thegrout properties during execu-tion of the grouting works onsite. Table 3 lists the testingrecommended for QC on site. Itincludes also a proposed testfrequency for acceptance tests.

4. Grout ing on Site

4.1 General

Grouting work on site is a com-plex activity. It needs to be well

prepared. Once it has started itshould not be interrupted. Theassessment of the quality ofgrout during injection is stillbased to some degree onjudgement of an individual, e.g.

for the decision when the qual-ity of grout is acceptable toclose a particular vent. Most, ifnot all, activities during groutingare on the critical path, in par-ticular for grout mixes whichshow an early start of setting.The actual grouting works canbe physically quite demanding,are dirty, and involve safetyrisks, e.g. if the human skin oreyes get in direct contact withthe grout. For all the above

reasons, grouting work needsto be planned, and supervisedby experienced technicians,with a thorough understandingof the behaviour of grout, andawareness of the potential im-plications of poor grouting onthe durability of a post-tensioned structure. Hence,only such experienced techni-cians should be qualified toplan and supervise groutingworks. These technicians

should be capable of trainingthe labour used for grouting,usually on site, as needed forthe anticipated activities.

A satisfactory quality of grout-ing work can only be achievedif grouting equipment of a suit-able capacity adapted to theparticular project is used. Suchequipment should be confirmedprior to the actual grouting workduring suitability testing to beable to produce a sufficientlyhomogeneous grout mix.

A post-tensioning tendon canonly reliably and completely befilled if the entire tendon andduct system, including anchor-ages, hoses, etc. is leak tight.Hence, careful detailing of the


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0

2

4

6

8

10

12

0 2 4 6 8 10 12

Mixing time of grout (minutes)

Sedimenta

tionofgrout(%)

StandardVSL Mixer

LaboratoryMixer

acceptance criterion

Fig. 12: VSL Grout Mixers have been confirmed in a proprietary optimisation procedure.

tendon and duct system is es-sential. Improvised connec-tions between ducts and an-chorages, or improvised seal-ing of anchorages and vents,present risks which may lead

to grouting defects.

The leak tightness of the ten-don system may be confirmedby air pressure testing [1].

Excess water in the grout hasbeen confirmed as a majorcause of grouting and durabil-ity problems. Hence, control ofthe water added to the grout isessential. This includes watereventually present in the duct

system. Therefore, duct sys-tems need to be kept ade-quately sealed on site at alltimes to avoid ingress of rainor other water before grouting.

The quality and reliability ofthe filling of the tendon ductand anchorages depends onthe suitability of the chosengrouting procedure. There-fore, only grouting proceduresshould be used which have

been proven through sufficientexperience and / or represen-tative testing. Whenever pos-sible, standard proceduresshould be applied to reducethe risk.

Actually, only the combinationof all these above mentionedingredients will assure a highquality of grouting on site.This fact has been recognizedby some organisations andcountries which have pro-posed and / or actually intro-duced an approval procedurecombining the product (post-tensioning system, grout) withthe qualification of the special-ist contractor carrying out theworks, and the equipment andprocedures / method state-

ments used by the specialistcontractor. FIP has producedrecommendations on the"Qualification and approval ofprestressing contractors andsystem suppliers", [22]. France

has recently introduced an"Avis Technique" on the ap-proval of grout. It is a two stepprocedure where an approval isneeded for the product, i.e. thegrout, and a separate approvalfor the specialist contractor,demonstrating that he is quali-fied to produce the approvedgrout with his personnel,equipment and procedures tothe specified performance andquality, [23]. The UK has also

introduced comparable re-quirements for companies as abasis for the lifting of the tem-porary ban of grouted post-tensioning tendons, [1].

The different aspects of thequality of grout as a product upto its approval have been dis-cussed in Section 3. This sec-tion will review the other essen-tial ingredients for high qualitygrouting on site.

4.2 Train ing and qual i f icat ion

of personnel

Any type of grout, whether sup-plied in bags as ready-mixed /pre-bagged grout or mixed on

site, is finally mixed with waterand injected by on site people.A consistent good quality ofgrout is only achieved by ex-perienced, well-qualified per-sonnel who receive regulartraining to re-fresh and up-datetheir knowledge. Therefore, thequalification and training ofgrouting personnel is of primeimportance. This applies to alllevels from labour to supervi-sor/foreman, and technician.

In the above terminology, thegrouting technician and super-visor / foreman assume quitesimilar responsibilities. They

should both be able to plan andorganize grouting, to select andoperate grouting equipment,and to carry out grouting onsite. In addition, grouting tech-nicians should be able to selectgrout constituents, prepare adesign for a new grout mix, andconfirm it by testing. The su-pervisor / foreman should inparticular be able to train labour


23/46


on site for their anticipated ac-tivity.

In parallel with the develop-ment work on the optimisationof grout mixes, the VSL Group

has introduced a specifictraining programme in grout-ing activities for VSL person-nel. This training programmeaddresses technicians andsupervisors/foremen. It is in-tended to refresh and up-datetheir knowledge on grouting,confirm their qualification, andassist them in training their la-bour on site.

4.3 Grout ing equipm ent

Suitable grout mixers adaptedto the size of a particular pro-ject, and capable of producinga homogeneous grout mix areessential for achieving qualitygrouting. A grout mix which isnot homogeneously mixed willtend to more easily flocculateand consequentially, is likelyto show excessive bleed andpossibly sedimentation and /or segregation. To produce

homogenous grout mixes,project specifications oftenmake reference to "colloidalmixers". According to the dic-tionary, a "colloid" is a liquidsubstance made up of verysmall, insoluble, non-diffusibleparticles (as single largemolecules or masses ofsmaller molecules) that re-main in suspension in a sur-rounding liquid medium of dif-ferent matter. While thisdefinition is correct in terms ofobjectives for a high qualitygrout for post-tensioning ten-dons, it provides unfortunatelylittle guidance in terms ofspecifying how this perform-ance can be quantified andconfirmed. Based on today'sknowledge there is at leastone indirect test method which

test method which may be usedto qualify a grout mixer forgrouting of post-tensioning ten-dons, and to confirm its per-formance and capability to pro-duce a homogeneous grout.

This is the sedimentation testintroduced in Section 3, and de-tailed in Appendix A.3. A par-ticular grout mix can be pre-pared in the mixer to be as-sessed according to a givenprocedure and specific mixingtime. After mixing, the sedimen-tation test specimen is pre-pared with this mix, and sedi-mentation measured after com-plete setting of the grout. Basedon our present knowledge,

sedimentation in this testshould be limited to a maximumof 5% to assure a sufficientlyhomogeneous and stable grout.

Apart from the above perform-ance requirement, grout mixerssuitable for grouting of tendonsneed to satisfy other more prac-tical requirements. These in-clude:

Device or method to accu-

rately weigh the grout con-stituents which will be usedto prepare a specific mix. Inparticular, this addressesthe weight of cement andwater. A weighing toleranceof 2% is recommended.Mixing reservoir with a high-speed mechanical mixer.For grouting of large volumetendons, two mixing reser-voirs and mechanical mix-ers are required to assurecontinuous production andflow of grout into the ten-don, at the anticipatedspeed of grout flow.Standby reservoir with aslowly moving agitator tokeep the grout continuouslyin motion. Mixing reservoirsare emptied into the

standby reservoir to allowthe preparation of a nextmix. The grout leaves fromthe standby reservoir intothe tendon during injection.The grout should not be ac-

tually mixed but just kept inmotion since excessive mix-ing may be harmful to thehomogeneity of the grout.Pumps of sufficient capacityto inject the grout at the an-ticipated speed into a ten-don of a given size and ge-ometry.

The VSL Group owns a largenumber of grouting equipmentwhich satisfy the above re-

quirements. As part of the re-search and development, VSLhas also introduced a verifica-tion procedure for the perform-ance of the mixers in terms ofgrout homogeneity as a func-tion of mixing time. The resultsof the procedure allow to de-termine on optimum mixingtime for a particular grout mixer,see Fig. 12.

4.4 PT System detail ing for

grou t ing

4.4.1 General

Correct detailing of the tendonprofile, ducts, grout vents (inletand outlet), connections of ductto anchorages, and anchoragecaps are of decisive importancefor high quality grouting. Thetendon profile is typically cho-sen for structural reasons tobalance applied loads. How-ever, the tendon profile shouldalso be detailed for optimumflow of the grout. Local highpoints where no vents can beplaced should, e.g. be avoided.The tendon profile needs to besecured with sufficiently strongtendon supports at a sufficientlyclose spacing. Inadvertent


24/46


Fig. 13: VLS CS 2000 Anchorage withpermanent grout cap, grout inlet andcap vent

Fig. 14: VSL EC anchorage withtemporary grout cap, grout inlet andcap vent

movements of the duct duringconcreting must be preventedunder all circumstances.Ducts need to be made ofsuitable material such assheet metal, polyethylene or

polypropylene in accordancewith relevant standards orrecommendations. Any tem-porary hole in the pipe of ex-ternal tendons need to beproperly sealed before grout-ing to assure reliable corro-sion protection. The ductneeds to have a sufficientlylarge cross section to allowproper flow of the grout. Forstrand tendons typically theduct size is chosen such that

the cross sectional area of theprestressing steel does notoccupy more than about 40-45% of the duct cross section.For bar tendons, this percent-age can be higher. Significantreductions of cross sectionswhich will cause significantchange of speed of grout flowduring injection need to beavoided since this may causeexcessive bleed and segrega-tion of grout, or even block-

ages. All connections of ducts,vents, anchorages, and capsneed to be leak tight to assure

complete filling of ducts withgrout.

4.4.2 Sealing of anc ho rages

Grouting can only be carried

out once the anchor head andanchorage are properly sealed.

The most suitable method ofsealing anchor heads and an-chorages is with the use oftemporary or permanent groutcaps on the anchorages. Per-manent grout caps are now be-ing specified more frequently, inparticular for fully encapsulatedtendons using plastic duct sys-tems and for external tendons.

Fig. 13 shows the VSL CS2000 - PLUS System which of-fers full encapsulation of thetendon with the VSL PT-PLUSTM plastic duct system.The encapsulation is completedby the plastic trumpet throughthe CS anchorage, and thepermanent CS cap. All connec-tions are made with specialcoupling devices to ensure leaktightness.

For tendons which are notspecified as fully encapsulated,temporary grout caps are suit-able. These can be sealedagainst the anchor head if theinterface of anchor head andbearing plate is leak tight. Fig.14 shows such an example forthe VSL EC System.

For both the above cases withpermanent and temporary solu-tion, the cap may be removed

after grout setting to verify thecomplete filling of the anchor-age. Even immediately aftergrouting, tapping on the capmay be used to verify the com-plete filling. If necessary, groutpumping and venting can becontinued through the cap till itis completely filled.

In the past, inexpensive sealingmethods of anchor heads oranchorages were used whichare no longer recommended.These include sealing of theanchor head with quick-setting

mortar or by pouring the an-chorage recess with concretebefore grouting, see Fig. 15.Both these methods do not al-low a proper control of the qual-ity of grouting at the anchorage,i.e. control of complete filling,and should not be used anymore.

The above sealing methodshave been presented forbonded multistrand tendons.

However, they apply similarly toexternal tendons, and tosmaller slab tendons.

4.4.3 Detail ing of vents

The term vent is used here tocover both grout inlet and out-let. The diameter of grout ventsshould be sufficiently large toallow easy flow of grout. Typi-cally a minimum diameter in therange of 19-25 mm is recom-mended for multistrand ten-dons. They need to be flexible


25/46


NOT RECOMMENDED

a) Quick-setting mortar

NOT RECOMMENDED

b) Concrete pour back

Fig. 15: Past sealing methods of anchorages which are no longer recommended

to accommodate a particulargeometry imposed by the pro-ject, and must be able to sus-tain the maximum expectedgrouting pressure. In view ofthe risk of ingress of water

and chlorides the vents shouldhave a leak tight valve or cap.Alternately, the vents shouldbe located such tha

Documents

Tendon Grouting- VSL