Project Summary Report 0-1405-S – 1 –

The University of Texas at Austin

C e n t e r f o rTransportation Research

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CENTER FOR TRANSPORTATION RESEARCH

THE UNIVERSITY OF TEXAS AT AUSTIN

Project Summary Report 0-1405-SDurability Design of Post-Tensioned Bridge Substructure Elements

Authors: Ruben M. Salas, Andrea J. Schokker, Jeffrey S. West,John E. Breen, and Michael E. Kreger

Center for Transportation Research, The University of Texas at AustinSeptember 2004

Durability Design of Post-Tensioned BridgeSubstructure Elements: Summary

Corrosion protection forbonded internal tendons can bevery effective. Within theelements, internal tendons can bewell protected by the multilayerprotection system— including asound design taking away thesurface water, surface treatments,high quality concrete, plastic orgalvanized duct, sound cementgrout, coatings and other internalbarriers in the prestressing steel,and good anchorage protectionmeasures. However, potentialweak links exist when, amongother conditions, the followinghold true: (1) the concrete hashigh permeability; (2) the concretehas cracking; (3) the ducts are notadequately spliced or adequatelyprotected by impermeableconcrete; (4) the Portland cementgrout has voids, bleed water, andcracks; and/or (5) the prestressingsteel is not adequately protectedand handled during construction.

One of the major problemsthat agencies face today is thedifficulty of providing goodmonitoring and inspectiontechniques for bonded post-tensioned structures. Conditionsurveys are often limited to visualinspections for signs of cracking,spalling and rust staining, whichcan often overlook thedeterioration of prestressing steeland fail to detect the potential forvery severe and suddencollapses. Similar limitations exist

when using advanced techniquesto assess localized voids or activecorrosion, since in these casesthe analysis is limited to specificareas in selected bridge elements,overlooking grout voids or evencorrosion of prestressing steel inother member locations.

What We Did...After an extensive literature

review, the project was dividedinto the following maincomponents:

1. Literature Review and Surveyof Existing Bridge SubstructureInspection Reports(BRINSAP),

2. Investigation of CorrosionProtection for InternalPrestressing Tendons inPrecast Segmental Bridges,

3. Development of ImprovedGrouts for Post-Tensioning,

4. Long-Term Corrosion Testswith Large-Scale Post-Tensioned Beam and ColumnElements, and

5. Development ofRecommendations and DesignGuidelines for Durable BondedPost-Tensioned Bridges.

Component 1 includedliterature review of a substantialamount of relevant informationthat could be applied to thedurability of post-tensionedbridge substructures and

superstructures. This informationallowed the scope of theexperimental portion of the projectto be narrowed.

Component 2 was developedand implemented under TxDOTProject 0-1264 and transferred toTxDOT Project 0-1405 for long-term testing. A total of thirty-eight macrocell specimens wereused to investigate the corrosionprotection of internal tendons atsegmental joints. See Figure 1(a).Half of the specimens wereautopsied after four and a halfyears of highly aggressiveexposure. The second half of thespecimens were autopsied withover eight years of veryaggressive exposure. Thevariables included: joint type,duct type, grout type, and level ofjoint compression.

Component 3 consisted intesting numerous grouts in threephases to develop a highperformance grout for corrosionprotection. The testing phasesincluded fresh property tests,accelerated corrosion tests, and alarge-scale clear draped parabolicduct test that allowed observationof the grout under simulated fieldconditions. See Figures 1(b) and1(c). Two grout mixes wererecommended and have alreadybecome widely used in practice.

Component 4 includedtesting of twenty-seven large-scale beam specimens, which

Project Summary Report 0-1405-S – 2 –

were constructed in two phases. SeeFigure 1(d). Phase I beams (16specimens) were used to investigatethe effect of prestressing levels, crackwidths, and high performance grout.Phase II beams (11 specimens) wereused to investigate duct splices, grouttype, concrete type, strand type, ducttype, and end anchorage protection.Full autopsies were performed on sixPhase I beams (after four and a halfyears of exposure) and also on sixPhase II beams (after three and a half

years of exposure). Partial autopsieswere performed on two Phase I beams.Full autopsies for the remainingspecimens will be performed at afuture date under TxDOT Project0-4562. Component 4 also includedtesting of five non-prestressed andfive post-tensioned column specimensto investigate corrosion mechanismsand chloride ion transport (“wickingeffect”) in various column connectionconfigurations and to evaluatecorrosion protection measures. See

Figure 1(e). Variables included columnto foundation connection, loading,concrete type, prestressing barcoatings, and post-tensioning ducts.Full autopsies were performed at theend of testing, after six and a halfyears.

What We Found...High-Performance Grouts andGrouting Procedures• A 30% fly ash grout with a 0.35 w/c

(water content) had excellentperformance in horizontalapplications.

• A 2% anti-bleed grout with 0.33 w/chad excellent performance in verticalapplications.

• The standard TxDOT grout hadbelow average performance.

• Grout voids, due to entrapped air,bleed water, incomplete grout fillingor lack of grout fluidity were foundto be detrimental not only to theprestressing strand, but also to thegalvanized duct, as shown inFigure 2.

• Grout is not prestressed and groutcracks were often found underservice loading.

• Calcium Nitrite corrosion inhibitoradded to the grout did seem toprovide some enhanced long-termstrand corrosion protection.

Ducts for Internal Post-Tensioning• Galvanized steel ducts performed

poorly (see Figure 2). Plastic ductswere superior.

• The use of completely filled epoxyjoints with unspliced plastic ductsshowed very good protection.

Cracking and Joints• Transverse cracking due to loading

had a definite effect on corrosiondamage. Larger crack widths andcrack density were the cause of verysevere localized and uniformreinforcement corrosion activity.

CE

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CE - Counter ElectrodeWE - Working ElectrodeRE - Reference Electrode

(SCE)

Specimen

ReactionBeam

1.37 m(4.5 ft)

1.37 m(4.5 ft)

1.37 m(4.5 ft)

Cross Section:457 x 610 mm(18 x 24 in.)

Ponded SaltSolution

4.62 m (15' 2")TubeSection

ChannelSection

16 mm PT Bar(5/8 in.)

Spring

4.5 ft 4.5 ft 4.5 ft4.5 ft

15’ 2”

5/8 in. PT Bar Cross Section

18 x 24 in.

Specimen

ReactionBeam

1.37 m(4.5 ft)

1.37 m(4.5 ft)

1.37 m(4.5 ft)

Cross Section:457 x 610 mm(18 x 24 in.)

Ponded SaltSolution

4.62 m (15' 2")TubeSection

ChannelSection

16 mm PT Bar(5/8 in.)

Spring

4.5 ft 4.5 ft 4.5 ft4.5 ft

15’ 2”

5/8 in. PT Bar Cross Section

18 x 24 in.

Dow eled Joint Post-Tensioned Joint No Dowel

Column reinforcement

at joint

6 dowels:#6 bars

4 PT bars5/8“ diameter

coupler

bearing plateand nut

2 in. cover

rubber gasket

Dow eled Joint Post-Tensioned Joint No Dowel

Column reinforcement

at joint

6 dowels:#6 bars

4 PT bars5/8“ diameter

coupler

bearing plateand nut

2 in. cover

rubber gasket

9.8 m

2.4 m

1.2 m 1.2 m

A B C

D E

2.9 m

0.6 m

F G H I J 0.9 m

Vent

Vent Vent

Inlet

0.9 m 0.9 m 0.9 m 0.9 m

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X

Figure 1. (a) Macrocell Specimen, (b) Schematic of ACTM Station, (c) Large-Scale Clear Draped Parabolic Duct Test, (d) Beam Test Setup, (e) Column-Foundation Joint

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Project Summary Report 0-1405-S – 3 –

.

• Longitudinal or splitting cracksalways indicated very severecorrosion within the member.

• Dry joints and incompletely filledepoxy joints in the macrocellspecimens showed very poorperformance.

Levels of Post-Tensioning• As the level of post-tensioning or

concrete precompressionincreased, the corrosionprotection increased. Lowerpermeability due to increasedprecompression also providedbetter resistance to wickingeffects.

Concrete Type• High performance concrete

appears to be effective inminimizing the chloridepenetration through concrete.

Concrete Cover• Small concrete cover was clearly

shown to be detrimental toreinforcement performance.

Galvanized Duct Splices• Neither the industry standard

splice (duct taped) nor the heat-shrink splices appear to besatisfactory to prevent moistureand chloride ingress.

Gaskets for Post-Tensioning• The use of gaskets in the joints to

avoid epoxy filling of the ducts insegmental construction, or theuse of rubber gaskets to seal theduct ends at column joints, weredetrimental to the performance ofthe specimens.

Post-Tensioning Bars or Strands• PT bar coatings showed

enhanced general corrosionprotection, in comparison to plainPT bars. However, under verysevere localized attack, as in acrack or joint location, corrosionactivity was severe.

Exposure Testing Methods• After using half-cell potential

readings, chloride contentdeterminations and corrosioncurrent readings, only the firsttwo showed some degree ofcorrelation with forensicexamination results.

The ResearchersRecommend...• For PT Tendons with small rises

under severe exposure conditions,the following grout should beused: 0.35 w/c, 30% fly ash(Class C) replacement and4 ml/kg superplasticizer.

• For PT Tendons with large risesunder severe exposure conditions,the following grout should beused: 0.33 w/c and 2% anti-bleedadmixture.

• Plastic ducts should be used in allsituations where even moderateaggressive exposure may occur.

• Epoxy joints should always beused with internal prestressingtendons.

• Stringent inspection andconstruction practices should beenforced to guarantee completegrouting and good epoxy filling atthe joints in segmentalconstruction.

• Duct gaskets in epoxy jointsshould be avoided.

• Mixed reinforcement membersshould be used in aggressiveexposures only if specialprovisions are made to sealcracks and to prevent concreteexposure to chlorides.

• Fully prestressed members arerecommended in aggressiveenvironments.

• High performance concrete(w/c=0.29) is recommended inaggressive environments.

• Fly ash (Class C) concrete(w/c=0.44) may also beconsidered when the environmentis less aggressive.

• Development of better ductsplicing systems should be a highpriority.

• Inspection of in-service bridgesshould identify potentialreinforcement corrosion from anynoted longitudinal or splittingcrack.

• A minimum of 2-inch cover toany reinforcement should be usedin any post-tensioning design.

• Fully plastic chairs to ensureproper concrete cover arerecommended to eliminatecorrosion damage, instead ofchairs made out of steel.

• Column elements should beprestressed to improve spiral andrebar corrosion protection in veryaggressive environments.

• Galvanized steel bars or epoxycoated are susceptible to severelocalized corrosion.

• Bar coatings can be used whenpositive sealing of cracks or jointsis attained.

• Half-cell potential readings andchloride content determinationscould be used to assess to somedegree the service condition ofthe specimens. Better corrosionassessment methods should bedeveloped and evaluated.

Figure 2. Bleed Water Voids and Galvanized Duct Corrosion

The University of Texas at AustinCenter for Transportion Research3208 Red River, Suite #200Austin, TX 78705-2650

Disclaimer

Research Supervisor: John E. Breen, P.E., (512) 471-4578, jbreen@mail.utexas.eduTxDOT Project Director: Bryan Hodges (TYL), (903) 510-9127, bhodges@dot.state.tx.us.

The research is documented in the following reports:− Research Report 0-1405-1, State of the Art Durability of Post-Tensioned Bridge Substructures, October 1999.− Research Report 0-1405-2, Development of High-Performance Grouts for Bonded Post-Tensioned Structures, October 1999.− Research Report 0-1405-3, Long-Term Post-Tensioned Beam and Column Exposure Test Specimens: Experimental

Program, October 1999.− Research Report 0-1405-4, Corrosion Protection for Bonded Internal Tendons in Precast Segmental Construction, October 1999.− Research Report 0-1405-5, Interim Conclusions, Recommendations and Design Guidelines for Durability of Post-

Tensioned Bridge Substructures, October 1999.− Research Report 0-1405-6, Final Evaluation of Corrosion Protection for Bonded Internal Tendons in Precast Segmental

Construction, October 2002.− Research Report 0-1405-7, Long-Term Post-Tensioned Beam Exposure Test Specimens: Final Evaluation, August 2003.− Research Report 0-1405-8, Long-Term Post-Tensioned Column Exposure Test Specimens: Final Evaluation, August 2003.− Research Report 0-1405-9, Conclusions, Recommendations and Design Guidelines for Corrosion Protection of Post-

Tensioned Bridges, August 2003.To obtain copies of the above reports, contact the Center for Transportation Research, The University of

Texas at Austin, (512) 232-3126, ctrlib@uts.cc.utexas.edu.

Results from research project 0-1405 have substantially impacted the design and construction of post-tensionedstructural elements for bridges, and have been implemented both within Texas and on a national level. For example,the TxDOT specifications for grout materials were changed based on this research, and prepackaged grouts are nowcommercially available. A national grouting certification program has been implemented through the AmericanSegmental Bridge Institute for construction personnel, and galvanized metal ducts are no longer specified for post-tensioning applications in corrosive environments in Texas. Epoxied segment joints are now an industry standard.Post-tensioning bar and prestressing strand coatings (epoxy, galvanized, and other) show enhanced corrosionprotection and are being conclusively evaluated under ongoing TxDOT research project 0-4562.

For more information please contact Tom Yarbrough, P.E., RTI Research Engineer, at (512) 465-7403 or email attyarbro@dot.state.tx.us.

This research was performed in cooperation with the Texas Department of Transportation and the U. S. Department ofTransportation, Federal Highway Administration. The contents of this report reflect the views of the authors, who are responsiblefor the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official view or policies of theFHWA or TxDOT. This report does not constitute a standard, specification, or regulation, nor is it intended for construction,bidding, or permit purposes. Trade names were used solely for information and not for product endorsement. The engineers incharge were John E. Breen, P.E. (Texas No. 18479), and Michael E. Kreger, P.E. (Texas No. 65541).

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