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692ACIStructuralJournal/September-October2002ACIStructuralJournal,V.99,No.5,September-October2002.MSNo.01-349receivedOctober23,2001,andreviewedunderInstitutepublicationpolicies.Copyright2002,AmericanConcreteInstitute.Allrightsreserved,includ-ingthemakingofcopiesunlesspermissionisobtainedfromthecopyrightproprietors.PertinentdiscussionwillbepublishedintheJuly-August2003ACIStructuralJournalifreceivedbyMarch1,2003.ACISTRUCTURALJOURNALTECHNICALPAPERAninnovative,uniaxialductilefiber-reinforcedpolymer(FRP)fabrichasbeenresearched,developed,andmanufactured(intheStructuralTestingCenteratLawrenceTechnologicalUniversity)forstrengtheningstructures.Thefabricisahybridoftwotypesofcarbonfibersandonetypeofglassfiber,andhasbeendesignedtoprovideapseudo-ductilebehaviorwithalowyield-equivalentstrainvalueintension.Theeffectivenessandductilityofthedevelopedfabrichasbeeninvestigatedbystrengtheningandtestingeightconcretebeamsunderflexuralload.Similarbeamsstrengthenedwithcurrentlyavailableuniaxialcarbonfibersheets,fabrics,andplateswerealsotestedtocomparetheirbehaviorwiththosestrengthenedwiththedevelopedfabric.Thefabrichasbeendesignedsothatithasthepotentialtoyieldsimultaneouslywiththesteelreinforcementofstrengthenedbeamsandhence,aductileplateausimilartothatforthenonstrengthenedbeamscanbeachieved.Thebeamsstrengthenedwiththedevelopedfabricexhibitedhigheryieldloadsandachievedhigherductilityindexesthanthosestrengthenedwiththecurrentlyavailablecarbonfiberstrengtheningsystems.Thedevelopedfabricshowsamoreeffectivecontributiontothestrengtheningmechanism.Keywords:concrete；ductility；fiberreinforcement；flexure.INTRODUCTIONTheuseofexternallybondedfiber-reinforcedpolymer(FRP)sheetsandstripshasrecentlybeenestablishedasaneffectivetoolforrehabilitatingandstrengtheningreinforcedconcretestructures.SeveralexperimentalinvestigationshavebeenreportedonthebehaviorofconcretebeamsstrengthenedforflexureusingexternallybondedFRPplates,sheets,orfabrics.SaadatmaneshandEhsani(1991)examinedthebehaviorofconcretebeamsstrengthenedforflexureusingglassfiber-reinforcedpolymer(GFRP)plates.Ritchieetal.(1991)testedreinforcedconcretebeamsstrengthenedforflexureusingGFRP,carbonfiber-reinforcedpolymer(CFRP),andG/CFRPplates.Graceetal.(1999)andTrian-tafillou(1992)studiedthebehaviorofreinforcedconcretebeamsstrengthenedforflexureusingCFRPsheets.Norris,Saa-datmanesh,andEhsani(1997)investigatedthebehaviorofconcretebeamsstrengthenedusingCFRPunidirectionalsheetsandCFRPwovenfabrics.Inalloftheseinvestigations,thestrengthenedbeamsshowedhigherultimateloadscom-paredtothenonstrengthenedones.Oneofthedrawbacksexperiencedbymostofthesestrengthenedbeamswasacon-siderablelossinbeamductility.Anexaminationoftheload-deflectionbehaviorofthebeams,however,showedthatthemajorityofthegainedincreaseinloadwasexperiencedaf-tertheyieldofthesteelreinforcement.Inotherwords,asignificantincreaseinultimateloadwasexperiencedwithoutmuchincreaseinyieldload.Hence,asignificantincreaseinservicelevelloadscouldhardlybegained.Apartfromtheconditionoftheconcreteelementbeforestrengthening,thesteelreinforcementcontributessignificantlytotheflexuralresponseofthestrengthenedbeam.Unfortunately,availableFRPstrengtheningmaterialshaveabehaviorthatisdifferentfromsteel.AlthoughFRPmaterialshavehighstrengths,mostofthemstretchtorelativelyhighstrainvaluesbeforeprovidingtheirfullstrength.BecausesteelhasarelativelylowyieldstrainvaluewhencomparedwiththeultimatestrainsofmostoftheFRPmaterials,thecontri-butionofboththesteelandthestrengtheningFRPmaterialsdifferwiththedeformationofthestrengthenedelement.Asaresult,steelreinforcementmayyieldbeforethestrengthenedelementgainsanymeasurableloadincrease.SomedesignersplaceagreaterFRPcrosssection,whichgenerallyincreasesthecostofthestrengthening,toprovideameasurablecontri-bution,evenwhendeformationsarelimited(beforetheyieldofsteel).Debondingofthestrengtheningmaterialfromthesurfaceoftheconcrete,however,ismorelikelytohappeninthesecasesduetohigherstressconcentrations.Debondingisoneofthenondesiredbrittlefailuresinvolvedwiththistechniqueofstrengthening.Althoughusingsomespeciallow-strainfiberssuchasultra-high-moduluscarbonfibersmayappeartobeasolution,itwouldresultinbrittlefailuresduetothefailureoffibers.Theobjectiveofthispaperistointroduceanewpseudo-ductileFRPfabricthathasalowstrainatyieldsothatithasthepotentialtoyieldsimultaneouslywiththesteelreinforcement,yetprovidethedesiredstrengtheninglevel.RESEARCHSIGNIFICANCEFRPshavebeenincreasinglyusedasmaterialsforrehabil-itatingandstrengtheningreinforcedconcretestructures.CurrentlyavailableFRPmaterials,however,lacktheductilityandhavedissimilarbehaviorstosteelreinforcement.Asaresult,thestrengthenedbeamsmayexhibitareducedductility,lackthedesiredstrengtheninglevel,orboth.Thisstudypresentsaninnovativepseudo-ductileFRPstrengtheningfabric.ThefabricprovidesmeasurablyhigheryieldloadsforthestrengthenedbeamsandhelpstoavoidthelossofductilitythatiscommonwiththeuseofcurrentlyavailableFRP.DEVELOPMENTOFHYBRIDFABRICToovercomethedrawbacksmentionedpreviously,aductileFRPmaterialwithlowyieldstrainvalueisneeded.Titleno.99-S71StrengtheningofConcreteBeamsUsingInnovativeDuctileFiber-ReinforcedPolymerFabricbyNabilF.Grace,GeorgeAbdel-Sayed,andWaelF.Ragheb693ACIStructuralJournal/September-October2002LiteraturereviewonhybridizationTodevelopthismaterial,hybridizationfordifferentfiberswasconsidered.Hybridizationofmorethanonetypeoffibrousmaterialswastheinterestofmanymaterialsscienceresearchers.Mostoftheirworkwasconcernedwithcombiningtwotypesoffiberstoenhancethemechanicalpropertiesofeithertypeactingaloneandtoreducethecost.ThishasbeenreportedinseveralpublicationssuchasBunsellandHarris(1974),Philips(1976),MandersandBader(1981),ChowandKelly(1980),andFukudaandChow(1981).HybridizationinterestedstructuralengineersasatooltoovercometheproblemofalackofductilityinFRPreinforcingbars.Nanni,Henneke,andOkamoto(1994)studiedbarsofbraidedaramidfibersaroundasteelcore.TamuzsandTepfers(1995)reportedexperimentalinvestigationsforhybridfiberbarsusingdifferentcombinationsofcarbonandaramidfibers.Somboonsong,Frank,andHarris(1998)developedahybridFRPreinforcingbarusingbraidedaramidfibersaroundacarbonfibercore.Harris,Somboonsong,andFrank(1998)usedthesebarsinreinforcingconcretebeamstoachievethegeneralload-deflectionbehaviorofconcretebeamsreinforcedwithconventionalsteel.DesignconceptandmaterialsTogenerateductility,ahybridizationtechniqueofdifferenttypesoffibershasbeenimplemented.Threefibershavebeenselectedwithadifferentmagnitudeofelongationsatfailure.Figure1showsthestress-straincurvesintensionfortheselectedcompositefibers,andTable1showstheirmechanicalproperties.Thetechniqueisbasedoncombiningthesefiberstogetherandcontrollingthemixtureratiosothatwhentheyareloadedtogetherintension,thefiberswiththelowestelongation(LE)failfirst,allowingastrainrelaxation(anincreaseinstrainwithoutanincreaseinloadforthehybrid).Theremaininghigh-elongation(HE)fibersareproportionedtosustainthetotalloaduptofailure.ThestrainvalueatfailureoftheLEfiberspresentsthevalueoftheyield-equivalentstrainofthehybrid,whiletheHEfiberstrainatfailurepresentsthevalueofultimatestrain.TheloadcorrespondingtofailureofLEfiberspresentstheyield-equivalentloadvalue,andthemaximumloadcarriedbytheHEfibersistheultimateloadvalue.Ultra-high-moduluscarbonfibers(CarbonNo.1)havebeenusedasLEfiberstohaveaslowastrainaspossible,butnotlessthantheyieldstrainofsteel(approximately0.2%forGrade60steel).Ontheotherhand,E-glassfiberswereusedasHEfiberstoprovideashighastrainaspossibletoproduceahigh-ductilityindex(theratiobetweendeformationatfailureanddeformationatyield).High-moduluscarbonfibers(CarbonNo.2)wereselectedasmedium-elongation(ME)fiberstominimizethepossibleloaddropduringthestrainrelaxationthatoccursafterfailureoftheLEfibers,andalsotoprovideagradualloadtransitionfromtheLEfiberstotheHEfibers.Basedonthisconcept,auniaxialfabricwasfabricatedandtestedtocompareitsbehaviorintensionwiththetheoreticalpredictedloadingbehavior.Thetheoreticalbehaviorisbasedontheruleofmixtures,inwhichtheaxialstiffnessofthehybridiscalculatedbyasummationoftherelativestiffnessofeachofitscomponents.Thefabricwasmanufacturedbycombiningdifferentfibersasadjacentyarnsandimpregnatingtheminsideamoldbyanepoxyresin.Figure2showsaphotoofoneofthefabricatedsamples.Wovenglassfibertabswereprovidedatbothendsofthetestcouponstoeliminatestressconcentrationsatendfixturesduringtesting.Thecouponshadathicknessof2mm(0.08in.)andawidthof25.4mmACImemberNabilF.GraceisaprofessorandChairoftheStructuralTestingCenter,DepartmentofCivilEngineering,LawrenceTechnologicalUniversity,Southfield,Mich.HeisamemberofACICommittee440,FiberReinforcedPolymerReinforcement；andJointACI-ASCECommittee343,ConcreteBridgeDesign.Hisresearchinterestsincludetheuseoffiber-reinforcedpolymerinreinforcedandprestressedconcretestructures.GeorgeAbdel-SayedisProfessorEmeritusintheDepartmentofCivilandEnvi-ronmentalEngineering,UniversityofWindsor,Windsor,Ontario,Canada.Hisresearchinterestsincludesoil-structureinteraction.WaelF.RaghebisaresearchassistantintheDepartmentofCivilEngineeringatLawrenceTechnologicalUniversity.HeisaPhDcandidateintheDepartmentofCivilandEnvironmentalEngineering,UniversityofWindsor,Windsor,Ontario,Canada.Fig.1Stress-strainbehaviorofcompositefibersandsteelreinforcingbars.*Compositepropertiesarebasedon60%fibervolumefraction.Table1Mechanicalpropertiesofcompositefibers*FibermaterialDescriptionModulusofelasticity,GPa(Msi)Tensilestrength,MPa(ksi)Failurestrain,%CarbonNo.1Ultra-high-moduluscarbonfibers379(55)1324(192)0.35CarbonNo.2High-moduluscarbonfibers231(33.5)2413(350)0.9to1.0GlassE-glassfibers48(7)1034(150)2.1Fig.2Testsamplefordevelopeduniaxialhybridfabric.Fig.3Resultsoftensiletestsfordevelopedhybridfabric.694ACIStructuralJournal/September-October2002(1in.)andweretestedintensionaccordingtoASTMD3039specifications.Theaverageload-straincurveforfourtestedsamplesisshowninFig.3togetherwiththetheoreticalprediction.Itshouldbenotedthatthebehaviorislinearuptoastrainof0.35%,whentheLEfibersstartedtofail.Atthispoint,thestrainincreasedatafasterratethantheload.Whenthestrainreached0.90%,theMEfibersstartedtofail,resultinginanadditionalincreaseinstrainwithoutasignificantincreaseinload,uptothetotalfailureofthecouponbyfailureoftheHEfibers.Ayield-equivalentload(thefirstpointontheload-straincurvewherethebehaviorbecomesnonlinear)of0.46kN/mmwidth(2.6kips/in.)andanultimateloadof0.78kN/mm(4.4kips/in.)areobserved.BEAMTESTSBeamdetailsThirteenreinforcedconcretebeamswithcross-sectionaldimensionsof152x254mm(6x10in.)andlengthsof2744mm(108in.)werecast.TheflexurereinforcementofthebeamsconsistedoftwoNo.5(16mm)tensionbarsnearthebottom,andtwoNo.3(9.5mm)compressionbarsnearthetop.Toavoidshearfailure,thebeamswereover-reinforcedforshearwithNo.3(9.5mm)closedstirrupsspacedat102mm(4.0in.).Fivebeamswereformedwithroundedcornersof25mm(1in.)radiustofacilitatetheinstallationofthestrengtheningmaterialontheirsidesandbottomfaceswithoutstressconcentrations.Figure4showsthebeamdimensions,reinforcementdetails,supportlocations,andlocationofloadingpoints.ThesteelusedwasGrade60withayieldstrengthof415MPa(60,000psi),whiletheconcretecompressivestrengthatthetimeoftestingthebeamswas55.2MPa(8000psi).StrengtheningmaterialsThedevelopedhybridfabricwasusedtostrengtheneightbeams.Twodifferentthicknessesoffabricwereused.Thefirst(H-system,t=1.0mm)hadathicknessof1.0mm(0.04in.),andthesecond(H-system,t=1.5mm)hadathicknessof1.5mm(0.06in.).Fourotherbeamswerestrengthenedwiththreecurrentlyavailablecarbonfiberstrengtheningmaterials:1)auniaxialcarbonfibersheetwithanultimateloadof0.34kN/mm(1.95kips/in.)；2)twolayersofauniaxialcarbonfiberfabricwithanultimateloadof1.31kN/mm(7.5kips/in.)forthetwolayerscombined；and3)apultrudedcarbonfiberplatewithanultimateloadof2.8kN/mm(16kips/in.).Thetestedload-straindiagramsforFig.4Detailsoftestbeams.Fig.5Comparisonbetweencarbonfiberplate,fabric,sheet,anddevelopedhybridfabric(H-System).ACIStructuralJournal/September-October2002695allthesematerialsareshowninFig.5.Table2showsthepropertiesofthestrengtheningmaterials,includingthedevelopedfabric.AdhesivesForthehybridfabric,anepoxyresin(EpoxyA)wasusedtoimpregnatethefibersandasanadhesivebetweenthefabricandtheconcretesurface.Thisepoxyhadanultimatestrainof4.4%toensurethatitwouldnotfailbeforethefailureofthefibers.Forthebeamsstrengthenedwithcarbonfibersheets,plates,andfabric,anepoxyresinwithanultimatestrainof2.0%wasused(EpoxyB).ThemechanicalpropertiesoftheadhesivesprovidedbytheirmanufacturesareshowninTable3.StrengtheningThebeambottomfacesandsidesweresandblastedtoroughenthesurface.Thebeamswerethencleanedwithacetonetoremovedirt.Twostrengtheningconfigurationswereused:1)strengtheningmaterialonthebottomfaceofthebeamonly(BeamGroupA)；and2)strengtheningmaterialonthebottomfaceandextendedup152mm(6in.)onbothsidestocoverapproximatelyalltheflexuraltensionportionsofthebeam(BeamGroupB).Thestrengtheningwasinstalledfor2.24m(88in.),centeredalongthelengthofthebeam.Theepoxywasallowedtocureforatleast2weeksbeforethebeamsweretested.Forthebeamsstrengthenedwiththedevelopedhybridfabric(H-system),twobeamswerefabricatedandtestedforeachconfigurationtoverifytheresults.Table4summarizesthetestbeams.InstrumentationTheFRPstrainatmidspanwasmeasuredbythreestraingageslocatedatthebottomfaceofthebeam.ThesteeltensilestrainwasmeasuredbymonitoringthestrainonthesidesurfaceofthebeamatreinforcingbarlevelusingaDEMEC(detachablemechanicalgage)withgagepointsforBeamGroupA,whilestraingageswereusedforBeamGroupB.Themidspandeflectionwasmeasuredusingastringpoten-tiometer.Thebeamswereloadedusingahydrau