Rehabilitation of rectangular simply supported RC beams.docx

Rehabilitation of rectangular simply supported RC beams.docx

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RehabilitationofrectangularsimplysupportedRCbeams

英文原文：

RehabilitationofrectangularsimplysupportedRCbeamswithsheardeficienciesusingCFRPcomposites

AhmedKhalifaa,*,AntonioNannib

aDepartmentofStructuralEngineering,UniversityofAlexandria,Alexandria21544,Egypt

bDepartmentofCivilEngineering,UniversityofMissouriatRolla,Rolla,MO65409,USA

Received28April1999;receivedinrevisedform30October2001;accepted10January2002

Abstract

Thepresentstudyexaminestheshearperformanceandmodesoffailureofrectangularsimplysupportedreinforcedconcrete（RC）beamsdesignedwithsheardeficiencies.Thesememberswerestrengthenedwithexternallybondedcarbonfiberreinforcedpolymer（CFRP）sheetsandevaluatedinthelaboratory.Theexperimentalprogramconsistedoftwelvefull-scaleRCbeamstestedtofailinshear.Thevariablesinvestigatedwithinthisprogramincludedsteelstirrups,andtheshearspan-to-effectivedepthratio,aswellasamountanddistributionofCFRP.TheexperimentalresultsindicatedthatthecontributionofexternallybondedCFRPtotheshearcapacitywassignificant.Theshearcapacitywasalsoshowntobedependentuponthevariablesinvestigated.Testresultswereusedtovalidateasheardesignapproach,whichshowedconservativeandacceptablepredictions.

2002ElsevierScienceLtd.Allrightsreserved.

Keywords:

Rehabilitation;Shear;Carbonfiberreinforcedpolymer

1.Introduction

Fiberreinforcedpolymer（FRP）compositesystems,composedoffibersembeddedinapolymericmatrix,canbeusedforshearstrengtheningofreinforcedcon-crete（RC）members[1–7].ManyexistingRCbeamsaredeficientandinneedofstrengthening.TheshearfailureofanRCbeamisclearlydifferentfromitsflexuralfailure.Inshear,thebeamfailssuddenlywithoutsufficientwarninganddiagonalshearcracksareconsid-erablywiderthantheflexuralcracks[8].

Theobjectivesofthisprogramwereto:

1.InvestigateperformanceandmodeoffailureofsimplysupportedrectangularRCbeamswithsheardeficien-ciesafterstrengtheningwithexternallybondedCFRPsheets.

2.Addressthefactorsthatinfluenceshearcapacityofstrengthenedbeamssuchas:

steelstirrups,shearspan-to-effectivedepthratio（a/dratio）,andamountanddistributionofCFRP.

3.Increasetheexperimentaldatabaseonshearstrength-eningwithexternallybondedFRPreinforcement.

4.Validatethedesignapproachpreviouslyproposedbytheauthors[9].

Fortheseobjectives,12full-scale,RCbeamsdesignedtofailinshearwerestrengthenedwithdifferentCFRPschemes.Thesemembersweretestedassimplebeamsusingafour-pointloadingconfigurationwithtwodifferenta/dratios.

2.Experimentalprogram

2.1.Testspecimensandmaterials

Twelvefull-scalebeamspecimenswithatotalspanof3050mm.andarectangularcross-sectionof150-mm-wideand305-mm-deepweretested.ThespecimensweregroupedintotwomainseriesdesignatedSWandSOdependingonthepresenceofsteelstirrupsintheshearspanofinterest.

SeriesSWconsistedoffourspecimens.ThedetailsanddimensionsofthespecimensdesignatedseriesSWareillustratedinFig.1a.Inthisseries,four32-mmsteelbarswereusedaslongitudinalreinforcementwithtwoattopandtwoatbottomfaceofthecross-sectiontoinduceashearfailure.Thespecimenswerereinforcedwith10-mmsteelstirrupsthroughouttheirentirespan.Thestirrupsspacingintheshearspanofinterest,righthalf,wasselectedtoallowfailureinthatspan.

SeriesSOconsistedofeightbeamspecimens,whichhadthesamecross-sectiondimensionandlongitudinalsteelreinforcementasforseriesSW.NostirrupswereprovidedinthetesthalfspanasillustratedinFig.1b.

Eachmainseries（i.e.seriesSWandSO）wassubdividedintotwosubgroupsaccordingtoshearspan-to-effectivedepthratio.Thiswasselectedtobea/d=3and4,resultinginthefollowingfoursubgroups:

SW3;SW4;SO3;andSO4.

ThemechanicalpropertiesofthematerialsusedformanufacturingthetestspecimensarelistedinTable1.FabricationofthespecimensincludingsurfacepreparationandCFRPinstallationisdescribedelsewhere[10].

Table1

2.2.Strengtheningschemes

Onespecimenfromeachseries（SW3-1,SW4-1,SO3-1andSO4-1）wasleftwithoutstrengtheningasacontrolspecimen,whereaseightbeamspecimenswerestrengthenedwithexternallybondedCFRPsheetsfollowingthreedifferentschemesasillustratedinFig.2.

InseriesSW3,specimenSW3-2wasstrengthenedwithtwoCFRPplieshavingperpendicularfiberdirections（90°/0°）.ThefirstplywasattachedintheformofcontinuousU-wrapwiththefiberdirectionorientedperpendiculartothelongitudinalaxisofthespecimen（90°）.Thesecondplywasbondedonthetwosidesofthespecimenwiththefiberdirectionparalleltothebeamaxis（0°）.Thisply[i.e.0°ply]wasselectedtoinvestigatetheimpactofadditionalhorizontalrestraintonshearstrength.

InseriesSW4,specimenSW4-2wasstrengthenedwithtwoCFRPplieshavingperpendicularfiberdirection（90°/0°）asforspecimenSW3-2.

FourbeamspecimenswerestrengthenedinseriesSO3.SpecimenSO3-2wasstrengthenedwithone-plyCFRPstripsintheformofU-wrapwith90°-fiberorientation.Thestripwidthwas50mmwithcenter-to-centerspacingof125mm.SpecimenSO3-3wasstrengthenedinamannersimilartothatofspecimenSO3-2,butwithstripwidthequalto75mm.SpecimenSO3-4wasstrengthenedwithone-plycontinuousU-wrap（90°）.SpecimenSO3-5wasstrengthenedwithtwo

CFRPplies（90°/0°）similartospecimensSW3-2andSW4-2.

InseriesSO4,twobeamspecimenswerestrengthened.SpecimenSO4-2wasstrengthenedwithone-plyCFRPstripsintheformofU-wrapsimilartospecimenSO3-2.SpecimenSO4-3wasstrengthenedwithone-plycontinuousU-wrap（90°）similartoSO3-4.

Fig.1.Configurationandreinforcementdetailsforbeamspecimens.

Fig.2.SchematicrepresentationofCFRPstrengtheningschemes.

2.3.Testset-upandinstrumentation

Allspecimensweretestedassimplespanbeamssubjectedtoafour-pointloadasillustratedinFig.3.Auniversaltestingmachinewith1800KNcapacitywasusedinordertoapplyaconcentratedloadonasteeldistributionbeamusedtogeneratethetwoconcentratedloads.Theloadwasappliedprogressivelyincycles,usuallyonecyclebeforecrackingfollowedbythreecycleswiththelastoneuptoultimate.Theappliedloadvs.deflectioncurvesshowninthispaperaretheenvelopesoftheseloadcycles.

Fourlinearvariabledifferentialtransformers（LVDTs）wereusedforeachtesttomonitorverticaldisplacementsatvariouslocationsasshowninFig.3.TwoLVDTswerelocatedatmid-spanoneachsideofthespecimen.Theothertwowerelocatedatthespecimensupportstorecordsupportsettlement.

Fig.3.Schematicrepresentationoftestset-upforspecimen（a）SW3-1,and（b）SW3-2.

ForeachspecimenofseriesSW,sixstraingaugeswereattachedtothreestirrupstomonitorthestirrupstrainduringloadingasillustratedinFig.1a.ThreestraingaugeswereattacheddirectlytotheFRPsheetonthesidesofeachstrengthenedbeamtomonitorstrainvariationintheFRP.Thestraingaugeswereorientedintheverticaldirectionandlocatedatthesectionmid-heightwithdistancesof175,300and425mm,respectively,fromthesupportforseriesSW3andSO3.ForbeamspecimensofseriesSW4andSO4,thestraingaugeswerelocatedatdistanceof375,500and625mm,respectively,fromthesupport.

3.Resultsanddiscussion

Inthefollowingdiscussion,referenceisalwaysmadetoweakshearspanorspanofinterest.

3.1.SeriesSW3

ShearcracksinthecontrolspecimenSW3-1wereobservedclosetothemiddleoftheshearspanwhentheloadreachedapproximately90kN.Astheloadincreased,additionalshearcracksformedthroughout,wideningandpropagatinguptofinalfailureataloadof253kN（seeFig.4a）.

InspecimenSW3-2strengthenedwithCFRP（90°/0°）,nocrackswerevisibleonthesidesorbottomofthetestspecimenduetotheFRPwrapping.However，alongitudinalsplittingcrackinitiatedonthetopsurfaceofthebeamatahighloadofapproximately320kN.

Fig.4.FailuremodesofseriesSW3specimens.

Thecrackinitiatedatthelocationofappliedloadandextendedtowardsthesupport.Thespecimenfailedbyconcretesplitting（seeFig.4b）attotalloadof354kN.Thiswasanincreaseof40%inultimatecapacitycomparedtothecontrolspecimenSW3-1.Thesplittingfailurewasduetotherelativelyhighlongitudinalcompressivestressdevelopedattopofthespecimen,whichcreatedatransversetension,ledtothesplittingfailure.Inaddition,therelativelylargeamountoflongitudinalsteelreinforcementcombinedwithover-strengtheningforshearbyCFRPwrapprobablycausedthismodeoffailure.Theloadvs.mid-spandeflectioncurvesforspecimensSW3-1andSW3-2areillustratedinFig.5,toshowtheadditionalcapacitygainedbyCFRP.

Fig.5.Appliedloadvs.mid-spandeflectionforseriesSW3specimens.

ThemaximumCFRPverticalstrainmeasuredatfailureinspecimenSW3-2wasapproximately0.0023mm/mm,whichcorrespondedto14%ofthereportedCFRPultimatestrain.Thisvalueisnotanabsolutebecauseitgreatlydependsonthelocationofthestraingaugeswithrespecttoacrack.However,therecordedstrainindicatesthatifthesplittingdidnotoccur,theshearcapacitycouldhavereachedhigherload.

ComparisonbetweenmeasuredlocalstirrupstrainsinspecimensSW3-1andSW3-2areshowninFig.6.Thestirrups1,2and3werelocatedatdistanceof175,300and425mmfromthesupport,respectively.Theresultsshowedthatthestirrups2and3didnotyieldatultimateforbothspecimens.Thestrains（andtheforces）inthestirrups