Improving shear capacity of existing RC beams using external bonding of steel plates

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EngineeringStructures27(2005)

781–791

/locate/engstruct

ImprovingshearcapacityofexistingRCbeamsusing

externalbondingofsteelplates

SinanAltin ,ÖzgürAnil,M.EminKara

DepartmentofCivilEng.,GaziUniversity,Maltepe,06570Ankara,Turkey

Received8October2003;receivedinrevisedform7December2004;accepted21December2004

Availableonline24February2005

Abstract

Variousmethodsaredevelopedforstrengtheningreinforcedconcretebeamsagainstshear.Strengtheningofreinforcedconcretebeamsusingexternalboundingofsteelplateswasoneofthepopularresearchareasofrecentyears.Thisstudypresentstestresultsonstrengtheningshearde cientreinforcedconcretebeamsbyexternalbondingofsteelplates.ElevenreinforcedconcretebeamswithaT-sectionweretestedundermonotonicloadingintheexperimentalprogram.Threemaintypesofsteelmemberswithdifferentarrangementswerebondedtothesideofthebeamwebsalongtheshearspanbyusingepoxy.Thepurposewastoobtainductilebehaviorforshearde cientreinforcedconcretebeams.Thetestresultcon rmedthatallsteelplatearrangementsimprovedthestrengthandstiffnessofthespecimenssigni cantly.Thetensionreinforcementofallstrengthenedreinforcedconcretespecimenswasyielded.Thefailuremodesandductilityofspecimenswereprovedtodifferaccordingtothetypeofthesteelmemberandarrangementalongthebeam.Beamsthatwerestrengthenedwithcontinuoussteelplatealongtheshearspanshowedductile exuralbehavior.©2005ElsevierLtd.Allrightsreserved.

Keywords:Reinforcedconcretebeam;Strengthening;Shear;Epoxy;Steelplate

1.Introduction

Ingeneral,reinforcedconcrete(RC)beamsfailintwomodes: exureandshear.ItiswellknownthattheshearfailureofaRCbeamissuddenandbrittleinnature.Soitislesspredictableandgivesnoadvancewarningpriortofailure.Shearfailureismoredangerousthan exuralfailure.ForthatreasonRCbeamsmustbedesignedtodeveloptheirfull exuralcapacityandassureaductile exuralfailuremodeunderextremeloading.However,manyRCstructuresencountershearproblemsforvariousreasons,suchasmistakesindesigncalculations,improperdetailingoftheshearreinforcement,constructionerrorsorpoorconstructionpractices,changingthefunctionofastructurefromalowerserviceloadtoahigherserviceload,andreductionofthe

Correspondingauthor.Tel.:+903122317400/2248;fax:+90312231

9223.

E-mailaddress:saltin@gazi.edu.tr(S.Altin).

0141-0296/$-seefrontmatter©2005ElsevierLtd.Allrightsreserved.doi:10.1016/j.engstruct.2004.12.012

shearreinforcementsteelareaduetocorrosioninserviceenvironments.

Theknownstrengtheningtechniquesofshearde cientbeamsareasfollows:CFRP,strengtheningwithexternallyappliedclamps,jacketingwithconcretelayersandexternalbondingofsteelplateswithepoxy.Forstrengtheningshearde cientbeams,althoughnumeroustestshavebeencarriedout,andshownthatcompositematerialsareanexcellentoptionforuseasexternalreinforcing,thesteelplatebondingtechniqueisbecomingpreferableforstrengtheningduetoseveraladvantagessuchaseasyconstructionwork,minimumchangeintheoverallsizeofthestructureafterplatebondingandbeinganeconomicaltechnique.Themajorityofresearchtodateonplatebondinghavefocusedon exuralstrengtheningofRCbeamsbybondingsteelplatestobeamsof ts.Previously,veryfewstudieshavebeencarriedoutontheshearstrengtheningofRCbeamsusingwebbondedsteelplates[1–8].TheultimateloadofthestrengthenedRCbeamdependsprincipallyonthecompressivestrengthoftheconcrete,theyield

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Fig.1.Reinforcementdetailsofspecimens.

strengthoftheshearandlongitudinalreinforcement,thetensilereinforcementratio,theshearspantodepthratio,thestrengthandratioofstrengtheningmaterialssuchascompositeorsteelplate.AdditionalresearchonsuchstrengtheningtechniqueswasconductedtodeterminetheperformanceofstrengthenedRCbeamsunderdifferentconditions,andtodeterminetheeffectofsteelplatetypesandarrangementsonaRCbeam’sbehaviorandfailuremode.

Thispaperpresentsresultsofanexperimentalstudyconductedonthestrengtheningofshearde cientbeamsbyusingexternalwebbondedsteelplates.Elevenspecimens,oneofwhichwasacontrolspecimenandtheremainingtenofwhichhadde cientshearreinforcement,weretestedinanexperimentalprogram[9–11].RCbeamswithde cientshearreinforcementswerestrengthenedwithdifferentarrangementsofbondedsteelelements.Theaimwastoobtainductile exuralfailureforallstrengthenedspecimens.Theresultsofthetestsonthesebeamswerecomparedwiththatforthecontrolbeam.Theeffectsofthetypeandarrangementofthesteelplatesthatwereusedforstrengtheningonthebehavior,strength,stiffness,failuremodeandductilityofthespecimenswereinvestigated.Theexperimentalresultsarecomparedwithanalyticalpredictions.

2.Experimentalprogram

2.1.Specimensandmaterialproperties

AtotalofelevenT-sectionRCbeamsweretestedintheexperimentalprogram.DimensionsandreinforcementdetailsareshowninFig.1.Thedistancebetweenthesupportswas3800mmandthesameforallspecimens,aswerethecrosssectionalgeometriesandlongitudinalreinforcements.Thelongitudinalreinforcementconsistsofthree20mmdiametersteeldeformedbarsatthebottomandtwo8mmdiameterbarsatthetopofthebeam.Theshearreinforcementconsistedof6mmdiameterclosedstirrups,spacedat300mmcentertocenterthroughoutthebeamexceptforBeam-1.TheclosedstirrupspacingforBeam-1was75mm.Theyieldstrengthsofthelongitudinalsteelbarsatthebottom,topandstirrupswerefsy=414MPa,fsy=304.2MPaandfsy=275MPa,respectively.Table1summarizesthespecimens’properties.Averagecompressivestrengthsofconcreteweredeterminedfromstandardtestsofcylindersthatwerecastfromthesameconcreteaswasusedforthebeams.AscanbeseenfromTable1,theaveragecompressivestrengthsoftheconcretewerethegreaterthan25MPa.Table2showsthemechanicalpropertiesofthereinforcingbarsandsteelplatesusedinthebeams.

Beam-1wasthecontrolspecimenthatwasdesignedsuchthatithadgreatershearstrengththan exuralstrength.Thus,ductile exuralfailurewasthedominantmodeoffailure.TheotherRCbeamsweredesignedtobede cientinshearcapacity;thus,shearfailurewastheirdominantmodeoffailure.Theratiooftheshearde cientbeams’stirrupreinforcementratiotothecontrolmember’sstirrupreinforcementratiowas0.25.Shearde cientbeamswerestrengthenedbybondingsteelstrapsandplatestobothsidesofthebeamwebalongthelengthoftheshearspan.Steelstrapsorplatesweredesignedsuchthattheycouldincreasetheshearforceuptothebeams’ultimate exuralcapacitieswithoutyielding.Thesteelstrapandplate

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Table1

SpecimenpropertiesSpecimen#(1)Beam-1Beam-2Beam-3Beam-4Beam-5Beam-6Beam-7Beam-8Beam-9Beam-10Beam-11

(Control)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)

fc(MPa)(2)25.827.027.627.326.526.525.825.626.726.026.4

Stirrupsρw(3)0.002240.000560.000560.000560.000560.000560.000560.000560.000560.000560.00056

RatioρW

WBeam1(4)1.000.250.250.250.250.250.250.250.250.250.25

ρ

783

SteelmemberusedforstrengtheningDimensionsType(5)(6)–

–

40×285×4040×405×4040×285×40150×285×40150×405×40150×285×401550×285×40310×285×401550×285×40

––

Narrowsteelstrap

NarrowLshapesteelstrapNarrowsteelstrapWidesteelstrap

WideLshapesteelstrapWidesteelstrapSteelplate

Widesteelstrap

Steelplatewithopening

Spacing(mm)(7)––80804015015075–––

Table2

MaterialpropertiesofreinforcementsandsteelmembersReinforcements(1)

6mmbar8mmbar

20mmdeformedbar4mmsteelplate

Yieldstrength(MPa)(2)275.0304.2414.0261.0

Ultimatestrength(MPa)(3)417.0443.1687.9402.8

Elasticmodulus×103(MPa)(4)192198205188

geometricdimensionsareshowninFig.2.Thethicknessofallsteelplateswas4mm.AscanbeseenfromFig.2:(a)Twotypesofnarrowsteelstrapswithdimensions40×

285×4mmand40×405×4mmweremanufactured.The40×405×4mmsteelstrapswerebentintoan“L”shape.Thelengthsofthearmsofthe“L”were120and285mm,respectively.

(b)Threetypesofwidesteelstrapswithdimensions150×

285×4mm,150×405×4mmand310×285×4mmweremanufactured.The150×405×4mmsteelstrapswerebentintoan“L”shape.Thelengthsofthearmsofthe“L”were120and285mm,respectively.

(c)Twotypesofsteelplateswithdimensions1550×285×

4mmandthesamedimensionalplateswithopeningsweremanufactured.Thesteelplateswithopeningsweremanufacturedfrom1550×285×4mmplatesbycuttingthreesymmetricalopeningswithdimensions317×125mm.SteelstrapandplatearrangementsofstrengthenedspecimensaregiveninFig.3.2.2.Bondingprocedure

Thesameapplicationstepswereusedforstrengtheningallspecimens.Beforebondingthesteelmemberstotheconcretesurface,specialconsiderationwasgiventopreparationofthebeam’swebsurface.Bothsidesofthebeamwebwereroughenedbyamechanicalgrindingmachineuntiltheaggregatewasexposed,brushedandthenthesurfacesvacuumcleanedtoremovelooseparticlesanddust.Thebondingfacesofthesteelplateswerealsoroughenedbyamechanicalgrindingmachineandcleanedthoroughlywithacetone.Thenepoxyresinwasmixedinaccordancewiththemanufacturer’sinstruction.Mixingwascarriedoutinametalcontainerandwascontinueduntilthemixturewasauniformcolor.Theepoxy(Sikadur32)wasspreadalloverthewebofthebeamsandthesteelplatestoathicknessof1.5mm.Theenvironmentalconditionofthesiteatwhichepoxywasappliedwascrucial.Conditionstobeobservedbeforeandduringinstallationincludethesurfacetemperatureoftheconcrete,airtemperature,relativehumidityandcorrespondingdewpoint.Thetemperatureduringapplicationwas20±2 Cinallcases.Thehumidityofthesurfacesatwhichtheepoxywasappliedandthecalculateddewpointsaccordingtothehumiditiesofthesurfacesmustbe ttedtotheregulations.DewpointcalculationsweremadeaccordingtohumiditymeasurementsandISO8502-4regulations[12].Theageoftheconcreteatthetimeofbondingoftheplateswasbetween80and90days.Thesuccessoftheepoxywascloselyrelatedtothestrengthoftheconcrete.Duetothefactthatthestrengthoftheconcretewascloselyrelatedtoitsage,aftercompletingallofthespecimens,epoxywasappliedtoallofthemtogether.Afterbondingoperationswascompleted,specimenswerecuredfor15daysunderlaboratoryconditionsbeforetesting.

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Fig.2.Steelstrapsandplatesusedfor

strengthening.

Fig.3.Steelstrapandplatearrangementsofstrengthenedspecimens.

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Fig.3.(continued).

2.3.Experimentalset-up

Aschematicviewoftheexperimentalset-upandthearrangementofthemeasurementdevicesisshowninFig.4.Beamsweretestedunderfour-pointloading.Theloadappliedtothemid-pointofthereactionbeamwasdividedsymmetricallyintotwoconcentratedloadsandappliedtothespecimens.Theratiooftheshearspanlength,1450mm,totheeffectiveheightofthebeam,335mm,was4.3andwasthesameforallspecimens.Specimensweretestedundermonotonicloadingtofailure.Loadwasappliedwitha600kNcapacityhydraulicjackandwascontrolledwitha300kNcapacityloadcell.Themid-pointde ectionandshearcracksofthespecimensweremeasured.ShearcrackwidthsweremeasuredelectronicallybyattachingeightLVDTsdiagonallytorectangularregionswithdimensionsof300×210mmatequalintervalsforshearspans.Shearcrackmeasurementsofthespecimensforwhichalltheshearspanwascoveredwithsteelplatesweretakenbydrillingholestothemeasurementpoints.Themeasurementdevicesweremountedsothattheydidnottouchthewebsofthebeamsorthesteelmembers.

3.Experimentalresultsandevaluation3.1.Observedbehaviorandfailuremodes

TestresultsaresummarizedinTable3.Fig.5showscrackingpatternsandfailuremodesofspecimens.Inallspecimens,the rstcrackalwaysappearedasa exuralcrackinthemaximumbendingmomentregionofthebeam.Ingeneral rst exuralcracksdevelopedat17%oftheultimatestrengthsofthespecimens.Shearcracksdevelopedatloadlevelsbetween40%and50%oftheultimateloadalongtheshearspanofallspecimensexceptBeam-2.ThreetypicalexamplesthatshowthediagonalcrackpropagationthatoccurredbytheseventhLVDTmeasurementrangearepresentedinFig.6.ControlspecimenBeam-1showedductile exuralbehaviorasaresultoflongitudinaltensionreinforcementyielding.Afteryielding,Beam-1developedlargemid-spandisplacements,andreachedanultimateloadvaluethatwas11%greaterthantheyieldload.Beam-1failedbecauseofcrushingoftheconcreteintheextremecompression ber.Beam-2withde cientshearreinforcementcollapsedwithabrittleshearfailureand

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Fig.4.Testset-upandinstrumentation.

Table3TestresultsSpecimen#(1)Beam-1Beam-2Beam-3Beam-4Beam-5Beam-6Beam-7Beam-8Beam-9Beam-10Beam-11

(Control)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)

Crackingload(kN)FlexureShear(2)(3)13.412.614.014.112.913.613.712.412.812.213.5

36.034.536.337.540.635.734.334.338.238.037.8

Yieldload(kN)(4)81.0–79.281.280.079.080.180.681.381.081.0

Ultimateload(kN)(5)90.455.381.079.783.679.980.280.188.687.584.7

Yielddisp.(mm)(6)23.5–25.224.924.822.822.225.122.020.723.5

Ultimatedisp.(mm)(7)84.920.550.433.076.040.733.546.093.788.067.9

Stiffnessatyield(kN/mm)(8)3.45–3.143.263.233.473.603.213.693.913.44

Ductilityratio(9)3.61–2.001.333.061.791.511.834.264.252.89

Failuremodeatultimate(10)FlexureShearShearShearFlexureShearShearShearFlexureFlexureShear

withoutreachingits exuralcapacity.Twoshearcracksdevelopedintheleftshearspan.Beam-2carried39%lessultimateloadthanBeam-1.

Flexuralcracksofspecimensstrengthenedwithsteelstrapspropagatedasobliquecracksbetweenthesteelstrapsalongtheshearspanwithincreasingload.Shearcracksreachedtothesteelstrapsataloadlevelof55%oftheultimatestrengthofthespecimens.Attheseloadslevelspropagationoftheshearcrackswasrestrictedbythesteelstrapsandcracksdidnotpassunderthesteelstraps.Someshearcracksfollowedtheedgeofthesteelstrapsandpropagatedtothebeam’stop.Thelongitudinaltensionreinforcementofallspecimensyielded.Noneofthesteelmembersseparatedfromthewebsidesofthespecimensbeforetheyieldloadwasreached.Afteryielding,thearrangementofthesteelmembersdeterminedthebehaviorofthespecimens.Theendsofthesteelstrapsseparatedfromthesidesofthespecimensatdifferentductilityratiosafteryielding.Atthesametime,shearcracksstartedtopropagatequicklyandpassedunderthesteelstraps.Atthispoint exuralbehaviorsofthespecimenceased,andthespecimencollapsedinshearbecausecriticalshearcrackspropagatedalongoneoftheshearspans.NoneofthesteelstrapsofBeam-5thatwasstrengthenedwith40mmspacedsteelstrapsseparatedfromthebeamsurface,andthespecimenshowedfullyductile exuralbehavior.Beam-5failedbecauseofcrushingoftheconcreteinthecompressionregion.Beam-4andBeam-7whichwerestrengthenedwith“L”shapedsteelstrapsshowedthelowestductility.Bothspecimenscollapsedinshearduetosuddenseparationofthelegsofthe“L”shapedsteelstrapsthatwerebondedtotheundersidesofthe angesofthebeams.

Beam-9strengthenedbybondingasteelplatealongthewholeshearspanandBeam-10strengthenedbybondingadjacentsteelplatesshowedductile exuralbehavior.Bothofthespecimensfailedduetocrushingoftheconcreteinthecompressionregionatmid-span(Figs.7and8).Shearcrackmeasurementsthatweretakenfrombothofthespecimensshowedthatwidthsoftheirshearcrackswereapproximately50%lessthanthewidthofthecontrolmember’sshearcracks.ShearcracksofBeam-11,strengthenedwithonesteelplatewithopenings,propagatedtothebeam’stop ange,andthespecimencollapsedinshear.Thesteelplatebondedtotherightshearspanbuckledonbothsidesof

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Fig.5.Failuremodesofspecimens.

thebeam.Ingeneral,therewerefewershearcracksforstrengthenedspecimensthanforthecontrolspecimen,andthestrengthenedspecimenscollapsedduetopropagationofonemainshearcrack.

3.2.Load–displacementbehavior

Load–displacementrelationshipsforthespecimensareshowninFigs.9–11.Allspecimensshowedthesamestiffnessatyielding.Thesecantstiffnessesofthespecimenswerecalculatedbyusingtheslopeofthelinesthatconnectedtheload–displacementcurveoriginandtheyieldpoint.Strengthenedspecimensallshowedapproximatelythesamedisplacementandloadvaluesatyield.AscanbeseenfromTable3,thecalculatedsecantstiffnesseswerecloseto

each

Fig.6.Typicalexamplesofcrackwidthmeasurements.

otherforallspecimens.Theaveragesecantstiffnessofthespecimenswas3.44kN/mmatyield.

AscanbeseenfromFig.9,thespacingofthesteelstrapswasasigni cantparameteraffectingtheultimateloadcarryingcapacity,displacementcapacityandfailuremodeofthespecimen.Beam-3andBeam-4carried11%lessloadthanthecontrolmemberultimately.Thespecimenshad41%and61%lessdisplacementcapacitythanthecontrolspecimen,respectively.Ofthespecimensstrengthenedwithnarrowsteelstraps,onlyBeam-5showedductile exuralbehavior.Beam-5behavedsimilarlytothecontrolmember,

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Fig.7.Beam-9after

failure.

Fig.8.Beam-10after

failure.

Fig.9.Load–displacementcurvesofspecimensstrengthenedwithnarrowsteelstraps.

whentheultimateloadcarryingcapacity,failuremodeanddisplacementcapacityaretakenintoaccount.AscanbeseenfromFig.10,theultimateloadanddisplacementcapacitiesforBeam-6,Beam-7andBeam-8thatwerestrengthenedwithwidesteelstrapsweresigni cantlylowerthanthecorrespondingquantitiesforthecontrolmember.Afteryielding,Beam-4andBeam-7thatwere

strengthened

Fig.10.Load–displacementcurvesofspecimensstrengthenedwithwidesteel

straps.

Fig.11.Load–displacementcurvesofspecimensstrengthenedwithsteelplates.

with“L”shapedsteelstrapslostloadsuddenly,whentheshortlegsof“L”separatedfromtheconcretesurface.Attheultimateloadbothspecimensreachedapproximately60%lessdisplacementthanthecontrolspecimen.AscanbeseenfromFig.11,specimensthatwerestrengthenedwithsteelplatesalongthewholeshearspanshowedverysimilarload–displacementbehaviortothecontrolspecimen.Beam-9andBeam-10hadslightlymoreductilitythanthecontrolspecimen.Beam-11hadtheleastultimateductilityofallspecimensstrengthenedwithsteelplatesalongthewholeshearspan.Beam-11had20%lessultimatedisplacementthanthecontrolspecimen.3.3.Ductility

DisplacementductilityratiosofthespecimensarepresentedinTable3.Thatratiowascalculatedasthedisplacementatthemaximumloaddividedbythatattheyieldload.Beam-9andBeam-10thatwerestrengthenedwithsteelplatesalongthewholeshearspanhadmore

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Table4

ComparisonoftestandcalculatedresultsSpecimen#(1)Beam-1Beam-2Beam-3Beam-4Beam-5Beam-6Beam-7Beam-8Beam-9Beam-10Beam-11

(Control)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)(Strengthening)

ExperimentalstrengthsMU(kNm)(2)130.581.5117.2113.1122.2115.7113.4118.8129.9127.9124.6

VU(kN)(3)90.455.381.079.783.679.980.280.188.687.584.7

VU(Beam1)

VU

VU(Beam2)

VU

789

CalculatedstrengthsMU(kNm)(6)121.9122.6122.8122.4122.5122.5121.9122.0122.4122.4122.3

VU(kN)(7)104.653.586.185.9101.7101.7101.2117.0118.1117.6117.9

(4)1.000.610.900.880.930.880.890.890.980.970.94

(5)1.631.001.461.441.511.441.451.451.601.581.53

Experimental/calculatedMexp./Mcal.(8)1.070.660.950.921.000.940.930.971.061.051.02

Vexp./Vcal.(9)0.861.040.940.930.820.790.790.680.750.740.72

ductilitythanBeam-1.TheductilityratioofBeam-11was32%lessthanBeam-9’sductilityratio.Beam-4andBeam-7thatwerestrengthenedwith“L”shapedsteelstrapshadthelowestdisplacementductilityratiosamongthestrengthenedspecimens.

Beam-3andBeam-5thatwerestrengthenedwithnarrowsteelstrapsshowed44%and15%lessductilitythanBeam-1,respectively.Beam-6andBeam-8thatwerestrengthenedwithwidesteelstrapshadapproximately50%lessductilitythanBeam-1.Thebehaviorofthespecimensthatwerestrengthenedwithsteelstrapsshowedthatthespacingofthesteelstrapswascloselyrelatedtotheductilityratio.3.4.Strength

Effectsofthestrengtheningtechniqueonthespecimens’ultimatestrengthsaresummarizedinTable4.Ratiosoftheultimatestrengthofstrengthenedspecimenstothecontrolmember’sultimatestrengthwerebetween0.88and0.98.TheultimatestrengthsofBeam-4andBeam-7thatwerestrengthenedwith“L”shapedsteelstrapswereobtainedas12%lessthantheultimatestrengthofthecontrolmember.Thelargestultimatestrengthswereforthespecimensstrengthenedwithsteelplatesalongthewholeshearspan.TheratiosoftheultimatestrengthsofBeam-9,Beam-10andBeam-11totheultimatestrengthofthecontrolspecimenwere0.98,0.97and0.94,respectively.ThelargestincreaseinstrengthattheultimatestagewasforBeam-5forthespecimensstrengthenedwithsteelstraps.TheratiooftheultimatestrengthofBeam-5tothecontrolmember’sultimatestrengthwas0.93.

parisonofexperimentalandanalyticalresultsComparisonsofcalculatedandexperimentalstrengthsarepresentedinTable4.Forcalculationsofthemomentcapacitiesofthespecimens,themaximumconcretestrainwastakenas0.003.Calculated exuralcapacitiesagreed

wellwiththeexperimentalresultsforallspecimensexceptforBeam-2thatfailedinshear.

Thestrengthenedspecimens’shearforcecapacities(VU)werecalculatedbysummingtheshearforcecarriedbyconcrete(VC),theshearforcecarriedbytheshearreinforcements(VS)andtheshearforcecarriedbythebondedsteelmembers(VP)(Eq.(1)):VU=VC+VS+VPwhere:VU:VC:VS:VP:

Shearcapacity

Shearforcecarriedbyconcrete

ShearforcecarriedbyshearreinforcementsShearforcecarriedbysteelplatesorstraps.

(1)

Eqs.(2)and(3)wereusedforcalculatingtheshearloadcarriedbythesteelstrapsandplates,respectively[8].TheshearforcecarriedbytheconcretewascalculatedaccordingtoACIregulations[13]:

tShS

2τave2d

(2)VP=

SPwhere:VP:Sp:ts:hs:d:τave:

ShearforcecarriedbysteelstrapsSpacingofsteelstrapsWidthofsteelstrapsHeightofsteelstraps

EffectiveheightofcrosssectionAverageshearstressofepoxy.

(3)

dhW

VP=2τave

2where:VP:hW:d:τave:

ShearforcecarriedbysteelplatesHeightofsteelplates

EffectiveheightofcrosssectionAverageshearstressofepoxy.

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Whilecalculatingtheshearloadcarryingcapacityofsteelplates,thefailureoftheconnectionbetweenconcreteandsteelplateswasassumedtobestartedbyexceedingtheshearstrengthoftheepoxy.Asaresult,thesesteelplatespeeledfromtheconcretesurfaces.Theconcretestrengthofthespecimenswaschosensuchthatthefailureoftheconcretewasprevented,andthecrosssectionofthesteelplateswaschosensuchthattheyieldingofthesteelwasprevented.Asaresultofthesechoices,thefailureofthespecimenoccurreduponexceedingtheshearstrengthoftheepoxy.Theshearforcecarriedbythesteelmemberswasassumedtodependonthestrengthofpeelingoftheepoxyfromtheconcretesurface.VPwascalculatedbyassumingthatthepeelingofthesteelstrapsoccurredwhenthemaximumshearstressτmaxattheendsofthestrapsreachedtheinterfaceshearresistanceτult.Themanufacturer’sguaranteedultimateshearstressfortheepoxywas3.0MPa.Thesheardistributionoftheepoxyundersteelplateswassimilartothebondingstressofthereinforcementanchorageintheconcrete.Thisstresswasdeterminedbyaveragingalongthesteelplates.Asaresult,theaverageshearstresswastakenasτ=0.8MPaforspecimensthatwereavestrengthened=1.2MPaandτavewithsteelstrapsandspecimensthatwerestrengthenedwithsteelplates,respectively[8].

Thecalculatedshearstrengthswerelargerthanthemeasuredshearstrengthsforallspecimens.TheratiosofthemeasuredshearstrengthtothecalculatedshearstrengthforBeam-3,Beam-4andBeam-5thatwerestrengthenedwithsteelstrapsrangedbetween0.82and0.94.Thecalculatedshearcapacitieswere25%largerthanthemeasuredvaluesonaverageforBeam-6,Beam-7andBeam-8thatwerestrengthenedwithwidesteelstraps.Thecalculatedshearcapacitieswere26%largerthanthemeasuredvaluesonaverageforBeam-9,Beam-10andBeam-11thatwerestrengthenedwithsteelplates.

Calculatedshearstrengthslargerthanthemeasuredonesweretobeexpectedbecausewhencalculatingtheshearforcescarriedbythesteelmembers,allthebondingsurfacesweretakenasfullyeffective.But,intherealcase,eventhoughallprecautionsweretakenandbestpracticewasfollowed,uniformbondingwasnotobtainedforthesteelandtheconcreteoverthefullbondingsurface.Soallbondedinterfacesdidnotcarrytheshearforceeffectively.

4.Conclusion

Inthisstudy,strengtheningofRCbeamsagainstshearbyusingepoxybondedsteelplateswithdifferentarrangementswasinvestigated.Thesuccessofthestrengtheningtechniquewascloselyrelatedtothequalityoftheconstruction.Rougheningoftheconcreteandsteelsurfaces,cleaningofthesesurfacesandcomplyingcompletelywiththespeci edepoxyapplicationprocedureswascrucialforsuccessfulbonding.Generalresultsobtainedfromtheexperimentalresearchareasfollows:

Allsteelmembertypesbondedexternallyhadimprovedbeamstrength,stiffnessandductility.

Strengthenedspecimensshowedsimilarbehaviortoacontrolspecimenupto exuralyield.Specimensreachedthe exuralyieldstrengthwiththesamestiffnessfornearlythesameloadanddisplacement.

Thetypeofsteelmemberanditsarrangementonthebeamwereamongtheeffectiveparametersdirectingtheductilitybehavioranddeterminingthefailuremode.Thedisplacementductilityratiowasincreasedwhenthespacingofthesteelstrapswasdecreased.Increaseinthebondingareaontheshearspanreducedthepropagationofshearcrackssigni cantly.Specimensthatwerestrengthenedwith“L”typesteelstrapshadthelowestductilityratioamongthespecimens.

Specimensthatwerestrengthenedwithsteelplatesshowedstrengthandductilityclosetothoseofthecontrolmember.Steelplatespreventedpropagationofshearcracks,clearly.Insteadofusingonelargesteelplatealongthewholeoftheshearspan,segmentingthesteelplatesandthenbondingthemadjacenttoeachothertothebeam’sshearspanshowedsuccessfulresults.

Thefollowingcanbesuggestedforfutureworkonthissubject:thestrengtheningtechniqueshouldbeinvestigatedundercyclicreversalloads;thebehaviorsofthespecimensshouldberesearchedunderlongtermloads;andanalyticalmethodsforcalculatingshearcapacitiesofstrengthenedbeamsshouldbeimproved.

Conversionfactors1mm=0.039in.1mm2=0.00152in.21kN=0.2248kips1MPa=145psi

Symbolsa:Shearspan

d:Effectiveheightofthecrosssectionfc:Compressionstrengthofconcretefsy:Yieldstrengthofreinforcement

fspy:YieldstrengthofsteelstrapsandplateshS,hW:Steelstrapheight,steelplateheightL:Restraintspanofbeam

Mcal.:CalculatedmomentcapacitiesofspecimensMexp.:ExperimentalmomentcapacitiesofspecimensMU:SpecimenmomentcapacitiesVC:ShearforcecarriedbyconcreteVU:Shearcapacitiesofspecimens

VS:ShearforcecarriedbyshearreinforcementsVP:ShearforcecarriedbysteelstrapsorplatesSp:

Steelstrapspacing

粘钢加固钢筋混凝土梁斜截面

S.Altinetal./EngineeringStructures27(2005)781–791791

ts:

SteelstrapwidthδDisplacements

ε1,...,δ11:CU:Maximumstrainofconcreteφρ:Diameterofreinforcementsw:

Ratioofshearreinforcements

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