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土木工程外文文献及翻译
外文文献:
MaterialsandStructures
?
?
RILEM?
2010
10.1617/s11527-010-9700-y
OriginalArticle
Impactofcrackwidthonbond:
confinedandunconfinedrebar
David?
W.?
Law1?
Denglei?
Tang2,Thomas?
K.?
C.?
Molyneaux3andRebecca?
Gravina3
(1)?
SchooloftheBuiltEnvironment,HeriotWattUniversity,Edinburgh,EH144AS,UK
(2)?
VicRoads,Melbourne,VIC,Australia
(3)?
SchoolofCivil,EnvironmentalandChemicalEngineering,RMITUniversity,Melbourne,VIC,3000,Australia
David?
W.?
Law
Email:
Received:
14?
January?
2010?
?
Accepted:
14?
December?
2010?
?
Publishedonline:
23?
December?
2010
Abstract
Thispaperreportstheresultsofaresearchprojectcomparingtheeffectofsurfacecrackwidthanddegreeofcorrosiononthebondstrengthofconfinedandunconfineddeformed12and16?
mmmildsteelreinforcingbars.ThecorrosionwasinducedbychloridecontaminationoftheconcreteandanappliedDCcurrent.Theprincipalparametersinvestigatedwereconfinementofthereinforcement,thecoverdepth,bardiameter,degreeofcorrosionandthesurfacecrackwidth.Theresultsindicatedthatpotentialrelationshipbetweenthecrackwidthandthebondstrength.Theresultsalsoshowedanincreaseinbondstrengthatthepointwhereinitialsurfacecrackingwasobservedforbarswithconfiningstirrups.Nosuchincreasewasobservedwithunconfinedspecimens.
Keywords:
?
?
bond?
;corrosion?
;?
rebar?
;?
cover?
;?
crackwidth?
;?
concrete
1?
Introduction
Thecorrosionofsteelreinforcementisamajorcauseofthedeteriorationofreinforcedconcretestructuresthroughouttheworld.Inuncorrodedstructuresthebondbetweenthesteelreinforcementandtheconcreteensuresthatreinforcedconcreteactsinacompositemanner.However,whencorrosionofthesteeloccursthiscompositeperformanceisadverselyaffected.Thisisduetotheformationofcorrosionproductsonthesteelsurface,whichaffectthebondbetweenthesteelandtheconcrete.
Thedeteriorationofreinforcedconcreteischaracterizedbyageneralorlocalizedlossofsectiononthereinforcingbarsandtheformationofexpansivecorrosionproducts.Thisdeteriorationcanaffectstructuresinanumberofways;theproductionofexpansiveproductscreatestensilestresseswithintheconcrete,whichcanresultincrackingandspallingoftheconcretecover.Thiscrackingcanleadtoacceleratedingressoftheaggressiveagentscausingfurthercorrosion.Itcanalsoresultinalossofstrengthandstiffnessoftheconcretecover.Thecorrosionproductscanalsoaffectthebondstrengthbetweentheconcreteandthereinforcingsteel.Finallythecorrosionreducesthecrosssectionofthereinforcingsteel,whichcanaffecttheductilityofthesteelandtheloadbearingcapacity,whichcanultimatelyimpactupontheserviceabilityofthestructureandthestructuralcapacity[12,25].
Previousresearchhasinvestigatedtheimpactofcorrosiononbond[2–5,7,12,20,23–25,27,29],withanumberofmodelsbeingproposed[4,6,9,10,18,19,24,29].Themajorityofthisresearchhasfocusedontherelationshipbetweenthelevelofcorrosion(masslossofsteel)orthecurrentdensitydegree(corrosioncurrentappliedinacceleratedtesting)andcrackwidth,orontherelationshipbetweenbondstrengthandlevelofcorrosion.Otherresearchhasinvestigatedthemechanicalbehaviourofcorrodedsteel[1,11]andthefrictioncharacteristics[13].However,littleresearchhasfocusedontherelationshipbetweencrackwidthandbond[23,26,28],aparameterthatcanbemeasuredwithrelativeeaseonactualstructures.
Thecorrosionofthereinforcingsteelresultsintheformationofironoxideswhichoccupyalargervolumethanthatoftheparentmetal.Thisexpansioncreatestensilestresseswithinthesurroundingconcrete,eventuallyleadingtocrackingofthecoverconcrete.Oncecrackingoccursthereisalossofconfiningforcefromtheconcrete.Thissuggeststhatthelossofbondcapacitycouldberelatedtothelongitudinalcrackwidth[12].However,theuseofconfinementwithintheconcretecancounteractthislossofbondcapacitytoacertaindegree.Researchtodatehasprimarilyinvolvedspecimenswithconfinement.Thispaperreportsastudycomparingthelossofbondofspecimenswithandwithoutconfinement.
2?
Experimentalinvestigation
2.1?
Specimens
Beamendspecimens[28]wereselectedforthisstudy.Thistypeofeccentricpulloutor‘beamend’typespecimenusesabondedlengthrepresentativeoftheanchoragezoneofatypicalsimplysupportedbeam.Specimensofrectangularcrosssectionwerecastwithalongitudinalreinforcingbarineachcorner,Fig.?
1.An80?
mmplastictubewasprovidedatthebarunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedduetoa(transverse)compressiveforceactingonthebaroverthislength.
Fig.?
1?
Beamendspecimen
Deformedrebarof12and16?
mmdiameterwithcoverofthreetimesbardiameterwereinvestigated.Duplicatesetsofconfinedandunconfinedspecimensweretested.Theconfinedspecimenshadthreesetsof6?
mmstainlesssteelstirrupsequallyspacedfromtheplastictube,at75?
mmcentres.
Thisrepresentsfourgroupsofspecimenswithacombinationofdifferentbardiameterandwith/withoutconfinement.Thespecimenswereselectedinordertoinvestigatetheinfluenceofbarsize,confinementandcrackwidthonbondstrength.
2.2?
Materials
Themixdesignisshown,Table?
1.ThecementwasTypeIPortlandcement,theaggregatewasbasaltwithspecificgravity2.99.ThecoarseandfineaggregatewerepreparedinaccordancewithAS1141-2000.MixingwasundertakeninaccordancewithAS1012.2-1994.Specimenswerecuredfor28?
daysunderwethessianbeforetesting.
Table?
1?
Concretemixdesign
Material
Cement
w/c
Sand
10?
mmwashedaggregate
7?
mmwashedaggregate
Salt
Slump
Quantity
381?
kg/m3
0.49
517?
kg/m3
463?
kg/m3
463?
kg/m3
18.84?
kg/m3
140?
±?
25?
mm
Inordertocomparebondstrengthforthedifferentconcretecompressivestrengths,Eq.?
1isusedtonormalizebondstrengthfornon-corrodedspecimensashasbeenusedbyotherresearcher[8].
(1)
where
isthebondstrengthforgrade40concrete,τexptlistheexperimentalbondstrengthandfcistheexperimentalcompressivestrength.
ThetensilestrengthoftheΦ12andΦ16?
mmsteelbarswasnominally500?
MPa,whichequatestoafailureloadof56.5and100.5?
kN,respectively.
2.3?
Experimentmethodology
Acceleratedcorrosionhasbeenusedbyanumberofauthorstoreplicatethecorrosionofthereinforcingsteelhappeninginthenaturalenvironment[2,3,5,6,10,18,20,24,27,28,30].Thesehaveinvolvedexperimentsusingimpressedcurrentsorartificialweatheringwithwet/drycyclesandelevatedtemperaturestoreducethetimeuntilcorrosion,whilemaintainingdeteriorationmechanismsrepresentativeofnaturalexposure.Studiesusingimpressedcurrentshaveusedcurrentdensitiesbetween100?
μA/cm2and500?
mA/cm2[20].Researchhassuggestedthatcurrentdensitiesupto200?
μA/cm2resultinsimilarstressesduringtheearlystagesofcorrosionwhencomparedto100?
μA/cm2[21].Assuchanappliedcurrentdensityof200?
μA/cm2wasselectedforthisstudy—representativeofthelowerendofthespectrumofsuchcurrentdensitiesadoptedinpreviousresearch.However,cautionshouldbeappliedwhenacceleratingthecorrosionusingimpressedcurrentastheaccelerationprocessdoesnotexactlyreplicatethemechanismsinvolvedinactualstructures.Inacceleratedteststhepitsarenotallowedtoprogressnaturally,andtheremaybeamoreuniformcorrosiononthesurface.Alsotherateofcorrosionmayimpactonthecorrosionproducts,suchthatdifferentoxidationstateproductsmaybeformed,whichcouldimpactonbond.
Thesteelbarsservedastheanodeandfourmildsteelmetalplateswerefixedonthesurfacetoserveascathodes.Sponges(sprayedwithsaltwater)wereplacedbetweenthemetalplatesandconcretetoprovideanadequatecontact,Fig.?
2.
Fig.?
2?
Acceleratedcorrosionsystem
Whentherequiredcrackwidthwasachievedforaparticularbar,theimpressedcurrentwasdiscontinuedforthatbar.Thespecimenwasremovedforpullouttestingwhenallfourlocationsexhibitedthetargetcrackwidth.Averagesurfacecrackwidthsof0.05,0.5,1and1.5?
mmwereadoptedasthetargetcrackwidths.Thesurfacecrackwidthwasmeasuredat20?
mmintervalsalongthelengthofthebar,beginning20?
mmfromtheendofthe(plastictube)bondbreakerusinganopticalmicroscope.Thelevelofaccuracyinthemeasurementswas±0.02?
mm.Measurementsofcrackwidthweretakenonthesurfacenormaltothebardirectionregardlessoftheactualcrackorientationatthatlocation.
Bondstrengthtestswereconductedbymeansofahandoperatedhydraulicjackandacustom-builttestrigasshowninFig.?
3.TheloadingschemeisillustratedinFig.?
4.Aplastictubeoflength80?
mmwasprovidedattheendoftheconcretesectionunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedbythereactive(compressive)force(actingnormaltothebar).Thespecimenwaspositionedsothatanaxialforcewasappliedtothebarbeingtested.Therestraintsweresufficientlyrigidtoensureminimalrotationortwistingofthespecimenduringloading.
Fig.?
3?
Pull-outtest,16?
mmbarunconfined
Fig.?
4?
Schematicofloading.Note:
onlytestbarshownforclarity
3?
Experimentalresultsanddiscussion
3.1?
Visualinspection
Followingtheacceleratedcorrosionphaseeachspecimenwasvisuallyinspectedforthelocationofcracks,meancrackwidthandmaximumcrackwidth(Sect.?
2.3).
Whileeachspecimenhadameantargetcrackwidthforeachbar,variationsinthiscrackwidth