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外文文献：

MaterialsandStructures

©RILEM2010

10.1617/S11527-010-970

0-y

OriginalArticle

Impactofcrackwidthonbond:confinedandunc

onfinedrebar

DavidW.Law\DengleiTang^ThomasK.C.Molyneaux^ndRebecca

Gravina3

(1SchooloftheBuiltEnvironment,HeriotWattUniversity,Edinburgh,E

)H144AS,UK

(2VicRoads,Melbourne,VIC,Au

)stralia

(3SchoolofCivil,EnvironmentalandChemicalEngineering,RMITUnive

)rsity,Melbourne,VIC,3000,Australia

DavidW.Law

Email:D.W.Law@hw.a

c.uk

Received:14January2010Accepted:14December2010Publis

hedonline:23December2010

Abstract

Thispaperreportstheresultsofaresearchprojectcomparingth

eeffectofsurfacecrackwidthanddegreeofcorrosiononthebon

dstrengthofconfinedandunconfineddeformedl2andl6mm

mildsteelreinforcingbars.Thecorrosionwasinducedbychlorid

econtaminationoftheconcreteandanappliedDCcurrent.Thep

rincipalparametersinvestigatedwereconfinementofthereinf

orcement,thecoverdepth,bardiameter,degreeofcorrosionan

dthesurfacecrackwidth.Theresultsindicatedthatpotentialrel

ationshipbetweenthecrackwidthandthebondstrength.There

sultsalsoshowedanincreaseinbondstrengthatthepointwhere

initialsurfacecrackingwasobservedforbarswithconfiningstirr

ups.Nosuchincreasewasobservedwithunconfinedspecimens

Keywords:bond;corrosion;rebar;cover;crackwidth;concret

e

llntroduction

Thecorrosionofsteelreinforcementisamajorcauseofthed

eteriorationofreinforcedconcretestructuresthroughoutthew

orld.Inuncorrodedstructuresthebondbetweenthesteelreinfo

rcementandtheconcreteensuresthatreinforcedconcreteactsi

nacompositemanner.However,whencorrosionofthesteelocc

ursthiscompositeperformanceisadverselyaffected.Thisisdue

totheformationofcorrosionproductsonthesteelsurface,whic

haffectthebondbetweenthesteelandtheconcrete.

Thedeteriorationofreinforcedconcreteischaracterizedb

yageneralorlocalizedlossofsectiononthereinforcingbarsand

theformationofexpansivecorrosionproducts.Thisdeteriorati

oncanaffectstructuresinanumberofways;theproductionofex

pansiveproductscreatestensilestresseswithintheconcrete,w

hichcanresultincrackingandspallingoftheconcretecover.This

crackingcanleadtoacceleratedingressoftheaggressiveagent

scausingfurthercorrosion.Itcanalsoresultinalossofstrengtha

ndstiffnessoftheconcretecover.Thecorrosionproductscanals

oaffectthebondstrengthbetweentheconcreteandthereinfor

cingsteel.Finallythecorrosionreducesthecrosssectionofthere

inforcingsteel,whichcanaffecttheductilityofthesteelandthel

oadbearingcapacity,whichcanultimatelyimpactupontheserv

iceabilityofthestructureandthestructuralcapacity[12,25].

Previousresearchhasinvestigatedtheimpactofcorrosion

onbond[2-5,7,12,20,23-25,27,29],withanumberofmodelsbe

ingproposed[4,6,9,10,18,19,24,29].Themajorityofthisresearc

hhasfocusedontherelationshipbetweenthelevelofcorrosion(

masslossofsteel)orthecurrentdensitydegree(corrosioncurre

ntappliedinacceleratedtesting)andcrackwidth,orontherelati

onshipbetweenbondstrengthandlevelofcorrosion.Otherres

earchhasinvestigatedthemechanicalbehaviourofcorrodedst

eel[l,ll]andthefrictioncharacteristics[13],However,littlerese

archhasfocusedontherelationshipbetweencrackwidthandbo

nd[23,26,28],aparameterthatcanbemeasuredwithrelativeea

seonactualstructures.

Thecorrosionofthereinforcingsteelresultsintheformatio

nofironoxideswhichoccupyalargervolumethanthatofthepar

entmetal.Thisexpansioncreatestensilestresseswithinthesurr

oundingconcrete,eventuallyleadingtocrackingofthecoverco

ncrete.Oncecrackingoccursthereisalossofconfiningforcefro

mtheconcrete.Thissuggeststhatthelossofbondcapacitycoul

dberelatedtothelongitudinalcrackwidth[12].However,theus

eofconfinementwithintheconcretecancounteractthislossofb

ondcapacitytoacertaindegree.Researchtodatehasprimarilyi

nvolvedspecimenswithconfinement.Thispaperreportsastud

ycomparingthelossofbondofspecimenswithandwithoutconf

inement.

2Experimentalinvestigation

2.1Specimens

Beamendspecimens[28]wereselectedforthisstudy.Thist

ypeofeccentricpulloutor'beamend'typespecimenusesab

ondedlengthrepresentativeoftheanchoragezoneofatypicals

implysupportedbeam.Specimensofrectangularcrosssection

werecastwithalongitudinalreinforcingbarineachcorner,Fig.l.

An80mmplastictubewasprovidedatthebarundemeaththetra

nsversereactiontoensurethatthebondstrengthwasnotenhan

cedduetoa(transverse)compressiveforceactingonthebarove

rthislength.

Fig.lBeamendspecimen

Deformedrebarofl2andl6mmdiameterwithcoverofthre

etimesbardiameterwereinvestigated.Duplicatesetsofconfin

edandunconfinedspecimensweretested.Theconfinedspeci

menshadthreesetsof6mmstainlesssteelstirrupsequallyspace

dfromtheplastictube,at75mmcentres.

Thisrepresentsfourgroupsofspecimenswithacombinati

onofdifferentbardiameterandwith/withoutconfinement.The

specimenswereselectedinordertoinvestigatetheinfluenceof

barsize,confinementandcrackwidthonbondstrength.

2.2Materials

ThemixdesignisshownJablel.ThecementwasTypelPortl

andcement,theaggregatewasbasaltwithspecificgravity2.99.

Thecoarseandfineaggregatewerepreparedinaccordancewit

hAS1141-2000.MixingwasundertakeninaccordancewithASl

012.2-1994.Specimenswerecuredfor28daysunderwethessia

nbeforetesting.

TablelConcretemixdesign

10mm7mmw

MaterCemenw/washeasheda

SandSaltSlump

ialtcdaggreggrega

gatete

Quant381kg/0.4517kg/463kg/463kg/18.84kg/

140±25mm

itym39m3m3m3m3

Inordertocomparebondstrengthforthedifferentconcret

ecompressivestrengths,Eq.lisusedtonormalizebondstrengt

hfornon-corrodedspecimensashasbeenusedbyotherresearc

her[8].

⑴

whereisthebondstrengthforgrade40concrete,Texptlisth

eexperimentalbondstrengthandfcistheexperimentalcompre

ssivestrength.

Thetensilestrengthofthe<J>12and<I>16mmsteelbarswasn

ominally500MPa,whichequatestoafailureloadof56.5andl00.

5kN,respectively.

2.3Experimentmethodology

Acceleratedcorrosionhasbeenusedbyanumberofauthor

storeplicatethecorrosionofthereinforcingsteelhappeningint

henaturalenvironment[2,3,5,6,10,18,20,24,27,28,30].Theseh

aveinvolvedexperimentsusingimpressedcurrentsorartificial

weatheringwithwet/drycyclesandelevatedtemperaturestore

ducethetimeuntilcorrosion,whilemaintainingdeterioration

mechanismsrepresentativeofnaturalexposure.Studiesusingi

mpressedcurrentshaveusedcurrentdensitiesbetweenlOO|jA

/cm2and500mA/cm2[20].Researchhassuggestedthatcurren

tdensitiesupto200|jA/cm2resultinsimilarstressesduringthee

arlystagesofcorrosionwhencomparedtol00|jA/cm2[21].Ass

uchanappliedcurrentdensityof200|jA/cm2wasselectedforthi

sstudy—representativeofthelowerendofthespectrumofsuch

currentdensitiesadoptedinpreviousresearch.However,cauti

onshouldbeappliedwhenacceleratingthecorrosionusingimp

ressedcurrentastheaccelerationprocessdoesnotexactlyrepli

catethemechanismsinvolvedinactualstructures.Inaccelerate

dteststhepitsarenotallowedtoprogressnaturally,andtherem

aybeamoreuniformcorrosiononthesurface.Alsotherateofcor

rosionmayimpactonthecorrosionproducts,suchthatdifferen

toxidationstateproductsmaybeformed,whichcouldimpacto

nbond.

Thesteelbarsservedastheanodeandfourmildsteelmetalp

lateswerefixedonthesurfacetoserveascathodes.Sponges(spr

ayedwithsaltwater)wereplacedbetweenthemetalplatesandc

oncretetoprovideanadequatecontact,Fig.2.

Fig.2Acceleratedcorrosionsystem

Whentherequiredcrackwidthwasachievedforaparticular

bar,theimpressedcurrentwasdiscontinuedforthatbar.Thespe

cimenwasremovedforpullouttestingwhenallfourlocationsex

hibitedthetargetcrackwidth.AveragesurfacecrackwidthsofO.

05,0.5,landl.5mmwereadoptedasthetargetcrackwidths.The

surfacecrackwidthwasmeasuredat20mmintervalsalongthele

ngthofthebar,beginning20mmfromtheendofthe(plastictub

e)bondbreakerusinganopticalmicroscope.Thelevelofaccura

cyinthemeasurementswas±0.02mm.Measurementsofcrack

widthweretakenonthesurfacenormaltothebardirectionregar

dlessoftheactualcrackorientationatthatlocation.

Bondstrengthtestswereconductedbymeansofahandope

ratedhydrauliqackandacustom-builttestrigasshowninFig.3.

TheloadingschemeisillustratedinFig.4.Aplastictubeoflength

80mmwasprovidedattheendoftheconcretesectionundernea

ththetransversereactiontoensurethatthebondstrengthwasn

otenhancedbythereactive(compressive)force(actingnormalt

othebar).Thespecimenwaspositionedsothatanaxialforcewas

appliedtothebarbeingtested.Therestraintsweresufficientlyri

gidtoensureminimalrotationortwistingofthespecimendurin

gloading.

Fig.3Pull-outtest,16mmbarunconfined

Fig.4Schematicofloading./Voteonlytestbarshownforclarity

3Experimentalresultsanddiscussion

3.1Visualinspection

Followingtheacceleratedcorrosionphaseeachspecimen

wasvisuallyinspectedforthelocationofcracks,meancrackwidt

handmaximumcrackwidth(Sect.2.3).

Whileeachspecimenhadameantargetcrackwidthforeach

bar,variationsinthiscrackwidthwereobservedpriortopulloutt

esting.Thisisduetocorrosionandcrackingbeingadynamicpro

cesswithcrackspropagatingatdifferentrates.Thus,whileindivi

dualbarsweredisconnected,oncethetargetcrackwidthhadbe

enachieved,corrosionandcrackpropagationcontinued(toso

meextent)untilallbarshadachievedthetargetcrackwidthandp

ullouttestsconducted.Thisresultedinarangeofdataforthema

ximumandmeancrackwidthsforthepullouttests.

Thevisualinspectionofthespecimensshowedthreestages

tothecrackingprocess.Theinitialcracksoccurredinaveryshort

period,usuallygeneratedwithinafewdays.Afterthat,mostcrac

ksgrewataconstantrateuntiltheyreachedlmm,3-4weeksafte

rfirstcracking.Aftercrackshadreachedlmmtheythengrewver

yslowly,withsomecracksnotincreasingatall.Fortheconfineda

ndunconfinedspecimensthesurfacecrackstendedtooccuront

hesideofthespecimens(asopposedtothetoporbottom)andto

followthelineofthebars.Inthecaseoftheunconfinedspecimen

singeneraltheseweretheonlycrackwhileitwascommoninthec

asesofconfinedspecimenstoobservecracksthatwerealignedv

erticallydowntheside-adjacenttooneofthelinks,Fig.5.

Fig.5Typicalcrackpatterns

Duringthepull-outtestingthemostcommonfailuremode

forbothconfinedandunconfinedwassplittingfailure—withth

einitial(pre-test)crackscausedbythecorrosionenlargingunde

rloadandultimatelyleadingtothesectionfailingexhibitingspal

lingofthetopcorner/edge,Fig.6.Howeverforseveraloftheconf

inedspecimens,asecondmodeoffailurealsooccurredwithdia

gonal(shearlike)cracksappearinginthesidewalls,Fig.7.Theap

pearanceofthesecracksdidnotappeartoberelatedtotheprese

nceofverticalcracksobserved(inspecimenswithstirrups)durin

gthecorrosionphaseasreportedabove.

Fig.6Longitudinalcrackingafterpull-out

Fig.7Diagonalcrackingafterpull-out

Thebarswereinitially(precasting)cleanedwithal2%hydr

ochloricacidsolution,thenwashedindistilledwaterandneutral

izedbyacalciumhydroxidesolutionbeforebeingwashedindist

illedwateragain.Followingthepull-outtests,thecorrodedbars

werecleanedinthesamewayandweighedagain.

Thecorrosiondegreewasdeterminedusingthefollowinge

quation

whereGOistheinitialweightofthesteelbarbeforecorrosio

n,Gisthefinalweightofthesteelbarafterremovalofthepost-tes

tcorrosionproducts,gOistheweightperunitlengthofthesteelb

ar(0.888andl.58g/mmfor<t>12and(P16mmbars,respectively),

listheembeddedbondlength.

Figures8and9showsteelbarswithvaryingdegreeofcorros

ion.Themajorityexhibitedvisiblepitting,similartothatobserve

donreinforcementinactualstructures,Fig.9.However,asmalln

umberofothersexhibitedsignificantoverallsectionloss,witha

moreuniformlevelofcorrosion,Fig.8,whichmaybeafunctionof

theaccelerationmethodology.

Fig.8Corrodedl2mmbarwithapproximately30%massloss

Fig.9Corrodedl6mmbarwithapproximatelyl5%massloss

3.2Bondstressandcrackwidth

FigurelOshowsthevariationofbondstresswithmeancrack

widthforl6mmbarsandFig.llforthel2mmbars.Figuresl2an

dl3showthedataforthemaximumcrackwidth.

Fig.lOMeancrackwidthversusbondstressforl6mmbars

Fig.llMeancrackwidthversusbondstressforl2mmbars

Fig.l2Maximumcracl<widthversusbondstressforl6mmbars

Fig.l3Maximumcrackwidthversusbondstressforl2mmbars

Thedatashowaninitialincreaseinbondstrengthforthel2

mmspecimenswithstirrups,followedbyasignificantdecreasei

nbond,whichisinagreementwithotherauthors[12,15].Forthe

16mmspecimensanincreaseonthecontrolbondstresswasobs

ervedforspecimenswith0.28and0.35mmmeancrackwidths,h

owever,adecreaseinbondstresswasobservedforatthemeancr

ackwidthof0.05mm.

Thel2mmbarswithstirrupsdisplayedanincreaseinbonds

tressofapproximately25%fromthecontrolvaluestothemaxim

umbondstress.Anincreaseofapproximatelyl4%wasobserve

dforthel6mmspecimens.Otherresearchers[17,24,25]havere

portedenhancementsofbondstressofbetweenl0and60%du

etoconfinement,slightlyhighertothatobservedintheseexperi

ment.Howevertheloadingtechniquesandcoverdepthshaven

otallbeenthesame.Variationsinexperimentaltechniquesinclu

deashorterembeddedlengthandalowercover.Thevariationo

ntheproposedempiricalrelationshipbetweenbondstrength,

degreeofcorrosion,barsize,cover,linkdetailsandtensilestren

gthpredictedbyRodriguez[24]hasbeendiscussedindetailinTa

ngetal.[28].Theanalysisdemonstratesthattherewouldbeanex

pectedenhancementofbondstrengthduetoconfinementofa

pproximately25%—correspondingtoachangeofbondstreng

thofapproximately0.75MPaforthel6mmbars(assessedata2

%sectionloss).Forthel2mmbarsthecorrespondingeffectofco

nfinementisfoundtobeapproximately35%correspondingtoa

1.0MPadifferenceinbondstress.Theexperimentalresults(14a

nd25%,above)are60-70%ofthesevalues.

Bothsetsofdataindicatearelationshipshowingdecreasin

gbondstrengthwith(visiblesurface)crackwidth.Aregressiona

nalysisofthebondstrengthdatarevealsabetterlinearrelations

hipwiththemaximumcrackwidthasopposedtothemeancrack

width(excludingtheuncrackedconfinedspecimens),Table2.

Table2Bestfitparameters,crackwidthversusbondstrength

UnconfinConfinUnconfinConfin

edl2mmedl2mmedl6mmedl6mm

UnconfinConfinUnconfinConfin

edl2mmedl2mmedl6mmedl6mm

Meancrackwidth

R20.9200.6370.6720.659

Slope(m)-3.997-3.653-2.999-8.848

Intercept

7.5608.1226.4968.746

(b)

Maximumcrackwidth

R20.9370.8550.7140.616

Slope(m)-2.719-2.968-1.815-5.330

Intercept

7.8058.4036.7079.636

(b)

Therewasalsoasignificantlybetterfitfortheunconfinedsp

ecimensthantheconfinedspecimens.Thisisconsistentwiththe

observationthatintheunconfinedspecimensthebondstrengt

hwillberelatedtothebondbetweenthebarsandtheconcrete,w

hichwillbeaffectedbythelevelofcorrosionpresent,whichitself

willinfluencethecrackwidth.Inconfinedspecimenstheconfini

ngsteelwillimpactuponboththebondandthecracking.

3.3Corrosiondegreeandbondstress

Itisapparentthat(Fig.l4)forcorrosiondegreeslessthan5%

thebondstresscorrelatedwell.However,asthedegreeofcorros

ionincreasedtherewasnoobservablecorrelationatall.Thiscon

trastswiththerelationshipbetweentheobservedcrackwidthan

dbondstress,whichgivesareasonablecorrelation,evenascrac

kwidthsincreaseto2and2.5mm.Apossibleexplanationforthis

variationisthatintheinitialstagesofcorrosionvirtuallyallthedis

solvedironionsreacttoformexpansivecorrosionproducts.This

reactionimpactsonboththebondstressandtheformationofcr

acks.However,oncecrackshavebeenformeditispossibleforth

eironionstobetransportedalongthecrackandoutoftheconcre

te.Asthebondhasalreadybeeneffectivelylostatthecrackanyir

onionsdissolvingatthecrackandbeingdirectlytransportedout

oftheconcretewillcauseanincreaseinthedegreeofcorrosio^b

utnotaffectthesurfacecrackwidth.Thelocation,orientationan

dchemistrywithinthecrackwillcontroltherelationshipbetwee

nbondstressanddegreeofcorrosion,whichwillvaryfromspeci

mentospecimen.Hencethelargevariationsincorrosiondegre

eandbondstressforhighlevelsofcorrosion.

Fig.l4Bondstressversuscorrosiondegree,12mmbars,unconfinedspecim

en

Significantlylargercrackwidthswereobservedfortheunco

nfinedspecimens,comparedtotheconfinedspecimenswithsi

milarlevelsofcorrosionandmasslost.Thelargestobservedcrac

kforunconfinedspecimenswas2.5mmcomparedtol.4mmfor

theconfinedspecimens.Thisisasexpectedandisadirectresulto

ftheconfinementwhichlimitsthedegreeofcracking.

3.4Effectofconfinement

Theunconfinedspecimensforbothl6andl2mmbarsdidn

otdisplaytheinitialincreaseinbondstrengthobservedforthec

onfinedbars.Indeedtheunconfinedspecimenswithcracksalldi

splayedareducedbondstresscomparedtothecontrolspecime

ns.Thisisinagreementwithotherauthors[16,24]findingsforcra

ckedspecimens.IncrackedcorrodedspecimensFangobserved

asubstantialreductioninbondstrengthfordeformedbarswith

outstirrups,whileRodriguezobservedbondstrengthsofhighly

corrodedcrackedspecimenswithoutstirrupswereclosetozero

,whilehighlycorrodedcrackedspecimenswithstirrupsretaine

dbondstrengthsofbetween3and4MPa.Inuncorrodedspecim

ensChananotedanincreaseinbondstrengthduetostirrupsofb

etweenl0and20%[14].HoweverRodriguezandFangobserve

dnovariationduetothepresenceofconfinementinuncorroded

bars.

Thedataisperhapsunexpectedasitcouldbeanticipatedth

atthecorrosionproductswouldleadtoanincreaseinbondduet

otheincreaseininternalpressures,causedbythecorrosionprod

uctsincreasingtheconfinementandmechanicalinterlockingar

oundthebar,coupledwithincreasedroughnessofthebarresult

inginagreaterfrictionbetweenthebarandthesurroundingcon

crete.However,thesepressureswouldthenrelievedbythesubs

equentcrackingoftheconcrete,whichwouldcontributetothed

ecreaseinthebondstrengthascrackwidthsincrease.Apossible

hypothesisisthatduetothelevelofcover,threetimesbardiamet

er,theeffectofconfinementbythestirrupsisreduced,suchthati

thaslittleimpactonthebondstressinuncrackedconcrete.How

ever,oncecrackinghastakenplacetheconfinementdoeshavea

beneficialeffectonthebond.

Itmayalsobethatthecompressivestrengthoftheconcrete

combinedwiththecoverwillhaveaneffectonthebondstressesf

oruncorrodedspecimens.Thedatapresentedherehasacovero

fthreetimesbardiameterandastrengthof40MPa,otherresearc

hrangesfroml.5tofourtimescoverwithcompressivestrengths

from40to77MPa.

3.5Comparisonofl2andl6mmrebar

Themaximumbondstressforl6mmunconfinedbarswas

measuredat8.06MPaandforthel2mmbarsitwas8.43MPa.The

sebothcorrespondedtothecontrolspecimenswithnocorrosio

n.Theunconfinedspecimensforboththel2andl6mmbarssho

wednoincreaseinbondstressduetocorrosion.Fortheconfined

specimensthemaximumbondstressforthecontrolspecimens

were7.29MPaforthel2mmbarsand6.34MPaforthel6mmbar

s.Themaximumbondstressforbothsetsofconfinedspecimens

correspondedtopointoftheinitialcracking.Themaximumbon

dstresseswereobservedatameancrackwidthofO.Olmmforthe

12mmbarsand0.28mmforthel6mmbars.Thecorresponding

bondstresseswere,8.45and7.20MPa.Overallthel2mmbarsdi

splayedhigherbondstressescomparedtothel6mmbarsatallc

rackwidths.Thisisattributedtoadifferentfailuremode.Thel6

mmspecimensdemonstratesplittingfailurewhilethel2mmba

rsbondfailure.

3.6Effectofcastingposition

Therewasnosignificantdifferenceofbondstrengthduetot

hepositionofthebar(toporbottomcast)oncecrackingwasobs

erved,Fig.l5.Forcontrolspecimens,withnocorrosion,howeve

r,thebottomcastbarshadaslightlyhigherbondstressthanthet

opcastbars.Theseobservationsareinagreementwithotheraut

hors[4,ll,15,22].Itisgenerallyacceptedthatuncorrodedbotto

mcastbarshavesignificantlyimprovedbondcomparedtotopc

astbarsduetothecorrosionproductsfillingthevoidsthatareoft

enpresentundertopcastbarsasthecorrosionprogresses[14].T

hecorrosionalsoactsasan'anchor',similartotheribsondefo

rmedbars,toincreasethebond.Overall,themeanvalueofbond

stressforallbars(corrodedanduncorroded)locatedinthetopw

erewithinl%ofthemeanbondstressofallbarslocatedinthebot

tomofthesection——forbothunconfinedandconfinedbars.This

isprobablyduetothelevelofcover.Theresultsreportedpreviou

slyareonspecimenswithonetimescover[14],However,atthree

timescoveritwouldbeanticipatedthatgreatercompactionwo

uldbeachievedaroundthetopcastbars.Thustheareaofvoidsw

ouldbereducedandthustheeffectofthecorrosionproductfillin

gthesevoidsandincreasingthebondstrengthwouldbereduce

d.

Fig.l5Bondstressversusmeancrackwidthforl2mmbars,topandbottomca

stpositions,confinedspecimen

Conclusions

Arelationshipwasobservedbetweencrackwidthandbond

stress.Thecorrelationwasbetterformaximumcrackwidthand

bondstressthanformeancrackwidthandbondstress.

Confinedbarsdisplayedahigherbondstressatthepointofi

nitialcrackingthanwherenocorrosionhadoccurred.Ascrackwi

dthincreasethebondstressreducedsignificantly.

Unconfinedbarsdisplayedadecreaseinbondstressatiniti

alcracking,followedbyafurtherdecreaseascrackingincreased.

Topcastbarsdisplayedahigherbondstressinspecimenswi

thnocorrosion.Oncecrackinghadoccurrednovariationbetwe

entopandbottomcastbarswasobserved.

Thel2mmbarsdisplayedhigherbondstressvaluesthanl6

mmwithnocorrosion,controlspecimens,andatsimilarcrackwi

dths.

Agoodcorrelationwasobservedbetweenbondstressand

degreeofcorrosionwasobservedatlowlevelsofcorrosion(less

than5%).However,athigherlevelsofcorrosionnocorrelationw

asdiscerned.

Overalltheresultsindicatedapotentialrelationshipbetwe

enthemaximumcrackwidthandthebond.Resultsshownherei

nshouldbeinterpretedwithcautionasthisvariationmaybenot

onlyduetovariationsbetweenacceleratedcorrosionandnatur

alcorrosionbutalsoduetothecomplexityofthecrackingmecha

nisminreality.

中文译文：

约束和无约束的钢筋对裂缝宽度的影响

收稿日期：2010年1月14纳稿日期：2010年12月14日

线上发表时间：2010年1月23日

摘要

本报告公布了局限约束和自由的变形对粘结强度12、16毫

米钢筋的表面腐蚀程度和裂纹影响的比较结果。腐蚀是氯化物污

染的混凝土的诱导和外加直流电流的引起的。调查的主要参数有

钢筋剥离，保护层厚度，钢筋直径，腐蚀程度和表面裂缝宽度。

结果表明了裂缝宽度和粘结强度之间的潜在关系。同时还发现在

围箍筋处发现表面裂纹的地方粘结强度增加，而无侧限的样本中

没有观察到粘结强度增加。

关键词：粘结；腐蚀；螺纹钢；保护层；裂缝宽度；混凝土

引言

在世界各地，钢筋的腐蚀是钢筋混凝土结构的恶化的重要原

因。在未腐蚀的结构中钢筋和混凝土之间的粘结使钢筋混凝土处

于有利状态。然而，当钢铁的腐蚀发生时，会对这种积极性能产

生不利影响。这是由于钢表面形成了腐蚀产物，从而影响了钢和

混凝土之间的粘结。

钢筋混凝土恶化是由钢筋和形成的膨胀腐蚀产物造成的局部

损失。这种情况的恶化在许多方面影响结构;膨胀产品的产生造成

混凝土的拉应力，这可能会导致混凝土保护层开裂和剥落的。这

种开裂可导致更严重的恶化和进一步的腐蚀。它也可以导致在混

凝土保护层的强度和刚度的损失。腐蚀产物也可以影响混凝土与

钢筋之间的粘结强度。最终腐蚀减少钢筋截面面积，影响钢筋的

延展性和承载能力，从而最终影响结构适用性和结构承载力[12,

25]0

以往的研究调查腐蚀对粘结的影响[2-5,7,12,20,23-25

，27,29],提出了数据模型[4,6,9,10,18,1924,29]o

本研究主要研究腐蚀（钢材质量损失）水平或电流密度程度（腐

蚀电流在加速测试中的应用）和裂缝宽度之间的关系，或粘结强

度和腐蚀程度之间的关系。其他研究已调查的锈蚀力学性能［1,

11］和摩擦特性［13］。然而，很少有人研究都集中在裂缝宽度与粘

结［23,26,28］之间的关系上，此参数易与实际结构相联系。

加强钢筋的腐蚀导致生成铁氧化物，它的体积大于原钢材。

这种扩张造成周围的混凝土内的拉应力，最终导致混凝土保护层

开裂。一旦开裂发生，混凝土紧箍力就会损失。这表明粘结能力

的损失可能与纵向裂缝宽度有关［12］。然而，以混凝土的剥离可

以在一定程度上抵消粘结力的损失。最新研究主要与剥离样本有

关。本文报道的一项研究比较了有侧限和无侧限样本的粘结力损

失。

2.实验研究

2.1样本

梁端样本［28］被选定为这项研究的研究对象。这种撤去偏心

或"梁端”模式样本以一个典型的简支梁锚固区的粘结长度支撑。

样本的矩形截面投在纵向钢筋的各处，如图1。由于没有增强下

方横反应的钢筋，试样提供了一个80毫米的塑料管,以确保粘结

强度（横向）压缩力超过这个长度的钢筋。

图1梁端试样

试验调查了由3倍直径厚的保护层保护的12和16毫米直径

的钢筋。重复测试有侧限和自由样本。在密闭的塑料管中有3套

6毫米的不锈钢箍筋从其间穿过，在75毫米中心。

这代表了四组不同钢筋直径和有侧限/无约束的样本。以调查

钢筋规格，混凝土剥离和裂缝宽度对粘结强度的影响。

2.2材料

配合比设计,如表1所示。水泥是I型硅酸盐水泥，骨料为

玄武岩溶重2.99。根据AS1141—2000进行粗、细集料的制备。

拌合根据AS1141—1994进行。测试前水浴养护28天。

表1混凝土配合比设计

材水泥w/c砂10mm7mm盐含量塌落度

料集料集料

结381kg/m30.4517kg/m3kg/m3kg/m3kg/m3140±

果25mm

为了比较不同的混凝土抗压强度，粘结强度，Eq。公式1已

被其他研究者用于正常化粘结强度的非腐蚀样本。

1

为40级混凝土的粘结强度，exptl为实验粘结强度和R是实

验抗压强度。

①12和①16毫米钢筋的抗拉强度是500兆帕，分别相当于一

个56.5和100.5kN的破坏载荷。

2.3实验方法

加速腐蚀已被许多作者用于重现在自然环境中发生的腐蚀钢

筋钢［2,3,5,6,10,18,20,24,27,28,30］,这些相关

实验使用外加电流或干湿周期人工风化和升高温度延缓腐蚀时

间，同时保持恶化机制处于自然状态。采用外加电流的研究使用

的电流密度在100kiA/cm2与500mA/cm2之间［20］。有研究表

明，电流密度200|jA/cm2与100pA/cm2相比，200的结果与

早期阶段的腐蚀更相似［21］。随着施加电流密度200pA/cm2被

选定为研究使用电流，这在以前的研究中成为电流密度频谱的低

端代表。然而，应谨慎应用外加电流的加速腐蚀，加速过程并不

完全复制在实际结构中所涉及的机制。在加速测试中不允许违背

自然的发展，并有可能在表面上更均匀腐蚀。腐蚀率也可能会影

响腐蚀的产品，这些产品可能会形成不同的氧化状态，这可能会

影响粘结强度。

钢筋作为阳极和四个碳钢金属板固定在表面作为阴极。金属

板和混凝土之间放置海绵（用盐水喷洒）提供足够的接触，如图2。

图2加速腐蚀系统

当裂缝宽度要求需适应特殊钢筋时应该终止施加外加电流。

当所有四个位置出现规定的裂缝宽度，试样就会被拆除撤离测试。

平均表面裂缝宽度0.05,0.5,1和1.5毫米作为目标裂缝宽度。

表面裂纹宽度沿钢筋长度测量间隔20mm,从约束（塑料管）末

端开始20mm用断路器光学显微镜测量。测量精度为±0.02毫米。

从钢筋表面测量裂缝宽度，不考虑裂缝实际方位在何处。

粘结强度测试通过手动操作液压千斤顶和一个定制的试验装

置，如图3所示。加载方案见图4。长80毫米的塑料管在末端提

供了一个横向反应的具体部分，以确保粘结强度不会因为内力（压

力）提高而增加。样本定位使轴向力，适用于被测试的钢筋。给

样本足够刚性的约束可以确保在加载过程中最小的旋转或扭曲。

图3拉出测试，16毫米钢筋不承压

图4加载示意图。注：只测试显示棒

3实验结果与讨论

3.1目视检查

加速腐蚀阶段后，检查每个样本的裂缝的位置，平均裂缝宽

度和最大裂缝宽度（第2.3款）。

虽然每个钢筋样本都有平均目标裂缝宽度，但是裂缝宽度的

变化在观察前拉出测试。这是由于腐蚀和开裂是一个动态的过程,

裂缝是以不同的速度传播的。因此，当个别钢筋被拉断的时候，

一旦目标裂缝宽度已经达到，腐蚀和裂纹在一定程度上继续扩展,

直到所有的钢筋已达到目标的裂缝宽度，再终止试验进行。这产

生了一系列的最大裂缝和终止测试时的平均裂缝宽度数据。

视觉检测的样本显示了三个阶段的裂解过程。初始裂缝发生

在很短的时间内，通常在几天之内产生。在此之后，大多数裂缝

以一个恒定的速度增长，直到3-4周后首次开裂，他们达到1毫

米。裂缝达到了1毫米后，它们的增长速度非常缓慢，甚至一些

裂缝一点都不增加。侧限和自由的样本表面裂纹往往发生在侧面

（如对侧的顶部或底部），并沿钢筋方向发展。一般情况下无侧

限的样本只有仅有的一部分裂缝，而自由的样本裂缝的发展却十

分常见，观察到的裂缝垂直对齐下边，垂直向下侧相邻的链接，

如图5.

图5典型裂纹模式

在拉出测试时最常见的侧限和自由的故障是剥离失败，这是

由于随着在荷载作用下腐蚀的扩大形成裂缝，最终导致右上角/边

缘剥落，如图6。但是一些侧限的样本，存在第二种破坏模式，

在侧墙对角线出现裂缝，如图7。在腐蚀阶段，这些裂缝的出现

与观察到的垂直裂缝如上面报道的，并不相关。

图6拉出后纵向开裂

图7角开裂后拉出

钢筋最初（预制）由12%的盐酸溶液清洗，然后在蒸憎水清

洗，另外蒸悟水洗涤之前由氢氧化钙溶液中和。锈蚀钢筋拉出来

测试之后，以同样的方式进行清洗，并再次称重。

使用下列公式确定的腐蚀程度

其中Go是钢筋腐蚀前的初始重量，G是最终去除腐蚀产物后的测

试后的钢筋重量，g。是每单位长度的钢筋重量（①12和①16毫米

钢筋分别0.888和1.58g/«米）,I是嵌入式的键长。

图8和图9显示有不同程度的腐蚀钢筋。多数表现出可见的

凹陷，类似的实际结构，如图9。然而，少数其他钢筋表现出显

着的整体部分损失，更均匀的腐蚀水平，如图8,这可能是一个

加速方法的功能。

图812毫米钢筋腐蚀、约30%的质量损失

图916毫米钢筋腐蚀、约15%的质量损失

3.2粘结应力和裂缝宽度

图10显示了16毫米的钢筋粘结应力与平均裂缝宽度的变

化。图11为12毫米的钢筋的。图12和图13显示的最大