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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   Accepted:14 December   Publishedonline:23 December 
Abstract  
Thispaperreportstheresultsofaresearchprojectcomparingtheeffectofsurfacecrackwidthanddegreeofcorrosiononthebondstrengthofconfinedandunconfineddeformed12and16 ,thecoverdepth,bardiameter,.
Keywords  Bond - Corrosion - Rebar - Cover - Crackwidth - Concrete
1 Introduction
,,whichaffectthebondbetweenthesteelandtheconcrete.
;theproductionofexpansiveproductscreatestensilestresseswithintheconcrete,,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,[
1,11]andthefrictioncharacteristics[13].However,littleresearchhasfocusedontherelationshipbetweencrackwidthandbond[23,26,28],aparameterthatcanbemeasuredwithrelativeeaseonactualstructures.
,[12].However,.
2 Experimentalinvestigation
 Specimens
Beamendspecimens[28]‘beamend’,Fig.  mmplastictubewasprovidedatthebarunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedduetoa(transverse)compressiveforceactingonthebaroverthislength.
Fig. 1 Beamendspecimen
Deformedrebarof12and16  mmstainlesssteelstirrupsequallyspacedfromtheplastictube,at75 mmcentres.
Thisrepresentsfourgroupsofspecimenswithacombinationofdifferentbardiameterandwith/,confinementandcrackwidthonbondstrength.
 Materials
Themixdesignisshown,Table ,-.- daysunderwethessianbeforetesting.
Table 1 Concretemixdesign
Material
Cement
w/c
Sand
10 mmwashedaggregate
7 mmwashedaggregate
Salt
Slump
Quantity
381 kg/m3

517 kg/m3
463 kg/m3
463 kg/m3
 kg/m3
140 ± 25 mm
Inordertocomparebondstrengthforthedifferentconcretecompressivestrengths,Eq. 1isusedtonormalizebondstrengthfornon-corrodedspecimensashasbeenusedbyotherresearcher[8].
(1)
whereisthebondstrengthforgrade40concrete,τexptlistheexperimentalbondstrengthandfcistheexperimentalcompressivestrength.
ThetensilestrengthoftheΦ12andΦ16 mmsteelbarswasnominally500 MPa, kN,respectively.
 Experimentmethodology
Acceleratedcorrosionhasbeenusedbyanumberofauthorstoreplicatethecorrosionofthereinforcingsteelhappeninginthenaturalenvironment[2,3,5,6,10,18,20,24,27,28,30].Thesehaveinvolvedexperimentsusingimpressedcurrentsorartificialweatheringwithwet/drycyclesandelevatedtemperaturestoreducethetimeuntilcorrosion, μA/cm
2and500 mA/cm2[20].Researchhassuggestedthatcurrentdensitiesupto200 μA/cm2resultinsimilarstressesduringtheearlystagesofcorrosionwhencomparedto100 μA/cm2[21].Assuchanappliedcurrentdensityof200 μA/cm2wasselectedforthisstudy—,,,suchthatdifferentoxidationstateproductsmaybeformed,whichcouldimpactonbond.
(sprayedwithsaltwater)wereplacedbetweenthemetalplatesandconcretetoprovideanadequatecontact,Fig. 2.
Fig. 2 Acceleratedcorrosionsystem
Whentherequiredcrackwidthwasachievedforaparticularbar,,,  mmintervalsalongthelengthofthebar,beginning20 mmfromtheendofthe(plastictube)± .
Bondstrengthtestswereconductedbymeansofahandoperatedhydraulicjackandacustom-builttestrigasshowninFig. .  mmwasprovidedattheendoftheconcretesectionunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedbythereactive(compressive)force(actingnormaltothebar)..
Fig. 3 Pull-outtest,16 mmbarunconfined
Fig. 4 :onlytestbarshownforclarity
3 Experimentalresultsanddiscussion
 Visualinspection
Followingtheacceleratedcorrosionphaseeachspecimenwasvisuallyinspectedforthelocationofcracks,meancrackwidthandmaximumcrackwidth(Sect. ).
Whileeachspecimenhadameantargetcrackwidthforeachbar,,whileindividualbarsweredisconnected,oncethetargetcrackwidthhadbeenachieved,corrosionandcrackpropagationcontinued(tosomeextent).
,,mostcracksgrewataconstantrateuntiltheyreached1 mm,3–4  mmtheythengrewveryslowly,(asopposedtothetoporbottom)—adjacenttooneofthelinks,Fig. 
5.
Fig. 5 Typicalcrackpatterns
Duringthepull-outtestingthemostcommonfailuremodeforbothconfinedandunconfinedwassplittingfailure—withtheinitial(pre-test)crackscausedbythecorrosionenlargingunderloadandultimatelyleadingtothesectionfailingexhibitingspallingofthetopcorner/edge,Fig. ,asecondmodeoffailurealsooccurredwithdiagonal(shearlike)cracksappearinginthesidewalls,Fig. (inspecimenswithstirrups)duringthecorrosionphaseasreportedabove.
Fig. 6 Longitudinalcrackingafterpull-out
Fig. 7 Diagonalcrackingafterpull-out
Thebarswereinitially(precasting)cleanedwitha12%hydrochloricacidsolution,-outtests,thecorrodedbarswerecleanedinthesamewayandweighedagain.