w w w . j m r t . c o m . b r
Availableonlineatwww.sciencedirect.com
Original Article
Enhancing the tensile performance of
ultra-high-performance concrete through strategic use of novel half-hooked steel fibers
Doo-Yeol Yoo
a, Han-Kyu Sohn
a, Paulo H.R. Borges
b, Roman Fediuk
c, Soonho Kim
a,∗aDepartmentofArchitecturalEngineering,HanyangUniversity,222Wangsimni-ro,Seongdong-gu,Seoul,04763,RepublicofKorea
bDepartmentofCivilEngineering,FederalCentreforTechnologicalEducationofMinasGerais(CEFET-MG),Av.Amazonas7675,Belo Horizonte,30510-000,Brazil
cDepartmentofHydraulicEngineeringandtheTheoryofConstructions,FarEasternFederalUniversity,8,SukhanovaStr.,Vladivostok, 690950,Russia
a r t i c l e i n f o
Articlehistory:
Received21August2019 Accepted10January2020 Availableonline23January2020
Keywords:
Ultra-high-performanceconcrete Hybridreinforcement
Novelhalf-hookedsteelfiber Straightsteelfiber
Tensileperformance
a bs t r a c t
Thisstudyimprovedthetensileperformanceofultra-high-performanceconcrete(UHPC) throughhybridreinforcing system.Fourtypes ofsteelfibers, i.e.,smooth-straight (SS), hooked-end(HE),half-hooked(HH),andstraightenedhalf-hooked(SH),wereconsidered atreplacingratiosof0.5%and1.0%.Topreciselyevaluatethetensileperformance,their pulloutbehaviorsfromUHPCweresimultaneouslyanalyzed.Testresultsindicatedthatthe HEfiberprovidedthebestpulloutperformanceforthebondstrengthandpulloutwork,4.7 and3.7timeshigherthanthoseoftheSSfiber,respectively.Theorderofpulloutresistance wasasfollows:HEfiber>SHfiber>HHfiber>SSfiber.TheUHPCmatrixwithhybrid0.5%SH fibersand1.5%SSfibersshowedthebesttensileperformanceregardingthedeformability andenergyabsorptioncapacity,about45%and47%higherthanthoseofthecontrolspeci- menwith2%SSfibers,whereastheultimatetensilestrengthwasnotaffectedbythehybrid reinforcingsystem.Thehybrid0.5%HE(orSH)fiberand1.5%SSfiber,and1.0%HHfiber and1.0%SSfiberwereeffectiveinimprovingthetensileperformanceofUHPCavailableon themarket.Thehybridfiberspecimensalsoexhibitedbettercrackingbehaviorsthanthe controlspecimen.
©2020TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The American Concrete Institute (ACI) committee 239 [1]
definesultra-high-performanceconcrete(UHPC)asaconcrete
∗ Correspondingauthor.
E-mail:[email protected](S.Kim).
havingaminimumcompressivestrengthof150MPaandspec- ified durability, ductility,and toughness.In ordertosatisfy theductilityandtoughnessrequirements,fibersareincluded in general[2]. Theexcellent mechanicalstrength and self- compactingcharacteristics ofUHPC canbeattributedtoits finenesscomponents,i.e.,Portlandcement(PC),silicafume (SF),fine quartzsand,filler, andhigh-range water-reducing admixture [3].Graybeal [4]hasalsoreportedthatthe com-
https://doi.org/10.1016/j.jmrt.2020.01.042
2238-7854/©2020 The Authors. Publishedby Elsevier B.V. This is anopen access articleunder the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
mercial UHPC product available on the market in the U.S.
containsahighvolumefraction(2%)ofmicrosmooth-straight (SS) steel fibers with a diameter of 0.2mm and a length of13mm.Numerousresearchers[4–7]worldwide(e.g.,U.S., Europe,SouthKorea, etc.)have thus evaluatedthe various materialandstructuralpropertiesofthismicrostraightfiber- reinforcedUHPC,andthelatterhasbeensuccessfullyusedto buildseveralbuildingsandpedestrianbridges[8].
TofurtherimprovethetensileperformanceofUHPC,sev- eral types of deformed steel fibers, e.g., Torex (or called twisted) fibers [9], hooked-end (HE) fibers [10,11], crimped fibers[11–13],andhalf-hooked(HH)fibers[14,15],havebeen introduced. Wille et al. [16] successfully developed strain- hardening ultra-high-performancefiber-reinforced concrete (UHPFRC)withatensilestrengthof13MPaandstraincapac- ityof0.6%byusingTorexfibersevenatalowerfibervolume fractionof1.5%.Thismaterialexhibitedabettertensileperfor- mancethanUHPCcontainingahighermicroSSfibervolume fractionof2%[16].Yooetal.[17]recentlycomparedtheinflu- encesoftwisted,HE,andHHfibersonthepulloutandtensile behaviorsofUHPCandnotedthatthetwistedfiberismost effective in terms of the tensile performance, followed by theHHfiber,thentheHEfiberatafibervolumefractionof 2%.Muralietal.[11]alsoreportedthat,althoughtheUHPC matricesreinforcedwithbothHEandcrimpedsteelfiberscan absorbmuchhigherimpactenergythanaplainmatrix,theHE fiberismoreeffectiveinenhancingtheimpactenergydissipa- tioncapacitythanthecrimpedoneatvariousdosagesranging from0.5to2%.Theliteraturereviewrevealsthattheorderof effectivenessinenhancingthetensileperformanceofUHPC isasfollowsforahighvolumefractionofabout2%:twisted fiber>HHfiber>HEfiber>crimpedfiber.
Thedeformedsteelfibersprovidebetterbondresistance thanSSfibers[17],butsuchanexcellentbondstrengthcan- notbeachievedatthecompositelevelbecausetheeffective matrixvolumethatpreventssplittingcrackformationisinsuf- ficient.Flanders et al.[18] observedthatinclinedHE fibers inUHPCareprematurelypulledoutfromthematrixdueto the splittingcracks formedbyinsufficientconfinement for thestraightening the fibers’end hooks,and Caoet al. [19]
reported that splitting cracks are more frequently formed ata shorterembedment lengthfrom the reduced confine- ment.Sincethefibersareorientedanddispersedrandomlyin thecomposites,mostofthemarepotentiallyinclinedalong thedirectionoftensile forceand haveashortembedment lengthfrom thecrackplane.Topotentially solvethedraw- backsofusingdeformedsteelfibersintheUHPCcomposites, researchershavedecreasethevolumeofthedeformedfibers andsimultaneouslyincludedmicroSSfibers,whichbridgethe splittingcracks.Severalstudies[20–25]havethusbeencon- ductedtoexaminetheeffectofthehybriduseoftwisted(or hooked-end)andstraightsteelfibersonthemechanicalprop- ertiesofUHPC.Kwonetal.[24]investigatedtheeffectofthe hybridmacroHEand microSSfiberson thetensilebehav- iorofUHPCandachievedahightensilestrengthof20.1MPa, straincapacityof1.06%,and energyabsorption capacityof 153kJ/m3withatotalvolumefractionof3%(2%ofHEfibers and1%ofSSfibers).Magureanuetal.[20]alsousedHEandSS fibersinUHPCmatrixatavolumefractionof2.55%andeval- uatedtheirmechanicalpropertiesanddurability.Theauthors
reported that the UHPC mixture exhibits excellent freeze- thawresistance;inaddition,thecompressive,splittingtensile, and flexuralstrengthsofplainUHPC areimproved by14%, 70–174%, and1.0%,respectively[20].Thedynamicmodulus ofelasticitywasalsoenhancedby4–5%.Parketal.[23]con- ducted directtensile tests using UHPC reinforced with1%
macro steel fibers and various amounts ofmicro SS fibers (0–1.5%)andfoundthatthehybridsystemwithtwistedand straightfibersprovidesthebesttensileperformance,witha tensilestrengthof18.6MPaandstraincapacityof0.64%ata totalvolumefractionof2.5%.Furthermore,theorderofhybrid systemeffectivenessisasfollows:thetwisted>HE>SSfibers.
A hybrid reinforcing system based on SS fibers could also improvetheflexuralperformanceofUHPCthatcontainsHE fibers,asreportedbyYooetal.[25].ChunandYoo[21]found that thetensilestrength and energyabsorptioncapacityof UHPCwithHEandtwistedfibersimprovedbyreplacingapor- tionofthefiberstothemicroSSfibers:forexample,thetensile strengthandenergyabsorptioncapacityimprovedbyasmuch as39%and14%and39%and18%,respectively,forthecaseof HEandtwistedfibers.
Likewise,hybridreinforcingsystemsusingthedeformed (e.g., twisted and HE) and SS fibers have been broadly investigatedtoenhancethemechanicalpropertiesofUHPC.
However,fromtheauthors’extensiveliteraturereviews,there is nopublished study regarding thehybrid use ofrecently developedmoderatelydeformedHHfibersinUHPCwithSS fibers.IthasbeenreportedthattheHHfibercanbeconsidered asthesecondbestdeformedsteelfibertypeintermsofthe flexural-tensilebehaviorofUHPCcomposites[11,16,17];there- fore,theapplicationofahybridreinforcingsystemcontaining HHfibersmayenhancethetensileperformance.Accordingly, twotypesofHHfibershavingdifferentend-hooklengths,leh, of2.5and5.0mmwereadoptedinthisstudy,andaportionof themwasreplacedwithmicroSSfibersatthefixedtotalvol- umefractionof2%.UHPCwithhybrid(conventional)HEand SSfiberswasalsoadoptedforthecomparison;ascontrolspec- imen,UHPCreinforcedwithsingleSSfiberswasconsidered.
Theoveralltensileresponses,i.e.,thestrength,straincapac- ity,andenergyabsorptioncapacitywereestimatedalongwith thenumberoffiberslocatednearthelocalizedcracksforthe preciseevaluationofthehybridreinforcingeffect.
2. Test program
2.1. IngredientsandmixingprocessofUHPC
TheSF and Type IPC were usedto fabricate UHPC matrix asabinder.Theprimary chemicalconstituentsofPCwere calciumoxide(CaO)andsilicondioxide(SiO2),whilethatof SFwasSiO2.Thedetailedchemicalcompositions,densities, andmeanparticlesizesoftheusedPCandSFcanbefound in a reference [26]. As afine aggregate, silica sand with a mean particle size of 337m was used, and a silica flour, asafiller,consistingof98%SiO2 andwithameanparticle sizeof4.2mwasalsoadoptedtoachieveafillingeffectand improve the packingdensityofthe mixture.Similar tothe commercial UHPC mixture[4,27], the coarseaggregate was not used although it has several benefits, suchas shorter
Table1–MixproportionofUHPC.
W/Bb Mixdesign[kg/m3]
Water Cement Silicafume Silicasand Silicaflour SPa
0.2 160.3 788.5 197.1 867.4 236.6 52.6
W/B=water-to-binderratioandSP=superplasticizer.
a Superplasticizerincludes30%solid(=15.8kg/m3)and70%water(=36.8kg/m3).
b W/Biscalculatedbydividingtotalwatercontent(160.3kg/m3+36.8kg/m3)bytotalamountofbinder(788.5kg/m3+197.1kg/m3).
mixingtime,highercosteffectiveness,andhigherfluidity.A lowwater-to-binder(W/B)ratioof0.2wasadopted;polycar- boxylatesuperplasticizer(SP) wasadditionallyincorporated toachieveself-consolidatingcharacteristics.Itcontained30%
solidsandpresentedadensityof1.01g/cm3.Thedetailedmix proportionofUHPCisgiveninTable1.Accordingtotheflow tabletestresults(ASTMC1437[28]),theaverageflowvalueof freshUHPCwasfoundtobeabout240mm.
Allofthedry componentswere initiallyplacedintothe Hobart-typemixerandpre-mixedfor10mintoensuresuffi- cientdispersal.Then,watermixedwithSPwasaddedtothe pre-mixeddrypowderandmixedforanother10minuntilit hadachievedasufficientflowability.Afterthat,2%steelfibers werecarefullyplacedintothemixerandmixedfor5min.From thismixingprocess,thefreshUHPFRCmixturecouldbefab- ricated.
2.2. Propertiesofsteelfibers
For thecommercial UHPC products, microSS fiberswith a diameterof0.2mmandalengthof13mmwereadopted[4].In thisstudy,toimproveitspost-crackingtensileperformance, threetypesofdeformedsteelfiberswereconsidered,i.e.,com- mercialHEfiber,HHfiberwithalehof2.5mm,andstraightened aplastichingeofhookedfiber(SH)withalehof5.0mm.The geometrical and physical properties ofthe steel fibers are given inTable 2. The HE, HH, and SH fibers had identical diametersof0.375mm,butdifferentlengthsof30and25mm, respectively.ThenovelHHandSHfibersweremadefromthe HEfiberbycuttingandstraighteningoneplastichingeatthe end-hooks,respectively.ThelengthoftheHHfiberwasthus shorter(i.e.,25mm)thanthoseoftheothers.Thegeometries ofthesteelfibersareshowninFig.1.
2.3. Strategicdeterminationofsteelfiberamounts
TheHHfiberfortheUHPCmatrixwasfirstintroducedbyXu etal.[14]in2016,anditspullout characteristicshavebeen broadlystudiedbyKimandYoo[15]andYooetal.[26]under variousloadingrates.TheyverifiedthefeasibilityoftheHH fiberasanovelreinforcementfortheUHPCmatrixfromsingle fiberpullouttestresults.However,thereareveryfewstudies [17]examiningtheeffectivenessofusingitatthecomposite leveleventhoughthisisfundamentaltoitspracticalappli- cation.They [17]reportedthat usingthe HEand HHfibers wasineffectiveinimprovingthetensileperformanceofcom- mercial UHPFRCata high volume content of2%owing to theexcessivemechanicalanchorageeffectformedlocallyat the end-hooks.Thus, boththe HE and HH fibers were not
Fig.1–Steelfibergeometry:(a)Smooth-straightfiber(SS);
(b)Hooked-endfiber(HE);(c)Straightenedhalf-hookedfiber (SH)and(d)Half-hookedfiber(HH).
straightenedevenafterthecompletepulloutfromthematrix due to prematuresevere matrix damages,not observedin thesinglefibertests.Prematurematrixspallingcouldoccur if the volume ofthe surrounding matrix is insufficient to resist the expansive pressure generated bythe end-hooks.
If the volumefraction ofsuchsteelfibers decreases,caus- ing a higher effective matrix volume, it may thus prevent prematurematrix failureand excellentfiber bridgingcapa- bilitymaybeachieved.Besides,thehybriduseofmicrosteel fiberscanbeefficientinlimitingtheformationofcracksin thesurroundingmatrix,delayingmatrixfailurecausedbythe radialpressure atthe end-hooks.Consequently,the hybrid HEorHHfibersandmicroSSfiberscanovercomethedraw- backsofsinglefiberuses foundinapreviousstudy [17]by apreventionoftheirexcessiveuseandfurtherimprovethe tensile performanceofUHPFRC. Moreover,the half-hooked steelfibersarenotcommercialproductsbutspeciallyfabri- cated,sothattheiramountswerelimited.Giventhatthetotal volumefractionofthefiberswas 2%,identicaltothecom- mercialone,0.5%and1.0%ofthemicroSSfiberswerethus replaced bythe HE andHH (orSH) fibers.Thedesignation ofvariablesusedinthisstudywasthus determinedasfol- lows.TheinitialletterSSdenotesthesmooth-straightsteel fiber and sequentialnumeralindicates its volumefraction, whilethesecondlettersHE,HH,andSHindicatethecommer- cialhooked-end,half-hooked,andstraightenedhalf-hooked steelfibers,respectively,andthefollowingnumeraldenotes theirvolumefraction.Forexample,theSS1.0–HH1.0specimen includes1.0%microSSfibersand1.0%HHfibers.
Table2–Detailedgeometricalandphysicalpropertiesofsteelfibers.
Typeoffiber df[mm] lf[mm] Aspectratio[lf/df] leh[mm] Density[g/cm3] ft[MPa] Ef[GPa]
SSfiber 0.200 13.0 65.0 – 7.90 2500 200
HEfiber 0.375 30.0 80.0 5.0 7.90 2900 200
SHfiber 0.375 25.0 66.7 2.5 7.90 2900 200
HHfiber 0.375 30.0 80.0 5.0 7.90 2900 200
SSfiber=microstraightsteelfiber,HEfiber=commercialhooked-endsteelfiber,HHfiber=half-hookedsteelfiber,SHfiber=straightenedhalf- hookedsteelfiber,df=fiberdiameter,lf=fiberlength,leh=longitudinallengthofendhook,ft=tensilestrengthoffiber,andEf=elasticmodulus offiber.
2.4. Specimenpreparation
To evaluate the direct tensile response of UHPFRC, five dog-bonespecimens were fabricated for each test variable in accordance with the JSCE recommendations [29]. Their cross-sectionaldimensionandlengthwere30×13mm2and 330mm,respectively.Itiswellknownthattheflexuralorten- silebehavior ofUHPFRCisstrongly influencedbythe fiber orientation, which varies according to the casting method [30,31].KangandKim[30]reportedthatmuchbettertensile performanceisobtainedasthefibersarecastparallel,rather thantransverse,tothelongitudinaldirectionofthespecimen duetoabetterfiberalignment.Thus,toobtaingoodandcon- sistentalignmentoffibersparalleltothedirectionoftensile force,allthespecimenswereidenticallycastintheparallel direction.Immediatelyafterfillingthemolds,thecastingsur- facewascoveredwithavinylsheettopreventsuddenwater evaporation.Thecastsampleswereinitiallycuredataroom temperaturefor24h;then,theyweredemoldedandimmersed inawatertank(ahightemperatureofabout90◦C)for48h.The heat-curingprocesssatisfiedtherecommendationsofFederal HighwayAdministration(FHWA)inU.S.[27],notingthatthe standardandmanufacturer-recommendedcuringtreatment issteamingtheUHPCat90◦Cand95%relativehumidity(RH) for48h.Aftertheheatingcuringprocesswascompleted,all thespecimensweretakenoutofthewatertankandstoredin alaboratoryroomuntilthetestingdate.
2.5. Testsetup
ThedetailsofthedirecttensiletestsetupareshowninFig.2, followingtheJSCErecommendations[29].Theprefabricated dog-bonespecimenwasinsertedintothesteelgripjip,andits alignmentwasthenchecked.Analuminumframecontain- ingtwolinearvariabledifferentialtransformers(LVDTs)was affixedwithagaugelengthof80mm. Auniaxialforcewas monotonicallyappliedthroughauniversal testingmachine withamaximumcapacityof25tonandtheloadingrateof 0.4mm/minwasdeterminedbythespeedofthestroke.The tensileforcewasmeasuredfromaloadcellatthecrosshead, andboththe load andelongationdatawere collected bya staticdatalogger(TDS-540).Thetensilestresswasthencal- culatedbydividingthemeasuredloadbythecross-sectional area,and the strain was obtained bydividing the elonga- tionbythegaugelength.Apin-fixedboundaryconditionwas recommended for direct tensile tests of high-performance fiber-reinforcedcementcompositestoensureacentricload- ingcondition[32],andvanZijletal.[33]insistedthefixed-free
Fig.2–Directtensiletestsetup(Pin-fixedsupport condition).
boundaryconditionifageometricalimperfectionisobtained.
Thus, a pin-fixed boundary condition was adopted in this study(Fig.2).
AstheUHPFRCcompositesshowstrain-hardeningcharac- teristics,theirmicrocrackingbehaviorneedstobeevaluated.
Owingtotheirverytinywidth,themicrocrackswerebarely detectedbythenakedeye.Toclearlyobservethemicrocracks andevaluatetheirproperties,suchasthenumberofcracks and averagecrackwidth,all ofthespecimenswere coated withpolyurethanebeforethetests;thewidersurfacesofdog- bonespecimenswere thinlycoatedbysprayingthemtwice andthenallowingthemtodry.
3. Test results and discussion
3.1. Pulloutresponseofstraight,hooked,and half-hookedsteelfibersembeddedinUHPC
TohelpbetterunderstandthetensilebehaviorsofUHPCrein- forcedwiththeSSandHE(orHH)fibers,theirfundamental pulloutresponsewasexaminedinFig.3a[15,26].Owingtothe differentgeometriesofthestraightanddeformedsteelfibers, theaveragebondstressandnormalizedslipcurveswereused basedonthefollowingequations:=P/dfLEandsN=s/LE, wherePisthepulloutloadinN,df isthediameteroffiberin
Fig.3–PulloutbehaviorsofSS,HE,HH,andSHfibersfromUHPCmatrix:(a)averagebondstressandnormalizedslipcurve and(b)summaryofpulloutparameters.
mm,LEistheinitialembedmentlengthinmm,sNisthenor- malizedslip,andsistheendslipinmm.Muchhigherbond strengthswerefoundintheHE,HH,andSHfibersthaninthe SSfiberowingtotheanchorageeffectformedbytheirend- hooks.TheSSfiberresiststhepulloutforcefromthematrix basedonitschemicaladhesionandfrictionalshearresistance.
Therefore,oncethefiberisfullydebondedfromthematrix,it startstobepulledout withdecreasingbondstress.Onthe otherhand,theHE,HH,andSHfibersresistthepulloutforce fromthechemicaladhesion,frictionalshearresistance,and mechanicalanchorageduetotheend-hooks.Therefore,even afterthefibersarefullydebondedfromthematrix,thepullout resistancecanbecontinuouslyincreaseduntilthetwoplastic hingesattheend-hookarestraightened[34],resultinginmuch
higherbondstrengths,asreportedbypreviousstudies[14,15].
Duetotheeliminationofoneofthetwoplastichingesinthe HHandSHfibers,asmootherdecreaseinthepost-peakbond stresswasobservedinFig.3acomparedtothatofHEfiber.
Justafternearthenormalizedslipof0.4,theHEandSHfibers presentedaslightincreaseofthebondstressduetobend- ingthefiberintheoppositedirectionatthelocationofthe secondplastichinge,consistentwiththefindingsofMarkovi ´c [34].TheaveragebondstressesofH,HH,andSHfibersbecame similartothatofSSfiberapproximatelyatthenormalizedslip of0.6sincetheplastichingeswereallstraightened.
Some important pullout parameters, i.e., average bond strengthandnormalizedpulloutwork,aregiveninFig.3b.The highestaveragebondstrengthof27.8MPa,whichisapproxi-
Fig.4–Summaryoftensilestress-straincurvesandtypicalcrackcharacteristic.
Fig.5–Picturesonlocalizedcracksurface.
mately4.7timeshigherthanthatofstraightone,wasfound intheHEfiberspecimen.TheHHfiberexhibitedintermedi- ate bondstrength betweenthe HE and SSfibers as oneof twoplastichingeswas cutoff.TheSH fibersampleexhib- itedabondstrengthof22.6MPa,whichisabout81%oftheHE fiber’sstrength,eventhoughthecontributionofoneplastic hingeontheanchorageeffectwasmitigated.Thisindicates thatthe lengthofthe end-hook portionis moredominant thanthenumberofplastichingeswithregardtothepullout resistanceofthefibersintheUHPCmatrix[26].Theresults of normalized pullout work (WP*) exhibited similar trends tothe bondstrength: the highestvalue ofWP* was found tobe11.8×10−3J/mm3intheHEfiberandisapproximately 1.2,2.0,and3.7timeshigherthan thoseofSH,HH,andSS fibers,respectively.Thus,itcanbeconcludedthattheenergy requiredtocompletelypullouttheHEandHH(orSH)fibers fromtheUHPCmatrixishigherthanthatrequiredtopullout theSSfiber,whichmayleadtobetterductilityofthecompos- ites.
3.2. Generaltensilebehavior
Fig.4summarizesthetensilestressversusstrainresponsesof allUHPFRCcompositeswithSS,HE,HH,andSHfibers.Allthe testedspecimensclearlyexhibitedastrain-hardeningbehav- iorwith the formation multiplemicrocracks, which agrees tothefindings ofotherstudies [30,35]on theconventional UHPFRC.Beforethegenerationofthefirstthroughcrackin the matrix, a linear elastic tensile behavior was observed.
Immediatelyaftertheinitialmatrixcrackformation,theten- silestressslightlyreducedwithincreasingstrainbecausethe tensileforceatthecracksurfacewasresistedbyonlythebridg- ingfibers,whichissuddenlychangedfromboththematrix andembeddedfibers.However,owingtotheexcellentfiber bridgingcapabilityandinterfacialbondresistance,thetensile stressincreasedcontinuouslyuptothepeakpoint,whichis calledastrain-hardeningzone.Thetensilestiffnessinthis zone islower than the initialstiffnessinthe linear-elastic zoneduetothedecreaseinthe effectivearea resistingthe tensileforcegeneratedbythecrackformation.Afterreaching thepeakvalue,afewofthemultiplemicrocracks,mostlyone ortwo,widenedwiththepullingoutprocessofthefibersin aprocesscalledthecracklocalizationphenomenon,causing agradualdecreaseinthetensilestress.Suchaspecialtensile stress–strainresponsewasnotaffectedbythehybriduseof steelfibers.
Forthecontrolspecimen(SS2.0),thetensilestressgradu- allydecreasedafterreachingthemaximumvalue,owingto theprogressivepullingoutprocessofsteelfibersatthelocal- izedcracksurface.Becauseoftheconstantkineticfrictionof SSfiberintheUHPCmatrix[36]anditsslightenddeforma- tion formedbyacuttingprocess [37],its post-peakpullout loadgraduallydecreaseduptoneartheembedmentlength [37]with asudden increase inthe interfacial shearstress.
Thus,agradualsofteningbehaviorwasobservedinthecon- trol specimen.By replacing aportion ofthe SS fibers with HEorHH,SHfibers(hybridspecimens),asteeperreduction inthetensile stress inthesoftening zonewasfound. This isbecauseofseverematrix damage(e.g.,spalling,splitting cracks,etc.)occurredduetothestressconcentrationatthe
Fig.6–Summaryoftensileparameters.
end-hookportion,leadingtoasuddenseparationofthefibers fromthematrix.Toverifythisexplanation,thelocalizedcrack surfaces wereexamined asshown inFig.5. Itwasnoticed thataportionoftheHE,HH,andSHfibersdidnotstraighten evenaftertheycompletelyseparatedfromthematrixandthe localizedcracksbecomemuchrougherthanthecontrolspeci- men.TheHHfiberspecimensufficientlysustainedthetensile stresses up toquite largestrain valuessimilar tothe con- trolspecimenregardlessofthereplacingratio(0.5and1.0%) (Fig.4b),whereastheHE(Fig.4a)andSHfiber(Fig.4c)speci- mensresultedinanearlierdecreaseinthetensilestressesat areplacingratioof1.0%owingtoinsufficientmatrixvolume duetotherandomfiberdispersion.Thenon-straightenedHE fibersfromtheUHPCmatrixandtheseverematrixdamage atthehighfibervolumefractionof2%werealsoreportedin apreviousstudy[17].Ithasbeenreported[26]thatthedou- bleplastichingesoftheHEfiberandthelongerlengthofthe end-hookportionoftheSHfiberleadtothehigheranchor- ageeffectthanthatofHHfibers,resultinginexcessivestress concentrationandasteepersofteningresponse.
3.3. Comparisonoftensileparameters
Fig.6showstheimportanttensileparameters,i.e.,thetensile strength, strain capacity, and g-value, of all tested speci- mens.TheseparametersarealsosummarizedinTable3.The hybridSSandHE(orHHandSH)fibersdidnotinfluencethe tensilestrengthofUHPFRCsignificantly.Thehighesttensile strength of15.7MPa,whichwasonlyabout4%higherthan thatoftheSS2.0specimen,wasfoundintheSS1.5–HE0.5and SS1.0–HH1.0specimens.Thisisinconsistentwiththesingle fiber pullouttestresults:muchhigherbondstrengths were obtainedinthe HE, HH,and SH fibersthan inthe SS fiber (Fig.3b).Thisisbecausethedevelopmentoftheirultimate bondstrengthswaslimitedbyprematurefiberandmatrixfail- ures.Thestraincapacity,however,wasgreatlyaffectedbythe steelfibergeometry.For HEandSH fibers,strain capacities higherthanthoseinthecontrolsamplewereobservedatthe replacingratioof0.5%,whiletheireffectivenesssubstantially decreasedbeyondthat.Thestraincapacitiesof0.45and0.49%
foundintheSS1.5–HE0.5and–SH0.5specimenswereapproxi- mately34%and45%higherthanthatofcontrolspecimen,but
Table3–Summaryofdirecttensiletestresults.
Tensilestrength,ft[MPa] Straincapacity,εu[%] g-value[kJ/m3]
SS2.0 15.1(0.47) 0.34(0.079) 45.6(12.48)
SS1.5-HE0.5 15.7(1.48) 0.45(0.124) 65.7(16.89)
SS1.0-HE1.0 15.2(0.80) 0.22(0.138) 28.4(18.95)
SS1.5-HH0.5 15.5(0.68) 0.28(0.073) 38.1(10.10)
SS1.0-HH1.0 15.7(1.76) 0.40(0.165) 55.6(18.76)
SS1.5-SH0.5 15.3(0.35) 0.49(0.125) 66.8(17.48)
SS1.0-SH1.0 15.4(0.68) 0.19(0.064) 25.6(8.73)
SSfiber=microstraightsteelfiber,HEfiber=commercialhooked-endsteelfiber,HHfiber=half-hookedsteelfiber,SHfiber=straightenedhalf- hookedsteelfiber,andsequentialnumeral=fibervolumefraction.
Itemsinparentheses()indicatestandarddeviation.
theSS1.0–HE1.0and–SH1.0specimensexhibitedpoorerstrain capacitiesof0.22and0.19%,respectively.TheHHfiberspeci- menexhibitedanincreaseinstraincapacitywithanincrease inthereplacingratioandahigherstraincapacityatthereplac- ingratio of1.0%than thecontrolspecimen. Thismight be causedbytherelativelymitigatedanchorageeffectandthe higher amountof HHfibers helped the specimensachieve betterstraincapacities.
Oneofthe biggestbenefitsofusingUHPFRCas abuild- ingmaterialisitsexcellent ductility,whichisrelatedtoits energyabsorptioncapacity.Theg-valueistheenergyabsorp- tioncapacityundertensionuptothepeakstrengthperunit volume[38].Theg-valuesofthetestedspecimensaresum- marizedinFig.6c.BasedonthestrategichybriduseofHE,HH, andSHfiberswiththeSSfiber,theenergyabsorptioncapacity ofUHPFRCcouldbeimproved.TheSS1.5-HE0.5,SS1.5-SH0.5, andSS1.0-HH1.0specimensachievedhigherg-valuesthanthe controlspecimen,which were44,47,and 22%higher than thatofthecontrolspecimen,andtheSS1.5-SH0.5specimen exhibitedthe highestg-valueof66.8kJ/m3. Asimilar trend wasobservedforthestraincapacity.Thisismainlyduetothe increasedstraincapacityratherthanthetensilestrength.The meang-valueofthecontrolUHPFRCwasreducedbyreplac- ing1.0%oftheSS fiberswithHEandSH fibers,whichwas attributedtotheprematurefailureassociatedwiththelatter.
Hybridfiberscontaining0.5%HEandSHfibersand1.5%SS fiberor1.0%HHfiber and1.0%SSfiber werethuseffective inimprovingthetensileperformanceofUHPFRCwithregards tothemeanenergyabsorptionandstraincapacities.Further- more,thehybridsystemcontaining1.5%SSfiberand0.5%SH fiberprovidedthebestperformance.
3.4. Crackingbehavior
Thetypicalcracking patterns ofallthe tested samplesare showninFig.7,andthecrackingparameters,i.e.,thenum- berofcracksandaveragecrackspacing,aresummarizedin Fig.8.Itcanbeseenthatallthesamplesexhibitedthestrain- hardeningresponseandtheyproducedmultiplemicrocracks alongwithoneortwolocalizedcracks.Theconceptofcrit- icaltransferdistance,which isrelated tothe formation of multiplemicrocracks,wasfirstintroducedbyAvestonetal.
[39,40]basedonthe maintenanceoftheequilibriumatthe cracksurfaceandstresstransferfromthebridgingfibersto thesurroundingmatrix.Ifthetensilestresstransferredtothe matrixexceedsthetensilestrength,anothercrackisformed,
and this limitpoint iscalled the critical transferdistance.
Thus,ifthemagnitudeofthepulloutforceappliedtothefibers atthecrackplaneislowerthantheultimatebondstrength, a higherexternalload isrequired, leading tofurthercrack formation.Thecontrolspecimen(SS2.0)exhibitedasmooth localizedcracksurfacebecausetheSSfiberscouldbepulled outofthematrixwithoutanysignificantmatrixdamages.One oftheprimarymechanismstohavesuchanexcellentbond characteristicofSSfiber from UHPCmatrix isits high and steeplyincreasedshrinkagestrain[41].Thematrix spalling observed inthe fiber pullout tests ofthe SS fiber [26] was minorcomparedtothatobservedinthefiberpullouttestsof theHEandSHfibers.However,byreplacingaportionofSS fiberwithHE,HH,andSHfibers,thelocalizedcracksurface becamerougherowingtothematrixdamagecausedbythe anchorageeffectoftheend-hooks.Thecracksurfacerough- nessseemedtobeslightlymitigatedbyusingtheHHandSH fibersinsteadoftheHEfiber(Fig.7)owingtothereducedstress concentrationneartheend-hooks asreportedbyYooetal.
[26].
AsgiveninFig.8,replacingaportionofSSfiberswithHE, HH,andSHfibers resultedinanincreaseinthenumberof cracksinUHPFRC,regardlessofthetensileperformance.For example,theaveragenumber ofcracksformedinthe con- trol specimenwasfoundto be10.0ea,whichisthe lowest valueamongallthetestedsamples. Althoughsomehybrid specimensexhibitedevenpoorertensileperformancesthan thecontrolspecimenintermsofthetensilestrengthandg- value,theyproducedmoremicrocracksowingtothehigher pulloutstiffnessofthedeformedsteelfibersthatmoreeffec- tivelytransmittedthetensilestresstothesurroundingmatrix.
Thisisconsistentwiththefindingsofpreviousstudies[16,17], i.e.,UHPCmatrixreinforcedwithdeformedsteelfiberspro- ducesahighernumberofcracksthanthatwithstraightfibers foridenticalfibervolumecontents.ThespecimenSS1.5-HE0.5 producedthehighestnumberofcracks,i.e.,22.5ea,whichis muchhigherthanthatofthecontrolspecimen.For theHE fiber,thenumberofcracksdecreasedwithincreasingreplac- ing ratio, whereas it increasedoratleast maintainedwith thereplacingratioforthecasesofHHandSHfibers(Fig.8).
These observations are consistent with the tensile perfor- manceresults.Thehighernumberofcracksledtoadecrease intheaveragecrackspacings,whichindicatesthatthenew cracksareformedbetweenthepre-existingcracksaswellas besidethem.Therefore,thesmallestaveragecrackspacingof about4.3mmwasfoundinthespecimenSS1.5–HE0.5,while
Fig.7–Picturesofcrackingpatterns.
thelargestspacingof8.4mmwasobservedinthecontrolspec- imen(SS2.0).
3.5. Numberoffibersdetectedinthelocalizedcrack plane
Inordertoevaluatetheimplicationofhybridusesofmacro HE,HH,and SHfibers onthedistributioncharacteristics of microSSfibersinthecomposites,thenumberoffibersatthe localizedcracksurfacewasexaminedasshowninFig.9.Com- paredtothecontrolspecimen(SS2.0),whichhadthenumber offibersperunitareaas35.8ea/cm2,thehybridfiberspeci- mensexhibitedsmallernumbersofSSfibersatthelocalized
crackplanesinceaportionofthemwerereplacedbymacroHE, HHandSHfibers.Theoretically,if0.5%and1.0%(byvolume)of microSSfibersarereplaced,thenumberofSSfibersperunit areawillbe26.9and17.9ea/cm2,respectively.Asrevealedby the theoreticalcalculation,thehigherreplacingratioledto smallernumbers ofSSfibers forall casesofmacroHE,HH andSHfibers.Inaddition,althoughthereisawidevariation in thenumber ofSS fibers according tothe typeofmacro fibers,theaveragenumbersofSSfibersperunitareaofthe hybridsampleswerefoundtobe27.4and18.9ea/cm2atthe replacingratiosof0.5%and1.0%,respectively,onaverage,and thesearequitesimilartothetheoreticalvalues.Fig.9sum- marizesthenumberofmacroHE,HHandSHfibersperunit
Fig.8–Summaryofcrackingbehaviors.
Fig.9–SummaryofthenumbersofmicroSandmacroHE, HH,andSHfibersperunitarea.
area.Thehighernumberoffiberswasobtainedatthehigher replacingratio:forexample,thenumberofHEfibersperunit areaincreasedfrom2.8to6.9ea/cm2whenthereplacingratio increasedfrom0.5%to1.0%.Furthermore,thenumbersofHE, HHandSHfiberswerequitesimilaratthesamereplacingratio, althoughthelatterhadaloweraspectratioof66.7thanthe others.Herein,theaspectratioindicatestheratiooflength anddiameteroffiber(lf/df).Thus,itcanbeconsideredthatthe effectoftheend-hookgeometry(doublehookorsinglehook) onthenumberoffibersatthecracksurfaceareinsignificant, andthe tensiletest resultsreportedaboveare thusmostly affectedbythepulloutresistanceandmechanismofthesteel fibersused.
3.6. Examinationofthepercentageofstraightened hookedandhalf-hookedfibersinthecomposites
The ultimate pullout resistances of HE, HH and SH fibers canbeachievedwhentheend-hookportionisfullystraight-
Fig.10–PercentageoffullystraightenedmacroHE,HH,and SHfibersaftertensiletests.
ened.Itisassumedthatfiberswithend-hooks,thatarenot fullyorpartiallystraightened,wereprematurelypulledoutof thematrixbeforetheyachievedtheultimatebondstrength owingtoseverematrixdamage.Thus,thepercentageoffully straightened HE, HHand SH fibers locatedatthe localized cracksurfacewasanalyzedasshowninFig.10.Itwasobserved thatthehalf-hookedfibers,i.e.,HHandSH,weremoreeffec- tively straightenedatallreplacing ratiosthan theHE fiber.
For example, the percentageofstraightened HE fibers was only5%in specimensSS1.0–HE1.0,which wasmuchlower than those(16%and 23%)ofthe SHand HHfibers,respec- tively,atthesamereplacingratio.Thisiscausedbythehigh stressconcentrationattheend-hookportionoftheHEfiber, which resultsineasier formationofsplittingcracksin the surroundingmatrixandprematurefailure.Furthermore,the percentage of straightened fibers generally decreased with increasingreplacingratio:thepercentagesoftheHEandSH fibersdecreasedfrom9%to5%andfrom37%to16%,respec- tively,asthereplacingratiodecreasedfrom0.5%to1.0%.This isrelatedtothelowereffectivematrixvolumesurroundingthe fibers.Asahigherfiberamountdecreasestheareaofmatrix coveringeachfiber,theinsufficientvolumeofthematrixcan- notwithstandtheradialpressureneartheend-hooksduring thepulloutprocess,inhibitingthestraighteningoftheend- hooks.Therefore,thetensileperformanceofthespecimens withhybridHE(orSH)andSSfibersdeterioratedatthehigher replacingratio.AlthoughthepercentageofstraightenedHH fibersalsoreducedatthehigherreplacingratioof1.0%(30%
→ 23%), it was relativelyinsignificant comparedto thatof SHfibers.Inaddition,theactualnumberofstraightenedHH fibersincreasedwithincreasingthereplacingratiothatisfrom 0.97ea/cm2to1.38ea/cm2.Thismightbeattributedtothemit- igatedanchorageeffectcausedbyareductioninthelength oftheend-hookportion.Thevalueofthebondstrengthof the HHfiberinUHPC matrixwas betweenthoseofthe HE andSSfibers(Fig.3).Owingtotherelativelyminordecrease in the percentage and higher number of straightened HH fibers atthehigherreplacing ratioof1.0%,the tensileper- formanceoftheUHPCcompositesreinforcedwithhybridHH andSSfibersimprovedwiththereplacingratio,whichdoes
Fig.11–Picturesforevaluatingthefiberlocationeffecton straightening.
not agreewith the findings of the HE and SH fiber cases.
Itisthus notedthat, if themore straightenedhooked and half-hookedsteelfibers are obtainedinthe UHPC compos- ites,thebettertensile performanceoftheUHPC composite willhave.
Toanalyzetheimplicationoffiberlocationonthedegree ofend-hookstraightening,thelocalizedcracksurfaceswere evaluated(Fig.11).TheSSfiberswereall completelypulled out of the matrix without any breakage (Fig. 11), leading to a smooth localized crack surface. The HE, HH, and SH fibers located near the center of the cross-section were mostly well straightened,whereas they were not straight- ened near the surface of specimen (Fig. 11) owing to the insufficient matrix volume. This increased the roughness ofthe localizedcrack surface. Therefore, byincreasingthe cross-sectional dimension of the UHPC composites, the effectiveness of the hybrid hooked (or half-hooked) and straight steel fibers in enhancing the mechanical perfor- mance of the composite can be increased because more fibers are potentially located at the center and are better straightened.
4. Conclusions
Inthis study,thehybrideffects ofhooked (orhalf-hooked) andstraightsteelfibersonthetensilebehaviorofUHPCwere investigated.Fortheexperiments,fourdifferenttypesofsteel fibers,i.e.,HE, HH,SH,and SS,and twodifferentreplacing ratiosbasedontheSSfiber,i.e.,0.5and1.0%,wereconsid- ered.Tobetterunderstandthetensiletestresults,thepullout behaviorsofsteelfibersembeddedinUHPCmatrixandthe
stateandnumberofthefibersatthelocalizedcracksurface werealsoevaluated.Basedontheaboveresultsanddiscus- sion,thefollowingconclusionscanbedrawn:
1) TheHEfiberprovidedthebestpulloutperformance,with thebondstrengthandnormalizedpulloutworkof27.8MPa and 11.8×10−3J/mm3, respectively, which are approxi- mately4.7and3.7timeshigherthanthoseoftheSSfiber, respectively.Thelengthofthe end-hook portion,rather thanthenumberofplastichinges,predominatedthepull- out resistance. The order of pullout resistance was as follows:HEfiber>SHfiber>HHfiber>SSfiber.
2) Theend-hookgeometryhadnoeffectsonthenumberof fibersatthelocalizedcrackplaneofUHPFRC.
3) ThetensilestrengthofUHPFRCwasinsignificantlyinflu- encedbythehybriduseofHE(orSHandHH)andSSfibers:
thehighesttensilestrengthofthehybridspecimenswas foundtobe15.7MPa,whichisonly4%higherthanthatof thecontrolspecimen(SS2.0).
4) TheUHPCmatrixreinforcedwith0.5%SHfibersand1.5%
SSfibersledtothebesttensileperformance,withastrain capacityandg-valueof0.49%and66.8kJ/m3,respectively, whichwereabout45%and47%higherthanthoseofthe controlspecimenwith2%SSfibers,respectively.
5) Thehybridreinforcingsystemswith0.5%HE(orSH)fibers and1.5%SS fibers,and1.0%HHfiber and1.0%SS fiber wereeffectiveinenhancingthetensileperformanceofthe controlspecimenavailableonthemarket.
6) Bettercrackingbehaviorsintermsofthenumberofcracks formedandaveragecrackspacingwerefoundinthehybrid fiberspecimensthaninspecimenSS2.0regardlessoftheir tensileperformances.
Data availability
Theraw/processeddatarequiredtoreproducethesefindings cannotbesharedatthistimeasthedataalsoformspartofan ongoingstudy.
Conflict of interest
Theauthorsdeclarenoconflictsofinterest.
Acknowledgements
ThisworkwassupportedbytheNationalResearchFoundation ofKorea(NRF)grantfundedbytheKoreagovernment(MSIT) (No.2017R1C1B2007589).
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