Influence of cellulose fiber addition on self-healing and water permeability of concrete

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Citation for this paper:

Singh, H. & Gupta, R. (2020). Influence of cellulose fiber addition on self-healing

and water permeability of concrete. Case Studies in Construction Materials, 12,

e00324.

https://doi.org/10.1016/j.cscm.2019.e00324

UVicSPACE: Research & Learning Repository

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Influence of cellulose fiber addition on self-healing and water permeability of

concrete

Harshbab Singh, Rishi Gupta

June 2020

© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under

the CC BY-NC-ND license (

https://creativecommons.org/licenses/by-nc-nd/4.0/

).

This article was originally published at:

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Case

study

In

ﬂuence

of

cellulose

ﬁber

addition

on

self-healing

and

water

permeability

of

concrete

Harshbab

Singh

a,

*

,

Rishi

Gupta

b

a_Department_of_Civil_Engineering,_University_of_Victoria,₃₈₀₀_Finnerty_Road,_Victoria,_B.C,_V8P_5C2,_Canada

b_Engineering_and_Computer_Science_(ECS),_314,_Department_of_Civil_Engineering,_University_of_Victoria,₃₈₀₀_Finnerty_Road,_Victoria,_B.C,

V8P5C2,Canada

ARTICLE INFO

Articlehistory:

Received3September2019

Receivedinrevisedform27November2019 Accepted5December2019

Keywords: Self-healing Celluloseﬁbers Ultrasonicpulsevelocity Waterpermeability Cracks

ABSTRACT

Crackformationundertensileforcesisamajorweaknessofconcrete.Cracksmakeconcrete vulnerabletoextremeenvironmentduetoingressofwaterandharmfulcompoundsfrom surroundingenvironment.Eventhoughconcreteissusceptibletocrackingithasabilityto sealitscracksbyitselftosomeextentduetoautogenousself-healing.Sofaronlyfew studieshavebeendoneonautogenoushealingoffiberreinforcedconcrete.So,thisstudy aims toevaluate the self-healing potential and water permeability of cellulose fiber reinforced concrete(CeFRC).Thetwotypesofcomposites; controlandcellulose fiber reinforcedconcretehavebeeninvestigated.Compressivestrengthandflexuraltestswere performedtomeasurethemechanicalpropertiesofthecomposites,waterpermeability testwasusedtoevaluatethecoefficientofpermeabilityandtheself-healingperformance wasinvestigatedbyusingultrasonicpulsevelocity(UPV)andapatentedself-healingtest. Theresultsindicatethatthewaterpermeabilitycoefficientdecreasedby42%whereasthe healingratioincreasedatahigherratefortheinitialdaysofhealingwhencellulosefibers wereaddedinthemix.CeFRCalsoresultsina7.8%increaseintheflexuralstrengthand demonstrateahigherself-healingratiobasedontheUPVtest.

1.Introduction

Concreteisstilloneofthewidelyusedmaterialsintheconstructionindustry.Traditionalconcretehasadrawback,it tendstocrackwhensubjectedtotensilestresses.Thereareseveralmethodsusedtodecreasethecrackinginconcretesuchas providingenoughsteelreinforcementor_ﬁberreinforcement.However,stillsomecracksareexpectedandtheyleadtoan increaseinpermeability,decreaseindurabilityandstrengthoftheconcretestructure.Duetotheincreaseinpermeability, thewatereasilypassesthroughtheconcretematrixandcomesinthecontactwiththereinforcementleadingtocorrosion initiation.Duetothis,thestrengthoftheconcretestructurefurtherdecreases,necessitatingmonitor,control,andrepairof cracks.Repairingofconcretecracksisnotalwaysarealistictaskascracksarenotalwaysvisibleoreasilyaccessible.Itis estimatedthatinEurope,costrelatedtorepairworksishalfoftheannualconstructionbudget[1].TheUShasaverageannual maintenancecostforexistingbridgesthroughtheyearisestimatedto$5.2billion[2].Additionally,indirectcostduetotrafﬁc jamsandinconveniencearealsoassociatedwiththeconcretecrackrepairworks.Eventhoughconcretemaybevulnerableto cracking,ithasaninherentabilitytohealingthecracksbyitselftosomeextent.Thisphenomenonisknownasautogenous

*Correspondingauthor.

E-mailaddresses:harshbabsingh@uvic.ca(H.Singh),guptar@uvic.ca(R.Gupta).

https://doi.org/10.1016/j.cscm.2019.e00324

ContentslistsavailableatScienceDirect

Case

Studies

in

Construction

Materials

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self-healingofconcrete.Mainmechanismsofautogenousself-healingareduetofurtherhydrationofunhydratedcement; recrystallizationofportanditeleachedfromthebulkpasteandformationofcalcite[3].Besidesnaturalautogenous self-healinginconcrete,therearevariousapproachestoimprovetheself-healingofconcreteincludingtheuseofbacteria[4]or chemicaladmixtures[5]intheconcretematrix.

Theconceptofusingfiberstoreinforceconcretehasbeenusedforhundredsofyears.However,onlyafewstudieshave beenperformedtoinvestigate the self-healing ef_ficiency of _fiberreinforcedconcrete [6_–8]. Commonlyused _fibers in concretearesteel,glass,synthetic(polypropylene,polyethylene,nylon,and polyester)and naturalsfibers(wood,fruit, cellulose),etc.Steelfibersimprovetheductility,flexuralstrengthandfracturetoughnessofconcreteduetohighermodulus ofelasticityofsteel.Kimetal.foundintheirstudythatsteelfibercanhelptocompletelyrecovertheflexuralperformanceof concrete exposure to cryogenic temperature [9] and concrete with micro steel fibers exhibited higher resistance to microcrackformationundercryogenicconditions[10].However,theyaresubjectedtocorrosionwhencomestocontactwith waterthroughcracksandtheirdurabilityreduces.LiandYangfoundintheirstudythatengineeredcementcompositeswith PVAfibersareabletoreducethecrackwidthdownto50

m

m[7], whichcanfurtherhelptoimprovetheself-healing propertiesofconcrete.Althoughglassfiberenhancesthetensileandimpactstrengthofconcrete,theybecamefragilewith timeduetothealkalinityofconcrete[11].Naturalfibersusedinconcreteareeco-friendly,recyclableandwidelyavailable throughouttheworldatamuchcheaperratecomparedtoother_fibertypes.SinghandGuptausedcellulose_fiberasacarrier forbacteriainself-healingmortartoimprovetheself-healingefficiencyofbacterialmortar[12].

Onephenomenonincementitiousmatricesthatpromotesself-healingincludesusingmaterialsthatrestrictthecrack widthandimprovetheautogenousself-healing.Literaturereportsuseofpolyethylene(PE)_fibers[13],polyvinylalcohol (PVA)fibers[8]andpolypropylene(PP)fibers[14,15]etc.toimprovetheautogenoushealingofconcrete.However,limited workhasbeenreportedwherecellulosefibershavebeenusedtoimprovetheautogenoushealingofconcrete.Inthisstudy, cellulosefibersareusedasreinforcementandtheirimpactontheautogenousself-healingofconcreteisinvestigated.

VanTittelboomandBelie[16]explainedintheirstudythatself-healinginconcreteismoreeffectivewhenthecrackwidth isrestricted(Fig.1A),waterisavailableinthecracks(Fig.1B),andcrystallizationofhydrationproductstakesplace(Fig.1C). Authorshypothesizethatcellulosefibercanassistinimprovingallthreemechanismsnotedabove(A,B,C).ForA,cellulose fiberscanlimitcrackwidthintheplasticshrinkagephase[17]andcanreducethecrackingby85%morethannormal concrete[18,19].Cellulosefibershaveahigh-waterabsorptionof85%[20],thusimprovinginternalcuringandassistingin mechanismB.Finallysincecellulosefibershavehighalkaliresistance[21]andcanworkasawaterreservoirwhichleadsto thecrystallizationof cementhydrationproducts due tocontinuoushydration.Inaddition, cellulosefibersin concrete increasethefreeze-thawdurability[22]andprovideanicefinishedsurface[23].Moreover,theyaresuitableforready-mix plantswhichmakesthemeasytouseinthesmalltolargescaleconcreteconstruction.

Thepurposeofthispaperistoexperimentallyinvestigatetheself-healing,durabilityandstrengthpropertiesofcellulose fiber-basedandnormalconcretecomposites.Theself-healingperformanceofcompositeswasevaluatedbyself-healingtest andultrasonicpulsevelocitytest(UPV).Waterpermeabilitytestwasusedtofigureoutthecoefficientofpermeability. Compressionandflexuraltestswerealsoperformedtoanalyzethemechanicalpropertiesofconcrete.Thispaperformsa partofalargerprojectwhereself-healingandself-sealingtechniques(suchascrystallinewaterproofingadmixtures[5,24] arebeingimplementedinaparkingstructureundergoingrepairinVictoria,Canada.Thispaperpresentspartofthework beingundertakeninthelaboratoryofthiscasestudy.

2.Experimentalprogram

2.1.Materialproperties

2.1.1.Cement

GeneraluseTypeGUOrdinaryPortlandCement(OPC),whichalsomeetstherequirementsoftype-Iandtype-IIcementas perASTMC150[25]speciﬁcations,wasusedinthemakingofconcretesamples.

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2.1.2.Aggregates

AggregatesusedforthepreparationofconcretewereobtainedfromtheSecheltpitinB.C.Coarseand_ﬁneaggregateshad arelativedrydensityof2.695and2.651respectively,relatedabsorptionratioof0.69%and0.79%.

2.1.3.Celluloseﬁber

FibersusedinthisstudywereobtainedfromSolomoncolours,INC.Thesefibersareaspecialtypeofnaturalcellulose fiberscalledUltraFiber500madefromSlashpinesandLoblollyinNorthAmerica.Asperthemanufacturer’sdeclaration, UltraFiber500isanalkaliresistantcellulosebasedmicrofibersusedforsecondaryreinforcement,providecrackcontroland havebetterhydrationandbondingproperties[26].Aclose-upviewofcellulosefibersisshowninFig.2.

Useofthesefibersinconcretealsosupportsthepurposeofsustainability astheycomefromnaturalrenewableresources.Apart fromthis,highsurfaceareaandclosespacingofcellulosefibersmakethemquiteeffectiveinthesuppressionandstabilizationof microcracksintheconcretematrix[27].PhysicalandmechanicalpropertiesofcellulosefibersarepresentedinTable1. 2.2.Concretemixdesign

Thecement/sandratioandwater/cementratiousedwas0.41and0.53respectivelyinalltypesofconcretemixes,which representsamixwithatargetstrengthof32MPa,normallyusedinthefield.Inthisstudy,0.5%volumefractionofcellulose fiberswasusedtoincreasethedispersionoffibersthroughoutthematrix.Moreover,Banthiaetal.alsousedthesame volumefractionofcellulosefibersintheirstudyonfiberreinforcedconcreteforflexuralanddirectsheartests[29].Two typesofmixeswereformulated:

1Cxx:Controlconcrete.

20.5Cxx:Celluloseﬁberconcretewithﬁbervolumefractionequalto0.5%.

WhereCandxxrefertoconcreteandsamplenumberrespectively.Thedecimalfractionindicatesthevolumefractionof celluloseﬁbersusedintheconcrete.MixproportionusedforconcreteispresentedinTable2.

2.3.Mixingcuringandsettingprocedure

Foreachmixture,intotaltwelvecylindersofsize

F

100200mm,threebeamsofsize355101101mmandsix cylindersofsize

F

150175mmforwaterpermeabilitytestwerepreparedaspertherecommendationsofASTMC192[30].

Fig.2.Microcelluloseﬁbers.(AdoptedfromSinghandGupta[46]).

Table1

GeneralPropertiesofUltraFiber500[28].

NameofFiber UltraFiber500

MaterialType HighAlkaliResistant,naturalcelluloseﬁbers

AverageLength 2.1mm

AverageDenier 2.5g/9,000m

AverageDiameter 0.00063inch

Count,ﬁber/lb 720,000,000

Density 1.10g/cm3

SurfaceArea 25,000cm2

/g

TensileStrength 750N/mm2

AverageElasticModulus 8500N/mm2

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Ingredientsforbothmixeswerebatchedoutasperthefinalvolumeofconcreterequiredbyusinganelectronicweighing balance.ForCeFRC,mixingprocedurewasstartedbyfirstsoakingcellulosefibersin20%oftotalmixwaterfor15min.This wasdonetoprepareaslurrypasteofcellulosefiberstoimprovetheuniformmixingoffiberswithotheringredients.Afterthe formationofaslurry,itwasmixedwithcoarseaggregatesfortwominutes,onceslurrywasthoroughlymixed,remaining ingredientswereaddedtothedrummixerandmixedforthreeminutesfollowedby2minrestandatwominutesfinal mixing.Oncetheuniformmixwasachieved,aslumptestwasperformedwithin15minasperASTMC143[31],afterward, theconcretewasplacedintodesiredmolds.

Toensuresufﬁcientcompaction,removalofentrappedairandtoavoidhoneycombstructure,ﬁlledmoldswereplacedon avibratingtablefor30s.Aftercompaction,moldswerecoveredwithaplasticsheetandplacedatroomtemperatureforthe next24h.Demoldingwascarriedoutafter24handsampleswereplacedinacuringchambermaintainedat232.Table3

summarizesthespecimens’typeandquantityaswellascuringageusedfordifferenttestmethods. 2.4.Testingprocedure

Threetypesoftestswereconductedtoinvestigatetheself-healingandstrengthcharacteristicsofallmixes.TheUPVand self-healingtestswereperformedtoevaluatetheself-healingefﬁciencyofconcrete.Compressionandﬂexuraltestswere usedtodeterminethestrengthcharacteristicsoftheconcretemixes.

2.4.1.Compressiontest

After14and28daysofcuring,todeterminethecompressivestrengthofconcrete,threecylindersfromeachmixwere testedaspertheproceduredeﬁnedinASTMC39[33].Uniaxialcompressiontestingmachinewasusedtotestthecylinders

Table2

Mixproportionsforconcrete.

Material Quantities Units

Cement 340 Kg/m3

Aggregates 1120

Sand 820

Water 181

CelluloseFibers0%and0.5% 0and5.5

Table3

Detailsofspecimensusedindifferenttestmethods.

Testmethod Numberofspecimen Typeofspecimen CuringAge Standard CompressiveStrength 3 Cylinder(F100200mm) 14 ASTMC39 CompressiveStrength 3 Cylinder(F100200mm) 28 ASTMC39 FlexuralStrength 3 Beam(355101101mm) 28 ASTMC293 WaterPermeability 6 Cylinder(F150175mm) 28 DIN1048 Self-healing 3 Cylinder(F100200mm) 2(DryCured) Patentedtest[32]

UPV 3 Cylinder(F100200mm) 28 C597

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andthepeakloadwasrecordedatthetimeoffailure.Theloadingratewasusedwithintherangespeciﬁedbythestandard. 14and28daycuredsampleswereusedtoinvestigatethechangeincompressivestrengthwithcuringtime.

2.4.2.Flexuraltest

Prismaticbeamsofsize355101101mmweretestedat28daysbyusingcenterpointloadingtest asperASTM C293-16[34], todeterminethe_flexuralstrengthofbothconcretemixes.A spanof304.8mm(12_”)was usedoverthe supports.Theuniversaltestingmachinewasusedtotestthebeamsforflexuralandpeakloadwasrecordedatthepointof failure.Fig.3showsthearrangementfortheflexuraltestofconcreteusingcenterpointloadingmethod.Themodulusof rupturewascalculatedusingtheformulabelow:

R¼ 3PL

2bd2 ð1Þ

WhereR=modulusofrupture(MPa),P=maximumappliedloadindicatedbythetestingmachine(N),L=spanlength(mm), b=averagewidthofthespecimenatthefracture(mm),d=averagedepthofspecimenatthefracture(mm).Bysubstituting appropriatevalues,Eq.(1)reducestoR=0.000435P.

2.4.3.Waterpermeabilitytest

Fig.4showsthetestingarrangementusedforthepermeabilitytest.Thetestwasperformedforallmixesusing DIN-1048-Part5standard[35].Sixspecimensfromeachmixweresubjectedto0.5MPawaterpressureforthreedays(72h).Priorto testing,thesurfaceofthesamplesubjectedtowaterpressurewasroughenedbywirebrushingasrecommendedbythe standard.Thespecimensweremountedontherubbergasketwitha100mmdiametertoavoidanyleakageduringtesting. Afterthetestingwas_ﬁnished,thesamplesweresplitintotwohalvesfromthemiddle,perpendiculartothewaterinjected surface byusingthecompression testingmachine. Thewaterpenetration depthwas markedfor brokensamplesand measuredimmediately.

Assumingtheﬂowofwaterthroughconcreteporesislaminarandstationary,thecoefﬁcientofpermeabilitycanbe calculatedusingDarcy’slaw[36]asfollows:

dx dt¼Kw

h

x  ð2Þ

Wherexisthedepthofwaterpenetrationinmeters,tisthetimeforthetestinsec,histhewaterpressureheadandKwisthe

waterpermeabilitycoefﬁcient.TheKwcanbeﬁguredoutbyintegratingEq.(2)toyieldEq.(3):

Kw¼

x2 t

2ht ð3Þ

Wherextisthepenetrationdepthattimet.Sincethewaterﬂowisunsteadyandassociatedwiththesorptivity,itismore

reasonabletouseaveragedepthinsteadofmaximumpenetrationdepthtocalculateKw[5,36].Thexavgwascalculatedby

ﬁrstmeasuringthewetarea(Aw)andmaximumwidth(Wmax)ofthewettedregionbyusingimageJsoftwareasshownin

Fig.5.ThexavgwascalculatedasanaverageofAwdividedbywmaxforeachhalfofthetestedsample.

2.4.4.Ultrasonicpulsevelocity(UPV)test

TheUPVtestisaverysensitiveindicatorofthepresenceofdamage(cracks/ﬂaws)inconcretewhenperformedunder laboratoryconditions[37].Arif_ﬁnetal.[38],Bahrinetal.[39]andSarkaretal.[40]alsousedtheUPVtesttoevaluatethe self-healingperformanceofmortars.UPVtestwasusedtodeterminetheinternalhealingofcracks.TheUPVtestwasperformed onsamplesasperC597-16[41].Thelongitudinalstresswaveispropagatedthroughtheconcretesamplesandtimerequired totravelthewaveacrossthediameterofthecylinderwasrecorded.Thetraveltimeofthewavevariesasafunctionofthe densityofthematerial,allowingtheestimationofthediscontinuitiesinthesamples.

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Thespecimenspreparedtomonitorself-healingusingUPVwerepre-crackedafter28daysofcuring.Crackswereinduced inthecylindersbyusingastandardcrackinducingjig(SCIJ)[32]showninFig.6.ThisjigusestheVshapedcuttingedgesthat actasstressconcentrators.Thecylinderwasassembledinsidethejigandthecompressionmachinewasusedtosubject compressiveloadingtillvisiblecracksappearedonthesurfaceofthecylinder.Thepre-crackedspecimenswereallowedto cureinwatertoallowanyself-healing.

TheUPVtestwasperformedontheuncrackedandpre-crackedsamplesattheageof28days,followedby21daysof healing.Figs. 7and 8shows thesequenceand arrangementfor theUPV testrespectively. Thedeviceconsistsof two transducersonetotransmittheultrasonicwaveandtheothertoreceiveit.Bothtransducerswereconnectedwiththe surfaceofthecylinderandtimerequiredtotravelthesoundwaveperpendiculartothecrackdirection(acrossthecrack)as showninFig.7wasrecordedwithaprecisionofatleast0.1

m

s.Thecouplinggelwasappliedonthesurfacetogetabetter contactarea,requiredforaccurateresults.Thetestwasperformedatafrequencyof150kHz.Thevelocityoftheultrasonic wavewascalculatedusingtheEq.(4):

V=D/T (4)

D=diameterofthesample,100mm,T=Timerequiredtotravelthedistance,D.

2.4.5.Self-healingtest

Theself-healingtestwasperformedbyusingtheinnovativetechniquedevelopedbyGuptaetal.[32].Thecylindersof

F

100x200mmsizewerecrackedusingSCIJ[32].Pejmanetal.[5]alsousedthesamemethodtoevaluatetheself-healing efﬁciencyofconcretecylinders.Thecylinderswerecuredinambienttemperaturepriortocracking.Aftercracking,surface crackwidthofeachcylinderwasmeasuredontopandbottomofthesurfacesbyusingopticalcrack-detection-microscopeat sixequal distancepointsalong thecrack: threealongthetopfaceandthreealong thebottomface.Allreadingswere

Fig.5.Methodtocalculatetheaveragewidth(xavg)inthewettedregion.

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averaged to calculatetheaverage width of thecrack for a cylinder. Thecracked cylinders wereinserted intospecial rubbersleevesandsealedusingsiliconsealantandepoxyresintomakesurewateronlypassesthroughthecrackduringthe self-healingtest.Theoneendofthecylinderwithrubbersleeveswasexposedtoaconstantwaterheadof1.7m.The_ﬂowof waterthroughthecrackofthecylinderwascollectedinawatercontainerandmeasuredatfrequentintervals.Fig.9shows theself-healingtestarrangementforconcretecylinders.

3.Resultsanddiscussion

3.1.Slumpandcompressivestrength

Theslumpvaluesof75mmand35mmwereobservedfortheCxxand0.5Cxxmixrespectively.A53.3%decreaseinthe slumpwasobservedfor0.5CxxwhencomparedtoCxx.Thedecreaseintheslumpisduetofactthatcelluloseﬁbersare hydrophilicandtheytendtosoakmostofthewaterduringmixing(about85%oftheirweight).

Fig.7. SequenceofUPVtesttoevaluateself-healingefﬁciencyofconcretecylinders.

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Table4showstheaveragecompressivestrength(f’c)ofeachmixalongwithaveragesandstandarddeviationvaluesfor14 and28dayscuredsamples.

ThedatainTable4wasanalyzedforthe%changeincompressivestrengthofCeFRCfromcontrolconcrete.The%changein compressivestrengthfor14and28dayscuredmixesisshowninFig.10.Adecreaseincompressivestrengthof37.39%and 25.59%isnoticedwithadditionof0.5%fibersafter14and28daysofcuringrespectively.Thedecreaseincompressive strengthispossiblyattributedtolossofworkabilityofconcretewhen0.5%byvolumecellulosefibersareadded,which causeslowercompactionofconcretesamples.However,alowerreductionincompressivestrengthisobservedfor28days curedsamplesascomparedto14dayscuredsamplesbecauseatlongeragecellulosefibersactasawaterreservoirthathelps toimprovethehydrationofunreactedcementparticlesbymeansofinternalcuring.Thisinternalcuringinfiberconcrete somehowcompensatesforthelossoff_’c.

Fig.9.Self-healingtestarrangementforconcretecylinders.

Table4

Compressivestrengthtestresultsforconcrete.

MixDesign 14daysCompressiveStrength(f’c)(MPa) 28daysCompressiveStrength(f’c)(MPa) Individual AverageSTDV Individual AverageSTDV

Cxx 47.62 48.112.16 48.71 48.520.9 50.97 49.52 45.74 47.34 0.5Cxx 30.96 30.120.63 37.53 36.210.99 29.97 35.96 29.43 35.15

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3.2.Flexuralstrength

Table5showstheaverage_flexuralstrengthandcorrespondingmaximumde_flectionforeachmixalongwithaveragesand standard deviationvalues for 28 days cured samples when tested in the formof beams under center point loading configuration.

ThedatainTable5wereanalyzedforthe%changeinflexuralstrengthofCeFRCfromnormalconcrete.Eventhoughthef’c ofthismixislowertheadditionofcellulosefibersresultsina7.84%increaseinflexuralstrengthofnormalconcrete.A considerableincreaseinthemaximumdeflectionatthetimeoffailureisalsoobserved,whichindicatescellulosefiberhasa goodbondstrengthwiththeconcretematrixandabletotransferloadundertensileforces.Additionally,cellulosefiberhasa significantlyhighertensilestrength(750MPa)ascomparedtonormalconcrete(5MPa),whichalsoleadstoanincreasein flexuralstrengthofconcretewiththefiberaddition.

3.3.Waterpermeabilitytest

ThepermeabilitytestwasconductedonthesamplesbasedonDIN1048.Thepenetrationdepthofsamplesthatdeviated morethan20%ofthemeanvalueofsixsampleswereremovedfromtheresults.Figs.11and12showsthemaximumand averagepenetrationdepthsrespectivelyforbothconcretemixes.

HedegaardandHansen[42]explainedintheirstudythatforallpracticalpurposesconcreteisconsideredaswatertight when themaximumpenetration depthisless than50mm(2inch).Bothmixes indicatedless than50mm maximum penetrationdepthswhich rangesfrom24.11mmto28.33mmforcontrolmixand 17.57mm–21.54mmforCeFRC. The additionofﬁberssigniﬁcantlyreducedthemaximumandaveragewaterpenetrationdepthforconcrete.Figs.11and12

exhibitsthatadditionofﬁberresultsina24%and23.4%reductioninmaximumandaveragewaterpenetrationdepth respectively.Also,whenresultsarecomparedformaximumandaveragevaluesofpenetrationdepths,itwasobservedthat standard deviationwas lowerfor theaverage penetrationdepths, demonstratingaverage depthis morereliable than maximumdepth.

CoefficientofpermeabilitywasalsocalculatedusingEq.(3)basedonDIN1048testresults.Figs.13and14showsthe permeability coefficient calculated based onmaximum and average penetration depths respectively. Again,it can be observedthattheadditionoffibersresultedinadecreaseinpermeabilitycoefficientby42%and41%calculatedbasedon maximumandaveragepenetrationdepthsrespectively.Considerabledecreaseinthepenetrationdepthsandcoefficientof permeabilityindicatestheeffectivenessofcellulosefibersforwaterproofingpurposesinconcrete.

Table5

Flexuralstrengthtestresultsforconcrete.

MixDesign 28daysFlexuralStrength(MPa) 28daysMaximumMid-SpanDeﬂection(mm) Individual AverageSTDV Individual AverageSTDV

Cxx 5.60 5.620.03 1.33 1.510.18 5.60 1.43 5.67 1.76 0.5Cxx 5.97 6.060.09 1.767 2.180.32 6.02 2.22 6.19 2.56

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Theself-healingpotentialofbothconcretemixeswasinvestigatedbytheUPVtest.TheUPVtechniqueusedtoevaluate self-healingisthesameasusedbyArif_ﬁnetal.[38],Bahrinetal.[39]andSarkaretal.[40].TheUPVtimewasrecordedfor uncracked,pre-crackedand21dayshealedconcretecylindersacrossthecracktostudyanyinteriorhealingofthecracks.It shouldbenotedthatcrackcharacteristicssuchaswidthandlengthcanalsobemeasuredonthesurface,butthismaynot necessarilycapturethetortuosityofcracksinthe3rddimension(throughthedepth)ofthespecimen.Foreachsample,test wasperformedatthreelocationsofthecylinder;top,middleandbottom,andrepeatedatleastthreetimesforeachlocation. Datawerepresentedasaveragestandarddeviation.Table6showstheUPVresultsforallconcretemixesuncrackedand

Fig.12.AveragewaterpenetrationdepthbasedonDIN1048testresults.

Fig.13.CoefﬁcientofpermeabilityusingmaximumdepthbasedonDIN1048testresults.

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pre-crackedat28days.SameasdonebyZhongandYao[43],themicrostructurechangesinconcreteareinferredfromthe decreaseofUPVbyintroducingadamageddegreedeﬁnedas:

D¼1Vp

V0 ð5Þ

WhereDisthedamagedegreeoftheconcrete,VpisUPVofthepre-crackedsample,andV0istheUPVofanuncrackedsample.

Aself-healingratioofconcrete,SH,thatincorporatesUPVafter21daysofself-healingandpre-crackedsamplecanbe introducedas:

SH¼ðV21VpÞ

Vp ð6Þ

WhereV21istheUPVafter21daysofself-healing,andVpistheUPVofpre-crackedsample.

Mixeswerecomparedbasedonthedamageddegreeinsteadofcrackwidthsbecauseinducedcrackshavevaryingwidths anddepthsasexplainedearlier.Basedontheavailabilityanddistributionofresults,thedatawereaveragedfordamaged degreebetween0.6–0.7 onlyfor bothmixes. Fig.15shows thecuringdaysvs UPV relationandSH for28 days aged pre-crackedsamplesforDbetween0.6–0.7forbothmixes.

ZhongandYao[43]explainedintheirstudythattheUPVofconcreteisacombinedeffectofthematrix,microcracks,and macrocracks,whichcouldberepresentedas:

Vc¼_i 1 1 V1þ i2 V2þ i3 V3 ð7Þ Table6

UPVresultsforsamplespre-crackedat28days.

Mixdesign Samplename UPV(Km/s)STDV) Damageddegree, D=1-(Vp/V0)

Selfhealingratio SH=(V21-Vp)/Vp

28daysuncracked(V0) 28dayspre-cracked(Vp) 21dayshealed

Cxx C1Top 5.080 1.630.01 2.790.03 0.68 0.71 C1Middle 5.130.02 1.510 2.660.02 0.71 0.77 C1Bottom 5.180 1.870.16 2.910.01 0.64 0.55 C2Top 5.140.04 2.130.12 3.670.02 0.59 0.72 C2Middle 5.080.01 2.990.11 4.060.36 0.41 0.36 C2Bottom 5.130.03 1.410.06 2.850 0.72 1.02 C3Top 5.150 2.160.04 2.470 0.58 0.14 C3Middle 5.130.02 1.270.07 2.390.03 0.75 0.89 C3Bottom 5.130.03 2.150.02 2.850 0.58 0.32 0.5Cxx 0.5C1Top 4.960.01 0.610.13 4.640.01 0.88 6.64 0.5C1Middle 4.750.01 1.50.01 4.590.01 0.68 2.06 0.5C1Bottom 4.910.01 1.190.05 4.570.01 0.76 2.84 0.5C2Top 4.930 1.770.13 4.940.01 0.64 1.79 0.5C2Middle 4.840.03 1.750.08 3.030.08 0.64 0.73 0.5C2Bottom 4.850.02 1.460.07 2.50.02 0.70 0.72 0.5C3Top 4.930.01 0.790 4.770.08 0.84 5.08 0.5C3Middle 4.820.01 1.420 2.730.02 0.71 0.93 0.5C3Bottom 4.830 1.110.01 4.410.02 0.77 2.96 0.5C4Top 4.930.01 1.790.09 4.830 0.64 1.70 0.5C4Middle 4.780.01 1.990.06 4.390.25 0.58 1.21 0.5C4Bottom 4.830.02 1.830.02 3.790.09 0.62 1.07

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WhereVcistheUPVofconcrete,i1,i2,i3and V1,V2,V3arethevolumefractionandUPVofthematrix,microcracks,and

macrocracksrespectively.V1is afunction ofelasticmodulus,Poisson’sratio,inner frictionangleandmaterial fraction

toughnessofthematrix.V2canbeexpressed asafunctionofmicrocracksparameterssuchashalflength,shaperatio

anddensityofmicrocracks.Whereas,V3formacrocracksisultrasonicwavevelocityintheair(340m/s).Increaseofelastic

modulusandmaterialfracturetoughnessresultsintheincreaseofV1,whiletheincreaseofdensityofmicrocracksleadsto

decreaseofV2.

UPVismostlyinﬂuencedbyV1foruncrackedsamplesanditisclosetotheintactconcretematrixduetolessvolumeof

microcracksintheuncrackedsample.Oncetheconcreteiscracked,theuncrackedmatrixvolumei1decreasesgreatly,while

i2andi3increaseconsistently.CrackingofconcreteresultsintheriseofmicrocracksdensitythatleadstoadecreaseofV2,

thusUPVofcrackedconcretedecreasesgreatly.Oncethecrackedsamplesarecuredinwaterforself-healing,re-hydration productsofunreactedcementparticlescrystallizedinsidethecracksthatresultinadecreaseofi2andi3.Thecrystallization

alsochangestheinnermicrocracksandporousstructurethatresultsinanincreaseofV2,thusUPVofself-healedconcreteis

expectedtoincrease.

FromFig.15,itappearsthatthepresenceofﬁbersdecreasestheUPVslightlyforuncrackedsamples.Thisisexpectedas UPVthroughcelluloseislowerthanconcrete.CrackingdecreasestheUPVforbothmixeswhichindicatesthediscontinuity duetocracking.ItcanbeobservedthatcuringtimeresultsinanincreaseinUPVforbothmixeswhichindicatesthe autogenoushealingprocessworkingwellinsidethecracks.CeFRCresultsin48.7%higherUPVthannormalconcreteafter 21daysofhealingthatindicatesanenhancementofautogenousself-healingduetotheadditionofﬁbersforDbetween 0.6_–0.7.

ItcanalsobeobservedthatcontrolledandonlyﬁbermortarsamplesshowedaSHof0.62and1.46respectively.Thiscan beattributedduetocontinuedhydrationprocessofunhydratedcementparticlesandprecipitationofcalciumcarbonatedue tocarbonationofcalciumhydroxide,oneofthemajorhydrationproductsofcement.FiberconcreteresultsinmoreSHas comparedtocontrolconcreteindicatingthatcelluloseﬁberareeffectiveinimprovementofautogenoushealingofconcrete becausetheyhavethetendencytoabsorbwaterupto85%oftheirweightandactasawaterreservoirinsidetheconcrete matrixforimprovedinternalcuring.

Fig.16representstherelationbetweendamageddegreeandself-healingratioforsamplespre-crackedattheageof28 days.ItappearsthatSHisnearlydependentonthedamageddegree.ItcanbeobservedthatSHisincreasedasDincreasesfor bothmixes.CeFRCshowshigherSHthannormalmixforalldamageddegrees,indicatestheeffectivenessofcellulosefiber additionontheautogenoushealingofconcrete.However,asignificantlylargerSHathigherdamageddegreewasobservedin CeFRC than normal concrete.This is becausehigher Dresults inmore availability of waterthrough microcracks and availabilityofcellulosefibersinsidethecrackimprovesthebridgingofnewhydrationproductsacrossthecracks.However, thenewhydrationproductsinnormalconcretearenotabletobridgethecrackathigherdamageddegreethatresultsin lowerSHascomparedtoCeFRC.

3.4.Self-healingtestresults

Thesurfacecrackwidth,theoreticalcrackwidth,initialﬂowrateandhealingratioofthreecylindersforbothconcrete mixesarerepresentedinTable7.Theeffectofhealingwascalculatedbyintroducingahealingratio(HR)parameterasshown below[5,44]:

Healing Ratio¼1 Final flow Initial flow¼1

qF

q0>

0 ð8Þ

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Whereq0istheinitialwaterflow(lit/min)andqFisthefinalwaterflowmeasuredafterahealingperiodof28days.Edvardsen

proposedamodeltodeterminetherelationbetweencrackwidthandthewaterﬂowpassingthroughthecrack[45].The modelisshowninEq.(9).

q0

litres hour

¼740 ICW3avgKt ð9Þ

Whereq0istheinitialwaterleakagepermetervisiblecracklength(lit/h);Iisthehydraulicgradient,mofwaterhead/m;

CWavgistheaveragecrackwidthatthesurface(mm);ktisfactorcomprisingdifferentwatertemperature(kt=1forwaterat

20_C_with_viscosity_of₁_mm2_/s)._This_expression_can_be_modi

ﬁedtotheparametersofthisstudybychangingthecracklength (considered75mm,assomepartofcrackcoveredwithsealant)andotherunitsandcanbere-writtenasshowninEq.(10).

q0 litres min ¼740 1₀:7 :2CW 3 avg10:075 1 60¼7:863CW 3 avg  CWavg¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q0=₇_:863 3 q   ð10Þ

ThetheoreticalsurfacecrackwidthofallsampleswascalculatedfromEq.(10)byusingtheinitialﬂowrateofwater. SampleswithalmostsimilaractualandtheoreticalcrackwidthinTable7werecomparedforhealingratio.Table7showsthe initialﬂowrateincreaseswiththecrackwidthandsampleswithsmallercrackwidthexhibitsthehigherhealingratioas comparedtowidercracksforbothmixes.Thesmallercrackwidthsamplesdemonstrateahigherhealingratiobecause hydrationproductshavemoretendencytobridgethecrackwithasmallerwidthascomparedtowidercracks.SpecimenC6 withactualsurfacecrackwidthof0.865mmandtheoreticalcrackwidthof0.48mmwascomparedwithaveragedactualand theoreticalcrackwidthfor0.5C6and0.5C7,0.8625mmand0.49mmrespectively.ItcanbeobservedthatCeFRCexhibitsa slightlylessaveragehealingratioof0.609ascomparedto0.628ofnormalconcreteforcrackwidthof0.86mmafter28days ofthehealingperiod.

Fig.17showstherelationshipbetweenhealingratioandtimeforbothconcretemixes.Itcanbeobservedhealingratio increasedwithtimeofhealingforbothmixtures.However,celluloseﬁberconcreteexhibitsahigherrateofhealinginthe initial8days,followedbyamoderatelyhigherhealingratefromday8–20ascomparedtocontrolconcrete.After20days, bothmixesdemonstratealmostsimilarhealingrate.

AuthorshypothesizethatrapidhealingrateofCeFRCintheinitialhealingperiodcanbeimprovedbyusingother self-healingingredientsinconcretelikecrystallizeadmixtures,bacterialandcapsule-basedingredients,etc.

4.Conclusions

10.5 % volumefraction of cellulose ﬁbers results in a decrease in compressive strength of control concrete due to replacementofpartofconcretematrixwithcellulose_ﬁbersthatleadstolowerworkabilityandcompactionofconcrete.

Table7

Measuredcrackwidth,theoreticalcrackwidth,initialﬂowandhealingratioofconcretemixes. SampleID Surfacecrackwidth(mm) Theoreticalcrackwidth

fromequation(10)(mm)

Realinitialﬂow q0(lit/min)

Healingratio (HR) Top Bottom Average

Control C5 0.7 0.2 0.45 0.578 0.35 0.34 0.765 C6 0.43 1.3 0.865 0.48 0.86 0.628 C7 0.33 0.51 0.42 0.36 0.36 0.806 CeFRC 0.5C5 0.43 0.34 0.385 0.703 0.21 0.07 0.857 0.5C6 1.25 0.54 0.895 0.52 1.11 0.459 0.5C7 0.73 0.93 0.83 0.46 0.75 0.76

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2 Cellulosefiberresultsinanincreaseinflexuralstrengthofnormalconcreteby7.84%andaconsiderableincreasein maximumdeflectionarealsoobserved.Thisindicatesgoodfiberbondandtheirsuitabilityforuseasareinforcementin concrete.

3 Concretewithcellulosefibershada 24%lowerwaterpenetrationdepth and42%lowercoefficientof permeability comparedtocontrolconcrete,indicatedtheimprovementinwatertightnessofconcrete.Averagewaterpenetration depthsseemedtobeabetteroptiontocalculatethecoef_ficientofpermeabilityascomparedtomaximumpenetration depths.

4 Cellulosefiberactasawaterreservoirandresultinbetterinternalcuringofconcrete.AhigherSHand48.7%higherUPV velocitywereobservedinCeFRCascomparedtonormalconcreteafter21daysofself-healingwithdamageddegree between0.6to0.7,indicatesimprovementofautogenoushealingofconcreteduetotheadditionofcellulosefiber. 5 Self-healingratioofCeFRCincreasessignificantlythannormalconcreteathigherdamageddegreebecausefibersinside

thecrackimprovethebridgingofnewhydrationproductsacrossthecracksathigherdamageddegree,whichisnot possibleinnormalconcrete.

6 CeFRChasarapidhealingrateintheinitialdaysascomparedtonormalconcreteandthispropertycanbeenhancedby usingwithotherself-healingconcreteingredientslikecrystallineadmixtures,bacteriaandcapsule-basedingredients,etc. toimprovetheself-healingef_ﬁciencyofconcrete.

Fundingbody

ThisresearchwasfundedbytheNationalSciencesandEngineeringResearchCouncilofCanada,NSERC-CRDgrant.

Ethicalstatement

Weconﬁrmthatneitherthemanuscriptnoranypartsofitscontentarecurrentlyunderconsiderationorpublishedin anotherjournal.

DeclarationofCompetingInterest

Theauthorsdeclarenoconﬂictofinterest. Acknowledgement

Thetechnicalexpertiseandin-kindsupportprovidedbyMarkRyanfromSolomonUltraﬁberisgreatlyappreciated. References

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