Dynamic Properties of High Volume Fly Ash Nanosilica ... · Dynamic Properties of High Volume Fly Ash Nanosilica HVFANS ... The extensive use of high strength concrete : ... Moreover,

Original Article

LatinAmericanJournalofSolidsandStructures,2018,15 1 ,e06

DynamicPropertiesofHighVolumeFlyAshNanosilica HVFANS ConcreteSubjectedtoCombinedEffectofHighStrainRateand

Temperature

AbstractThestudyaimstodeterminethedynamicpropertiesofhighvolumeflyashnanosilica HVFANS concrete exposed to strain rates between 30.12 to101.42s‐1andtemperaturesof25,400,and700oCbyusingsplitHopkinsonpressure bar SHPB machine. The static and dynamic compressivestrengthsofHVFANSconcretewereslightlylowerthanplainconcrete PC atroomtemperature,whileitsvalueswerehigherat400and700oC.Theresults proved that the CEB model of dynamic increase factor is morereliable to estimate the behaviour of HVFANS concrete at studiedtemperatures. The toughness, critical strain, and damage of HVFANSconcrete recorded a superior performance than PC under studied strainrates and temperatures thatwould reflect the possibility of useHVFANSconcreteinstructurestoimproveitsresistantoffireandimpactloads,aswell as to decrease the demand on Portland cementwhich could lead torestricttherisksofliberatedgasesduringcementproduction.Furthermore,equations were proposed to estimate the dynamic increase factor,toughness, and critical strain of both concretes under investigatedconditions.

KeywordsHVFAconcrete;Nanosilica;DIF;SHPB;toughness;criticaldamage.

1.INTRODUCTION

Sinceearlydecadesof20thcentury,numerousstudieswereconductedtoassesstheconcretepropertiesunderelevated temperature to ensure the safety of engineering structures 1‐3 . The extensive use of high strengthconcrete HSC inmodernconstructionmotivatedseveralscholarstostudyitsthermalproperties.Theresultswerecontradictoryand indicated that thestrengthofHSCmightdeteriorateahigheror lesser thannormalstrengthconcrete NSC atelevatedtemperature 4‐7 .Recently,severalalternativesbinderswerewidelyusedinHSCasapartialcementreplacementtoreducetherisksofCO2emissionsliberatedduringcementproduction 8 .

Fly ash FA was broadly utilized as a Portland cement replacement owing to it’s a great performance inconcreteandlowprice.FAisaby‐productofthepulverizedcoalignitioninthermalpowerplants.Sincethe1980s,highvolumeflyash HVFA concretestartedtouseinCanadaviareplacingthePortlandcementby55to60%ofFA 9 . There is no a unified definition for HVFA concrete until now. Sivasundaram et al. 10 stated thatreplacementofPortland cementwith30%of FAmaydefine asHVFA concrete.However, other standards andresearchersdeclaredthatFAreplacementpercentageshouldbeabove40or50% 11‐15 .

ThemaindisadvantageofusingHVFAinconcreteisdecreasingofcompressivestrengthatearlyagesduetotheslowpozzolanicreactionofFA 16‐20 .Hence,nanosilica NS wereextensivelyusedinHVFAconcreteduetoitshighpozzolanicreactivitywithCHliberatedfromcementhydrationtoproducecalciumsilicatehydrate CSH whichgreatlyenhancestheconcretestrength 21‐24 .SeveralstudieswerecarriedoutonHVFAconcretewithNStoinvestigateitsthermalproperties.Ibrahim 25,26 reportedthatHVFANSconcretecanbeusedasafireproofmaterialundertemperaturesreachupto700oCviareplacingthePortlandcementby2.5and52.5%ofNSandFA,respectively.Whereas the HVFANS concretemaintained about 94.54% of its room compressive strength afterexposingtoatemperatureof700oC.Inthesamecontext,Radzi 27 usedthesameaboveproposedmixtureof

MohamedH.Mussa1*AzrulA.Mutalib1RoszilahHamid1SudharshanN.Raman2

1DepartmentofCivilandStructuralEngineering,UniversitiKebangsaanMalaysia,43600UKMBangi,Selangor,Malaysia2DepartmentofArchitecture,UniversitiKebangsaanMalaysia,43600UKMBangi,Selangor,Malaysia

*[email protected] [email protected] [email protected] [email protected]

http://dx.doi.org/10.1590/1679-78254900

Received:February04,2018InRevisedForm:February26,2018Accepted:March11,2018Availableonline:March20,2018

MohamedH.Mussa,AzrulA.Mutalib,RoszilahHamid,andSudharshanN.RamanDynamicPropertiesofHighVolumeFlyAshNanosilica HVFANS ConcreteSubjectedtoCombinedEffectofHighStrainRateandTemperature

LatinAmericanJournalofSolidsandStructures,2018,15 1 ,e06 2/19

HVFANSconcretetofabricateaslabwithdimensionsof 1850 1700 200 mmandtestedundertemperatureof 1100°C.The results revealed that the residual compressive strengthwas62.2%of theoriginal compressivestrengthatroomtemperature.Moreover,thetemperaturesofconcretecoverandbottomreinforcementwerelessthan300°Cwithamaximumspallingdepthandcoverageareaof23mmand34.3%,respectively.ItcanbenotedthatthesestudiesonHVFANSconcretewereconductedunderstaticloadcondition.However,theknowledgeaboutthedynamicbehaviourofthisconcreteathighstrainratesandelevatedtemperatureshasnotbeendetermined.

Several engineering structures such as tunnels and high‐rise buildings might expose to an explosion byterroristattacksduringtheservicetimesuchasthedetonationsofMoscowMetroin2004,LondonSubwayin2005,andNewYorkWorldTradeCentrein2001 28,29 .Theseincidentsandothershighlightedafactthatconcretematerialofstructuresmightbeindangeroffireandblastloadssimultaneously.Theblastorimpactloadingusuallyresultsinahighstrainrateeffectontheconstructionmaterialofstructures,whereasfirecausesahightemperatureeffect. These effects might cause a great change in the mechanical behaviour of concrete 3 . Therefore,understanding of HVFANS concrete behaviour under various conditions will increase the confidence of itsapplicationintheengineeringsector.

The Split Hopkinson Pressure Bar SHPB machinewas broadly used to study the dynamic properties ofconcreteandothermaterialsunderstrainratesof10to104s‐1 30,31 .Severalstudiesutilizedthismachinetodeterminethepropertiesofdifferentconcretetypessubjectedtothecouplingeffectofelevatedtemperatureandhighstrainrate.Li 3 determinedthedynamicpropertiesofHSCconcretecontained4.5%ofsilicafumeand23%ofFAundertemperaturesof25,200,400,600,and800oC.Theresultsrevealedthat thedynamiccompressivestrength improvedwith the increase of strain rate and evidently decreased after a temperature of 400 oC. Inaddition, the dynamic increase factor DIF was increased linearly with the strain rate. Huo 32 studied thedynamic properties of NSC under elevated temperatures reach up to 700 oC. The outcomes indicated that thetemperature and strain rate remarkably affected the dynamic increase factor DIF of concrete.Moreover, theresidualdynamicstrengthofconcreteevidentlydecreasedwiththeincreaseoftemperature,nevertheless,aclearinfluenceofthetemperatureandstrainratedidnotobserveontheshapeofnormalizedstress‐straincurves.Ziyanetal. 33 evaluatedthedynamicbehaviourofNSCconcreteattemperaturesof25,400,600and800oCunderstrainraterangeof30s‐1 to220s‐1.Theresultsproved that thecompressivestrengthofheatedconcretewashighlydegradationascomparedwithconcreteatambienttemperature.Inaddition,theeffectofhightemperatureonthedynamicpropertiesofconcretewasmorenotablethanthatofstrainrate.

Heetal. 34 statedthatNSCconcreteunderhightemperaturesandblastloadstendedtobemoresensitivetostrainratewiththeincreaseofhightemperature.Thedynamicpropertiesofconcreteatroomtemperaturewasclearlydifferentfromthatatelevatedtemperaturesmorethan200oC.Suetal. 35 demonstratedthatthedynamiccompressive strengthofNSC increasedbynearly14%ata temperatureof400 oCas compared to theambienttemperature.Chen 36 studiedthedynamicpropertiesofNSCatelevatedtemperaturereachupto950oC.Theexperimental results appeared that the dynamic compressive strength and stress‐strain curve at elevatedtemperature still experienced remarkable strain rate effects. Moreover, the failure modes of rapidly loadedspecimensatelevatedtemperatureswereconsiderablydifferentfromthoseatroomtemperature.Zhai 37 studiedtheeffectsofstrainraterangedbetween10‐4to300s‐1onNSCunderelevatedtemperaturereachupto1000oC.TheresultsprovedthattheDIFofNSCafterexposingtotheelevatedtemperaturesof600to800oCstillappearedacloserelationshipwithstrainrate.

Fromthepreviousstudies,itcanbeconcludedthatthefireandstrainratehadnotableeffectsonthedynamicstrengthofanytypeofconcrete.Therefore,themutualeffectsofelevatedtemperatureandhighstrainrateonthemechanicalbehaviourofHVFANSconcretewasevaluated.TherecommendedmixtureproportionbyIbrahim 25,26 was used to fabricate theHVFANS concrete samples due to its excellent thermal properties. The dynamicpropertiesofHVFANSconcreteweredeterminedattemperaturesof25,400and700oCbyusingSHPBmachineunder average strain ratebetween30.12 to101.42 s‐1 and theobtained resultswere comparedwith theplainconcrete PC .

2.EXPERIMENTALTESTPROCEDURE

2.1MaterialsofConcreteMixture

Portlandcement TypeI manufacturedbasedonMalaysianstandard MS522 usedintheconcretemixture38 .Theflyash FA wascollectedfromtheJimahpowerplantlocatedatMalaysianPortDicksonandcategorizedasclassFbasedonBritishstandard BSEN450:2005 39 .ThechemicalcompositionsofPortlandcementandFAwereshowninTable1.



Table1.ChemicalcompositionsofPortlandcementandflyash.

Items Clinker% Cement% Flyash%SiO2 21.66 21.28 44.16

Al2O3 5.8 5.6 24.6

Fe2O3 3.68 3.36 12.5

CaO 65.19 64.64 5.34

Mgo 2.86 2.06 2.5

TiO2 ‐ ‐ 4.1

K2O ‐ ‐ 1.5

SO3 0.2 2.14 0.3

Totalalkalis 0.07 0.05 ‐

Insolubleresidue 0.1 0.22 ‐

Lossofignition,LOI 0.27 0.64 5

Table 2 showed the properties of colloidal nanosilica NS type Cembinder W8 provided by AkzoNobel

Company,whichwasused in the concretemixture to improve theearly compressive strengthof concrete thatcontainedahighvolumeofFA.

Table2.Propertiesofcolloidalnanosilicamaterial.

Surfacearea m2/g Averagesize nm Silicaconcentration % Bulkdensity kg/m3 pH80 35 50 1050 10

Thefineaggregatewasnaturalriversandpassedthrougha4.75mmsievesizewithafinenessmodulusof2.98

andbulkspecificgravityof2.53 40,41 .Thelooseandcompactedbulkdensityoffineaggregatewere1510.18and1721.83kg/m3,respectively 42 .Thecoarseaggregatewascrushedgraniteprovidedfromlocalsourceswithamaximumsizeof10mmandbulkspecificgravityof2.07.

Superplasticiser SP typeDarexSuper20wasutilizedtoacquirethedesiredworkabilityofconcretemixtureandincreasethecompressivestrengthbydecreasingtherequiredvolumeofmixingwaterbasedonnaphthalenesulphonate.Fibrillatedpolypropylene PP fibresmanufacturedbyTimuranCompanywithalengthof12mmandspecificgravityof0.9hadutilizedtoincreasetheductilityofconcreteandeliminatethesurfacespalling 43‐47 .ThedosageofSPwasequalto1%ofcementitiousmaterialsweightandPPwasequalto1kg/m3ofconcretevolume.

2.2MixturePreparationandCuringProcess

Americanconcreteinstitutestandard ACI211.4R‐93 48 wasappliedtoselectthemixtureproportionsofPCandHVFANSconcretesampleswithwatertocementitiousmaterialsratio w/c p equalto0.29andtargetedstatic compressive strengthof 60MPaas shown inTable3.TheHVFANS concrete sampleswerepreparedviareplacingthecementwith52.5%ofFAand2.5%ofNSbasedontherecommendationsofIbrahim’sstudyconductedinUniversitiKebangsaanMalaysia UKM 25,26 .

Table3.Mixturematerials kg/m3 ofPCandHVFANSconcrete.

Concretetype Cement Sand Gravel Water Sp PP FA NSPC 531.30 780.06 942.64 170.16 5.31 1 ‐ ‐

HVFANS 225.80 682.01 942.64 154.85 5.31 1 278.93 26.56

During the study, the serving sequence of materials according to ASTM C192 49 caused cement balls

particularlyforHVFANSconcretebecausethemixturewasquitehotandstiff.Toavoidthisproblem,thefineandaportionofcoarseaggregatewithcementandFAwerefirstlymixedviahorizontalpanmixer.Then,thecolloidalNSandwaterwereaddedtothemixturetakingintoconsiderationthewatercontentofNSparticles.Finally,therestofcoarseaggregateandPPwereaddedtobreakupanynoodlesofthemixture 50 .TheslumptestwasperformedbeforeaddingSPandkeptbetween25to50mm.Themixturewascastintocylindricalandcubicsteelmouldswithdimensionsof 50 Ø50 mmand 150 150 150 mm,respectively.Thesampleswerede‐mouldedafter24h,



excepttheHVFANSconcretespecimenswhichstayedinmouldsfor48h 26 .Then,allthesampleswereplacedintoawatertankandcuredfor28days.

2.3HeatingProcess

ThestaticanddynamicpropertiesofPCandHVFANSconcretesamplesweredeterminedathightemperaturesof400and700oC,thesetemperatureswereselectedaccordingtopriorstudiesperformedbyIbrahim 25,26 .TheheatingprocessfollowedISO834firecurve 51 andcarriedoutbyusinganelectricfurnacewiththecapacityof3concretecubesandheatingrateof9oC/min.After2hofheatingatthementionedtargettemperatures,theheatingprocesswasstoppedandthenthesampleswereremovedandcooleddowntotheambienttemperature.

2.4StaticTest

The static test was performed based on British standard BS EN 12390‐3 52 by using an automaticcompressiontestmachineproducedbyUnitTestScientificCompany UTS withaloadingcapacityof5000kN.Theaveragecompressivestrengthvaluesofthethreespecimenswererecordedunder25,400and700oCatcuringageof28days.

2.5DynamicTest

Thedynamic testwasconductedbyusingsplitHopkinsonpressurebar SHPB machineavailable inUKMlaboratoryasshowninFig.1aand1b 53,54 .TheSHPBmachineconsistedofstrikerbarmovinginsidelaunchercylinder,incidentbarandtransmitterbar.AllthebarsarefabricatedfromstainlesssteelwithYoungmodulusof210GPaandwavevelocityof5190m/s.Thebarsdesignedwitha largeratioof lengthtodiameter inordertoensure uniaxial wave propagation through the bars. The bar diameter of 50 mm was found to be the mostappropriatetoachievetheminimumfrictionbetweenbarsandsupports toreducethewavesdispersionwhichsignificantlyimprovedtherecordedsignalsofstraingaugesasshowninFig.1c 53 .Agoodcontactbetweenbarsandconcretesampleswasachievedtominimizefrictionbysmoothingthesamplesurfacewithahandgrinderandsandpaper.Inaddition,thebarswerewellcoaxialbyusinglaseralignmentwithinanaccuracyupto0.01mmtosatisfytheforceequilibriumconditionasshowninFig.1d 53 .

Atotalof150cylindricalconcretesampleswith 50 Ø50 mmweretestedwithinanaverageoffivesamplesforeachappliedstrainrateunderdifferentthermalconditionsforPCandHVFANSconcrete.Lindholm 55 statedthatthesampleshavetobeshortenoughtorapidlyachieveauniformstressstatealongthe lengthofconcretesampleduringloading.However,usingofveryshortsamplesmayleadtoincreasethefrictionbetweenthebarsandsampleends,henceanapparentriseinstrengthcouldoccur 56 .ThespecimendiameteralsorequirestobesmallcomparativetothewavelengthoftheappliedloadpulsetosuccessfullyachievethetheoryofonedimensionalwavewithoutthecomplicationofPochhammer‐Chreeradialoscillations 57 .Inmostcases,theoptimumratiooflengthtodiameter L/D is0.5accordingtotherelationshipsuggestedbyDaviesandHunter 57 topreventwavedispersion. Nevertheless, the experimental results of Davies andHunter did not observe any obvious effect ofspecimensizeonstress–straincurveswithinL/Dratiosrangebetween0.31to1.55 57 .Consequently,theL/Dratiowaschosentobe1byusingspecimenswithdiameterof50mmandlengthof50mm.SimilarL/Dratiowasusedbyotherresearchers 53,54 .

Duringthetest,thecylindricalconcretesampleissandwichedbetweentheincidentandtransmitterbars,andthenthestrikerbarwaslaunchedataknownvelocitytowardtheincidentbartoachievethedesiredstrainrate.Newton’s second law applied to determine the striker’s acceleration according to the pressure exerted on thestrikerbar 58,59 .Acompressionstresspulsewasgeneratedintotheincidentbarduringtheimpactandtravelledinauniaxialdirectiontowardtheinterfacebetweentheincidentbarandconcretespecimen.Itsamplitudemainlyreliedonthestrikerbarvelocity 60,61 .Atthispoint,apartofthepulseisreflectedbackalongtheincidentbarasa tensile pulse, and the remaining part is transmitted from the concrete sample to the transmitter bar as acompressivepulseandthenrecordedbyastraingauge.



Fig.1. a SHPBmachineview b ComponentsofSHPB mm c Frictioneffectonincidentwave d Recordedsignals

ofincidentandtransmittedbarsduringthetest

TwoOmegastraingauges’model SGD‐3/350‐LY11 wereusedtomimicthematerialelongationofincidentandtransmitterbars.Thechangeintheresistanceofstraingaugewastoosmallduetothelowvaluesofstrainandtheonlywaytodetect itschangeisbyusingavoltagemeasurement.Therefore, thestraingaugewasquarterlyconnectedtoOmegabridgesensorsmodel OM2‐163 whichconsidersasacompletesignalconditioningsystemand consistofhighperformance instrumentationamplifier.Thebridge sensorswere connected toOmegadata



acquisitionmodule OMB‐DAQ‐3000 whichused OMB‐DaqView‐XL computersoftwaretoconverttheanalogyvoltagesignalstodigitaldatathatcouldbestoredandanalysed.MATLABsoftwarewasusedtoanalysethedigitaldataandobtainedthestress‐straindiagramsofconcretesamples.Thestraingaugeoftheincidentbarrecordedtheincident pulse and reflected pulse , while the strain gauge on the transmitter bar measured thetransmitted pulse 62 . Once all the pulses are recorded, the stress , strain rate t , and strain historiesofthesamplecanbecomputed,respectively,byusingthefollowingequations 54,62,63 :

s

AE 2A i r tt t t t 1

0

s

Ct

L i r tt t t 2

t

0

s 0

C dt

L i r tt t t t 3

where , , arethecross‐sectionalarea,theYoung’smodulusandthewavevelocityofthebarmaterial,and, ,arethelengthandcross‐sectionalareaofthespecimen.

3.RESULTSANDDISCUSSIONS

3.1StaticTestResults

Table4andFigure2indicatedthatthecompressivestrengthofHVFANSconcreteatroomtemperaturewasmarginallydecreasedby4.21%ascomparedwithPCduetothehighquantityofflyash FA classFwhichcontainedalowamountofcement‐likeproperties CaO 64 .HVFANSconcreteappearedaclearincreaseincompressivestrengthatexposingtemperatureof400oCbecausetheheatingprocesssignificantlyenhancestheNSreactivityaswellasincreasethehydrationprocessviaproducingahigh‐densitycalciumsilicatehydratestructurewhichgreatlyimprovestheconcretestrength 64 .

Table4.StaticcompressivestrengthofPCandHVFANSconcreteatvarioustemperatures.

SampleNo.

Temperature oC

25 400 700

PC HVFANS PC HVFANS PC HVFANS

1 62.66 57.95 58.61 67.79 38.87 51.6

2 59.08 61.55 56.48 61.01 33.54 52.13

3 61.82 56.68 59.48 64.86 36.71 46.26

Average MPa 61.19 58.72 58.19 64.55 36.37 49.99

MaximumRelativeError % 3.43 4.82 2.94 5.48 7.78 7.46



Fig.2.BehaviourofPCandHVFANSconcreteatdifferenttemperatures

At temperature of 700 oC, a dramatic reduction in compressive strength ofHVFANS andPC concretewasobservedduetotheextremeaccumulationofvapourpressureinsidethespecimenswhichcausedalargenumberof cracks, besides the dehydrate of cement paste binder products at this temperature. Nevertheless, HVFANSconcrete revealed a higher residual compressive strength by 85.13% from their original strength at roomtemperatureascomparedwithPCwhichmaintainedonly59.44%,thisbehaviourmayattributetothefillereffectofNSinthemixturewhichincreasedthecalciumsilicahydratecontentforHVFANSconcrete.Therelativeerrorofeachcompressivestrengthgroupwaslessthan7.78%whichrevealedthattheexperimentalrecordswerefairlyreliable.

3.2DynamicTestResults

3.2.1CompressiveStrengthanddynamicincreasefactor DIF

Theultimatedynamiccompressivestrengthrepresentsthepeakstressdeterminedfromstress‐straincurvesofPCandHVFANSconcretespecimenssubjectedtofiveaveragestrainratesandtemperaturesof25,400,and700oCasshowninFig.3.

Thedynamicstress‐straincurvesappearedacloserelationshipwiththeincreasingofaveragestrainrateforbothconcretetypesandthedescendingpartofcurveswaspointedtothesamplesdestructionandstrainrelaxingafterultimatestress 65 .Thedynamiccompressivestrengthincreasedapproximatelylinearlywiththeincreaseofaveragestrainrateforbothconcretetypesundertemperaturesof25,400,and700oCasshowninFig.4.



Fig.3.Stress‐straincurvesofPCandHVFANSconcreteatvarioustemperatures.

Fig.4.DynamiccompressivestrengthofPCandHVFANSconcreteatdifferentstrainratesandtemperatures.



Smallvariationsincompressivestrengthwereobservedat400oCforbothconcretetypes;however,arapidreductionwasdetectedat700oCparticularlyforPCsamples.TheHVFANSconcreteshowedagoodresistantfordynamicloadsatexposingtemperaturesof400and700oCthanPCinalltheinvestigatedcasesofstrainrate,thatreflectedthegreateffectofNSonconcretestrengthwhichworksasafillerforsmallvoidsthatmayincreasethecalciumsilicahydratecontentinmixture.

Dynamicincreasefactor DIF broadlyusedtoevaluatetheeffectofstrainrateonthecompressivestrengthanddefinedastheratiobetweendynamictostaticcompressivestrength 3,34 .TheresultsprovedthatDIFmainlydependedon the strain rate and increasedalmost linearlywith the increaseof strain rate forPC andHVFANSconcretesamplesatalltheinvestigatedtemperaturesasshowninFig.5.

Fig.5.DIFofPCandHVFANSconcreteatvariousstrainratesandtemperatures.

Theresponsesurfacemethodology RSM availableintheDesignExpertSoftware version11 66 wasusedforthestatisticaldesignandanalysisofexperimentaldata.ThemaingoalwastoderiveanequationthatcouldsignificantlyrepresenttherelationshipofDIFwiththeexposingtemperature T anddynamicstrainrate .Thelinearand2FIsolvermodelswiththeinversetransformationwerethemostaccuratetofittheexperimentaldataofDIFforPCandHVFANSconcrete,respectively,asfollows:ForPC:

10.943249 0.000028 T 0.003796 dDIF

4

ForHVFANSconcrete:

610.980249 0.000156 T 0.004310 2.08303*10 * *d dT

DIF 5

where:Tistheexposingtemperaturebetween25to700oC,and isthedynamicstrainratewithinarangeof30.12to101.42s‐1.

Theresultsofvarianceanalysis ANOVA wereusedtoevaluatetheaccuracyoftheadoptedmodelsasshowninTable5.TheoutcomesrevealedthatthecurrentmodelscanbeefficientlyusedtopredictthevaluesofDIFforbothtypesofconcretewithinthementionedrangesoftemperatureandstrainrate.Thequalityofthefitpolynomialmodelwasexpressedbythevalueofcorrelationcoefficient R2 whichshouldbecloseto1withaminimumvalueof0.8 67,68 .Moreover,thedifferencesbetween“PredictedR2”and“AdjustedR2”shouldbelessthan0.2andtheAdequatePrecision AP whichmeasuresthesignaltonoiseratioofthemodelsshouldbegreaterthan4 69 .



Table5.TheresultsofANOVAanalysisforDIFmodelsofPCandHVFANSconcrete.

Concrete Transformation Model R2 AdjustedR2 PredictedR2 AdequatePrecision APPC inverse linear 0.9826 0.9797 0.9736 45.6237

HVFANS inverse 2FI 0.9764 0.9700 0.9516 35.9775

Several experimental tests had extensively used the DIF to evaluate the effects of dynamic strain rate on

concrete in tension or compression 70, 71 . Based on to their studies, expressions were recommended todeterminetheDIFofconcreteincompressionasfollows:a Inthe1993s,CEB 72 proposedanequationtocalculatetheDIFofconcreteincompressionunderhighstrain

ratesreachupto300s‐1:

1.026 1

1/3 1

/ 30

/ 30

s

d s dCEB

s d s d

For sDIF

For s

6

where isdynamicstrainrate, isstaticstrainratewhichequalto30 10 , 1 5 9 10⁄⁄ ,log6.156 2,and isstaticcompressivestrengthinMPa.b Inthe1997s,Tedescoetal. 73 derivedanequationtodeterminetheDIFofconcrete,whereasthetransition

fromlowstraintohighstrainrateswasoccurredat63.1s‐1:

0.000965 log 1.058 63.10.758 log 0.289 63.1

7

c Inthe2008s,ZhouandHao 74 proposedanequationtoevaluatetheDIFofconcrete‐likematerialsasfollows:

&0.0225 og 1.12 100.2713 og 0.3563 log 1.2275 10

8

Inthepresentstudy,thedynamicincreasefactorcurvesoftheexperimentaltestwereextendedbehindthestrainrateof30s‐1untilthestaticconditionwhentheDIFisequalto1.Theexperimentaldynamicincreasefactorsof both concrete types under temperatures of 25, 400, and 700 oC were compared with the results of therecommendedexpressionsofdynamicincreasefactorasdescribedinFig.6.TheoutcomesprovedthattheCEBmodel expression may efficiently estimate the dynamic increase factor of both concrete types under roomtemperature.However, thedifferencesbetween theexperimentalandCEBresultsclearly increasedatelevatedtemperatures particularly in case of PCunder400 and700 oC thatmay attribute to the great loss in concretestrength. Nevertheless, the CEBmodelwasmore reliable to predict the behaviour of HVFANS concrete undermentioned temperatures that reflect the excellent role of nanosilicamaterialwhich significantly enhances thestrengthofconcreteunderhightemperature.

ItcanbeconcludedthattheCEBexpressioncouldsuccessfullyusetoobtainafullrangeofdynamicincreasefactorversusstrainraterelationshipforHVFANSconcreteincompressionundertemperaturesof25,400,and700oCwhichcouldsignificantlyutilizeasinputdataforthenonlinearconcretematerialmodelinnumericalanalysisofstructureconstructedbyusingthistypeofconcreteandsubjectedtofireandblastloads.



Fig.6.ComparisonbetweenDIFoftestresultsandexistingexpressionsatdifferenttemperatures

3.2.2Toughness

Specific energy absorption SEA defines as the energy absorption per unit volume of concretematerial.ScholarsusedtheSEAtocharacterizethetoughnessofconcretespecimensasfollows 3,75,76 :

2 2 2

0

T

i r ts s

AEcSEA t t t dt

A l 9

whereA,E,carethecrosssectionalarea,Young’smodulus,andelasticwavespeedofbars,respectively,and , arethelengthandcrosssectionalareaofthespecimen: , , aretheincidentpulse,reflectpulseandtransmitpulse,respectively;Tisthecompletefailuremomentofspecimen.Figure7indicatedthattheSEAofPCandHVFANS



concrete specimens increased linearly with increasing of strain rate, similar behaviour was observed byresearchersonconcretematerialtestedbyusingSHPBmachineunderroomorelevatedtemperatures 76,77 .Smallvarianceswerenotedintoughnessofbothconcretetypesatroomtemperature.However,theincreaseoftemperatureuntil400oCledtoreducingthetoughnessofPC,whilethesametemperaturecausedanincreaseintoughnessofHVFANSconcreteby9.79%underaveragestrainrateof100.67s‐1ascomparedwiththetoughnessatroomtemperature.

Fig.7.ThetoughnessofPCandHVFANSconcreteatvariousstrainratesandtemperatures.

Ontheotherhand,anotablereductionwasobservedinthetoughnessofPCandHVFANSatatemperatureof700oCby84.02and28.83%underaveragestrainratesof100.81and100.54s‐1,respectively,ascomparedwiththe toughness at room temperature.DesignExpert softwarewas used to suggest the relationship of SEAwithdynamic strain rate and exposing temperature T byusingquadratic solvermodelwith square root andnaturalalgorithmtransformations forPCandHVFANSconcrete,respectively,withahighaccuracyasshowninTable6.ForPC:

10.28522 0.013370 0.343809 0.000096 ∗ 0.0000250.000769 10

ForHVFANSconcrete:

ln 5.13192 0.001126 0.030767 2.86175 ∗ 10 ∗ ∗ 1.57476 ∗ 10 ∗ 0.000105 11

where:Tistheexposingtemperaturebetween25to700oC,and isthedynamicstrainratewithinarangeof30.12to101.42s‐1.

Table6.TheresultsofANOVAanalysisforSEAmodelsofPCandHVFANSconcrete

Concrete Transformation Model R2 AdjustedR2 PredictedR2 AdequatePrecision APPC Squareroot quadratic 0.9972 0.9956 0.9924 82.4900

HVFANS NaturalLog quadratic 0.9910 0.9861 0.9774 41.0776

3.2.3Criticalstrain

Critical strain defined as the strain of concrete corresponding to peak stress, which used to describe theductilityofPCandHVFANSconcretesamplesexposedtohighstrainratesandtemperaturesof25,400,and700oC



asshowninFig.8.Thecriticalstrainincreasedapproximatelylinearlywithincreasingofstrainrateandrecordedsmallvaluesattemperaturesof25and400oCforbothtypesofconcreterangedbetween0.005to0.0075duetotheexistentofPP fibreswhich considerablyenhance theductilityof concreteandprovideescape channels forvapourpressureinsidethevoidsatelevatedtemperatures 78‐80 .

Fig.8.CriticalstrainofPCandHVFANSconcreteatvariousstrainratesandtemperatures.

ThesmallestvaluesofcriticalstrainwereobservedforHVFANS400thatmayattributetothehighreactivityofNSinthemixturewhichincreasedataheatingtemperatureof400oC.Otherwise,thecriticalstrainobviouslyincreasedatatemperatureof700oCforbothconcretetypesandthehighestincrementofcriticalstrainwasnotedforPC700by47.75%ascomparedwithPC25underaveragestrainrateof100.81s‐1.Therelationshipofcriticalstrain ε withdynamicstrainrate andtemperature T wasproposedbyusingDesignExpertsoftwarewithaquadraticsolvermodelandinversetransformationforbothconcretetypeswithinagoodaccuracyasshowninTable7.ForPC:

171.21120 0.040541T 0.055351 0.000199T ∗ 0.000119 0.002501

12

ForHVFANSconcrete:

170.76193 0.122916T 0.256219 0.000015T ∗ 0.000196 0.001491

13

where:Tistheexposingtemperaturebetween25to700oC,and d isthedynamicstrainratewithinarangeof30.12to101.42s‐1.

Table7.TheresultsofANOVAanalysisfor modelsofPCandHVFANSconcrete

Concrete Transformation Model R2 AdjustedR2 PredictedR2 AdequatePrecision APPC inverse quadratic 0.9949 0.9920 0.9811 59.4065

HVFANS inverse quadratic 0.9919 0.9874 0.9760 50.2862

3.2.4CriticaldamageandFailuremodes

Severalstudieshadusedthecontinuousdamagemechanicstheorytodescribethecriticaldamageofconcreteunderdynamicloadsaccordingtothedifferencesinstress‐straincurveswiththeincreaseofstrainrate 81‐83 .Thecriticaldamagevariable waspresentedanddefinedasascalar:



0 0

1 0

d

pcrd

o cr

For

DFor

E

14

where isthepeakstress, isthecriticalstrainand istheinitialmodulusofelasticity.Thegivenexpressiondefinedthecriticaldamageasakinematicvariablethatmainlyreliesonthedensityofvoidsorcracksinaspecificcrosssectionwhichmayresult in thegradualdamageofconcrete 84,85 .Thecaseof 1means that theconcreteisnotabletoresisttheapplyingloads.Figure9indicatedthatthecriticaldamageincreasedalmostlinearlywiththeincreaseofstrainrateandprovedthatthedamageofHVFANSconcreteatroomtemperaturewasslightlyhigherthanPCundersimilarconditionsofstrainrate,thatmayattributetothehighvolumeofflyashinthemixture.However,theHVFANSconcreteappearedlowervaluesofcriticaldamageattemperaturesof400and700oCby6.26 and9.14%under average strain ratesof 100.67 and100.54 s‐1, respectively, as comparedwithPCundersimilarrangeofstrainrate.

Fig.9.CriticaldamageofPCandHVFANSconcreteatvariousstrainratesandtemperatures.

Duringexperimentalstudy,theinfluencesofstrainrateandtemperatureonthedamageofPCandHVFANSconcretewerevisible.Thefailureofconcretesampleswassoviolentandcausedseveredestructionofcylindricaltest samples intosmallpiecesathighstrainratesasshown inFig.10a.Thepatternof shear failurecouldonlyobserve at the range of strain rate between 30.12 to 70.91 s‐1 under studied cases of room and elevatedtemperaturesexceptforthecaseofPCunder700oCwhichwastotallydestructedatanaveragestrainrateof70.17s‐1.ThevariationofconcretefailurewithincreasingofstrainrateswasshowninFig.10b.

Itcanbenotedthattheshearfailureofconcretesamplescouldpassthroughthreestagesaccordingtotheincreaseofstrainrate.Inthefirststagewhentheaveragestrainratebetween30.12to50.91s‐1,thecrack’spathwaspassedthroughouttheconcretemortaronly.Foraveragestrainratesof70.17to71.32s‐1,thepathofcrackswasmore straight andpassed through a rough fracture surfacewhichhad amorebroken aggregate thatmayattributetotherapidincreaseofstressinsampleswhichwasquitesufficienttobreaktheaggregateandconcretemortarzonesalongthepathoflessresistance.Inthefinalstageofstrainrates 85.17s‐1 ,thecracksamountwasgreatly increasedand resulted in a severe fracturingof concrete samples into small fragments todissipate theinternalenergy. In the samecontext,numerous researcherswereobtainedsimilarpatternsof shear failure forcement‐basedmaterialsaccordingtotheincreaseofstrainrate 75,86‐88 .



Fig.10.FailuremodesofPCandHVFANSconcreteatvariousstrainrates.

4.CONCLUSION

Theresultscanbesummarizedasfollows:1.ThestaticcompressivestrengthofPCwashigherby4.21%thanHVFANSconcreteatroomtemperature.A

dramaticreductionincompressivestrengthwasobservedat700oCforbothtypesofconcrete;nevertheless,theHVFANSconcreterecordedahighresidualstrengthof85.13%ascomparedwithPCwhichmaintained59.44%.

2.HVFANSconcreteachievedanexcellentperformancethanPCintermsofdynamiccompressivestrength,toughness,criticalstrainanddamageunderelevatedtemperaturesof400and700oCsubjectedtoaveragestrainratebetween30.12to101.42s‐1.

3.TheCEBmodelwasthemostappropriatetopredictthedynamicincreasefactorsofHVFANSconcreteunderthestudiedtemperatures.

4.Equationswereproposedtopredictthedynamicincreasefactor,toughness,andcriticalstrainofbothconcreteswithinanexposingtemperaturerangeof25to700oCandaveragestrainrateof30.12to101.42s‐1.

ThementionedresultsprovedagoodpossibilitytouseHVFANSconcreteinstructurestoimproveitsresistanttofireanddynamicloads,besidesitseco‐friendlyproperties.

Acknowledgments

TheauthorswouldliketoacknowledgethefinancialsupportfromtheFundamentalResearchGrantSchemegrantnumberFRGS/1/2015/TK01/UKM/02/4 ,ArusPerdanaGrant AP‐2015‐011 andDanaImpakPerdanagrantnumberDIP‐2014‐019 toperformthisresearch.

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