Mechanicalpropertiesof high-strengthsteelber-reinforcedconcreteP.S.Songa,*,S.HwangbaDepartmentofCivilEngineering,DahanInstituteofTechnology,Sincheng,Hualien971,Taiwan,ROCbDepartmentofSystemEngineering,ChungChengInstituteofTechnology,NationalDefenseUniversity,Dashi,Taoyuan335,Taiwan,ROCReceived7August2002;receivedinrevisedform13April2004;accepted14April2004Availableonline17June2004AbstractThemarkedbrittlenesswithlowtensilestrengthandstraincapacitiesofhigh-strengthconcrete(HSC)canbeovercomebytheadditionof steel bers. This paper investigatedthemechanical properties of high-strengthsteel ber-reinforcedconcrete. Theproperties included compressive and splitting tensile strengths, modulus of rupture, and toughness index. The steel bers were addedatthevolumefractionsof0.5%,1.0%,1.5%,and2.0%.Thecompressivestrengthoftheber-reinforcedconcretereachedamax-imum at 1.5% volume fraction, being a 15.3% improvement over the HSC. The splitting tensile strength and modulus of rupture ofthe ber-reinforced concrete improved with increasing the volume fraction, achieving 98.3% and 126.6% improvements, respectively,at 2.0% volume fraction. The toughness index of the ber-reinforced concrete improved with increasing the fraction. The indexes I5,I10,and I30registeredvaluesof6.5,11.8,and20.6,respectively,at2.0%fraction.Strengthmodelswereestablishedtopredictthecompressiveandsplittingtensilestrengthsandmodulusofruptureoftheber-reinforcedconcrete. Themodelsgivepredictionsmatchingthemeasurements.2004ElsevierLtd.Allrightsreserved.Keywords:Steelbers;High-strengthconcrete;High-strengthsteelber-reinforcedconcrete1.IntroductionThe engineering characteristics and economic ad-vantages of high-strengthconcrete (HSC) are distinctfromconventional concrete, thereby popularizing theHSCconcreteinalargevarietyof applications intheconstruction industry. Used for high-rise buildings,HSCavoidstheunacceptableoversizedcolumnsontheloweroors, allowinglargecolumnspacingandusableoorspace,orincreasingthenumberofpossiblestorieswithoutdetractingfromloweroors[1].Usedforlong-span bridges, HSCreduces the dead load of bridgegirdersforfewerandlighterbridgepiersandthusen-ablesgreaterunderpassclearancewidths. HSCinspiressubstantial savings in expenditure on bridge mainte-nance, while prolonging the serviceable life of thebridges[2]. Further, HSCpossessesuniformhighden-sityandverylowimpermeability, endowingitself withexcellentresistancetoaggressiveenvironmentsanddis-integrating agencies, and beneting the durability ofconcretebuildingsandstructures[3,4].The comparatively higher compressive strength ofHSCis anattractive prot; whereas, the strengthbe-haves against the ductility of the concrete by welcomingbrittlenesspronouncedly[5].TheHSCalwayspossessesasteeperdescendingstressstraincurveincompressionthandoesthenormal strengthconcrete. Therapidde-crease in compressive strength in the post-peak loadregion brings about a pronouncedly brittle mode offailure[6]. Tofoster thecompressivestrengthwithoutsacricing the ductility, a strategy is to add discrete steelbersasreinforcementinHSC[7].Asthehigh-strengthsteel ber-reinforced concrete (HSFRC) hardens,shrinks,orbearsserviceloadstodevelopcracksandtopropagate them, the bers evenly distributed throughoutthe composite intersect, block, and even arrest thepropagating cracks. This way, the addition of berscontributes strengthtotheconcrete[8]. First, KhalooandKim[9] investigatedthestrengthimprovement to*Correspondingauthor.Tel./fax:+886-3-8263936.E-mailaddress:pssong@ms01.dahan.edu.tw(P.S.Song).0950-0618/$-seefrontmatter 2004ElsevierLtd.Allrightsreserved.doi:10.1016/j.conbuildmat.2004.04.027ConstructionandBuildingMaterials18(2004)669673Constructionand BuildingMATERIALSwww.elsevier.com/locate/conbuildmatHSC containing 0.5%, 1.0%, and 1.5% volume fractionsof steel bers, declaringthat compressiveandsplittingtensile strengths improved to 1.0% fraction, whereas themodulusofrupturedidupto1.5%.ErenandCelik[10]studiedthestrength-producingeectofsteel bersandsilica fume in HSC, indicating that the ber volume andberaspectratiogovernedthecompressivestrengthofthe concrete. ChunxiangandPatnaikuni [8] indicatedthat the compressive strength of HSFRC increased withmaturity, which increased 24%in the aged 76 dayHSFRC. AccordingtoMarar et al. [11], at eachberaspect ratio, the compressive strengthof HSFRCim-provedwiththeincreaseinbervolume.AndasfarasDaniel and Loukili [12] declared, the compressivestrengthof HSFRCheld15%advantageoveritsHSCpartner. Theforegoingdiscussionsindicatethesteel -ber additions primarily exerting the pick-up eect on thecompressivestrength. However,theadditionsplayalsodevotedly in developing splitting tensile and exuralstrengths.This paper further investigated the strength im-provingpotentials of HSFRCcontaining0.5%, 1.0%,1.5%, and2.0%volume of hooked-endsteel bers incomparison with the plain high-strength counterpart,and established models predicting the behavior ofHSFRC under compression, splitting tension, andexure.2.Experimentalprogram2.1.MaterialsTypeIcement,riversandwithanenessmodulusof3.1, andcrushedbasalt of 19mmmaximumsizewereused. Silica fume used was a commercially availablebyproduct of theproductionof siliconmetal andfer-rosiliconalloys, whichimprovedconcretepropertiesinfreshandhardenedstates. Toimprovetheworkabilityof concrete, a high-range water-reducing admixture(superplasticizer) was employedduring mixing opera-tions. Thecement, silicafume, water, superplasticizer,river sand and crushed basalt of 430, 43, 133, 9, 739 and1052kg/m3wereusedtomaketheHSC. Theslumpofthe concrete was 60mm. The hooked-endsteel bersweremadeofmildcarbonsteel. Thebershaveanav-eragelengthof35mm, nominal diameterof0.55mm,andtheaspectratioof64.Thesebersareavailableinbundlesof about 30bers, whichwerebrillatedwithwater-soluble glue to ensure immediate dispersion inconcreteduringmixing.2.2.PreparationofsamplesIntheproductionofconcrete, theconstituentmate-rialswereinitiallymixedwithoutbers.Theberswerethenaddedinsmallamountstoavoidberballingandtoproducetheconcretewithuniformmaterial consis-tencyandgoodworkability. Forconcretemixeswitha2.0%volume of bers, extra time was required formixing. Thefreshlymixsteel ber-reinforcedconcretewasplacedintwoequal layersintoacylindermoldtocast a standard 150 300 mm cylindrical concretespecimenforacompressivestrengthtestandasplittingtensiletest, andintoa150 150 530mmbeammoldforaexurestrengthtest. Eachlayerwasconsolidatedusingavibratingtable. At theendof 24hafter con-solidating, the specimenwas removedfromthe moldand curedin waterat73 3 Ffor28 days. Andthenastrengthtestwasperformed.2.3.TestmethodsThe compressive strengthtest, performedon15ofthe standardtest cylinders, followedASTMC39 testfor compressive strengthof cylindrical concrete speci-mens. Thecylinderswereloaded, inatestingmachineunder load control, at the rate of 0.3 MPa/s untilfailure.Thesplittingtensiletest, runon15of thetest cyl-inders,wasinaccordancewiththeASTMC496testforsplitting tensile strengthof cylindrical concrete speci-mens, althoughACI committee544.2Rhardlyrecom-mendstheuseof thetest onber-reinforcedconcrete.Therunningarosebecausetheratioof berlengthtocylinderdiametertookalowvalueof0.23intheworkand because some investigators have shown that theASTMC496test isapplicabletober-reinforcedcon-crete specimen [9,13]. In the test, load applicationswere continuous and shockless, at a constant rateof 900 kPa/min splitting tensile stress until specimenfailure.The exural strength (modulus of rupture, MOR)test, conductedusing15test beams under third-pointloading, followed the ASTMC1018 test for exuraltoughness and rst-crack strength of ber-reinforcedconcrete. The mid-span deection was the average of theonesdetectedbythetransducersthroughcontact withbrackets attachedtothe beamspecimen. The testingmachine ran to increase the deection at a constant rate;the loaddeection relation recorded using an XYplotter.3.ResultsanddiscussionTable1presentsthestrengthtestresultsonHSFRCandHSC.Eachstrengthtestresultwastheaveragefor15test specimens. The compressive strength, splittingtensile strength, and modulus of rupture of HSFRCimprovedtodierent extents inresponse tothe bervolumefractions.670 P.S.Song,S.Hwang/ConstructionandBuildingMaterials18(2004)6696733.1.CompressivestrengthThe compressive strength development of HSFRCversus HSCappears Fig. 1, declaring that the com-pressive strength f0cof HSC was 85 MPa and of HSFRCprovidedanimprovementateachvolumefraction.Theimprovement, as the strength-eectiveness inTable 1,was 7.1%at 0.5%fraction, 11.8%at 1.0%fraction,15.3%at1.5%fraction, andreducedto12.9%at2.0%fraction, being a reduction small compared to themaximumimprovement at 1.5%fraction. The com-pressivestrengthimprovementofHSFRCrangedfrom7.1%to15.3%atthevolumefractionsof0.5%to2.0%,comparabletotheimprovementsof4.310.4%fornor-mal-strengthconcreteatthesamefractions[14].Followingfromthecompressivestrengthtestresults,thecompressivestrengthf0cfof HSFRCwas predictedusingthecompressivestrength f0cofHSCandthebervolumefraction Vf,andwasexpressedasf0cfMPa f0c AVf BV2f: 1Substitutingf0c 85MPainEq. (1) andapplyingtheregressionanalysisgavef0cfMPa 85 15:12Vf 4:71V2f: 2The compressive strength predictions using Eq. (2)agreed favorably with the test results, as in Table 2. Thepredictionerrorsrunbelow1.02%.3.2.SplittingtensilestrengthThe development of splitting tensile strength ofHSFRCatvariousvolumefractionsisshowninFig.2;comparedtoHSC, the strengthof HSFRCimprovedwithincreasingthevolumefraction.Fromthestrength-eectivenessinTable1, theimprovement startedfrom19%at 0.5%fractionandexpandedto98.3%at 2.0%fraction.Thesplittingtensilestrength ftfofHSFRCwaspre-dictedbyusingthecompressivestrengthf0cpof HSCandthevolumefraction Vf,andwasgivenasfollows:0.0 0.5 1.0 1.5 2.0Fiber volume fraction Vf (%)7580859095100Compressive strength (MPa)MeasurementsPredictions f'cf = 85 + 15.12Vf - 4.71Vf2Fig.1.Eectofbervolumeoncompressivestrength.Table2ComparisonofpredictedandmeasuredvaluesforcompressiveandsplittingtensilestrengthsandmodulusofruptureFibervolumefraction(%)Compressivestrength Splittingtensilestrength ModulusofrupturePredicted(MPa)Measured(MPa)Predictionerrora(%)Predicted(MPa)Measured(MPa)PredictionError(%)Predicted(MPa)Measured(MPa)Predictionerror(%)0 85 85 0 5.8 5.8 0 6.4 6.4 00.5 91 91 0 7.3 6.9 5.80 8.2 8.2 01.0 95 95 0 8.8 8.7 1.15 10.2 10.1 0.991.5 97 98 )1.02 10.3 10.8 )4.63 12.3 12.3 02.0 96 96 0 11.7 11.5 1.74 14.5 14.5 0aPredictionerror predictedvaluemeasuredvaluemeasuredvalue100%.Table1Strengthtestresultsandstrength-eectivenessonHSFRCandHSCFibervolumefraction(%)Compressivestrength Splittingtensilestrength ModulusofruptureMeasured(MPa)Strength-eectivenessa(%)Measured(MPa)Strength-eectivenessa(%)Measured(MPa)Strength-eectivenessa(%)0 85 5.8 6.4 0.5 91 7.1 6.9 19.0 8.2 28.11.0 95 11.8 8.7 50.0 10.1 57.81.5 98 15.3 10.8 86.2 12.3 92.22.0 96 12.9 11.5 98.3 14.5 126.6aStrength-effectiveness HSFRCstrengthHSCstrengthHSCstrength100%.P.S.Song,S.Hwang/ConstructionandBuildingMaterials18(2004)669673 671ftfMPa A f0cpBVf CV2f: 3Substitutingf0c 85MPainEq. (3) andapplyingtheregressionanalysisgaveftfMPa 5:8 3:01Vf 0:02V2f: 4AtVf 0%, Eq. (4)givestheHSCavalueofftf 5:8MPa, equal tothatgivenby0:63f0cp 0:6385p. Thecoecient of 0.63runs near 0.54derivedbyACI 363[15], 0.58 by Wafa and Ashour [16], 0.67 by Khaloo andKim[9],and0.68byNilson[17].Eq.(4)showsasatis-factoryttothesplittingtensiletestresultsatvariousberfractions,asTable2shows.3.3.ModulusofruptureTheMORfor HSFRCat various volumefractionsappears in Fig. 3. And the strength-eectiveness inTable1indicatesthattheMORvalueswerehigherby28.1%, 57.8%, 92.2%, and126.6%at the fractions of0.5%, 1.0%, 1.5%, and2.0%, respectively, comparedtotheHSC.The MORvaluefrfof HSFRCwas relatedtothecompressive strengthf0cpof HSC and the volumefraction VfandwasgivenasfrfMPa A f0cpBVf CV2f: 5Again, substituting f0c 85 MPa in Eq. (5) and applyingtheregressionanalysisgavefrfMPa 6:4 3:43Vf 0:32V2f: 6TotheHSC, Eq. (6) giveanMORvalueof 6.4MPaequal to0:69f0cp 0:6985p. Thecoecient of 0.69lies slightly above that of 0:63f0cpobtainedby ACI318fornormal andHSCsandbelowthoseof 0:9f0cpand 1:0f0cpobtained by Nilson [17] and Wafa andAshour [16], respectively, andcome close to0.68 byKhaloo and Kim[9] for HSC. The MORvalues ofHSFRC,predicted usingEq. (6),arepresented in Table2.TheseMORvaluespredictedapproachthemeasuredones.3.4.FlexuraltoughnessFlexural toughness is the energy absorbed in de-ecting a beam a specied amount, being the area undera loaddeection(Pd) curve for the 150 150 530mmsteel brous beamtestedinthird-point bending.Index toughness I for steel ber-reinforced concretereectstheimprovementinexural toughnessoverthenonber-reinforcedconcrete, beingtheexural tough-nessataspecieddeectiondividedbythatattherst-crackdeectiondof nonber-reinforcedconcrete. Thewidely estimated indexes are I5at 3d, I10at 5:5d, and, I30at 15:5d [18]. All the three indexes reached unity,assumingthatthenonber-reinforcedmatrixiselastic-brittle. These indexes increased their values with in-creasingvolumefraction. The I5, I10and I30valueswere6.5,11.8,and20.6,respectively,atthefractionof2.0%(seeTable3).0.0 0.5 1.0 1.5 2.0Fiber volume fraction Vf (%)6810121416Modulus of rupture (MPa)MeasurementsPredictions frf= 6.4 + 3.43Vf+ 0.32Vf2Fig.3.Eectofbervolumeonmodulusofrupture.Table3ToughnessindexatvariousbervolumefractionsFibervolumefraction(%) ToughnessindexI5I10I300 1.0 1.0 1.00.5 3.0 4.8 8.21.0 3.3 6.2 12.41.5 4.2 8.3 17.82.0 6.5 11.8 20.60.0 0.5 1.0 1.5 2.0Fiber volume fraction Vf (%)56789101112Splitting tensile strength (MPa)MeasurementsPredictions ftf= 5.8 + 3.01Vf- 0.02Vf2Fig.2.Eectofbervolumeonsplittingtensilestrength.672 P.S.Song,S.Hwang/ConstructionandBuildingMaterials18(2004)6696734.Conclusions1. ThecompressivestrengthofHSCimprovedwithad-ditions of steel bers at various volume fractions. Thestrengthshowedamaximumat1.5%fractionbutaslight decrease at 2%fraction compared to 1.5%,still remaining 12.9%higher than before the beraddition.2. Thesplitting tensilestrengthandmodulusof ruptureofHSFRCbothimprovedwithincreasingbervol-ume fraction. The splitting tensile strengthrangedfrom19.0%to98.3%higher for thefractions from0.5%to2.0%. Andthemodulus of rupturerangedfrom28.1%to126.6%higher for thefractionfrom0.5%to2.0%.3. The strength-eectiveness showed at each volumefraction a maximumfor modulus of rupture, fol-lowedbysplittingtensilestrength, andcompressivestrength.4. ThestrengthmodelsdevelopedforHSFRCpredictsthe compressive andsplitting tensile strengths andmodulusofruptureaccurately.References[1] Swamy RN. High-strength concrete-material properties andstructural behaviors. ACI SP-87, Detroit: American ConcreteInstitute;1987.p.110146.[2] Rabbat BG, Russell HG. Optimizedsections for precast, pre-stressedbridgegirders. 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ACISP-87, Detroit: AmericanConcreteInsti-tute;1987.p.85109.[18] ACI Committee544, Stateof theart report of ber reinforcedconcrete.Concr.Int.:Des.Construct.1982;4(5):930.P.S.Song,S.Hwang/ConstructionandBuildingMaterials18(2004)669673 673