Micro Mechanical Measurement of Concrete Strain to ... of principle tensile strain distribution

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  • INTERNATIONALJOURNALOFAPPLIEDENGINEERINGRESEARCH,DINDIGULVolume1,No2,2010

    Copyright2010AllrightsreservedIntegratedPublishingAssociation

    RESEARCHARTICLEISSN09764259

    244

    MicroMechanicalMeasurement ofConcreteStraintoEvaluatePrincipleStrainDistributioninSteelFiberReinforcedCementConcreteModerate

    DeepBeamsacrossitswidthanddepthsVinuR.Patel , I.I.Pandya

    Department ofAppliedMechanics,M.S.University,Baroda.zarnaasso@yahoo.com

    ABSTRACT

    Thispaperinvestigatesthedistributionofprincipalstrainbymicromechanicalmeasurementof concrete axial strain in Steel Fiber Reinforced Cement Concrete moderate deep beamsacrossitswidthanddepthsforvariousspantodepthratios.Acompletesheardeformationalbehavioralongwithloaddeflectionresponse,crackpatternsandmodesoffailurewasstudiedexperimentally.Sixteenbeamsweretestedandresultsofprincipalstrainwereplotted.Natureofprinciple tensile strain distributionwas found similar to elliptical pattern of distributionalongthelinejoiningtheloadandthesupportpoint.Theassumptionofplanestressconditionunderestimates the significance of the small compressive or tensile stresses develop in thetransversedirection from incompatible responseofconsecutiveconcreteelements subjectedtodifferent statesof stress.Therefore, itwasconsidered that themodesof shear failureareassociatedwithmultiaxialstressconditionsexist intheregionofthepathsalongwhichthecompressiveforcesaretransmittedtothesupports.

    Keywords:SteelFiberReinforcedconcrete,moderatedeepbeams,Principalstraindistribution.

    1.Introduction

    AsperIS:456,beamscanbeclassifiedintothreecategoriesnamelynormalorshallowbeams,moderatedeepbeams,anddeepbeams.Theseclassificationsarebasedontheirspantodepthorshearspantodepthratio.Moderatedeepbeamsanddeepbeamshaveawideapplicationin structural engineering field. Numerous investigations have been carried out and reportswerepublished regarding the strength and the loaddeformationbehaviorofmoderatedeepanddeepbeams.Avery littleworkshavebeenreportedonsheardeformationalbehaviorofSteel FiberReinforced CementConcretemoderate deep beams, particularly, related to thestudyofprincipalstraindistribution.Moderatedeepanddeepbeamsareshearpredominantmembers and generally fail in brittle shearmode. Hence the study of strain deformationalbehavior isan importantsteptowardsthedeeperunderstandingofbehaviorofbeamsunderapplied loading. It is essential to know the complete loaddeformational behavior beforedesignofanystructure.Thecompleteandcarefulanalysisisrequiredtopredictthebehaviorof any structure. Micromechanical measurement and analysis of an individual structuralelementisveryimportantforpredictionofcomplexbehaviorofstructuresuchasitsloadingcapacity, rotational capacity, strain deformational behavior and cracking mechanismactivatedunderappliedloading.

    In the present investigation,Visualizationwasmade regarding the distribution of principalstraininshearzoneandvariationofthesamealongthelinejoiningtheloadandthesupport

  • INTERNATIONALJOURNALOFAPPLIEDENGINEERINGRESEARCH,DINDIGULVolume1,No2,2010

    Copyright2010AllrightsreservedIntegratedPublishingAssociation

    RESEARCHARTICLEISSN09764259

    245

    point. The shear capacity of Steel Fiber Reinforced Cement Concrete section can bedeterminedbyadditionofshearresistanceofferedbytwocomponentsnamelyconcreteandreinforcement.Shearcapacityofconcretedependsonsplittingofanellipticalsectionwhosemajoraxisliesonthelinejoiningtheloadandsupportpoint.Thenatureofstraindistributionalongtheinclinedaxiswasfoundsimilartoellipticalsection(FIG1)asdescribedabove.Theprimeaimwastoprovideasystematicandcomprehensivestudyonthesheardeformationalbehavior and principal strain distribution in Steel Fiber Reinforced Cement Concretemoderatedeepbeamsusingsimpleinstrumentationapproach.AninnovativehexagonalgridarrangementofDemectargetsconsideringtriangularrosettepatternwasusedtomeasuretheconcrete axial strain. Evaluation of principal strain from concrete axial strain readingsconfirmsthestraindistributionalongtheinclinedaxisjoiningtheloadandsupportpoints.

    2.Objective

    Theobjectiveof thepresentexperimental investigationwas tomicromechanicallymeasurethe concrete axial strain to evaluate principal strain distribution in Steel Fiber ReinforcedCement Concrete moderate deep beams across its width and depths to obtain the clearinformation regarding the strain distribution at various planes, which provides deeperunderstandingof thefundamentalnatureofshearbehaviorduringtheprecrackingstage,atthetimeofcrackinitiationandduringthepostcrackingstage.

    3.ExperimentalProgram

    3.1TestMaterials

    OrdinaryPortlandcementof53grade,naturalriversandhavingfinenessmodulusof2.8andmaximum size of4.75mmas a fine aggregate, and naturalbasalt gravel ofmaximum size20mmas coarse aggregatewere used.The concretemix proportionwas1:1.5:3.0 (cement:fineaggregate:coarseaggregate)byweightwithwatercementratioof0.45keptconstantforallbeams.LongitudinaltensionreinforcementconsistsofHighyieldstrengthdeformedbars(Fe 415), corrugated semicircular steel fibers of embedded length 50mm and equivalentdiameter0.625mm,havinghightensilestrength825N/mm2washomogeneouslymixedwithcement paste.Therewere four series ofbeams and for each series six cubes (150mm)andeight cylinders (four cylinders for compressive strength and four cylinders for splittingstrength, 150 mm diameter and 300 mm height) were cast as control specimens. Allspecimenswerecuredatleastfor28days.

    3.2SpecimenDetails

    Testing was carried out on sixteen beams, simply supported on constant effective span of1200 mm under two point concentrated symmetrical loads. All the beams were havingconstantoverallspanandwidthof1300mmand150mmrespectively.

  • INTERNATIONALJOURNALOFAPPLIEDENGINEERINGRESEARCH,DINDIGULVolume1,No2,2010

    Copyright2010AllrightsreservedIntegratedPublishingAssociation

    RESEARCHARTICLEISSN09764259

    246

    Compressive Stress

    Trajectories

    Elliptical Stress

    Pattern

    ELLIPTICAL STRESS PATTERN IN MODERATE DEEP BEAM

    PATH OF COMPRESSIVE STRESS TRAJECTORIES

    IN MODERATE DEEP BEAM

    P P PP

    Figure1: Conceptofstressdistributioninmoderatedeepbeam

    Therewerefourseriesofbeamshavingdifferentdepthsof300mm,400mm,500mm,and600mm and each series comprised of four beams on which the axial concrete strain wasmeasuredonsurfaceplanessituatedat differentdepthsof0.0cm(i.e.,atthesurface),2.5cm,5.0cm,and7.5cmacrossthewidthof thebeams.ThebeamnotationD50S2.5denotesthe beam having overall depth Dof 50 cm and axial concrete strain measured on surfaceplanesituatedatadepthof2.5cmacrossthewidthofthebeam.

    DIALGAUGES

    BEAM

    LOADPOINT FORM

    Figure 2: Testsetup3.3TestingProcedure

    Allthebeamsweretestedundertwopointconcentratedloadingspositionedatonethirdspans.All the beams were simply supported with a effective span of 1200 mm. Beams werecenteredonplatformandleveledhorizontallyandverticallybyadjustingthebearingplates.Loadwas applied gradually. Three dial gaugeswereused tomeasure the deflection at thecenterandunder thepointsof loadings.Here,Delta rosette isused formeasuring strainsatdifferentdemecpoints inwhich three strainsare required tocalculate the shearstrainsandshearstresses.InD30andD40beamsdemectargetsnumbersare6,7,8andinD50andD60beams demec targets numbers are 7,8,11,12,13,16,17. Readings were taken at proper loadinterval.Deflections,concretesurfacestrainsandindepthstrainsinflexureandshearzonesweremeasuredusingmechanicalstraingauge.Crackpropagationsweretracedbypencilandtheirtipsweremarkedcorrespondingtotheloadreadings.

  • INTERNATIONALJOURNALOFAPPLIEDENGINEERINGRESEARCH,DINDIGULVolume1,No2,2010

    Copyright2010AllrightsreservedIntegratedPublishingAssociation

    RESEARCHARTICLEISSN09764259

    247

    BEAMS OF SERIES D30 & D40

    BEAMS OF SERIES D50 & D60

    200

    100

    100

    100100100

    120

    60

    54321

    6 7 8 9

    10 11 12 13 14

    18171615

    19 20 21 22 23

    1 2 3 4

    6 7 8 9

    10 11 12 13

    5

    1

    2

    3

    4

    5

    1

    2

    3

    Figure 3: Hexagonalgridarrangementsofdemectargets

    4.DiscussionofTestResult

    4.1CrackPatterns,Propagation,andModesofFailure

    Inall the beam specimens, initiation of flexure cracks (Fig 4)was from the bottomof thebeams. Inmostof the cases, all the flexure crackswere almost vertical,whilemostof theshear cracks (Figure 4) were inclined and their direction of propagation was towards thenearestloadpointirrespectiveofitsplaceoforigin.

    BeamsofseriesD30andD40failedinpureflexurefailurebyyieldingoflongitudinaltensilereinforcement.Sizeandpropagationoftheflexurecrackswerenoticeable,whileshearcrackswerefewandveryfine.Majorityofflexurecracks(Figure4)propagatedbeyond 4

    3

    heightofthebeamandwereconsiderablywideat thebottom.BeamsofseriesD50failedinflexureshearmode.The phenomenon of all of a sudden formationofmajordiagonal shear cracks

    emerging from 3D

    to 4D

    height of the beam frombottom and its rapidpropagation towardsthe nearest load and supportpointwas firstly seen in beamsofD50 series.Although, theshearcrackswerecomparabletoflexurecrackstheultimatefailureofallthebeamsofD50serieswasduetotheyieldingofthelongitudinaltensilereinforcement.BeamsofseriesD60failedinpureshearfailure.Flexurecrackswerefewandveryfine,andhardlyreacheduptothemidheight of thebeam,while shear crackswerenoticeable in size and itspropagationcoveredmore than 4

    3

    height of the beam. After sudden formation ofmajor diagonal shearcracks (Figure4)beam sustained somemore loadbefore ultimate failure,which shows itsreservestrength.

  • INTERNATIONALJOURNALOFAPPLIEDENGINEERINGRESEARCH,DINDIGULVolume1,No2,2010

    Copyright2010AllrightsreservedIntegratedPublishingAssociation

    RESEARCHARTICLEISSN09764259

    248

    Figure4: (a) D30specimen

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