experimental investigation of reinforced concrete · pdf fileexperimental investigation of...

K.V. VENKATESHA, K. BALAJI RAo, S.V. DINESH, B.H. BHARATKUMAR, M.B. ANooP, BALASUBRAMANIAN, S.R., NAgESH R. IYER

ExpErimEntal invEstigation of rEinforcEd concrEtE BEams With and Without cfrp Wrapping

KEY Words

• Reinforced concrete,• Carbon fibre,• Shear strength,• Failure mode,• Cracking load,• Ultimate load.

aBstract

This paper presents the results of experimental investigations on six reinforced concrete beams, with three different shear span-to-depth ratios, which were tested under two-point loading. The aim of the work was to study the efficacy of Carbon Fibre Reinforced Polymer (CFRP) strips in enhancing shear capacity and/or changing the failure mode from brittle shear failure to ductile flexural failure. The results of the study indicate that while there is a marginal increase in first crack and ultimate loads, it is possible to achieve a change in the failure mode, and the monitored strain gauge data can be used to explain the failure pattern observed.

K.V. VENKATESHAemail:[email protected]:Reinforcedconcrete;CFRPwrapping

S.V. DINESH email:[email protected]:SoilDynamics;Discreteelementmod-elling;Fractureofconcrete

Address:DepartmentofCivilEngineering,SiddagangaInstituteofTechnology,Tumkur,Karnataka,India–572103.

K. BALAJI RAoemail:[email protected];[email protected] field:Stochasticmechanics;Risk-consistentdesign;Hybriduncertaintymodelling

B.H. BHARATKUMAR email:[email protected] field: High performance concrete com-posites including fibre reinforced concrete; Fracturemechanicsofconcrete

M.B. ANooP email:[email protected]:Computationalmechanics;Fuzzyandfuzzy-randommodelling;Durability-basedservicelifedesign

BALASUBRAMANIAN, S.R. email:[email protected] field: Structural brick masonry buildings:lateralloadanalysis,vulnerabilityanalysisandretrofit-tingstrategies

NAgESH R. IYER email:[email protected] field: Computational structural mechanics;Damage tolerant design; Innovative sustainable and/ornewengineeredmaterialsandnovelmethodologiesforretrofitting

Address:CSIR–StructuralEngineeringResearchCentre,CSIRCampus,Taramani,Chennai,TamilNadu,India–600113.

Vol. XX, 2012, No. 3, 15 – 26

2012 slovaK univErsitY of tEchnologY 15

16 ExpErimEntal invEstigation of rEinforcEd concrEtE BEams With and ...

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1. introduction

TheuseofFibre-ReinforcedPolymers(FRPs)fortherehabilitationof existing reinforced concrete (RC) structures has grown veryrapidlyoverthelastfewyears.ResearchhasshownthatFRPscanbeusedveryefficiently in strengtheningRCbeams that areweakin flexure, shear or torsion [1-14]. Strengthening with externallybondedFRPsheetshasbeenshowntobeapplicabletomanytypesof RC structural elements. FRP sheets may be: i) adhered to thetensionsideofstructuralmembers(e.g.,slabsorbeams)toprovideadditionalflexuralstrength,ii)adheredtothesidesofwebsofjoistsandbeamstoprovideadditionalshearstrength,iii)wrappedaroundcolumns to provide additional shear strength, and iv) wrappedaroundcolumnstoincreaseconcreteconfinementandthusincreasethe strength and ductility of columns. FRP wraps increase theshear strength ofRC beams and columns [15-18].The additionalshear strength is obtained by orienting the fibres in adirectionthat is transverse to the axis of the RCmember or perpendicularto the shear cracks [19-20].The primary advantage of the use ofFRPreinforcement inRCstructures is that ithasbettercorrosion-resistantpropertiescomparedtoconventionalsteel.Thepurposeofthepresentstudywastoinvestigatethestrengthandbehaviourofreinforcedconcretebeamswrapped/strengthenedwithadiscreteU-typewrappingofonelayerofCarbonFibre-ReinforcedPolymer(CFRP)strips.Towardsthis,sixsimply-supportedsingly-

reinforcedconcretebeamsofarectangularcross-sectionwerecastand tested under two-point loading. Three shear span-to-effectivedepthratios(a/d)werechosensothattwocontrolbeamswouldfailinshear,whilethethirdonewouldfailinflexure.Foragiven(a/d)ratio,onecontrolbeamandonewrappedbeamwereconsidered.

2. ExpErimEntal invEstigations

Themain focus of this investigationwas to examine the efficacyofdiscreteCFRPwrapsinimprovingtheflexural/shearstrengthofsimply-supportedsingly-reinforcedconcretebeams.Fromareviewof the literature it has been found that shear reinforcement andshearspan-to-depthratiosare,amongstotherthings,twoimportantparameterstobeconsideredinstudyingtheefficacyofCFRPwraps.Keeping these in mind, acomprehensive experimental programinvolvingthetestingofsixbeams(threewrappedandthreecontrolbeams) in two-point bending, was planned. All the beams weredesigned according to IS 456-2000 [21]. The dimensions of thebeams were 100 mm × 200 mm × 1500 mm, with two 12 mmdiameterbarsasmain reinforcements, two6mmdiameterbarsashanger bars, and 6mmdiameter bars as two-legged stirrupswithaclear cover of 25 mm. Three stirrup spacings were consideredin the present investigation (i.e., 100mm, 120mm and 125mm,with adesignmaximumspacingof 126mm).Figure1 shows the

Fig. 1 Longitudinal and cross-sections of the beam.

Fig. 2 Strengthening scheme using CFRP wrapping.

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17ExpErimEntal invEstigation of rEinforcEd concrEtE BEams With and ...

longitudinalandcross-sectionaldetailsofatypicalbeam.ThemixwasdesignedaccordingtoACI211.4R:1993,andthemixproportionwasobtainedas1:2.31:2.60withawater-cementratioof0.45.Twodifferent types of coarse aggregateswere used in the proportion:60%of20mmand40%of10mm.TheCarbonFibreReinforcedPolymer (CFRP) strips of awidth of 100 mm and alength of500mm (i.e., 2 × 200 + 100 mm) were wrapped (U-wrapped)around three beams with acentre-to-centre spacing of 125 mm(Fig.2).TheCFRPwrapsusedhadaYoung’smodulusof330GPa

andatensilestrengthof1.80GPa[9].Table1givesthedetailsofthe specimens considered in this experimental investigation, andTable2givesthepropertiesofthereinforcingbarsobtainedbasedon the experimental investigations. Strain gauges of alength of2mmwereembeddedonthestirrupreinforcementintheshearspanformeasuringthestrain(seeFig.3).Foreachcasting(consistingofacontrolbeamandthebeamtobewrapped),three100mmcubeswerecastascompanionspecimens.Thecastingofthebeamsandthecompanionspecimenswerecarried

Fig. 3 Locations of embedded strain gauges (2mm gauge length) on stirrup (N and S denote the North and South faces of the beam, respectively; L and R denote left and right ends, respectively).

Tab. 1 Details of the beams considered.

Beam designation

a/d*Mean cube

compressive strength of concrete (MPa)**

Beam typeShear reinforcement

No. of CFRP layerSteel stirrups CFRP

SB12.57 47.52

Control 6mmф@100mmc/c ¬¬¬ -WSB1 Wrapped 6mmф@100mmc/c U-wrap# oneply90°SB2

1.85 46.57Control 6mmф@120mmc/c ¬¬¬ -

WSB2 Wrapped 6mmф@120mmc/c U-wrap# oneply90°SB3

1.71 48.77Control 6mmф@125mmc/c ¬¬¬ -

WSB3 Wrapped 6mmф@125mmc/c U-wrap# oneply90°(Note:*-shearspan(a)toeffectivedepth(d)ratio** -basedontestsonthreenominallysimilarcubes# -U-wrappingwithstrips100mmwide@125mmc/c)

Tab. 2 Properties of reinforcing bars.

Reinforcing barMean elastic modulus

E (MPa)*Mean diameter

(mm)Mean yield stress

fy (MPa)*Mean ultimate

strength fu (MPa)*Mean %

elongation*Main 2.02x105 11.94 473.33 620.26 24.33

Stirrup/Hanger 2.22x105 6.24 387.0 555.86 11.81(Note:*-basedontestsonthreenominallysimilarbars)


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out at the Advanced Materials Laboratory of CSIR-SERC. Thebeamand the companion specimenswerekept in their respectivemouldsfor24hoursandlaterdemoulded.Theywerecuredfor28daysinacuringpond.The beam specimens were tested in the loading frames of theStructuralTestingLaboratoryofCSIR-SERC. In the caseof thecontrol beams, itwas planned tomeasure the surface strains onthe concrete in the shear zone alongX-Y-Zas shown in Fig. 4.APfender gauge with aleast count of 1/1000 strain units wasused to measure the surface strains. However, for the wrappedbeams,thedemecpointscouldnotbepastedsincethesurfaceissmooth,andrubbingthesurfacewoulddamagethewrap.Hence,for thewrappedbeams,electrical resistancegauges (havinga30mm gauge length) were pasted on the surface to measure thesurfacestrainsintheshearspan(inthedirectionofthefibre)andin the region of the constant bendingmoment (perpendicular tothedirectionofthefibre)asshowninFig.5.Threestraingauge-based transducers, having aleast count of 0.01mm (capable ofmeasuring amaximum deflection of 50mm)were placed underthe load points and at the central section for measuring thedeflections. All theelectricalresistancestraingauges(sixinthecaseofthecontrolbeamsandsixteeninthecaseofthewrappedbeams)andthedisplacementtransducerswereconnectedtoadata

logger.Inthecaseofthecontrolbeams,thewidthsofthecracksweremeasured(ifandwhen thecracksformed)at fourdifferentlevels in the shear dominant region and at the level of the steelin the flexure dominant region (see Fig. 6).Acrack gaugewithaleast count of 0.01mmwas used tomeasure thewidth of thecracks. In the case of the wrapped beams, the widths of thecrackswerenotmeasuredsincethecrackingcouldnotbetraced.Atypicaltestset-upisshowninFig.7.The loading regime appliedon the control beamwasdesigned insuchawaythat therewereat least threeloadingstagesbeforetheestimated first crack load and about five to six stages thereafter.However,whitewashingcouldnotbedoneinthewrappedbeams,and the demec points could not be pasted. Hence, the strain anddeflectionreadingsweretakenusingtheautomaticstraingaugedataloggeratincrementsofevery5kNload.

3. ExpEctEd BEhaviour of thE BEams tEstEd

Control Beams:Shear failurewasnotaconsiderationfor theSB1beamsincethestirrupspacingadoptedwas100mm,whichislessthanthemaximumallowablespacing.Also,the(a/d)ratioforthis

Fig. 4 Arrangement of demec points for measuring surface strains on north and south faces of Beam SB1.

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Fig. 6 Sections marked on north and south faces of Beam SB1 for measuring crack widths.

Fig. 5 Test set-up and locations of electrical resistance strain gauges (30 mm gauge length) on the surface of the wrapped beams.


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beamwas2.57,andhenceaflexuralfailurewasexpected.However,intheSB2andSB3beams,ashearfailurecouldhaveoccurredsincethestirrupspacingadopted(120mmforSB2and125mmforSB3)werecloser to themaximumallowablespacingof126mm.Also,

the (a/d) ratio for SB2was 1.85, and ashear compression failurewasexpectedinthisbeam.ForSB3, the(a/d)ratiowas1.71,andadiagonaltension(shear)failurewasexpectedinthisbeam.Wrapped Beams: The provision of the strips would enhance theshearstrengthofthebeamsandaddtotheshearresistanceofferedby the stirrups. Noting that aflexural failure was likely in thecontrol beam (SB1), it was felt that theWSB1 beammight alsohavefailedinflexuresincethefibres in theflexurezonemaynothave provided significant resistance to the first crack formation.Also, the shear reinforcementprovided in theWSB1beam in theform of stirrups was less than the maximum allowable spacing.Thus, aflexure failure was expected with enhanced ductility andamarginalincreaseinthefirstcrackstrengthfortheWSB1beam.Fromthevaluesofthe(a/d)andstirrupspacings,shearcompressionfailureanddiagonalshearfailurewereexpectedintheSB2andSB3control beams, respectively. However, it was felt that theWSB2andWSB3wrappedbeamswouldnotfailinthesamemodessincetheshearzonehadbeenstrengthened.Thus,afailureinthezoneofconstantbendingwasmorelikely,perhapswithenhancedductilityandamarginalincreaseinthestrengthatthefirstcrack.

Fig. 7 Typical test set-up.

Fig. 8 Load-deflection curves for the three control beams.

Fig. 9 Load versus strain (in µε) curves for the three control beams (measured from the strain gauges pasted on stirrups; strains: positive – tension).

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4. rEsults and discussions

Thedeflectionsrecordedat threedifferentsectionsandthestrainsrecorded on the stirrups for the three control beams are shownin Figs. 8 and 9, respectively. The deflections measured at threedifferent sections (measured using displacement transducers), thestrainsonthestirrupsandthesurfacestrainsforthethreewrappedbeamsareshown inFigs.10,11and12, respectively.The failurepatternsof thecontrolbeamsareshowninFig.13alongwiththefailurepatternsofthewrappedbeams.As afirst step it was proposed to compare the first crack andultimateloadsofthecontrolandthecorrespondingwrappedbeamsobtained from the tests.While testing the control specimens, thefirst crack load could not be exactly ascertained through visualexaminationofthesurfaceofthebeamsincethetestingframewascongested.Hence,theload-deflectioncurvesandthereadingsfromthe strain gauges (whichwere located closer to the flexure zone)wereconsideredfordeterminingthefirstcrackload.ThevaluesofthefirstcrackandultimateloadsarepresentedinTable3.Itisnotedfrom this table that there was no significant improvement in the

firstcrackandultimatestrengthsduetothewrapping.However,thefailuremodechangedwithreferencetothesecondandthirdcontrolbeams (for which (a/d) were 1.85 and 1.71, respectively). Thepositionof thedominantcrack leading to failuredependedon the(a/d)ratio,andthepresenceorabsenceofthewraps.WhilefailureoccurredintheSB2andSB3beamsduetothepropagationofthedominant crack from the shear zone, the failure of the respectivewrapped beams (namely,WSB2 andWSB3) occurred due to thepropagation of the cracks (almost vertically) in the flexure zone,albeitmore or less under one of the load points. In the case ofthewrappedbeams, it is noted that at the timeof the failure, thewrap, along with the thin layer of concrete, became separated(in the compression zone)ononeorbothof the faces,which ledto an explosive failure due to the crushing of the concrete in thecompressionzone.Infact,inthecaseoftheWSB1beam,alateralbendingwasalsoobservedaftertheseparationofthewrap.Alongwith thefact that therewasnosignificant increase in theultimatestrengths of the beams, these observations suggest that thewrapsintheshearzoneshouldperhapshavebeencontinuousandthattheorientationofthefibresoftheCFRPintheflexurezoneshouldhave

Fig. 10 Load-deflection curves for the three wrapped beams.

Fig. 11 Load versus strain (in µε) curves for the three wrapped beams (measured from the strain gauges pasted on stirrups; strains: positive – tension).


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beenparalleltotheneutralplaneofthebeam;theseschemesneedfurtherstudy.However,itisnotedthatthenatureofthefailuremodechangedfromafailuremodeofsheartooneofalmostflexurebythewrappingschemeusedinthepresentstudy.Acritical examination of the behaviour of both the control andwrappedbeamswasmadeusingthestrainsrecordedbytherelevantelectricalresistancestraingauges.Thesearepresented inFigs.14and15.Thefollowingaretheobservationsmade:SB1:Whileaflexuralfailurewasexpectedforthisbeam,sincethe(a/d)value(i.e.,2.57)wasclosetothe(a/d)valueof2.5separatingtheflexuralandshearfailures,thestrainsrecordedbytheelectricalstraingaugesintheshearspanregionwereexamined(Fig.14(a)).Itmaybenotedthatthespacingofthestirrupsforthisbeamwas100mmandthattheshearspanwas450mm.Thesg1,sg2,sg5andsg6

straingaugeswerelocatedintheshearspan.Itwasnotedthatthesg2andsg5showedhighervaluesof strain than thesg1andsg6.Both the sg2 and sg5 strain gauges showed asimilar trend (seeFig. 14(b)). Even though the sg5 showed ahigher strain aroundtheexperimentalpeakload,thefinalfailureoccurredatalocationnearertothesg2.SB2:The spacingof the stirrups for thisbeamwas120mm,andthe shear spanwas325mm.The sg1and sg6 straingaugeswerelocated in the shear span. It is noted that the sg1 and sg6 showhigher values of strain (Fig. 14(c)) compared to the other straingauges,whichindicatesthatthestrainsthatdevelopedintheshearzone were significant. This was expected, since this beam wassupposedtohaveashearcompressionfailure.Itisalsonotedthatthe sg2 exhibited fewer fluctuations than the sg5 gauge (see Fig.

Fig. 12 Load versus strain (in µε) curves for the three wrapped beams (measured from the strain gauges pasted on surface; strains: positive – tension).

Tab. 3 Experimental first crack- and ultimate- loads and the nature of failure.

Beam designation

Load at initial crack Pcr (kN)Ultimate load

(kN)Nature of failureFrom load-deflection

curveFrom load-strain

curveSB1 10-15 10-15 100.92 Flexuralfailure

WSB1 20(60%) 20 106.08(5.11%)Failureduetoburstingofconcretein

compressionnearloadpointSB2 18 18 134.92 Shearcompressionfailure

WSB2 20-25(25%) 20-25 157.08(16.42%)Failureduetoburstingofconcretein

compressionzoneunderoneoftheloadpointsSB3 25 25 149.08 DiagonalShearFailure

WSB3 30(20%) 30 176.08(18.11%)Failureduetoexplosiveburstingofconcreteincompressionzoneunderoneoftheloadpoints

(Note:Valuesinbracketsindicatethepercentageincreasefromthatofthecorrespondingcontrolbeam)

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14(d)).Althoughthestrainsareapproximatelythesamearoundthepeakload(andthesg5alsoshowedasteepincreaseinstrain),thesg2relaxed,perhapsduetoyieldingorfailureclosertothispoint.Infact,failurewasnotedundertheloadpointclosertosg2.SB3:Thestirrupspacingwas125mm,andtheshearspanwas300mmforthisbeam.Thesg1andsg6straingaugeswerelocatedintheshearspan.Itwasnotedthatthestrainsinthesg1andsg2exhibitedasteepincreaseafteraloadof60kN(seeFig.14(e)).Thestrainsrecordedbythesestraingaugeswerehighercomparedtotheotherstraingauges.Thisisbecauseadiagonaltension(shear)failurewasexpectedinthisbeam;hence,thestrainsthatdevelopedintheshearspanregionwouldbehigh.ItcanbeseeninFig.14(f)thatthesg5recorded asteep increase in strain around the experimental peakload,which suggests that the failureoccurredaround the locationof this strain gauge. The P-δ diagram of this specimen (Fig. 8)indicatesthatthesectionclosertosg5wasmoreflexiblecomparedto themiddlesectionand thesectioncloser tosg2. In fact,as theP-δdiagramindicates, themiddlesectionof thisbeamwasstifferthanthesectionscloser to the loadpoints.Thefinalfailureof thebeamoccurredatasectionclosertosg5andclosertotheloadpoint.

WSB1: Thesg3andsg4straingauges,whichwerepastedonstirrups,wereclosertotheloadpoints.Thesurfacesofthebeamcorrespondingtothelocationoftheseembeddedstraingaugeswerenotwrapped.Asexpected,thesg3andsg4straingaugeswereundertension.Fig.15showsthatexceptatthepeakload,thesg4wasrelativelylessstrainedcompared to the sg3.When focusing on the sg10 and sg13 straingauges,itcanbeseenthatbothofthesestraingauges(whichareonthesurfaceoftheconcrete)werealsoundersignificanttension.Thefinalcrackpatternsuggests that thecrackspropagatedvertically intheflexurezone.However,mostofthesecracksformedbetweenthewraps.Insomeofthestrips,althoughcracks(vertical)formed,theywerenotdominant.Thefinalfailureoccurredduetothecompositefailure of thewrap and the concrete (together) in the flexure zoneclosertoaloadpoint.Afterthecompositefailure,itseemsthebeamunderwentout-of-planebending/buckling.WSB2:Itisnotedthat,asinthecaseofthecontrolbeam(SB2),thesg1andsg6straingaugesshowedhigherstrainvaluescomparedtotheotherelectricalstraingauges(seeFig.11).Thestrainvariationwithloadinginthesg2andsg5straingaugesareshowninFig.15.The sg2 and sg5 strain gaugeswere pasted on stirrups andwere

Fig. 13 Failure modes for the control beams and the three wrapped beams.

Tab. 4 Stiffness values (kN/mm) based on intial and second segments of P-δ curves. Quantity SB1 WSB1 SB2 WSB2 SB3 WSB3Stiffnessbeforecracking 48.39 83.33 50.0 Veryhigh 50.0 100.0Stiffnessaftercracking 15.63 24.02 24.79 34.59 23.53 48.91


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closer to the load points. These embedded strain gauges were atlocationswheretheexternalsurfaceoftheconcretewaswrapped.Fig.15 shows that the sg5wasalways in compression,while thesg2 gauge alternated between tension and compression. Close tothe peak load, the sg2 gaugewas under tension,which indicatesapossiblede-bondingandthusmakingthestirruptakethetension.In fact, the final de-bonding failure occurred near the sg2 gauge.When focusing on the surface pasted sg10, sg12 and sg13 straingauges(whichwereonthesurfaceoftheconcrete),itcanbeseenthat the sg10 and sg13 were less strained compared to the sg12(see Fig. 15).This beam developed significant flexural cracks inaddition to the dominant crack under the load point before thewrap was de-bonded. The final failure pattern showed that thecrackspropagatedverticallybothinbetweenandwithinthewraps.However,thecrackpropagationwithinthewrapoccurrednearthefailure zone. The final failure was due to the composite failurecloser toaloadpoint. It shouldbekept inmind that the (a/d) forthisbeamwas1.85andthattheSB2failedinshearcompression.WSB3:The sg1 and sg6 strain gauges (located in the shear spanregion)showedhigherstrainvaluescomparedtotheotherelectricalstrain gauges (see Fig. 11), similar towhatwas observed for the

SB3controlbeam.Thestrainvariationwiththeloadinginthesg2andsg5straingaugesareshowninFig.15.Thesg2andsg5straingaugeswerepastedonstirrupsandwereclosertotheloadpoints.These embedded strain gauges were situated at locations wherethe external surface of the concretewaswrapped. Fig. 15 showsthatthesg5wasmostlyincompression,whilethesg2wasalwaysunder tension.The sg2 gauge near the peak load carried ahigherdegreeof tension,which indicatesapossiblede-bondingand thusputtingademandonthestirruptotakethetension.Infact,thefinalde-bondingfailureoccurrednearthesg2gauge.Whenfocusingonthesurfacepastedsg10,sg11andsg12straingauges(thesg11wason thewrap in the flexurezone,while theother twowereon thesurfaceoftheconcretebetweenthewraps),itisnotedthatthesg11gaugewasunderalargedegreeof tensionnear thepeak loadandthatthesg10gaugewasincompression,whilethesg12gaugewasundertensionatthepeakload.Thetensionintheconcreteindicatedthat the wrap had possibly de-bonded near this gauge. The finalfailuredidoccurunderthepointloadclosertothisgauge.Itwasnotedthatforallthecontrolbeamsandthewrappedbeamsexcept for the WSB1, the P-δ diagram could be idealised asabi-linear curve,while for theWSB1, atri-linear curvemight be

Fig. 14 Strain variations (in µε) in electrical resistance strain gauges (located in the shear span region and located closer to the point loads) for the control beams (sg1, sg2 – north face; sg5, sg6 – south face; all the gauges on the stirrup; strains: positive – tension).

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abetterapproximation.Acarefulstudyoftheload-deflectioncurvesofthecontrolandwrappedbeams(seeTable4)showsthatwiththewrapping, the stiffness values of all three beams almost doubledbothbeforeandafterthecracking.Thus,whiletheremaynothavebeen asignificant increase in strength, the serviceability controlcouldbebetterexercisedthroughwrapping.

5. conclusions

Basedontheexperimentalinvestigationscarriedout,thefollowinggeneralconclusionsweredrawn:• Thethreecontrolbeams,eachofwhichwasdesignedtofail inaparticularfailuremode,behavedasexpected.

• Thewrappingschemeadoptedinthisstudy(i.e.,discreteCFRPstripswith fibres oriented at 900 to aneutral plane) resulted in achangeoffailuremodefromthesheartoalmostflexuralmode.TheCFRPwrappingandconcretewasintactuptothefailureofthebeam,whichclearlyindicatesthecompositeactionwasduetotheCFRPwrap.

• Theenhancementinthefirstcrackloadwas20-60%,whilethatintheultimateloadwasbetween5-18%,dependingonthefailuremodeofthecontrolbeam.

• Theload-deflectionbehaviourofthewrappedandtheunwrappedbeams considered in this investigation could be idealised byabi-linearcurve(atleastuptotheworkingloadlevel).

• Whiletheincreaseinstrengthwasmarginal,therewasasignificantimprovement in thestiffnessgainedby thewrapping(italmostdoubled,dependingontheregionoftheload-deflectioncurve).This observation indicates that the wrapping scheme tested inthis investigation could be used to satisfy the serviceabilityrequirementsand,toanextent,alterthefailuremode.

• Thestrainsmeasuredbothonthestirrupsandonthesurfaceofthebeamsusingtheelectricalresistancestraingaugescanbeusedtoexplainthebehaviourofthebeamsuptofailure.However,ithastobeborneinmindthatwhilethestraingaugemeasurementsarepointmeasurements,theload-deflectionbehaviourisanintegraleffect(hencetheload-deflectioncurvesaresmooth).

Theseconclusionsarebasedon investigationsononlysixbeams; inordertogeneralisetheconclusions,agreaternumberoftestsforagivencombinationofvariablesmaybeneeded.Itisnotedthatthefinalfailureofthewrappedbeamsoccurredduetothede-bondingofthewrapinthecompressionzoneatthecriticalpointandthatthefailurewassudden.Inordertoavoidsuchfailures,theuseofaU-wrapintheformofstripswithendanchorscouldpossiblybeexplored.Also,thereisaneedto

Fig. 15 Strain variations (in µε) in the electrical resistance strain gauges (located closer to the centre of the beam and closer to the supports) for the wrapped beams (strains: positive – tension).


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studytheefficacyofbothdiscreetandcontinuouswrappingwithafibreorientationof0°totheneutralplaneoftheunloadedbeam.

acknowledgements

This paper is being published with the kind permission of theDirector, CSIR-SERC, Chennai, India. The authors gratefully

acknowledge the support extended by Dr. K. Ravisankar, ChiefScientist, Shri. K. Kesavan, Principal Scientist, and Ms. Pavithaof theStructuralHealthMonitoringLaboratory,CSIR-SERC.Theauthors are grateful to the members of the Advanced MaterialsLaboratory and Mr. Kumarappan, Senior Technical Officer,Structural Testing Laboratory, CSIR-SERC, for their unstintinghelpincarryingouttheexperimentalwork.

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