K. Gajdošová, j. BilčíK

Slender reinforced concrete columnS Strengthened with fibre reinforced PolymerS

Key wordS

• strengthening,• slender concrete columns, • fibre reinforced polymers,• CFRP sheet,• CFRP strips.

AbStrAct

The requirement for a long life with relatively low maintenance costs relates to the use of building structures. Even though the structure is correctly designed, constructed and maintained, the need for extensions of its lifetime can appear. The preservation of the original structure with a higher level of resistance or reliability is enabled by strengthening. Conventional materials are replaced by progressive composites – mainly carbon fibre reinforced polymers (CFRP). They are used for strengthening reinforced concrete columns in two ways: added reinforcement in the form of CFRP strips in grooves or CFRP sheet confinement and eventually their combination. This paper presents the effect of the mentioned strengthening methods on slender reinforced concrete columns.

Katarína Gajdošováe-mail:katarina.gajdosova@stuba.skResearchfield:StrengtheningofConcreteStructuresJuraj Bilčík e-mail:juraj.bilcik@stuba.skResearch field: Reliability, Diagnosis and Repair ofConcreteStructures

DepartmentofConcreteStructuresandBridges,FacultyofCivilEngineering,STU,Radlinského11,81368Bratislava

Vol. XIX, 2011, No. 2, 27 – 31

introduction

The design of slender load-bearingmembers is themost assertedtrendtoday.Thesamerequirementisthenimposedonstrengthenedstructures and members. That is the main reason for replacingstrengthening techniques using conventional materials (concrete,steel) with new ones. Progressive composites – fibre reinforcedpolymers(FRP)–havemanyadvantagessuchashighstrength-to-weight and stiffness-to-weight ratios, corrosion resistance, easeofinstallation,etc.;theirdisadvantageistheirrelativelyhighcost.Forstaticstrengtheningpurposes,unidirectionalcompositeswithcarbonfibresandepoxyresinarethemostwidelyused.Thereare two formsofcarbon fibre reinforcedpolymersused forstrengthening.Polymers in strip formarebonded to the structuralmember’ssurface or into the pre-cut grooves in the concretecover – the well-known near surface mounted reinforcement

method (NSMR). They transfer tensile forces and perform asaddedreinforcementwithrelatedcharacteristics.Polymersheetsinaconfinementformcanbeshapedlikestirrupsorcontinuouslylikeaspiral.Theconfinementeffectisbasedonthewell-knownfactthatthecontainmentofthelateraldeformationofconcreteincreasesitsstrength.Theconfinementeffectscanbeconsideredbytheincreasein the concrete’sstrength and themodification of the stress-strainmodel.The confinement of concrete has amajor effect on columns – itseffectonaxiallyloadedshortsquaredandcircularconcretecolumnshasbeendemonstratedinnumeroustests(alsoOlivová(2007)).Theresearch on eccentrically loaded slender concrete columns is stillquitelimited,andtherearefewpublicationsonthistopic,whichiswhythisapplicationisnotadvanced.Mirmiran, et al. (2001) started the research in this field withconcrete-filled fibre-reinforced polymer tubes (CFFT), which

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showed that as the slenderness ratio is increased, the columns’strengthrapidlydrops[1].Pan,etal.(2007)andTaoandHan(2007)investigated slender reinforced concrete columns wrapped withFRP; the behaviour of these columns differs from that ofCFFTs,eventhoughthestrengtheningeffectdecreaseswithanincreaseintheslendernessratioandtheinitialendeccentricity[2],[3].Confinement with aunidirectional and also abidirectional CFRPjacket was the subject of Tao’sand Yu’s(2008) investigation.The ultimate strength measured for the columns (slenderness ofabout 70) strengthened by unidirectional CFRP is quite close tothatofunstrengthenedones.The longitudinal fibresbecomemoreeffective when bending becomes predominant [4]. Asmall-scaletest of Fitzwilliam and Bisby (2010) contained columnswrappedwith CFRP in ahoop and also in alongitudinal direction. Forslender columns, wrapping in ahoop direction resulted in onlyamodest increase in capacity. Longitudinal CFRPwraps improvethe behaviour of slender concrete columns and allow for theachievementofhigherstrengthsandcapacities[5].To enable awider application of strengthening techniques forslenderconcretecolumns,itisnecessarytoengageinmoreresearchinthisfieldandtrytoderivedesignmethodsforthestrengtheningeffectsofCFRPs.

exPerimentAl inVeStigAtion

InordertoassesstheeffectivenessoftheCFRPlaminatestripsinthegroovesmethodandwrappingwithaCFRPsheetforstrengtheningslender concrete columns submitted to an eccentric compressionload, four series of specimens were tested. The specimens werecompoundedof4mlongcolumns,withacrosssectionof150x210mmandreinforcedwitha2x4φ10hingesupportedatbothends(Fig.1).Thefirstseriesconsistedofnon-strengthenedcolumns;thesecondserieswascomposedofconcretecolumnsstrengthenedwithCFRPlaminatestripsinthegrooves;thethirdserieswascomposedof columns confined with aCFRP sheet; and the last series wascomposed of columns strengthened with acombination of thesemethods.All the columnswere loaded in the sameway – the compressionforceactedontheinitialeccentricityof40mm,anditsvaluegrewin ten loading steps to the resistance force expected from thetheoreticalanalysis.Theexperimentaltestset-upisshowninFig.2.Strains of the concrete, steel and composite reinforcements anddeflection from the increasing bendingmoment at themid heightweremonitoredduringtheincreaseinthecompressionforce.The observation of the relationship between the deflection at themidheightandtheeccentricallyactingcompressionforcewastheprimeobjectiveoftheexperimentalinvestigation.Themostaccurate

measurementallowingfordescendingbranchplottingwasenabledby linear variable displacement transducers (LVDTs) – Fig. 3.Theincreaseintheresistanceofthecompressionforcebesidethenon-strengthened reinforcedconcrete columncanbe estimatedasfollows:12.9%forthecolumnsstrengthenedbyaddingtheCFRPstrips into the grooves in the concrete cover layer; 2.4% for thecolumnsstrengthenedbyconfinementwithonelayeroftheCFRPsheet; 15.4% for the columns strengthened by acombination ofCFRP strips and aCFRP sheet. These main measurements showgood agreement with the check scaling (direct: theodolite andindirect: tensometers, deformeters). The indirect measurementsallowforcalculatingthefinaldeflectionofthemoststressedcross-section from the strains measured. Strains of the concrete, steelreinforcementandcompositematerialswereobserved.FromacomparisonofthesteelreinforcementandCFRPlaminatestrips and the tension and compression strains in the columnstrengthenedbyaddingCFRPstripsintothegroovesintheconcretecover,itcanbeseenthattheCFRPstripstrainsalwayshavehigher

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Fig. 1 Geometry, reinforcement and fitting of a column.

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values, which result from their greater distance from the neutralaxis.Theruptureof theCFRPstripswasobservedat theultimatestrain of 2.5 – 2.8‰. Similar values were measured by Olivová(2007) [6], which was five times less than the values from thetensiontest,whichcanbecausedbyanothertypeofstress:thereisalargecurvatureinthemoststressedcross-sectionof thecolumn,and the strip is not stressed in pure tension as during the tensiontestbutinsomethinglikedirectandbendingstress.ThatiswhytheutilizationofthestraincapacityofCFRPstripsislimited.The measured values of the CFRP sheet strains in the columnsstrengthened with the CFRP sheet confinement were very low –almost zero on the compressed face and amaximal 0.1‰ on thetensile face of the column’scross-section. The same values arelistedinTaoandYu(2008)[4].TheselowstrainvaluesintheCFRPsheetledtoalimitedconfinementeffect–thenecessaryincreaseintheconcrete’strengthwasnotachieved.

theoreticAl And numericAl AnAlySiS

Along with the experimental investigation, numerical modellingwas executed; the ATENA 3D program was used. The materialpropertiesandstaticactionswereadaptedfromfull-scaletests.Thenumerical and experimental procedures give equivalent results. Itwasdemonstrated that strengtheningwithCFRPstrips ingroovesisamoreeffectivemethodforslenderreinforcedconcretecolumns.Atheoretical analysis was carried out before testing the columnswith the material properties measured according to anumber ofanalyticalmodelsfromsourcesabroadandthenmodifiedfollowingtheresultsoftheexperimentalinvestigation.Themodelsthatmostlyapproximatetheexperimentalresultsarefurtherrecommendedforordinaryuse.Thetheoreticalanalysisconsistsoftwoparts.Atfirsttheresistanceofashortcolumnrepresentedbyacross-sectioninteractiondiagram(ID)isconstructed.Inthesecondpart,thisdiagramcanbeturnedinto acolumn interaction diagram to determine the resistance ofaslender columnwithout the additional second order effects. Forthe dimensions of the known column’scross-section, thematerialproperties, slenderness and end eccentricity, the resistance axialforceNRcanbetakenfromthecolumninteractiondiagramfromthelineardependenceof theaxial forceNand thefirstorderbendingmomentMR,0.Inacriticalcross-section,theaxialforceNRandtotalbendingmomentMR (denoting thesumof the firstordermomentMR,0andthesecondordermomentMR,II)act(seeFig.4).Fortheordinaryuseofthementionedstrengtheningtechniques,itisnecessarytoderivethemethodoftheCFRPmaterials’inclusiveeffect on the section and column capacity.When calculating thestrengthening effect, the CFRP strips in grooves can be included

Fig. 2 Set-up of the experimental test.

Fig. 3 Relation between the axial force and bending moment – results from the LVDT measurements.

LVDT

LVDT

LVDT

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as an additional reinforcement near the cross-section’ssurface;thevaluesof their strainsaredeterminedbasedupon theconcretestrains, which depend on their distance from the neutral axis.FollowingthestrainvaluesoftheCFRPstrips,stressisdeterminedfrom the linear stress-strain relation. It is very important tomakeprovision for the initial strains in the reinforced concrete sectionwhentheCFRPstripsareapplied;duringstrengtheningthestructureismaximallyrelievedeventhoughtherearesomelowstrainsintheconcrete and steel reinforcement, and it is necessary to calculatewiththem.CFRPsheetconfinementcanbeincludedasanincreasingconcretestrengthandmodifiedstress-strainrelation.LamandTeng’sstress-

strain model (2003) [7] mostly approximates the experimentalresults.Thisdiagramconsistsofaparabolicpartfollowedbyalinearpartthatendsatapointdefinedbytheconfinedconcrete’sstrengthandultimatestrain.ThesevaluesarecalculatedaccordingtoTeng(2007) [7]. The confined concrete’sfinal stress and strain resultfrom the confining pressure provided by theCFRP jacket,whichdepends onmany factors that provision has to bemade for.Themost important factors are: the material properties of the CFRPsheet, the shapeof thecross-section, thediameterof the roundededges, and the continuous or stirrup-form confinement. Becauseof the linear σ-ε diagram of the CFRP materials, the confiningpressure continuously increaseswith the increasing strains of theCFRP sheet.The confinement’seffectiveness is highly dependentonthesestrains.Acomparison of the short (cross-section ID) and slender columninteraction diagrams of the four series of specimens is shown inFig. 5:As aresult of the interaction diagrams, the effect of thestrengtheningtechniquesisdifferentforshortandslendercolumns.

concluSionS

The results of all three investigations presented indicate thatstrengthening columns with the use of fibre reinforced polymersallowsforanincreaseinload-carryingcapacitywithoutasignificantincreaseinacross-section.The evident difference in the effect of strengthening techniquesontheshortandslenderreinforcedconcretecolumnsisespeciallyconclusive from the theoretical analysis.As aresult of the short

Fig. 4 Interaction diagram of slender column.

Fig. 5 Comparison of short and slender column interaction diagrams.

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columns’ interaction diagrams (cross-section ID), the increase inload-carrying capacity is 1.3% for columns strengthened by nearsurfacemountedCFRPstrips,10.8%forcolumnsstrengthenedbyconfinementwith aCFRP sheet, and 12.6% for acombination ofthesetwomethods.Forslendercolumns,whereprovisionismadeforslenderness,theaverageincreaseintheresistancecompressionforcebesideanon-strengthenedreinforcedconcretecolumncanbeestimated as follows: 11.4% for columns strengthened by addingCFRPstripsintothegroovesintheconcretecoverlayer;1.8%forcolumnsstrengthenedbyconfinementwithonelayeroftheCFRPsheet;15.3%forcolumnsstrengthenedbyacombinationofCFRPstripsinthegroovesandconfinementwithonelayeroftheCFRPsheet. Generally it can be said that the most effective strengtheningmethod depends on the predominant type of stress.The confinedconcretewould bemost active in the case of compressive stress;longitudinal strains activate transverse strains and increase theconfinementeffect.Thepredominantcompressivestressisassumedfor short columns, which is why confinement by aCFRP sheet

hasamarkedlyhighereffectonashortcolumn’sresistance.Whencalculating astrengthening action, CFRP sheet confinement canbe included to increase the concrete’sstrength. CFRP strips actmost efficiently if they are in tension; this case occurs on thetensionsideofacross-sectionofacolumnstrengthenedwithCFRPlaminate strips in longitudinal grooves in aconcrete cover alongacolumn’saxis.The typeofstress is removedto thepredominantbendingregionbyslendercolumns,wherethesecondordereffectscauseanincreaseinthebendingmomentatthesamevalueofthecompressiveforce.CFRPstripsingrooveshaveagreatereffectonthe resistance of aslender column. For calculating strengtheningactions, CFRP strips in grooves can be included as an additionalreinforcementwithrelatedmaterialproperties.

AcknowledgementThisresearchwasdevelopedwithinandwiththesupportofresearchprojectVEGANo.1/0306/09ApplicationofProbabilisticMethodstoImprovetheReliabilityofConcreteStructures.

REFERENCES

[1]Mirmiran,A., Shahawy,M.,Beitleman,T.: SlendernessLimitforHybridFRP–ConcreteColumns.In:JournalofCompositesforConstruction,Vol.5,2001,No.1,pp.26-34.

[2] Pan,J.L.,Xu,T.,Hu,Z.J.:ExperimentalInvestigationofLoadCarryingCapacityoftheSlenderReinforcedConcreteColumnWrapped with FRP. In: Construction and BuildingMaterials,Vol.21,2007,No.11,pp.1991–1996.

[3] Tao,Z.,Han,L.H.:BehaviourofFire-ExposedConcrete-FilledSteelTubularBeamColumnsRepairedwithCFRPWraps. In:Thin-WalledStructures,Vol.45,2007,No.1,pp.63–76.

[4] Tao, Z., Yu, Q.: Behaviour of CFRP-strengthened SlenderSquareRCColumns.In:MagazineofConcreteResearch,Vol.60,2008,No.7,pp.523–533.

[5] Fitzwilliam, J., Bisby, L.A.: Slenderness Effects on CircularCFRPConfinedReinforcedConcreteColumns. In: Journal ofComposites forConstruction,Vol. 14,2010,No.3,pp.280–288.

[6] Olivová, K.: Zosilňovanie betónových stĺpov lamelami atkaninouzuhlíkovýchvlákien:Dizertačnápráca(Strengtheningof concrete columns with the use of carbon fibre polymerlaminate strips and sheet : Dissertation thesis). Bratislava:SlovakUniversityofTechnology,2007,207pp.(inSlovak)

[7] Hollaway, L.C., Teng, J.G.: Strengthening and rehabilitationof civil infrastructures using fibre-reinforced polymer (FRP)composites.Cambridge:WoodheadPublishing,2008,416pp.