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Cyclic responses of reinforced concrete composite columns strengthened in the plastic hinge region by HPFRC mortar

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Cyclic responses of reinforced concrete composite columns strengthened in the plastic hinge region by HPFRC mortar
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  Cyclic responses of reinforced concrete composite columns strengthenedin the plastic hinge region by HPFRC mortar Chang-Geun Cho a , Yun-Yong Kim b, ⇑ , Luciano Feo c , David Hui d a School of Architecture, Chosun University, Seosuk-Dong 375, Dong-Gu, Gwangju 501-759, South Korea b Dept. of Civil Engineering, Chungnam National University, South Korea c Dept. of Civil Engineering, University of Salerno, Italy d Dept. of Mechanical Engineering, University of New Orleans, USA a r t i c l e i n f o  Article history: Available online 2 February 2012 Keywords: High Performance Fiber ReinforcedCementitious composites (HPFRCs)Reinforced concrete composite columnsMultiple-microcracksSeismic strengthening a b s t r a c t The brittleness of concrete raises several concerns due to the lack of strength and ductility in the plastichinge region of reinforced concrete columns. In this study, in order to improve the seismic strength andperformance of reinforced concrete columns, a new method of seismic strengthened reinforced concretecomposite columns was attempted by applying High Performance Fiber Reinforced Cementitious com-posites (HPFRCs) instead of concrete locally in the plastic hinge region of the column. HPFRC has high-ductile tensile strains about 2–5% with sustaining the tensile stress after cracking and develops multiplemicro-cracking behaviors. A series of column tests under cyclic lateral load combined with a constantaxial load was carried out. Three specimens of reinforced concrete composite cantilever columns byapplyingtheHPFRCinsteadofconcretelocallyinthecolumnplastichingezoneandoneofaconventionalreinforced concrete column were designed and manufactured. From the experiments, it was known thatthe developed HPFRC applied reinforced concrete columns not only improved cyclic lateral load anddeformation capacities but also minimized bending and shear cracks in the flexural critical region of the reinforced concrete columns.Crown Copyright   2012 Published by Elsevier Ltd. All rights reserved. 1. Introduction Due to the increase in number of severe earthquakes in the late20th century, disaster from the damage/failure of buildings orinfrastructures with huge losses of both human life and propertyis unavoidable. Building or infrastructures built in the past wereeither designed with a relatively lower level of seismic design loador with no consideration of the seismic design concept, especiallyin low-rise building/housing structures. Following the increase of damage caused by severe earthquakes all over the world, there isan increased interest in the need for an effective seismic strength-ening and rehabilitation of reinforced concrete columns.Theperformanceof buildingstructuresrequiredtoresistsevereearthquakes mainly depends on the ability of the columns in thelower-stories to sustain relatively large inelastic deformationswithout a significant loss of load-carrying capacity. However, asshowninFig.1,thebrittlenessandcrackingofconcreteintheplas-tic hinge region of reinforced concrete columns raises serious con-cerns due to the lack of lateral load-carrying and deformationcapacitiesofthecolumn, withthecolumnleadingtofailurecausedby flexural cracks of the concrete, yielding and buckling of the lon-gitudinal bars as well as crushing of the concrete in the plastichinge zone [1–3].Steel jacketing in the plastic hinge region of the reinforced con-crete column was one of the most well-known strengtheningmethods, particularly improving the flexural deformation and load-carrying capacities of the columns [4,5]. Fiber wrapping was alsoperhaps one of the most successful applications of fiber-reinforcedpolymer(FRP),duetothestrengthenhancementbeingaccompaniedbyconsiderablecostsavingsovertraditionalretrofittingalternatives.Fiberreinforcedpolymer(FRP) couldimprovethestrengthandduc-tilityofconcretebyconfiningtheconcrete[6–9].Anumberofstudiesdealing with improving the strength and ductility of confined con-cretewrappedbyFRP jackets havebeencarriedout [10–15].On the other hand, a number of studies have reported that theuse of high ductile and high performance cementitious fiber-reinforced composites mortar such as High Performance FiberReinforced Cementitious composites (HPFRCs) or EngineeredCementitiousComposites(ECCs)cansignificantlyincreasethebrit-tleness of concrete in tension. In comparison to normal concrete,the material characteristics of HPFRC, as shown in Fig. 2, retain ahigh ductile deformation capacity, with a tensile strain of about2% being caused by multiple fine cracks [16–18]. The developmentofECCorHPFRCwasprimarilymotivatedbytheneedtoimprovea 0263-8223/$ - see front matter Crown Copyright   2012 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.compstruct.2012.01.025 ⇑ Corresponding author. Tel.: +82 42 821 7004; fax: +82 42 821 0318. E-mail address:  yunkim@cnu.ac.kr (Y.-Y. Kim).Composite Structures 94 (2012) 2246–2253 Contents lists available at SciVerse ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct  quasi-brittle tensile behavior, which was typical for normal con-creteandmortar. HPFRCcanbeconsidereda special familyof fiberreinforcedconcretethatexhibitspseudostrain-hardeningbehaviorunder uniaxial tension. By increasing the tensile loads, HPFRC gen-erally shows multiple fine cracks of reduced width, without strainlocalization. Unlike normal concrete, HPFRC can sustain tensilestresses corresponding to high strains [17–19].In order to improve the seismic performance of reinforced con-crete columns, Fischer and Li [20,21] were the first to investigatethe effect of the ductile deformation behavior of ECC on the re-sponse of both steel reinforced and FRP reinforced columns underreversed cyclic loading conditions. The advantage of applying ECCmortar to concrete flexural members can significantly improve theseismic behavior of concrete building structures, which includesgreater deformation and load-carrying capacity, smaller membersize, reduced reinforcement and longer span.The aimof this research is to develop a new approach of seismicstrengthened reinforced concrete composite columns by applyingHPFRCinsteadofconcretelocallyintheregionofthecolumnplastichinge. A series of column tests under cyclic lateral load combinedwithconstantaxialloadwasconductedbymanufacturingfourspec-imens: three specimens of reinforcedconcrete composite cantilevercolumnsbyapplyingHPFRClocallyinthecolumnplastichingezonewith/without tied bars in the zone and one specimen of a conven-tional reinforced concrete column. By taking into consideration theexperimental variables such as applying HPFRC with or withouttransversebars,theplacedlengthofHPFRCandthePVAfibervolumefraction in mixing of the HPFRC, the strengthened columns werecompared to the conventional reinforced concrete column under areversed cyclic lateral load. The experimental results reported wereusedtoevaluatetheeffectivenessoftheproposedstrengtheningcon-ceptintheseismicperformanceenhancementofreinforcedconcretecolumns. 2. Mixing design and properties of HPFRC In order to have a high-ductile tensile behavior with multiplemicrocracks, the HPFRC was freshly mixed in this study by using1.5% fiber volume fractions with high-tensile Polyvinylalchol(PVA) fibers, ordinary Portland cement (OPC), fine aggregates(maximum grain size 0.25mm), water, a high-range water-reduc-ing admixture, and admixtures to enhance the fresh properties of the mortar, as shown in Table 1. To increase the matrix fluidityof the cement and fiber dispersibility, a polycarboxylate superp-lasticizer (PCSP) was used, while hydroxypropyl methylcellulose(HPMC) was also applied to prevent the segregation of materialssuch as silica, fly ash (FA), blast-furnace slag (BFS) fine powderand fibers. In addition, an antifoaming agent was used to finishthe surface as well as control the air content, as shown in Table 2.PVAfibers with a length of 12mm, a diameter of 39 l m and a sur-face treated by an oiling agent were used as a reinforcing materialto mix the HPFRC mortar in order to improve the brittle nature of the concrete or mortar. The material properties of the PVA fibersare shown in Table 3. The HPFRC as shown in Table 4 had a water/binder ratio (W/B) of 45%, a sand/cement ratio (S/C) of 71%, and a PVA fiber volume fraction of 1.5%.As shown in Fig. 3, a direct uniaxial tensile test was carried out to evaluate the high-ductile tensile performance of the HPFRC. Asshown in the figure, a series of uniaxial tensile tests was carriedout using a 10kN capacity universal testing machine (UTM) bycontrolling the displacement of 0.2m/min. The LVDT was attachedto two sides of the specimens in order to obtain the tensile strainsfrom the measured displacements. The hardened specimens wereremoved from the molds 1day after placing and cured in waterfor 28days, with a 30  30mm cross-section and length of 330mm.Fromthedirectuniaxialtensiletest,thedirecttensilestressandstrain relationships of the HPFRC could be measured as shown inFig. 4. The HPFRC had a high-ductile tensile characteristic, with ameasured tensile strain of about 2.5–5.0%. This explained that Fig. 1.  Plastic hinge in reinforced concrete column. ' 0.002 c ε   ≥ ' c  f  0.02 ≥ t   f  stressstrain HPFRC concrete Fig. 2.  High-ductile tensile characteristic of HPFRC.  Table 1 Properties of mixed materials. Types Density (g/mm 3 ) Fineness (cm 2 /g) SiO 2  Al 2 O 3  Fe 2 O 3  CaO MgO SO 3  Ig. lossOPC 3.14 3.200 21.24 5.97 3.34 62.72 2.36 1.97 1.46Silica sand 2.64 0.2 a 96.9 1.44 0.34 0.11 0.03 – –FA 2.16 3.645 50.5 – – – – – 3.04BFS 2.94 4.310 34.7 13.8 0.11 44.6 5.62 0.23 0.64 a Diameter (mm).  Table 2 Chemical admixtures. PCSP HPMC DefoamerDensity (g/mm 3 ) 0.37 0.60 0.26Type Brownish powder White powder White powder C.-G. Cho et al./Composite Structures 94 (2012) 2246–2253  2247  the high-ductile tensile behavior after cracking of the HPFRC couldbe caused by the appearance of multiple microcracks as shown inFig. 5. An apparent curing strain behavior was also observed afterthe premature cracks, which seemingly displayed the propertiesof high toughness since it had a relatively improved resistance toconcrete when the cracks appeared. The premature crackingstrengthwasapproximately1.7–3.7MPa,whilethemaximumten-sile strength measured in the interval of the curing behavior was3.7–4.4MPa.TheresultshighlightthattheHPFRChadthemechan-ical characteristics of a high-ductile tensile strain with multiplemicrocracks, thus improving the brittleness of the concrete. 3. Experimental program of strengthened columns  3.1. Design and manufacture of column specimens In order to improve the lateral load-carrying and deformationcapacities of the reinforced concrete columns, the currentstrengthening method is that the column section from the columnbasetoabovethelengthoftheplastichingeregionisplacedbytheHPFRC instead of concrete with and without transverse reinforce-ments. Four specimens were manufactured, representing thefirst-storey column between the footing and the inflection point,with the column being fixed to the column base as the cantilevercolumn. Three specimens were strengthened with reinforced con-crete composite columns by applying HPFRC locally in the columnplastic hinge zone and one specimen as the conventional rein-forced concrete column. Table 5 summarizes the four specimenswith the design variables and Fig. 6 illustrates the geometry andreinforcement details of the specimens. The main variables in this  Table 3 Properties of PVA fiber. Ingredient Density (g/mm 3 )Length(mm)Diameter( l m)SurfacetreatmentTensile strength(MPa)Young’s modulus(GPa)Elongation(%)AlkaliresistancePolyvinylalchol(PVA)1.3 12 39 Oiling agent 1600 40 3–113 High  Table 4 Mixing properties of HPFRC. W/B wt.% S/C (wt.%) FA/B (wt.%) Slag/B (wt.%) Unit: kg/m 3 W B a OPC FA BFS Silica sand PCSP HPMC Defoamer PVA (vol.%)45 71 20 20 375 833 500 167 167 692 0.37 0.18 0.45 1.5 a B: OPC+FA+BFS. Fig. 3.  Setup for direct tensile test of HPFRC specimens. Fig. 4.  Measured tensile stress–strain behaviors of HPFRC specimens.2248  C.-G. Cho et al./Composite Structures 94 (2012) 2246–2253  experiment were the PVA fiber volume fraction, the length of HPFRC and the amount of shear reinforcements. Each columnhad a 300mm  300mm cross-section, height of 1540mm, a400mm  400mm cross-section of the head part of the columnandthecolumnbasewhichwasconnectedtoareinforcedconcretefooting, measuring 900mm  900mm  700mm. The cross-sectionfor all of the specimens had a main longitudinal reinforcementsas eight units of D13 bars.The standard specimen RC-0 is a conventional reinforced con-crete column. In order to evaluate the strengthening effect of HPFRC, with a detailed design variable of each specimen as shownin Table 5, the specimens HPFC-0, HPFC-1, and HPFC-2, the HPFRCwere manufactured by assembling and placing the HPFRC with alengthof1.5–2.0 d fromthecolumnbasewithorwithouttransversereinforcement,where d istheeffectivedepthonthecross-sectionof  Fig. 5.  Multiple micro-cracks of HPFRC specimens.  Table 5 Variables of column specimens. SpecimennameFiber volumefraction ( V   f  )Height of HPFCReinforcements (main/shear)RC-0 – – 8-D13/D10 @ 100HPFC-RC-0 PVA (1.5%)  H   =2.0 d  8-D13/D10 @ 100HPFC-RC-1 PVA (1.5%)  H   =2.0 d  8-D13/no stirrupHPFC-RC-2 PVA (2.0%)  H   =1.5 d  8-D13/D10 @ 100 Fig. 6.  Geometry and reinforcement details of column specimens. (a) Form work (c) attaching gauges(e) Mixing HPFRC (g) Placing topping concrete and curing (b) Assembling reinforcements (d) Placing reinforcements (f) Placing HPFRC Fig. 7.  Manufacturing process of column specimens. C.-G. Cho et al./Composite Structures 94 (2012) 2246–2253  2249  thecolumn.Inthecolumnbase,inordertoavoidfailureinducedbyconstruction joint between the concrete in the footing and theHPFRC, the HPFRC was placed about 50mm inside the footing. Forspecimen HPFC-1, the shear reinforcements were not placed atthe height of the HPFRC in order to evaluate the control of shearcracks by HPFRC, while for all the other cases, the shear reinforce-ments were assembled with D10 bars with a space of 100mm.The topping concrete was placed with finishing after placing theHPFRC. The practical manufacturing process of the specimens isshown in Fig. 7.  3.2. Properties of the concrete and reinforcing steel bars TwotypesofreinforcingsteelbarsproducedinKoreawereusedin the column specimens. The yielding stresses of the reinforcingbars for the main longitudinal bars (D13) and the transverse bars(D10) measured 385MPa and 383MPa, respectively. The concretewas mixed with OPC, crushed stones with a maximum aggregatesize of 20mm, sand and admixtures. The combinations of the con-crete mixtures were designed to satisfy the required strength,workability, and mechanical characteristics of the selected con-crete. Cylindrical specimens were cast to test the compressivestrength of the concrete, with each specimen using  £ 100  200mm molds by placing the concrete in three layers and thenexternally vibrated. The specimens were wrapped with plasticimmediately after production and moist-cured for 28days. Theuniaxial compressive strength of the concrete was measured asthe average of 28.7MPa.  3.3. Experimental procedure The installation of the test frame for the column specimens isshown in Fig. 8. To provide cantilever-type loading conditions,the bottom stub of each specimen was fixed to the base in orderto achieve fully fixity at the base.This loading configuration was chosen to promote a flexuraldeformation mode in all the specimens as well as investigate theeffect of the HPFRC material properties on the expected plastichinge region in particular. Lateral loading was applied through areaction wall equipped with a 100kN-capacity actuator accordingto a predetermined displacement-controlled loading sequence.Fig.9illustratestheunidirectionallateraldisplacementhistoryfol-lowed in the testing specimens. The cyclic lateral load was con-trolled by the top-displacement of the column by  l , the laterallydisplacement ductility ratio defined as the ratio of the current dis-placement to the yielding displacement of the column. To applyaxial loading, external steel tendons were attached between thepin and the loading frame and tensioned by hydraulic actuators.Theaxialloadofthecolumnwassetto196.2kNduringtheloadinghistory. The specimens were equipped with a displacement trans-ducer at the top of the column to measure and control the lateraldisplacement of the column. The strain gauges were attached tothe longitudinal and transverse reinforcing bars, with the spaceof the transverse reinforcements from the column base to theheight being 2.0 d . 4. Evaluation of cyclic load test of column 4.1. Hysteretic behaviors, displacement ductility and evaluation of seismic strengthening  Inordertoevaluatetheseismicresponsesofthereinforcedcon-crete columns in severe earthquake regions, it is necessary tounderstand the seismic performance characteristics of the column,such as ductility, energy dissipation capacity, strength deteriora-tion, and stiffness degradation. For this reason, the hystereticbehavior of the members should be thoroughly investigated.Fig. 10 shows the lateral load–top displacement hysteretic behav-ior of each column. For the specimens HPFR-0, HPFR-1, and HPFR-2, since the limited displacement capacity of the actuator is upto±150mm, the displacement-controlled cyclic loading wasstopped if the lateral top displacement of the column reachedabout 120mm.In comparison to the conventional reinforced concrete column,RC-0, the developed strengthened columns, HPFR-0, HPFR-1, andHPFR-2, could provide excellent seismic improved responses inimproving the load-carryingand deformation capacities of the col-umn during cyclic load reversals.The primary curve of each specimen as shown in Fig. 11 can beobtained from the lateral load–top displacement hysteretic behav-ior of each specimen, and Table 6 provides a summary of the (a) Side view (b) Frontal view Fig. 8.  Setup for cyclic load test of columns. Fig. 9.  Cyclic lateral loading history by displacement control.2250  C.-G. Cho et al./Composite Structures 94 (2012) 2246–2253
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