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In: 8th RILEM International Symposium on Fibre Reinforced Concrete: Challenges and Opportunities (BEFIB 2012), Guimarães, Portugal, 19 – 21 September 2012.

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  1. 1. BEFIB2012 – Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 2012 COMPRESSIVE BEHAVIOUR OF FLAX FRP TUBE CONFINED COIR FIBRE REINFORCED CONCRETE * † †† ††† L. Yan , X. Yuan , C. Nguyen , N. Chouw * Dep. Civil & Environ. Eng., University of Auckland Private Bag 92019, Auckland 1142, New Zealand e-mail:, web page: † Dep. Mechanic. Eng., University of Auckland Private Bag 92019, Auckland 1142, New Zealand e-mail:, web page: †† Mater. Accelerator, University of Auckland Private Bag 92019, Auckland 1142, New Zealand e-mail:, web page: ††† Dep. Civil & Environ. Eng., University of Auckland Private Bag 92019, Auckland 1142, New Zealand e-mail:, web page: coir fibre, flax fibre, FRP, stress-strain behaviour, confinement.Summary: This study addresses the contribution of flax fibre reinforced polymer (FFRP) and coir fibrereinforced concrete (CFRC) to the compressive strength of the composite structure. FFRP tubes madeby flax fabric reinforced epoxy resin composites were fabricated with infilled plain concrete (PC) orCFRC. The considered coir fibre content is 1 wt. % of concrete and the fibre length is 40 mm. Uniaxialcompression tests were performed on the FFRP tube confined PC and CFRC specimens. The axialstress-strain relationship, confinement effectiveness, failure mode and ultimate axial strain of theFFRP confined PC and CFRC were investigated. The test results showed that FFRP tube significantlyenhance the compressive strength and ultimate axial strain of both PC and CFRC, i.e. PC confined by4-layers of FFRP tube experiences respectively 199 % and 1047 % enhancement in the compressivestrength and ultimate axial strain. The stress-strain curves of FFRP confined PC and CFRC exhibit abi-linear manner. The failure mode of FFRP tube confined PC and CFRC is dominated by the ruptureof FFRP tube when its tensile strength in the hoop direction exceeds its tensile strength obtained fromFFRP flat coupon tests. It was also found that coir fibre inclusion reduces the concrete cracks width forboth PC and FFRP confined PC.1 INTRODUCTION In recent years, fibre reinforced polymer (FRP) composite materials, with their high stiffness andstrength-to-weight ratios, design flexibility and corrosion resistance performance, have been widelyapplied in civil engineering to strengthen concrete structures [1-10]. FRP materials were firstly used forconfining steel reinforced concrete columns as external wrapping for seismic retrofit and rehabilitation.In the 1990s, Mirmiran and Shahawy [1, 8] proposed the concrete filled FRP tubes (CFFT) system.FRP materials in the form of pre-fabricated tubes were considered in this new construction system.The FRP tubes were manufactured using filament-winding techniques or hand lay-up process. Thesetubes, acting as stay-in-place structural formwork for fresh concrete, can also protect the encasedconcrete from the potential aggressive environment, e.g. de-icing salts and other chemicals [2-5]. Thelaminate structure of the FRP tube can generally be engineered to offer the desired strength and
  2. 2. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouwstiffness in a specific direction by controlling the numbers of layers and angles at which layers of fibresare oriented. Therefore, the FRP tubes can effectively replace conventional steel reinforcement toincrease concrete compressive strength and ductility [2-7]. Fardis and Khalili [7] proposed the concept of the FRP tube encased concrete structures in the1980s, they investigated the behaviour of circular and rectangular FRP confined concrete beams andconcluded that FRP tubes increased concrete compressive strength while decreasing structuralweight. So far the compressive behaviour of FRP confined concrete has been widely studied by manyresearchers [9-18]. Mirmiran and Shahawy [1], Xiao and Wu [9] and Lam and Teng [10] examined the failuremechanism of FRP confined concrete. They reported that the failure of FRP confined concrete wasdominated by the rupture of FRP at an average stress much smaller than the ultimate tensile stressobtained from the FRP flat coupon tensile test. Turgay et al. [12] developed a stress-strainconfinement model to predict the ultimate comrpessive strength and strain for FRP confined concrete. Berthet et al. [13] investigated the compressive behaviour of concrete wrapped by E-glass FRPcomposite jackets with concrete compressive strength varying from 20 MPa to 200 MPa. The resultsshowed that the confinement effect of FRP on normal strength concrete was larger than that on highstrength concrete. Nowadays FRP materials are widely accepted in the practical infrastructure construction industrysuch as highway bridge decks and piles. Fam et al. [14] reported the construction details and fieldtests of precast CFFT piles which were used for the first time in the construction of the Route 40highway bridge over the Nottaway River in the United States. Although FRP composites have a promising future as construction materials, the wider application ofFRP in structures is limited mainly by its high initial cost and the concern that the non-yieldingcharacteristics of FRP could result in wide cracks in confined concrete after the rupture of FRP. Thismay lead to sudden brittle failure of FRP confined concrete structures [15, 16]. Nowadays the increasing of environmental consciousness promotes a rapid development in bio-composites with high mechanical performance. Use of bio-fibres to replace synthetic carbon/glassfibres as reinforcement has gained popularity in engineering applications [17-19]. Compared withglass and carbon fibres, natural fibres are more cost effective and abundantly available, and have highspecific strength and stiffness [20, 21]. Among natural fibres, flax, hemp, jute and coir are the mostpopular reinforcement materials in composites due to their good mechanical properties and availability[22]. Polymer matrix reinforced by woven fabric is probably the form of composites used mostcommonly in structural applications such as aircrafts, boats and automobiles. Test by Assarar et al.[22] compared the tensile properties of flax fabric reinforced composites with glass fabric reinforcedcomposites (GFRP). They confirmed that the tensile stress and strain of flax fabric reinforced epoxycomposites were close to GFRP composites. The flax composites can be recommended in structuralapplication. Natural fibres are also considered in the cementitious matrix to increase concrete tensile and flexuralstrengths, increase toughness, impact resistance and fracture energy [23]. Generally, natural fibresare cellulose materials with relatively lower specific densities. They can be used to decrease concreteweight when they are at high fibre volumes. They can be optimised in the concrete to design aconstruction material with high performance–to-cost ratio [24]. Coir fibre, as reinforcement material in the cementitious matrix, has been investigated by manyresearchers because coir fibres are most ductile among natural fibres, nearly cost-free and abundantlyavailable worldwide. Satyanarayana et al. [25] and Munawar et al. [26] presented stress-strain curvesfor 12 natural fibres confirming the higher ductility of coir fibre up to 24 % and 39 %, respectively. Forother natural fibres, their strains at failure range from 3 % to 6 %. Baruah and Talukdar [27] analysedthe mechanical properties of coir fibre reinforced concrete (CFRC) with different volume fractions of0.5 %, 1 % and 2 % of concrete, the fibre length is 4 cm. The study showed that the compressive,splitting tensile and shear strengths were increased with an increase in fibre volume fraction. TheCFRC with 2 % coir fibre possessed the best mechanical properties, whose compressive, splitting 2
  3. 3. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouwtensile and shear strengths were increased up to 13.7 %, 22.9 % and 32.7 %, respectively, comparedto those of PC. Experiments by Ali et al. [28] showed that concrete compressive strength wasincreased 9 % by an addition of 1 % coir fibre (by weight of cement). In the splitting tensile test, plainconcrete specimens were broken into two halves without contact, while for CFRC, the two halves werestill held by the coir fibre as a whole. In this study, novel flax FRP (FFRP) tube confined coir fibre reinforced concrete (CFRC) structurewas investigated. This new system consists of a FFRP tube and a CFRC core. In this system, therelatively cheaper flax fibre as reinforcement material of FRP tube to confine concrete for enhancingconcrete compressive strength and ductility. Coir fibre in the cementitious matrix to further increaseconcrete compressive strength and control concrete crack patterns, as the non-yielding behaviour ofFRP materials could result in brittle failure of the concrete core. The behaviour of FFRP confined PCand FFRP confined CFRC specimens under axial compression was investigated.2 EXPERIMENTS2.1 FFRP tubes The reinforcement material of the flax FRP composites is the bidirectional woven flax fabric. TheEpoxy system used is the SP High Modulus Ampreg 22 resin and Ampreg 22 slow hardener. Thetensile strength and modulus of the resin is 70 MPa and 3.7 GPa, and strain to failure and linearshrinkage is 3.0 and 1.7 %, respectively. The FFRP tubes were fabricated using the hand lay-upprocess at the Centre for Advanced Composites Materials (CACM) at the University of Auckland. oFabric fibre orientation was at 90 from the axial direction of the tube. Composites tensile propertieswere determined by flat coupon tensile test on Instron 5567 machine according to ASTM D3039 [29]and their flexural properties were determined on the Instron 5567 machine in accordance with ASTMD790 [30]. The mechanical properties of the FFRP composites are listed in Table 1. Fibre volumefraction is the ratio of fibre volume divided by the total volume of the composite. Table 1: Mechanical properties of flax FRP composites No. of flax FRP Tensile Tensile Tensile Flexural Flexural Fibre fabric layers thickness strength Modulus Strain strength Modulus volume (mm) (MPa) (GPa) (%) (MPa) (GPa) fraction (%) 2 2.65 102 5.0 3.6 103 5.9 53.8 4 5.30 125 9.2 4.4 128 8.5 55.7 The fabrication process of flax FRP tubes has nine steps: (1) Cutting: Flax fabrics were cut into aspecified size. The length of a piece was equal to the perimeter of a concrete cylinder (100 mm indiameter), with an overlap of 100 mm. A width is 200 mm as the cylinder height. (2) Surfacepreparation: Hollow aluminium tube mould was wrapped with a thin release film for easy demouldingof the tube. (3) Epoxy resin mixture: The Ampreg 22 resin and hardener were mixed with a ratio of100:28 by mass. (4) Impregnation of flax fabrics: Fabric pieces were impregnated into the resin for 30min. (5) Primer application: A coat of epoxy primer was applied to the release film surface to cure for30 min. at the room temperature. (6) First fabric application: The first epoxy-impregnated fabric wasthen applied. The prepregnated fabric was carefully rolled around the mould to insure good adhesion.(7) Second fabric application: The second fabric layer was applied. This step was repeated for thetargeted layers. (8) Curing: After consolidation for 24 h at room temperature, the FFRP tube with the omould was cured in an oven at 65 C for 7 h. (9) After curing, the tube was removed from the mouldwith the help of a press machine. 3
  4. 4. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouw2.2 Concrete specimens A total 18 concrete cylinders were constructed: three PC and three CFRC specimens (100 mm indiameter and 200 mm in height) as the control group, half of the 12 remaining cylinders were FFRPconfined PC and the other six are FFRP confined CFRC specimens, with 100 mm core diameter and200 mm height. The matrix of the specimens prepared for this study is listed in Table 2. Two differentlayer arrangements of FFRP tube were used: two layers and four layers (Figure 1).Figure 1: Flax FRP tubes (a) flax fabrics, (b) FFRP tubes with mould, (c) demould and (d) FFRP tubes Coir fibres were immersed in water for 6 h to remove dust. The washed fibres were rolled with aradial toothed roller and then combed to divide the fibres smoothly and orientated them in one odirection using a steel comb. Next, the fibres were dried in an oven at 70 C for 24 h. Later, the fibreswere cut to length of 40 mm and a sieve was used to separate the very short fibres from thedesignated fibres. Figure 2: CFRC is poured in a FFRP tube 4
  5. 5. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouw Table 2: Test matrix Specimen No. of FFRP Core Height Tube * type specimens layers diameter (mm) (mm) thickness (mm) PC 3 -- 100 200 -- CFRC 3 -- 100 200 -- 2-FFRP-PC 3 2 100 200 2.65 4-FFRP-PC 3 4 100 200 5.30 2-FFRP-CFRC 3 2 100 200 2.65 4-FFRP-CFRC 3 4 100 200 5.30 In column *, 2- for 2-layer and 4- for 4-layer Two batches of concrete were prepared. Both batches were designed as plain concrete with a 28-day compressive strength of 25 MPa. The concrete mix design followed ACI Standard 211. 1 [31]. Themix ratio by weight was cement: water: gravel: sand = 1: 0.58: 3.72: 2.37. For the second batch, coirfibre was added during mixing. The coir fibre length is 40 mm and weight content is 1 % of concrete. For each FFRP tube, one end was capped with a wooden plate. Then concrete was cast, poured,compacted and cured in a standard curing water tank for 28 days. Both ends of FFRP specimenswere treated with high quality mortar to have a uniform surface bearing and then a blade was used tocut the edges of the FFRP tube to avoid it from directly bearing the axial compression (Figure. 3). Figure 3: Specimens (a) without and (b) with surface treatment2.3 Experimental instruments Compressive test was conducted to investigate the FFRP confinement effect on the concretespecimens. For each cylinder, four strain gauges were used. Two strain gauges were mounted at themid-height of a cylinder aligned along the hoop direction to measure the hoop strain and two straingauges were mounted at the mid-height of a cylinder aligned along the axial compressive direction tomeasure the axial strain, as displayed in Figure 4. An Avery-Dension test machine was used toperform the compression test using stress control based on ASTM C39 [32]. Each sample was axiallycompressed up to failure. Readings of the load, displacement, gauges and LVDTs were taken using adata logging system and were stored in a computer. 5
  6. 6. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouw Figure 4: Schematic of axial compression test3 RESULTS AND DISCUSSION3.1 Test results The average static compressive properties of each concrete type are listed in Table 3. f co is the compressive strength of unconfined concrete and f cc is the compressive strength of confinedconcrete. Confinement effectiveness of FRP confined concrete is defined as compressive strength of the confined concrete f cc divided by the corresponding compressive strength of unconfined concrete f co . ε co and ε cc are the ultimate axial strain for unconfined concrete and FFRP confinedconcrete with respect to the ultimate strength, respectively. ε h.rup is the hoop strain at the rupture ofFFRP. Table 3 shows that the coir fibre slightly decreased the compressive strength of CFRC. Thismay attributable to the non-uniform distribution of the fibres in the concrete which leads to voidsresulting in decreased strength. However, coir fibre enhanced the ultimate axial compressive strain to0.0047, compared to that of 0.0019 for plain concrete. This fact indicated that the post-crackingtoughness of CFRC is much higher than that of PC, and more energy is dissipated to damage theCFRC. The increase of toughness is believed attributable to the fibre bridging effect which resistsfurther opening of initial cracks in the concrete. Table 3. Test results for all the specimens Properties PC CFRC 2 layer 4 layer 2 layer 4 layer FFRP-PC FFRP-PC FFRP-CFRC FFRP-CFRC 25.3 ± 1.6 * 23.9 ± 1.0 * 38.7 ± 1.4 50.4 ± 2.7 33.2 ± 1.8 48.9 ± 2.2 f co or f cc (MPa) ε co or ε cc (%) 0.19 ± 0.02 * 0.47 ± 0.05 * 1.5 ± 0.2 1.9 ± 0.1 1.5 ± 0.1 2.2 ± 0.2 -- -- 2.6 ± 0.2 4.5 ± 0.1 3.5 ± 0.2 4.2 ± 0.1 ε h.rup (%) -- -- 1.53 1.99 1.39 2.05 f cc / f co -- -- 7.89 10.47 2.96 4.36 ε cc / ε co The * values were used for calculating the f cc / f co and ε cc / ε co for FFRP tube confined PC and CFRC 6
  7. 7. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouw For both PC and CFRC, FFRP tube confinement enhanced the ultimate strength and axial strainremarkably. Both structural ductility and carrying capacity increased noticeably with the thickness ofFFRP tube. For confined PC, the average confinement effectiveness and axial strain ratio of twolayers of FFRP is 1.53 and 789 %, and of four layers of FFRP is 1.99 and 1047 %. For confinedCFRC, the average confinement effectiveness and axial strain ratio by two layers and four layers is1.39 and 296 %, and 2.05 and 436 %, respectively. The results confirmed that the effect ofconfinement on CFRC is larger than that on PC for concrete confined by 4-layer FFRP. This may beattributable to the non-uniform distribution of fibres in the concrete which leads to voids resulting instrength reduction. The average ultimate axial strains for confined PC and CFRC with 2-layer FFRP are both 1.5 %, andwith 4-layer FFRP are 1.9 % and 2.2 %, respectively. This indicates that the effect of coir fibre on theultimate strain of FFRP confined concrete is insignificant than that on unconfined one. The possiblereason is that the third linear region (see Figure 5) in the stress-strain curve of FFRP confinedconcrete is mainly dominated by the behaviour of FFRP tube. As for hoop strain, it was clear that the coir fibre significantly increased the hoop strains of concretedue to the fibre bridging effect. For FFRP confined PC, the growth of tube thickness increased thehoop strain remarkably, compared to that on the confined CFRC. In general, the effect of FFRP tubeon increasing the hoop strain is larger than that on axial strain, for both confined PC and CFRC.3.2 Stress-strain relationships All previous studies [1-12] on FRP confined concrete displayed that, subjected to an axialcompressive load, FRP confined concretes behave bi-linearly, two linear regions connected by atransition zone. The stress-strain curve of 4 layer FFRP tube confined PC is presented in Figure 5. Ingeneral, the curves of FFRP confined PC are similar to those of FFRP confined CFRC, which are alsosimilar to those of G/CFRP confined concrete. Figure 5: Stress-strain curves: FFRP confined PC and FFRP confined CFRC Stress-strain curves obtained from this study can be divided approximately into three zones. In thefirst purely linear region, the stress-strain behaviour of both FFRP confined PC and CFRC specimen issimilar to their corresponding unconfined PC and CFRC. In this region the applied axial stress is low,lateral expansion of confined PC and CFRC is inconsiderable and confinement of FFRP tube is notactivated. When the applied stress approaches the ultimate strength of unconfined PC or CFRC, thecurve enters the second nonlinear transition region where considerable micro-cracks are propagatedin concrete and the lateral expansion increased. With the growth of micro-cracks, the tube starts to 7
  8. 8. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouwconfine the concrete core and counteracts the stiffness degradation of the concrete. The thirdapproximately linear region is mainly dominated by the structural behaviour of FFRP compositeswhere the tube is fully activated to confine the core, leading to a considerable enhance in compressivestrength and ductility of concrete when the core is subjected to tri-axial compression. When axialstress increases, FFRP tube hoop tensile stress also increases. Once this hoop stress exceeds theultimate tensile strength of FFRP tube, failure of the first layer starts, subjected to sustained loading, asequential layer failure will occur, leading to a total structural failure.3.3 Failure mode Figure 6(a) displays that failure of PC is more severe than that of CFRC. It was observed that thecrack widths of PC are clearly larger than those in CFRC; also some parts of concrete are crushedand spalled. In contrast, the cracks in CFRC are held by the coir fibres. The coir fibres bridge themacro-cracks in the concrete and provides an effective secondary reinforcement for crack widthcontrol, thus the macro-cracks are prevented and blocked. In Figures 6(b) and 6(c), failure modes of FFRP confined PC and CFRC are presented. 2 FFRP-PCand 4 FFRP-PC stands for 2 layer and 4 layer FFRP tube confined plain concrete, respectively. 2FFRP-CFRC and 4 FFRP-CFRC stands for 2 layer and 4 layer FFRP tube confined coir fibrereinforced concrete, respectively. For all the confined PC and CFRC, the specimens failed by suddenrupture of FFRP tube accompanied by a heavy popping noise. The single and straight fracture crack in othe tube parallels to the axial stress direction. This linear crack is attributed to the 90 orientation of theflax fabric in the tube, i.e., hoop direction. The tube crack location is strongly dependent on thedevelopment of concrete cracks. 8
  9. 9. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. Chouw Figure 6: Failure mode: (a) PC vs. CFRC, (b) FFRP confined PC and CFRC, and (c) FFRP confined PC and CFRC after removing tubes FFRP tubes separated from the concrete cores after failure are given in Figure 6(c). The tubes wereremoved from the cores with ease. This indicates that the tubes have no attachment to the core. Allthe specimens possessed cracks on the concrete, which implies that the concrete has already failedbefore the rupture of the tube. Therefore, once the concrete core was cracked, the FFRP would beactivated gradually due to the lateral expansion caused by the gradual crushing and compaction of theconcrete, and FRP tube dominated the stress-strain behaviour in the third linear region in Figure 5.However, it is clear that PC specimens confined by FFRP have larger cracks width compared to theconfined CFRC specimens. The PC core has been crushed and spalled but the CFRC only hadcracks. Thus, the addition of coir fibre in FRP confined concrete can effectively reduce concrete widecracks after failure. This is important for changing the failure mode of FRP confined plain concrete dueto the non-yielding behaviour of FRPs.4 CONCLUSIONS The compressive behaviour of flax FRP (FFRP) tube confined plain concrete (PC) and coir fibrereinforced concrete (FFRP confined CFRC) was studied. The study reveals:• The axial stress-strain behaviour of flax FRP confined plain concrete and coir fibre reinforced concrete is bilinear.• The use of coir fibre increased the ultimate axial strain of the plain concrete significantly, from 0.0019 to 0.0047.• For both unconfined and confined concrete, coir fibre can prevent and control the macro-cracks of concrete effectively. The coir fibre can be used to reduce the wider cracks in FRP confined plain concrete core.• FFRP enhanced the compressive strength and ductility of PC and CFRC considerably, as well as resulting in large energy absorption capacity. As for PC, the confinement effectiveness and ultimate axial strain ratio are 1.53 and 789 % for 2-layer FFRP tube and 1.99 and 1047 % for 4- layer FFRP tube. As for CFRC, these values are 1.39 and 296 % for 2 layer-FFRP tubes and 2.05 and 4.36 % for 4 layer-FFRP tubes.• The effect of FFRP confinement on the hoop strain enhancement is greater than that on the axial strain of both FFRP confined PC and CFRC.• The failure mode of flax FRP confined concrete is dominated by the rupture of flax FRP in the hoop directly when the hoop tensile strength exceeds the tensile strength of FFRP tube. 9
  10. 10. BEFIB2012: L. Yan, X. Yuan, C. Nguyen and N. ChouwREFERENCES[1] A. Mirmiran and M. Shahawy. “Behavior of concrete columns confined by fiber composites”. JStruct Eng ASCE, 23, 583-590 (1997).[2] M.S. Samaan. “An analytical and experimental investigation of concrete filled fiber reinforcedplastics (FRP) tubes”. Ph.D Thesis, University of Central Florida, Orlando, Florida (1997).[3] A. Mirmiran, M. Shahawy, M. Samaan, H.E. Echary, J.C. Mastrapa and O. Pico. “Effect of columnparameters on FRP-confined concrete”. J Compos Constr ASCE, 2,175-185 (1998).[4] A. Fam and S.H. Rizkalla. “Confinement model for axailly loaded concrete confined by circularfibre-reinforced polymer tube”. ACI Struct J, 98, 451-461 (2001).[5] H.M.H. Mohamed. Axial and flexural behaviour of reinforced concrete-filled FRP tubes:Experimental and theoretical studies. Ph.D Thesis, University of Sherbrooke, Sherbrooke (2010).[6] A. Fam and S. Rizkalla. “Flexural behavior of concrete-filled fiber-reinforced polymer circulartubes”. J Compos Constr ASCE, 6, 123-132 (2002).[7] M.N. Fardis and H.H. Khalili. “FRP-encased concrete as a structural material”. Mag Concrete Res,34,191-202 (1983).[8] A. Mirmiran and M. Shahawy. “A new concrete-filled hollow FRP composite column”. Composites:Part B, 27, 263-268 (1995).[9] Y. Xiao and H. Wu. “Compressive behaviour of concrete confined by carbon fiber compositesjackets”. J Mater Civil Eng ASCE, 12, 139-146 (2000).[10] L. Lam and J.G. Teng. “Design-oriented stress–strain model for FRP-confined concrete”. ConstrBuild Mater, 17, 471-489 (2003).[11] J. Berthet, E. Ferrier and P. Hamelin. “Compressive behavior of concrete externally confined bycomposite jackets Part B: modeling”. Constr Build Mater, 20, 338-347 (2006).[12] T. Turgay, H.O. Köksal, Z. Polat and C. Karakoc. “Stress–strain model for concrete confined withCFRP jackets”. Mater & Des,30, 3243-3251 (2009).[13] J. Berthet, E. Ferrier and P. Hamelin. “Compressive behavior of concrete externally confined bycomposite jackets Part A: experimental study”. Constr Build Mater, 19, 338-347 (2005).[14] A. Fam, M.P. Pando, G. Filz and S. Rizkalla. “Precast piles for route 40 bridge in Virginia usingconcrete filled FRP tubes”. PCI J, 48, 31-40 (2003).[15] US-FHWA. U.S. Department of Transportation Federal Highway Administration. 2011. Web site::[16] C.E. Bakis, L.C. Bank, V.L. Brown, E. Cosenza, J.F. Davalos, J.J. Lesko, A. Machida, S.H.Rizkalla and T.C. Triantafillou. “Fiber-reinforced polymer composites for construction-State-of-the-artreview”. J Compos Constr ASCE, 6, 369-384 (2002). 10
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