NATURAL FLAX FRP TUBE ENCASED COIR FIBRE REINFORCED CONCRETE COLUMN: EXPERIMENTAL

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1 Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) December 2013, Melbourne, Australia 2013 International Institute for FRP in Construction NATURAL FLAX FRP TUBE ENCASED COIR FIBRE REINFORCED CONCRETE COLUMN: EXPERIMENTAL L. Yan 1 and N. Chouw 2 1 Department of Civil and Environmental Engineering, The University of Auckland, New Zealand. lyan118@aucklanduni.ac.nz 2 Department of Civil and Environmental Engineering, The University of Auckland, New Zealand. ABSTRACT This study investigated the mpressive and flexural behavior of FFRP tube nfined plain ncrete (PC) and ir fibre reinforced ncrete (CFRC) mposite lumns. The mass ntent of ir fibre nsidered was 1% of cement. Eighteen cylinders were tested under uniaxial mpression and 12 beams were tested under four-point bending. In axial mpression, two types of FFRP and ncrete bond were nsidered, i.e. ncrete nfined by (i) FFRP tube and (2) by FFRP wrappings. They were termed as naturally and mechanically bonded, respectively. Effect of ir fibre inclusion and FFRP nfinement on the stress-strain relationship, nfinement effectiveness, and load bearing capacity, deflection, and failure mode were studied. Test results show that in mpression, both FFRP tube and FFRP wrapping nfinements enhance the axial mpressive strength and ultimate strain of ncrete significantly. In flexure, the FFRP tube can increase the lateral load carrying capacity and the deflection several times larger than the unnfined ncrete lumns. The flexural behavior of FFRP tube nfined CFRC is highly dependent on the tube thickness. Additionally, ir fiber inclusion reduced the numbers and widths of the cracks in the ncrete, thus significantly affect the failure mode of FFRP tube nfined CFRC in flexure. KEYWORDS Natural fibre reinforced polymer tube, Coir fibre reinforced ncrete, bond, slippage, ductility. INTRODUCTION Recently glass/carbon fibre reinforced polymer (G/CFRP) material became popular in civil engineering for structural applications due to its high strength-to-weight ratio and rrosion-resistance (Smith and Teng, 2002; Teng et al., 2002). It is well known that G/CFRP material as lateral nfinement of ncrete, in both the seismic retrofit of existing reinforced ncrete lumns and in the nstruction of ncrete-filled FRP tubes (CFFT) as earthquake-resistant lumns in new nstruction, can enhance ncrete mpressive strength and ductility significantly (Ozbakkaloglu et al., 2012). However, currently a wider application of G/CFRP materials in civil infrastructure is limited by the high initial st, the insufficiency of long term performance data, the lack of standard manufacturing techniques and design standards, risk of fire, and the ncern that the non-yielding characteristic of FRP materials uld result in sudden failure of the structure without prior warning (Bakis et al., 2002; Yan and Chouw, 2012). Among these limitations, st and ncern of brittle failure of FRP materials are probably the most influential factors when assessing the merits of FRP as a nstruction material. Recently, the use of natural fibres to replace carbon/glass fibres as reinforcement in FRP mposites (Yan, 2012; Yan and Chouw, 2013a) and the use of natural fibres as reinforcement of ncrete (Pache-Torgal and Jalali, 2011) have gained popularity due to increasing environmental ncern. Natural fibres such as flax, hemp, jute, ir and sisal, are st effective, have low density with high specific strength and stiffness, and are readily available (Yan et al., 2012). Dittenber and GangaRao (2012) reviewed more than 20 mmonly used natural fibres and ncluded that flax fibre offers the best potential mbination of low st, light weight, and high strength and stiffness as the reinforcement of fibre reinforced polymer mposites for structural applications. Among natural fibres, ir fibre, as reinforcement fibre in ncrete, was investigated widely due to its highest toughness among natural fibres and the extremely low st, as well as availability. Li et al. (2004) stated that flexural toughness and flexural toughness index of cementitious mposites with ir fibre increased by more than 10 times. Reis (2006) reported that ir fibre increased ncrete mposite fracture toughness and the use of ir fibres showed even better flexural properties than synthetic fibres (glass and carbon). Therefore, steffective natural fibres as reinforcement of ncrete to replace the expensive, highly energy nsumed and non-

2 renewable reinforced steel rebar and natural fibres as reinforcement of mposites to replace the glass/carbon fibres are the major steps to achieve a more sustainable nstruction (Yan and Chouw, 2013b). Therefore, in this study a new natural flax FRP (FFRP) tube nfined ir fibre reinforced ncrete (CFRC) structure (termed as FFRP tube nfined CFRC) was nsidered. In this system, two natural fibres are used, i.e. flax and ir fibres. The relatively inexpensive flax fibre is used as reinforcement of FFRP tube nfining the ncrete. Coir fibre as the reinforcement in the cementitious matrix increases the fracture properties of the ncrete. The mpressive and flexural properties of FFRP tube nfined CFRC are experimentally investigated based on the uniaxial mpression and four-point bending test. In addition, the mpressive behaviour of FFRPwrapped PC and CFRC were investigated and mpared with the FFRP tube nfined PC and CFRC. EXPERIMENTAL Test Specimens and Materials Table 1 gives the test matrix of the specimens for this study. Two types of ncrete were nsidered, plain ncrete (PC) and ir fibre reinforced ncrete (CFRC). The test matrix nsidered nsists of 18 short cylindrical specimens and 12 long cylindrical specimens. For the short nfined ncrete specimens, there are divided into four types: FFRP tube nfined PC (FFRP-T-PC), FFRP tube nfined CFRC (FFRP-T-CFRC), FFRP-wrapped PC (FFRP-W-PC) and FFRP-wrapped CFRC (FFRP-W-CFRC). Therefore, the mpressive properties of FFRP tube ncrete and FFRP-wrapped ncrete were studied and mpared. For the long specimens, only the type of FFRP tube nfinement was nsidered in this study. For all the FFRP nfined ncrete specimens, the fabric layer arrangement used was four layers. Table 1. Test matrix of the specimens nsidered in this study Specimen group No. of specimens Core diameter D (mm) Length (mm) Tube thickness t (mm) PC CFRC FFRP-T-PC FFRP-T-CFRC FFRP-W-PC FFRP-W-CFRC PC CFRC FFRP-T-PC FFRP-T-CFRC Materials and Specimen Fabrication Commercial bidirectional woven flax fabric (550 g/m 2 ) was used for this study. The fabric has a plain woven structure with unt of 7.4 threads/cm in warp and 7.4 threads/cm in the weft direction (Yan and Chouw, 2012). The epoxy used was the SP High Modulus Ampreg 22 resin and slow hardener. Table 2 gives the mechanical properties of flax fibre and epoxy resin. For fabrication of FFRP tubes, an aluminum mould was first cut longitudinally, and then taped tightly to make a formwork for FRP wrapping, while allowing easy removal of the tube after the curing of FFRP. Then the aluminum mould was vered with a layer of infusion sheet, so that the cured FFRP tubes can be easily detached. More details about the fabrication were given in the study (Yan and Chouw, 2013c). Fabric fiber orientation was at 90 o from the axial direction of the tube. Table 2. Mechanical properties of flax fibre and epoxy Material Diameter (mm) Density (g/cm 3 ) Elastic modulus (GPa) Tensile strength (MPa) Elongation (%) Flax fibre Epoxy Two types of ncrete were prepared, PC and CFRC. Type I Portland cement, gravel, natural sand and water were used to prepare ncrete. Concrete with 28-day mpressive strength of 30 MPa was designed. The mix ratio by weight was 1:0.55:3.82:2.27 for cement: water: gravel: sand, respectively. This mix design followed the ACI Standard For CFRC, ir fibres were added during the mixing. The fibre length was 50 mm with fibre ntent of 1% of cement by mass. For each FFRP tube nfined ncrete specimen, one end of the tube

3 was capped with a wooden plate before ncrete pouring. Then ncrete was cast, poured, mpacted and cured in a standard curing water tank for 28 days. Both end sides of the specimens were treated with plaster to have a uniform bearing surface and a blade was used to cut the upper and lower edges of tube nfined specimen to avoid it directly from bearing the axial mpression. For FFRP-wrapped ncrete specimen, PC and CFRC specimens were cast and cured for 28 days. After drying out, the surface of the specimen was polished and impregnated with a thin layer of epoxy using a brush. Then, resin-impregnated flax fabrics were placed on the ncrete surface with epoxy. The specimens were dried at room temperature for 24 h and then placed into an oven and cured at 65 o C for 7 h. Instrumentation and Test Setup For axial mpression test, two strain gauges were attached at the mid-height of each short cylindrical specimen in the hoop direction to monitor lateral strains. Two linear variable displacement transducers (LVDTs) were attached apart and vered and spaced 130 mm centred at the mid-height to measure axial strain. Compression test was nducted on an Avery-Denison machine under stress ntrol with a nstant rate of 0.20 MPa/s based on ASTM C39. Each sample was axially mpressed up to failure. Readings of the strain gauges and LVDTs were taken using a data logging system. For four-point bending test, three strain gauges were mounted at the mid-span of each long cylindrical specimen aligned along the hoop direction to monitor the lateral strain and another three strain gauges at the axial direction of tube to measure the axial strain. One LVDT was vered the lower boundary of the mposite lumn at the mid-span to measure the deflection of the lumn. The four-point bending test was nducted on Instron testing machine aording to ASTM C78 standard. Readings of the load, strain gauges and LVDT were taken using a data logging system and were stored in a mputer. RESULTS AND DISCUSSION Axial Stress-Strain Relationship Axial mpressive stress versus axial strain curves of FFRP nfined ncrete are displayed in Figure 1. In general, the axial stress-strain response of FFRP tube PC and FFRP tube nfined CFRC are similar to that of FFRP-wrapped PC and FFRP-wrapped CFRC, all the nfined specimens exhibit an approximate bi-linear behavior with a sustainable ascending branch at the send linear stage. Figure 1. Axial mpressive stress-strain responses of the short cylindrical specimens In all cases of FFRP tube and FFRP-wrapped ncrete, the initial purely linear response is similar to the rresponding unnfined PC or CFRC. When the applied stress exceeds the ultimate strength of unnfined PC or CFRC, the curve enters the send linear region where nsiderable micro-cracks are developed in the ncrete, the lateral expansion significantly increased and the FFRP tube or wrapping starts to nfine the ncrete re. This send linear region is mainly dominated by the structural behavior of FFRP mposites where the tube is fully activated to nfine the re, leading to a nsiderable enhancement in the ncrete mpressive strength. When axial stress increases, the hoop tensile stress in the FFRP tube or wrapping also increases. Once this hoop stress exceeds the ultimate tensile strength of FFRP jacket the failure starts.

4 Confinement Performance Table 3 gives the average mpressive results of specimens. f represents the peak mpressive strength of unnfined ncrete and f is the ultimate mpressive strength of nfined ncrete. ε is the axial strain of unnfined ncrete at peak strength, ε is the ultimate axial strain of the nfined ncrete, f f is the nfinement effectiveness and ε ε is the axial strain ratio, The ductility of FFRP nfined ncrete can be represented by the axial strain ratio. Table 3 shows that the ir fibre inclusion slightly reduced the mpressive strength of the CFRC, mpared to the unnfined PC. However, mpared with PC, ir fibre significantly increased the axial strain at the peak strength. In general, the nfinement provided by the two different FFRP systems enhances the ultimate mpressive strength and ultimate axial strains of both PC and CFRC. The nfinement effectiveness of FFRP tube and FFRP-wrapped PC are 1.94 and 1.65, the values are 1.94 and 1.62 respectively for the rresponding nfined CFRC. The average axial strain ratios of PC nfined by FFRP tube, FFRP wrapping are and respectively and are 4.01 and 3.10 respectively for the rresponding nfined CFRC. This data indicates that the two different types of FFRP nfinements increase the ductility of the ncrete cylinder remarkably. Table 3. Average axial mpressive results of the short specimens Specimens f (MPa) ε (%) f (MPa) ε (%) ε ε PC 32.0* 0.22* CFRC 30.1* 0.62* FFRP-T-PC FFRP-T-CFRC FFRP-W-PC FFRP-W-CFRC The rresponding * values were used for f f f / f and ε / ε calculation Compared with FFRP tube nfined PC and CFRC, both FFRP-wrapped PC and CFRC had lower nfinement effectiveness. The reduction in ultimate strength of FFRP-wrapped PC and CFRC are 15.0% (from 62.2 to 52.9 MPa) and 16.3% (from 58.2 to 48.7 MPa), respectively. In the case of FFRP tube nfined ncrete, the FFRP tube only subjects to the tension in the hoop direction, while in the case of FFRP-wrapped ncrete, the FFRP is subject to the hoop tension as well as axial mpression due to the good interfacial bond between the FFRP and the ncrete. A mbination of the stresses reduces the nfinement level of the FFRP jacket and thus leads to a lower ultimate stress of the FFRP-wrapped ncrete. Figure 2 gives the failure modes of the specimens. Figure 2. Failure modes of FFRP-T-PC, FFRP-T-CFRC, FFRP-W-PC and FFRP-W-CFRC specimens Load-deflection Response Figure 3 gives the lateral load versus mid-span deflection responses of the long cylindrical specimens in flexure. PC beam exhibited a linear response up to failure showing a pure brittle failure of the unreinforced ncrete. Since no reinforcement was used in the PC specimen, the ability to carry the lateral load was negligible (7.4 kn). For CFRC, the curve also showed a linear response up to the peak load and followed by a post-softening

5 response, indicating a relatively ductile behaviour of the specimen. The ir fibre inclusion increased the peak load to 10.1 kn mpared to the unnfined PC, with an increase of 36.5%. For FFRP tube nfined ncrete specimens, both the nfined PC and CFRC specimens possessed a nonlinear load-deflection response before the peak load. FFRP-PC specimen exhibited a brittle failure as the result of the non-yielding characteristics of FFRP materials. However, on the other hand, the FFRP-CFRC specimen had a post-softening curve with a ductile response. It is believed that the post-peak ductile response of FFRP-CFRC specimen is attributed to the fibre bridge effect. In the case of FFRP tube nfined ncrete, the 4-layer flax FRP tube nfinement enhanced the load carrying capacity significantly, with the increase is 1066% and 946% for the nfined PC and CFRC, mpared to the rresponding unnfined PC and CFRC, respectively. Additionally, the ir fibre inclusion slightly increased the peak load of FFRP-CFRC (84.7 kn) mpared to the FFRP-PC (78.9 kn). Failure Mode Figure 3. Load versus deflection responses of long cylindrical specimens In flexure, the failure of both FFRP-PC and FFRP-CFRC specimens started by the tensile rupture of the FFRP tube at the lowest point in its bottom section (the nstant moment zone); the tensile cracks appeared on the bottom section of FRP tube and then progressed toward the upper section resulting in the development of a major crack (Figure 4(a)-(b)). The major crack was perpendicular to the longitudinal direction of the tube as a nsequence of pure bending. No mpression failure was observed at the ncentrated loading points. After tested, the failure pattern of the nfined ncrete res was evaluated by removing the outer FFRP tube. Figure 4 shows the failure modes of the nfined PC and CFRC res, see Figure 4(c)-(d). It was observed that the PC re was broken into two halves after the removal of the tube and there were several large cracks along the ncrete re. For CFRC re, there was a major crack in the zone between the two ncentrated loads. However, it was clear that the ir fibres bridged the adjacent surfaces of the major crack, as displayed in Figure 5(e). Under flexure, the ir fibre and ncrete interface debonding, frictional sliding, fibre fracture and fibre pull-out all ntributed to the energy dissipation under the post-peak response in the load-deflection curve. Therefore, the mparison in failure modes of the nfined PC and CFRC res gives credence to the support that ir fibre bridging dominated the post-peak ductile response for FFRP-CFRC lumn under flexure in Figure 3. Figure 4. Failure modes: nfined (a) FFRP-PC, (b) FFRP-CFRC, (c) CFRC re and (d) PC re, and (e) ir fibre bridge effect

6 CONCLUSIONS In this study, the mpressive and flexural properties of flax fabric reinforced polymer (FFRP) tube nfined ir fibre reinforced ncrete (CFRC) were experimentally investigated. The test results reveals: (1) In axial mpression, both FFRP tube and FFRP wrapping nfinement enhanced the mpressive strength and ductility of both PC and CFRC cylinders significantly. The ultimate strength of FFRP tube ncrete is larger than the FFPP-wrapped ncrete. (2) In flexure, FFRP tube nfinement increases the ultimate lateral load and mid-span deflection of the PC and CFRC members remarkably, e.g. the ultimate lateral load of 4-layer FFRP nfined PC and CFRC are 1066% and 946% larger than the rresponding unnfined PC and CFRC specimens. However, FFRP-PC lumns exhibit a brittle failure mode while FFRP-CFRC lumns behave a ductile manner due to ir fibre bridging effect. In general, FFRP tube nfined ir fibre reinforced ncrete lumns have the pontential to be mpressive and flexural structrual memebers. The use of ir and flax fibres as nstruction materials will be benefit to build a nstruction industry with more environmentally-friendly and lower carbon footprint. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Engineering Faculty Research Development Fund (FRDF ID: ) of the University of Auckland. The first author also wishes to thank the University of Auckland to provide the Doctoral Scholarship for supporting his PhD study. REFERENCES Bakis, C.E., Bank, L.C., Brown, V.L., Cosenza, E., Davalos, J.F., Lesko, J.J., Machida, A., Rizkalla, S.H., and Triantafillou, T. C. (2002). Fiber-reinforced polymer mposites for nstruction-state-of-the-art review, Journal of Composites for Construction, 6, Dittenber, D.B. and GangaRao H.V.S. (2012). Critical review of recent publications on use of natural mposites in infrastructure, Composites Part A, 43, Li, Z., Wang, L., and Wang, X. (2004). Flexural characteristics of ir fibre reinforced cementitious mposites, Fibers and Polymer, 7, Ozbakkaloglu, T., Lim, J.C., and Vincent, T. (2012). FRP-nfined ncrete in circular sections: Review and assessment of stress-strain models, Engineering Structures, 49, Pache-Torgal, F. and Jalali, S. (2011). Cementitous building materials reinforced with vegetable fibres: A review, Construction and Building Materials, 25, Reis J. (2006). Fracture and flexural characterization of natural fibre-reinforced polymer ncrete, Construction and Building Materials, 20, Smith, S.T. and Teng, J.G. (2002). FRP-strengthened RC beams-i: Review of debonding strength models, Engineering Structures, 24, Teng, J.G., Chen, J.F., Smith, S.T. and Lam, L. (2002). FRP strengthened RC Structures, Wiley, Chichester, U.K. Yan, L.B. (2012). Effect of alkali treatment on vibration characteristics and mechancial properties of natural fabric reinforced mposites, Journal of Reinforced Plastics and Composites, 31, Yan, L.B., Chouw, N., Yuan, X.W. (2012). Improving the mechanical properties of natural fibre fabric reinforced epoxy mposites by alkali treatment, Journal of Reinforced Plastics and Composites, 31, Yan, L.B., Chouw, N. (2012). Behavior and analytical modeling of natural flax fibre reinforced polymer tube nfined plain ncrete and ir fibre reinforced ncrete, Journal of Composite Materials, DOI: / Yan, L.B., Chouw, N. (2013a). Crashworthiness characteristics of flax fibre reinforced epoxy tubes for energy absorption application, Materials & Design, 51, Yan, L.B., Chouw, N. (2013b). Dynamic and static properties of flax fiber reinforced polymer tube nfined ir fiber reinforced ncrete, Journal of Composite Materials, DOI: / Yan, L.B., Chouw, N. (2013c). Experimental study of flax FRP tube encased ir fibre reinforced ncrete mposite lumn, Construction and Building Materials, 40,