Natural Flax Frp Tube Encased Coir Fibre Reinforced Concrete Column: Experimental

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. Email: [email protected] 2 Department of Civil and Environmental Engineering, The University of Auckland, New Zealand.

ABSTRACT

This study investigated the compressive and flexural behavior of FFRP tube confined plain concrete (PC) and coir fibre reinforced concrete (CFRC) composite columns. The mass content of coir fibre considered was 1% of cement. Eighteen cylinders were tested under uniaxial compression and 12 beams were tested under four-point bending. In axial compression, two types of FFRP and concrete bond were considered, i.e. concrete confined by (i) FFRP tube and (2) by FFRP wrappings. They were termed as naturally and mechanically bonded, respectively. Effect of coir fibre inclusion and FFRP confinement on the stress-strain relationship, confinement effectiveness, and load bearing capacity, deflection, and failure mode were studied. Test results show that in compression, both FFRP tube and FFRP wrapping confinements enhance the axial compressive strength and ultimate strain of concrete significantly. In flexure, the FFRP tube can increase the lateral load carrying capacity and the deflection several times larger than the unconfined concrete columns. The flexural behavior of FFRP tube confined CFRC is highly dependent on the tube thickness. Additionally, coir fiber inclusion reduced the numbers and widths of the cracks in the concrete, thus significantly affect the failure mode of FFRP tube confined CFRC in flexure.

KEYWORDS

Natural fibre reinforced polymer tube, Coir fibre reinforced concrete, 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 corrosion-resistance (Smith and Teng, 2002; Teng et al., 2002). It is well known that G/CFRP material as lateral confinement of concrete, in both the seismic retrofit of existing reinforced concrete columns and in the construction of concrete-filled FRP tubes (CFFT) as earthquake-resistant columns in new construction, can enhance concrete compressive 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 cost, the insufficiency of long term performance data, the lack of standard manufacturing techniques and design standards, risk of fire, and the concern that the non-yielding characteristic of FRP materials could result in sudden failure of the structure without prior warning (Bakis et al., 2002; Yan and Chouw, 2012). Among these limitations, cost and concern of brittle failure of FRP materials are probably the most influential factors when assessing the merits of FRP as a construction material.

Recently, the use of natural fibres to replace carbon/glass fibres as reinforcement in FRP composites (Yan, 2012; Yan and Chouw, 2013a) and the use of natural fibres as reinforcement of concrete (Pacheco-Torgal and Jalali, 2011) have gained popularity due to increasing environmental concern. Natural fibres such as flax, hemp, jute, coir and sisal, are cost 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 commonly used natural fibres and concluded that flax fibre offers the best potential combination of low cost, light weight, and high strength and stiffness as the reinforcement of fibre reinforced polymer composites for structural applications. Among natural fibres, coir fibre, as reinforcement fibre in concrete, was investigated widely due to its highest toughness among natural fibres and the extremely low cost, as well as availability. Li et al. (2004) stated that flexural toughness and flexural toughness index of cementitious composites with coir fibre increased by more than 10 times. Reis (2006) reported that coir fibre increased concrete composite fracture toughness and the use of coir fibres showed even better flexural properties than synthetic fibres (glass and carbon). Therefore, cost- effective natural fibres as reinforcement of concrete to replace the expensive, highly energy consumed and non- renewable reinforced steel rebar and natural fibres as reinforcement of composites to replace the glass/carbon fibres are the major steps to achieve a more sustainable construction (Yan and Chouw, 2013b).

Therefore, in this study a new natural flax FRP (FFRP) tube confined coir fibre reinforced concrete (CFRC) structure (termed as FFRP tube confined CFRC) was considered. In this system, two natural fibres are used, i.e. flax and coir fibres. The relatively inexpensive flax fibre is used as reinforcement of FFRP tube confining the concrete. Coir fibre as the reinforcement in the cementitious matrix increases the fracture properties of the concrete. The compressive and flexural properties of FFRP tube confined CFRC are experimentally investigated based on the uniaxial compression and four-point bending test. In addition, the compressive behaviour of FFRP- wrapped PC and CFRC were investigated and compared with the FFRP tube confined PC and CFRC.

EXPERIMENTAL

Test Specimens and Materials

Table 1 gives the test matrix of the specimens for this study. Two types of concrete were considered, plain concrete (PC) and coir fibre reinforced concrete (CFRC). The test matrix considered consists of 18 short cylindrical specimens and 12 long cylindrical specimens. For the short confined concrete specimens, there are divided into four types: FFRP tube confined PC (FFRP-T-PC), FFRP tube confined CFRC (FFRP-T-CFRC), FFRP-wrapped PC (FFRP-W-PC) and FFRP-wrapped CFRC (FFRP-W-CFRC). Therefore, the compressive properties of FFRP tube concrete and FFRP-wrapped concrete were studied and compared. For the long specimens, only the type of FFRP tube confinement was considered in this study. For all the FFRP confined concrete specimens, the fabric layer arrangement used was four layers.

Table 1. Test matrix of the specimens considered in this study Specimen group No. of Core diameter Length Tube thickness specimens D (mm) (mm) t (mm) PC 3 100 200 -- CFRC 3 100 200 -- FFRP-T-PC 3 100 200 6.50 FFRP-T-CFRC 3 100 200 6.50 FFRP-W-PC 3 100 200 6.50 FFRP-W-CFRC 3 100 200 6.50 PC 3 100 520 -- CFRC 3 100 520 -- FFRP-T-PC 3 100 520 6.50 FFRP-T-CFRC 3 100 520 6.50

Materials and Specimen Fabrication

Commercial bidirectional woven flax fabric (550 g/m2) was used for this study. The fabric has a plain woven structure with count 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 covered 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 o Chouw, 2013c). Fabric fiber orientation was at 90 from the axial direction of the tube.

Table 2. Mechanical properties of flax fibre and epoxy Material Diameter Density Elastic modulus Tensile strength Elongation (mm) (g/cm3) (GPa) (MPa) (%) Flax fibre 0.708 1.43 16.4 153.8 3.2 Epoxy - 1.09 3.6 87.8 4.5

Two types of concrete were prepared, PC and CFRC. Type I Portland cement, gravel, natural sand and water were used to prepare concrete. Concrete with 28-day compressive 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 211. 1. For CFRC, coir fibres were added during the mixing. The fibre length was 50 mm with fibre content of 1% of cement by mass. For each FFRP tube confined concrete specimen, one end of the tube was capped with a wooden plate before concrete pouring. Then concrete was cast, poured, compacted 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–confined specimen to avoid it directly from bearing the axial compression. For FFRP-wrapped concrete 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 concrete surface with epoxy. The specimens were dried at room temperature for 24 h and then placed into an oven and cured at 65oC for 7 h.

Instrumentation and Test Setup

For axial compression 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 1800 apart and covered and spaced 130 mm centred at the mid-height to measure axial strain. Compression test was conducted on an Avery-Denison machine under stress control with a constant rate of 0.20 MPa/s based on ASTM C39. Each sample was axially compressed 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 covered the lower boundary of the composite column at the mid-span to measure the deflection of the column. The four-point bending test was conducted on Instron testing machine according to ASTM C78 standard. Readings of the load, strain gauges and LVDT were taken using a data logging system and were stored in a computer.

RESULTS AND DISCUSSION

Axial Stress-Strain Relationship

Axial compressive stress versus axial strain curves of FFRP confined concrete are displayed in Figure 1. In general, the axial stress-strain response of FFRP tube PC and FFRP tube confined CFRC are similar to that of FFRP-wrapped PC and FFRP-wrapped CFRC, all the confined specimens exhibit an approximate bi-linear behavior with a sustainable ascending branch at the second linear stage.

Figure 1. Axial compressive stress-strain responses of the short cylindrical specimens

In all cases of FFRP tube and FFRP-wrapped concrete, the initial purely linear response is similar to the corresponding unconfined PC or CFRC. When the applied stress exceeds the ultimate strength of unconfined PC or CFRC, the curve enters the second linear region where considerable micro-cracks are developed in the concrete, the lateral expansion significantly increased and the FFRP tube or wrapping starts to confine the concrete core. This second linear region is mainly dominated by the structural behavior of FFRP composites where the tube is fully activated to confine the core, leading to a considerable enhancement in the concrete compressive 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.

Confinement Performance

' Table 3 gives the average compressive results of specimens. fco represents the peak compressive strength of unconfined concrete and ' is the ultimate compressive strength of confined concrete. is the axial strain of fcc ε co unconfined concrete at peak strength, is the ultimate axial strain of the confined concrete, ' ' is the ε cc f cc f co confinement effectiveness and ε cc ε co is the axial strain ratio, The ductility of FFRP confined concrete can be represented by the axial strain ratio.

Table 3 shows that the coir fibre inclusion slightly reduced the compressive strength of the CFRC, compared to the unconfined PC. However, compared with PC, coir fibre significantly increased the axial strain at the peak strength. In general, the confinement provided by the two different FFRP systems enhances the ultimate compressive strength and ultimate axial strains of both PC and CFRC. The confinement 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 corresponding confined CFRC. The average axial strain ratios of PC confined by FFRP tube, FFRP wrapping are 10.95 and 10.32 respectively and are 4.01 and 3.10 respectively for the corresponding confined CFRC. This data indicates that the two different types of FFRP confinements increase the ductility of the concrete cylinder remarkably.

Table 3. Average axial compressive results of the short specimens ' ' ' ' Specimens f (MPa) ε (%) f (MPa) ε (%) f cc f co ε cc ε co co co cc cc PC 32.0* 0.22* - - - - CFRC 30.1* 0.62* - - - - FFRP-T-PC - - 62.2 2.41 1.94 10.95 FFRP-T-CFRC - - 58.2 2.49 1.94 4.01 FFRP-W-PC - - 52.9 2.27 1.65 10.32 FFRP-W-CFRC - - 48.7 1.93 1.62 3.11 '' The corresponding * values were used for fcc/ f co and ε cc/ ε co calculation

Compared with FFRP tube confined PC and CFRC, both FFRP-wrapped PC and CFRC had lower confinement 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 confined concrete, the FFRP tube only subjects to the tension in the hoop direction, while in the case of FFRP-wrapped concrete, the FFRP is subject to the hoop tension as well as axial compression due to the good interfacial bond between the FFRP and the concrete. A combination of the stresses reduces the confinement level of the FFRP jacket and thus leads to a lower ultimate stress of the FFRP-wrapped concrete. 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 concrete. 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 response, indicating a relatively ductile behaviour of the specimen. The coir fibre inclusion increased the peak load to 10.1 kN compared to the unconfined PC, with an increase of 36.5%. For FFRP tube confined concrete specimens, both the confined 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 confined concrete, the 4-layer flax FRP tube confinement enhanced the load carrying capacity significantly, with the increase is 1066% and 946% for the confined PC and CFRC, compared to the corresponding unconfined PC and CFRC, respectively. Additionally, the coir fibre inclusion slightly increased the peak load of FFRP-CFRC (84.7 kN) compared to the FFRP-PC (78.9 kN).

Figure 3. Load versus deflection responses of long cylindrical specimens

Failure Mode

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 constant 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 consequence of pure bending. No compression failure was observed at the concentrated loading points. After tested, the failure pattern of the confined concrete cores was evaluated by removing the outer FFRP tube. Figure 4 shows the failure modes of the confined PC and CFRC cores, see Figure 4(c)-(d). It was observed that the PC core was broken into two halves after the removal of the tube and there were several large cracks along the concrete core. For CFRC core, there was a major crack in the zone between the two concentrated loads. However, it was clear that the coir fibres bridged the adjacent surfaces of the major crack, as displayed in Figure 5(e). Under flexure, the coir fibre and concrete interface debonding, frictional sliding, fibre fracture and fibre pull-out all contributed to the energy dissipation under the post-peak response in the load-deflection curve. Therefore, the comparison in failure modes of the confined PC and CFRC cores gives credence to the support that coir fibre bridging dominated the post-peak ductile response for FFRP-CFRC column under flexure in Figure 3.

Figure 4. Failure modes: confined (a) FFRP-PC, (b) FFRP-CFRC, (c) CFRC core and (d) PC core, and (e) coir fibre bridge effect CONCLUSIONS

In this study, the compressive and flexural properties of flax fabric reinforced polymer (FFRP) tube confined coir fibre reinforced concrete (CFRC) were experimentally investigated. The test results reveals: (1) In axial compression, both FFRP tube and FFRP wrapping confinement enhanced the compressive strength and ductility of both PC and CFRC cylinders significantly. The ultimate strength of FFRP tube concrete is larger than the FFPP-wrapped concrete. (2) In flexure, FFRP tube confinement 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 confined PC and CFRC are 1066% and 946% larger than the corresponding unconfined PC and CFRC specimens. However, FFRP-PC columns exhibit a brittle failure mode while FFRP-CFRC columns behave a ductile manner due to coir fibre bridging effect. In general, FFRP tube confined coir fibre reinforced concrete columns have the pontential to be compressive and flexural structrual memebers. The use of coir and flax fibres as construction materials will be benefit to build a construction 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: 3702507) 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.

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