Design and characterization of natural flax fibre reinforced polymer tube encased coir fibre reinforced concrete composite structure

Abstract

Construction industry is responsible for the depletion of large amounts of non-renewable resources and for 30% of greenhouse gas emissions. With an increase of environmental concern, a sustainable construction industry is urgently needed. Reducing raw materials consumption by using renewable or waste materials is considered as a significant step to achieve a construction industry with sustainability. Natural fibres are renewable resources and readily available in many countries all over the world. Most importantly, the specific mechanical properties of natural fibres, i.e. flax, are comparable to those of glass fibres being used as reinforcement materials in fibre reinforced polymer (FRP) composites. The use of natural fibres, i.e. coir, as reinforcement within concrete structures will help to achieve a sustainable consumption pattern of building materials. Based on this fact, steelfree concrete structure using natural fibre reinforcements is developed, i.e., natural flax fibre reinforced polymer (FFRP) tube encased coir fibre reinforced concrete (CFRC) structure (FFRP-CFRC). This composite structure is composed of an outer FFRP tube and a CFRC core. In this composite structure, flax fibre is considered as the reinforcement of FRP tube because the comparable mechanical properties of flax to glass fibre. Coir fibre is considered as reinforcement of concrete because of its highest toughness amongst all natural fibres. In a FFRP-CFRC, the pre-fabricated FFRP tube serves as permanent formwork for fresh concrete and also protects the concrete core from possible outer aggressive environments. In addition, as confinement of the concrete core, it increases concrete strength and ductility. Coir fibres within concrete are used to reduce concrete cracks and modify the failure mode of concrete. The composite structure becomes ductile because of coir fibre bridging effect. This study provides a comprehensive understanding of design and characterization of this steel-free FFRP-CFRC composite structure for infrastructure application. Initially, three different fibre fabric reinforced polymer composites, i.e. flax, bamboo and linen, were fabricated using a vacuum bagging technique and their mechanical properties, i.e. tensile, flexural, compressive, vibration and in-plane shear, were studied. The results confirmed flax fabric to be used as the reinforcement material in the outer FRP tube. Then, FFRP tubes were fabricated using a hand lay-up process, the axial compressive, flexural and vibration properties of FFRP tubes with different geometries were experimentally investigated. These studies showed that FFRP tubes had good energy absorption capabilities to be axial and flexural structural members. Next, axial compressive test on FFRP tube confined plain concrete (FFRP-PC) and FFRP-CFRC showed that the FFRP tube confinement increased ultimate compressive stress and strain for both PC and CFRC remarkably, compared with the unconfined PC and CFRC. Coir fibre inclusion had an insignificant effect on ultimate compressive stress but modified the failure mode of FFRP-CFRC to be ductile, compared with the FFRP-PC. In addition, experimental results obtained from axial compression were compared with the predictions using existing stress and strain models for glass/carbon FRP (G/CFRP) confined concrete, and two strain models were developed for FFRP-PC and FFRP-CFRC for a practical design purpose. After that, four-point bending test on FFRP-PC and FFRP-CFRC beams indicated that the FFRP tube confinement increased lateral load carrying capacity and energy absorption capability of both PC and CFRC beams significantly. However, compared with the FFRPPC beams, coir fibre inclusion led to a ductile failure mode of the concrete core after the rupture of the outer FFRP tube for FFRP-CFRC specimens. Based on linear elastic theory and an assumption of Bernoulli’s theory, a simplified analytical method was developed and predicted the ultimate bending moment of FFRP-PC and FFRP-CFRC beams under flexure. Flexural test also confirmed that slippage between FFRP tube and concrete core could be an issue which may compromise the structural performance of the composite structures. Hence, a novel interlocked FFRP tube and CFRC interfacial profile was developed to impede the slippage between the tube and the concrete core, which in turn increased the interfacial bond stress and composite action between the tube and the concrete core effectively. Then, the effect of this new interfacial profile on the axial and flexural behaviour of FFRP-CFRC composite was investigated. The effect of different parameters of the interfacial profile on the bond behaviour of FFRP panel and CFRC block specimens were studied. The results will be used to develop a FFRP panel and CFRC overlay bridge deck for the future study. Next, hammer-induced vibration tests were performed on FFRP-PC and FFRP-CFRC beams, which showed that both the FFRP tube and coir fibre increased the damping ratio of the concrete significantly, thus reducing the impact of dynamic loading on the composite structure. Finally, future works and recommendations were provided for designing this kind of steel-free composite structure for future infrastructure application.