Compressive and flexural behaviour and theoretical analysis of flax fibre reinforced polymer tube encased coir fibre reinforced concrete composite

The use of cost-effective natural fibres, i.e. flax in fibre reinforced polymer composites and coir in concrete as building materials is a step to achieve a sustainable construction. Both flax fibre reinforced polymer tube encased-plain concrete and the tube encased-coir fibre reinforced concrete composites have potential to be axial and flexural structural members. However, in flexure, slippage between tube and the concrete core may compromise the performance of the composites. In this study, thin flax fibre reinforced polymer band rings were embedded into the tube inner surface in order to eliminate the slippage because a better tube and concrete interlocking was achieved. Therefore, one purpose of this study is to investigate the effect of band rings on the compressive and flexural properties of flax fibre reinforced polymer tube encased-plain concrete and the tube encased-coir fibre reinforced concrete. The ductility of these composites was evaluated using an energy ductility index based on measurement of fracture energy. Next, the cracking strength and neutral axis depth of the composite beams without band rings under flexure were analysed. Finally, based on linear elastic analysis, a simplified analytical method was developed to predict the resisting moment capacities of these composite beams. The test results indicate that in axial compression, the use of band rings reduced the ultimate compressive strength and ductility of both composites. In flexure, the band rings eliminated the slippage but increase the load carrying capacity and deflection. The predicted ultimate moment capacities of these composite beams match the experimental results well.