Effect of column parameters on flax FRP confined coir fibre reinforced concrete

• Push out test was performed to investigate FFRP tube and CFRC interfacial bond.
• Level of confinement decreases with an increase in FFRP/CFRC interfacial bond.
• Confined concrete strength was predicted and compared with experimental results.
• Slip between FFRP tube and CFRC core was eliminated.

This paper experimentally investigated various column parameters on axial compressive and flexural behaviour of a new type of FRP-confined concrete, i.e. natural flax fibre reinforced polymer (FFRP)-confined coir fibre reinforced concrete (CFRC), which is termed as FFRP–CFRC. This new composite structure was composed of an outer FFRP jacket and a CFRC core. Under axial compression, the column parameters examined include types of FFRP/CFRC interfacial bond (i.e. naturally and mechanically bonded), confinement (i.e. FFRP tube and wrapped), and specimen end condition (i.e. flat and grinded tube ends) and tube thickness (i.e. 2, 4, and 6 layers). Push-out test was further performed to find out the relationship between bond strength and level of confinement. In addition, the experimental results were compared with predictions obtained from 25 existing strength models. In flexure, the effects of bond on load–deflection response, ultimate load, energy absorption, failure mode, load–slip behaviour and load–strain responses of the composite beams were evaluated. Results showed that in compression, with an increase of tube thickness, the strength, energy absorption capacity and ductility increased remarkably. The ultimate stress and strain of FFRP-wrapped specimen were lower than the tube-confined specimen and the influence of tube end condition on confinement performance cannot be ignored. It was also found that there was a strong link between FFRP/CFRC interfacial bond and the confinement performance, i.e. with an increase of bond strength, the level of confinement reduced noticeably. In flexure, both naturally and mechanically bonded FFRP tube-confined CFRC composite beams exhibited high load carrying capacity, energy absorption and ductility. Slip between the tube and the concrete was eliminated in the mechanically bonded beams without compromising the peak load, compared to the naturally bonded beams.