Behaviour of FRP spiral-confined concrete and contribution of FRP longitudinal bars in FRP-RC columns under axial compression

Fibre-reinforced polymer (FRP) bar-reinforced concrete (RC) columns (FRP-RC columns) have become increasingly popular since they are durable against harsh environmental attacks. For a reliable and cost-effective design of FRP-RC columns, it is essential to gain an in-depth understanding on the compressive behaviour of FRP spiral-confined concrete and the role of FRP longitudinal bars in resisting the axial stresses in the columns. Although extensive studies have been carried out on FRP-RC columns, few studies have been conducted to clarify the axial compressive behaviour of each component (i.e., FRP spiral-confined concrete and FRP longitudinal reinforcing bar). To this end, an experimental study was conducted to explore the compressive behaviour of FRP spiral-confined concrete in FRP-RC columns with and without longitudinal reinforcing FRP bars under axial compression. The contribution of the FRP bars in the FRP-RC columns is also investigated. The key variables of the experimental study include the spiral pitch and the diameter of FRP longitudinal bars. Test results show that the FRP spiral-confined concrete in FRP-RC columns exhibits a bilinear axial stress–strain behaviour. The threshold of sufficient confinement ratio of FRP spiral-confined concrete is 0.12, which is larger than that of the FRP jacket-confined concrete documented in the literature. The axial load carried by the FRP bars with a diameter of 16 mm (corresponding to a reinforcement ratio of 4.6%) accounts for 25% ∼ 35% of the total axial load in the column at the ultimate state, while the contribution of the FRP bars is significantly reduced with the decrease in the bar diameter. The value of the tensile elastic modulus of the longitudinal FPR bars should be reduced when estimating their axial load contribution to the FRP-RC columns. The axial load–strain behaviour of FRP-RC columns can be well predicted by the existing model with some modifications.