Behaviour of circular concrete columns reinforced with GFRP bars and GFRP helices

Glass Fibre Reinforced Polymer (GFRP) bar has emerged as a preferable alternative to steel bar in Reinforced Concrete (RC) members in harsh, corrosive, and coastal environments. This is particularly because steel bars may corrode in such environments and may cause damage and deterioration of RC members. FRP bars are noncorrosive and possess high tensile strength to weight ratio. In spite of their high tensile strength, FRP bars are not recommended to reinforce concrete columns because of their low compressive strength and low modulus of elasticity in comparison to the steel bars. The behaviour of FRP bar reinforced concrete (FRPRC) columns under axial compression and particularly under eccentric axial loads was not addressed adequately in the previous studies. Moreover, the effects of FRP wrapping on the behaviour of FRP-RC columns was not investigated in the previous studies.\nThis study aims to investigate experimentally and analytically the behaviour and performance of GFRP bar reinforced concrete (GFRP-RC) circular columns under different loading conditions. A total of 18 circular concrete specimens with 205 mm in diameter and 800 mm in height were cast and tested. The influences of reinforcing materials (steel and GFRP bars), helix pitches (30 mm and 60 mm), loading conditions (concentric, 25 mm eccentric, 50 mm eccentric and flexural loadings) and wrapping with CFRP sheets were investigated. In addition to the experimental works, analytical studies were conducted for the axial load-bending moment interactions of FRP-RC columns. The developed analytical model well predicted the axial load-axial deformation and moment-curvature behaviours of FRP-RC columns with reasonable agreements to the experimental results.\nThe experimental and analytical results showed that GFRP bars can be used as longitudinal reinforcements to improve the performance of RC specimens in terms of axial load carrying capacity and bending moment. Also, the GFRP helices considerably confined the concrete core to sustain loads. In addition, well confined GFRP-RC columns can obtain two peak loads where the first peak load represents the capacity of unconfined cross-section and the second peak load represents the capacity of confined concrete core of the column.