Innovative FRP-confined GFRP bar reinforced concrete columns: Compression tests and theoretical analysis

Fiber-reinforced polymer reinforced concrete columns (FRP-RC columns) have attracted increasing attention in recent years. However, conventional FRP bars remain susceptible to compressive buckling. Building on previously developed FRP-confined GFRP bars with high compressive strength, this study investigates concrete columns incorporating these bars as longitudinal reinforcement to mitigate this issue. Axial compression tests were carried out on 26 FRP-RC columns to examine the effects of GFRP bar type, GFRP bar diameter, and spiral pitch. The results indicate that, compared with columns reinforced with conventional GFRP bars, those incorporating FRP-confined GFRP bars showed improved ultimate load capacity and ductility, with maximum increases of 18.0% in strength and 18.1% in ductility. The FRP-confined GFRP longitudinal bars also helped reduce the risk of longitudinal bar buckling and contributed up to 36% of the overall axial load capacity. This contribution was strongly influenced by the confinement provided by the FRP spirals. In addition, existing load capacity and confinement models were reviewed, and the experimental results indicated that the model proposed by Ye et al. provided suitable predictions for the FRP-confined GFRP bar-reinforced concrete columns tested in this study.