Behavior of FRP Ring-Confined CFST columns under axial compression

Although concrete-filled steel tube (CFST) columns have been widely accepted in applications, they suffer deterioration due to various reasons. On the other hand, the concrete and the steel in CFST columns subjected to compression are not perfectly bonded at the initial loading stage. To this end, fiber-reinforced polymer (FRP) rings are proposed to strengthen the CFST columns in the present study. By strengthening CFST columns with FRP rings, it is expected that their nominal yield strength is increased and the strengthened CFSTs exhibit a monotonic ascending load-strain behavior under compression. An experimental study on axial compressive behavior of FRP ring-confined CFST (FRCCFST) columns is carried out and the effects of the FRP type, the FRP ring thickness (0, 2, 3, 4 layers) and the FRP ring clear spacing (40, 80, 120 mm) on the axial compressive behavior of FRCCFST columns are investigated in this study. The experimental results show that FRP rings substantially enhance the ultimate load capacity and slightly enhance the yield strength of CFST columns. The ultimate axial strain of polyethylene terephthalate (PET) FRP ring-confined specimens is much better than that of CFRP ring-confined specimens due to the large ultimate tensile strain of PET FRP. The ultimate stress and strain of concrete in CFST specimens are obtained and compared with those provided by existing stress-strain models to examine their accuracy and capability.