Behavior of hybrid CFST with FRP-confined UHPC core under axial compression

A fiber-reinforced polymer (FRP)-confined concrete core that provides high strength and ductility under axial compression can act as strength enhancement in a hybrid column. In the present study, ordinary concrete was replaced with ultra-high-performance concrete (UHPC) to form an FRP-confined UHPC core (FCUC). The FCUC was embedded in square concrete-filled steel tube (CFST) columns to form a high-performance hybrid column (SCF-UHPC column for short). The axial compressive behavior of the SCF-UHPC was experimentally investigated using 12 SCF-UHPC columns and two ordinary CFST columns for comparison. The advantages of the SCF-UHPC include excellent axial load-bearing capacity, good ductility, and stable residual load-bearing capacity. The results show that failure of an SCF-UHPC column was caused by FRP rupture of FCUC, which occurred after steel tube buckling that results in the degraded stiffness. It was also shown that the load displacement behavior of the SCF-UHPC composite column was determined by the UHPC core diameter and the corresponding confinement provided by the outer steel tube and inner FRP jacket. A hardening effect could be achieved when the confinement demand of the UHPC core was satisfied, whereas a plateau effect appeared if the confinement was insufficient. Furthermore, the load-bearing capacity and ductility of the SCF-UHPC columns improved with increased thickness of the steel tube and the FRP.