Push-out tests of CFST columns strengthened with self-compacting and self-stressing concrete filled square steel tube

Abstract

There are two dominant interfaces in concrete-filled steel tube (CFST) columns strengthened with CFST jacketing: (a) interface between the sandwiched concrete and the outer wall of the inner steel tube (interface Ti); (b) interface between the sandwiched concrete and the inner wall of the outer steel tube (interface To). 16 push-out tests were conducted on CFST columns strengthened with self-compacting and self-stressing concrete filled square steel tube to study the bond behaviour of the two interfaces. The experimental parameters included the sandwiched concrete strength (C50, C60 and C70), thickness of outer steel tube (3.5, 4.5 and 5.5 mm), self-stressing (with and without) and interface type (interface Ti and interface To). The influence of different parameters on the bond behaviour was assessed and the longitudinal strain distribution of interfaces To and Ti along the length of the outer and inner steel tubes was analyzed, respectively. The results indicate that the interfacial bond stress presents as an exponential function in the direction of interface length. Interface Ti has higher average ultimate bond strength compared with corresponding interface To. The bond strengths of both interfaces increase with increasing sandwiched concrete strength and thickness of outer steel tube. However, the self-stressing only has evident effect on the bond strength of interface To. In addition, formulas for calculating the bond strengths of both interfaces in retrofitted columns were proposed, and the experimental results from previous studies were also compared with predictions from proposed formulas and design codes.