Non-linear distributions of bond-slip behavior in concrete-filled steel tubes by the acoustic emission technique

This study aims to investigate the non-linear distribution of bond-slip behavior in steel-concrete interface of concrete-filled steel tubes (CFST). An analytical model was proposed to account for the attenuation effect in boundaries of load transfer. In this analysis, a modified governing equation with the second-order ordinary differential form was deduced on the basis of elastic theory. Solutions of slippage, bond stress and potential energy in the steel-concrete interface were then obtained. For the model validation, measurements of acoustic emission (AE), as well as strain gauge, were performed in a push-out test of CFST. Test results agree well with the solutions of the analytical model proposed. The interfacial bond stress exhibits a highly non-linear distribution along the direction of load transfer. The maximum stress appears nearby the position of 0.2L away from the specimen end, and is about 1.2 times of the average shear strength in steel-concrete interface. In addition, the AE peak frequencies of bond failure have two main bands, including 75-125 kHz and 290-340 kHz. It indicates that there are mainly two failure modes during the debonding of steel-concrete interface in CFST.