Experimental investigation on the interface of concrete-filled steel tubular (CFST) structures based on roughness and geometric deviation

The bonding behaviour at the interface between the steel tube and the core concrete of concrete-filled steel tubular (CFST) structures is crucial for their composite performance. While previous studies have primarily focused on the effects of the steel tube geometry and material strength, key factors influencing interface bond strength remain underexplored, such as micro-interlocking and friction. To address these issues, this study performs seven push-out tests on CFST specimens, with precise measurements of the geometric deviation and roughness of the steel tube. Consequently, these results are used to characterize the micro-interlocking mechanism and frictional resistance, considering key influences such as geometric features, friction coefficients, and confinement effects. In addition, the volumetric deformation during the push-out process is identified as a novel phenomenon, providing direct evidence of the confinement effect exerted by the steel tube. Based on the experimental findings, a novel method is proposed to calculate the interface bond strength for CFST structures, incorporating quantified micro-interlocking and friction. This approach provides a theoretical foundation and a practical method for the reasonable evaluation of bond strength in similar interfaces.