Experimental research on mechanical behavior of UHPCFST under repeated axial compression

This paper investigates the mechanical behavior of ultra-high performance concrete-filled steel tubes (UHPCFST) subjected to repeated axial compression. A total of 34 specimens of UHPCFST were systematically designed, constructed, and evaluated experimentally. The design parameters encompassed steel tube wall thickness, UHPC type, specimen size (varying diameters while preserving a consistent diameter-to-thickness ratio), and loading scheme. The failure patterns, stress-strain relationships, axial load-bearing capacity, and stiffness were meticulously examined. Predominantly, shear failure and drum-shaped upsetting failure were identified as the primary failure mechanisms in the specimens. The axial load-bearing capacity was found to increase notably with the use of thicker steel tubes and higher-grade UHPC. Under repeated loading, a reduction in stiffness was noted, which was dependent on factors such as the steel content, tube diameter, and the volume of coarse aggregate of UHPC. Current predictive equations for the axial load-bearing capacity of CFST were assessed using the experimental results of UHPCFST and were determined to over-predict the axial load-bearing capacity of UHPCFST. Consequently, a refined equation is proposed to yield a more precise estimation of the axial load-bearing capacity for UHPCFST. Furthermore, an empirical model was developed to characterize the stress-strain behavior of UHPCFST under repeated axial compression, offering a tool for practical engineering design and analysis.