Compressive behavior of prestressed SFRCFST stub columns after heating: Effect of fresh concrete compression technique

One innovative and effective technique to enhance the compression response of concrete-filled steel tube (CFST) columns is the compression of the fresh concrete, which actively confines the concrete core by prestressed steel tubes. To utilize active confinement in structural design, the behavior of prestressed CFST columns under different conditions should be investigated. In this research, the compression response of steel fiber-reinforced concrete-filled steel tube (SFRCFST) stub columns prestressed by the fresh concrete compression (FCC) technique was assessed before and after experiencing heat. Toward this goal, 60 cylindrical SFRCFST specimens were fabricated, with key variables of the prestressing ratio, volume percentage of steel fibers, applied temperature, external diameter-to-wall thickness ratio (D/t) of steel tube, and water/cement ratio. Post-heating axial compression tests examined the loading capacity, ductility, peak strain, toughness, and load-strain response. It was revealed that due to prestressing and the temperature rise to 600 °C, the heat-induced reduction in load capacity was negligible, while this drop was notable in the specimens without prestressing. At a temperature of 600°C, the load-bearing capacity of the specimens with a prestressing ratio of 0.5 f_y increased by 40 % compared to the non-confined specimens, while the axial and lateral strain capacities decreased by 40 and 49 %, respectively. With an increase in the prestressing ratio to 0.7 f_y, the load-bearing capacity increased by 50 %, and the axial and lateral strain capacities decreased by 26 and 25 %, respectively. In prestressed and non-prestressed SFRCFST columns, with or without thermal exposure, a steel fiber content of up to 1.5 % improved toughness and increased peak strain and ductility in most specimens. Finally, the nonlinear regression analysis was employed to present a proper prediction model for the loading capacity of SFRCFST columns.