Steel tube filled with steel-fiber-reinforced recycled aggregate concrete subjected to coupled compression-torsion

Recycled aggregate concrete-filled steel tube (RCFST) is a wise way to utilize demolition concrete, nevertheless its torsional behavior suffers from the distinct shrinkage and friability of recycled aggregate concrete (RAC). This study adopts adding steel fibers to improve the torsional behavior of RCFST columns. A total of 17 steel-fiberreinforced recycled aggregate concrete filled circular steel tube (SRCFST) columns were tested, in which 7 ones under pure torsion and 10 ones subjected to combined compressive-torsional load. Test results showed that adding 1.2 % similar to 1.8 % steel fibers improved the torsional properties of RCFST columns with the promotion of 27.62 %-34.27 % in ultimate torsional capacity. Steel fibers enhanced the shear resistance of the concrete core, delayed the cracking, and postponed the failure of RCFST columns. The increase of axial compression ratio (n) led to the first increase and then decrease trend in ultimate torsional capacity. Because the relative small axial compression (n <= 0.2) was applied, the concrete strength was improved due to the triaxial stress state, and the steel-concrete interface was enhanced due to the normal stress. Moreover, a modified finite-element model (FEM) was established based on the "cohesive" contact model. Compared with conventional "penalty" contact model, the "cohesive" contact model can capture the delamination and destruction of the steel-concrete interface, thereby accurately predicting the pure torsion behavior of RCFST columns.