Square concrete-filled steel tube strengthened with carbon fiber-reinforced polymer grid-reinforced engineered cementitious composite under axial loading

This paper presents an investigation on axial compression behavior of square concrete-filled steel tube (CFST) columns strengthened with carbon fiber-reinforced polymer (CFRP) gird-reinforced engineered cementitious composite (CFGRE). A total of 19 columns, including 3 un-strengthened CFST columns, 1 CFST column strengthened with CFRP sheets, and 15 CFST columns strengthened with CFGRE, were prepared and tested. The main test parameters involved the strengthening strategy, number of CFRP grid layers, width-to-thickness ratio of the steel tube, and concrete strength grade. The CFST columns strengthened with CFGRE showed a ductile failure, with the CFRP grid rupturing at the corner. The CFGRE provided confinement to the original column in the transverse direction and, along with the bearing load contribution of engineered cementitious composite in the longitudinal direction. This resulted in a significantly greater enhancement in load-bearing capacity compared to the CFST columns strengthened with CFRP sheets. The CFST columns strengthened with CFGRE demonstrated a similar developing trend in load-deformation curves as the CFST columns during the entire loading process, indicating its strong adaptability to the original structural system. A finite element model was established to evaluate the failure process of the strengthened columns. A calculation formula was proposed to predict the load-bearing capacity of the CFST columns strengthened with CFGRE, taking into account the effect of the uneven confining pressure on the confinement provided by CFGRE.