Concentric axial load test for square CFST columns strengthened with steel jackets

This study introduces an innovative method to strengthen the existing square concrete-filled steel tube (CFST) columns without altering the elastic stiffness of the original column. This approach uses a rounded square steel jacket to confine the CFST column, along with a thin layer of low-friction material wrapped around the steel tube. This setup prevents the axial stress transfer between the steel tube and the confining jacket. The effectiveness of the proposed method was evaluated by testing nine steel-jacketed concrete-filled steel tube (SJCFST) columns and three CFST counterparts subjected to axial compression. The test variables included the thickness and strength of the steel tube, the corner radius ratio, and the thickness of the jacket. The test results demonstrated that the polytetrafluoroethylene layer effectively mitigated the axial stress transfer between the steel tube and the confining jacket. The effective confining stress provided by the jacket developed gradually at the beginning; however, it increased substantially after the axial load reached between 70 % and 90 % of the peak load. The introduction of a confining jacket did not alter the initial axial stiffness of the columns but significantly improved the load-carrying capacity and deformation capacity of the column. Notably, the increments in the axial load capacity and ductility were proportional to the effective jacket confinement index. Based on the test results, an average stress-strain model was developed for the concrete under the dual confinement of the steel tube and jacket. An expression to predict the axial load capacity of SJCFST columns was also suggested.