Axially loaded square CFST columns strengthened with circular steel tubes and sandwiched concrete jackets

A large number of concrete-filled steel tube (CFST) structures that have been in service for an extended period suffer from severe performance degradation. Therefore, their repair and reinforcement have become an urgent and essential task. This paper proposes a novel composite reinforcement technique—strengthening square CFST short columns with an outer circular steel tube and sandwiched concrete. Nine strengthened columns and one non-strengthened control column were designed and fabricated, followed by axial compression testing of these specimens. Compared to the non-strengthened column, the strengthened columns exhibited an over 180% increase in load-bearing capacity, confirming the strong and effective confinement provided by the outer circular steel tube. In addition, the effects of the outer steel tube’s diameter-to-thickness ratio and the strength of the sandwiched concrete on the mechanical behavior of the strengthened columns were analyzed, including load-displacement curves, relative load-strain curves, and ductility. By establishing a finite element model, the working mechanism of the strengthened columns and the interaction between their components were revealed. The stress development trends and evolution patterns of each component were analyzed. Based on the experimental and finite element results, a predictive formula for the axial compressive bearing capacity of strengthened columns was proposed.