Axial Compressive Performance of RC Columns Strengthened with Self-Compacting Expansive Concrete (SCEC)-Filled Steel Tubes

To address the performance degradation of reinforced concrete (RC) columns, this study experimentally investigated the axial compressive performance of RC columns strengthened with self-compacting expansive concrete (SCEC)-filled steel tubes. A realistic loading protocol was adopted in which only the RC column was directly compressed, compelling the steel tube and SCEC layer to serve exclusively as confinement. A total of 16 specimens were tested, considering seven key parameters, including the height and cross-sectional shape of the RC columns, the strengthening method, the steel tube thickness, the infilling concrete type, the interface treatment method, and the compressive loading region. The results demonstrated that the external steel tube changed the brittle failure mode of un-strengthened RC columns into a ductile one, with the ultimate axial capacity increasing by up to 184%. Steel tube thickness was identified as the most influential parameter, with an increase from 2 mm to 6 mm improving the capacity by 58.5%. Interface roughening was found to be essential for preventing premature debonding. Based on the Mander model and superposition theory, a simplified analytical formula was proposed, yielding a conservative mean calculated-to-experimental ratio of 0.823. The proposed system offers a reliable high-performance solution for rehabilitating bridge piers and similar RC structures.