Active and sustainable confinement of circular hollow reinforced concrete columns using shape memory alloy strips: experimental and numerical insights

Circular hollow reinforced concrete columns (CHRCCs) are widely employed in seismic regions due to their high strength-to-weight and stiffness-to-weight ratios. However, the structural enhancement of compromised CHRCCs remains challenging, particularly under eccentric load scenarios. This study proposes a direct confinement technique using shape memory alloy (SMA) strips to improve the mechanical performance of CHRCCs. In this work, an innovative confinement concept is introduced, where SMA strips are activated through their martensite-to-austenite phase transformation to provide active confinement without adhesives. This mechanism relies on the shape memory effect rather than the superelastic response, where recovery stress is generated during restrained martensite-to-austenite transformation prior to structural loading, establishing active circumferential prestress. The SMA-confined CHRCCs exhibited significant enhancement in concentric and eccentric load-bearing capacity (up to 36.5%), while the ductility improved by 139.4% under high-eccentricity loading, validating the effectiveness of the active confinement generated by recovery stress developed during the transformation stage. The experimental results were validated through finite-element analysis calibrated against observed damage modes, and the potential for further performance optimisation was examined through parametric analysis of strip configuration. This cost-efficient and high-performing SMA confinement approach offers a sustainable, reusable and durable retrofitting solution with superior thermal stability, addressing limitations of fibre-reinforced polymer systems and contributing to resilient infrastructure design.