The Monotonic Behavior of Existing Bridge Piers Retrofitted by Expansive Concrete-Filled Steel Tubes: An Experimental and Numerical Study

With the growing traffic volume in China, numerous existing highway bridges in seismic zones are constrained by outdated design standards that are inadequate against current seismic requirements. To address this issue, this study proposes a novel reinforcement technique using expansive concrete-filled steel tube (ECFST) for bridge piers. Through combined experimental and numerical investigations on ECFST columns, the effect of expansive agent (EA) content on steel tube strain was systematically examined. The monotonic quasi-static tests were conducted to evaluate the influence of steel tube thickness, concrete strength, reinforcement thickness, and EA content on the ultimate bearing capacity. The proposed method was implemented in a case study involving a reinforced concrete pier, with analysis focused on the “confinement–self-stress coupling mechanism” of ECFST. Results demonstrated good agreement between numerical simulations and experimental data. The optimal EA content was identified as 15%, achieving the most effective reinforcement. ECFST-reinforced piers exhibited significantly enhanced seismic performance, achieving up to 22.6% increase in peak bearing capacity compared to non-expansive concrete filling. While steel tube thickness considerably affected the reinforcement efficiency, concrete strength grade showed minimal impact. This research provides theoretical support and practical design guidelines for seismic retrofitting of similar bridge piers.