Experimental study on bonded-bolted hybrid joints for concrete structures strengthened with Fe-SMA

In structural strengthening applications using iron-based shape memory alloy (Fe-SMA), efficient prestress transfer between the Fe-SMA plates and the concrete is essential. However, conventional bonded or bolted joints often suffer from strain concentration or premature failure. This study presents a hybrid connecting system that integrates adhesive bonding with mechanical fastening to achieve stable and efficient load transfer. Thirty-two single-lap shear tests were conducted to investigate the effects of adhesive type, bond length, adhesive thickness, bolt diameter, quantity, and arrangement. Compared with bolted joints, the proposed hybrid joints can significantly mitigate local strain concentration through the synergistic shear resistance between the adhesive layer and bolts, thereby ensuring a more uniform stress distribution along the interface. Besides, the hybrid joints exhibit three distinct stages under shear: an adhesive-dominated elastic stage, a collaborative adhesive-bolt softening stage, and a bolt-dominated failure stage. High-strength SK-adhesive alleviates strain concentration at bolt holes, restrains Fe-SMA deformation, and synergizes with bolts to enhance both initial stiffness and ultimate shear capacity, whereas low-strength HM-adhesive reduces joint strength due to premature brittle failure. Increasing the number of bolts or placing bolts closer to the loading end promotes early adhesive-bolt synergistic interaction and more uniform stress distribution. A predictive model considering adhesive-controlled stiffness and bolt-dominated capacity was proposed, with errors below 20% for stiffness and 15% for ultimate shear capacity. The predictions show reasonable agreement with the experimental results within the tested dataset and parameter ranges. This study provides mechanistic insight into the shear behavior of Fe-SMA-concrete hybrid joints and may serve as a reference for future studies on Fe-SMA-based strengthening systems. In addition, the findings may also provide a reference for similar hybrid joints between metallic plates and concrete structures subjected to shear-dominated load transfer.