Strengthening damaged reinforced concrete pipes using a novel sandwich fiber composite liner: Experimental and numerical studies

This paper investigates the structural performance of damaged reinforced concrete pipes (RCPs) strengthened with a novel sandwich fiber-reinforced polymer (FRP) liner incorporating a thin, flexible bulkermat spacer, providing necessary thickness and stiffness for the liner without requiring multiple stacked FRP layers. RCPs with a 380 mm inner diameter and 500 mm length were pre-cracked using the three-edge bearing (TEB) test to simulate damage. The RCPs were then strengthened using layered FRP systems in which a 4 mm bulkermat core was sandwiched between glass FRP (GFRP) and carbon FRP (CFRP). Experimental results showed that the hybrid GFRP–bulkermat–CFRP (G-B-C) liner achieved the highest load capacity, increasing the peak load by 306 % relative to the control, and by 25 % and 9 % compared to the C-B-C and G-B-G sandwich liners, respectively. The hybrid configuration provided the best balance between stiffness compatibility and strength enhancement, enhancing the load-carrying capacity of the damaged RCPs under the tested conditions. A detailed three-dimensional finite element (FE) model was developed, incorporating concrete damage plasticity (CDP) for concrete, continuum shell elements for the FRP liners, embedded steel reinforcement, and a cohesive zone model (CZM) for the FRP–concrete interface. The FE results showed strong agreement with the experiments in stiffness, peak load, and failure mode, demonstrating the capability of the numerical model to reproduce the observed behavior of the strengthened pipes.