Interfacial Mechanics and Bond Behavior of GFRP-Strengthened RC Beams with Tri-Layer Substrates

Structural degradation of reinforced concrete (RC) caused by spalling requires rehabilitation strategies that often create heterogeneous tri-layer systems comprising old concrete, grout, and GFRP, where interface incompatibility may trigger premature debonding. This study investigates strain redistribution and interfacial bond performance in five RC beam specimens representing control, repair-only, and three GFRP-strengthened configurations: longitudinal strips (BSPS), U-wraps (BSPST), and full-wraps (BSPSF). An integrated multi-modal monitoring framework combining strain measurement and real-time acoustic emission monitoring was employed to evaluate the influence of restraint geometry on bond behavior. Results demonstrate that the U-wrap configuration achieved the highest bond capacity of 110 kN, increased ultimate load by 18.9% (34.39 kN), and enhanced GFRP strain capacity by 35.6% (15,759 με) compared with the BSPS configuration. The full-wrap configuration provided the highest confinement level but exhibited lower bond stress (0.070 N/mm²) and slightly lower bond capacity (107 kN), indicating that excessive lateral restraint may introduce stiffness incompatibility within the tri-layer interface. These findings demonstrate that U-wrap strengthening provides an effective balance between confinement, interfacial compatibility, and load-transfer efficiency for rehabilitated RC beams with heterogeneous substrates. The study is limited by the laboratory-scale specimen size and controlled testing conditions, highlighting the need for further validation at larger structural scales.