Time-dependent bond performance of CFRP-to-concrete joints exposed to elevated temperatures: A comparative study of EBROG and EBR techniques

Premature debonding of externally bonded fiber-reinforced polymer (FRP) composites is exacerbated when concrete is exposed to elevated temperatures, causing significant degradation of the FRP-concrete interface. While the externally bonded reinforcement on grooves (EBROG) method improves bond behavior by transferring shear stresses into deeper concrete layers, its performance after thermal damage remains largely unexplored. Existing studies have focused on specimens strengthened immediately after heating, overlooking the practical reality that heat-damaged structures are rarely repaired without delay. Consequently, no study has investigated the time-dependent bond performance after high-temperature exposure using EBROG with groove classifications. This study addresses these gaps by examining the time-dependent bond behavior of FRP-concrete joints using EBR and EBROG methods with two groove classes (5 × 5@15 and 10 × 10@20 mm). Forty-eight single-lap shear tests were conducted on specimens exposed to 25, 200, 400, and 600 °C, strengthened either immediately or after a two-month delay. Results show that elapsed time significantly accelerates bond degradation in EBR joints, decreasing bond strength by approximately 18%, 34%, and 36% at 200, 400, and 600 °C after two months, respectively. In contrast, EBROG substantially suppressed this deterioration, limiting bond strength loss to 25%. Notably, at 200 °C, EBROG produced a 7–12% increase in bond strength after two months, indicating superior stress transfer within thermally affected concrete. These findings provide the first evidence that EBROG, particularly with optimized groove classes, can preserve or even enhance the long-term bond performance of heat-damaged RC members, offering practical guidance for post-heating strengthening.