Effect of groove classes on flexurally strengthened RC beams with FRP using the EBROG method

A key challenge in flexural strengthening of reinforced concrete (RC) beams using the externally-bonded reinforcement on grooves (EBROG) method is determining the optimal groove class to achieve efficient, economical designs. In this study, six RC beams (1400×120×160 mm) were experimentally tested: one reference beam, one beam strengthened with the externally-bonded reinforcement (EBR) method, and four beams strengthened with the EBROG method. All strengthened beams were reinforced with fiber-reinforced polymer (FRP) sheets, with identical material properties and widths; the only variables were the strengthening method and groove class. The concept of “ groove class ”, defined as groove width × depth @ center-to-center spacing, with optimized spacing, was implemented to investigate four groove classes: 5×5@15, 5×10@15, 10×5@20, and 10×10@20 mm. The results showed that the EBROG method increased the load-carrying capacity by 68–86% compared to the reference beam and by 24–37% relative to the EBR beam. Among the EBROG specimens, the beam with a 5×10@15 mm groove class exhibited the highest FRP utilization, with theoretically estimated strains of 0.01170 in the pure bending region and 0.0080 at the debonding initiation section, both corresponding to the experimentally measured peak load of 63.1 kN (representing 73% and 50% of the FRP rupture strain, respectively). This specimen outperformed the EBR beam by 37% and the other EBROG beams by 8–11% in terms of load-carrying capacity. In contrast, the beam with a 5×5@15 mm groove class showed the highest energy absorption (614 kN.mm) and ductility index (2.54). These findings highlight the significant influence of groove classes on FRP utilization, load-carrying capacity, and ductility in RC beams strengthened in flexure.