Effects of groove angle and pattern on CFRP-to-concrete bond behavior of EBROG joints: Comparison of diagonal with longitudinal and transverse grooves

Recently, the novel method of externally bonded reinforcement on grooves (EBROG) has been introduced as an alternative to the conventional externally bonded reinforcement (EBR) technique to attach the fiber-reinforced polymer (FRP) sheets to concrete. Although extensive research has been carried out on the EBROG joints with longitudinal and transverse grooves, no single study exists to investigate the diagonal grooves with different angles and patterns. Besides, the function of longitudinal, transverse, and diagonal grooves with similar conditions has not been yet compared. This paper is the first attempt to compare the bond strength, effective bond length, and bond behavior (i.e., load-carrying mechanism and strain field distribution) of EBROG specimens with longitudinal (zero degrees), transverse (90 degrees), and diagonal grooves (21, 37, 45, and 63 degrees). Twenty specimens were strengthened with the EBROG method through different groove angles and patterns, and two specimens were strengthened with the EBR method. The total grooves' length was kept almost the same in all the strengthened specimens. The specimens were then subjected to a single-lap shear test. From the results, the optimum groove angles were found to be zero (longitudinal groove) and 90 degrees (transverse groove); e.g., compared to EBR, in the specimens with 30 mm groove spacing, the longitudinal and transverse grooves increased the bond strength up to 84.5% and 89.7%, while a 52.6% increase was observed for diagonal grooves with 45-degree groove angle. The particle image velocimetry (PIV) analysis revealed that the specimens with longitudinal and transverse grooves had greater effective bond lengths compared to those with diagonal grooves. PIV results showed that although diagonal grooves in both zigzag and parallel patterns yielded almost the same bond resistance, they led to different bond behavior.