Evaluation of bond strength between existing concrete and UHPC repair material using slant-shear tests with grooved and bar-anchored interfaces

This study examined the peak-load bond-related response of existing C30 concrete repaired with ultra-high-performance concrete (UHPC) using inclined-interface compression tests. Eighteen composite specimens were tested at interface inclinations of 20°, 30°, and 40°, with three replicates per angle and interface configuration. The configurations comprised mechanically grooved interfaces without connectors (G-series) and nominally smooth interfaces incorporating 8 mm reinforcing bars (R-series). Peak axial forces were converted to nominal normal and shear stresses using angle-specific specimen areas. The G-series exhibited greater mean calculated shear stress than the R-series at each tested angle. At 30°, mean shear stresses were 12.65 and 10.91 MPa for G and R, respectively; at 40°, they were 14.27 and 11.56 MPa. Mohr–Coulomb regression was used to describe the peak-load stress states. Because specimen dimensions differed among angles and surface preparation was confounded with connector use, neither inclination nor treatment effects can be independently isolated. The results highlight the importance of interface preparation for the investigated UHPC repair configurations, while identifying the need for factorial testing and direct interface-slip measurements.