Detection of interfacial bonding defects in FRP-strengthened concrete using thermal deformation analysis and DIC

Interfacial spalling is a common failure mode in fiber-reinforced polymer (FRP) strengthened concrete structures, and interfacial bonding defects are one of the major causes of such damage. These early-stage defects are difficult to identify through visual inspection, highlighting the importance of developing non-contact, non-destructive testing (NDT) methods for detecting interfacial bonding. In this study, the digital image correlation (DIC) technique was employed to detect early bonding at the interface of FRP-strengthened concrete beams. Deformation was induced by varying the surface temperature of the specimens, and image stitching and analysis were performed to obtain second principal strain maps during thermal loading. The interfacial bonding defects were identified based on strain concentration zones in the strain field. A finite element model was developed to simulate the thermal loading process, and the simulation results were found to be consistent with experimental observations. The study confirms the feasibility of using DIC-based thermal deformation analysis for detecting interfacial bonding in FRP-strengthened concrete structures. The results show that larger defect size and greater defect thickness lead to more pronounced strain concentration. Thinner FRP layers improve defect detectability, while the effect of anchorage method on detection results is negligible.