Investigation on interface bonding damage of epoxy coating-concrete composite beam based on DIC technology

Epoxy coatings are widely used to repair and strengthen concrete structures. However, the influence of coating thickness on post-cracking load transfer and interfacial damage evolution remains insufficiently understood. This study investigated the flexural behavior and progressive damage mechanisms of epoxy coating-concrete composite beams using four-point bending tests combined with digital image correlation (DIC). Five nominal coating thicknesses–0, 2.5, 5.0, 7.5, and 10.0 mm–were considered. Full-field strain distributions, interfacial strain profiles, crack trajectories, and crack-opening displacements were analyzed and correlated with the mechanical responses. The results showed that the epoxy coating enhanced both the flexural resistance and energy-absorption capacity of the beams. Compared with the uncoated specimen, the specimen with a 10 mm coating exhibited a 34.1% increase in maximum flexural stress and a 498.8% increase in total flexural toughness, from 3.790 to 22.696 J. The coating thickness had a particularly pronounced effect on the post-cracking response. Specimens with coating thicknesses of 2.5–7.5 mm exhibited stress softening after initial concrete cracking. The 10 mm coating produced a post-cracking strengthening response by maintaining tensile load transfer and promoting stress redistribution. DIC observations revealed a corresponding transition from rapid strain localization and single-crack-dominated failure in thin-coated beams to distributed strain concentration, multiple-crack formation, and crack coalescence in thick-coated beams. Under the investigated material, geometric, and loading conditions, the transition from post-cracking softening to strengthening occurred between coating thicknesses of 7.5 and 10.0 mm. These findings provide a quantitative basis for selecting the thickness of epoxy coating in concrete repair application.