Interfacial bond enhancement between ECC overlay and concrete substrate: mechanism and strength model

The effectiveness of Fiber-Reinforced Polymer (FRP) reinforced with Engineered Cementitious Composites (ECC) for strengthening concrete structures is critically dependent on the interfacial bond between the ECC overlay and the substrate concrete. This study investigates the efficacy of interfacial agents in enhancing this bond and delves into the underlying mechanisms through single-sided shear tests and micro-scale measurements. The results demonstrate that the application of a silicate-based activator including titanium fluoride (Z3 agent) yielded the most significant improvements, with increases of 70.8% in shear strength compared to untreated specimens. Micro-scale analysis revealed that the Z3 agent deeply penetrates the interfacial transition zone, reacting with free Ca(OH)2 to form solid precipitates that densify the pore structure and enhance mechanical properties. Based on the experimental data, a highly accurate practical model for predicting bond strength is proposed, which explicitly incorporates the contributions of surface roughness, concrete strength, and the interfacial agent.