Evaluation method of the synergistic service performance between ultrahigh-performance concrete and normal strength concrete based on the parameter of environmental equivalent force ratio

This study investigated the synergistic service performances of ultrahigh-performance concrete (UHPC) and normal-strength concrete (NSC). Restrained specimens used in this study were designed to assess the non-equivalent deformation of a substrate and UHPC under specific service conditions and shrinkage. Various UHPC repair materials characterized by different shrinkage deformation gradients were developed using expansive agents and lightweight aggregates. The study explored the effects of shrinkage on the synergistic service properties of UHPC-NSC. In addition, the bond fatigue performance of UHPC-NSC was investigated, and a bond fatigue-life equation was formulated. Under static loads and a stress ratio of 0.9, specimens exhibited both substrate and bond failures. By contrast, specimens under stress ratios of 0.8 and 0.7 displayed substrate failure. The synergistic effect of a 10 % dosage of expansive agent and a 15 % dosage of lightweight aggregate resulted in a 67.2 % reduction in UHPC shrinkage after 28 d. Internal stress derived from the non-equivalent deformation of the substrate and UHPC due to shrinkage was quantified. A reduction in UHPC shrinkage led to a 33.0–67.3 % decrease in tensile stress for repair materials. Internal stresses in the UHPC-NSC under alternating environments, including wet–dry, hot–cool, and freeze–thaw cycles, were calculated, where the ratio of internal stress to interfacial strength was defined as the interfacial stress-intensity ratio. This study presents an evaluation method for the synergistic service performance of UHPC-NSC based on an environmental equivalent-force ratio, thereby providing guidance for practical concrete-repair service-life design.