Fatigue crack propagation for Ultra-High-Performance Concrete-normal strength concrete interface with restrained strain

As an advanced construction material, ultra-high-performance concrete (UHPC) has gained prominence in rehabilitating aging concrete infrastructure. When UHPC is bonded to normal strength concrete (NC), the interface between two types of concrete becomes a potential weak link, especially under fatigue loading. This vulnerability is further compounded by restrained shrinkage, which arises from the significant mismatch in shrinkage behavior between UHPC and NC, leading to stress concentrations at the interface. This research delves into the fatigue crack propagation behavior at the UHPC-normal strength concrete (NC) interface under restrained strain conditions. The study’s novelty stems from its focus on the relationship between restrained strain and fatigue crack propagation at the UHPC-NC interface, an area largely unexplored in previous research. The experimental program encompassed free and restrained shrinkage tests, as well as Mode I and Mode I-II mixed fatigue tests under restrained strain conditions. Variables such as the addition of shrinkage-mitigating additives, curing conditions, and mode types were investigated. The interfacial restrained strain between UHPC and NC was examined, and the fatigue behavior of UHPC-NC interfaces was analyzed, covering fatigue life, failure patterns, compliance behavior, fatigue strain analysis, and fatigue damage evolution. The Spearman’s correlation coefficient was utilized to explore the link between restrained strain at the UHPC-NC interface and the maximum crack mouth opening displacement (CMOD). Findings reveal that adding shrinkage-mitigating additives effectively reduces the free shrinkage of UHPC, thereby decreasing the interfacial restrained strain at the UHPC-NC interface. For instance, in Mode I specimens cured under natural conditions, the restrained strain decreased from 143.4 με to 2.4 με. This reduction significantly alleviates interfacial stress concentrations and reduces the potential of crack initiation. Specimens without shrinkage-mitigating additives (NC-w/o-I, SC-w/o-I, NC-w/o-MIX and SC-w/o-MIX) show crack propagation characterized by initial deviation into the NC substrate, followed by vertical progression and slight extension toward the loading point. In contrast, specimens with shrinkage-mitigating additives (NC-w/-I and NC-w/-MIX) exhibit interfacial crack propagation confined to the UHPC side, despite lower restrained strain values. For Mode I-II mixed specimens, a strong correlation exists between restrained strain and crack mouth opening displacement (CMOD), with a Spearman’s coefficient of 1. By clarifying the mechanisms underlying interfacial fatigue performance, this study enhances the understanding of UHPC-NC composite systems and lays the groundwork for improving their design and durability. The results hold significant implications for the construction and maintenance of infrastructure, particularly in applications where UHPC is used to enhance the performance of existing NC structures.