Damage evolution and residual shear resistance of toughness-improved UHPC-NSC interface under high-stress level of fatigue shear loading

Shear properties between ultra-high performance concrete (UHPC) and normal strength concrete (NSC) are critical factors influencing the efficiency of UHPC strengthening. However, existing studies primarily focus on the properties of the toughness-improved UHPC-NSC interface under static load, the accumulative fatigue damage induced degradation of shear properties remains inadequately investigated. To address this, the damage evolution and residual performance of the toughness-improved UHPC-NSC interface were explored through fatigue push-off tests. Then, the calculation method for the residual shear resistance of UHPC-NSC interface was established. Results indicate that the lower limit of fatigue stress exhibited a limited effect on the damage evolution of the UHPC-NSC interface. The UHPC-NSC interface specimens experienced two phases of damage evolution under high-stress fatigue loading: crack formation and crack propagation. In phase I, a tiny crack was formed in the center of the interface. Phase II was characterized by slow propagation of the interfacial crack, and this phase exhibited stable fatigue damage evolution comprising more than 80% of the fatigue life. The interface reinforcement ratio and lower limit of fatigue stress exhibited insignificant effects on the secant modulus (Dn). Dn=0.50 and Dn=0.85 can be used to characterize the damage initiation and endurance limit of the UHPC-NSC interface under high-stress fatigue loading. Additionally, the residual shear performance of the UHPC-NSC interface decreased significantly with the opening of the fatigue crack tip, and the residual strength can be predicted based on the degradation model of material property.