Fatigue shear failure mechanism and prediction method for UHPC-NC bond interfaces

This paper explores the fatigue failure mechanism and prediction methods for the bonded interface between ultra-high performance concrete ( UHPC ) and normal concrete ( NC ) under shear loading, featuring an experimental program that includes two sets of direct shear tests on a total of 19 Z-shaped UHPC-NC combined specimens. The first set consists of eight static shear tests, from the authors’ previous study. The second includes the eleven fatigue shear tests, key aspects explored include fatigue failure modes, residual shear strength , stiffness degradation , and fatigue life of the UHPC-NC interface, considering different bond sizes, load amplitudes, and pre-existing defects ( e.g., 0 %, 15 %, and 30 % ). Experimental results indicate that interfaces with preset bond defects are more susceptible to fatigue shear damage due to stress concentrations, with fatigue life decreasing as defect dimension increases. Damage accumulates at the UHPC-NC interface with additional loading cycles, leading to persistent slippage and reduced bonding strength. The results also reveal that the fatigue residual bond between UHPC and NC decline as interfacial size increases. A semi-empirical model, combining static shear calculation with non-linear cumulative damage method, was developed to predict the UHPC-NC interface’s residual bond strength and validated against experimental results. This research provides valuable experimental data and theoretical insights for enhancing the fatigue design of shear bonds at UHPC-NC interfaces. • Fatigue performance of UHPC-NC interface is experimentally evaluated. • Failure mechanism of UHPC-NC interface under cyclic loads is analyzed. • Impacts of critical parameters on the UHPC-NC bond in fatigue are determined. • Analytical model to calculate the interface’s residual resistance is derived.