Shear damage evolution of grooved UHPC-NC interfaces under salt freeze-thaw cycles

Abstract

This study first evaluated chloride concentration effects through mechanical tests and scanning electron microscopy (SEM). The most damaging concentration (3.5 % NaCl) was then used in freeze-thaw tests on shear specimens with different groove densities. After 300 freeze-thaw cycles, both strength loss and dynamic elastic modulus loss of UHPC first increased and then decreased with increasing salt concentration. SEM crack analysis revealed that chloride concentration affected damage severity in the order: 3.5 % > 0 % > 5.0 %. Under 3.5 % chloride concentration freeze-thaw cycles, the UHPC-NC interface shear strength exhibited an initial increase followed by degradation. Specifically, the strength increased by 11–13 % within the first 40 cycles, mainly attributed to pore-filling by hydration products, but decreased by 34–44 % after 100 cycles, primarily due to frost heaving stress and chloride ion penetration. Higher groove density improved interface compactness, with the 2-groove interface showing 44.8 % greater strength than the single-groove. However, during severe salt freeze-thaw cycles, NC matrix deterioration became the main cause of interface failure, reducing the strength differential between the two interfaces to 33.4 %. Finally, based on Mohr's stress circle theory, an interface damage prediction model incorporating salt freeze-thaw damage of the material was developed, demonstrating computational accuracy with less than 4.7 % deviation from experimental values.