Interfacial strength characteristics of steel fibers embedded in ultra-high-performance concrete under salt freeze-thaw environments

This study investigates the interfacial bonding behavior of straight steel fibers embedded in ultra-high-performance concrete (UHPC) subjected to corrosion from salt freeze-thaw (S-FT) cycles. The effect of freeze-thaw media (water and 3 % NaCl solution) and the number of freeze-thaw (F-T) cycles on fiber bonding performance are analyzed through pullout tests. Moreover, backscattered electron imaging (BSEM), laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) are employed to examine the structural deterioration and micro-component of the interface including pore distribution, morphology, surface corrosion and product characteristics. The correlation between porosity in different interfacial transition zone (ITZ) regions and interfacial bond performance is also analyzed. Results show that F-T cycles reduce the bond strength significantly. Specifically, after 100 water freeze-thaw (W-FT) and 100 S-FT cycles, bond strength decreased by 53.0 % and 67.9 %, respectively, compared to samples not exposed to F-T cycles, primarily due to surface groove loss and loosening of corrosion layers. The substantial reduction in bond strength is closely linked to pore structure changes near the fiber surface. Region 1 (within 10 μm of the fiber surface) is particularly sensitive to porosity variations, with a slope of approximately −0.91. Under W-FT and S-FT conditions, the average pore circularity in Region 1 decreased by 0.9 % and 4.0 %, respectively, indicating that the S-FT cycles significantly increase pore irregularity. These irregular mesoscopic pores create localized stress concentrations at their sharp edges, promoting crack initiation and propagation, which ultimately leads to severe interfacial bond degradation. These findings elucidate the fiber-interface deterioration mechanism under S-FT conditions and emphasize the critical need to control porosity within the 10 μm interfacial zone to enhance the long-term durability of UHPC in cold and chloride-rich environments.