Ultra-High Performance Concrete (UHPC) is widely recognized for its exceptional mechanical properties and durability, making it a cornerstone material in modern infrastructure projects. However, the long-term service performance of UHPC structures with initial cracks under freeze-thaw (F-T) cycling remains a safety concern, particularly in harsh environments such as cold regions and coastal zones. While extensive research has focused on the properties of uncracked UHPC, the study on degradation of cracked UHPC under coupled F-T cycling and self-healing conditions is limited. Existing studies highlight UHPC's intrinsic self-healing potential through secondary hydration and carbonation reactions, yet the interplay between these healing mechanisms and cyclic F-T-induced deterioration remains unclear. To address this, the present study investigates the tensile performance evolution of pre-cracked UHPC under F-T cycling and water-curing conditions. By integrating macroscopic mechanical tests with microscopic analyses, this work aims to unravel the dual effects of self-healing and F-T-induced damage on UHPC's structural integrity.

UHPC specimens were prepared using white cement, silica fume, quartz powder/sand, steel fibers (2% by volume), and a polycarboxylate superplasticizer. The mix design followed a water-to-binder ratio of 0.22. After casting and demolding, specimens underwent 48 h hot-water curing at 90 ℃. Dog-bone-shaped specimens (30 mm×13 mm×80 mm) were pre-notched and subjected to pre-tensioning to introduce controlled initial cracks (100 μm width) via displacement-controlled loading. Pre-cracked specimens were divided into two groups, including 1) F-T cycling groups: Exposed to 100, 200, or 300 F-T cycles (-17 ℃ to 8 ℃ per cycle); 2) water-curing groups: immersed in 20 ℃ water for 14, 30 d, or 60 d. Secondary tensile tests were conducted to evaluate residual strength and crack recovery. Single-fiber pull-out tests assessed interfacial bond performance, while scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) characterized microstructural evolution and hydration products.

Water-cured specimens exhibited remarkable mechanical recovery. After 60 d, tensile strength exceeded the undamaged control group by 32%, attributed to C-S-H gel and Ca(OH)₂ filling microcracks. SEM revealed dense microstructures with nearly closed cracks, confirming the role of secondary hydration in enhancing matrix integrity. Single-fiber pull-out tests showed interfacial bond strength fully recovered within 30 d, though prolonged immersion led to steel fiber corrosion, reducing post-peak ductility. F-T cycling initially promoted low-temperature self-healing. After 200 cycles, tensile strength increased by 14% due to partial crack closure via hydration. However, beyond 300 cycles, cumulative damage dominated: surface spalling, fiber corrosion, and interfacial debonding caused a 7.1% decline in tensile strength. TGA confirmed reduced Ca(OH)₂ and CaCO₃ content under F-T conditions, indicating suppressed hydration and carbonation compared to water curing. Progressive densification of the matrix with crystalline hydration products sealing cracks. Initial healing at 100-200 F-T cycles was counteracted by interfacial microcrack propagation and fiber-matrix debonding at 300 cycles. EDS analysis highlighted localized CaCO₃ precipitation at crack surfaces, insufficient to offset F-T-induced damage. The study revealed a critical threshold (200 F-T cycles) where self-healing and deterioration mechanisms compete. While water ingress facilitates secondary hydration, prolonged F-T exposure disrupts the healing process through ice crystallization pressure and moisture redistribution, exacerbating matrix degradation.

Water curing significantly enhances UHPC's self-healing capacity, with complete tensile strength recovery, which was driven by continuous secondary hydration and carbonation, forming dense, crack-resistant matrices. Freeze-thaw cycling exhibited a dual role: healing occurred at early stages (≤200 cycles), but 300 cycles led to irreversible strength loss. Microstructural analysis underscored the importance of hydration products (C-S-H, CaCO₃) in healing cracks, while F-T cycling disrupts interfacial bonding and accelerates matrix spalling. For UHPC structures in cold climates, proactive crack sealing and controlled curing are essential to maximize self-healing benefits before F-T damage accumulates. Future work should explore hybrid curing regimes and corrosion-resistant fibers to extend service life.