In alpine saline soil environments, concrete has a multi-factor coupling degradation due to the freeze-thaw cycles, as well as sulfate and chloride corrosion. This degradation results in a significant decline in structural lifespan primarily due to reduced material durability. The maintenance process is complex and incurs substantial economic losses, attracting considerable attentions. Under the influence of these coupled factors, the presence of salt facilitates the migration of water toward the salt. Upon reaching the critical freezing point, ice jams occur, leading to concrete damage from ice pressure. Also, the formation of numerous expansion products generates an expansion pressure during dry-wet cycles, further contributing to concrete deterioration. Although the use of fiber-reinforced concrete has a potential to mitigate these issues, tight construction timelines can result in an initial damage, such as cracks during winter construction, which are often unavoidable. The existing research on the durability and mechanisms of fiber-reinforced concrete in real crack states within this environment remains insufficient, indicating a need for a further research.

This study examined the fiber-reinforced concrete used in a project located in the alpine saline soil region of Tibet, China. Three distinct types of fiber concrete specimens were designed and manufactured with polypropylene fiber (PPF), polyacrylonitrile fiber (PANF), and modified polyester fiber (PEF). After 150 d outdoor curing, the actual cracks were analyzed by a microscope. Two accelerated degradation tests were conducted, i.e., composite salt-freeze-thaw and composite salt-dry-wet cycles. The durability damage mechanisms and patterns were investigated in detail through measurements of ultrasonic wave velocity and splitting tensile strength, as well as by scanning electron microscopy.

According to the crack information statistics of the specimens after outdoor exposure and curing, the cracks in plain concrete are straight and singular, whereas the cracks in fiber-reinforced concrete are interconnected, forming a ring network structure. The presence of these cracks accelerates the connection rate between surface and internal cracks during composite salt-freeze-thaw and dry-wet cycles, leading to a reduction in wave velocity and splitting tensile strength. The mechanistic analysis indicates that under the influence of salt-freeze-thaw, the saturation of the concrete pore solution increases, and the combined effects of crystallization pressure, stress, and chemical expansion products contribute to the initiation and propagation of internal cracks within the concrete. Damage can be categorized into surface damage and internal damage. During the composite salt-dry-wet cycle, the water pressure in the wet state gradually diffuses the salt solution from the concrete surface to its interior, resulting in the formation of expansive products. In the dry state, the solution is heated and evaporated, causing soluble salts to crystallize due to temperature effects. The alternating dry-wet conditions produce a compounded damage. The fibers exhibit a bridging-cracking-toughening effect, which enhances crack propagation resistance. Furthermore, an analysis model correlating the relative values of ultrasonic wave velocity damage with splitting tensile strength, which is proposed based on material decay theory, demonstrates a correlation coefficient of exceeding 0.95. This model effectively reflects the attenuation behavior of the splitting tensile strength of fiber concrete under conditions of composite salt-freeze-thaw and dry-wet cycles, highlighting its significance in predicting the tensile properties of concrete structures.

The initial crack width of plain concrete ranged from 6 μm to 15 μm with a maximum length of 20 mm. In comparison, the early crack resistance of polyester fiber reinforced concrete was superior, exhibiting a width of less than 10 μm and a maximum length of approximately 10 mm, alongside the maximum anti-deterioration capability. Upon reaching the designated number of cycles in the durability test, the wave velocity was decreasesd by 29.36% and 16.05%, respectively, while the splitting tensile strength was diminished by 43.80% and 37.19%, respectively. This deterioration in durability resulted in the expansion of cracks within the interfacial transition zone of the concrete. However, the fibers could form a network that bridged these cracks, thereby reducing tensile stress, enhancing crack propagation resistance in the interfacial transition zone, ultimately delaying the overall damage.