To investigate the effects of alpine freeze-thaw cycles and saline soil environments on the microstructure of fiber-reinforced concrete, a comprehensive durability test was conducted on polyacrylonitrile fiber-reinforced concrete (PANFRC) under combined salt corrosion and freeze-thaw (salt-freezing) conditions. Using both ultrasonic plane testing and opposite-side measurement methods, the thickness of the damaged layer and the development of internal defects in PANFRC were evaluated. Furthermore, the evolution of the pore structure was analyzed using NMR technology and pore fractal theory. Results indicate that under the combined action of salt-freezing cycles, the thickness of the damaged layer increases progressively, and its compressive strength decreases, leading to a corresponding reduction in the overall compressive strength of the concrete. A linear relationship was observed between these two parameters. The ultrasonic velocity in the central region of the PANFRC specimens decreased sharply, and the area of the defect zone expanded significantly, making this region the most vulnerable part under load and prone to fracture. When the PANF content exceeds 1.5 kg/m³, the loss in ultrasonic velocity is minimized, and the fibers effectively inhibit the growth of internal defects, thereby enhancing the toughness of the concrete, particularly during the middle to late stages of salt-freezing cycles. As the number of salt-freezing cycles increases, the proportion of micropores in PANFRC decreases, while the number of macropores and microcracks increases. The total proportion of small and medium-sized pores also rises. Overall, the development of the pore structure follows a trend of increasing complexity, with small pore throats interconnecting and transforming into super-large and fissure-type pore throats.

salt-freezing coupling  /  fiber-reinforced concrete  /  internal defects  /  pore structure