By the end of 2024, China's railway operating mileage has reached 162 000 kilometers, including over 48 000 kilometers of high-speed railway lines. High-speed railways, characterized by diverse structural types and ribbon-shaped distribution, are directly exposed to atmospheric environments. As the structure directly bears high-speed train loads, the durability of ballastless track determines the service life and safe operation of high-speed railways. With the gradual expansion of high-speed railway construction to high-altitude and frigid regions, the durability of concrete structures faces some challenges. The unique planar structure of ballastless track leads to significantly different degradation patterns under harsh environments, compared to vertical structures like conventional bridge piers. Under a prolonged exposure to surface rainwater accumulation, snow coverage, and freeze-thaw cycles, ballastless track concrete surfaces gradually exhibit typical freeze-thaw damage characteristics such as surface pulverization and layered spalling. Note that the cast-in-place bi-block ballastless track slabs demonstrate higher risks of surface freeze-thaw damage, compared to precast CRTS III track slab. Therefore, greater attentions should be paid to environmental impacts on the durability of cast-in-place concrete structures in ballastless tracks. The vacuum dewatering technology as a surface physical modification method offers distinct advantages for cast-in-place ballastless track concrete, including operational simplicity and significant frost resistance improvement. However, the effectiveness of this technology in enhancing surface frost resistance for low-air-content cast-in-place concrete under low-pressure environments remains unclear. This study was to focus on cast-in-place ballastless track concrete under low-pressure environments. The surface frost resistance of concrete with different air contents from 2.0% to 6.0% under different vacuum levels of 0.06 MPa and 0.08 MPa for different treatment durations of 1, 3, 5 min, and 10 min was analyzed. In addition, an evaluation model considering concrete air content, vacuum level, and duration was also proposed via the theoretical analysis of bubble influence mechanisms on vacuum dewatering effects. This research could provide a theoretical support for enhancing surface frost resistance of cast-in-place ballastless track concrete structures in high-altitude and frigid environments.

According to the requirements of TB/T 3275-2011 standard, all raw materials were first loaded into a mixer within 60 s. The mixer was then operated continuously for 180 s before stopping, and the mixture was poured into molding forms. A self-developed vacuum negative pressure dewatering device was used to perform vacuum dewatering on the surface of self-compacting concrete. The device was run at different vacuum levels of 0.06 MPa and 0.08 MPa for different treatment durations of 1, 3, 5 min, and 10 min, respectively, to prepare cast-in-place concrete specimens treated under different vacuum levels and durations. Untreated specimens were retained as control samples. Immediately after vacuum dewatering, the cast-in-place concrete specimens were covered with sealing film and transferred to a constant-temperature environment of (20±2) ℃ for 24 h of initial curing. After demolding, the specimens were moved into a standard curing chamber with ≥95% relative humidity and maintained until reaching the designated curing age. The vacuum-dewatered specimens were subjected to surface frost resistance tests and vacuum dewatering ratio tests to validate the enhancement effect of vacuum dewatering technology on the frost resistance of cast-in-place concrete surfaces.

Vacuum dewatering technology can effectively enhance the surface frost resistance of cast-in-place ballastless track concrete. For 2.0% to 6.0% air-content vacuum-dewatered concrete under low-pressure environments, the mass loss per unit area after 28 single-sided freeze-thaw cycles is reduced by 5.0%-94.6%, compared to reference group specimens. However, the "over-vacuum" phenomenon caused by excessive vacuum levels or prolonged duration demonstrates the existence of an optimal vacuum dewatering regime for cast-in-place concrete.

During vacuum dewatering, the pore water pressure generated in fresh concrete drives free water migration and drainage under effective stress. The dewatering ratio continuously increases with higher vacuum levels, extended duration, and increased concrete air content. The key mechanisms for frost resistance improvement lie in a reduced water-to-binder ratio (i.e., 3.8%-11.2% reduction) and an enhanced surface strength (i.e., 4.4%-13.5% increase compared to reference group) in the concrete surface layer. Note that the strength improvement becomes more pronounced with a higher air content. These modifications collectively contribute to the enhanced frost resistance of the concrete surface through a vacuum dewatering treatment.

Vacuum dewatering technology could enhance surface strengthening of cast-in-place concrete through compaction and consolidation. During this process, the bubbles ruptured under the pressure when the vacuum-induced a negative pressure in fresh concrete exceeded a critical threshold that air bubbles could withstand. This rupture increased the number of connected pores-acting as "water drainage channels" in the fresh concrete surface layer-thereby accelerating the expulsion ratio of internal moisture along with air. This work could establish a comprehensive evaluation model for surface frost resistance of vacuum-dewatered concrete that simultaneously considered vacuum level, duration, and air content in fresh concrete. The model enabled a quantitative prediction of vacuum dewatering effectiveness and surface frost resistance improvement for ballastless track cast-in-place concrete under varying low-pressure conditions. This could provide a theoretical support for applying vacuum dewatering technology to ballastless track cast-in-place concrete structures in low-pressure environments.