IntroductionLow-vacuum tunnels are the core infrastructure of ultra-high-speed low-vacuum magnetic levitation transportation systems. The internal environment of these tunnels is characterized by extremely low pressure, alternating airflows, and very low humidity. Based on a comprehensive review of existing research, concrete is considered a highly competitive material for low-vacuum tunnels due to its cost-effectiveness and technological maturity. However, studies have shown that under conditions of extremely low pressure and fluctuating atmospheric pressure, concrete experiences rapid moisture loss, resulting in significant changes to its pore structure and leading to the deterioration of its mechanical properties and long-term performance. Additionally, the high-velocity airflow caused by alternating pressure in low-vacuum tunnels can induce aerodynamic fatigue damage to the internal and external surfaces of the concrete, generating severe dust that poses a significant threat to the safe operation of high-speed trains. To address these challenges and ensure the reliable performance of concrete in low-vacuum environments, protective measures or the use of high air-tightness concrete is necessary. Coating materials have been identified as an effective solution to enhance the service safety of concrete in vacuum tunnels.MethodsThe cement used was P·I 42.5 Portland cement produced by Fushun Cement Co., Ltd. Fine aggregates consisted of natural river sand with a fineness modulus of 2.8. Coarse aggregates were crushed limestone with continuous gradation, sized between 5 mm and 20 mm, with a mud content of 0.4%. Water used was municipal tap water. The water-reducing agent was a non-air-entraining polycarboxylate- based superplasticizer. The coating system was a two-component fluorocarbon material jointly developed by Beijing Jiaotong University, consisting of a solvent-free epoxy penetrating primer and a fluorocarbon resin topcoat. The primary film-forming material was fluorocarbon resin, with hexamethylene diisocyanate as the hardener, and the pigment-to-binder ratio of the coating was 0.4.To better understand the effect of coating materials on concrete performance, the coating was applied during the early hydration stage of concrete. After casting, the specimens were stored at (20 ± 2) ℃ for one day before demolding, after which the primer coat was applied. Following two days of air drying, the topcoat was applied, and the specimens were further air-dried for two days. The compressive and flexural strengths of the concrete were tested using specimens sized 300 mm × 100 mm × 100 mm and 400 mm × 100 mm × 100 mm, respectively. To evaluate the impact of surface coating on concrete mass loss, specimens of 40 mm × 40 mm × 40 mm were prepared. Adhesion strength tests were conducted to examine the effects of low-vacuum environments on the coating's adhesion. Hardened cement paste samples with the same water-to-cement ratio as C50 concrete (0.37) were also prepared, cured to the specified age, cut into cylinders with heights of 20 mm and 10 mm, and embedded in epoxy resin for analysis. Hydration was terminated by immersing these samples in isopropanol.The surface coating's effects on C50 concrete were further analyzed using scanning electron microscopy (SEM) and backscattered electron (BSE) analysis. Atmospheric curing was conducted in a standard curing chamber at (20 ± 2) ℃ and ≥95% relative humidity. A U-shaped vacuum chamber with a pressure range of 1000-2000 Pa and a temperature consistent with the local outdoor environment (-5 ℃ to 20 ℃) provided the low-vacuum exposure environment.Results and discussionThe application of surface coatings significantly reduced the mass loss of concrete under low-vacuum conditions. Compared to uncoated concrete, coated specimens exhibited a noticeably lower mass loss rate, demonstrating that the surface coating effectively suppressed the loss of free water within the concrete.The surface coating markedly improved the flexural strength of concrete under both atmospheric and low-vacuum conditions. Under atmospheric curing, the flexural strength of coated specimens at 14, 28 d, and 60 d increased by 28.2%, 23.9%, and 36.2%, respectively, compared to uncoated specimens. For coated specimens exposed to a low-vacuum environment after six days of atmospheric curing, flexural strength at 14, 28 d, and 60 d increased by 33.3%, 25.0%, and 15.4%, respectively, compared to uncoated specimens under the same conditions. Furthermore, coated specimens exposed to low vacuum after 28 d of atmospheric curing showed a 47.5% increase in flexural strength compared to uncoated specimens.In contrast, the compressive strength of coated specimens was generally lower than that of uncoated specimens under both atmospheric and low-vacuum exposure. This reduction is attributed to the adverse effects of low vacuum on the compressive strength of coated specimens. Additionally, the application of surface coatings reduces friction between the concrete specimens and the testing machine, diminishing end effects, which may also contribute to the lower compressive strength observed.Microstructural analysis using SEM revealed similar hydration products in both coated and uncoated specimens, including ettringite and C-S-H. However, uncoated specimens exhibited more needle-like ettringite and network-like C-S-H, whereas coated specimens showed fewer needle-like structures and more plate-like C-S-H. BSE analysis indicated an 18.1% reduction in porosity for coated specimens under low-vacuum conditions compared to uncoated ones, reflecting improved internal density. Nevertheless, the unhydrated cement content in coated specimens was 7.5% higher than in uncoated specimens, suggesting partial hydration inhibition by the coating.The adhesion strength of the coating increased with time under both atmospheric and vacuum conditions. Vacuum exposure slightly enhanced the coating's adhesion strength, indicating that low-vacuum environments promote better bonding between the coating layer and the concrete surface over time.ConclusionsThe surface coating effectively mitigated free water loss in low-vacuum environments, resulting in significantly reduced mass loss rates for coated concrete compared to uncoated specimens. Coated specimens demonstrated notable improvements in flexural strength under both atmospheric and low-vacuum conditions, although compressive strength was slightly reduced. Early-age exposure to low vacuum negatively impacted both compressive and flexural strength, regardless of coating protection. Microstructural analysis showed that the coating reduced porosity and enhanced internal density, mitigating the detrimental effects of low-vacuum environments on concrete's internal structure. Over time, the adhesion strength of the coating increased under both atmospheric and vacuum conditions, with slight improvements observed following vacuum exposure.