1. Introduction
The Jiuma Expressway is located in the northwest of Sichuan Province, China, with an average altitude of 3000–3800 m. The climate and environment are characterized by a large temperature difference between day and night, with frequent alternation between positive and negative temperatures and repeated freeze–thaw cycles. At present, the standard method for evaluating the frost resistance of concrete is the rapid freezing method, but many studies have shown significant differences in the frost resistance of concrete under natural service conditions compared to those obtained using this method. Yuan Bin et al. studied the relationship between rapid freeze–thaw conditions and natural freeze–thaw conditions in hydraulic concrete indoor environments. Their investigation indicated that the deterioration phenomenon worsened as the lowest temperature of the cycle decreased [
1]. Li Haoyu et al. established a freeze–thaw relationship between laboratory concrete and natural hydraulic structural concrete in the Jilin region of China based on the parameters of the dynamic modulus of C25 specimens and the ultrasonic speed index [
2]. Studies have found that existing concrete freeze–thaw life prediction models are mostly based on the regression of experimental data to obtain the relationship between a certain indicator and the number of freeze–thaw cycles or modeling methods that use theoretical derivation to obtain evolution equations and then fit the parameters of the evolution equations with experimental data [
3,
4,
5,
6,
7]. However, due to the assumptions of the model and the fact that projections are unverified by real service life, the applicability of frost resistance evaluation models is relatively limited. Currently, most engineering construction projects do not use the estimated frost resistance service life of models.
At present, the common methods used to improve the frost resistance of concrete include air entrainment and the internal mixing of admixtures. There seem to be inconsistent conclusions in the literature regarding the use of the air entrainment method to improve the frost resistance of plateau concrete. Li et al. compiled meteorological data from 20 meteorological stations in the Qinghai Tibet Plateau region over the past 40 years and obtained a formula for calculating the average annual freeze–thaw cycles through statistical regression. Experiments showed that air entrainment in concrete decreases linearly with a decrease in air pressure. Therefore, it is recommended to increase the air content of ready-mixed concrete in high-altitude areas compared to that used for plain areas [
8]. Zhang et al. used nuclear magnetic resonance to analyze the pore structure of concrete in the Qinghai Tibet Plateau region before and after freeze–thaw cycles. They believed that concrete with a gas content of 3.2–3.8% had the best frost resistance. Nuclear magnetic resonance showed that the gas content in this range effectively enhanced the internal pore structure of hardened concrete [
9]. Dai et al. showed that negative temperature curing can reduce the frost resistance of high-strength concrete, making it unable to achieve the same frost resistance and service life as positive temperature curing. The optimal initial air content for the frost resistance of high-strength concrete is 4–5% [
10].
Some studies have also improved the frost resistance of concrete by adding admixtures. Qu et al. showed that the addition of basalt fibers and silica fumes can improve the frost resistance of concrete by 25% [
11]. Sumanta Das et al. studied a microencapsulated phase-change material, which was incorporated into concrete layers to reduce the freeze–thaw effects on bridge decks. The effectiveness of the microencapsulated phase change for the protective layer of concrete was mainly demonstrated using finite element analysis and multi-scale simulations [
12]. Dong et al. used n-tetradecane and expanded perlite to produce the composite phase-change material EPC14, and the study also showed that concrete with 20% addition of this material experienced the smallest amount of damage after 200 freeze–thaw cycles [
13]. Sun et al. showed that the addition of 1–2 mm rubber fine aggregate, being raised from 0% to 5.6%, can increase the F100 of concrete from 76.6% to 86.5% [
14]. Arasteh-Khoshbin et al.’s research showed that adding up to 6% nano-SiO
2 improved the compressive strength before and after freeze–thaw cycles [
15]. Mohammed et al. studied the beneficial effects of anti-freeze–thaw measures by mixing fly ash, blast furnace slag, silica fume, and metakaolin with concrete to enhance the concrete’s performance [
16]. Lee et al.’s research showed that concrete in which 10% of the ordinary Portland cement was replaced with air-cooled slag and water-cooled slag exhibited a similar durability against the freeze–thaw cycle as that of concrete in which 40% of the ordinary Portland cement was replaced with water-cooled slag only [
17].
Chen et al. showed that saturation and water migration have a significant impact on the freeze–thaw damage of cement-based materials. In water environments, unsaturated specimens are more susceptible to frost damage before a freeze–thaw event [
18].
In the literature, there is relatively little experimental research on the differential damage to concrete materials caused by the natural environment and rapid freezing environment under high-frequency freeze–thaw cycles. Model research is generally biased toward theory, and its guiding significance is limited for practical engineering. In this study, in order to explore the actual service status of concrete along the entire Jiuma Expressway in small regional environments and adopt targeted measures to improve the frost resistance and durability of concrete, we conducted freeze–thaw tests, simulating the natural temperature and humidity environment of the region. To compare the difference in influence on concretes under the rapid freezing method and the simulated natural freeze–thaw environment of concrete, the weight loss and the relative dynamic modulus of elasticity were tested every 25 freeze–thaw cycles. Moreover, a computerized tomography (CT) scan and a mercury injection test (MIP) were carried out. The reasons for the differences in the changes to the micro-pore structure of concrete before and after exposure to different freeze–thaw environments and the rapid natural freezing method were explained from a microscopic perspective. Finally, according to the research rules, the environment where the bridges were located—along the Jiuma Expressway—was analyzed. Measures to improve frost resistance and durability are provided for every kind of concrete facility along the Jiuma Expressway.