In regions characterized by excessively high diurnal temperature variations and strong radiation (high-temperature-difference environments), concrete structures face heightened risks of cracking during service. Excluding the influence of hydration heat released by the concrete itself, energy absorption from the external environment during service is identified as the direct cause of concrete cracking. Surface cracking represents one of the primary failure modes for concrete in plateau regions with large temperature differences. In such environments, concrete structures are susceptible to cracking not only due to wind-induced drying shrinkage but also from intense solar radiation and excessive diurnal temperature fluctuations. Notably, vertical structures like bridge piers exhibit complex cracking patterns, with cracks often forming a crisscross network on the surface. As a non-homogeneous composite material with low thermal conductivity, concrete experiences internal/external temperature gradients and thermal stresses arising from differences in the coefficients of thermal expansion between cementitious systems and aggregates, which constitute the principal mechanisms for structural cracking. Existing methods for detecting and identifying cracks in concrete structures include manual visual inspection, infrared thermography, and Digital Image Correlation (DIC) techniques. Manual visual inspection, while the most direct and simplest method, suffers from low accuracy due to human subjectivity. Infrared thermography identifies cracks based on thermal conductivity differences between cracked and intact concrete; however, its performance is significantly affected by ambient temperature, humidity, and wind speed, making it unsuitable for regions with dramatic climatic variations like high-temperature-difference environments. DIC, a widely used image analysis tool, precisely captures strain field distributions in concrete under load, enabling accurate crack localization and evaluation. Nevertheless, DIC is limited to surface observations and cannot monitor the development of internal micro-cracks or damage. Additionally, its stringent requirements for ambient lighting hinder its application in long-term field monitoring of actual structures.

Acoustic Emission (AE) Technology exhibits unique advantages in monitoring concrete cracking under high-temperature-difference environments. Compared to other detection/monitoring methods, AE offers real-time crack capture, high precision, crack localization, fracture type classification, and minimal environmental interference. Radhika et al. employed AE analysis to investigate fracture behavior and damage mechanisms in concrete beams under static and fatigue loads. Li et al. utilized AE to study damage degradation patterns and crack types in concrete under varying fatigue stress levels, categorizing the fatigue failure process into four stages. Jiao et al. applied AE to analyze the damage characteristics of steel fiber, basalt fiber, and glass fiber-reinforced manufactured sand concrete under load. However, current AE applications predominantly focus on damage assessment under mechanical loads, with its feasibility for internal damage monitoring and cracking state evaluation in concrete structures under high-temperature-difference environments remaining unverified.

The raw materials used in this study included cement, coarse aggregates, fine aggregates, and water. The cement was P·O 42.5 ordinary Portland cement. The coarse aggregates consisted of continuous graded crushed stones with particle sizes ranging from 5 mm to 10 mm and 10 mm to 20 mm. The fine aggregates were natural river sand with a fineness modulus of 2.8 and graded in Zone Ⅱ. To accelerate the simulation of concrete cracking characteristics under extreme temperature environments, a "square-inner-circle-outer" composite concrete structure was designed for temperature cycling tests. During the temperature cycling process, stress concentration induced by temperature gradients occurs at the edges and corners of the internal prismatic component, amplifying the thermal stress-induced damage to the concrete.

First, a prismatic specimen with dimensions of 100 mm × 100 mm × 400 mm was cast according to the mix design and cured in a standard curing environment for 28 d. The cured prismatic specimen was then placed at the center of a cylindrical mold with a diameter of 200 mm and a height of 400 mm. Fresh concrete with the same mix design was cast into the mold. After 1 day of indoor membrane-covered curing, the specimen was demolded and transferred to a standard curing environment for an additional 7 d, completing the preparation of the "square-inner-circle-outer" composite concrete specimen. Prior to testing, concrete specimens cured for 7 d were dried for 21 d under constant temperature and humidity conditions (20±2) ℃ and relative humidity (60±5)%. Subsequently, the specimens were polished smooth and thoroughly cleaned to ensure effective sensor adhesion and facilitate observation of surface crack development. Eight sensors were attached to both the upper and lower sections of the lateral surface of cylindrical specimens, coupled to the sample interface area using high-temperature-resistant silicone couplers. To investigate the evolution of cracking damage in concrete under extreme temperature cycling, this study set the environmental chamber temperature to cycle between 0 ℃ and 80 ℃, with each cycle lasting 4 h. Concrete specimens standardly cured for 7 d underwent 40 consecutive temperature cycles in the chamber, with AE data acquisition covering the entire duration of each temperature cycle.

During the 40 temperature cycles, the acoustic emission AE equipment captured a total of 539 events generated by concrete stress release or crack propagation, which occurred throughout the entire temperature cycling process. High-amplitude signals indicate intense energy release at crack tips, typically associated with rapid crack propagation or unstable cracking. Most AE signals exhibited stable amplitudes between 50 dB and 80 dB during temperature cycling, reflecting the gradual accumulation of damage and crack extension under thermal fatigue stress induced by repeated expansion-contraction cycles. High-amplitude AE signals predominantly occurred during temperature rises to 60–80 ℃, demonstrating that concrete under thermal cycling is more susceptible to rapid damage accumulation or stable crack propagation at elevated temperatures. A concentrated increase in AE event counts was observed during the 16th temperature cycle, coinciding with a peak cumulative energy of 1943 aJ, indicating significant macroscopic cracking. Furthermore, the cumulative AE energy effectively characterizes the overall structural damage process. A sharp rise in cumulative energy over time correlates with the formation of fracture process zones, and abrupt changes in the energy-time curve slope serve as a criterion for microcrack coalescence into macrocracks.

Based on energy characteristics, the concrete damage and cracking process can be divided into three stages: microcrack accumulation, macrocrack initiation, and crack propagation. After multiple temperature cycles, internal microcracks interconnect during the cooling phase due to shrinkage stresses, ultimately forming macrocracks. This phenomenon is validated by the concentrated energy release and abrupt slope changes in the cumulative energy curve during cooling. Both microcracks and macrocracks in concrete under thermal cycling are predominantly tensile in nature, with over 90% of cracks attributed to thermal fatigue tensile stresses. In summary, the composite concrete structure designed in this study can effectively evaluate the temperature cycling resistance of concrete. The number of temperature cycles corresponding to the onset of macrocracks (Stage Ⅱ) serves as a quantitative metric for assessing concrete cracking resistance.

This paper addresses the damage and cracking of bridge pier concrete under plateau high-temperature-difference environments. A testing method simulating concrete cracking in such environments was proposed, combining AE technology to achieve real-time monitoring and quantitative analysis of internal damage evolution. The damage mechanisms under high-temperature-difference conditions were elucidated. The key conclusions are as follows: 1) Micro-cracks initiate in bridge pier concrete during the early stages of temperature cycling under high-temperature-difference environments. The cracking process can be divided into three stages: micro-crack (damage) accumulation, macroscopic cracking, and crack propagation. 2) During the micro-crack (damage) accumulation and crack propagation stages, both temperature rise and fall exacerbate damage or crack development in the concrete. However, macroscopic cracking predominantly occurs during the cooling phase from 80 ℃ to 60 ℃. This is attributed to the faster surface temperature reduction compared to the interior, creating a non-uniform deformation effect characterized by "outer shrinkage and inner expansion." 3) Temperature fatigue tensile stress is identified as the primary cause of bridge pier concrete cracking under high-temperature-difference environments. Over 90% of the total cracks observed under temperature cycling are tensile cracks. Furthermore, the failure mode exhibits continuous damage accumulation and propagation along macroscopic primary cracks.

By the end of 2024, China's railway operating mileage has reached 162000 kilometers, including over 48000 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.