Introduction
The wide range and strip-like distribution of high-speed rail crossings, as well as the exposed service characteristics, result in ballastless track concrete not only directly bearing train loads, but also facing freeze–thaw damage in severely cold or cold areas such as high-latitude cold regions and high-altitude mountainous areas. In the service process, the coupling beween environmental effects and train loads has significantly affected the ballastless track concrete at the micro- and macro-levels. However, current standards do not consider the effects of high-speed train fatigue loads and freeze-thaw cycles on the service performance of concrete. During the operation of high-speed trains, the flexural fatigue load frequency that causes damage to the concrete exceeds 20 Hz, which is a significant difference from the test system commonly used in current fatigue tests.To reflect the actual service conditions of ballastless track more realistically in cold regions, a coupled test system of ‘freeze-thaw cycles and 20 Hz fatigue loads’ was designed. The effects of freeze–thaw cycles on the performance of concrete were analyzed from the perspectives of mechanical properties, fatigue life, and stiffness. The freeze–thaw cycles were divided into freezing and thawing stages, and their damage characteristics and energy transfer laws were analyzed. A life prediction model considering the coupling of freeze-thaw and fatigue was established from a probabilistic perspective to predict the service life of ballastless track manufactured sand concrete in cold regions.
Methods
The study used C60 manufactured sand concrete as the research object, with specimen dimensions of 100 mm (width) × 100 mm (height) × 400 mm (length). A SUNS-890 electro-hydraulic servo static and dynamic universal testing machine was used for fatigue tests. The stress level (maximum stress fmax/flexural strength ff) was fixed at 0.6, stress ratio (minimum stress fmin/peak stress fmax) was 0.1, loading frequency was 20 Hz, and displacement data was simultaneously recorded during loading. After being saturated, the concrete underwent a series of freeze–thaw cycles, including 0, 150, 300, 450 cycles, and 600 cycles, before undergoing fatigue testing. To analyze the evolutionary of concrete performance during different stages, the freeze-thaw cycles were divided into two stages: freezing (–20 ℃) and thawing (0 ℃ and 20 ℃). After saturation, the specimens were placed at –20, 0 ℃, and 20 ℃ to conduct bending fatigue tests. Simultaneously, in order to analyze the temperature rise caused by fatigue loads at a constant temperature, a temperature measurement point was taken every 25 mm along the side of the concrete during the loading process, and a thermometer was used to measure its temperature.
Results and discussion
After 300 freeze–thaw cycles, the ballastless track concrete exhibits accelerated brittle failure characteristics. Both flexural strength and fatigue performance show a trend of accelerated decline with freeze–thaw cycles exceeding 300 times, resulting in a 46.3% decrease in fatigue life and a 12.9% increase in stiffness degradation after 600 freeze–thaw cycles. The internal and surface defects caused by freeze–thaw cycles serve as initiation points for fatigue damage propagation, with the increased actual stress level making the concrete more prone to fatigue fracture. Defect connectivity induced by the cycles becomes the main factor contributing to the deterioration of concrete performance. From an energy perspective, the loading process involves both potential and internal energy, with concrete dissipating energy mainly through deformation energy generated by deformation. The decrease in maximum strain and increase in residual strain caused by freeze–thaw cycles result in reduced deformability of the concrete, thereby negatively affecting its fatigue performance. Ice within the concrete pores during the freezing stage has a reinforcing effect, leading to an improvement in fatigue performance. Additionally, ice helps alleviate internal friction and accelerates internal energy dissipation, thereby mitigating fatigue damage during the freezing stage. By introducing a fatigue damage factor, a life analysis model for concrete under freeze–thaw + fatigue service conditions are established. Predictions of concrete life under different environments are made based on freeze–thaw environmental criteria, revealing that C60 manufactured sand concrete can meet the freeze-thaw design requirements of cold region at level D1 and level D2, but considerations must be made for the intensified damage from fatigue loads.
Conclusions
A test regime of ‘freeze–thaw cycle + high-frequency fatigue’ was established to investigate the damage mechanism of concrete under the coupled action of freeze–thaw and fatigue from the perspectives of damage progression and energy transfer. It was found that high-speed train fatigue loads exacerbated the freeze–thaw damage of ballast-free track concrete, with defect connectivity caused by freeze–thaw cycles being the main reason for the decline in concrete fatigue performance. Freeze–thaw cycles lead to a decrease in maximum strain and an increase in residual strain, resulting in reduced deformability of the concrete. Compared to fatigue performance at room temperature, ice within the concrete pores during the freezing stage has a reinforcing effect, and ice in the frozen state can accelerate internal energy dissipation, thereby alleviating fatigue damage. A life analysis model for concrete under ‘freeze-thaw + fatigue’ service conditions, considering a fatigue damage factor, was successfully established to predict the service life of ballast-free track C60 manufactured sand concrete.