1. Introduction
Asphalt mixtures are typical thermal-sensitive materials, greatly influenced by temperature variations. Changes in atmospheric temperature induce alteration in the temperature field of the asphalt layer, leading to thermal stresses. Consequently, the impact of thermal stresses on the cracking resistance of pavement cannot be overlooked. Research has identified two main causes for the generation of thermal stresses. Firstly, differences in thermal properties among various layers of road structure result in varying contracted deformation when temperature changes. The road layers are mutually constrained, thus generating thermal stresses in asphalt pavement. Secondly, asphalt pavements are significantly affected by solar radiation and atmospheric convection heat exchange. The combined effects of these factors create a temperature gradient within the pavement structure, thereby generating thermal stress. Fluctuation in the thermal stress can trigger thermal cracks in asphalt pavement, with cracks potentially extending downward until they penetrate the entire structure, thereby affecting road driving comfort. Existing methods for determining thermal stresses in asphalt pavement typically involve numerical simulation, theoretical analyses, and laboratory experiments. Although the use of polymer-modified asphalt can enhance the low-temperature crack resistance of asphalt mixtures [
1], due to the complex service environment of asphalt pavement, theoretical methods may fail to analyze the influence of complex temperature history. Consequently, the numerical simulation method is widely employed to analyze thermal stress in asphalt pavement [
2,
3].
Apeagyei et al. [
4] employed the finite element method to analyze thermal stresses in asphalt pavements, revealing a significant correlation between thermal stress and initial temperature as well as cooling rate. Tian et al. [
5] conducted thermal stress restrained specimen tests (TSRSTs) to examine the thermal stresses of two types of asphalt mixtures under various initial temperature and cooling rates, thereby refining the temperature–thermal stress relationship. Their analysis indicated that when the initial temperature was above 5 °C, the asphalt mixture exhibited excellent relaxation capabilities, resulting in no thermal stress regardless of the cooling rate. Similarly, Tan [
6] analyzed the thermal stress in asphalt pavement above 0 °C under the non-periodic cooling condition, revealing that regardless of the initial temperature and cooling rate, when the temperature drop interval is above 0 °C, thermal stress in asphalt pavement is very small. The accumulated thermal stress is far less than the ultimate tensile strength of the asphalt mixture, thus preventing low-temperature-induced cracking. Fan et al. [
7] conducted a virtual TSRST simulation, indicating that the asphalt mixture exhibited good relaxation capability at middle temperature, resulting in minimal cumulative thermal stress. As the temperature gradually decreased below 0 °C, thermal stress in asphalt pavement increased. Pszczola et al. [
8] determined the master curve of modulus using bending creep experiments and direct tensile creep experiments. They calculated the thermal stresses of the asphalt mixture using different methods, pointing out that the strain caused by tensile creep at low temperature is smaller, and the thermal stress of the asphalt mixture can be calculated using the results of the direct tensile creep test with good reliability. Yang et al. [
9] proposed an equivalent fracture temperature (EFT) corresponding to the critical fracture temperature in the TSRST, and the results showed that the EFT is as accurate as the critical fracture temperature in evaluating the low-temperature cracking performance. Compared with fracture energy and critical fracture temperature, the EFT is more sensitive to the type of mixture and can effectively distinguish the low-temperature performance of different mixtures. Wang et al. [
10] studied the low-temperature performance of different mixtures using a low-temperature bending test and TSRST experiment, pointing out that the fracture temperature is the best indicator for describing low-temperature performance and can effectively differentiate asphalt mixtures in high-altitude and cold regions. Arabzadeh et al. [
11] developed a setup to measure the thermal fatigue performance of the asphalt mixture under a constant strain cyclic loading pattern, and the results indicated that the asphalt content, aggregate type, and binder type have the strongest influence on the thermal fatigue resistance of the asphalt mixture. Zhou et al. [
12] pointed out that at a constant initial temperature, thermal stress in the mixture increased with the cooling rate, a lower cooling rate yielding a smoother thermal stress trend because of the relaxation. For the constant cooling rate, a lower initial temperature resulted in a higher thermal stress upon reaching the same temperature [
13]. Sun et al. [
14] noted that thermal stresses in various layers of the road increased with the temperature fluctuation range, and the surface layer is the most affected by the daily temperature variation. Xiao et al. [
15] studied thermal stresses in asphalt surface layers under the influence of daily temperature, indicating that the thermal stresses in asphalt surface layers exhibited a sinusoidal trend. In the rapidly cooling segment, thermal stress accumulated due to insufficient relaxation ability. Prolonged low-temperature conditioning decreased the stress relaxation capability, accelerating pavement thermal damage.
Meanwhile, Das et al. [
16] analyzed the effects of critical fracture energy and the nonlinear thermal shrinkage coefficient on the thermal cracking of the asphalt mixture based on the finite element method, which was used to predict the thermal stress and fracture temperature. The results indicated that the thermal shrinkage coefficient and cooling rate had significant impacts on the fracture temperature of the asphalt mixture. Gu et al. [
17] conducted pavement cracking analysis using a numerical method, suggesting that when the strain energy caused by the load exceeds the ultimate strain energy of the asphalt mixture, cracks will occur, thus determining the initial life of top-down cracks. Ling et al. and Onifade et al. [
18,
19,
20] studied the non-load-related failure mode of asphalt pavement, specifically thermal cracking. They utilized finite element simulation to determine the J-integral at the tip of thermal cracks, applying it to the Paris law to predict the crack extension over time and fatigue life. Wang et al. [
21] investigated crack propagation behavior in the asphalt mixture, validating the reliability of the finite element method through indirect and direct tensile tests on asphalt mixtures. Hasni et al. [
22] and Ling et al. [
23] studied the crack propagation law of asphalt pavement under the coupling effect of thermal and traffic loads and analyzed the evolution of top-down cracks in asphalt pavement. Multiple indicators considering the mixture damage were used to depict the crack propagation process of asphalt layers. Das et al. [
24] investigated the influence of the fracture energy of different mixtures on thermal cracking through numerical simulation and experiments. They concluded that the thermal shrinkage coefficient, cooling rate, and creep compliance significantly contribute to thermal stress. A nonlinear thermal shrinkage coefficient provided a much better prediction than the linear approach. In 2018, Rahbar-Rastegar et al. [
25] employed three methods to analyze the fatigue and crack resistance performance of nine asphalt mixtures. The results indicated that the integration of simulation models is crucial for establishing a reliable connection between lab-measured properties and the cracking behaviors of the asphalt mixture. Additionally, the research demonstrates that fatigue and low-temperature cracking performances exhibit no correlation.
Based on the above studies, it can be observed that thermal stress in asphalt pavement primarily occurs at low temperature (<0 °C). The asphalt mixture has good relaxation capability and will not generate thermal stress when the temperature is above 0 °C. The cooling rate significantly affects thermal stress, and a higher cooling rate results in larger thermal stress. The concept of low-temperature cracking suggests that as long as thermal stress does not exceed the ultimate tensile strength of the asphalt mixture, cracking will not occur [
2]. However, Song et al. [
26] pointed out that while thermal stress did not exceed the ultimate strength, thermal fatigue damage may occur within the mixture. Lv et al. [
27] indicated that the fatigue life of the asphalt mixture is in a power function relationship with the ratio of thermal stress, high thermal stress leads to a shorter fatigue life of the asphalt mixture. Khan et al. [
28] confirmed that temperature variations are diverse in different regions, with regions located in higher latitudes experiencing lower temperature. Additionally, cold waves differ across regions, inevitably exerting a significant influence on the thermal stress of the asphalt layer. Therefore, studying the characteristics of cold waves in different regions and analyzing their impacts on thermal stress is helpful for predicting the thermal fatigue life of the asphalt layer.
In light of this, this paper takes some regions of China as an example, conducts statistical analysis on the daily temperature, determines the occurrence frequency of cold waves, performs an indirect tensile relaxation experiment to obtain the viscoelastic parameters of the asphalt mixture, analyzes the variations in thermal stress based on FE simulation, and predicts the thermal fatigue life of the asphalt mixture. This work provides a new perspective for predicting the thermal fatigue life of the asphalt layer. This makes it possible to estimate the thermal fatigue performance of asphalt pavement using cold wave statistical data, developing a method for the design of the thermal fatigue performance of the asphalt layer in different regions.
4. Conclusions
This work uses atmospheric temperature data from five typical regions in China from 2012 to 2019 to analyze the frequency and moment of cold waves in different regions. Based on typical cold waves, road surface temperatures are calculated and used to analyze the thermal stress of the asphalt mixture. The thermal fatigue life of the asphalt mixture in different regions is then predicted. The following conclusions can be drawn.
There are significant differences in the frequency of cold waves in the selected five regions. Among them, the temperature drop in North China and Northeast China is the largest, with the highest frequency of severe and extreme cold waves. Cold waves mainly occur from November of one year to March of the next year, with the cooling rate ranging from 0.2 °C/h to 0.63 °C/h.
Severe and extreme cold waves result in higher thermal stress, with a clear accumulation of thermal stress during the cooling process. The peak thermal stress in different regions is not the same, and it does not exceed the ultimate tensile strength of the asphalt mixture. The maximum thermal stress in Northeast and North China can reach 1.6 MPa, while it is approximately 0.6 MPa in Central China, 1.19 MPa in East China, and 0.98 MPa in Northwest China.
Severe and extreme cold waves are the main causes of thermal fatigue damage in asphalt mixtures. There are significant differences in the thermal fatigue lives of asphalt mixtures in different regions. The thermal fatigue lives of asphalt mixtures in Northeast and North China are short, while the thermal fatigue life of asphalt mixtures in Central China, East China, and Northwest China is long. Within the designed service life, asphalt mixtures in these regions will not experience thermal fatigue damage. It is recommended that the thermal fatigue stress threshold in Northeast and North China be set at 0.77 MPa, while in other regions, it should be 0.39 MPa.
This work clarifies the distribution of cold waves, determines the impact of cold waves on the thermal fatigue life of the asphalt mixture, and provides a technical approach for the design of thermal fatigue resistance of asphalt mixtures. In the future, the influence of asphalt with greater penetration, aggregate types, gradations, and binder content should be investigated, and more variables such as pavement structures in the FE simulation should also be conducted.