1. Introduction
As the global economy grows and people’s consumption levels rise, the automotive industry and global tire production proliferate. Relevant studies show that about 4 billion tires are discarded globally every year [
1]. Traditional waste rubber disposal methods, such as landfills and incineration, cause environmental pollution and damage human health [
2,
3]. Grinding waste tires into rubber powder and adding them to asphalt to make rubber asphalt pavement solves the above problems and significantly improves pavement performance [
4,
5]. Therefore, the technical means of rubber asphalt is increasing in attention and is gradually being used in road engineering. However, the production and construction of rubber asphalt produces a lot of harmful gases due to the high temperature, which limits its popularity and application.
At present, warm-mix technology mainly includes foaming warm-mix technology, emulsified asphalt warm-mix technology, and organic viscosity reduction warm-mix technology [
6,
7,
8,
9]; among them, the organic viscosity-reducing warm mix, represented by the Sasobit warm mix and the emulsified asphalt warm mix represented by Evotherm warm mix are widely used. Suppose waste tire rubber powder is added to asphalt using warm-mix technology. In that case, it not only reduces greenhouse gas emissions during the production and construction process but also reduces energy loss [
10,
11,
12]. Not only that, but with further research scholars have found that the use of warm-mix technology can improve the performance of rubber asphalt. Guo et al. [
10] found that warm-mix additives improved the low-temperature crack resistance of asphalt mixtures through freeze–thaw cycle tests. Shi et al. [
13] used dynamic mechanical analysis (DMA) and found that an organic viscosity-reducing temperature mix improves the high-temperature performance of warm-mix rubber asphalt. Bilema et al. [
14] found that Sasobit improves the stiffness and workability of rubber asphalt by comparing the effects of three warm mixes on the physical properties of rubber asphalt.
In addition to conventional performance studies, the fatigue performance of asphalt materials has emerged as a research priority and is aimed at enhancing the service life and road performance of asphalt pavements [
15,
16]. Under the long-term influence of vehicle loads, fatigue cracks develop in asphalt pavements, which seriously impacts their service life. Therefore, in recent years, scholars have successively carried out related research on the fatigue performance of warm-mix rubber asphalt and its mixtures. Wang et al. [
17] found that the addition of a warm mix can effectively improve the long-term fatigue performance of rubber asphalt by testing the fatigue performance of warm-mix rubber asphalt after long-term aging. Xiao et al. [
18] evaluated the effects of warm-mix asphalt additives on the fatigue characteristics of the rubber asphalt mixture from the perspectives of low temperature, aging, and fatigue. They found that warm-mix agents can effectively extend the fatigue life of rubber asphalt mixture. Kumar et al. [
19] compared the fatigue performance of different types of warm-mix rubber asphalt and its mixtures through various fatigue testing methods and found that the chemical warm-mix asphalt (WMA) additive improved the fatigue resistance of rubber asphalt more than the organic WMA additive. Wang et al. [
20] revealed the fatigue performance enhancement mechanism of warm asphalt rubber by removing crumb rubber modifier (CRM) particles through filtration, obtaining the liquid phases of asphalt rubber (AR) and four WAR binders, and performing fatigue tests and gel permeation chromatography (GPC) tests on them.
Based on the above literature analysis, current research on the fatigue performance of WMRA pavements by scholars mainly focuses on two directions: Firstly, the use of macroscale testing to analyze the influence of the type and dosage of warm-mix additives on the fatigue performance of WMRA and its mixtures, and secondly, the employment of microscale testing to explore the mechanisms by which warm-mix additives enhance the fatigue performance of modified asphalt. However, there has been relatively little research on the impact of asphalt material’s self-healing properties on the fatigue damage process. The asphalt material has a certain self-healing function (i.e., self-healing performance). Under certain conditions, the generated microcracks can be gradually recovered [
21,
22]. As the microcrack surface undergoes reconstruction, contact, and wetting, the stiffness of the asphalt begins to recover and the cracks gradually close. After the cracks close, the asphalt molecules begin to diffuse into each other, and eventually, the stiffness and strength of the asphalt become comparable to their original state [
23,
24,
25]. This lack of research contributes to the specific differences between fatigue life measured in the laboratory and fatigue life observed under actual pavement conditions. Moreover, asphalt mastic is the basis for forming asphalt mixtures, a key factor affecting the road performance of asphalt mixtures [
26,
27,
28]. In addition, the aging effect on the performance of asphalt pavement is unavoidable in practical applications. Therefore, to more accurately assess the service life of warm-mix rubber asphalt materials, it is necessary to carry out a systematic study on the fatigue performance and self-healing performance of asphalt mastic under different aging effects.
In summary, the aim of this paper is to analyze and compare the fatigue and self-healing properties of virgin asphalt mastic (VAM), rubber asphalt mastic (RAM), Sasobit rubber asphalt mastic (SRAM), and Evotherm rubber asphalt mastic (ERAM) under different degrees of aging, in order to assess the fatigue durability and self-healing capacity of warm-mixed rubber asphalt pavements in practical applications. To achieve this objective, four materials were aged in this study through short-term and long-term aging tests simulating everyday aging environments. The frequency scanning test, linear amplitude test, and multiple intermittent loading time scanning test were further carried out by a dynamic shear rheometer, and theoretical analyses were carried out by using the simplified viscoelastic continuum damage theory to provide theoretical basis and technical support for the application of warm-mix rubber asphalt pavements.
4. Conclusions
Through experimental tests and theoretical analyses, this paper evaluated the fatigue and self-healing performances of four asphalt mastics, VAM, RAM, SRAM, and ERAM, under simulated aging conditions, as well as the effect of self-healing behavior on fatigue performance. The following conclusions can be drawn:
- (1)
Both Sasobit and Evotherm can significantly enhance the fatigue performance of asphalt mastics, thereby extending their fatigue life. This is due to the fact that the former can form a solid lattice structure in the bitumen, while the latter can effectively increase the bond between the bitumen and the mineral powder. Specifically, under the long-term aging condition of 7% strain, the fatigue life of SRAM is 9.9 times as long as that of VAM and 2.4 times as long as that of RAM; the fatigue life of ERAM is 8.7 times as long as that of VAM and 1.7 times as long as that of RAM. In addition, SRAM shows better fatigue performance.
- (2)
The self-healing properties of four asphalt mastics under different aging states were tested and analyzed by multiple intermittent loading TS tests based on and RCM. In particular, after ten fatigue intermittent loadings in the long-term aging condition, the for VAM is only 43.2% of the for SRAM and 48.2% of the for ERAM, while the for RAM is only 76.1% of the N for SRAM and 84.8% of the for ERAM. Moreover, the values of RCM for long-term aging VAM and RAM after ten fatigue intermittent loading tests are 0.65 and 0.70, respectively, which are lower than the values of RCM for SRAM. This indicates that adding a warm-mix agent can improve the self-healing properties of asphalt mastic.
- (3)
The fatigue life of asphalt mastic at a 5% strain level before and after considering the effect of self-healing was calculated by the fatigue life conversion factor μ. A comparison reveals that the fatigue life of asphalt mastic is significantly prolonged after considering self-healing, which indicates that the self-healing property of asphalt mastic significantly enhances its fatigue performance.
- (4)
This indicates that aging has some adverse effects on fatigue performance and self-healing performance, and the deeper the aging, the more pronounced the adverse effects. This is because aging hardens the asphalt and reduces its fluidity, which affects the road’s life and the asphalt’s self-healing performance. At a 7% strain, the fatigue life of SRAM after long-term aging is only 30.7% of the fatigue life in the unaged state and 49.4% of the fatigue life in the short-term aging state. Moreover, after ten fatigue intermittent loads in the long-term aging state, the of the SRAM is only 53.2% of the in the unaged state and 58.8% of the in the short-term aging state.
- (5)
The comprehensive results of the study concluded that Sasobit has a better effect on improving the fatigue and self-healing performances of rubber asphalt mastic. Therefore, Sasobit warm mix is preferable in practical applications to improve the performance of rubber asphalt pavements. This study can provide a theoretical basis for the promotion and application of WMRA pavements and contribute to the sustainable development of road construction.
Based on the S-VECD theoretical system, this study provides an in-depth analysis of the fatigue durability and self-healing capacity of warm-mix rubber asphalt mastic under different aging conditions. This paper not only systematically examines how aging affects the fatigue properties and self-healing potential of this material but also further explores the positive role of self-healing performance on fatigue performance. This paper aims to lay a theoretical foundation for the wide application of warm-mix rubber asphalt pavements through this series of studies and to help promote the wide application of warm-mix rubber asphalt technology in the field of road construction.