1. Introduction
Fatigue failure is the primary form of distress in asphalt pavements, and researchers have investigated the fatigue performance of asphalt mixtures extensively [
1,
2,
3,
4]. Strain sweep tests reveal that the maximum stress at an increased strain corresponds to the shear strength, and the principle of thermal equivalence also applies to the shear strength and shear rate [
5]. The measured fatigue performances of asphalt mixtures under controlled conditions in accelerated loading facilities exhibit discrepancies from the actual fatigue performances of field materials [
6,
7]. Aging significantly affects the fatigue performance of asphalt mixtures, as evidenced by indirect tension and four-point bending tests [
2]. Moreover, several different experimental methods describe fatigue damage, with some scholars conducting direct tensile fatigue tests under stress-controlled modes [
8] or analyzing the fatigue performance of asphalt mixtures through fatigue tests based on energy methods [
9,
10]. Researchers have independently designed alternate bidirectional splitting fatigue tests to effectively simulate stress conditions in asphalt pavements [
11]. Numerous experiments have investigated the fatigue performance of asphalt mixtures through three-point and semicircular bending tests [
12,
13,
14,
15,
16,
17]. In addition to the diversity of fatigue test methodologies, multiple evaluation indicators exist for the fatigue damage of asphalt mixtures. Researchers have suggested that the stiffness modulus can serve as an evaluation index for the fatigue performance of asphalt mixtures. For instance, when the modulus decreases to 50% of its initial value during asphalt material fatigue testing, the material is considered fatigued [
18]. However, some scholars argue that reducing the stiffness modulus to 50% of the initial value may not entirely represent the fatigue failure criterion for asphalt mixtures, proposing that a reduction of 65% or within the range of 15–25% of the initial value would be more appropriate [
19,
20].
Currently, researchers enhance the fatigue performance of asphalt mixtures primarily through the modification of component materials [
21,
22,
23,
24,
25]. Investigations using fatigue performance tests have analyzed the fatigue resistances of conventional asphalt, rubber-powder-modified asphalt, and SBS-modified asphalt. The results demonstrated a significant enhancement in the fatigue performance of rubber powder- and SBS-modified asphalt compared to that of conventional asphalt, with SBS-modified asphalt exhibiting the best fatigue performance [
24]. Furthermore, polyurethane-modified asphalt mixtures display a good high-temperature performance and resistance to water damage. Compared to conventional asphalt, polyurethane-modified asphalt mixtures have lower production costs, presenting a measurable cost advantage [
26]. The addition of basalt fibers or diatomite to conventional asphalt also enhances the fatigue performance of asphalt mixtures. Basalt fibers improved the fatigue performance of the asphalt mixtures, whereas diatomite significantly enhanced their high-temperature stability. The simultaneous use of basalt fibers and diatomite resulted in an overall enhancement in the performance of asphalt mixtures [
27]. The use of multiple materials for asphalt mixture modification is a common and effective method in experimental studies [
22,
28,
29,
30]. For instance, nano-CaCO
3/SBR composite-modified asphalt mixtures exhibit superior high-temperature and fatigue performances compared to SBR-modified asphalt mixtures. Additionally, nano-ZnO/TiO
2/SBS composite-modified asphalt mixtures display enhanced water stability, resistance to aging, and splitting strength compared with SBS-modified asphalt mixtures [
31]. In addition to the improvement in asphalt mixture properties using different materials, some studies have approached modification through aggregates. For instance, the use of recycled aggregates derived from construction waste to prepare asphalt mixtures has been investigated [
32,
33,
34,
35]. Research indicates that when traffic volumes are low, recycled aggregates can replace 75% of natural aggregates to prepare asphalt mixtures, which are easier to compact, potentially saving resources during the construction compaction stages. However, indices such as the elastic and dynamic elastic moduli of asphalt mixtures decline with the increase in recycled aggregates [
36]. Several studies have found that asphalt saturation, the elastic modulus, and the void content deteriorate in asphalt mixtures made with recycled aggregates compared to those made with pure natural aggregates. Nevertheless, they still meet technical requirements and can be used depending on actual engineering needs [
37,
38,
39,
40,
41].
Gradation design is an important approach to achieve different fatigue performances in asphalt mixtures. Asphalt mixtures with different gradation curves exhibit notable differences in their sensitivity to test loading frequencies [
42,
43], and the gradation design also influences the void content of asphalt mixtures [
44,
45,
46]. Asphalt mixtures designed using the Bailey method, which is a gradation design approach, demonstrated a commendable fatigue performance [
47]. Moreover, a comparative study of two discontinuous graded rubberized asphalt mixtures revealed that when the 4.75 mm sieve passage rate was controlled to between 24% and 34%, rubber-modified asphalt displayed the optimal fatigue performance [
48]. Beyond common gradations such as AC-13 and AC-20, researchers have introduced LSAM-30, a large-sized asphalt mixture (LSAM), as a gradation type [
49]. In the design of LSAM-30, particular attention was paid to its drainage performance, ensuring smooth drainage of water from the pavement structure and thus effectively reducing the water damage and pavement deterioration issues caused by water accumulation [
30]. Through an in-depth investigation and testing using three molds with improved thicknesses, the rutting resistance of LSAM-30 was proven to be excellent [
50]. Additionally, an improved freeze–thaw splitting test further verified its good water damage resistance [
51]. Another study found that different compaction methods and coarse-to-fine aggregate ratios significantly affected the performance of LSAM-30 [
52]. When the asphalt content reached approximately 15%, LSAM-30 exhibited the most superior high-temperature rutting resistance [
53,
54]. Furthermore, through precise material proportioning optimization and reduced asphalt usage, LSAM-30 not only improves the production efficiency but also effectively reduces carbon emissions and energy consumption during production [
55]. These features endow LSAM-30 with significant advantages in terms of overall production costs and service life. Therefore, the widespread application of LSAM-30 will contribute to promoting the construction of a sustainable transportation infrastructure, reducing resource waste, and mitigating environmental pressures.
In conclusion, LSAM-30 demonstrates the capability to significantly enhance the longevity of roads, thereby having crucial implications for the sustainable advancement of road engineering. However, there are few studies specifically investigating the fatigue performance of LSAM-30. Moreover, there are no comparative studies on LSAM-30, AC-13, and AC-20 asphalt mixtures under identical test conditions. Therefore, this study uses four-point bending tests to investigate the fatigue characteristics of LSAM-30 by independently controlling the strain, frequency, and temperature. The aim was to establish fatigue equations based on different influencing factors and subsequently to conduct a comparative analysis of the fatigue performance of the three different gradations of asphalt mixtures. This analysis aimed to identify the disparities in fatigue performance among asphalt mixtures with different gradations and their underlying causes.
4. Conclusions and Outlook
4.1. Conclusions
In this study, four-point bending tests were conducted to compare and analyze the fatigue performances of LSAM-30, AC-20, and AC-13 under different frequencies (5, 10, and 15 Hz), temperatures (5, 10, and 15 °C), and strains (200, 400, 600, and 800 ). This investigation aimed to establish the fatigue characteristics of LSAM-30 and subsequently develop fatigue prediction equations based on the frequency and temperature factors for LSAM-30. The major conclusions drawn from this study are as follows:
- (1)
The fatigue performance of LSAM-30 varied under different loading frequencies. At lower frequencies, the internal material of LSAM-30 tends to maintain a stable structure, and the asphalt in LSAM-30 has more time to relax, which can help alleviate the displacement and friction between material particles, thereby reducing the impact of fatigue damage.
- (2)
A comparison of the fatigue results of LSAM-30, AC-20, and AC-13 under all conditions revealed that the fatigue performance ratio of the three asphalt mixtures did not vary significantly with strain but exhibited significant changes with variations in temperature and frequency.
- (3)
With an increase in temperature or frequency, the fatigue performance ratio of the three asphalt mixtures increased, indicating that the increase in temperature and frequency weakened the supporting effect of large-sized aggregates in LSAM-30 asphalt mixtures. In comparison to AC-13 and AC-20, the temperature or frequency is more likely to affect the fatigue performance of LSAM-30.
4.2. Outlook
In summary, under low-frequency conditions, LSAM-30 can effectively leverage the supportive role of its large aggregate particles. However, its fatigue performance tends to deteriorate under high-temperature or high-frequency conditions. Therefore, in practical engineering scenarios characterized by lower temperatures and lower load frequencies, prioritizing the selection of LSAM-30 is advisable. This approach will aid in enhancing the longevity of roads in low-temperature or low-frequency environments, consequently reducing the consumption of maintenance resources and funds while bolstering the sustainability of the road infrastructure.
This study determined the fatigue characteristics of LSAM-30 by selecting specific experimental parameters and established an exponential fatigue prediction equation for LSAM-30 based on temperature and frequency factors. However, the applicability of this equation requires further investigation. In addition, the limited number of experimental variables selected in this study for LSAM-30 suggests the potential for considering additional influencing factors. This could involve experimenting with suitable combinations of factors such as particle sizes, temperatures, frequencies, strains, asphalt types, and asphalt concentrations. These extended experiments significantly enhance the accuracy of the exponential prediction model. However, the predictive equations obtained in this study have not yet been widely applied or validated. In future work, they can be applied to predict the fatigue performance of other types of asphalt mixtures and will be continuously revised to enable their extensive application in fatigue performance studies of different asphalt mixtures.