1. Introduction
In response to the challenges of limited asphalt pavement service life and early deterioration, polymer-modified asphalt, predominantly SBS-modified asphalt, has gained widespread usage. Extensive applications and prolonged observations have affirmed its positive impact on road performance, garnering recognition within the transportation industry. However, the burgeoning market demand has led to a substantial surge in modifier prices, directly contributing to escalated road construction costs [
1]. Furthermore, the production of polymer-modified bitumen necessitates significant investment in process equipment, and issues such as modifier separation during transportation and storage introduce considerable fluctuations in the quality of modified asphalt. This has prompted the exploration of alternative modified asphalt technologies.
The utilization of natural bitumen to enhance matrix asphalt performance emerges as a pivotal avenue [
2]. Rock asphalt, formed under high temperature, high pressure, and diverse natural conditions, exhibits remarkable stability. Rigorous testing and engineering practices have substantiated its significant role in bolstering asphalt mixture stability [
3]. North American rock asphalt, a subject of extensive study, distinctly enhances the high-temperature stability, load resistance, and long-term stability of asphalt pavement. Its efficacy is particularly pronounced in regions with high temperatures, heavy loads, and special sections featuring substantial slopes. Consequently, North American rock bitumen has found extensive engineering applications in countries such as the United States, Japan, Australia, Germany, and Singapore [
4].
KOKBV et al. assessed the high-temperature performance of North American rock asphalt-modified asphalt using dynamic shear rheology and rotational viscosity tests, with SBS-modified asphalt as a reference. By substituting a portion of SBS-modified asphalt with an equivalent amount of rock bitumen, a rock asphalt content comparable to the high-temperature stability level of SBS-modified asphalt mixture was achieved [
5].
Mahmoud Ameri et al. incorporated varying proportions of EVA and North American rock bitumen into bitumen, conducting a comprehensive examination of high-temperature, low-temperature, and rheological properties through SHRP tests. Dynamic shear rheological tests revealed significant increases in rutting resistance factors for both modified asphalts, indicating improvements in high-temperature stability. However, bending creep stiffness tests demonstrated a compromise in low-temperature performance for rock bitumen. Simultaneously, rotational viscosity test results indicated that neither modifier adversely affected the viscosity of the matrix asphalt [
6].
Ruixia Li et al. conducted rheological and bending creep stiffness tests on diverse natural rock bitumen-modified bitumens, determining the PG classification of modified bitumens with varying rock bitumen content. Frequency temperature sweep and creep relaxation tests indicated a partial reduction in the relaxation potential of asphalt mixture at low temperatures due to rock asphalt. Nevertheless, rock asphalt demonstrated the capability to synthesize hard billet with excellent performance. Long-term aging tests underscored the significant role of natural rock bitumen in retarding the aging rate of asphalt mixture [
7].
Mehmet Yilmaz et al. compared the properties of three natural bitumens (Trinidad Lake bitumen, North American rock bitumen, and Iranian rock bitumen) with SBS-modified bitumen. The determination of the PG fraction through rutting resistance factor assessment revealed that Trinidad Lake bitumen achieved optimal modulus, stability, tensile strength, and resistance to permanent deformation at 60% content. North American rock bitumen exhibited superior water damage resistance at 9.5% [
7].
Nene Kusnianti et al. conducted a comparative analysis of Lawele rock bitumen from Indonesia and Kabungka rock bitumen, assessing springback modulus, high-temperature performance, and water stability. Lawele rock bitumen outperformed Kabungka rock bitumen across various technical indicators. Furthermore, the study highlighted that, when designing the gradation of rock asphalt mixture using the Fuller curve, opting for gradation above the curve yielded superior road performance [
8].
Williams G extensively investigated the rutting resistance and water stability of budunite bitumen. However, with the progressive incorporation of the active agent, its low-temperature crack resistance exhibited a nonlinear change [
9].
Karami M investigated the impact of Budunite asphalt as an active agent on pavement performance, revealing that the segmented addition of Budunite asphalt modifier to the asphalt mixture significantly altered its properties and extended pavement life [
1]. Liu S et al. delved into the specific properties of Buton-modified rock bitumen as an active agent, demonstrating that the addition of an appropriate active agent could enhance the fracture resistance of the pavement [
3]. Lv S et al. conducted an analysis and verification of the modification mechanism of Butonite asphalt. The study showcased its effectiveness in improving the pavement’s high-temperature performance and conducted comprehensive research on its freeze–thaw effect, providing further evidence of the modification impact of Butunite asphalt [
10]. Li Y et al. performed mechanical experiments on asphalt mixture with rock asphalt as an active agent, resulting in significantly improved aging resistance and high-temperature resistance of the pavement modified by Budun rock asphalt [
7].
In the pursuit of enhancing pavement performance, Zhong K et al. explored the impact of adding various doses of modifiers, such as 0%, 5%, 10%, 15%, and 20%, to petroleum asphalt in Xinjiang. The study revealed diverse changes in fatigue resistance, low-temperature resistance, high-temperature resistance, tensile strength, compressive strength, and moisture sensitivity. The asphalt mixture exhibited good bonding with the asphalt binder, leading to a general improvement in pavement high-temperature resistance [
11]. Li Y et al. utilized atomic microscopy to examine the microscopic properties of Budunite bitumen before and after activation. The viscosity of the modified bitumen post-activation significantly increased, highlighting a more pronounced modification effect [
12]. To enhance the aging resistance of Butunite asphalt, Lv S et al. scrutinized its surface flatness, void-free internal structure, fracture-free aging degree, and other related properties. This research not only reduced material costs to a certain extent but also generated improved ecological benefits [
13]. Zou G and Wu C, through rheological properties analysis of Budunite bitumen, found that BRA-modified bitumen as a modifier had distinct effects on asphalt concrete and asphalt binder. Under different proportions in the penetration test, the softening point of the binder and mixture showed varying trends. Extensive experiments revealed that the overall rheological properties and characteristics of BRA-modified mixture surpassed those of unmodified asphalt mixture [
14].
In the realm of enhancing the physical and chemical properties of mixed pavement materials, Zamhari K A et al. employed activation treatment and other methods for in-depth analysis and research on the aging resistance, mechanical properties, and high-temperature resistance of asphalt pavement. The addition of 30% content of Budunite modifier optimized the workability and performance of the modified asphalt [
15]. Jing L et al. investigated the physical and chemical properties of BRA asphalt, applying their findings to the optimization of road pavement and subsequently improving the overall road performance of asphalt pavement [
16].
Waste rubber, a polymeric elastic material, poses a formidable challenge in its natural state due to its resistance to degradation for decades or even centuries. This not only depletes natural resources but also poses severe environmental hazards. Recycling of rubber can be achieved through various methods, including surface treatment, grinding, shredding technologies, and desulfurization technologies involving chemical, ultrasonic, and microwave processes, among others. In civil engineering practice, the applications of recycled rubber primarily include the following: (1) serving as modifiers in asphalt paving mixtures; (2) acting as an additive to Portland cement concrete; (3) functioning as lightweight fillers [
17].
The exceptional elasticity and viscosity of waste rubber make it suitable for grinding into specific rubber powder, offering a cost-effective solution in the modification of road petroleum asphalt [
18]. In contrast to conventional rubber, desulfurized rubber, characterized by a relatively low molecular weight and the presence of a binder, exhibits favorable high-temperature storage stability. It undergoes chemical reactions with the binder, addressing issues such as asphalt binder segregation and ensuring improved compatibility with asphalt. Notably, despite the larger particle size of desulfurized rubber powder, minimal debris remains at the conclusion of the expansion process, effectively absorbing the light oil component of the asphalt [
19,
20].
Consequently, the utilization of North American rock asphalt and desulfurized rubber particles in modifying matrix asphalt has been explored. Indoor tests, including high-temperature rutting, low-temperature trabecular bending, indirect tensile fatigue, and freeze–thaw splitting, have been conducted to assess the performance changes in dry and wet mixture specimens. The aim is to offer insights into the promotion and application of natural rock asphalt-modified asphalt mixtures and contribute to the development of technical standards.
3. Results and Discussion
3.1. Microstructure Performance of Rock Asphalt-Modified Asphalt
A study was conducted on the modification mechanisms of rock asphalt and devulcanized rubber [
26]. Fourier transform infrared (FTIR) spectroscopy tests were performed on single rock asphalt-modified asphalt, single rubber-modified asphalt, and devulcanized rubber–rock asphalt composite-modified asphalt, as shown in
Figure 8. The infrared spectra of the composite-modified asphalt made from devulcanized rubber and rock asphalt did not exhibit any new peaks compared to the single modified asphalts, indicating that the mixing of devulcanized rubber, rock asphalt, and base asphalt is a physical process without chemical reactions. The distinct peak observed at 956 cm
−1 in SBS-modified asphalt relative to PG 64-22 base asphalt is associated with the bending vibration in the butadiene double bond. Butadiene is a component of styrene–butadiene–styrene (SBS) copolymers, which are commonly used as modifiers in asphalt to improve its properties. The presence of a peak at this wavelength in modified asphalt containing devulcanized rubber suggests that the devulcanized rubber contains components similar to butadiene in SBS. This indicates that the devulcanized rubber may have some chemical similarity to SBS, which could contribute to similar properties in the modified asphalt. The infrared spectra of the composite-modified asphalt made from devulcanized rubber and rock asphalt did not exhibit any new peaks compared to the single modified asphalts. This suggests that the mixing of devulcanized rubber, rock asphalt, and base asphalt is a physical process without chemical reactions. In other words, the components are blended together without altering their chemical structures [
27].
The absence of new peaks in the composite-modified asphalt indicates that there are no new chemical bonds formed between the devulcanized rubber, rock asphalt, and base asphalt. Instead, the properties of the composite are likely due to the physical interaction and blending of the components.
The microscopic morphologies of rock asphalt and devulcanized rubber in the modified asphalts were also investigated, as shown in
Figure 9. For the single rock asphalt-modified asphalt (64-10G2-0R-U), ash particles (yellow bright spots in the image) were observed under the microscope, indicating that the asphalt in the rock asphalt had merged with the base asphalt, leaving the remaining free ash suspended in the asphalt. As for the devulcanized rubber-modified asphalt (64-0G2-18R-U), it was observed under a microscope that a portion of the devulcanized rubber had dissolved in the base asphalt and released carbon black, but there was still some rubber suspended in the base asphalt. The dispersion of devulcanized rubber in the asphalt was relatively uniform, with only a small portion of the rubber adhering together without dispersion. For the composite-modified asphalt with both devulcanized rubber and rock asphalt (64-10G2-18R-U), the microscope images revealed that the dispersion of devulcanized rubber in the asphalt was even more uniform, and the interfaces between the base asphalt, rock asphalt, and devulcanized rubber were also more uniformly blended. This indicated that rock asphalt contributed to the binding with devulcanized rubber and the dispersion of devulcanized rubber to some extent.
3.2. High-Temperature Stability Result
According to the results of the rutting tests, the development of high-temperature rutting deformation with loading durations is shown in
Figure 10, and the results of dynamic stability are detailed in
Table 11 and
Figure 11.
In asphalt mixtures, which are crucial materials in road construction, high-temperature stability is directly linked to road durability and driving safety. Under high-temperature conditions, asphalt mixtures are prone to rutting deformation, which affects the road surface’s smoothness and service life. Rock asphalt, serving as a natural modifier, can effectively enhance the high-temperature performance of asphalt mixtures due to its unique physicochemical properties. Based on the results of high-temperature rutting tests, this paper provides an in-depth analysis of the high-temperature stability of rock asphalt-modified asphalt mixtures [
28].
As shown in
Figure 10, the relationship curve between high-temperature rutting deformation and time visually displays the deformation of each sample at different time points. It can be observed from
Figure 10 that the deformation of all samples increases with time, but the growth rates vary. Specifically, Groups A, B, and C exhibit relatively rapid deformation growth in the early stages of the test, while Group D shows a relatively slow deformation growth.
Table 11 lists the specific deformation data for each sample at t
1 = 45 min and t
2 = 60 min. Notably, Group D has the smallest deformation, which is 2.199 mm and 2.337 mm, significantly lower than the deformation of the other three groups. This indicates that the rock asphalt modifier significantly reduces the high-temperature deformation of asphalt mixtures, thereby enhancing their rutting resistance.
Dynamic stability (DS) is a critical indicator for measuring the high-temperature stability of asphalt mixtures.
Table 11 shows that Group D has the highest DS, reaching 4621 times/mm, far higher than the other three groups. The increase in DS indicates that the asphalt mixture has stronger resistance to deformation under high-temperature conditions. Further analysis reveals that as deformation decreases, DS exhibits an upward trend, indicating a clear negative correlation between them.
Figure 11 visually presents the DS test results for each sample. It can be seen that the DS of rock asphalt-modified asphalt mixtures is significantly better than that of unmodified or low-content-modified asphalt mixtures, further confirming the positive effect of rock asphalt modifiers on enhancing the high-temperature stability of asphalt mixtures. Under high-temperature conditions, the increase in the deformation of asphalt mixtures over time is inevitable. However, the addition of rock asphalt modifiers significantly slows down this growth trend. Through comparative analysis, it is evident that the deformation of modified asphalt mixtures within the same time frame is significantly reduced, indicating enhanced high-temperature deformation resistance. This may be attributed to certain components in rock asphalt that chemically react or physically interlock with the asphalt, thereby improving the overall strength and stiffness of the mixture [
10].
As a key indicator for evaluating the high-temperature stability of asphalt mixtures, a higher DS value indicates stronger resistance to high-temperature deformation. The test results show that the DS of rock asphalt-modified asphalt mixtures is significantly improved, primarily due to the modifier’s enhancement of asphalt properties. On the one hand, the modifier enhances the viscosity and cohesion of the asphalt; on the other hand, the interaction between the modifier and the aggregate also contributes to improving the overall stability of the mixture. Therefore, in practical applications, adding an appropriate amount of rock asphalt modifier can significantly enhance the high-temperature stability of asphalt mixtures.
Through the analysis of high-temperature rutting test data, it is found that rock asphalt-modified asphalt mixtures exhibit excellent high-temperature stability under high-temperature conditions. Specifically, the growth rate of deformation over time is reduced, and the final deformation is relatively small; simultaneously, the significant increase in DS indicates enhanced resistance to high-temperature deformation. Therefore, it can be concluded that rock asphalt modifiers are an effective way to enhance the high-temperature stability of asphalt mixtures. In future road engineering projects, the further promotion and application of rock asphalt-modified asphalt mixtures should be considered to improve road service life and driving safety.
3.3. Cracking Resistance Result at Low Temperatures
The results of the low-temperature bending test (as shown in
Table 12 and
Figure 12) reveal a significant impact of different processing methods and material compositions on the low-temperature crack resistance of the samples. Under the condition of a rubber-based asphalt ratio of 18%, the A-group samples, processed through the wet method, exhibit superior low-temperature crack resistance compared to the dry method. This is primarily manifested in their higher flexural tensile strength and flexural modulus. Specifically, the flexural tensile strength of the A-group reaches 11.94 MPa, significantly higher than that of the B-group (9.94 MPa) and the C-group (9.07 MPa). Although the D-group also shows a relatively high flexural tensile strength (10.50 MPa), it still does not surpass the A-group. This trend is also reflected in the flexural modulus, with the A-group leading with 4431.79 MPa, followed by the D-group (3957.01 MPa), while the values for the B-group and C-group are relatively lower.
The reason why the A-group samples exhibit such excellent performance in the low-temperature bending test is mainly attributed to the uniqueness of their modification and processing method. This method may contribute to optimizing the dispersion of rubber particles in asphalt, enhancing the interaction between the rubber and asphalt and thereby strengthening the overall strength and toughness of the asphalt mixture. In contrast, although the B-group and C-group also adopt different processing methods, their effects are not as significant as those of the A-group, resulting in lower levels of flexural tensile strength and flexural modulus [
29].
Further analysis reveals that as the content of devulcanized rubber increases (from the A-group to the D-group), the low-temperature crack resistance of the samples shows a downward trend. This phenomenon can be explained in two ways: Firstly, the increase in rubber content leads to a relative reduction in the content of base asphalt in the mixture, which subsequently reduces the thickness of the asphalt film covering the aggregate. The thickness of the asphalt film is one of the key factors affecting the bonding performance between asphalt and aggregate. A thinner film means a weaker bonding force, thereby reducing the crack resistance of the samples. Secondly, when the rubber content is too high, not all rubber particles can be completely dissolved in the base asphalt, and the undissolved rubber particles become weak points in the mixture, further weakening the overall performance of the samples [
30].
The results of the low-temperature bending test not only reflect the impact of different processing methods and material compositions on the low-temperature crack resistance of the samples but also reveal the complex relationship between rubber content and the performance of asphalt mixtures. To achieve better low-temperature crack resistance, it is necessary to ensure the complete dissolution of rubber particles, reasonably control the amount of rubber added, and optimize the modification or processing methods to enhance the bonding performance between the asphalt and the aggregate.
3.4. Tensile Strength Result
The load–displacement curve of the IDEAL-CT for the rock asphalt–devulcanized rubber compound-modified asphalt mixture is depicted in
Figure 13. The calculation results of IDEAL-CT indicators are presented in
Table 13 and
Figure 14. By comparing the samples from Groups A and B, it is evident that Group B exhibits higher indirect tensile strength and CT index values compared to Group A, indicating that the dry preparation method for the rock asphalt–devulcanized rubber compound-modified asphalt mixture yields slightly better mid-temperature crack resistance than the wet preparation method. Furthermore, upon contrasting the samples from Groups B, C, and D, it is observed that as the dosage of devulcanized rubber powder increases, both the indirect tensile strength and CT index of the mixture gradually decrease. Notably, the CT index appears to be more sensitive to changes in the dosage of devulcanized rubber powder compared to indirect tensile strength [
10,
31].
Specifically, Group B’s indirect tensile strength of 1.56 MPa surpasses that of Group A (1.53 MPa), suggesting that the dry preparation method imparts superior mid-temperature crack resistance to the mixture. This advantage may stem from the improved mixing effect and uniform distribution of asphalt and devulcanized rubber achieved through the dry preparation process. Additionally, as the dosage of devulcanized rubber powder increases from Group B to Group D, the indirect tensile strength decreases sequentially (1.56 MPa → 1.46 MPa → 1.44 MPa). This suggests that while the inclusion of devulcanized rubber powder enhances the flexibility of the material, excessive amounts weaken its crack resistance [
32].
Concurrently, Group B’s CT index of 69.72 is higher than that of Group A (67.10), further substantiating the superiority of the dry preparation method. Notably, the CT index decreases significantly with the increase in devulcanized rubber powder dosage (69.72 → 47.23 → 43.82), and its sensitivity to dosage changes is more pronounced than that of the indirect tensile strength. This indicates that the CT index serves as a more refined indicator, capable of earlier reflecting variations in the material’s crack resistance toughness.
The dry preparation method (Group B) outperforms the wet preparation method (Group A) in terms of mid-temperature crack resistance. While a moderate dosage of devulcanized rubber powder, such as 1%, optimizes the material’s crack resistance, excessive amounts lead to performance degradation. This phenomenon could be attributed to the interaction between devulcanized rubber powder and asphalt, as well as its distribution within the mixture. Therefore, rigorous control of the devulcanized rubber powder dosage is crucial in practical applications [
33].
The heightened sensitivity of the CT index to changes in devulcanized rubber powder dosage underscores its effectiveness as an evaluation metric for material crack resistance toughness. This suggests that the optimal dosage of devulcanized rubber powder for the rock asphalt–devulcanized rubber compound-modified asphalt mixture is 1%, as any increase beyond this point diminishes crack resistance and accelerates crack propagation. When optimizing the anti-fatigue properties of this mixture, a comprehensive consideration of factors such as preparation method, devulcanized rubber powder dosage, material temperature sensitivity, and actual application environment is necessary. By adjusting these factors, a comprehensive enhancement of material properties can be achieved [
34].
3.5. Tensile Fatigue Performance
The fatigue curves from indirect tensile tests on the mixture specimens are shown in
Figure 15. The vertical strain curves of asphalt mixtures in the indirect tensile fatigue test exhibit a typical three-stage characteristic. In the first stage, the vertical strain increases sharply under cyclic loading, indicating the initiation of damage accumulation and the formation of initial cracks within the material. The second stage is characterized by a linear growth trend in the fatigue curve, where the development rate of micro-cracks remains constant, reflecting the material’s stability under continuous loading. In the third stage, the vertical strain accelerates rapidly, growing exponentially, until the sample undergoes fatigue failure, signifying that the material can no longer withstand further loading [
1].
As shown in
Table 14 and
Figure 16, the fatigue curve of the wet process group (Group A) exhibits the highest slope in the second stage, indicating the fastest development rate of micro-cracks, the shortest fatigue life, and the poorest fatigue resistance. The dry process groups include B, C, and D. Group B (with 1% desulfurized rubber powder content) shows slightly weaker fatigue performance than Groups C and D but exhibits better fracture toughness and stronger resistance to deformation. Group C (with 1.5% desulfurized rubber powder content) demonstrates the longest fatigue life and the strongest ability to resist repeated loading, but it exhibits weaker toughness and resistance to deformation in the ultimate failure state. Compared to Group C, Group D (with 2% desulfurized rubber powder content) has an increased desulfurized rubber powder content of 0.5%, yet its CT index and fatigue life decrease, suggesting that the desulfurized rubber powder content should not exceed 1.5%. The experimental results indicate that the rock asphalt–desulfurized rubber composite-modified asphalt mixture prepared using the wet process (Group A) performs significantly worse in the fatigue test than those prepared using the dry process. Specifically, Group A exhibits the highest slope in the second stage of the fatigue curve, indicating the fastest development rate of micro-cracks, which leads to the shortest fatigue life and poorest fatigue resistance. In contrast, the samples prepared using the dry process (Groups B, C, and D) exhibit more stable behavior during fatigue testing, especially Group C, which demonstrates the longest fatigue life and best fatigue resistance.
The desulfurized rubber powder content has a significant impact on the fatigue performance of asphalt mixtures. The experimental data show that when the desulfurized rubber powder content is 1.5% (Group C), the fatigue life of the samples reaches its maximum, and the data stability is the best (with the lowest coefficient of variation). This indicates that a content of 1.5% may be the optimal content for this material system, effectively enhancing the material’s fatigue resistance. However, when the desulfurized rubber powder content increases to 2% (Group D), the fatigue life and CT index of the samples decrease, suggesting that excessively high content is not beneficial for improving the material’s fatigue performance.
Table 14 visually presents the fatigue life (N
f) of different sample groups. Group A has the lowest fatigue life (21,449 cycles), indicating the poorest fatigue resistance. Group C has the highest fatigue life (48,257 cycles), indicating the best fatigue resistance. Groups B and D are intermediate, but Group B is slightly higher than Group D. Group A has the largest coefficient of variation (21.68%), indicating the highest data dispersion and poor stability. Group C has the smallest coefficient of variation (2.06%), indicating the most stable data and good repeatability.
Samples prepared using the wet process exhibit significantly weaker fatigue performance than those prepared using the dry process. When the desulfurized rubber powder content is 1.5%, the samples exhibit the longest fatigue life and best fatigue resistance. Beyond 1.5%, the fatigue life and CT index of the samples decrease, indicating the existence of an optimal content level. Samples with longer fatigue life (such as Group C) typically have a lower coefficient of variation, indicating better data stability and repeatability.
Table 14 and
Figure 16 reveal a close relationship between fatigue life and data stability. Specifically, samples with longer fatigue life (such as Group C) typically have a lower coefficient of variation, indicating more stable and repeatable experimental data. This finding further confirms the reliability and validity of the experimental results and provides strong support for subsequent material optimization and engineering design [
35].
The fatigue performance and influencing factors of rock asphalt desulfurized rubber composite modified asphalt mixture were systematically studied through indirect tensile fatigue testing. The experimental results indicate that dry method preparation is superior to wet method preparation; there is an optimal value for the dosage of desulfurization rubber powder (1.5%), and a dosage that is too high or too low is not conducive to improving the fatigue performance of the material. In addition, specimens with longer fatigue life usually have better data stability and repeatability. These findings provide important references for the modification research of asphalt mixtures and scientific basis for the selection and application of road engineering materials [
36].
3.6. Freeze–Thaw Splitting Test Result
The TSR (tensile strength ratio after freeze–thaw cycles) is a crucial indicator for evaluating the water stability of asphalt mixtures. As evident from
Table 15 and
Figure 17, TSR values are generally high, fulfilling the regulatory requirements across different groups, albeit with notable variations. This variation stems from the intrusion of water into the interface between asphalt and aggregates during freeze–thaw cycles, which weakens their adhesion, subsequently reducing the splitting tensile strength. As the dosage of desulfurized rubber powder increases (from Group B to Group D), TSR values exhibit a downward trend. This indicates that, while the addition of rubber content may enhance the high-temperature stability of asphalt mixtures, it concurrently deteriorates their water stability [
37].
Specifically, Group A’s splitting tensile strength decreases from 0.509 MPa before freeze–thaw cycles to 0.455 MPa afterward, a reduction of approximately 0.054 MPa, or 10.6%. This significant decrease underscores the profound impact of water intrusion on the asphalt–aggregate interface during freeze–thaw processes. Conversely, Group B’s strength diminishes to a lesser extent (0.015 MPa or 2.9%), possibly attributable to the moderate dosage of desulfurized rubber powder, hinting at its potential positive role in maintaining water stability [
32,
38].
Group C’s initial strength, slightly lower than Groups A and B, drops by 0.044 MPa (9.1%) post freeze–thaw, which is comparable to Group A’s decrease but more pronounced in relative terms due to its lower starting point. Group D, with the highest rubber content, experiences the most substantial decrease (0.063 MPa or 12.6%), emphasizing the detrimental effect of excessive desulfurized rubber powder on water stability.
Asphalt–desulfurized rubber composite-modified asphalt mixtures prepared by the dry process exhibit superior water stability compared to the wet process. This superiority stems from the more uniform mixing of asphalt, aggregates, and desulfurized rubber powder during the dry process, fostering stronger interfacial adhesion and enhancing the water stability of the mixture. Group B boasts the highest TSR value of 97.18%, underscoring its exceptional water stability performance. Group C, with a TSR of 90.99%, also demonstrates good water stability, albeit slightly lower than Group B. While Groups A and D’s TSRs of 89.46% and 87.46%, respectively, are relatively lower, they still meet regulatory standards, suggesting acceptable water stability in practical applications.
The relationship between TSR and the dosage of desulfurized rubber powder is evident from the declining TSR values (97.18% > 90.99% > 87.46%) as the rubber content increases (Group B < Group C < Group D). This trend reinforces the notion that increased rubber dosage weakens asphalt mixtures’ water stability by potentially enhancing porosity, facilitating water intrusion and compromising asphalt–aggregate adhesion.
Despite the adverse effect of higher desulfurized rubber powder content on water stability, all tested mixtures’ TSR values surpass regulatory thresholds, indicating satisfactory performance in real-world applications. However, selecting an appropriate rubber dosage tailored to specific environmental conditions and project requirements is crucial for achieving optimal overall performance [
39].
4. Conclusions
In this study, North American rock asphalt and desulfurized rubber particles were used to modify base asphalt. Through laboratorial tests, including high-temperature rutting tests, low-temperature bending tests, indirect tensile tests, and freeze–thaw splitting tests, the service performance of the mixture specimens with different rubber-to-asphalt ratios from 18 to 36% was investigated. The effects of blending methods (i.e., the dry method and the wet method) were also compared. Based on the results of this study, several conclusions can be drawn, as follows:
- (1)
The blending method used significantly influences the performance profile of modified asphalt mixtures. The dry blending method excels in terms of fatigue life and water stability, while the wet blending method shows better low-temperature crack resistance. This finding underscores the importance of selecting the appropriate blending method based on the specific pavement requirements and climatic conditions.
- (2)
The use of desulfurized rubber particles notably improves the anti-aging properties of the modified asphalt, suggesting improved durability against UV radiation and oxidative stress. This enhanced resilience can translate into longer-lasting pavement surfaces, reducing maintenance costs and the environmental impacts associated with frequent repairs.
- (3)
Microscopic analysis reveals the uniform dispersion of desulfurized rubber in the presence of rock asphalt, facilitating better interaction with the asphalt binder. This insight offers a foundation for further optimizing mix designs through targeted additive selection and blending protocols to maximize dispersion and adhesion at the microscale.
- (4)
While increased rubber content enhances high-temperature stability, it compromises water stability.
- (5)
The successful incorporation of desulfurized rubber waste into asphalt mixtures underscores its potential as a sustainable construction material. This approach not only reduces waste disposal challenges but also contributes to circular economy initiatives by repurposing end-of-life products.
- (6)
This study presents a comprehensive evaluation framework encompassing high-temperature rutting, low-temperature cracking, fatigue life, and water stability tests. This framework serves as a valuable reference for future studies exploring novel asphalt modification strategies, emphasizing the need for multi-faceted performance analysis.
In summary, this paper presents a groundbreaking investigation into the performance of desulfurized rubber and rock asphalt-modified asphalt mixtures, offering innovative insights into blend optimization, environmental resilience, microstructural engineering, climate-specific tailoring, waste utilization, and comprehensive performance assessment. These conclusions lay the groundwork for the development of next-generation asphalt mixtures tailored to meet the evolving challenges of modern pavement engineering.