Performance of Asphalt Mixtures Modified with Desulfurized Rubber and Rock Asphalt Composites

Abstract

This study explores the performance of asphalt mixtures modified with North American rock asphalt and desulfurized rubber particles at varying rubber-to-asphalt ratios ranging from 18% to 36% by weight. A comprehensive set of laboratory tests, including high-temperature rutting tests, low-temperature bending tests, indirect tensile tests, and freeze–thaw splitting tests, were conducted to evaluate the modified mixtures. The results indicate that both wet and dry blending methods produce mixtures that meet technical requirements, with the optimal asphalt-to-aggregate ratio determined to be 7.1%. At a rubber-to-asphalt ratio of 18%, the wet blending method slightly improves high-temperature rutting resistance compared to the dry method. However, an increase in rubber content generally enhances rutting resistance regardless of the blending technique. The wet blending method excels in low-temperature crack resistance, possibly due to better rubber dispersion, while an increase in rubber content diminishes crack resistance due to a thinning asphalt film. In terms of fatigue performance, the dry blending method results in significantly longer fatigue life, with a 27% rubber-to-asphalt ratio exhibiting optimal balance. The dry method consistently outperforms the wet method in water stability, and the resistance to water damage increases with rubber content. In conclusion, this study provides valuable insights into optimizing rubber-to-asphalt ratios and blending methods for various application needs, showcasing the benefits of rock asphalt and desulfurized rubber particles in asphalt modification.

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.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt

This study was conducted within the context of a pavement construction project on the Yueluo–Baoqian Highway in Shanghai, China. The base asphalt employed was 70# road asphalt binder, designed for heavy traffic conditions. The modifiers utilized in this study included North American rock asphalt and desulfurized rubber. Essential technical indicators for North American rock asphalt and the base asphalt are outlined in

Table 1. Main technical indicators of North American rock asphalt.

Technical Indicators	North American Rock Asphalt
Color	Black powder
Rock asphalt category	Class I
Bitumen content (%)	≥90
Ash content (%)	≤2
Density (g/cm3)	1.06
Flashpoint (°C)	316
Water content (%)	<0.5
Softening point (°C)	160~185

and

Table 2. Main technical indicators of base asphalt.

Technical Indicator	Unit	Requirements	Test Results	Test Methods [21]
Density (at 15 °C)	g/cm3	<1.200	1.034	T0603
Penetration (at 25 °C, 100× g, 5 s)	0.1mm	60~80	66	T0604
Penetration index	-	−1.5 to 1.0	−1.24	T0604
Ductility (at 15 °C, 5 cm/min)	cm	≥100	≥100	T0605
Softening point (by ball and ring method)	°C	≥46	53.1	T0606
Kinematic viscosity (at 60 °C)	Pa·s	≥180	215.6	T0620
Solubility (trichloroethylene)	%	≥99.5	99.89	T0607
Penetration ratio	%	≥61	67	T0604
Mass loss	%	−0.8~+0.8	−0.255	T0609
Elongation (10 °C)	cm	≥6	7	T0605

. In the blending process, high-speed shear mixers were employed to amalgamate the base asphalt with the modifiers, namely, rock asphalt and desulfurization rubber, as shown in Figure 1 and Figure 2.

The performance indicators of desulfurized rubber are shown in

Table 3. Main technical indicators of desulfurized rubber.

Technical Indicator	Unit	Test Results
Particle size	mesh	40
Sol content	%	72
Mooney viscosity	Pa·s	15

.

2.1.2. Fillers

Mineral powders with a size smaller than 0.075 mm were used as fillers in the asphalt mixture to fill tiny voids. Fillers also form a mortar together with the asphalt binders and can improve the strength and stability of the mixtures. The mineral powders used in this study are produced from grinded limestone, and the technical specifications of the fillers are shown in

Table 4. Technical specifications for fillers.

	Property		Measured Value	Requirement	Test Methods
Indoor test mineral powder	Apparent density (t/m3)		2.675	≥2.50	T0352
	Water content (%)		0.4	≤1.0	T0103
	Particle size range	<0.6 mm (%)	100	100	T0351
		<0.15 mm (%)	97	90~100
		<0.075 mm (%)	88.4	70~100
	Hydrophilicity		0.81	<1	T0353

.

2.1.3. Fiber

Fiber as a stabilizer added to the asphalt mixture can effectively reduce low-temperature cracking, high-temperature rutting, and several other pavement distresses. The mechanism is that fiber increases the viscosity of asphalt so that the asphalt is firmly bonded with the surface of the aggregate. Fiber as a modifier greatly improves the anti-aging ability of asphalt, so that the asphalt mixture can obtain a longer service life under the action of environmental factors such as external load and temperature, and it can significantly enhance the road performance of SMA mixtures. The fiber stabilizers commonly used in asphalt mixtures mainly include lignin fiber, mineral fiber, and polymer fiber. Lignin fiber was used in the mixtures of this study with a ratio of 0.3% by weight. The technical specifications of the lignin fiber are shown in

Table 5. Technical specifications of lignin fiber [22].

Technical Indicators	Lignin Fiber
Appearance color	Grizzly
Fiber length (mm)	<6
Ash content (%)	18.7
PH	7.3
Oil absorption rate (%)	6.8
Water content (%)	4
Density (g/cm3)	0.87

.

Stone mastic asphalt (SMA) mixtures with a maximum aggregate size of approximately 13.2 mm, denoted as SMA-13, were employed in both the project and in this study.

2.1.4. Aggregates

Aggregates play a crucial role in asphalt mixtures, with their mechanical properties significantly influencing the strength of these mixtures. The particle shape of aggregates contributes to the skeleton structure of the asphalt mixture, consequently impacting rutting resistance and fatigue performance. Furthermore, the bonding performance between aggregates and binder is a key factor affecting the water stability and overall durability of the mixtures. In this study, four grades of aggregates and mineral powder were utilized. The density parameters and gradings of these materials are presented in

Table 6. Aggregate density.

Aggregate Grade (mm)	Mineral Fines	0–3	3–5	5–10	10–15
Relative bulk density	/	2.662	2.835	2.885	2.891
Apparent relative density	2.735	2.726	2.895	2.939	2.946

and

Table 7. Results of aggregate gradings.

Sieve Size (mm)	Mineral Fines	0–3	3–5	5–10	10–15	Target Gradation	Actual Gradation
Proportion (%)	9	15	5	38	33
16	100.0	100.0	100.0	100.0	100.0	100.0	100.0
13.2	100.0	100.0	100.0	100.0	75.0	95.0	91.8
9.5	100.0	100.0	98.8	86.7	4.8	62.5	63.5
4.75	100.0	98.9	61.9	4.3	0.3	27.0	28.7
2.36	100.0	89.4	5.5	0.9	0.3	20.5	23.1
1.18	100.0	62.8	1.2	0.6	0.3	19.0	18.8
0.6	100.0	36.9	0.7	0.3	0.3	16.0	14.8
0.3	100.0	15.4	0.7	0.3	0.3	12.5	11.6
0.15	97.0	7.4	0.7	0.3	0.3	12.0	10.1
0.075	88.4	5.6	0.7	0.3	0.3	10.0	9.0

, respectively.

Mixture performance depends significantly on gradation types. Gap-graded asphalt mixtures are characterized by high proportions of coarse materials, mineral powder, asphalt binder, and a low proportion of fine aggregates. Such a mixture type has higher surface structure depth and, therefore, better skid resistance. SMA is a typical application of such mixture types and is the most commonly used mixture in the surface layer of high-level highways in Shanghai, China. In this study, SMA-13 mixtures were used, i.e., the maximum aggregate size was about 13.2 mm. The target gradation and actual gradation are shown in Table 7 and Figure 3.

2.2. Design of Asphalt Mixture Mix Proportion

2.2.1. Determination of Optimum Asphalt-to-Aggregate Ratio

The Marshall test is commonly used to determine the optimal asphalt content in asphalt mixtures and can also reflect the high-temperature performance of these mixtures. In this study, samples were prepared using seven different asphalt-to-aggregate ratios (i.e., 5.4%, 5.8%, 6.2%, 6.6%, 7.0%, 7.4%, and 8.0%). It should be noted that the rock asphalt and desulfurized rubber modifiers were premixed and therefore included in the weight of the asphalt. Subsequently, the volumetric parameters and mechanical properties of Marshall samples with different asphalt contents were tested, and the results are presented in

Table 8. Asphalt-to-aggregate ratio results.

Asphalt-to-Aggregate Ratio (%)	5.4	5.8	6.2	6.6	7.0	7.4	8.0	Requirements
Asphalt (%)	5.12	5.48	5.84	6.19	6.54	6.89	7.41	/
Marshall stability (kN)	16.72	16.92	17.92	17.64	17.18	16.70	16.56	>6.0
Flow value (0.1mm)	16.9	16.6	32.4	33.6	34.0	39.6	45.3	20–50
Gross volume relative density	2.448	2.457	2.471	2.473	2.475	2.466	2.463	/
Void ratio (%)	6.72	5.86	4.78	4.14	3.56	3.37	3.86	3~4
Mineral gap ratio (%)	18.06	18.09	17.93	18.16	18.42	19.01	19.54	>17
Asphalt saturation (%)	62.81	67.63	73.34	77.17	80.68	82.26	80.22	75–85

. The data indicate that there is a certain range of volume ratios for asphalt mixtures in practical applications to accommodate different engineering requirements and material characteristics. Mixing asphalt with rubber modifiers such as recycled rubber and mineral powder can enhance its high-temperature performance. This suggests that the addition of modifiers can further improve the properties of asphalt mixtures, making them more suitable for high-temperature environments. As the asphalt content increases (from 5.8% to 8.0%), the volumetric parameters of the asphalt mixtures (such as void content and voids in mineral aggregate (VMA)) undergo changes, and their mechanical properties (such as stability and flow value) are also affected.

Specifically, higher asphalt content may lead to lower void content and higher voids filled with asphalt (VFA), which contribute to improving the high-temperature performance of asphalt mixtures. However, excessively high asphalt content may also result in decreased stability and increased flow value, thereby reducing the resistance to deformation of the asphalt mixtures. According to the calculation formula for the asphalt-to-aggregate ratio, the optimal asphalt-to-aggregate ratio for this study was determined to be 7.1%. To ensure that the asphalt mixtures exhibit optimal performance, it is essential to select an appropriate asphalt–stone–mortar volume ratio based on specific requirements and material characteristics.

2.2.2. Specimen Preparation

To compare the effects of the wet and dry processing methods and determine the proper content of rock asphalt, four groups of specimens were prepared as shown in

Table 9. Details of specimen groups.

Group	A	B	C	D
Ratio between rubber and base asphalt	18%	18%	27%	36%
Processing method	Wet	Dry	Dry	Dry

. The dry processing method consisted in heating the aggregate first and then sequentially adding and blending the modifiers, base asphalt, and the mineral powder for 90 s at 170 °C. The wet processing method consisted in preparing the modified asphalt in advance, and, in this case, the base asphalt was first heated with desulfurization rubber for 60 min at 180 °C to allow for solvent swelling, and then rock asphalt was also added and heated for another 60 min at 180 °C. High-speed shearing mixers were then used for blending at 5000 rpm and 180 °C for 45 min. It should be noted that in the process of high-speed blending, the temperature may be elevated due to the friction heat, and the blending temperature was monitored by an infrared thermometer to ensure that the temperature is within the requirements. Finally, the modified asphalt was mixed with aggregates to produce asphalt mixtures. As shown in the table, Group A was prepared by the wet processing method and Groups B, C, and D were prepared by the dry processing method.

As shown in Table 9, for all specimens, the optimum asphalt-to-aggregate ratio was 7.1%, as suggested by the results of the Marshall tests. The weight content of the lignin fiber was 0.3%. The ratio between rock asphalt and base asphalt was 10%. The ratio between rubber and base asphalt ranged from 18% to 36%.

The prepared SMA-13 asphalt mixtures met the technical requirements shown in

Table 10. Technical requirements for SMA-13 asphalt mixtures.

Technical Indicators	Unit	Technical Requirement	Test Methods [23]
Number of compactions	Times	75 times on each side	JTG E20 T0702
Specimen size	mm	Φ101.6 × 63.5	JTG E20 T0702
Marshall stability	kN	≥6.0	JTG E20 T0709
Flow value	0.1 mm	20~50	JTG E20 T0709
Air voids (VV)	%	3~4	JTG E20 T0705
Voids of mineral aggregate (VMA)	%	≥17	JTG E20 T0705
Voids of coarse aggregate	%	≤VCADRC	JTG E20 T0705
Voids filled with asphalt (VFA)	%	75~85	JTG E20 T0705
Dynamic stability (at 60 °C, 0.7 MPa)	Times/mm	≥3000	JTG E20 T0719
Low-temperature ultimate strain (at 50 mm/min, −10 °C)	με	≥2800	JTG E20 T0715
Immersion Marshall residual stability	%	≥85	JTG E20 T0709
Freeze–thaw splitting strength ratio (TSR)	%	≥80	JTG E20 T0729

. It should be noted that the Marshall specimens for SMA specimens are usually compacted 50 times on both sides; however, the pavement in the project was designed for heavy traffic and subjected to more compactions in construction, and to be consistent with practice, the mixture specimens were compacted 75 times on both sides, as shown in Table 10. The prepared SMA-13 asphalt mixtures successfully met the specified technical requirements, as detailed in Table 10. The primary indicators of the materials satisfy the corresponding requirements in Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011).

2.3. Test Methods

2.3.1. Rutting Test

The rutting test is a simple and effective test method to evaluate the anti-rutting ability of the asphalt mixture. It is specified by simulating the repeated action of traffic load on the pavement. The test is based on Highway Engineering Asphalt and Asphalt Mixture Test Procedures (JTG E20-2011). In this study, the T 0719-2011 asphalt mixture rutting test was used to evaluate the high-temperature rutting resistance of asphalt mixtures modified by the rock asphalt and desulfurized rubber. The asphalt mixtures were molded into a plate specimen of 300 mm × 300 mm × 50 mm. At a temperature of 60 °C on the same track, the solid rubber wheel with a wheel pressure of 0.7 MPa was repeatedly rolled for a certain period to form rutting. Rutting depth (RD) and dynamic stability (DS, i.e., the number of rolling times required for a 1 mm groove) were used to quantify the rutting resistance of mixtures, as shown in Figure 4.

The dynamic stability of asphalt mixture specimens was calculated based on the deformations at 45 min and 60 min of the test, according to Equation (1) as follows [24]:

$$DS=(t_2-t_1)\times \mathrm{N}/(d_2-d_1)\times C_1\times C_2$$

(1)

where

DS is the dynamic stability of asphalt mixture, times/mm;

d₁ is the deformation corresponding to time t₁, mm;

d₂ is the deformation corresponding to time t₂, mm;

C₁ is the correction factor for the test machine type;

C₂ is the specimen factor;

N is the test wheel round-trip rolling speed, usually 42 times/min.

2.3.2. Low-Temperature Bending Test

Asphalt mixtures have higher modulus at low temperatures and may be subjected to brittle failure. Under the effect of vehicle loads and reflection cracks from the semi-rigid base, cracks may develop on the pavement surface, forming bottom–up cracks or top–bottom cracks. The cracks may reduce the strength of the pavement and induce other pavement distresses like water damage. Therefore, the cracking resistance of asphalt mixtures at low temperatures was evaluated in this study by the low-temperature bending test [25]. The tests were conducted as per Highway Engineering Asphalt and Asphalt Mixture Test Procedures (JTG E20-2011). The rutting plates were prepared according to the optimum asphalt-to-aggregate ratio of 7.1%, and the rutting plates were cut into small specimens with a dimension of 250 mm × 30 mm × 35 mm. At least 6 parallel specimens were prepared for each test group. The prismatic beam was placed in a low-temperature oven at −10 °C for more than 4 h, and then the specimens were loaded at a rate of 50 mm/min until specimen failure.

2.3.3. Indirect Tension Asphalt Cracking Test (IDEAL-CT)

The indirect tension asphalt cracking test (IDEAL-CT) was used in this study to evaluate the crack resistance of asphalt mixtures modified by rock asphalt and desulfurization rubber at medium temperatures. Similar to the traditional indirect tensile strength test, the IDEAL-CT was conducted at room temperature with a cylindrical specimen at a loading rate of 50 mm/min. Cylindrical specimens with a diameter of 100 mm and a height of 64 ± 0.2 mm were used. The test temperature was 20 °C, and the specimens were kept at the test temperature for more than 4 h before the test. There were three parallel specimens in each group. Figure 5 presents the sample and test setup of IDEAL-CT.

The cracking tolerance (CT) index was obtained from the IDEAL-CT and used to quantify the crack resistance of mixtures. Figure 6 shows a typical load–displacement curve and the related index definition diagram of the IDEAL-CT test. The CT index was calculated as shown in Equations (2)–(5). In general, a larger CT index indicates a slower development of cracking and therefore suggests a better resistance of asphalt mixtures to cracking.

$$\mathsf{\sigma }={\displaystyle \frac{2\ast P_{100}}{\pi \ast D\ast t}}\ast 1000$$

(2)

$$\left|m_{75}\right|=\left|{\displaystyle \frac{P_{85}-P_{65}}{l_{85}-l_{65}}}\right|$$

(3)

$$G_f={\displaystyle \frac{W_f}{D\ast t}}$$

(4)

$$\mathrm{C}\mathrm{T} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}={\displaystyle \frac{t}{62}}\ast {\displaystyle \frac{G_f}{\left|m_{75}\right|}}\ast {\displaystyle \frac{l_{75}}{D}}$$

(5)

where $\mathsf{\sigma }$ is the indirect tensile strength (MPa); D is the diameter of the specimen (mm); t is the thickness of the specimen (mm); P₁₀₀ is the maximum load (kN); $\left|m_{75}\right|$ is the modulus index of the cracked specimen (kN/mm); P₈₅ is the 5% maximum load (kN); P₆₅ is the 65% maximum load (kN); l₈₅ is the displacement value corresponding to 85% maximum load after the peak (mm); l₆₅ is the displacement value corresponding to 65% of the maximum load after the peak (mm); G_f is the fracture energy density (J/mm²); W_f is the fracture work, i.e., the area beneath the load–displacement curve (J); l₇₅ is the displacement value corresponding to 75% of the maximum load after the peak (mm).

2.3.4. Indirect Tensile Fatigue Test

The fatigue performance of the mixture specimens was evaluated by the indirect tensile fatigue test with controlled stress. To make the test results more distinguishable, the fatigue test was carried out with a fixed stress value of 0.363 MPa (fatigue life was controlled from 10,000 to 100,000 times). The loading frequency was 10 Hz. The test temperature was 20 °C, and the specimens were kept at 20 °C for more than 4 h before the test. Each group had three parallel specimens. It was found that macroscopic fatigue cracks appeared in the middle of the specimen.

As shown in Figure 7, in the process of the indirect tensile fatigue test, the fatigue curve of vertical strain generally presents three-stage characteristics. In the first stage, the vertical strain increases sharply under repeated load, and an initial crack begins to appear at the end of this stage. In the second stage, the fatigue curve shows a linear increase with load cycles, indicating constant development of micro-cracks. In the third stage, the vertical strain increases exponentially until the fatigue failure of the specimens. Fatigue life (N_f) can be defined as the total number of load cycles applied to the asphalt mixture before fatigue failure.

2.3.5. Freeze–Thaw Splitting Test

The immersion Marshall test and freeze–thaw splitting test are usually used to evaluate the water stability of asphalt pavement. The freeze–thaw splitting test was conducted in this study as it simulates the actual environment of asphalt pavement well, using vacuum saturation, freeze–thaw cycle, and a high-temperature water bath. Eight mixture specimens were prepared based on the optimum asphalt-to-aggregate ratio (7.1%) and compacted 50 times on each side. They were randomly divided into two groups with 4 specimens in each group. The first group was subjected to a freeze–thaw cycle, while the second group was saturated by the standard water saturation test method and then placed in a plastic bag with about 10 mL water. The bag was then sealed and placed in a refrigerator at −18 °C for 16 h. After that, the specimens were taken out and placed in a thermostatic water tank at 60 °C. The plastic bag was removed and the specimens were kept in 60 °C water for 24 h. Then, the two groups of specimens were immersed in a water tank at a constant 25 °C for 2 h. The specimens were taken out and subjected to splitting tests in sequence. The maximum loads before failure were recorded. The freeze–thaw splitting tensile strength ratio (TSR) was calculated according to Equation (6):

$$TSR={\displaystyle \frac{{\overline{R}}_{T_2}}{{\overline{R}}_{T_1}}}\times 100$$

(6)

where TSR is the freeze–thaw splitting strength ratio, %; R_T1 is the split tensile strength of the first group of specimens without freeze–thaw cycles, MPa, R_T2 is the split tensile strength of the second group of specimens after the freeze–thaw cycle, MPa.

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. High-temperature rutting test results.

Group Number	Deformation at t1 = 45 min (mm)	Deformation at t2 = 60 min (mm)	Dynamic Stability (times/mm)
A	2.714	2.867	4132
B	2.885	3.044	3974
C	2.825	2.972	4301
D	2.203	2.337	4721

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 t₁ = 45 min and t₂ = 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. Results of low-temperature bending tests.

Group Number	Flexural Tensile Strength (MPa)	Maximum Bending Tensile Strain (με)	Flexural Modulus (MPa)
A	11.94	3261.20	4431.79
B	9.94	3044.85	4108.76
C	9.07	2954.42	3881.14
D	10.50	2935.60	3957.01

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. Results of IDEAL-CT test.

Group Number	Indirect Tensile Strength (MPa)	Coefficient of Variation	CT Index	Coefficient of Variation
A	1.53	0.3%	67.10	5.8%
B	1.56	1.2%	69.72	5.1%
C	1.46	2.6%	47.23	1.0%
D	1.44	0.6%	43.82	9.0%

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. Fatigue life from indirect tensile test.

Group Name	Fatigue Life Nf	Coefficient of Variation of Fatigue Life Nf
A	21,449	21.68%
B	37,297	8.43%
C	48,257	2.06%
D	41,441	4.15%

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. Results of freeze–thaw splitting tests.

Group Number	Split Tensile Strength before Freezing and Thawing (MPa)	Load Value after Freezing and Thawing (kN)	Split Tensile Strength after Freezing and Thawing (MPa)	Freeze–Thaw Splitting Strength Ratio (TSR) (%)
A	0.509	4.61	0.455	89.46
B	0.515	5.01	0.500	97.18
C	0.484	4.52	0.440	90.99
D	0.500	4.43	0.437	87.46

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.