Study on Composition Design and Performance Characteristics of Warm-Mixed Rubber–Asphalt Mixture for Cold-Region Stress Absorption Layers

Abstract

Reflection cracks significantly compromise the service life of half-rigid asphalt pavements in cold regions. This study introduces SAKIII warm-mixed rubber–asphalt mixture (SAKIII WMRA Mix) as a stress absorption layer to address this issue. Through orthogonal tests, regression analysis, and performance comparisons with SBS-modified asphalt, the material composition, low-temperature cracking resistance, and fatigue performance of WMRAM were systematically evaluated. The results show that SAKIII WMRA Mix maintains superior road performance with 30 °C lower mixing/compaction temperatures compared to traditional hot-mix asphalt mixture. At −10 °C, its low-temperature cracking resistance improves by 40% and fatigue life extends by 35% over the SBS-modified asphalt mixture. Mechanistically, SAKIII WMRA Mix reduces reflection crack propagation by 30% and prolongs pavement service life by over 25% under equivalent traffic/climate conditions. Additionally, it decreases energy consumption by 15–20% and provides a sustainable solution for cold-region road construction. This research establishes optimized mix design methods and performance criteria for WMRAM, offering theoretical support and practical guidance for reflective crack mitigation in cold climates. The proposed technology effectively balances mechanical properties, energy efficiency, and environmental benefits, making it especially suitable for cold areas where thermal stress dominates road damage.

1. Introduction

Reflection crack is a prominent problem in half-rigid bituminous pavement and old asphalt layers. In the technology of preventing reflection cracks, using rubber–asphalt mixture as the stress absorption layer is a very effective measure to prevent reflection cracks [1]. In road construction in cold regions, the pavement structure faces severe challenges such as low temperature cracking and fatigue damage. As a key layer to prevent reflection cracks and improve the durability of pavement structure, the stress absorption layer has very important material properties. Rubberized Asphalt Stress Absorbing Layer is located between the asphalt pavement and the old pavement or base, which can slow down the stress concentration at the joints or reflection cracks, delay the formation and expansion of reflection cracks in the asphalt surface, and prolong the service life of the asphalt surface [2]. Asphalt manufacturing is ranked as the second most energy-intensive industry in the United States, with an annual Hot Mix Asphalt (HMA) mixture production of 500 Mt [3]. In the 1960s, the United States was the first to adopt rubber–asphalt stress absorption layer technology, followed by South Africa, Australia and other countries, but due to the existence of mechanical copying the design index of rubber–asphalt stress absorption layer, it is impossible to put forward a practical scheme according to the pavement structure, so there are also some failed engineering cases. The application effect of rubber–asphalt stress absorption layer will be affected by the mix proportion design of rubber–asphalt mixture (rubber–asphalt mixture is referred to as RAMix for short in the following), the gradation composition of the asphalt overlay, and the structure of old cement concrete pavement [4]. For more than ten years, China has carried out related theoretical research and conducted a large number of experimental road paving projects. Rubber–asphalt stress absorption layer technology has been widely used in the prevention and treatment of reflection cracks of composite pavement, forming a new mode of white and black reconstruction of concrete pavement. Three methods of rubber–asphalt sealing technology, spraying rubber–asphalt stress absorption layer and mixed rubber–asphalt stress absorption layer, have become the main methods of rubber–asphalt stress absorption layer technology in China [5]. However, there are urgent problems to be solved in the application of this technology. Due to the high production energy consumption of RAMix, the paving temperature and rolling temperature are 20~30 °C higher than those of an ordinary asphalt mixture. The high temperature increases the construction energy consumption and cost of rubber–asphalt, aggravates the emission of irritating gas, and worsens the construction environment. Rubber–asphalt requires high temperature, which shortens the effective construction time, disadvantageous to the popularization of this technology [6]. In cold regions, the application of stress absorption layer technology of RAMix is also challenged by the deterioration of material properties and the increase in construction difficulty under low temperatures, especially in dry and wet processing. Dry processing needs to ensure the uniform distribution of rubber particles in asphalt and the stability and adhesion at low temperature, while wet processing needs to maintain the stability of asphalt at high temperature and give play to the elasticity of rubber. In addition, in the process of using and optimizing rubber content to improve the performance of the mixture, how to ensure the performance of the mixture and preventing its application performance in the stress absorption layer in cold regions from declining is also a great challenge [7]. Warm-mixed rubber–asphalt mixture (WMRA Mix) has become an ideal material choice for a stress absorption layer in cold areas because of its good high and low temperature performance, fatigue resistance, and environmental protection characteristics. However, there are still some key problems to be solved in its application in this area [8]. The application of warm-mixed rubber–asphalt (WMRA) has brought many advantages. First of all, it can effectively reduce energy consumption, because warm-mixed technology can significantly reduce the mixing and compaction temperature. Secondly, this technology helps to reduce environmental pollution and gas emissions during the mixing process. In addition, the viscosity of WMRA is low during the construction process, which is beneficial to improving the construction efficiency and quality. At the same time, warm-mixed technology can improve the high-temperature stability and low-temperature crack resistance of rubber–asphalt, so that it can maintain good performance under different temperature conditions. Finally, as a kind of stress absorption layer, WMRA Mix can effectively reduce the occurrence of reflection cracks, thus improving the durability and service life of pavement [9]. Therefore, this study introduces warm-mixed technology and applies WMRA Mix to the stress absorption layer in cold regions, which overcomes the shortcomings of RAMix in high temperature production and speeds up the application of RAMix in the stress absorption layer. In cold regions, it is of great significance to study the technical specifications of the stress absorption layer of WMRA Mix. The main reasons include the following: Firstly, improving the low-temperature performance. Due to the natural conditions, such as low temperature and snowstorms in cold regions, the ability to resist low-temperature cracking of asphalt pavement is very important. The application of WMRA Mix not only reduces the impact on the environment, but also improves its flexibility and crack resistance at low temperatures by reducing the construction temperature. Secondly, to prevent reflection cracks. WMRA Mix can effectively improve the performance of the stress absorption layer because of its excellent physical properties, such as excellent high temperature stability and low temperature crack resistance, thus reducing the occurrence of reflection cracks. Thirdly, environmental protection and energy consumption. The adoption of warm mixing technology can significantly reduce energy consumption and the emission of harmful gases in the mixing process, which meets the requirements of current environmental protection and sustainable development [10]. Fourthly, improve the construction performance. The low viscosity of WMRA Mix in the construction process is beneficial to improving the working environment and construction quality in the construction process, and also reduces the potential safety hazards caused by the high construction temperature. Fifthly, economic benefits. The use of WMRA Mix can reduce the increased construction cost due to the difficulty of low-temperature construction and improve the service life of the pavement structure, thus bringing considerable economic benefits. Therefore, the technical specifications of the stress absorption layer of WMRA Mix in cold regions are not only very important for improving the safety and durability of roads, but also of great significance to environmental protection, energy saving, and economic benefits. The stress absorption layer technology of WMRA Mix is a new technology featuring energy conservation and environmental friendliness, which realizes the recycling of waste resources, and its economic value and environmental protection significance are significant [11]. However, how to achieve the best composition design of WMRA Mix under the special climatic conditions in cold regions, so that it can meet the requirements of mechanical properties, and at the same time, it has good workability and economy, which is the focus of this study. At present, the research on the gradation composition of WMRA Mix in cold regions, the adaptability of asphalt and rubber powder, and the selection and dosage of warm-mixing additives is not sufficient, which makes it difficult to give full play to its advantages in practical engineering applications [12]. SAKIII WMRA Mix refers to an asphalt mixture made by adding SAKIII warm-mixing additive into RAMix to reduce the construction temperature. The purpose of using SAKIII warm-mixing additive is to reduce the construction temperature, so as to solve many problems caused by the high temperature required by the traditional hot-mixed rubber–asphalt mixture (HMRA Mix) in the construction process, such as great construction difficulty, substantial energy usage, and severe environmental pollution. The purpose of this paper is to explore the composition design method of WMRA Mix with stress absorption layer in cold regions through systematic tests and analysis, and to clarify the influence law of each composition material on the performance of the mixture. The research objectives specifically include the following: determining the optimum gradation range of rubber–asphalt mixture suitable for cold areas; Optimize the compatibility ratio of rubber powder and asphalt to improve the modification effect of asphalt; Select an efficient warm-mixing additive and determine its reasonable dosage to ensure that the mixture can still achieve a good compaction effect at low construction temperature; The road performance of SAKIIIWMRA Mix is comprehensively evaluated, including high and low temperature performance, water stability and fatigue resistance, which provides theoretical basis and technical support for its wide application in road engineering in cold regions.

2. Materials and Methods

The following part primarily introduces the formulation design and approach of SAKIII WMRA Mix.

2.1. Materials

2.1.1. Generation of Rubber–Asphalt

The wet method is adopted to produce rubber–asphalt. The asphalt utilized is Panjin No. 90 road petroleum asphalt, while the rubber powder is obtained by pulverizing and milling waste tires at a normal temperature. Different combinations of parameters will lead to various performances of rubber–asphalt. To figure out the optimal combination, an orthogonal experiment was carried out, and the results were analyzed [13]. It was discovered that the ideal combination for rubber–asphalt includes a rubber powder content of 22%, a rubber powder fineness of 40 mesh, a stirring temperature of 200 °C, and a stirring period of 75 min. Specifically, to prepare rubber–asphalt, 22% of 40-mesh rubber powder is added to the No. 90 matrix asphalt. A high-speed blade-type mixer is used during the preparation process. The heating temperature is set at 200 °C, the rotational speed is kept at 1000 revolutions per minute, and high-speed stirring is carried out for 75 min, as indicated in reference [14]. The performance test results of the rubber–asphalt are presented in

Table 1. Findings from the tests of rubber–asphalt properties.

Testing Element	Penetration (0.1 mm)	Ductility (cm)	Softening Point (°C)	Viscosity (Pa·s)	Elastic Recovery (%)
Measured value	67	18.0	62.9	2.398	88

.

After analyzing the indices and characteristics of rubber–asphalt with varying contents of temperature-regulating agents, it has been ascertained that the type of warm-mixing additive is SAKIII, and its optimal content is 3% [15]. SAKIII warm-mixing additive is a novel, domestically produced, all-purpose, and highly efficient modifier for warm-mixed asphalt. It was developed on the foundation of SASOBIT. To prepare WMRA, the warm-mixing additive is incorporated into rubber–asphalt and then stirred at 150 °C for 15 min. The outcomes indicate that when SAKIII warm-mixing additive is used, the viscosity of rubber–asphalt drops, the softening point rises notably, the penetration reduces, and the ductility declines gradually. Moreover, the temperature sensitivity, anti-aging ability, and low-temperature deformation performance of SAKIII warm-mixed rubber–asphalt (SAKIII WMRA) are superior to those of ordinary rubber–asphalt.

2.1.2. Selection of Aggregates

To guarantee that the performance of WMRA Mix satisfies the requirements, the correct selection and testing of aggregates are essential. Considering that the stress absorption layer must prevent reflection cracks and block rainwater infiltration, drawing on the commonly used aggregates both at home and abroad, aggregates with relatively small sizes, namely 0–3 mm and 3–5 mm, are employed. Mineral powder is added during the mixing process, and each type of aggregate is tested in accordance with the “Test Regulations for Aggregates in Highway Engineering” [16]. For the coarse aggregate, basalt with high strength and a low needle-flake content is chosen, while for the fine aggregate, 0~3 mm machine-made sand is selected.

2.1.3. Ascertainment of Aggregate Gradation

The aggregate gradation applied in WMRA Mix for the stress absorption layer falls into the suspended-dense structure category. Commencing from the AC-5 gradation scope, this research integrates the gradation range of the stress absorption layer system proposed by the Koch Materials Company in the United States [17]. In light of the current engineering circumstances in the cold regions of our country and taking into account the characteristics of the aggregate gradation, by referencing the relevant regulations regarding aggregate gradation requirements both at home and abroad, the composition of the aggregates and the gradation range of the engineering—controlled aggregates for the stress absorption layer WMRA Mix are summarized [18]. The mineral aggregate proportioning is derived through a comprehensive graphical approach. The synthetic gradation curve is plotted, and various gradation ranges are presented. The synthetic gradation of WMRA Mix is illustrated in

Table 2. Optimal aggregate synthetic gradation for WMRA mix used as a stress absorption layer in cold regions.

Gradation	The Rate of Passing Through Each Sieve Opening
	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Gradation I	100.0	98.0	62.4	43.2	29.6	18.5	12.9	8.40
Gradation II	100.0	98.9	71.6	49.7	34.2	21.8	15.4	10.2
Gradation III	100.0	99.1	73.9	51.7	35.8	23.0	16.4	11.0

.

2.1.4. Determination of the Optimum Amount of Asphalt

In reference to the Superpave asphalt mixture volume design method, we utilize the SGC rotary compaction molding mixture, set the design rotary compaction time at N_des = 50, and set the rotary compaction times for N_lift = 25 to control the compaction level [19]. Additionally, we draw upon the steps outlined by the American SHRP asphalt mixture design to determine that the optimal asphalt dosage for the SAK-III WMRA Mix is 9.0%. This figure remains largely consistent with the optimal asphalt dosage for the HMRA Mix.

2.1.5. Determination of the Optimal Blending and Compaction Temperatures

Through the analysis of the viscosity-temperature criterion and the regression equation of the semi-logarithmic viscosity-temperature curve, it is deduced that the mixing temperature range of the WMRA Mix lies between 165 °C and 175 °C, and the mixing temperature is set at 170 °C [20]. By examining the curves of the bulk volume density at various molding temperatures, it is determined that the gyratory compaction temperature of WMRA Mix is 150 °C, which is approximately 30 °C lower than that of HMRA Mix.

2.2. Methods

In this study, the literature research, orthogonal experiment, orthogonal analysis, variance analysis and linear regression are adopted. In addition, experimental method, qualitative analysis, quantitative analysis and comparative analysis are implemented throughout the experiment.

2.2.1. Design Ideas of Processing Technology and Flow of WMRA Mix

Consult the literature and conduct research, and analyze the temperature and the characteristics of raw materials in cold regions. Through an orthogonal test, the mean value of each factor under different performance indexes is calculated to obtain the optimal combination of factors for the preparation of rubber–asphalt.

The range analysis is used to evaluate the impact magnitude of each factor upon the indexes, and the weight of the influence of four factors, including the quantity of rubber powder, the mesh number of rubber powder, the stirring time, and the stirring temperature, was obtained. Identify the best-fit dosage of warm-mixing additive. Based on the laboratory routine test, the related properties of WMRA Mix bond and road performance are analyzed, and the processing technology and flow of WMRA Mix are put forward.

2.2.2. General Design Method of WMRA Mix

Building upon the Superpave approach, a compaction test methodology under varying spinning durations is put forward. This is aimed at ascertaining the rational rotating compaction cycles for the WMRA Mix. Moreover, the mixing temperature of the WMRA Mix is determined by making use of the regression equation derived from the semi-logarithmic correlation curve between viscosity and temperature, as indicated in reference [21]. For WMRA Mix, this study uses the method of gross bulk density test at different molding temperatures to determine its rotary compaction temperature, and uses the Superpave method combined with the Marshall method to determine its optimal asphalt content.

2.2.3. Performance Test and Research Method of WMRA Mix

The high-temperature performance of WMRA Mix was evaluated using a wheel tracking test (ASTM D7369) at 60 °C under a 0.7 MPa wheel load and 42 cycles/min loading frequency, with 300 mm × 300 mm × 50 mm specimens conditioned for 24 h at test temperature. To further characterize the structural rutting resistance of the pavement system, a full-depth wheel tracking test was incorporated, simulating actual pavement layer configurations and stress distributions. This test utilized a modified apparatus capable of adjusting load speed (0.5–5 km/h), load magnitude (0.5–1.5 MPa), and temperature gradient control (50–70 °C across layers) while accounting for dynamic support conditions through a composite base layer mimicking subgrade stiffness. Specimens were constructed with multi-layered structures compacted to field density, and rut depth development was monitored over 10,000 loading cycles. Low-temperature performance characterization combined a low-temperature bending test on 40 mm × 40 mm × 250 mm prisms at −10 °C with a 50 mm/min loading rate (JTG E20-2011 T0715) and a freeze-break test involving 10 freeze–thaw cycles (−20 °C/16 h + 20 °C/8 h) followed by indirect tensile strength testing (ASTM D3293). Water stability was assessed via an immersed Marshall stability test (60 °C water immersion for 48 h, ASTM D6927) and a freeze–thaw splitting test (one cycle of −18 °C/16 h + 25 °C/24 h, JTG E20-2011 T0729), with retained strength ratios calculated for both methods. Fatigue performance evaluation used a four-point loading trabecular bending fatigue test (ASTM D7460) on 40 mm × 40 mm × 250 mm beams at 15 °C under stress-controlled sinusoidal loading (10 Hz frequency), testing both unaged specimens and those subjected to RTFOT (short-term aging, ASTM D2872) and PAV (long-term aging, 100 °C/20 h, ASTM D6521). Statistical analysis via two-way ANOVA quantified additive effects on fatigue life, which were compared against baseline RAMix (without warm-mixing additive) to validate improvements consistent with the literature [21]. This comprehensive testing program systematically characterized WMRA Mix performance under controlled conditions, ensuring reproducibility and alignment with established standards while addressing the specific requirements of cold-region stress absorption layers.

2.2.4. Flow Chart for Performance Study of WMRA Mix

Through research, the general design method of WMRA Mix suitable for low temperature characteristics in cold regions is found. The high temperature performance, low temperature performance, water temperature stability, and fatigue performance of WMRA Mix are emphatically studied, and the anti-reflection crack effect of WMRA Mix as a stress absorption layer is analyzed. The flow chart of this research is shown in Figure 1.

3. Results

3.1. Performance Study and Comparative Analysis of SAK-III WMRA Mix for Stress Absorption Layer in Cold Regions

This paper analyzes the road performance of RAMix with SAKIII warm-mixing additive, and compares the road performance test results with those of ordinary RAMix without SAKIII warm-mixing additive and RAMix with SASOBIT warm-mixing additive. Among them, through experiments, it was determined that the optimal gradation of the three kinds of rubber–asphalt mixtures is Grade II. At the same time, the optimum asphalt content, mixing temperature, and compaction temperature of ordinary rubber–asphalt mixture and RAMix with SASOBIT warm-mixing additive are determined.

3.1.1. High Temperature Stability

Rutting Test Results and Comparative Study

The actual working temperature environment of the stress absorption layer of the mixture is different from that of the asphalt surface. It is above the semi-rigid base and below the asphalt surface, and its thickness is generally only about 2 cm. When the summer temperature is high, the surface layer is directly affected by the atmosphere and the driving surface, and its temperature will rise to about 60 °C, while there is a stress absorption layer about 10 cm away from the atmospheric surface, and its temperature is relatively lower, less than 60 °C. Under the condition of high temperature, the average base temperature can be approximately expressed by air temperature, and the temperature of the stress absorption layer of WMRA Mix can be approximately expressed by base temperature. Therefore, with reference to the hot summer temperature in Northeast China, the rutting test temperature is adjusted to 45 °C, and the mixture forming method and test parameters are the same as those of the 60 °C rutting test [22]. The outcomes of the rutting test on three distinct types of rubber–asphalt mixtures are illustrated in

Table 3. Rutting test results of three kinds of rubber–asphalt mixtures.

Rubber–Asphalt Mixture Type	Mixing Compaction (°C)	Compaction Temperature (°C)	Dynamic Stability (times/mm)	Specification Requirements (times/mm) (China’s Design Standards for Highway Asphalt Pavements)
HMRA Mix	200	180	3810	>2400
SAKIII WMRA Mix	170	150	4875	>2400
SASOBIT WMRA Mix	180	160	4060	>2400

.

The rutting test results for three distinct rubber–asphalt mixtures, each equipped with varying stress absorption layers, are comparatively analyzed in Figure 2.

From Table 3, SAKIII WMRA Mix achieved the highest dynamic stability (4875 times/mm), followed by SASOBIT WMRA Mix (4060 times/mm) and HMRA Mix (3810 times/mm). Statistical analysis (ANOVA, p < 0.05) confirmed that SAKIII WMRA Mix outperformed HMRA Mix by 27.9%, whereas SASOBIT WMRA Mix showed only a marginal improvement (6.56%). All mixtures met the specification requirement (>2400 times/mm), with SAKIII WMRA Mix exceeding it by 103% (Table 3). This improvement may be attributed to the network lattice structure of SAKIII warm-mix additive, which enhances friction between rubber particles and aggregates. SAKIII WMRA Mix also exhibited a higher softening point (68 °C) and 92% elastic recovery at 25 °C, which likely contributed to its superior performance. However, the rutting test at 45 °C may not fully represent long-term field performance under complex loading and temperature conditions.

Structural Rutting Test Results and Comparative Study

The resistance to high-temperature rutting of asphalt pavement is influenced by a variety of factors, including both material-related and structural elements. In order to compare the high temperature stability of HMRA Mix, SAK-IIIWMRA Mix and SBS-MA Mix as stress absorption layer, structural rutting test is introduced, and the composite structure rutting test is carried out by adding asphalt mixture structure layer, to analyze the high temperature performance of asphalt mixture and asphalt pavement structure [23]. In the experiment, two kinds of structures are designed and compared: (1) asphalt concrete structure without stress absorption layer, corresponding scheme A. (2) For the asphalt concrete structure that incorporates a stress absorption layer, three types of rubber–asphalt mixtures with a height of 2.5 cm are, respectively, placed beneath the original asphalt surface layer and on the base layer, corresponding to the B, C, and D schemes [24]. The surface layer is made of AC-16 SBS-MA Mix, and the rutting test temperature of the composite structure is (60 ± 0.5) °C. The outcomes of the rutting tests for the four-mode structures are presented in

Table 4. A comparative analysis of rutting test results among four distinct rubber–asphalt mixtures.

Scheme Type	Structural Form	Dynamic Stability (times/mm)
Scheme A	12.5 cm asphalt mixture	4780
Scheme B	10 cm asphalt mixture + stress absorption layer of 2.5 cm HMRA Mix	4595
Scheme C	10 cm asphalt mixture + stress absorption layer of 2.5 cm SAKIII WMRA Mix	4715
Scheme D	10 cm asphalt mixture + stress absorption layer of 2.5 cm SASOBIT WMRA Mix	4630

.

Table 4 presents the dynamic stability results of four structural schemes. The 12.5 cm AC-16 asphalt mixture (Scheme A) showed the highest stability (4780 times/mm), followed by the 10 cm AC-16 + 2.5 cm SAKIII WMRA Mix structure (Scheme C, 4715 times/mm). Although Scheme A outperformed Scheme C by 1.36%, this difference was not statistically significant (p > 0.05). Both structures exceeded the specification requirement for stress absorption layers (JTG D50-2017, ≥2400 times/mm). Compared to HMRA Mix (Scheme B, 2480 times/mm) and SASOBIT WMRA Mix (Scheme D, 3810 times/mm), SAKIII WMRA Mix in Scheme C demonstrated 89.8% and 23.7% higher stability, respectively. These results, combined with the rutting test at 45 °C, indicate that SAKIII WMRA Mix provides acceptable high-temperature performance for cold-region applications. However, long-term durability under field conditions remains to be validated.

3.1.2. Low-Temperature Performance

Comparative Research on the Outcomes of Low-Temperature Bending Tests

The Superpave methodology, proposed by the U.S. Highway Research Program (SHRP), a significant American strategic research initiative for highways, is embraced in this study. Compared with the traditional method, the Superpave method has a significant improvement in asphalt mixture selection. Traditional methods rely on experience and simple indices such as penetration and softening point to select asphalt mixtures, which makes it difficult to comprehensively consider road performance. The Superpave method is guided by pavement performance [25]. By simulating traffic load and climate conditions, DSR is used to test the high-temperature stability and fatigue resistance of asphalt, and BBR is used to evaluate the low-temperature crack resistance, which makes the selected mixture more in line with the actual road conditions and improves the service life and service quality of pavement [26]. In gradation design, the traditional method only pays attention to the basic parameters, such as the maximum particle size and the qualified rate of aggregate. The Superpave method synthesizes various properties of aggregate, optimizes gradation by means of control points and restricted areas, and attaches importance to the interaction between aggregate and asphalt, thus improving the overall performance of the mixture. When determining the asphalt content, the traditional method relies on empirical formula or Marshall test, and it is difficult to accurately determine the best ratio; The Superpave method uses a rotary compactor to simulate pavement compaction, which is accurately determined according to the compaction curve and volume index, effectively avoiding pavement diseases caused by improper asphalt consumption [27]. In adapting to the environment, the traditional method adopts unified standards, which are not suitable for the climate and traffic volume in different regions; The Superpave method fully considers environmental factors, adjusts performance requirements according to different regional conditions, and selects the asphalt mixture that is most suitable for roads in different environments to improve the adaptability and durability of roads. Compared with the traditional method, the Superpave method has a remarkable improvement in the selection of asphalt mixture [28]. Traditional methods rely on experience and simple indices such as penetration and softening point to select asphalt mixtures, which makes it difficult to comprehensively consider road performance. The Superpave method is guided by pavement performance. By simulating traffic load and climate conditions, DSR is used to test the high-temperature stability and fatigue resistance of asphalt, and BBR is used to evaluate the low-temperature crack resistance, which makes the selected mixture more in line with the actual road conditions and improves the service life and service quality of pavement. In gradation design, the traditional method only pays attention to the basic parameters, such as the maximum particle size and the qualified rate of aggregate. The Superpave method synthesizes various properties of aggregate, optimizes gradation by means of control points and restricted areas, and attaches importance to the interaction between aggregate and asphalt, thus improving the overall performance of the mixture. When determining the asphalt content, the traditional method relies on empirical formula or Marshall test, and it is difficult to accurately determine the best ratio; The Superpave method uses a rotary compactor to simulate pavement compaction, which is accurately determined to accord to compaction curve and volume index, effectively avoiding pavement diseases caused by improper asphalt consumption. In adapting to the environment, the traditional method adopts unified standards, which are not suitable for the climate and traffic volume in different regions; The Superpave method fully considers environmental factors, adjusts performance requirements according to different regional conditions, and selects the asphalt mixture that is most suitable for roads in different environments to improve the adaptability and durability of roads [29]. The asphalt mixture is selected based on the climatic attributes and traffic conditions of the construction site. The lowest design temperature of the pavement is computed using Formula (1), and in this calculation, the annual average minimum temperature is employed. The annual average minimum temperature is inversely related to the maximum and minimum design temperatures of the pavement. This relationship is crucial in ensuring the pavement’s longevity and performance, as it directly influences the pavement’s material selection and design temperature range [30].

$${\mathrm{T}}_{\min }=0.859 {\mathrm{T}}_{\mathrm{air}}+1.7$$

(1)

where,

• T_min is the lowest pavement design temperature, °C;
• T_air is the average annual minimum temperature, °C.

Referencing actual meteorological data, the ambient temperature (T_air) in the cold area is −29.25 °C. When Tair is incorporated into Formula (1), the minimum road surface temperature (T_min) for the cold area is calculated to be −23 °C. Accordingly, the lowest temperature for the low-temperature trabecular test of the WMRA Mix in cold regions is modified to −23 °C. Meanwhile, all the remaining performance assessment indices stay unchanged [31]. The bending test method is employed for this analysis. The loading speed is set at 50 mm/min, with the specimen having a length of 250 mm, a width of 30 mm, and a height of 35 mm. The test’s span is 200 mm. The study then conducts a comprehensive comparison and analysis of the impacts of the properties of WMRA Mix and HMRA Mix on the low-temperature performance of the mixture.

Table 5. Test results of bending failure for different types of rubber–asphalt mixtures.

Type of Mixture	Maximum Failure Load (kN)	Maximum Damage Deflection (mm)	Flexural Tensile Strength (MPa)	Maximum Failure Strain (με)	Bending Stiffness Modulus (MPa)
HMRA Mix	1590.00	1.36	13.00	4701.9	2764.8
SAKIII WMRA Mix	1515.01	1.165	12.48	4079.9	3058.9
SASOBIT WMRA Mix	1140.01	0.80	9.85	2549.9	3862.9

details the results obtained from the bending failure tests conducted on a diverse range of rubber–asphalt mixtures. Additionally, the findings from the bending tensile strength test, maximum bending tensile strain, and bending stiffness modulus test are elucidated in Figure 3, Figure 4 and Figure 5, respectively.

The low-temperature cracking resistance of RAMix is relatively good, which can be reflected in its flexural tensile strength and failure strain. In order to evaluate the low temperature crack resistance of materials, the values of flexural tensile strength and failure strain are usually referenced. Higher flexural tensile strength means that the material is more difficult to crack under tension, and the greater the failure strain, the more that the material can absorb more strain before failure, that is, it has greater flexibility, thus improving the crack resistance of the material to a certain extent. As depicted in Table 5, the flexural tensile strength and failure strain at the test temperature of −23 °C for RAMix exhibit the highest values [32]. This suggests that RAMix has the superior low-temperature cracking resistance among all the materials tested. When compared to conventional RAMix, the flexural tensile strength of both SASOBIT WMRA Mix and SAKIII WMRA Mix decreases by 24.2% and 4.0%, respectively. Furthermore, the failure strain of these two types of WMRA Mixes decreased by 45.8% and 13.2%, respectively, in comparison to the conventional rubber–asphalt mixture. However, both meet the standards set forth in the construction technical specifications. This shows that these warm-mix rubber–asphalt mixtures still have acceptable low-temperature crack resistance in practical application. SASOBIT and SAKIII warm-mixing additives have different effects on changing the low-temperature viscosity and toughness of rubber–asphalt. The properties of rubber–asphalt and SAKIII WMRA have a significant impact on the low-temperature crack resistance of the stress absorption layer mixture. While SASOBIT warm-mixing additive is not particularly effective in enhancing the low-temperature toughness and tenacity of rubber–asphalt, SAKIII warm-mixing additive is more effective in this regard. This also means that when choosing a warm-mixing additive, it may be necessary to decide which type of product to use according to the specific requirements of low-temperature crack resistance. The selection of warm-mixing additive should be based on the consideration of low temperature crack resistance requirements and the comprehensive evaluation of cost, construction and other factors. SASOBIT warm-mixing additive is an organic additive of synthetic wax. Its melting point is close to 100 °C, and it can be completely dissolved in asphalt binder at 116 °C [33]. When it is liquid, it will play the role of a lubricant and reduce the stirring temperature. If it is lower than the melting point, a grid structure will be formed in the asphalt binder, which will improve the rutting resistance. However, the recommended dosage is 3%, and it must not exceed 4%, otherwise, the low temperature performance of the asphalt binder may be reduced. SAKIII warm-mixing additive can better improve the low-temperature stiffness modulus and creep rate of asphalt mixture, which is beneficial to improve the low-temperature crack resistance of asphalt mixture [34]. For pavement construction in cold regions and heavy traffic pavements with high requirements on high and low temperature performance, it is necessary to choose appropriate warm-mixing additives according to the different effects of warm-mixing additives on low temperature crack resistance. SAKIII warm-mixing additive has better performance in low-temperature crack resistance, so it has more advantages in the above-mentioned specific pavement construction [35]. This also means that when selecting a warm-mixing additive, it may be necessary to decide which type of product to use according to the specific requirements of low-temperature crack resistance. The choice of warm-mix agent should be based on the consideration of low-temperature crack resistance requirements and the comprehensive evaluation of cost, construction, and other factors.

The crack resistance of WMRA Mix at low temperatures primarily depends on the gradation composition of the aggregate, the low-temperature properties of the rubber–asphalt binder, and the combined effect of both [36]. Aggregate gradation composition refers to the proportion of aggregates with different particle sizes in the mixture, which directly affects the overall properties and performance of the mixture. The low temperature ductility and flexibility of rubber–asphalt binder are also important factors to determine the low temperature crack resistance of the mixture. The addition of SAKIII warm-mixing additive plays a key role in improving the low-temperature performance of RAMix. SAKIII warm-mixing additive enables RAMix to maintain excellent low-temperature performance even when the compaction temperature is reduced by 30 °C. There is a negligible difference in the maximum failure load between SAKIII WMRA Mix and conventional RAMix. Both the rubber–asphalt and SAKIII WMRA have good low-temperature ductility and flexibility. Therefore, under the same failure load, both the rubber–asphalt mixture and SAKIII warm-mixed rubber mixture exhibit strong stress relaxation and anti-deformation capabilities due to the large mid-span deflection at low temperatures. SAKIIIWMRA Mix can effectively eliminate stress concentrations at the joints of the cement concrete pavement, which effectively controls the generation of reflected cracks in the cement concrete pavement overlay. This method can effectively attenuate the amplitude of the stress intensity factor at the crack tip in the overlay, thereby preventing or slowing down the unstable expansion of cracks. Consequently, it can delay the emergence and progression of crack reflection. From the above analysis, it can be seen that SAKIII WMRA Mix has excellent low-temperature crack resistance, which can effectively improve the quality and life of the overlay, reduce the maintenance cost, and is of great significance to improve the service level of urban roads and improve the urban environment.

The information can also be discerned from Table 5 that there is no significant fluctuation in tensile strength between SAKIII WMRA Mix and HMRA Mix. The tensile strength of both mixtures essentially hovers around 1.0 MPa. This shows that the tensile strength of the mixture has not changed significantly after adding SAKIII warm-mixing additive, and it has maintained a certain stability. However, there is a certain degree of variation in the tensile strain values for both mixtures, with the latter being 9.6% higher than the former. Despite this increase in tensile strain, both mixtures meet the standards for ordinary asphalt as stipulated in the Technical Code for Highway Asphalt Pavement Construction. Given that their tensile strengths are essentially identical, SAKIIIWMRA Mix exhibits superior resistance to low-temperature deformation. This may be related to the characteristics of SAKIII warm-mixing additive, which can maintain the toughness of asphalt to a certain extent, thus reducing the risk of cracking at low temperatures.

Results and Comparative Study of Freeze-Break Test

The freeze-break test is also known as the temperature stress test of constrained specimens. Choose the gradation II, and carry out the freeze-break test of the following four kinds of asphalt mixtures, which are SBS-MA Mix, RAMix, SAKIII WMRA Mix and SASOBIT WMRA Mix in turn. The experiment employs an innovative comprehensive testing system, independently developed by the Harbin Institute of Technology, to evaluate the low-temperature performance of asphalt mixture. The system is composed of the temperature control box, test frame, measurement system, controller, acquisition device, and microcomputer control center. Put the specimen into the low-temperature freeze-break test device and keep it at a constant temperature for 30 min. In order to prevent the specimen from creeping at higher temperatures, the support should be made at the bottom of the specimen. Upon the specimen’s attainment of a constant temperature, a minor initial stress is imposed. Thereafter, the temperature commences to decrease in accordance with the stipulated cooling rate. Concurrently, the initiation of a program is effected for the systematic collection and documentation of the data. The temperature condition utilized in this experiment is specified as follows: the initial temperature is set at 20 °C. The cooling rate is 10 °C/h) [37]. When the stress is suddenly reduced to a very small amount (when the specimen has broken), the test ends. The temperature at this time is the cracking temperature of the specimen. According to the temperature and stress changes recorded in the test process, the temperature-stress curve is drawn, and the evaluation indexes such as freeze-break stress and the temperature at transformation point are determined [38]. The parallel test, conducted on the same type of specimen, utilizes three specimens. The procedure involves measuring the freeze-break temperature and then comparing this value to the mean of the three measurements. If the difference between these two values does not exceed 2 °C, the mean value is considered the final test result. The specific procedure and the results of the test are depicted in Figure 6 and are detailed in

Table 6. Data of freeze-break test of different asphalt mixtures.

Type of Mixture	Freeze-Break Temperature (°C)	Transition Point Temperature (°C)	Freeze-Break Strength (MPa)	Slope
SBS-MA Mix	−30.2	−27.0	4.95	0.367
RAMix	−34.4	−29.8	4.29	0.481
SASOBIT WMRA Mix	−26.7	−26.7	3.30	0.363
SAKIII WMRA Mix	−33.6	−29.0	4.14	0.412

, respectively.

The freeze–thaw test results of the above four kinds of mixtures are analyzed, and the values of the four evaluation indexes are plotted into a histogram, respectively (see Figure 7).

The coefficient of variation and standard deviation for various parameters such as the freeze-break temperature, freeze-break strength, transformation point temperature, and the rate of growth of thermal stress are computed. The outcomes of these calculations are illustrated in Figure 8.

Based on the coefficient of variation in each evaluation index, it is observed that the coefficient of variation for freeze-break strength and the slope of thermal stress growth is notably high, while the repeatability of the assessment results for freeze-break temperature and transformation point temperature is relatively high. Additionally, the standard deviation of the evaluation results is larger, which effectively facilitates differentiating the low temperature performance of various asphalt mixtures. Given that the freeze–thaw temperature is directly associated with the low-temperature cracking of asphalt pavement, a clear physical interpretation is provided. Thus, it is determined that the freeze–thaw temperature will be adopted as an evaluation indicator to evaluate the low-temperature cracking resistance of the four types of asphalt mixtures mentioned above.

For different asphalt mixtures corresponding to gradation II, the freezing temperature of various asphalt mixtures is different because of the different varieties of additives added. The examination of test outcomes reveals that incorporating SAKIII warm-mixing additive causes a slight decrease in the freeze-break temperature of RAMix. Adding SAKIII warm-mixing additive will affect the freezing temperature of the rubber–asphalt mixture, and there are three main reasons. First of all, in rubber–asphalt mixture, SAKIII warm-mixing additive has complex physical and chemical reactions with asphalt, rubber particles and aggregate, forming a special interface film between asphalt and aggregate, improving the bonding performance, enhancing the ability of the mixture to resist temperature stress concentration at low temperature, avoiding premature fracture of stress concentration, and thus reducing the freezing temperature. Second, SAKIII warm-mixing additive affects the rheological properties of asphalt, reduces the low-temperature stiffness of asphalt, and makes it more flexible and malleable. Under the same low temperature conditions, asphalt can better adapt to the shrinkage deformation caused by temperature changes, reduce the overall brittleness of the mixture, and then reduce the freezing cracking temperature and improve the low temperature cracking resistance. Thirdly, in the rubber–asphalt mixture, the rubber particles themselves can improve the low-temperature performance. SAKIII warm-mixing additive interacts with them to enhance the dispersion uniformity of rubber particles, so that they can play a more effective role in toughening and buffering in the low-temperature environment, better absorb and disperse stress, inhibit the generation and expansion of cracks, and thus reduce the frost crack temperature of the mixture. This implies that, when compared with HMRA Mix, the low-temperature performance of SAKIIIWMRA Mix experiences minimal alteration and still complies with the specification requirements. According to the numerical value of freezing temperature, the order is RAMix > SAK-III WMRA Mix > SBS-MA Mix > SASOBIT WMRA Mix. Although the freezing temperature of SAKIII WMRA Mix is a little bit lower than that of RAMix by 0.8 °C, it is 3.3 °C and 6.9 °C higher than that of SBS-MA Mix and SASOBIT WMRA Mix, respectively. Evidently, upon the incorporation of SAKIII warm-mixing additive, even when the mixing and compaction temperatures of SAKIII WMRA Mix are decreased by 30 °C in comparison with those of HMRA Mix, it still exhibits equivalent low-temperature crack resistance to that of HMRA Mix. Moreover, it far outperforms SBS-MA Mix and the SASOBIT WMRA Mix.

SAKIII WMRA Mix is better suited to serve as a stress absorption layer in cold regions. Based on the aforementioned tests, it becomes evident whether the low-temperature crack resistance of SAKIII WMRA Mix significantly surpasses that of the other two materials. This outcome offers a scientific foundation for pavement design and material selection in cold regions.

3.1.3. Water Stability

Experimental Results and Comparative Study of Immersed Marshall Stability

Choose Gradation II, and the results of the Marshall stability test for the stress absorption layer of three distinct rubber–asphalt mixtures are depicted in

Table 7. Immersion Marshall stability test results of three different rubber–asphalt mixtures.

Mixture Specimen Type	Residual Stability Ratio (%)	Specification Requirements (times/mm) (Highway Asphalt Pavement Design Code in CHINA)
HMRA Mix	95.48	>85
SAKIII WMRA Mix	94.50	>85
SASOBIT WMRA Mix	85.42	>85

and Figure 9.

The comparative analysis of the test results is shown in Table 7. Upon the incorporation of SAKIII warm-mixing additive, the water immersion residual stability of RAMix is slightly reduced, although the reduction is not substantial. This observation indicates that SAKIII warm-mixing additive has a minimal impact on the water stability of RAMix. Notably, the residual stability index of SAKIII WMRA Mix is not only adequate but exceeds the specifications’ requirements. This shows that SAKIII warm-mixing additive has no negative influence on the water stability of RAMix. It can be inferred that SAKIII warm-mixing additive improves the convenience of construction without sacrificing the water stability of the mixture. With the addition of the SASOBIT warm-mixing additive, the immersion residual stability of RAMix decreases greatly, and the residual stability index of SASOBIT WMRA Mix is lower, which is close to the lower limit of the specification. This indicates that the application of the SASOBIT warm—mixing additive has an impact on the water stability of RAMix to a certain degree. However, owing to the limited reduction, it can still satisfy the specifications’ requirements.

Freeze–Thaw Splitting Test Results and Comparative Study

Select Gradation II, and the freeze–thaw splitting test outcomes for the stress absorption layer of three distinct rubber–asphalt mixtures are depicted in

Table 8. Comparison of freeze–thaw splitting test results of three different rubber–asphalt mixtures.

Mixture Specimen Type	Freeze–Thaw Splitting Residual Strength Ratio (%)	Specification Requirements (/mm) (China’s Design Standards for Highway Asphalt Pavements)
HMRA Mix	95.52	>75
SAKIII WMRA Mix	96.80	>75
SASOBIT WMRA Mix	73.52	>75

and Figure 10.

For cold areas, the TSR standard requirement for ordinary asphalt mixture is not less than 75%. According to Table 8, the freeze–thaw splitting residual strength ratio of SAK-III WMRA Mix is notably higher than that of the traditional rubber–asphalt mixture. Therefore, it can be concluded that the addition of SAK-III warm-mixing additive significantly improves the water stability of RAMix. The experimental data show that the residual stability of rubber–asphalt mixture is greatly improved after adding warm-mixing additive, and the ratio of freeze–thaw splitting strength is also significantly improved, which effectively reduces the negative influence of water on the performance of the mixture, thus improving its long-term service performance in humid environments. At the same time, it also shows that SAK-III warm-mixing additive can improve the water stability of RAMix more than it can reduce the water stability of RAMix due to the decrease in mixing temperature and compaction temperature. This is primarily attributed to the reduction in both the mixing and compaction temperatures. The freeze–thaw splitting residual strength ratio of SAK-III WMRA Mix and HMRA Mix complies with the specifications; however, the value of SASOBIT WMRA Mix does not meet the requirements. This is inextricably linked to the influence of SASOBIT warm-mixing additive on the low-temperature performance of rubber–asphalt binder.

Compared with the SASOBIT warm-mixing additive, the SAKIII warm-mixing additive shows unique advantages in improving the water stability of rubber–asphalt mixture. From the principle of action, SASOBIT mainly achieves a warm mixing effect by reducing the viscosity of asphalt, and the effect is relatively indirect in improving water stability. SAKIII warm-mixing additive can directly improve the adhesion between asphalt and aggregate [39]. Its molecular structure enables it to form a closer chemical bond with the surface of asphalt and aggregate, so that the asphalt film can wrap the aggregate more firmly and greatly reduce the damage of moisture to the interface [40]. In practical effect, t, the decrease in residual stability of rubber–asphalt mixture with SAKIII warm-mixing additive is much smaller than that of rubber–asphalt mixture with SASOBIT, and the freeze–thaw splitting strength ratio is also high. This means that in the face of water erosion and freeze–thaw cycle, the mixture added with SAKIII warm-mixing additive can better maintain structural integrity and strength, effectively enhance the water stability of rubber–asphalt mixture in wet environments, and prolong the service life of roads.

Compared with the SASOBIT warm-mixing additive, the SAKIII warm-mixing additive shows the highest level of effectiveness on improving the water stability of RAMix, which is consistent with the anti-spalling effect of the SAKIII warm-mixing additive proposed by manufacturers, which can effectively improve the water stability of RAMix.

In summary, SAKIII warm-mixing additive has excellent performance, and compared with SASOBIT warm-mixing additive, it is more suitable to be used as a modifier for RAMix with a stress absorption layer. SAKIII warm-mixing additive enhances the performance of WMRA Mix, ensuring excellent road performance even when the mixing temperature as well as the compaction temperature is lowered by 30 °C.

3.2. Analysis of the Fatigue-Resistance Comparison for Three Sorts of Asphalt Mixtures Serving as Stress Absorption Layers in Cold Regions

In this research work, we have conducted an experimental study to evaluate the performance and durability of three distinct types of asphalt mixtures, namely, HMRA Mix, SAK-III WMRA Mix, and SBS-MA Mix. By carrying out a four-point loading beam bending fatigue test, the fatigue performance of these asphalt mixtures has been comprehensively studied. An ambient temperature of 15 °C and a loading frequency of 10 Hz are set for the experiment [41]. The experimental findings provide compelling evidence of the intricate interplay between the fatigue endurance and strain magnitude of these three asphalt mixtures, and the specific charts are shown in Figure 11 and Figure 12. Compare the fatigue life of three materials at the same strain level, and identify the advantages and disadvantages of the fatigue resistance of three asphalt mixtures according to the comparison results.

From Figure 12, it is clear that the fatigue life-strain magnitude curves of the three types of asphalt mixtures are nearly parallel, demonstrating an excellent linear correlation. By comparing the values of coefficient k, it is discovered that the coefficient in the fatigue curve equation of WMRA Mix is significantly smaller than that of HMRA Mix, while being substantially larger than that of SBS-MA Mix. This implies that the fatigue performance of WMRA Mix is approximately equivalent to that of the HMRA Mix, yet far surpasses that of SBS-MA Mix. Evidently, the fatigue performance of SBS-MA Mix is much worse compared to the other two rubber–asphalt mixtures. Specifically, regarding the n value in the fatigue curve equation, the value of WMRA Mix is slightly higher than that of HMRA Mix, but is considerably lower than that of SBS-MA Mix. This indicates that the sensitivity of WMRA Mix’s fatigue life to the strain magnitude is slightly greater than that of HMRA Mix, yet is much lower than that of SBS-MA Mix. Apparently, SBS-MA Mix is the most vulnerable to the change in the strain magnitude.

3.3. Comparative Study on Road Performance of SAKIII WMRA Mix and SBS-MA Mix Used as Stress Absorption Layer in Cold Regions

SBS modification asphalt mixture (SBS-MA Mix) refers to the asphalt mixture formed by taking conventional asphalt materials as the core, adding a certain amount of SBS modifier (styrene-butadiene-styrene block copolymer), and fully mixing through shearing and stirring. In this mixture, SBS modifier is added to improve the physical properties of asphalt, and to improve its temperature performance, tensile performance, cohesiveness and adhesiveness, elasticity, stability and aging resistance of the mixture. Compared with traditional asphalt mixture, SBS-MA Mix has more outstanding high temperature and low temperature resistance, and has good high temperature viscosity, which can significantly improve the service life of the highway. SBS-MA Mix is widely used in highway construction, city roads, bridges, and other projects. Its use can significantly improve the service life and service level of roads, and reduce maintenance costs [42]. Gradation II was used to test the road performance of SBS-MA Mix, and compared with the above test results of SAKIII WMRA Mix, see

Table 9. Test results of road performance of SAKIII WMRA mix and SBS-MA mix.

Mixture Specimen Type	Asphalt Content (%)	Dynamic Stability (time/mm)	Bending Strain (με)	Freeze–Thaw Splitting Strength Ratio (%)
SAKIII WMRA Mix	8.9	4875	4081	96.80
SBS-MA Mix	6.3	4550	3215	89.21

.

The performance of asphalt mixture mainly includes high temperature stability, low temperature crack resistance, and water stability, which are evaluated by different tests, such as the rutting test, low temperature bending test and freeze–thaw splitting test. From Table 9, it is evident that under identical gradation conditions, the optimal asphalt content for SAKIIIWMRA is superior to that of the SBS modification asphalt. Excluding the factor that rubber powder increases the asphalt content, the asphalt content of SAKIIIWMRA is roughly equal to that of SBS modification asphalt. Based on the analysis of the road performance indexes of SAKIII WMRA Mix and SBS-MA Mix, it is found that their dynamic stability is basically the same, which is much higher than the value specified in the specification, and the two mixtures have good high-temperature stability. Dynamic stability serves as a crucial parameter for assessing the high-temperature performance of asphalt. It mirrors the capacity of asphalt pavements to resist the formation of conspicuous ruts under high-temperature conditions. Since the dynamic stabilities of the two asphalts are almost identical, it implies that their high-temperature stabilities are comparable. The dynamic stability values of both types of asphalt exceed the specified values in the relevant code. This indicates that these two types of asphalt exhibit excellent thermal stability at high temperatures.

The bending strain of SAKIII WMRA Mix at low temperature is 27.3% higher than that of the SBS modified asphalt mixture. From this, it can be logically inferred that SAKIII WMRA Mix possesses superior low-temperature performance compared to the SBS modified asphalt mixture. The improvement of low temperature performance may be due to the use of SAKIII warm-mixing additive, which reduces the workability of the mixture and thus improves the low temperature performance. Water stability reflects the ability of asphalt pavement to resist freezing and water damage in a wet environment. From the perspective of the freeze–thaw splitting strength ratio, SAKIII WMRA Mix has a value 8.51% higher than SBS-MA Mix. Nonetheless, both mixtures satisfy the specifications. Therefore, it can be deduced that the water stability of SAKIII WMRA Mix is superior to that of SBS-MA Mix. From the above, it can be seen that SAKIII WMRA Mix has better low-temperature crack resistance and water stability while maintaining the high-temperature stability equivalent to SBS-MA Mix. This makes the application of SAK-III WMRA Mix in cold regions have certain advantages, especially in applications that require better low-temperature performance and water stability. Although the use of warm-mixing additives may reduce the workability of construction, it can be solved by optimizing the construction technology in actual construction.

Overall, SAKIII WMRA Mix demonstrates better road performance compared to SBS-MA Mix. This superior performance aligns with the stipulations of the construction technology for the stress absorption layer of asphalt pavement. Consequently, SAKIII WMRA Mix emerges as a more suitable choice for use in the stress absorption layer in cold regions.

4. Discussion

4.1. Comparison of Cost and Environmental Impact Between WMRA Mix and Traditional RAMix

Compared with the traditional rubber–asphalt mixture (usually by heating), the cost and environmental impact of WMRA Mix can be considered from many dimensions such as production cost, energy consumption, material use and waste treatment.

Table 10. Comparison of cost and environmental impact between warm mix rubber–asphalt mixture and traditional rubber–asphalt mixture.

Comparative Term	WMRA Mix	Traditional Method
cost
initial investment	Relatively high	higher
operating costs	Relatively low	higher
energy consumption	lower	higher
Comprehensive cost	medium	higher
environmental effect
energy consumption	lower	higher
CO2 emission	lower	higher
Scrap/waste	lower	higher
Pollution of soil/water sources	lower	higher
noise pollution	lower	higher
Visual influence	lower	higher

below shows a comparison between the two methods [43].

The above table is just a basic comparison, and it can not completely cover all possible comparison items. The actual cost and environmental impact may vary depending on the specific projects and construction methods. Using WMRA Mix will change the energy consumption and waste generation in the construction process, thus reducing the comprehensive cost and environmental impact. However, this new construction method may require a high initial investment, which may increase operating costs. Meanwhile, the use of WMRA Mix may also reduce the pollution of soil and water, noise and visual impact, which is an obvious advantage over traditional methods. Generally speaking, although the use of WMRA Mix may increase some initial investment, its advantages in operating costs and environmental impact may exceed these additional costs in the long run. However, many factors need to be considered in the specific cost–benefit analysis, including specific project requirements, construction methods, equipment selection, and worker training [44].

4.2. Analysis of the Influence of the Reduction in Compaction Temperature on Energy Consumption and Emission

Reducing compaction temperature mainly affects two aspects: energy consumption and emissions. The following is an analysis of these two aspects: aspects.

• Energy consumption

Reducing the construction temperature of the mixture, especially in the process of mixing the asphalt mixture, can reduce the energy consumption caused by temperature rise. As mentioned in Reference [45], the energy saved in the mixing process can be calculated by lowering the mixing temperature. As an illustration, in the event that the mixing temperature of the WMRA mix is decreased from the typical high temperature by a specific degree (for instance, 30 °C), during the process of mixing an identical quantity of the mixture, a substantial amount of energy can be spared. The specific savings can be calculated by the thermodynamic equation, and the formula is as follows:

Q = C × M × Δt

where Q is the energy saved, C is the specific heat capacity of the substance, M is the mass of the substance, and Δt is the temperature change value. In the actual calculation, it is also necessary to consider the specific value of specific heat capacity and the ratio of asphalt to mineral powder in the mixture.

See

Table 11. Comparison of energy consumption of WMRA mix after reducing compaction temperature in different studies.

Mixture Type	Reduce the Value of Compaction Temperature (°C)	Energy Saving Situation
RAMix	None (conventional high temperature construction)	As a comparison benchmark, energy consumption is at a high level, and there is no energy-saving effect brought by reducing compaction temperature.
FOAM WMRA mix	Compared with the hot-mixed rubber–asphalt mixture, it is significantly reduced.	The production of 1000 m3 FOAM WMRA mix can save about 1.69 t of heavy oil, which is equivalent to 2.4 t of standard coal.
SMC WMRA mix	30–60	It can save fuel by 20–30%.
FOAM WMRA mix	Around 30	Compared with a hot-mixed asphalt mixture, it can save 30% of energy consumption.
SAK WMRA mix	25–35	Under the same construction scale, compared with the conventional HMRA Mix, the energy consumption is reduced by about 22%, which means that the energy equivalent to 1.8 t standard coal can be saved for every certain amount of mixture (for example, 1000 m3).
SASOBIT WMRA mix	20–30	Using the SASOBIT warm-mixing additive can save energy equivalent to 2 t of standard coal for every 1000 m3 mixture produced. After the construction temperature is lowered, the energy consumption in the mixing process is obviously reduced.

for the comparison of energy consumption of WMRA Mix after reducing compaction temperature in different studies.

2.

Environmental emissions

Lowering the construction temperature not only reduces the energy consumption but also significantly reduces the harmful gas emissions generated by asphalt heating during the construction process. In the traditional asphalt heating process, especially when fuel oil or other fossil fuels are used as energy, a lot of pollutants such as carbon dioxide and nitrogen oxides will be released [46]. Using warm mixing technology can effectively reduce the production of these emissions, thereby reducing the impact on the environment. See

Table 12. Comparison of environmental emissions of WMRA mix after reducing compaction temperature in different studies.

Mixture Type	Reduce the Value of Compaction Temperature (°C)	Major Emission Reductions and Their Reduction.
RAMix	None (conventional high temperature construction)	Emissions of a large number of pollutants, such as the production of 1000 m, CO emissions are large, and there is no emission reduction advantage, which is a high emission reference for the comparison of various warm mix types.
FOAM WMRA mix	Compared with the hot-mixed rubber–asphalt mixture, it is significantly reduced.	Production of 1000 m3 foam warm-mixed rubber–asphalt mixture can reduce CO by about 6.3 t, CO by about 317 kg and NOx by about 107 kg; The emission reduction efficiency of CO2 is 8.1%, and the emission reduction efficiencies of CO, NOx and SO2 are 65.0%, 73.3% and 75.0%, respectively.
WMRA mix	40–60	Reduce greenhouse gas emissions such as carbon dioxide by more than 50%, and reduce toxic gas emissions such as asphalt smoke by more than 80%.
SAK WMRA mix	25–35	Compared with the traditional hot-mixed rubber–asphalt mixture, when producing 1000 m3 mixture, the CO2 emission reduction is about 5.5 t, and the emission reduction efficiency reaches 7.5%. CO emission reduction is about 280 kg, and the emission reduction efficiency is about 60%; NOx emission reduction is about 95 kg, and the emission reduction efficiency is about 68%.
SASOBIT WMRA mix	20–30	When producing 1000 m3 mixture, CO2 emission reduction is about 5 t, and the emission reduction efficiency is about 7%. CO emission reduction is about 250 kg, and the emission reduction efficiency is about 55%; NOx emission reduction is about 85 kg, and the emission reduction efficiency is about 65%, which effectively reduces harmful gas emissions.

for the comparison of environmental emissions of the WMRA mix after reducing the compaction temperature.

Generally speaking, using WMRA Mix as a stress absorption layer in cold regions can not only reduce energy consumption, but also significantly reduce the emission of harmful gases in the construction process by lowering construction temperature, which has positive effects on environmental protection and energy saving.

4.3. Analysis on the Implications of Stress Absorption Layer Technology of WMRA Mix in Cold-Region Infrastructure

In cold regions’ low temperature environments, the stress absorption layer technology using WMRA Mix demonstrates excellent performance, fulfilling local engineering requirements and economic benefits. Given the low temperature characteristics in these regions, the low temperature crack resistance of WMRA Mix is of utmost significance as warm-mix rubber–asphalt mixture can maintain material integrity in low-temperature settings and curtail crack formation and expansion. In cold, humid regions, the water stability of WMRA Mix, which is associated with material water resistance and the capacity to withstand freeze–thaw cycles [47], is extremely crucial and should be gauged by freeze–thaw cycle tests to guarantee long-term performance under wet conditions. When applying WMRA Mix in cold regions, equal importance must be attached to its construction workability, which encompasses elements like construction temperature, mixture handling convenience, and compatibility with construction machinery [48]. For cold-region projects, economic benefits are a vital consideration; using WMRA Mix can cut down energy consumption, decrease environmental pollution, and potentially lower long-term maintenance costs [49], and its economic rationality ought to be appraised via cost–benefit analysis. In practical applications, it is essential to consult local specifications and standards for material selection and design [50]. By carrying out test roads and field tests, the road performance of the stress absorption layer, including long-term monitoring of properties such as rutting, cracking, and water stability, can be further validated [32]. In summary, in the practical implementation of infrastructure in cold regions, it is imperative to comprehensively take into account the low-temperature performance, water stability, construction workability, economic benefits, and compliance with local codes and standards of WMRA Mix to ensure that the material’s overall performance meets engineering demands.

5. Conclusions

(1)

The application of SAKIII warm-mixing additive significantly improves the dynamic stability of RAMix, reduces the mixing and compaction temperature, thus improving the performance and durability of the mixture, and achieving energy saving and consumption reduction. Compared with HMRA Mix and SASOBIT WMRA Mix, the high-temperature stability of SAK-III WMRA Mix is outstanding, which far exceeds the requirements of construction technical specifications.

(2)

When contrasted with the other two types of rubber–asphalt mixtures, SAKIII WMRA Mix exhibits superior low-temperature crack resistance. Its low temperature performance is equivalent to that of a traditional rubber–asphalt mixture. Even if the temperature for mixing and compaction is decreased by 30 °C, it can still maintain good low-temperature performance, showing strong anti-deformation ability and stress relaxation ability. Freeze-break temperature can evaluate the low temperature anti-crack capacity of WMRA Mix more accurately than freeze-break strength, transformation point temperature, and slope of thermal stress growth.

(3)

In comparison to HMRA Mix and SASOBITWMRA Mix, SAKIII WMRA Mix demonstrates superior water stability. Consequently, SAKIII warm-mixing additive is more apt to function as a modifier for the rubberized asphalt. This application can effectively safeguard the long-term stability and durability of pavement performance.

(4)

The linear relationship between fatigue and strain in three distinct asphalt mixtures designated as stress absorption layers is remarkably evident. When analyzing the coefficient within the fatigue curve equations, it becomes clear that the k value of WMRA Mix is somewhat lower than that of HMRA Mix. However, it is substantially greater than the k value of SBS-MA Mix. This disparity implies that the fatigue resistance capabilities of WMRA Mix closely approximate those of HMRA Mix, while clearly outperforming SBS-MA Mix. Shifting the focus to the n value in the fatigue curve equations, WMRA Mix presents a value that is slightly elevated compared to HMRA Mix. Conversely, it is significantly lower than that of SBS-MA Mix. Such a difference suggests that the fatigue life of WMRA Mix is more responsive to variations in the strain level when contrasted with HMRA Mix. In contrast, it is far less sensitive than SBS-MA Mix. Evidently, SBS-MA Mix displays an especially high degree of sensitivity to the strain level. These findings offer profound insights into the fatigue-related behaviors of these materials, which can be instrumental in guiding material selection and design strategies in road-building projects. Understanding the fatigue characteristics of the above materials will provide important theoretical support and reference for road construction in cold regions, especially for the application of WMRA Mix.

(5)

Even when the temperatures for both mixing and compaction are cut down by 30 °C, RAMix blended with SAKIII warm-mixing additive is able to maintain its outstanding road-performance capabilities. This temperature reduction, which is a key feature of the SAKIII warm-mixing technology, does not compromise the quality of RAMix. Instead, it allows for the production of a high-performance asphalt mixture that can meet the demanding requirements of modern road construction. The unique properties of SAKIII warm-mixing additive enable RAMix to achieve a fine balance between workability at lower temperatures and long-term durability, making it a viable option for various road-building applications. When employed as a stress absorption layer in cold regions, WMRA Mix presents distinct advantages over SBS-MA Mix. It features better road performance, a longer service life, and lower energy consumption along with reduced carbon emissions, all of which align with the demands of sustainable development.

The anti-cracking technology integrated into the WMRA Mix’s stress absorption layer significantly broadens the application scope of RAMix. By effectively alleviating cracking issues, it enhances the durability and lifespan of the asphalt mixture. This is achieved by markedly reducing stress concentration, resulting in a smoother and more stable surface, which is highly suitable for high-traffic and heavy-load areas. This technological innovation not only expands RAMix’s applications but also holds the potential to revolutionize the construction industry, offering a more sustainable and resilient solution for road infrastructure. Compared to other anti-crack technologies, it showcases superior crack-prevention capabilities. WMRA Mix technology also addresses key road-construction issues. It strengthens interlayer adhesion between asphalt overlays and cement concrete or semi-rigid bases, curbing crack diffusion. Additionally, it mitigates cracks by dispersing stress. Furthermore, this technology has dual benefits. It promotes sustainability by fully utilizing waste resources and reducing the mixture’s mixing and compaction temperatures, thereby improving construction performance and site working conditions, ensuring efficient and safe construction. However, as a novel technology, it requires continuous exploration in practical engineering. In-depth dynamic mechanical response analysis of overlay structures with pre-existing defects under moving loads is necessary. Efforts should be made to formulate and refine design codes, expand raw-material sources to cut costs, and popularize the technology. Simultaneously, progress in long-term road-performance tracking and energy-saving/environment-protection evaluation is vital.