Rheological Properties and Performance Evaluation of Different Types of Composite-Modified Asphalt in Cold Regions

Abstract

In low-temperature environments, asphalt materials harden easily and become brittle, and the repeated action of traffic load further aggravates the cracking of and damage to the asphalt mixture. In order to explore high-performance asphalt pavement materials that are more suitable for cold climates, this paper selected four modifiers, namely SBS, rubber powder, SBR and TPS. With SBS as the main agent, combined with other modifiers, three types of base asphalts with grades of 70#, 90# and 110# were compositely modified to prepare 12 different combinations of composite-modified asphalt samples. The optimal dosage of the modifier was determined by the basic performance test of asphalt, and the compatibility, interaction energy and mechanical properties of the modifier and base asphalt at different temperatures were analyzed by molecular dynamics simulation. Subsequently, the high- and low-temperature rheological properties of various modified asphalts were systematically evaluated using a dynamic shear rheology test (DSR) and a bending beam rheology test (BBR), and the rheological properties and road performance indicators of each composite-modified asphalt were comprehensively compared so as to select the road materials most suitable for cold areas. The research results show that different grades of base asphalt and modifiers show good compatibility in the range of 160–175 °C. Among them, rubber powder and TPS modifier significantly improve the high-temperature mechanical properties of SBS-modified asphalt, while rubber powder and SBR modifier significantly improve its low-temperature mechanical properties. The DSR and BBR test results further show that SBS/rubber powder composite-modified asphalt exhibits excellent rheological properties under both high- and low-temperature conditions, and is the preferred solution for road materials in cold regions.

1. Introduction

Severely cold regions refer to regions where the average temperature of the coldest month is below −10 °C and the average daily temperature is below 5 °C for more than 145 consecutive days. Asphalt pavement is still the preferred pavement construction material in such regions due to its good bearing capacity, comfortable driving, anti-skid, low noise and other functional characteristics. However, asphalt is an organic cementitious material and is easily affected by the external temperature [1]. Asphalt pavement often suffers from rutting, cracks, congestion and other problems under the influence of various external factors, such as overload, heavy load, temperature and extreme climate. The more complex climate environment and frequent freeze-thaw phenomena in cold regions seriously affect the service life of asphalt pavement [2]. Therefore, how to effectively improve the road performance of asphalt pavement in cold regions has become another problem that needs to be solved urgently.

SBS can reduce the temperature sensitivity of asphalt, making SBS-modified asphalt suitable for road paving, construction engineering and industrial applications. However, with the rapid increase in traffic volume, the road performance of SBS-modified asphalt can no longer meet the current traffic needs. Therefore, improving the road performance of asphalt and increasing the service life of asphalt pavement have become important research directions [3,4,5]. However, single-modified asphalt has certain limitations in improving pavement performance. In order to enhance the road performance of asphalt, Tan et al. [6,7,8] proposed a composite modification of asphalt to extend the service life of the pavement and reduce the occurrence of road diseases.

Lu et al. [9] studied the physical hardening law of base asphalt and modified asphalt under low-temperature conditions and believed that the physical hardening of asphalt under a low-temperature environment would affect its low-temperature creep stiffness and creep rate over time. The hardening behavior of modified asphalt under low-temperature conditions depends on the base asphalt. Stastna et al. [10] studied the viscosity change of SBS-modified asphalt at different temperatures and found that when SBS was added to asphalt by high-speed shearing, it would form a stable three-dimensional network structure with asphalt, and the shear rate would affect the stability of the three-dimensional network structure. This “enhancement behavior” would significantly enhance the high-temperature performance of asphalt, but would have little effect on the low-temperature performance. In order to study the effect of SBS modifier on the high-temperature performance of asphalt, Stangl et al. [11] characterized SBS-modified asphalt from the aspects of chemical composition, microstructure, micromechanical properties and thermal analysis behavior, and summarized the effect of SBS modifier on the chemical composition, microstructure and high-temperature performance of asphalt. Larsen et al. [12] blended SBS copolymers of different molecular weights with matrix asphalt, and studied the rheological and morphological changes of modified asphalt during the manufacturing process using rotational viscosity, fluorescence microscopy and FT-IR spectroscopy. It is believed that at higher shear rates and temperatures, SBS copolymers will degrade and distribute into the matrix asphalt, and the optimal rheological properties of modified asphalt depend on the composition, shear rate and shear time of the modified asphalt. Moreno-Navarro et al. [13] studied the effect of polymer modifiers on the long-term mechanical properties of asphalt mixtures at different temperatures. The rheological properties of SBS-modified asphalt with different dosages at different temperatures were studied. It is believed that SBS has a greater impact on the rheological properties of asphalt only at appropriate temperatures [14].

In order to further study how to enhance the high- and low-temperature performance of asphalt, Zhang et al. [15,16] found that styrene-butadiene rubber (SBR) and polyphosphoric acid (PPA) have good compatibility with asphalt, can also improve the adhesion between asphalt and stone, and can also enhance the high- and low-temperature rheological properties. Compared with the shortcomings of single-SBS-modified asphalt in low-temperature performance, PPA/SBR not only has better road performance, but also has good storage stability. SUN et al. [17] summarized the influence of rubber powder, SBR and SBS on the rheological properties of asphalt in order to improve the performance of matrix asphalt at high and low temperatures. According to the softening point of asphalt, 5 °C ductility and 60 °C viscosity, nanomaterials and polymers were selected to compositely modify the asphalt. The test results of DSR and BBR showed that the aging resistance, high-temperature performance and low-temperature performance (crack resistance and water stability) of NANO/SBS composite-modified asphalt are all better than those of SBS-modified asphalt. Kok et al. [18] studied the effects of styrene-isoprene-styrene (SIS) used alone and in combination with CR on the general properties and rheological properties of asphalt. For this purpose, viscosity, softening point, penetration, BBR and DSR tests were conducted. The elastic properties and performance classification were evaluated using the multi-stress creep recovery test (MSCR). It is believed that compared with SIS modifier, SIS/CR composite modifier can further improve the high- and low-temperature properties of asphalt.

In summary, SBS modifier is still the main choice in composite-modified asphalt due to its unique polymer properties, while other auxiliary modifiers are mainly used to make up for the performance deficiencies of SBS-modified asphalt. Based on this, this article takes SBS modifier as the core, supplemented by rubber powder, SBR and TPS modifiers, to study the properties of asphalt of different grades. Determine the composition design and technical parameters of different types of composite-modified asphalt and analyze the high- and low-temperature properties of the modified asphalt based on DSR tests and BBR tests. The research results have important application value for the development of high-performance pavement materials that are more durable and adaptable to extreme climate conditions, especially in cold areas. They can significantly improve the service life and construction quality of pavements and promote the advancement of green and environmentally friendly road construction technologies.

2. Material Design and Methods

2.1. Test Materials

2.1.1. Asphalt

The 70# matrix asphalt used in this study was produced in Jinshi, Shandong, and the 90# and 110# matrix asphalts were produced in Panjin, Liaoning. The basic indicators of the three matrix asphalts are shown in

Table 1. Basic indicators of base asphalt.

Asphalt Type	25 °C Penetration /0.1 mm	Softening Point /°C	10 °C Ductility /cm	60 °C Dynamic Viscosity /Pa·s
70#	65	47.2	94	184
90#	92	45.2	96.2	189
110#	115	43.5	98.0	162

.

2.1.2. Modifier

This article uses four modifiers: SBS, SBR, rubber powder and TPS. The appearance of the modifier is shown in Figure 1.

(1)

SBS

SBS modifier is a polymer material composed of styrene-butadiene-styrene copolymer and other complexes. It appears as a white granular solid. The main feature of SBS modifier is its good flexibility and elasticity, which can significantly improve the high- and low-temperature properties of asphalt. This article uses the linear SBS modifier LG-501 produced in Seoul, South Korea. The technical specifications are shown in

Table 2. SBS modifier technical indicators.

S/B	Density (g/cm3)	Tensile Strength (MPa)	Elongation at Break (%)	Melt Index (g/10 min)	Hardness (Shaw A)
31/69	0.94	24	750	<1	76

.

(2)

Rubber powder modifier

Rubber powder modifier is mainly processed from recycled waste tires and appears as a black powdery solid. Rubber powder modifier has an excellent modification effect and can significantly improve the mechanical properties and durability of asphalt mixtures, especially the crack resistance at low temperatures. The technical indicators are shown in

Table 3. Technical indicators of rubber powder (80 mesh) modifier.

Ash Content (%)	Acetone Extract (%)	Carbon Content (%)	Fiber Content (%)	Metal Content (%)	Rubber Diameter Content (%)	Natural Rubber Content (%)	Density (g/cm3)
≤7	≤10	≥28	<0.5	≤0.03	≥48	≥25	1.1~1.2

.

(3)

SBR modifier

SBR (styrene-butadiene rubber) modifier is a synthetic rubber used to modify asphalt. It is a white powdery solid. It is synthesized by the copolymerization of styrene and butadiene. SBR modifier has a high elastic modulus, wear resistance, weather resistance, oil resistance and other characteristics. It can significantly improve the high-temperature performance of asphalt and reduce the brittleness of asphalt. Its technical indicators are shown in

Table 4. SBR modifier technical indicators.

Density (g/cm3)	Tensile Strength (100%)	Tensile Strength (200%)	Breaking Strength	Wear and Tear	Hardness
1.37	6.0 MPa	12.0 MPa	15.2 MPa	0.47	78

.

(4)

TPS modifier

TPS modifier (TAFPACK-SUPER) is a thermoplastic elastomer with a translucent yellow granular appearance. It can improve the shear resistance and durability of asphalt, especially its stability at high temperatures. It is widely used in road projects such as highways, airport runways and parking lots. Its technical indicators are shown in

Table 5. Technical indicators of TPS modifier.

Density (g/cm3)	Tensile Strength (MPa)	Elongation at Break (%)	Heat Resistance (°C)	Cold Resistance (°C)	Tg (°C)
1.20	≥10 Mpa	≥500	200	−45	78

.

2.2. Design of Composite-Modified Asphalt

2.2.1. Modifier Combination Design

This paper mainly uses SBS modifier and three other modifiers as auxiliary agents to carry out composite modification of different types of asphalt (distinguished by grades). The final combination scheme is shown in

Table 6. Modifier combination scheme.

Asphalt Type	70#	90#	110#
Combination plan	70#SBS	90#SBS	110#SBS
	70#SBS/Rubber powder	90#SBS/Rubber powder	110#SBS/Rubber powder
	70#SBS/SBR	90#SBS/SBR	110#SBS/SBR
	70#SBS/TPS	90#SBS/TPS	110#SBS/TPS

.

2.2.2. Determination of Preparation Process Parameters

The shear time (t) and rate (v) required for preparing composite-modified asphalt are mainly affected by the type of modifier, while the modifier dosage and asphalt grade have no significant effect on the above variables [19]. In order to make the modifier evenly dispersed in the asphalt, this paper adopts a combination of low-speed stirring and high-speed shearing to prepare composite-modified asphalt. Due to different types of modifiers, the preparation process used is also different. The specific process parameters are shown in

Table 7. Preparation processes of different types of modified asphalt.

Modifier Type	Preparation Process
SBS	1. Heat the asphalt to a molten state in an oven at 170 °C. 2. Move to the mixer at 170 °C (1600 r/min), add the SBS modifier to the asphalt and stir evenly for 20 min. 3. Move to the shearing machine at 170 °C (5000 r/min) and shear at high speed for 50 min. 4. Place it in an oven at 170 °C to swell and then rest for 1 h to remove air bubbles in the modified asphalt.
SBS/Rubber powder	1. Heat the asphalt to a molten state in an oven at 170 °C. 2. Move to the mixer at 170 °C (1600 r/min), add the SBS modifier to the asphalt and stir evenly for 20 min. 3. Move to the shearing machine at 170 °C (5000 r/min) and shear at high speed for 50 min. 4. Move to the mixer at 175 °C (1600 r/min), add the rubber powder modifier into the asphalt and stir evenly for 30 min. 5. Move to the shearing machine at 175 °C (5000 r/min) and shear at high speed for 60 min. 6. Place it in an oven at 170 °C to swell and then rest for 1 h to remove air bubbles in the modified asphalt.
SBS/SBR	1. Heat the asphalt to a molten state in an oven at 170 °C. 2. Move to the mixer at 170 °C (1600 r/min), add the SBS modifier to the asphalt and stir evenly for 20 min. 3. Move to the shearing machine at 170 °C (5000 r/min) and shear at high speed for 50 min. 4. Move to the mixer at 170 °C (1600 r/min), add the SBR modifier to the asphalt and stir evenly for 20 min. 5. Move to the shearing machine at 170 °C (5000 r/min) and shear at high speed for 50 min. 6. Place it in an oven at 170 °C to swell and then rest for 1 h to remove air bubbles in the modified asphalt.
SBS/TPS	1. Heat the asphalt to a molten state in an oven at 170 °C. 2. Move to the mixer at 170 °C (1600 r/min), add the SBS modifier to the asphalt and stir evenly for 20 min. 3. Move to the shearing machine at 170 °C (5000 r/min) and shear at high speed for 50 min. 4. Move to the mixer at 180 °C (1600 r/min), add the TPS modifier to the asphalt and stir evenly for 30 min. 5. Move to the shearing machine at 180 °C (5000 r/min) and shear at high speed for 60 min. 6. Place it in an oven at 170 °C to swell and then rest for 1 h to remove air bubbles in the modified asphalt.

below.

2.2.3. Optimum Dosage of Modifier

According to the “Testing Procedures for Asphalt and Asphalt Mixtures for Highway Engineering (JTG E20-2011)”, the basic performance test of asphalt is carried out on asphalt, and the optimal dosage of modifier is determined by carrying out a basic performance test of asphalt (25 °C penetration, softening point, 5 °C ductility, 135 °C viscosity) on composite-modified asphalt. At high temperature, the smaller the needle penetration of asphalt, the higher the softening point, and the greater the viscosity at 135 °C, the better the high-temperature performance of asphalt; at low temperature, the greater the 5 °C ductility of asphalt, the better the low-temperature performance of asphalt. The optimal dosage of modifiers for various types of composite-modified asphalt is shown in

Table 8. The optimal dosage of the modifier.

Material Name	70#	90#	110#
Composite-modified asphalt	4%SBS	4%SBS	5%SBS
	4%SBS + 15%Rubber powder	4%SBS + 15%Rubber powder	5%SBS + 16%Rubber powder
	4%SBS + 3%SBR	4%SBS + 3%SBR	5%SBS + 3%SBR
	4%SBS + 7%TPS	4%SBS + 7%TPS	5%SBS + 7%TPS

below.

2.3. Molecular Dynamics Simulation

2.3.1. Introduction to MD Simulation

Molecular dynamics (MD) is a computational simulation method used to simulate the time evolution of atomic, molecular or other particle systems. By calculating and numerically integrating the interactions between particles, the dynamic behavior and structural changes of molecules in a specific environment can be simulated and predicted. By integrating these equations of motion over time, the evolution trajectory of the system can be calculated, and the behavior of the system under different conditions can be predicted.

2.3.2. Model Construction

Li and Greenfield [20] proposed a 12-component AAA-1 asphalt model based on the US Strategic Highway Research Program (SHRP). In this asphalt model, the asphaltene component is represented by a combination of three different molecules (namely, asphaltenephenol, asphaltene-pyrrole, asphaltene-thiophene). These three asphaltene molecules were proposed by Mullins [21] and have more reasonable structures. Resin (polar aromatic hydrocarbons) is composed of five molecules, including quinolinohopane, thioisorenieratane, benzobisbenzothiophene, pyridinohopane and trimethylbenzeneoxane. The ratio of resin to asphaltenes can determine the type of asphalt colloidal structure to a certain extent. The saturates are represented by (squalane and hopane), which are improved structures compared with the previous n-alkanes (n-C22) and are more consistent with the results of experimental studies [22]. Use PHPN and DOCHN to represent two types of aromatics.

(1)

Asphalt molecular model

The component models of asphalt are shown in Figure 2. The four-component test analysis of each base asphalt obtained its four-component ratio, as shown in

Table 9. The proportion of four components of asphalt of each grade.

Asphalt Component	70#	90#	110#
Asphaltenes	4.9%	7.29%	10.6%
Saturates	28.5%	28.71%	28.9%
Aromatic	27.6%	34.31%	43.3%
Resin	39%	29.69%	17.2%

. The ratio of the 12-component molecular model of asphalt obtained according to the four-component test is shown in

Table 10. The proportion of twelve components of asphalt of each grade.

Asphalt Component	Formula	70#		90#		110#
Asphaltenes	C51H62S	1	5.2%	2	8%	3	12
	C42H54O	1		1		2
	C66H81N	1		1		1
Saturates	C35H62	12	28%	11	29%	11	28
	C30H62	12		12		12
Aromatic	C30H46	14	28%	16	35%	20	43
	C35H44	11		13		17
Resin	C18H10S2	5	38.8%	3	28%	2	17%
	C36H57N	4		2		1
	C29H50O	17		15		10
	C40H60S	4		2		1
	C40H59N	4		2		1

.

(2)

Modifier molecular model

This article uses four modifiers: SBS, rubber powder, SBR and TPS. Among them, SBS is synthesized by the copolymerization of three monomers: styrene, butadiene and styrene in certain proportions. On the SBS molecular chain, styrene monomer units and butadiene monomer units are alternately arranged to form a styrene-butadiene-styrene block structure. SBR is a synthetic rubber made from the copolymer of butadiene and styrene. Its chemical formula is usually expressed as (C₈H₈)_x(C₄H₆)_y. Among them, x and y represent the molar ratio of styrene and butadiene monomers, which usually varies between 20:80 and 30:70. Rubber powder is mainly composed of natural rubber and other additives. Natural rubber is a polymer compound with polyisoprene as its main component. The molecular formula is (C₅H₈)_n, and 91% to 94% of its components are rubber hydrocarbons (polyisoprene). TPS is a high-viscosity asphalt modifier. Because there is no clear molecular formula for TPS in existing research, this article only uses double SBS to represent TPS and perform calculations in the mechanical properties section. The molecular formulas of each modifier are shown in Figure 3.

The molecular composition ratios of SBS and SBR modifiers are shown in

Table 11. SBS composition proportions.

Element	Mass Ratio	Molecular Mass	Number
Styrene	23.9%	106 g/mol	1
trans-butadiene	58.3%	56 g/mol	8
cis-butadiene	6.3%	56 g/mol	1
Styrene	12.7%	106 g/mol	1

and

Table 12. SBR composition proportions.

Element	Mass Ratio	Molecular Mass	Number
Styrene	14.8%	106 g/mol	2
trans-butadiene	62.6%	56 g/mol	9
cis-butadiene	7.8%	56 g/mol	1
1.2Butadiene	14.8%	56 g/mol	2

below, respectively. The molecular model diagram of the modifier is shown in Figure 4 below.

2.3.3. Modified Asphalt Molecular Model

The modified asphalt molecular model is constructed according to the optimal dosage of each modifier. Taking 90#SBS/rubber-powder-modified asphalt as an example, the optimal dosage of SBS is 4%, and the optimal dosage of rubber powder is 15%. The molecular model is built according to the molar mass ratio. The established model is geometrically optimized and annealed to minimize the energy of the asphalt system to ensure the convenience and accuracy of molecular dynamics calculations. First, the geometry optimization function in the Forcite block is used, the COMOASS force field is selected and the maximum number of iterations is 50,000 to converge the model and minimize the energy. Secondly, the annealing function in the Forcite block, the COMPSS force field, and the NVT ensemble are selected; the number of cycles is 10, and the annealing is performed. Finally, the annealed interface model is subjected to dynamic calculations for further research and analysis. The 90#SBS/rubber-powder-modified asphalt model is shown in Figure 5.

2.4. Test Methods

All tests in this paper were conducted in accordance with the Chinese standard “Test Procedure for Asphalt and Asphalt Mixtures in Highway Engineering” (JTG E20-2011) [23].

2.4.1. Dynamic Shear Rheology Test (DSR)

The DSR test of asphalt is a test method for evaluating the mechanical properties of asphalt materials under dynamic loads. By determining parameters such as the complex shear modulus (G*) and phase angle (δ) of the asphalt material, its high-temperature applicability range can be evaluated. The DSR test can be carried out at different temperatures to determine the dynamic shear properties of asphalt at different temperatures. The temperature test range of this article is 46–70 °C, the temperature interval is 6 °C and the loading frequency is 10 rad/s.

2.4.2. Bending Beam Rheology Test (BBR)

The BBR test is a commonly used method for testing the low-temperature rheological properties of asphalt materials. It is used to study the deformation behavior and rheological properties of solid materials under stress. The test temperatures are set at −12 °C, −18 °C and −24 °C.

3. Results and Discussion

3.1. Solubility Parameter Analysis

When using molecular dynamics to simulate modified asphalt, solubility parameters are usually used to describe the strength of the interaction between the modifier and asphalt in order to predict the solubility of the modifier and changes in the molecular structure. In addition, solubility parameters can also be used to evaluate the environmental adaptability of modified asphalt. The calculation results of solubility parameters for different modifiers and different types of asphalt are shown in Figure 6.

As shown in Figure 6, in the temperature range of 130–180 °C, the solubility parameters of the three grades of matrix asphalt change with temperature in roughly the same trend, showing a downward trend. Because when the temperature increases, the absorbed heat energy of the asphalt molecular system gradually increases, causing the kinetic energy of the molecules to increase and the molecules to become active. The cohesive energy density and solubility parameters of the molecular system gradually decrease. The minimum value of the solubility parameter difference Δδ of the two molecular systems is between 0 and 0.4 in the temperature range of 160–175 °C, indicating that the compatibility is best in this temperature range. The closer the solubility parameters of the two molecular materials are, the better the blending effect will be. If the difference between the two exceeds 0.5. It is generally difficult to blend uniformly. In actual road construction, the mixing temperature of the asphalt mixture is also between 160 and 180 °C. Considering only the influence of temperature, in the temperature range of 160–175 °C, the three grades of SBS-modified asphalt have good compatibility with rubber powder and SBR modifier. At the same time, it was also verified that the above temperature settings in the asphalt preparation process are reasonable. The modified asphalt prepared within this temperature range has the best performance. Taking 90# SBS/SBR-modified asphalt as an example, the SEM scanning morphology is shown in Figure 7.

As shown in Figure 7, the surface morphology of SBS-modified asphalt is in a piled state because the SBS modifier adsorbs the asphalt components, causing the asphalt to adhere to the surface of the modifier, thus presenting a cluster morphology. After adding the SBR modifier, the surface morphology is relatively flat, because SBR and SBS asphalt have a competitive mechanism, and the SBR modifier itself is in a granular state, which can better fill the gaps in the SBS-modified asphalt, so it presents the state shown in the above figure.

3.2. Analysis of Interaction Energy Calculation Results

In molecular dynamics simulations, interaction energy refers to the energy of interactions between molecules. It is an important parameter describing the interaction between molecules. It includes van der Waals interaction energy, electrostatic potential energy, non-bonding energy and other interaction energies between molecules. The magnitude of the interaction energy can help study the interaction mechanism and behavior between molecules, such as the attraction, repulsion and bonding between molecules. The larger the value of the interaction energy, the more stable the miscible system and the better the adhesion. The calculation method of interaction energy is as shown in the following formula. The interaction energy calculation results of modified asphalt are shown in Figure 8.

E_inter = E_ab − E_a − E_b

In the formula: E_inter—interaction energy (kJ/mol); E_ab—The total energy of removing a and b from the equilibrium structure (kJ/mol); E_a—The single point energy obtained by removing b from the balanced structure (kJ/mol); E_b—The single point energy obtained by removing a from the balanced structure (kJ/mol).

As shown in Figure 8. In different systems, the energy changes between molecules show a certain regularity. Comparing SBS/SBR-modified asphalts of different grades, the total energy shows an increasing trend. According to existing studies, the total energy in the asphalt system is the main factor determining the bonding strength of the asphalt interface. The greater the energy, the stronger the intermolecular interaction force and the more stable the molecular structure. This shows that after adding the SBR modifier to the 110#SBS-modified asphalt, the intermolecular interaction is enhanced and the structural system of the modified asphalt becomes more stable. However, the SBS-modified asphalt using the rubber powder modifier shows the opposite trend. After adding the rubber powder modifier to the 110#SBS-modified asphalt, the total energy shows a decreasing trend, indicating that the 70#SBS-modified asphalt has a more stable compatibility with the rubber powder modifier.

3.3. Mechanical Property Analysis

3.3.1. Analysis of High-Temperature Mechanical Properties

As shown in Figure 9, Figure 10 and Figure 11. When the temperature increases from 46 °C to 70 °C, the Young’s modulus (E), bulk modulus (K) and shear modulus (G) of the three grades of modified asphalt molecular models all decrease. Taking 90#SBS-modified asphalt as an example, after adding rubber powder or TPS modifier, the values of E, K and G have increased, while after adding SBR modifier, the values have decreased. It shows that rubber powder and TPS increase the viscosity and toughness of 90#SBS-modified asphalt, while SBR modifier increases the brittleness of 90#SBS-modified asphalt. When the temperature increased from 46 °C to 70 °C, the E, K and G of 90# SBS-modified asphalt decreased by 11.89%, 31.88% and 43.72%, respectively. It shows that as the temperature continues to increase, the viscosity of asphalt decreases and the restrictions on molecular chain movement are released, resulting in an increase in the displacement between chain segments. The modifier cannot play a supporting role in asphalt, resulting in a decrease in modulus. The modifiers are ranked according to their E, K and G improvement effects on asphalt at high temperatures: SBS/rubber powder > SBS/TPS > SBS > SBS/SBR.

3.3.2. Analysis of Low-Temperature Mechanical Properties

As shown in Figure 12, when the simulated temperature is −18 °C, the E, K and G values of asphalt change regularly, and the low-temperature mechanical properties of the composite-modified asphalt gradually become better as the asphalt grade increases. Taking 90#SBS-modified asphalt as an example, after adding rubber powder and SBR modifier, the values of E, K and G increase, while after adding TPS modifier, the values decrease. It shows that rubber powder and SBR modifier enhance the low-temperature toughness of asphalt, while TPS modifier weakens the low-temperature toughness of asphalt. The modifiers are ranked according to their E, K and G improvement effects on asphalt at low temperatures: SBS/SBR > SBS/rubber powder > SBS > SBS/TPS.

3.4. DSR Test Analysis

3.4.1. Complex Shear Modulus

The complex shear modulus (G*) describes the total amount of deformation that asphalt resists after repeated shearing, and is composed of two parts: elastic deformation and viscous deformation. G* is one of the important indicators for measuring the high-temperature rheological properties of asphalt, reflecting the ability of asphalt to resist deformation at high temperatures. The larger the G* value, the stronger the ability of asphalt to resist deformation at high temperatures, and the better the high-temperature resistance. The test results of the complex shear modulus of various types of modified asphalt are shown in Figure 13.

When the temperature gradually increases from 46 °C to 70 °C, the G* of various types of modified asphalt decreases to varying degrees. This indicates that the high-temperature deformation resistance of various types of modified asphalt decreases with increasing temperature. When the asphalt grade is the same and the modifiers are different. As the temperature increases, the G* of the SBS/rubber-powder- and SBS/TPS-modified asphalts always maintains a high level, and the extreme value difference is large. The modification effect of SBS/rubber powder modifier is greater than that of SBS/TPS modifier. This indicates that adding rubber powder and TPS modifier to SBS-modified asphalt will enhance the high-temperature deformation resistance of asphalt, and the modification effect of rubber powder modifier is greater than that of TPS modifier. The G* of the SBS/SBR- and SBS-modified asphalts remains at a low level, but the change trend is relatively stable. However, compared with SBS modifier, SBS/SBR modifier will weaken the G* of 70# and 90# and enhance the G* of 110#, indicating that the modification effect of SBS/SBR modifier is related to the asphalt grade.

The G* of SBS/rubber-powder- and SBS/TPS-modified asphalt is much higher than that of SBS- and SBS/SBR-modified asphalt. This shows that the SBS/rubber powder and SBS/TPS modifiers are better than SBS and SBS/SBR modifiers in modifying the high-temperature performance of asphalt. When the asphalt grades are different and the modifiers are the same. As the temperature rises, the G* of modified asphalts of different grades shows the same change trend, and the overall high-temperature performance ranking is 70# > 90# > 110#. This shows that the modification effect of various types of modifiers on asphalt mainly depends on the high-temperature performance of the matrix asphalt itself. Compared with the 90#SBS-modified asphalt currently used in cold regions, the G* of 70#SBS/rubber powder, 90#SBS/rubber powder, 70#SBS/TPS, 90#SBS/TPS, 70#SBS and 110#SBS/rubber powder increased by 291%, 226%, 197%, 158%, 39% and 10%, respectively. The G* of each type of modified asphalt meets the requirements for road asphalt use.

3.4.2. Phase Angle

The phase angle (δ) of asphalt refers to the phase difference caused by hysteresis between the stress and strain of asphalt. It is one of the important indicators by which to measure the ratio of viscosity to elasticity of asphalt, usually represented by the symbol δ. When δ is close to 0°, the viscosity of asphalt is small and it shows strong elastic characteristics; when δ is close to 90°, the viscosity of asphalt is large and it shows strong viscosity characteristics. The phase angle test results of various types of modified asphalt are shown in Figure 14.

As shown in Figure 14, with the increase in temperature, the phase angle (δ) of each type of modified asphalt increases. In 70#-modified asphalt, the δ of SBS-, SBS/rubber-powder- and SBS/TPS-composite-modified asphalt shows a slow growth trend, and the δ value does not change much, indicating that the change range of its viscosity and elastic properties is relatively similar. The δ of 70# SBS/SBR-composite-modified asphalt increased by about 15°, indicating that there is a transition process from elasticity to viscosity, the viscosity component increases, and the high-temperature deformation resistance is weakened. In 90#-modified asphalt, the δ of SBS-, SBS/SBR- and SBS/TPS-composite-modified asphalt shows a slow growth trend, and the δ value changes little, indicating that the change range of its viscosity and elasticity is relatively similar. The δ of 90# SBS/rubber-powder-composite-modified asphalt increased by about 5°, indicating that there is also a transition process from elasticity to viscosity; the viscosity component increases and the high-temperature deformation resistance is weakened.

In 110# asphalt, the overall change trend of δ is the same, all growing slowly. The δ values of SBS-, SBS/SBR- and SBS/TPS-composite-modified asphalts are close, indicating that the viscosity and elastic properties of the three modified asphalts are similar. The δ of 110#SBS/rubber-powder-composite-modified asphalt is smaller than that of other modified asphalts of the same type, indicating that among 110# asphalt, 110#SBS/rubber-powder-composite-modified asphalt has the best high-temperature deformation resistance. In 70# and 90# asphalts, SBS/TPS-composite-modified asphalt has the best deformation resistance. Compared with the SBS modifier, the SBS/TPS modifier did not change the growth trend of δ of asphalts of various grades, while SBS/rubber powder and SBS/SBR modifiers changed the change trend of asphalt δ. It shows that SBS/rubber powder and SBS/SBR modifiers have a great influence on the high-temperature viscosity and elastic properties of asphalt. The improvement effects of the viscosity and elastic properties of each modifier are ranked as follows: 70#SBS, 110#SBS/rubber powder, 70#SBS/SBR, 70#SBS/TPS and 90#SBS.

3.4.3. Rutting Factor

The rutting factor (G*/sin δ) is an important indicator that describes the asphalt’s ability to resist rutting deformation and stability at high temperatures. The larger the value of G*/sin δ, the stronger the asphalt’s ability to resist rutting deformation at high temperatures. The calculation results of the rutting factors of various types of composite-modified asphalt are shown in Figure 15.

As shown in Figure 15, as the temperature increases, the G*/sin δ of each grade of composite-modified asphalt shows a different downward trend, indicating that the high-temperature rutting resistance and high-temperature stability of each type of modified asphalt decrease with increasing temperature. Because G*/sin δ is calculated through G* and phase angle δ, the G*/sin δ temperature change curve of each type of modified asphalt is very close to the G* change curve. In 70#- and 90#-modified asphalts, the G*/sin δ values of SBS/rubber-powder- and SBS/TPS-modified asphalt are higher, while the G*/sin δ values of SBS- and SBS/SBR-modified asphalt are lower. This shows that SBS/rubber powder and SBS/TPS modifiers help to improve the high-temperature rutting resistance of 70# and 90# asphalts.

In 110#-modified asphalt. Compared with SBS-modified asphalt, the high-temperature resistance to rutting deformation of SBS/rubber powder, SBS/SBR and SBS\TPS has been improved, and the change trend is relatively stable. Rank the modifiers according to their modification effects: SBS/rubber powder > SBS/TPS > SBS/SBR. Compared with the 90# SBS-modified asphalt currently used in cold areas, the G*/sin δ of 70#SBS/rubber-powder-, 90#SBS/rubber-powder-, 70#SBS/TPS-, 90#SBS/TPS-, 110#SBS/rubber-powder- and 70#SBS-modified asphalt increased by 329% and 259%, 258%, 211%, 46% and 42%. The G*/sin δ of the above types of modified asphalt all meet the requirements for road asphalt materials.

3.5. BBR Test Analysis

3.5.1. Creep Stiffness

The creep stiffness of asphalt (S) refers to the ability of asphalt to deform under the action of stress within a certain period of time. Creep stiffness can be determined by a creep test, which is performed under constant stress and then records the deformation of the asphalt over time. The creep stiffness results of various types of composite-modified asphalt are shown in Figure 16.

As shown in Figure 16, as the test temperature increases, the creep stiffness of various modified asphalts shows a downward trend. Under the combined effects of low temperature and load, the greater the creep stiffness of asphalt materials, the more likely they are to produce low-temperature fracture and brittle fracture. Therefore, the smaller the creep stiffness of asphalt, the better the low-temperature flexibility of asphalt. When the modifiers are the same but the asphalt labels are different. The overall S level of 70# asphalt is high, the overall S level of 90# asphalt is in the middle and the overall S level of 110# asphalt is low. It shows that the low-temperature performance of composite-modified asphalt mainly depends on the material properties of matrix asphalt. The asphalt is ranked as 110# > 90# > 70# according to its low-temperature creep performance. When the modifiers are different and the asphalt grades are the same. Compared with SBS-modified asphalt, only SBS/rubber powder and SBS/SBR modifiers reduce the low-temperature creep stiffness of asphalt, while SBS/TPS modifier increases the creep stiffness of asphalt. That is, the low-temperature cracking resistance of SBS/rubber-powder- and SBS/SBR-modified asphalt is stronger than that of SBS-modified asphalt, and the low-temperature cracking resistance of SBS/TPS-modified asphalt is worse than that of SBS-modified asphalt. It shows that rubber powder and SBR modifier enhance the low-temperature flexibility of SBS-modified asphalt, allowing the asphalt to better adapt to low-temperature environments. The TPS modifier reduces the low-temperature toughness of SBS-modified asphalt and weakens the low-temperature adaptability of asphalt. The modifiers are ranked according to their creep stiffness improvement effects: SBS/SBR > SBS/rubber powder > SBS > SBS/TPS.

When the temperature is −12 °C, the creep stiffness of each type of modified asphalt meets the AASHTO requirement of creep stiffness ≤ 300 MPa. When the temperature is −18 °C, only 70#SBS/SBR-, 90#SBS/rubber-powder-, 90#SBS/SBR-, 110#SBS/rubber-powder- and 110#SBS/SBR-modified asphalt meet the requirements. When the temperature is −24 °C, all types of modified asphalt do not meet the requirements. When considering both the modifier and asphalt labels, the modified asphalt rankings (top five) according to low-temperature performance are: 110#SBS/SBR, 90#SBS/SBR, 110#SBS/rubber powder, 70#SBS/SBR and 90#SBS/rubber powder. And the creep stiffness of the above modified asphalt meets the requirements for use as pavement asphalt materials.

3.5.2. Creep Rate

The creep rate of asphalt (m) represents the rate of change of asphalt creep stiffness with time at low temperatures. Under the same loading time, the larger the m value, the better the low-temperature crack resistance of the asphalt material. The creep rates of various types of composite-modified asphalt are shown in Figure 17.

As shown in Figure 17, as the test temperature increases, the m values of each type of modified asphalt gradually increase, indicating that the low-temperature creep deformation of each type of asphalt increases with the increase in test temperature. When the modifiers are the same but the asphalt labels are different, under the same test temperature, the asphalt with a larger number has a greater creep rate. It shows that the creep rate of asphalt is related to the size of the asphalt mark. The ranking of asphalt grades according to the low-temperature creep rate is 110# > 90# > 70#, which shows that the low-temperature stress of 110# asphalt is not easy to accumulate; that is, it is not easy to cause low-temperature cracking.

When the modifiers are different and the asphalt grades are the same. The m values of SBS/rubber-powder- and SBS/SBR-modified asphalt are higher than those of SBS- and SBS/TPS-modified asphalt, indicating that SBS/rubber-powder- and SBS/SBR-modified asphalt are less prone to stress accumulation at low temperatures; it is not easy to crack under low-temperature conditions. It means that rubber powder and SBR modifier can effectively improve the low-temperature flexibility of SBS-modified asphalt. Ranking the modifiers according to the creep rate: SBS/SBR > SBS/rubber powder > SBS > SBS/TPS. When considering both modifier and asphalt designations, the modified asphalt rankings according to the creep rate m value (top six) from large to small are: 110#SBS/SBR, 110#SBS/rubber powder, 90#SBS/SBR, 110#SBS, 90#SBS/rubber Pink and 70#SBS/SBR. And the creep rates of the above modified asphalt meet the requirements for use as pavement asphalt materials.

4. Study on the Applicability of Modified Asphalt

According to the above analysis, the applicability of composite-modified asphalt with different grades was studied based on the simulation results and rheological test results. The results are shown in

Table 13. Composite-modified asphalt applicability results.

Study Variables		Best Choice	Applicability
Simulation Results	Solubility parameter difference	160–175 °C, base asphalt + SBS or rubber powder	Easy to mix
	Interaction Energy	Rubber powder + low grade SBS-modified asphalt	Easy to store
	Mechanical properties	SBS-modified asphalt + rubber powder	Best high-temperature mechanical properties
		SBS-modified asphalt + SBR	Best low-temperature mechanical properties
Test results	High-temperature rheological index	70#SBS + rubber powder	Best high-temperature rheological properties
	Low-temperature rheological index	110#SBS + SBR	Best low-temperature rheological properties

.

5. Conclusions

(1)

The solubility parameter Δδ analysis confirmed that SBS-modified asphalt of all three grades has good compatibility with rubber powder and SBR between 160–175 °C, validating the process design and parameters. Interaction energy results indicate that rubber powder stabilizes low-grade SBS-modified asphalt, while SBR provides better stability for high-grade SBS-modified asphalt.

(2)

Simulation of mechanical parameters shows that adding rubber powder or TPS enhances the high-temperature mechanical properties of SBS-modified asphalt, while rubber powder or SBR improves its low-temperature properties. Overall, SBS/rubber-powder-modified asphalt performs best at high temperatures, and SBS/SBR-modified asphalt excels at low temperatures.

(3)

The DSR results show that the composite-modified asphalt has better high-temperature deformation resistance than single SBS-modified asphalt; the addition of rubber powder and TPS significantly improves the high-temperature performance of SBS, among which SBS/TPS has the best improvement effect on 70# and 90# matrix asphalts, and SBS/rubber powder has the best effect on 110# asphalt; considering the G*, δ and G*/sin δ indicators comprehensively, the high-temperature performance is ranked as follows: 70#SBS/rubber-powder- > 90#SBS/rubber-powder- > 70#SBS/TPS- > 90#SBS/TPS- > 110#SBS/rubber-powder- > 70#SBS-modified asphalt.

(4)

BBR test results show that creep stiffness decreases with increasing temperature, and both creep stiffness and rate are related to asphalt grade. Higher grades exhibit lower creep stiffness and higher creep rates. Rubber powder and SBR enhance the low-temperature flexibility of SBS-modified asphalt, making it more resistant to cracking in cold conditions. The ranking for low-temperature creep performance is: 110#SBS/SBR > 110#SBS/rubber powder > 90#SBS/SBR > 90#SBS/rubber powder > 70#SBS/SBR.