Research on the Rheological Properties and Modification Mechanism of Epoxy Resin/SBS Composite-Modified Asphalt

Abstract

Both epoxy resin (ER) and SBS are considered effective pavement materials for avoiding ruts. However, epoxy resin asphalt suffers from poor low-temperate performance and a high material cost in practical applications. Aiming to tackle these issues, a new type of composite asphalt modifier (ER-SBS) has been fabricated by combining epoxy resin with SBS. This work prepared modified asphalt with different doping amounts using the above composite asphalt modifier (ER-SBS), intending to explore the properties of composite-modified asphalt and the modification mechanism of the modifier. Furthermore, the effects of the composite modifier at different doping amounts on the viscoelastic property of asphalt were explored through rheological tests, and then the prepared composite-modified asphalt was compared with matrix asphalt and SBS-modified asphalt. In addition, the modification mechanism of the composite modifier was investigated by fluorescence microscopy and infrared spectroscopy. The difference in pavement performance between the composite-modified asphalt and SBS-modified asphalt was compared by a rut test and dynamic modulus test. The research results showed that the composite modifier improved the high- and low-temperature performances and viscoelastic property of matrix asphalt. When the doping amount was raised to 9%, the composite-modified asphalt exhibited better a modification effect than SBS-modified asphalt. The rut test results indicated that composite-modified asphalt demonstrated a stronger deformation resistance than SBS-modified asphalt. The dynamic modulus test showed that the composite-modified asphalt has better viscoelastic properties and temperature sensitivity. Fluorescence microscopy suggested that the crosslinking between the composite modifier and asphalt forms a mesh structure which greatly improves its resistance to deformation. From infrared spectroscopy, the composite modifier clearly functions as a physical modifier.

1. Introduction

As the service life of asphalt pavements rises, their fractures and damage intensify [1,2,3]. Epoxy resin, as a kind of asphalt modifier, has been effectively implemented in asphalt pavements in recent years [4,5]. Numerous studies evidenced that epoxy resin asphalt possesses excellent anti-rut performance and fatigue resistance [6]. Epoxy resin modifier is composed of epoxy resin and a curing agent. It is fabricated by mixing the two at a certain ratio and stirring them. The epoxy resin and curing agent react with each other to produce a three-dimensional polymer with an interpenetrating network structure, which fundamentally changes the thermal plasticity of regular asphalt while significantly improving the adhesive force, tensile strength, and fracture elongation of the material [7].

At present, research on epoxy resin asphalt and its mixture has been continuously deepening at home and abroad. To identify the most appropriate crosslinking level and improve the mechanical properties of epoxy asphalt, researchers carry out their work mainly by starting from determining the doping amount of epoxy resin used in the system, the addition of composite-modified materials and the optimization of preparation methods. In terms of the doping amount of epoxy resin used in the system, M. Motamedi et al. [8] looked into the effect of epoxy resin on the property of asphalt using a dynamic mechanics analyzer, DSC, and TG. Their findings showed that epoxy resin could improve its tensile strength, hardness, and thermal stability, especially when the doping amount reached 5%. Jiahao et al. [9] carried out a tensile test, viscosity test, and fluorescence microscope test on epoxy asphalt with different epoxy resin contents. Their findings demonstrated that when the doping amount of epoxy resin was greater than 40%, epoxy asphalt exhibited excellent mechanical properties, and the growth rate of its viscosity increased with the doping amount. During the curing process of epoxy asphalt, epoxy resin particles exhibited a tendency to aggregate into clusters. When the doping level surpassed 40%, a crosslinked network structure was established within the asphalt colloid. Zhu et al. [10] examined the impact of the doping amount of epoxy resin on the high-temperature performance of epoxy asphalt through micro and macro tests. Their micro test showed that epoxy asphalt was in a dispersed phase at a low doping amount but suffered phase changes when the doping amount was in the range of 30~40%. The micro test indicated that epoxy asphalt demonstrated an improved high-temperature performance after being cured for 96 h, further proving that the doping amount of epoxy resin was positively correlated with the high-temperature performance of asphalt. Yu [11] discovered from a micro test that epoxy resin reacted relatively slowly in epoxy asphalt with both the curing agent and carboxylic acid present in the epoxy asphalt. Due to the escalation in the doping amount of epoxy resin, epoxy asphalt shifted from a dispersed structure to network structure, and, when doping amount was 40%, the phase of the asphalt began to change.

In terms of the addition of composite-modified materials, Xu [12] found that epoxy resin formed a mesh-like microstructure in the curing process of SBS-EA through fluorescence microscopy and micro-computed tomography. The addition of SBS with a mass content of 3% greatly enhanced the compatibility of EA, and the particle size of dispersed asphalt decreased with the increase in SBS. J Si et al. [13] performed a curing rheological analysis and discovered that ESA exhibited a longer service life and better machining performance at ambient temperatures compared with EA. The cured ESA had a higher glass transition temperature, superior thermal stability, and enhanced tensile strength and elongation at break compared to EA. Using polyethylene glycol composite-modified epoxy asphalt, Min et al. [14] studied the influence of the polyethylene glycol chain on the dynamic mechanical properties and flexibility of epoxy asphalt through a dynamic thermo-mechanical analysis, contact angle measurements, high-temperature performance tests, and moisture-proof tests, proposing that the low-temperature flexibility of epoxy asphalt could be effectively improved by adding polyethylene glycol with a manganese content of 400–800 g/mol. In terms of the optimization of preparation methods, with the goal of making epoxy resin and asphalt more compatible in epoxy asphalt, Ding et al. [15] created an epoxy soybean oil compatibilizer and used the cold mixing method to prepare the mixture. Their tensile test results demonstrated that the addition of the epoxy soybean oil compatibilizer to regular epoxy asphalt greatly enhanced its tensile strength, fracture elongation, and flexibility compared to the control group.

Due to the irreversible crosslinked network of epoxy resin formed after it is cured, which greatly limits the fluidity of asphalt, completely changes the thermoplastic properties of asphalt, and endows asphalt with good thermosetting elastomer characteristics, epoxy asphalt has unique performance advantages over other kinds of asphalt mixtures [16,17]. Zhang et al. [18] studied the mechanical properties of EAM under cyclic loads by conducting cyclic loading and unloading experiments on cylindrical samples of epoxy asphalt. The experimental results showed that under a medium confining pressure (less than 0.5 MPa), the elastic property of epoxy asphalt was improved, and permanent deformations such as ruts were effectively inhibited. By conducting performance experiments and laser texture detection experiments, Nie et al. [19] analyzed the performance indexes of epoxy asphalt, such as its watertightness, fatigue limit strain level, and slip resistance. The results implied that epoxy asphalt had strong watertightness, flexural and tensile resistance, durability, and adhesion to the steel bridge plate. Through the use of the pendulum test, the dynamic friction coefficient test, the permeability test, and the sound absorption coefficient test, Zhong et al. [20] investigated the functional properties of epoxy asphalt. They found that open-graded epoxy asphalt concrete possessed excellent permeability, an anti-skid property, and a noise reduction property. Using a composite study method that included both mechanical and microscopic mechanisms, Bahmani et al. [21] investigated the impact that epoxy had on the liquid sensitivity of the asphalt mixture. After conducting an analysis of the results of the indirect tensile strength test, they came to the conclusion that the epoxy asphalt mixture that contained 4% epoxy resin exhibited the highest water sensitivity, with its tensile strength greatly improved.

In light of the foregoing research on epoxy asphalt at home and abroad, epoxy asphalt is an irreversible curing system and a kind of paving material characterized by excellent high- and low-temperature performance and strong strength corrosion resistance [22]. Researchers have carried out a large amount of work on the compatibility of epoxy resin and asphalt, their microstructure and modification effects, and the performance of epoxy asphalt, with abundant results achieved. However, very little is known about the properties of epoxy resin/SBS composite asphalt and its mixture, and its rheological properties and modification mechanism remain unclear. On this basis, taking modified asphalt prepared using an epoxy resin/SBS composite modifier as the research object, this work discusses the modification effect and modification mechanism of the composite modifier by studying the asphalt’s high- and low-temperature performance and viscoelastic property, with the purpose of providing a theoretical basis for the engineering applications of this composite modifier.

Primarily, this study intends (1) to study the rheological and viscoelastic properties of the ER-SBS composite modifier; (2) to confirm that the asphalt with the composite modifier has viscoelastic qualities and is resistant to permanent deformation; and (3) to investigate how the composite modifier modifies the asphalt. Asphalt that has been composite-modified was prepared using a composite modifier prior to the test. We compare the rheological performance parameters of matrix asphalt, SBS-modified asphalt, and composite-modified asphalt using a dynamic shear rheometer (DSR) and bending beam rheometer (BBR). The rutting resistance and dynamic modulus of the composite-modified asphalt were determined by the rut test and a uniaxial compression dynamic modulus test, followed by the establishment of a connection with the composite-modified asphalt. The microscopic morphology of the composite-modified asphalt was observed by fluorescence microscopy (FM) and Fourier transform infrared spectroscopy (FTIR), and its chemical functional groups were analyzed to determine the modification method of the composite modifier. Figure 1 is a flow diagram depicting the procedures and activities that comprised this study.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt and Modifier

In this experiment, 70# base asphalt and SBS-modified asphalt were produced by the Jingbo petrochemical plant (Binzhou, China) in Shandong province as the control groups, and their content of SBS was 6%. The bitumen was tested using the domestic standard test method [23].

Table 1. Technical indexes of 70# asphalt and SBS-modified asphalt.

Test Properties	Test Results	70# Asphalt		SBS-Modified Asphalt
		Test Results	Technical Index	Test Results	Technical Index
P25 °C, 100 g, 5 s (0.1 mm)	-	63	60–80	52	40–60
TR&B (°C)	-	48.3	≥46	86	≥60
Ductility (cm)	5 °C	-	-	35.6	-
	10 °C	17.6	≥15	-	-
RTFOT (163 °C, 85 min)	Mass loss (%)	0.082	±0.8	−0.002	±1.0
	Penetration ratio	75	≥61	76	≥65
	Ductility (cm) 5 °C	-	-	27	≥15
	10 °C	6.3	≥6	-	-

displays the test results. Table 1 clearly shows that the main technical parameters of both asphalts are in accordance with the national standard [24].

The epoxy SBS composite modifier (ER-SBS) used in this study was provided by Henan Guolu hi-tech New Materials Co., Ltd. (Pingdingshan, China). Figure 2 shows the appearance of the composite modifier, which consists mainly of epoxy and SBS, and

Table 2. Technical indexes of composite modifiers.

Modifier	Technical Parameter	Unit	Measured Result	Technical Requirement
ER-SBS	Appearance	Dark green particle	Granular, uniform	Granular, uniform
	Individual particle mass	g	<0.001	≤0.5
	Ash content	%	0.96	≤1.0
	Melt index	g/10 min	14.2	-
	Dry mix dispersibility	-	No particle residue	-

displays its primary characteristics.

The use of a high-speed shear machine was necessary to prepare the modified asphalt. According to the recommendation of the manufacturer, the content of ER-SBS composite modifier was set at 3%, 6%, 9%, 12%, and 15%, while the percentage of asphalt content was taken as the reference. The preparation process is as follows: the base asphalt is heated to a state of full flow, the modifier is slowly poured into the base asphalt several times, and a glass rod is used to stir continuously until the modifier is added. The high-speed shear machine is turned on, and shear rate is controlled at 5000 r/min; the shear temperature is kept at 170~180 °C, and the mixture is sheared for 45 min. After shearing, the modified asphalt was prepared by swelling the asphalt in an oven maintained at a constant temperature at 180 °C for 60 min. This specific preparation process is shown in Figure 3. At the same time, to examine the impact of aging on composite-modified asphalt, short-term aging asphalt was produced using a rotating thin-film oven, while long-term aging asphalt was generated using a pressure aging oven.

2.1.2. Asphalt Mixture

The high-temperature stability of the asphalt mixture plays a key role in preventing ruts. In order to make judgments on the actual influence of the composite modifier on the pavement performance of asphalt, the effect of the composite modifier on its high-temperature performance was discussed using the AC-13 mixture.

Table 3. The mineral aggregate gradation in the AC-13 asphalt mixture.

Type of Asphalt Mixture		Percentage of Mass by the Following Screens (%)
		26.5	19	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
AC-13	Upper limit	100	100	100	100	85	58	50	38	28	20	15	8
	Lower limit	100	100	100	90	68	38	24	15	10	7	5	4
	Median	100	100	100	95	76.5	48	37	26.5	19	13.5	10	6

displays the gradations used in the tests. The doping amount of asphalt modifier in the mixture was 15% (of the mass fraction of the matrix asphalt), and the designed void rate was 4.1%.

2.2. Measurement Methods

2.2.1. Dynamic Shear Rheological Test

The Dynamic Shear Rheological (DSR) test fills an essential function in analyzing the viscoelastic property of asphalt [25]. In researching the impact of temperature on the rheological properties of asphalt, a DSR test was used to carry out temperature scanning tests at different temperatures. The parameter settings used for the tests are as follows: the temperature was in the range of 52~76 °C, with an interval of 6 °C; the strain was 1%; and the shear frequency was 10 rad/s. In addition, on-board frequency will also affect the rheological properties of asphalt, so it is imperative that you execute frequency scanning tests on asphalt. The parameter settings for the frequency scanning tests are listed as follows: the temperature was in the range of 52~76 °C, with an interval of 6 °C; the application frequency was in the range of 0.1~100 Hz; and the strain was 5%. According to the frequency scanning test results, the main curve of the composite shear modulus of asphalt was fitted by an S-type function, as shown in Formula (1). Taking 64 °C as the reference temperature, the displacement factor of the complex shear modulus at different temperatures was calculated by the WLF equation according to the time-temperature equivalent principle [26,27,28]. The WLF equation is expressed in Formulas (2) and (3).

$$\log \left|G^{*}\right|=\delta +{\displaystyle \frac{\alpha }{1+e^{\beta -\gamma \log f_r}}}$$

(1)

where G* is the complex shear modulus (KPa); f_r is the reduced frequency (H_Z); and α, δ, β, and γ are fitting parameters.

$$\log f_r=\log f+\log {\alpha }_T$$

(2)

$$\log {\alpha }_T={\displaystyle \frac{-C_1\left(T-T_r\right)}{C_2+\left(T-T_r\right)}}$$

(3)

where f_r is the reduced frequency (Hz); f is the loading frequency (Hz); a_T is the displacement factor; T is the testing temperature (°C); T_r is the reference temperature (°C); and C₁ and C₂ are the fitting parameters.

2.2.2. Multiple Stress Creep Recovery (MSCR) Test

The Multiple Stress Creep Recovery (MSCR) test is a method of testing the stress that was developed to take into consideration the ability of asphalt cement to recover from elastic deformation [29]. The specimens are subjected to two different stress levels, namely 0.1 kPa and 3.2 kPa, in order to simulate the dynamic cyclic loads of different stress levels. This brings the test closer to the actual state of a vehicle load, reflects the deformation of the road surface when a vehicle is loaded, and contributes to an improved evaluation of the high-temperature performance of asphalt.

This work performed an MSCR test on a variety of asphalt according to AASHTO MP19-10. Firstly, the asphalt specimen was circularly loaded to a stress of 0.1 KPa ten times with no stops. Subsequently, it was loaded to a stress of 3.2 KPa in the same way. The temperature during the tests was set to 64 °C.

2.2.3. Bending Beam Rheological Test

Asphalt pavement will crack due to temperature stress at low temperatures, which will affect driving safety. The SHRP Project invented the bending beam rheological test based on the “beam” theory, and proposed the use of two indexes, namely the stiffness modulus S and creep rate M, to reflect the deformation resistance and stress relaxation ability of asphalt materials at low temperatures. The stiffness of asphalt is calculated using its deflection value during the test by following Formula (4):

$$S\left(t\right)={\displaystyle \frac{P{L}^3}{4b{h}^3\sigma \left(t\right)}}$$

(4)

where b is the width of the beam (12.5 mm); h is the height of the beam (6.25 mm); P is the test load (980 Mn ± 50 mN); L is the spacing between beams (102 mm); and σ(t) is the mid-span deflection (mm).

The 1800 bending beam rheometer manufactured by Europe and America Dadi (China) Co., Ltd. (Jinjiang, China) was used to test various types of asphalt in this work. The test temperature was set to be −12~−24 °C, with an interval of 6 °C.

2.2.4. Fluorescence Microscope Test

A fluorescence microscope (FM) uses high-energy ultraviolet (UV) light to excite a fluorescence microscope sample. The reflection phenomenon occurs after the sample absorbs the UV light; the microstructure of samples can be magnified by the microscope system, and real-time monitoring and analysis by a computer can be used to study the microstructure of materials. The distribution of particles and shapes between the modifier and the interior of the asphalt were observed using IMAGER Z2 fluorescence microscopy (Shanghai Yuguang Instrument Co., Ltd.) (Shanghai, China), and their compatibility was explained according to the phase structure of asphalt.

2.2.5. Fourier Transform Infrared Spectroscopy Test

Fourier transform infrared spectroscopy (FT-IR) is mainly used in the detection of the structure of substances and the identification of their components, so it can be used in the analysis of the chemical structure of asphalt binder. The chemical groups of ER-SBS-modified asphalt were analyzed by the Fourier infrared spectrometer model Nicolet Continuum. Using the matrix asphalt as the control group, the changes in the characteristic groups of ER-SBS-modified asphalt were analyzed so as to identify the modification mechanism of the ER-SBS modifier on asphalt. Its wavelength scanning range is 4000~500 cm⁻¹, and its scanning times are 64.

2.2.6. Rut Test

Rutting can diminish a pavement’s service life and reduce safety and comfort; therefore, it is crucial to assess the high-temperature rutting resistance of asphalt mixtures. The rut test is a procedure used to assess the resistance of asphalt mixtures to plastic flow deformation at a specified temperature. The evaluation index employed in this study was dynamic stability (DS), and its calculation formula is presented in Equation (5). To better validate the high-temperature stability of various asphalt mixtures, two environmental conditions were constructed in the test. You can find the details of the parameters in

Table 4. The test conditions used in the rut test.

Test Parameters	Standard Rut Test	Rut Life Test
Specimen configuration	300 mm × 300 mm × 50 mm	300 mm × 300 mm × 50 mm
Specimen temperature	60 °C, 70 °C	60 °C, 70 °C
Tire configuration	Rubber tire, pressure is 0.7 MPa, 1.0 MPa	Steel tire, pressure is 1.4 MPa
Rolling distance	230 mm × 10 mm	230 mm × 10 mm
Rolling speed	42 times/min	30 times/min

.

$$DS={\displaystyle \frac{(T_2-T_1)\times N\times C_1\times C_2}{d_2-d_1}}$$

(5)

where DS is dynamic stability (times/mm); T₂ and T₁ are the duration of the tests (min); N is the test rate (times/min); C₂ and C₁ are constants; and d₂ and d₁ are the depth of the ruts corresponding to the test times T₂ and T₁ (mm).

This work also proposed a new method for evaluating the high-temperature performance of asphalt mixtures, which specifically is to quantitatively analyze the high-temperature performance of the mixture by taking its specific rut deformation (generally 1~1.5 cm) as the damage threshold and the amount of wheel grinding under different coaxial loads when the asphalt mixture reaches this threshold as the evaluation index. Meanwhile, a rut life tester was developed according to the proposed evaluation method, as shown in Figure 4. The rut life tester could autonomously control the test temperature, tire specifications, tire pressure, and roller speed. This work also took a rut deformation of 1 cm as the damage threshold to evaluate the high-temperature performance of asphalt mixtures by the rut life tester. The specific test conditions are shown in Table 4.

2.2.7. Dynamic Modulus Test

The dynamic modulus denotes the deformation of a material subjected to dynamic loading and is commonly used to characterize the dynamic stress–strain response of viscoelastic materials. In this test, a universal testing machine (UTM-100) was used to determine the dynamic modulus of the asphalt mixture. The 70# matrix asphalt, 6% SBS-, and 15% ER-SBS-modified asphalt mixtures were selected for the test. First, a Φ150 mm × 170 mm rotary compressed test piece was prepared. Then, its core was drilled, and it was cut into Φ100 mm × 150 mm test pieces. Third, displacement sensors were arranged around them. Finally, they were insulated and loaded. The test temperature was established at 5 °C, 20 °C, 35 °C, and 50 °C and the loading frequency was set to 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz, and 25 Hz. Taking 35 °C as the reference temperature, the sigmodial model is used to fit the dynamic modulus of the above-mentioned asphalt mixtures.

3. Results and Discussion

3.1. Rheological Properties of Asphalt

3.1.1. Analysis of Temperature Sweep Results

Figure 5 displays the outcomes of the temperature sweep. As evidenced by Figure 5a, the comparison of modified asphalt containing different doses of ER-SBS shows that with the increase in the amount of modifier, the rutting factor of the asphalt continues to increase. When the doping amount was low (3%, 6% and 9%), the rutting factor of ER-SBS-modified asphalt was always lower than that of SBS-modified asphalt, and when the doping amount was increased to 12% and 15%, the rutting factor of ER-SBS-modified asphalt was significantly higher than that of SBS-modified asphalt, which indicates that the ER-SBS-modified asphalt had undergone qualitative changes and had its high-temperature performance significantly improved.

It is evident from Figure 5b,c, which shows a comparison of the rutting factors of asphalt with different degrees of aging, that with deepened aging, the rutting factor of asphalt persists in escalating. Taking 15% ER-SBS at 52 °C as an example, its original rutting factor of 56.01 kPa was increased to 88.32 kPa in RTFOT aging and to 268.53 kPa after PAV aging. This trend is basically followed by the rest of the various asphalt, because asphalt will undergo a series of chemical reactions under the high-temperature and oxygen-containing conditions in the RTFOT aging process. For example, in the transformation of the light component (aromatic component) in the asphalt to a heavy component (asphaltene), the molecular weight of the asphalt increases, which results in greater hardness. The rutting factor after PAV aging is even greater. Asphalt undergoes a number of chemical reactions, including addition, polymerization, oxidation, and dehydrogenation, which result in the formation of complex cyclic compounds and polar functional groups like carbonyl and sulfoxide groups. This is the reason why asphalt is so popular. Not only will there be an increase in its molecular weight, but there will also be a strong attraction between its polar functional groups, which will result in an improvement in its ability to resist loads from the outside [30].

Figure 6 shows the phase angle of the asphalt. As we can see from Figure 6a, the rise in temperature coincides with the increase in the δ of asphalt modified with different doses of ER-SBS, which indicates that the proportion of elastic components in the asphalt decreases at high temperatures. This manifests as a decrease in elastic recovery, which easily causes more unrecoverable deformation and reduces asphalt’s resistance to deformation at high temperatures. At the same test temperature, as the dose of the ER-SBS modifier increases, the δ of the modified asphalt continues to decrease, which indicates that the doping of the ER-SBS modifier promotes the transformation of asphalt with more viscous components into an elastomer and improves the asphalt’s resistance to external loads. As can be seen from Figure 6b,c, the δ of RTFOT-aged asphalt is gradually reduced compared to the original asphalt sample, and the δ of PAV-aged asphalt is lower than that of RTFOT-aged asphalt. Under different aging states, the δ of 15% ER-SBS-modified asphalt is the smallest, which indicates that the aged 15% ER-SBS-modified asphalt has stronger elastic properties than the other modified asphalts and the optimal resistance to rutting at high temperatures.

3.1.2. Analysis of Frequency Scanning Results

At a reference temperature of 64 °C, the main curve of the composite shear modulus is as shown in Figure 7. The results are clear from Figure 7a; in its original state, as the ER-SBS doping amount increases, the complex modulus G* of the modified asphalt also increases. The complex modulus G* of 12% and 15% ER-SBS-modified asphalt is always higher than that of SBS-modified asphalt, which indicates that the resistance of ER-SBS-modified asphalt to external load-induced deformation is superior to that of SBS-modified asphalt, which aligns with the temperature scanning results. This is due to the fact that the high-doped ER-SBS-modified asphalt colloid can fully form a crosslinked three-dimensional mesh structure, allowing the asphalt phase to disperse evenly, with good compatibility with the epoxy phase. The G* main curve of 12% and 15% ER-SBS-modified asphalt is similar, which suggests that when the ER-SBS doping amount exceeds 12%, its effect on the complex modulus G* of asphalt decreases.

Figure 7b,c make it clear that the change trend in the complex modulus of the aged asphalt is the same as that of the original asphalt. It increases with the increase in the loading frequency and the deepening of aging, suggesting that there are benefits from aging for asphalt’s resistance to deformation [31]. The complex modulus of 12% and 15% ER-SBS-modified asphalt after aging is still higher than that of SBS-modified asphalt. Compared with RTFOT aging, the curvature of the main curve after PAV aging is more obvious. In the low-frequency region (with relatively higher temperatures), the difference from the matrix asphalt is obvious, while in the high-frequency region (with relatively low temperatures), the gap is smaller, which indicates that the ER-SBS modifier enhances asphalt’s resistance to high-temperature-induced deformation at a low frequency and high temperature well. The macro manifestation is an improved stability of asphalt at high temperatures. At a high frequency and low temperatures, local shrinkage occurs between asphalt molecules, causing damage to its molecular structure. The asphalt is mainly subjected to external loads, which reduces the improvement of asphalt with the ER-SBS modifier in high-frequency areas [32].

3.1.3. Analysis of MSCR Test Results

As observed from Figure 8, R_0.1 and R_3.2 gradually increase with the increased doping amount of the ER-SBS composite modifier, which indicates that with the increase in the doping amount, the deformation recovery of the modified asphalt is gradually improved. Furthermore, as R_0.1 gradually approaches 100% with the increased doping amount, the rate of this increase slows down. Consequently, the increase rate of R_3.2 is greater than that of R_0.1, and the stress sensitivity index R_diff gradually decreases with the increase in the doping amount, which indicates that the increased doping amount results in a gradual convergence of asphalt’s creep recovery rate under high stress and low stress.

J_nr0.1 and J_nr3.2 gradually decrease with the increased doping amount of the ER-SBS composite modifier, which indicates that with the increase in the doping amount, the modified asphalt’s resistance to permanent deformation gradually increases. The stress sensitivity index Jnr-diff gradually increases with the increase in the doping amount, which indicates that the increase in the doping amount increases the size of the unrecoverable softness of the asphalt under high and low stress. Since J_nr3.2 decreases more slowly with the increase in the doping amount than J_nr0.1, the stress sensitivity index Jnr-diff gradually increases with the change in the doping amount, which shows that the doping amount of the ER-SBS composite modifier improve the asphalt’s performance to different degrees at high temperatures under low stress and high stress. ER-SBS-modified asphalt is mainly improved in terms of its performance at high temperatures in the linear viscoelastic interval.

3.1.4. Analysis of BBR Test Results

Figure 9 presents the BBR test outcomes for ER-SBS-treated asphalt. As can be seen from Figure 9a, after ER-SBS is dosed into the matrix asphalt, the stiffness modulus of the asphalt first decreases and then increases, which suggests that a small doping amount (3%, 6%) of ER-SBS can improve the low-temperature flexibility of asphalt and thus improve its low-temperature performance. When the doping amount is 6%, the creep stiffness of the ER-SBS-modified asphalt is at its minimum, but it increases again with an increase in the doping amount. The primary reason for this is that a high dosage of ER-SBS induces crosslinking in the asphalt colloid, resulting in a three-dimensional mesh structure that restricts the mobility of asphalt molecules and negatively impacts the flexibility of the asphalt. Where the doping amount of ER-SBS is constant, as the test temperature decreases, the stiffness modulus of the asphalt gradually increases, which indicates that a lower temperature causes the hardening of asphalt and the elastic components in asphalt gradually become dominant. As can be seen from Figure 9b, the creep rate M decreases as the temperature decreases at a constant doping of ER-SBS; at the same temperature, as the doping amount of ER-SBS increases, an initial increase followed by a decrease in M indicates that a low doping concentration of ER-SBS enhances the stress relaxation properties of asphalt, but an excessively high doping amount causes the opposite effect.

In identical settings, the stiffness modulus S of the SBS-modified asphalt is less than that of the ER-SBS-modified asphalt and its creep rate M is greater than that of the ER-SBS-modified asphalt at different doping amounts, which indicates that the low-temperature performance of the SBS-modified asphalt is better than that of the ER-SBS-modified asphalt. This is mainly because the main component of the SBS modifier is a thermoplastic elastomer whose separated hard and soft phases balance the rigidity and low-temperature flexibility of the modified asphalt. In contrast, the ER-SBS modifier contains a considerable amount of epoxy resin, which is brittle after curing due to its thermosetting characteristics, which causes a poor crack resistance of the modified asphalt at low temperatures.

3.2. High-Temperature Performance of Asphalt Mixture

3.2.1. Analysis of Rut Test Results

The dynamic stability determined in the rut test can assess the high-temperature stability of the asphalt mixture. Increased dynamic stability correlates with enhanced high-temperature stability. It is evident from Figure 10a that after the ER-SBS modifier is added, the dynamic stability of the asphalt mixture is significantly improved compared to the matrix asphalt mixture, which indicates that ER-SBS can effectively improve the permanent deformation resistance of the asphalt mixture and thus improve the rutting resistance of asphalt pavements. The dynamic stability of ER-SBS-modified asphalt mixtures under the two test conditions is significantly higher than that of the SBS-modified asphalt mixture. Under test conditions of 60 °C and 0.7 MPa, the dynamic stability of the ER-SBS-modified asphalt mixture reached 13,786 times/mm, while that of the SBS-modified asphalt mixture was only 1/2 of that of the ER-SBS-modified asphalt mixture. It was found that the dynamic stability of the ER-SBS- and SBS-modified asphalt mixtures was reduced by 37.8% and 73.5%, respectively, under test conditions of 70 °C and 1.0 MPa. This was in comparison to the results obtained at 60 °C and 0.7 MPa. This indicates that the ER-SBS-modified asphalt mixture is more adaptable to the harsh environment of high temperatures and heavy loads than the SBS-modified asphalt mixture.

As a new evaluation index, the rutting life can also characterize the high-temperature stability of an asphalt mixture. The more ruts that can be sustained (the longer the rutting life) before a specified amount of rut deformation is reached, the better the high-temperature stability. From the analysis of Figure 10b, it can be seen that under test conditions of 60 °C and 1.4 MPa, the rutting life of the ER-SBS- and SBS-modified asphalt mixtures was 16,528 times and 3409 times, respectively. When the temperature was raised to 70 °C without changing the wheel pressure, the rutting life of the two modified asphalt mixtures was 11,436 times and 1855 times, respectively; a decrease of 30.8% and 45.9%, respectively. Compared with the ER-SBS-modified asphalt mixture, the effect of the temperature rise on the high-temperature performance of the SBS-modified asphalt mixtures was reduced by 30.8% and 45.9%, respectively. Compared with the ER-SBS-modified asphalt mixture, the effect of the temperature rise on the high-temperature performance of the SBS-modified asphalt mixture is greater. The primary reason is that the ER-SBS modifier comprises a higher concentration of epoxy resin. The epoxy resin and asphalt molecules undergo crosslinking at elevated temperatures to create a stable three-dimensional network structure. Simultaneously, the incorporation of the SBS polymer in the modifier markedly enhances the high-temperature stability of the asphalt mixture. Despite the external temperature reaching 70 °C, the effect remained negligible.

3.2.2. Analysis of Dynamic Modulus Test Results

Figure 11 shows the main dynamic modulus curve of the three asphalt mixtures. Low frequency is the same as high temperature, in line with the time–temperature equivalence principle. Here, the best dynamic modulus curves were ER-SBS, SBS, and 70# matrix asphalt, from high to low; that is, the ranking of their high-temperature performance is ER-SBS, SBS, and then the 70# matrix asphalt mixture, from high to low. The dynamic modulus gap between the three asphalt mixtures in the-low frequency zone is relatively obvious, which indicates that the high-temperature performance of the ER-SBS-modified asphalt mixture is significantly better than that of the rest of the three asphalt mixtures. At the same time, from the main curve of the dynamic modulus of the ER-SBS-modified asphalt mixture, it can be found that the slope increases and then decreases from a low frequency to high frequency, with a narrower range of changes, which suggests that the temperature sensitivity of the ER-SBS-modified asphalt mixture is smaller.

3.3. Modification Mechanism

3.3.1. Fluorescence Microscope Observation

Figure 12 shows the 40× fluorescence magnifications of 70# matrix asphalt and ER-SBS-modified asphalt containing different doping amounts. As can be seen from Figure 12, the microscopic image of the matrix asphalt is black. When the doping amount of the ER-SBS modifier is small, the light-colored part (epoxy phase) that represents the epoxy resin in the figure cannot form a dense spatial network, but shows a spotty distribution pattern. As the doping amount of the ER-SBS modifier increases, the particle size of the epoxy phase gradually increases, and the effective connection between the epoxy phases begins to increase and is dispersed more uniformly. However, the black part (asphalt phase) that symbolizes the matrix asphalt gradually decreases. When the doping amount of the ER-SBS modifier is less than 12%, the black part is dominant in the image, with dotted discrete light-colored spots displayed, which indicates that the matrix asphalt is the main phase in the ER-SBS-modified asphalt at this time and that the epoxy resin, after the curing reaction, is evenly dispersed in the matrix asphalt in the form of small particles. Since ER-SBS-modified asphalt still exhibits thermoplastic properties at this time, the change in the asphalt’s performance is insignificant. When the doping amount of the ER-SBS modifier is 12%, the light-colored parts of the image gradually gather, and the cured product of the epoxy resin undergoes initial crosslinking. When the doping amount of the ER-SBS modifier is 15%, the microscopic image is dominated by the light color, and the black spots are evenly dispersed in the three-dimensional spatial network generated by the polymerization reaction of the epoxy resin and curing agent. Nevertheless, the crosslinking network that is formed as a result of the reaction between the epoxy resin and curing agent is exhibited as a continuous phase, which restricts the asphalt particles to a certain extent. The properties of the ER-SBS-modified asphalt have undergone a fundamental transformation at this time. The high-temperature performance of the asphalt has significantly improved, while its crack resistance at low temperatures has significantly decreased. Although the epoxy in ER-SBS-modified asphalt cannot react chemically with the asphalt to form a chemical bond, the two intertwine with each other through continuous movement to form a physical crosslinking bond, and the epoxy resin molecules are also adsorbed onto the surface of the asphalt particles. In this way, the crosslinking between the asphalt molecules and epoxy resin molecules allows them to penetrate each other to form a spatial structure that can jointly resist the destruction of external forces. The spatial structure formed by the epoxy resin and asphalt is much more stable than the network formed by the crosslinking of simple epoxy resin molecules, which isolates the epoxy resin from other particles, and ensures the stability of the entire network system [33,34].

3.3.2. Infrared Spectroscopy Analysis

Figure 13 shows the infrared spectra of different types of asphalt. From the spectra of the matrix asphalt and ER-SBS-modified asphalt containing different doping amounts, it can be found that the newly added absorption peak of ER-SBS-modified asphalt is at 966 cm⁻¹. It is analyzed that this is the characteristic peak formed by the torsional vibration of the C=C bond of polybutadiene in the SBS of ER-SBS. In addition, there are no obvious new characteristic peaks in the infrared spectrum of ER-SBS-modified asphalt, with the only change being in the intensity of its absorption peaks. It can be seen that the doping of the ER-SBS modifier is mainly a physical miscibility. The doping of ER-SBS to the asphalt system improves the internal adhesion and stability of the asphalt [35].

The aromatic ring C=C double bond exhibits an excellent degree of correlation with the mechanical properties of the asphalt, and the greater the value, the better the macroscopic mechanical properties of the asphalt. As a result, Formula (6) shows the aromatic ring index calculation formula, which can be used to semi-quantitatively analyze the asphalt modification mechanism.

Table 5. Aromatic ring index of 6 types of asphalt.

Types of Asphalt	Aromatic Ring
70# matrix asphalt	0.0597
3% ER-SBS	0.0664
6% ER-SBS	0.0682
9% ER-SBS	0.0733
12% ER-SBS	0.0755
15% ER-SBS	0.0819

displays the results of these calculations.

$$I_{Aromaticity}={\displaystyle \frac{A_{1600{\mathrm{m}}^{-1}}}{{\displaystyle \sum A_{2000\sim 600{\mathrm{cm}}^{-1}}}}}$$

(6)

where A_1600m⁻¹ is the peak area, whose wavenumber is 1600 cm⁻¹, and ∑A_2000~600cm⁻¹ is the sum of the areas whose wavenumber is 2000~600 cm⁻¹.

Table 5 clearly shows that the aromatic ring index of ER-SBS-modified asphalt is greater than that of matrix asphalt and increases with the increase in the doping amount of ER-SBS. Among these samples, the improvement of ER-SBS-modified asphalt with a doping amount of 15% was the most obvious. The aromatic ring index of 9% ER-SBS-modified asphalt increased by 0.0069 compared to the 3% doping amount, but the increase rate is slow. Comparatively, the aromatic ring index of 15% ER-SBS-modified asphalt increased by 0.0086 compared to the 9% doping amount, and the increase is faster. This indicates that when the doping of ER-SBS is greater than 9%, its effect on asphalt improvement is enhanced. The aromatic ring C=C double bond is an important index for characterizing the mechanical properties of asphalt. The higher the number, the more superior the mechanical properties of asphalt. This also explains the excellent high-temperature performance of ER-SBS-modified asphalt in the DSR test.

4. Conclusions

Within the scope of this paper, rheological experiments were carried out on 70# matrix asphalt, SBS-modified asphalt, and ER-SBS composite-modified asphalt with different dosages, followed by the evaluation of the rheological properties of these various types of asphalt. With the help of microscopic test equipment, the modification mechanism of the composite modifier was discussed from the perspective of the composition of the modifier and the changes in the colloidal structure of the asphalt; indoor mixture performance tests were conducted to verify the high-temperature properties and mechanical properties of the composite-modified asphalt mixture, and the following conclusions were drawn:

• The doping of a composite modifier (ER-SBS) can greatly enhance the high-temperature performance and viscoelasticity of matrix asphalt. When the doping amount of the ER-SBS modifier is increased to 9%, the high-temperature performance of the composite-modified asphalt is better than that of SBS-modified asphalt.
• Compared to SBS, the resistance to permanent deformation in an ER-SBS composite-modified asphalt mixture is much higher under conditions of a high temperature and heavy load. When compared to SBS-modified asphalt, the viscoelastic characteristics and temperature sensitivity of the ER-SBS composite-modified asphalt mixture are superior across the whole temperature domain and at high load frequencies.
• ER-SBS composite modifiers are mainly physically miscible. The epoxy molecules in ER-SBS composite modifiers crosslink with asphalt molecules to form a spaced network structure which can resist external forces, greatly improving the anti-deformation ability of the asphalt.