Effects of Aging on Rheological Properties and Microstructural Evolution of SBS Modified Asphalt and Crumb Rubber Modified Asphalt Binders

Abstract

Focusing on the comparison of aging resistance between styrene-butadiene-styrene (SBS) modified asphalt (SBSMA) and crumb rubber modified asphalt binders (CRMA), the influences of aging on rheological properties and microstructural characteristics of different modified asphalts were investigated in this work. Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests were carried out, and the variations in rheological properties for different modified asphalts after the rotating thin film oven test (RTFOT) were analyzed with the rutting factor aging index (RAI) and creep rate aging index (CAI). By using Fluorescence microscopy (FM) and Fourier transform infrared (FTIR) spectroscopy, the evolutions of microstructure and chemical composition for two modified asphalts were analyzed with carbonyl index growth rate (CIR) and sulfoxide index growth rate (SIR). Then, The relationships between CIR/SIR and RAI/CAI were established to show the correlation between the deterioration of macroscopic performance and the evolution of micro-structure. The results indicated that the aging degree of asphalt increases with elevated temperatures, leading to decreasing low-temperature performance while improving high-temperature performance. Nevertheless, SBSMA exhibited strong sensitivity to aging temperature. Under thermo-oxidative aging, the RAI and CAI of SBSMA were lower than those of CRMA, whereas the regularities of CIR and SIR were opposite, indicating that CRMA was superior to SBSMA in terms of anti-aging properties due to the rupture of the cross-linked network structure of SBSMA. However, CRMA experienced aging accompanied by full swelling, and thus, relatively minor performance declined. The CIR and SIR exhibited a better correlation with the RAI and CAI, illustrating that both CIR and SIR could characterize the aging degree of modified asphalts well.

1. Introduction

The service performances of asphalt binders in pavement have been strongly influenced by heat, ultraviolet, oxygen, moisture, axle loads, and other factors, resulting in the degradation and damage of the polymer phase and the oxidation of base asphalt [1,2]. The aging process resulted in a decline in the rheological properties of asphalt, which can significantly impact the performance and service life of asphalt pavement [3]. Hence, as a viscoelastic material, the aging resistance of asphalt is closely related to its quality.

For CRMA and SBSMA, the thermal oxidative degradation of modifiers occurred simultaneously with the heavy matrix asphalt components. The ‘‘hardening’’ of matrix asphalt and the ‘‘softening’’ of modifiers interacted and shifted [4]. Rubber powder could enhance the anti-aging capability of base asphalt and play a more decisive role in the degradation of rubber-modified asphalt than base asphalt. Moreover, the sensitivity of rubber-modified asphalt to the aging temperature is relatively low [5].

To accurately assess the rheological properties of asphalt, the Strategic Highway Research Program (SHRP) has proposed the use of the Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR). These testing methods took into account the influence of loading modes and temperature on asphalt rheological properties [6] and were crucial in evaluating the high-temperature deformation resistance and low-temperature crack resistance of asphalt [7,8].

In a study by Li N. L. et al. [9], it was found that crumb rubber-modified asphalt binders (CRMA) exhibited better elasticity and high-temperature deformation resistance, with only minor changes in complex modulus and phase angle compared to base asphalt after aging. This suggests that CRMA has excellent anti-aging properties. However, Zhang Q. [10] discovered that CRMA showed a decrease in low-temperature ductility and fatigue resistance after both short-term and long-term aging, despite promoting its high-temperature deformation resistance. Zeng W. [11] also studied the high-temperature rheological properties of CRMA after aging and found that both the complex shear modulus and rutting factor increased with aging temperature and duration. Similarly, Yang Y. Q. et al. [12] observed that the softening point of CRMA increased with the lengthening of aging time and the rising temperature, and the change in softening point for activated rubber powder/SBS composite-modified asphalt was significantly smaller than that of ordinary rubber powder-modified asphalt and activated rubber powder-modified asphalt under different aging conditions. Li H. Y. et al. [13] demonstrated that penetration and ductility of CRMA decreased after aging, and these two indicators were highly sensitive to aging temperature, time, and pressure. However, CRMA exhibited better anti-aging performance compared to SBSMA. In another study by Li et al. [14], the impact of thermal aging on the properties of microwave-activated rubber-modified asphalt was evaluated. It was found that the rutting factor and phase angle of aged CRMA were significantly influenced by the aging temperature and time, with the aging time having a more pronounced effect. As for the microstructure evolution of asphalt during aging, Li Y. et al. [15] conducted microscopic tests on CRMA using infrared spectroscopy and fluorescence microscopy. Based on their results, they established correlations between parameters such as average core area, average particle size, and the percentage of total area with CRMA performance. Guo H. Y. et al. [16] investigated the composite modification mechanism of SBS/rubber composite modified asphalt using microscopic techniques, including fluorescence microscopy analysis, infrared spectroscopy analysis, and thermogravimetric differential scanning calorimetry (TG-DSC). Cui Y. N. et al. [17] utilized atomic force microscopy (AFM), to analyze the microstructure of asphalt binders and explored the effects of asphalt aging and self-healing properties on the properties of asphalt binders. They found that changes in asphalt microstructure could affect the self-healing performance of asphalt mastics, and modified asphalt binders exhibited a more stable microstructure and better performance after aging. Huang X. Y., Wang L. et al. [18,19] carried out the performances of warm mix rubber powder modified asphalt and used Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and rheological tests to investigate the relationship between micro-mechanical properties and macroscopic rheological properties. They established correlations between micro-mechanical indicators (such as microscopic Young’s modulus) and macroscopic rheological properties (such as improved rutting factor).

Currently, previous studies have primarily focused on the influence of film thickness and aging time on the degree of aging. However, there is a lack of further analysis about the effect of aging temperature of the performance deterioration of asphalt. Specifically, there are rare quantitative analyses on the changes in rheological properties during short-term aging processes for CRMA and SBSMA under different temperature conditions. Additionally, further investigation is necessary to understand the relationship between macroscopic rheological properties and the microstructural evolution of asphalt.

In this work, the rheological properties of CRMA and SBSMA were analyzed using Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests after undergoing short-term aging at various temperatures. The impact of temperatures on the rheological properties of modified asphalt binders after short-term aging was investigated. Furthermore, fluorescence microscopy and infrared spectroscopy were utilized to examine the changes in microstructure and chemical composition of CRMA and SBSMA. Moreover, to explore reasonable analytical methods to characterize the evolution of microstructure and properties of two modified asphalt binders during aging, the correlations between rutting factor aging index (RAI)/creep rate aging index (CAI) and carbonyl index growth rate (CIR)/sulfoxide index growth rate (SIR) were established to elucidate the relationship between macroscopic performance degradation and microstructural evolution of asphalt binders.

2. Materials and Methods

2.1. Preparation

The base asphalt of TIPCO 70# asphalt, which has performance indicators, is shown in

Table 1. Technical specifications of base asphalt.

Test Terms		Unit	Test Values	Request
Penetration (25 °C, 100 g, 5 s)		0.1 mm	66	60~80
Softening point (Ring and Ball)		°C	48	≥46
Ductility (15 °C, 5 cm/min)		cm	132	≥100
Dynamic viscosity (60 °C)		Pa·s	225	≥180
RTFOT	Mass changes	%	−0.09	≤±0.8
	Residual penetration ratio (25 °C)	%	76	≥61
	Residual ductility (10 °C)	cm	8.7	≥6

. The rubber powder was obtained by grinding truck tires, and its performance indicators are detailed in

Table 2. Material properties of rubber powder.

Project	Request	Result
Heating loss	≤1.0%	0.38%
acetone extract	≤8%	7%
Rubber hydrocarbon content	≥48%	56%
Carbon black content	≥28%	30%
Iron content	≤0.03%	0.02%
Fiber content	≤0%	0%
Residue	≤0%	0% (20 mesh)
	≤10%	4% (30 mesh)
Volume density	270~480 kg/m3	360 kg/m3

. The technical specifications of the linear SBS modifier are outlined in

Table 3. Material properties of SBS.

Structure Type	Block Ratio (S/B)	Stretch Rate (%)	Tensile Strength (MPa)	Melt Index (g·10 min−1)	Density (g/cm3)
Liner	30/70	800	25	0.6	0.93

. SBS is white particles with 2~6 mm in length, manufactured by Dushanzi Petrochemical Co., Ltd., Karamay, Xinjiang, China. Crumb rubber is a black powder with 30 mesh, produced by Guangxi Transportation Science and Technology Co., Ltd., Nanning, Guangxi, China. The geometry of the SBS and rubber power was shown in Figure 1.

Rubber powder and SBS were used as modifying additives to prepare CRMA and SBSMA via the following process as following: The base asphalt was heated to a flowable state at 140 °C, then the temperature was raised to 185 °C and a certain amount of rubber powder (or SBS) was added. The mixture was stirred for 40 min to achieve initial swelling and dispersion, followed by high-speed shearing at a rate of 4000 rpm for 20 min. Subsequently, the mixture was stirred for another 45 min to prepare CRMA and SBSMA. The dosages of rubber powder and SBS were 20% and 3%, respectively. The basic performance indicators of the two modified asphalts are presented in

Table 4. Properties of SBSMA and CRMA.

Sample	Brookfield Viscosity (Pa·s, 180 °C)	Penetration (0.1 mm, 25 °C)	Ductility (cm, 5 °C)	Softening Point (°C)	Elastic Recovery Rate (%)
CRMA	1.518	47.3	7.5	69.1	88
SBSMA	0.316	51.9	36.2	86.7	88

.

Considering that the RTFOT conducted at 163 °C for 85 min can simulate the aging performance of conventional hot-mix asphalt pavements, this study employed 163 °C as a reference condition. Short-term aging tests were then conducted on CRMA and SBSMA at temperatures of 163 °C, 173 °C, and 183 °C, heated for 85 min as the specimens for subsequent experiments.

2.2. Experimental Method for Asphalt Rheological Properties

2.2.1. Dynamic Shear Rheometer Test (DSR)

The rheological characteristics of asphalt at intermediate and high temperatures were investigated using a Dynamic Shear Rheometer (DSR) with temperature scanning at 52 °C, 58 °C, 64 °C, 70 °C, 76 °C, and 82 °C. To assess the rheological performance of modified asphalt at intermediate to high temperatures, the complex shear modulus |G*| and phase angle δ were measured for asphalt specimens’ various aging conditions. The complex shear modulus |G*| is determined by the ratio of stress and strain obtained from DSR tests, and the rutting factor (RF) can be calculated using Equation (1) with the δ and |G*|.

$$RF=\left|G^{*}\right|/\sin \delta$$

(1)

The DHR-1 Dynamic Shear Rheometer used in this work was produced by TA Company in the United States. The samples for DSR were shaped with a silicone rubber mold of a 25 mm diameter.

2.2.2. Bending Beam Rheometer Test (BBR)

In this study, BBR tests were conducted on two types of modified asphalt that had undergone RTFOT following the guidelines outlined in the “Test Methods for Asphalt and Asphalt Mixtures in Highway Engineering”. The BBR tests were performed at temperatures of −12, −18, and −24 °C, with two repeated experiments conducted for each temperature [20]. The average of the obtained data were considered the experimental result. The stiffness modulus (S) of the modified asphalt at a specific time and temperature is defined as the ratio of stress to total strain, which is automatically obtained by the bending beam rheometer. Drawing on the research findings of ASTM D6648, if the relationship between S and time (t) follows Equation (2), the creep compliance rate (m) can be calculated using Equation (3) [21].

$$\log \left(S\right)=A+B\cdot \log \left(t\right)+C\cdot \log {\left(t\right)}^2$$

(2)

$$m=\left|\frac{d\log \left(S\right)}{d\log \left(t\right)}\right|=\left|B+2C\cdot \log \left(t\right)\right|$$

(3)

where A, B, and C are regression coefficients; S is the stiffness modulus; m is the creep compliance rate; and t is time.

The samples for DSR were shaped with a silicone rubber mold of 25 mm diameter. The TE-BBR bending beam rheometer was produced by Cannon Instrument Company, State College, PA, USA. The sizes of the samples for BBR were 127 mm ± 2.0 mm in length, 12.7 mm ± 0.05 mm in width and 6.35 mm ± 0.05 mm in thickness, enclosed by three longer thin Aluminium sheets and two shorter Aluminium sheets in the forming process.

2.3. Methodology for Microscopic Characteristics Testing

2.3.1. Fluorescence Microscopy

The samples of CRMA and SBSMA with varying degrees of short-term aging were prepared in the following manner: first, the asphalt was heated until it reached a fluid state and then applied onto glass slides. Next, cover glasses were used to press the asphalt, ensuring the uniform thickness of the samples. Once cooled to room temperature, the samples were examined under a LW300LFT-LED fluorescence microscope.

The LW300LFT-LED fluorescence microscope was made by Shanghai Changsa Scientific Instrument Co., Ltd., Shanghai, China.

2.3.2. Fourier Infrared Spectrum Test

Infrared spectra of different samples were collected using the ALPHA II infrared spectrometer. Initially, a small amount of asphalt was dropped onto a silicone sheet while in a fluid state. After the asphalt cooled, it was placed on the infrared spectrometer for scanning. The wavenumber range used was 650 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹, and each sample was scanned 32 times.

The ALPHA II Infrared spectrometer was produced by Bruker Optics (China), Beijing, China, and the wavenumber ranged from 4000 to 400 cm⁻¹. The spectra were collected at 25 °C with a 4 cm⁻¹ wavenumber resolution after 32 continuous scans, and three replicates were tested for each group.

3. Results

3.1. DSR Test

The phase angle (δ) represents the ratio of the viscous component to the elastic component in asphalt binder. The smaller the phase angle (δ), the greater the elastic component in complex modulus (G*). The temperature scan results of δ are shown in Figure 2.

The phase angle of asphalt under various conditions exhibited a positive correlation with the scanning temperature. This means that as the temperature increases, the material becomes more viscous and less elastic. Due to the gradual softening of asphalt at higher temperatures, the amount of viscous components increased, producing a larger irrecoverable deformation in asphalt. Specifically, the phase angle of the original SBSMA declined as temperature increased, which was attributed to the new network structure generated by the SBS modification, which affected the viscoelastic characteristics of asphalt. However, when SBSMA underwent short-term high-temperature aging, the new network structure was broken down, and this phenomenon disappeared.

The viscoelastic characteristics of the two modified asphalts followed opposite patterns under different short-term aging conditions. As the aging temperature increased, the phase angle of SBSMA gradually increased at the same scanning temperature, while the phase angle of CRMA decreased. This indicates that during the short-term aging process, the elastic components of crumb rubber-modified asphalt binder increased significantly compared to styrene-butadiene-styrene-modified asphalt, resulting in an increase in the elastic recovery capability of CRMA. This is because the internal original network structure of SBSMA is disrupted after short-term high-temperature aging, weakening its binding effect on base asphalt and reducing the elasticity of the material. Additionally, the decrease in the light component in CRMA after high-temperature aging and the degradation of rubber powder enhanced the interaction between asphalt and rubber, forming a gel with strong rebound deformation capability.

Under the same short-term aging temperature, the phase angle of CRMA is significantly smaller than that of SBSMA (even smaller than that of the original SBSMA). This indicates that the elastic performance of CRMA is significantly superior to that of SBSMA.

Figure 3 and Figure 4 illustrate the variation patterns of the complex modulus and rutting factor of modified asphalt after RTFOT at different temperatures, presenting a comprehensive understanding of high-temperature rutting resistance for asphalt binders. Higher values of complex modulus and rutting factor indicated lower energy consumption and reduced flow deformation at high temperatures, indicating superior high-temperature rutting performance. The results are expressed in detail as follows.

After the RTFOT, the complex modulus and rutting factor of various asphalt binders exhibited a monotonically decreasing trend with the scanning temperature rising. The higher the aging temperatures, the more enhanced the high-temperature rutting resistance of modified asphalt. This trend is more pronounced at lower scanning temperatures and diminishes when the scanning temperature exceeds 70 °C.

According to the research findings of the SHRP, the rutting factor of original asphalt binder should not be less than 1.0 kPa, while a rutting factor of 2.2 kPa has been set as the standard for the high-temperature zone of Performance Grade (PG) after RTFOT. In this work, the rutting factor of original SBSMA dropped to 1.0 kPa at a scanning temperature of around 80 °C, while the rutting factor of non-aged rubberized asphalt remained at 2.4 kPa after reaching 85 °C. After undergoing short-term aging at 163 °C, 173 °C, and 183 °C, the rutting factors of SBSMA reached 2.2 kPa at 72 °C, 74 °C, and 76 °C, indicating that SBSMA would be classified as PG-70 or PG-76. However, CRMA exhibited a rutting factor of approximately 4.5 kPa at 82 °C after short-term aging at 163 °C, 173 °C, and 183 °C, indicating that CRMA would be classified as PG-82. This suggested that the use of crumb rubber for modification resulted in a more significant improvement in high-temperature rutting resistance compared to SBS.

However, the effect of this improvement gradually diminished as the RTFOT temperature increased. In terms of SBSMA, with the RTFOT temperature increasing, the high-temperature performance was further enhanced. This demonstrated that the impact of short-term aging at standard temperatures is more significant for CRMA, while the performance of SBSMA is more sensitive to the temperature of short-term aging.

To further evaluate the aging resistance of asphalt, the degree of change in the rutting factor of asphalt binder before and after aging was defined as the Rutting Factor Aging Index (RAI). RAI can be calculated via Equation (4), and the RAI variation curves of CRMA and SBSMA under different aging temperatures are shown in Figure 5.

$$RAI=RF/R{F}_0$$

(4)

where RAI is the rutting factor aging index, RF is the rutting factor of aged asphalt, RF₀ is the rutting factor of original asphalt.

After aging, the differences in RAI values for CRMA and SBSMA were greater, indicating that there was an improvement in the flow deformation resistance of asphalt. This could be attributed to the volatilization of lighter components and their conversions into heavier ones at high temperatures, resulting in a higher proportion of polar components in asphalt. The oxidizing reaction of asphalt affected oxygen in air, which also contributed to this increase in polar functional groups, leading to stronger intermolecular actions and bonding. However, when comparing the RAI values of SBSMA and CRMA at the same aging temperature, it was observed that CRMA had a lower RAI value at the same temperature. This could be explained by the fact that under aging conditions, the cross-linked structure between SBS and base asphalt is partially disrupted, causing flocculation and segregation, which weaken the structural stability. On the other hand, the swelling reaction in CRMA lasted for a longer period of time, and under the catalytic effect of heat, both aging and swelling reactions occurred simultaneously, inhibiting the superior anti-aging ability of CRMA to a certain extent.

3.2. BBR Test

The results of BBR tests are presented in Figure 6 and Figure 7. According to SHRP specification, a smaller stiffness modulus of asphalt binder indicated better adaptability to cracking resistance at low temperatures, while a higher creep rate allowed for the rapid release of stress-reducing damage. From Figure 5 and Figure 6, it can be observed that: (a) with TFOT temperature rising up, the stiffness modulus of modified asphalt increased while the creep rate decreased, indicating a significant deterioration in low-temperature cracking resistance; (b) at the temperature of −18 °C, the stiffness modulus of CRMA is less than 300 MPa, while the creep rates were close to 0.3, resulting in a low-temperature grading temperature below −18 °C. In contrast, the low-temperature grading temperature of SBSMA was between −12 °C and −18 °C, indicating better low-temperature cracking resistance for CRMA. (c) The addition of SBS modifier was mainly employed to improve the stiffness modulus of asphalt, which exacerbated the issue of insufficient creep rate. Therefore, the low-temperature cracking resistance of SBSMA was primarily controlled by the creep rate, demonstrating that the stiffness modulus of SBSMA still met engineering requirements at −18 °C. Yet, the creep rate was significantly less than 0.3.

Similar to the definition of RAI, the degree of change in creep rate before and after aging for asphalt was analyzed, which was called Creep Rate Aging Index (CAI). CAI can be calculated by Equation (5) and the CAI change curves of CRMA and SBSMA are shown in Figure 8.

$$CAI=m/m_0$$

(5)

where CAI is Creep Rate Aging Index, m is the creep rate of aged asphalt, and m₀ is the creep rate of original asphalt.

After undergoing aging at different temperatures, it could be observed that the CAI values of CRMA and SBSMA were less than 1, indicating a decrease in the creep rate at varying degrees. This can be attributed to the decrease in the content of light components and an increase in heavy components, resulting in a decrease in stress relaxation capacity due to thermal-oxidative aging. Furthermore, the CAI values of SBSMA consistently remained lower than those of CRMA under the same aging conditions, suggesting that CRMA had a better anti-aging ability. This could be attributed to the cross-linked network structure of SBSMA being damaged to a certain extent after aging, leading to reduced flexibility. On the other hand, CRMA continued to undergo a swelling reaction, which further improved the homogeneity of rubber powder and enhanced its ability of deformation resistance, demonstrating its superior stress relaxation effect.

3.3. Fluorescence Microscopic Test

The results of the fluorescence microscopy test for original SBSMA and CRMA are displayed in Figure 9 and Figure 10. All observation images were magnified 100 times. Commonly, SBSMA contained 5% content of SBS, and CRMA contained about 20% content of crumb rubber, mainly including polybutadiene rubber, styrene butadiene rubber, and natural or synthetic isoprene rubber.

The presence of polymer phases, such as SBS and rubber in modified asphalt, caused them to reflect longer-wavelength fluorescence (yellow spots), while asphalt phases do not emit any light under fluorescence excitation. This allowed for a clear distinction between the polymer phase (bright yellow spots) and the asphalt phase (bright green spots) in images. In the original modified asphalt, SBS appeared as small particles that were uniformly and densely distributed within the asphalt, while rubber powder was distributed in large block-shaped “islands”. This indicated particle expansion and the breakdown of internal structure by modification. Due to the high-speed shearing process, SBS was broken down into smaller particles and dispersed evenly within the asphalt matrix. However, rubber powder underwent swelling, absorbing the light components in asphalt and causing the particles to enlarge and agglomerate. Moreover, a new gel film was formed by interface interaction between base asphalt and crumb rubber, which enhanced the overall resilience and deformation resistance. This was consistent with the conclusions drawn from the temperature scan test. As a result, the internal skeleton structure of CRMA is less prone to deformation, making it more resistant to permanent deformation compared to SBSMA.

With aging temperatures increasing, SBS particles gradually became finer and more densely distributed. This was particularly evident in the images of SBSMA at different temperatures. Additionally, rubber powder transferred from large block-shaped “island” structures to multiple fibrous structures, gradually becoming more compatible with asphalt and interconnecting. However, the dispersion remained uneven, with the overall agglomeration state remaining unchanged under different aging temperatures. This phenomenon could be attributed to the evaporation of some light components in asphalt, while others were absorbed and swollen by SBS during the thermal-oxidative aging process. This led to the decomposition of SBS into smaller particles. As the interaction between SBS and asphalt aggravated at higher temperatures, the decomposition of SBS became more severe, resulting in finer particles coming out.

The block-shaped “island” structure of rubber powder absorbed more light components from asphalt during high-temperature aging, leading to expansion and decomposition. This caused some cross-linking nodes and molecular chains to rupture, transforming the originally compact block-shaped structure of rubber powder into a relatively loose fibrous structure. As rubber powder continued to swell, polymer chains on the surface of the powder cracked and diffused into asphalt. A few remaining short polymer chains dissociated from rubber powder and dissolved into asphalt. The diffused polymer chains are saturated with the light components in asphalt, forming a relatively stable structure. This reduced the rate of light component evaporation during aging. Therefore, CRMA demonstrated superior anti-aging performance compared to SBSMA. Besides, short-term aging had a greater impact on CRMA, while the performance of SBSMA is more sensitive to aging temperatures.

3.4. Fourier Infrared Spectroscopy Test

The infrared spectra of SBSMA and CRMA under different temperature aging conditions were obtained through FTIR experiments, as shown in Figure 11.

Figure 11 shows the absorption peaks of two modified asphalt binders at generally similar positions in the functional group region, ranging from 4000 cm⁻¹ to 650 cm⁻¹, although with varying intensities.

The peaks at 2920 cm⁻¹ and 2850 cm⁻¹ corresponded to the antisymmetric and symmetric stretching vibrations of aliphatic CH₂ groups, indicating the presence of saturated aliphatic compounds in asphalt. The peak at 1700 cm⁻¹ and 1600 cm⁻¹ corresponded to the carbonyl stretching vibration (C=O), suggesting the presence of oxygen-containing functional groups and aromatic compounds in the asphalt, respectively. The peaks at 1460 cm⁻¹ and 1376 cm⁻¹ corresponded to the asymmetric and symmetric bending vibrations of CH₃ groups, indicating the presence of aliphatic compounds. The peak at 1030 cm⁻¹ corresponded to the sulfoxide stretching vibration (S=O). The peaks at 863 cm⁻¹, 810 cm⁻¹, and 743 cm⁻¹ represented the out-of-plane bending vibrations of C-H bonds, while the peak at 724 cm⁻¹ represented the in-plane wagging vibration of CH₂. It was evident that both SBSMA-modified asphalt and CRMA contained saturated fractions, aromatic fractions, aliphatic compounds, and other small molecular derivatives.

Additionally, it could be observed that the characteristic peaks of two asphalt binders were consistent beyond the wavenumber of 1000 cm⁻¹. SBSMA exhibited additional characteristic peaks at 965 cm⁻¹ and 699 cm⁻¹ compared to CRMA.

The peaks at 965 cm⁻¹ and 699 cm⁻¹, corresponding to the twisting vibration of C=C bonds in polybutadiene and the absorption of C-H bonds in the benzene rings of polystyrene, were unique to SBSMA. To quantitatively analyze the aging degree of SBSMA and CRMA at different temperatures, the area ratios of characteristic functional group peaks at 1700 cm⁻¹ (C=O) and 1030 cm⁻¹ (S=O) to the total areas were calculated separately, as shown in Equation (6). The carbonyl index and sulfoxide index of CRMA and SBSMA before and after aging are presented in

Table 5. Carbonyl index and sulfoxide index of asphalt under different aging degree.

Sample	I1700 (C=O)	I1030 (S=O)
SBSMA Original	0.00130	0.00615
Aged SBSMA 163 °C	0.00168	0.00719
Aged SBSMA 173 °C	0.00194	0.00772
Aged SBSMA 183 °C	0.00232	0.00870
CRMA Original	0.00174	0.00934
Aged CRMA 163 °C	0.00206	0.01047
Aged CRMA 173 °C	0.00223	0.01098
Aged CRMA 183 °C	0.00233	0.01138

.

$$I_x=\frac{A_x}{{\displaystyle \sum A_{2000~650 {\mathrm{cm}}^{-1}}}}$$

(6)

where I_x is the carbonyl index (when x = 1700) or sulfoxide index (when x = 1030), A_x is the peak area at wavenumber, ∑A_{(2000~650 cm⁻¹)} is the total peak area sum within the wavenumber range of 2000 to 650 cm⁻¹.

According to Table 5, it was evident that the carbonyl index and sulfoxide index of various asphalt samples differ, indicating varying levels of aging [22]. Both the carbonyl index and sulfoxide index of both types of asphalt increase as the aging temperature increases. This suggests that the carbon and sulfur in the asphalt react with oxygen in the air at high temperatures, resulting in the formation of more C=O and S=O bonds. In order to accurately assess the anti-aging performance of the asphalt after exposure to different temperatures, the growth rates of the carbonyl index and sulfoxide index (calculated using Equation (7)) at 163 °C, 173 °C, and 183 °C were compared to those of the original asphalt. The calculation formulas are as follows. The results of the growth rates of the carbonyl index and sulfoxide index are presented in Figure 12.

$$HI{R}_x=\frac{I_{aged}-I_{orig}}{I_{orig}}$$

(7)

where HIR_x is the growth rate of carbonyl index (when x = 1700) or the growth rate of sulfoxide index (when x = 1030), I_aged is the carbonyl index or sulfoxide index of aged asphalt, I_orig is the carbonyl index or sulfoxide index of original asphalt.

From Figure 12, it was evident that the concentrations of the carbonyl and sulfoxide groups in two modified asphalt binders increased with higher aging temperatures. This indicated that as the temperature increased, the number of these groups in asphalt molecules also increased. This is due to the reaction between the asphalt molecules and oxygen from the air, resulting in the formation of C=O and S=O bonds as asphalt ages at different temperatures. The higher the temperature, the more vigorous the molecular motion, leading to more intense and rapid reactions among molecules. Additionally, at the same temperature, the growth rates of carbonyl and sulfoxide groups in SBSMA were higher than those in CRMA, indicating that CRMA had better resistance to aging compared to SBSMA, which is consistent with the macroscopic indicators. Furthermore, as the aging temperature increased, the growth rates of carbonyl and sulfoxide groups in SBSMA showed a significant increase, while those in CRMA showed a slower upward trend. This suggested that SBSMA was more sensitive to aging temperature compared with CRMA.

The reason for the difference in aging behavior between SBSMA and CRMA was due to the volatilization of lighter components during the aging process. This resulted in a higher proportion of heavier components, such as resins and asphaltene. The lower content of SBS made it less effective in absorbing these lighter components. In contrast, CRMA contained a higher content of rubber powder (up to 20%), which had a strong ability to adsorb lighter components. This reduced the loss of these components during short-term aging and led to more stable changes in carbonyl and sulfoxide growth rates. Additionally, the degradation of rubber molecules caused them to become more tightly arranged, reducing the contact area between the rubber powder and air. This decreased the number of asphalt molecules that were oxidized into polar molecules. As a result, the anti-aging performance of CRMA was significantly superior to that of SBSMA.

3.5. Analysis of the Correlation between Microscopic Evolution and Macroscopic Indicators

The macroscopic aging and microscopic aging indicators of two modified asphalt binders were conducted using linear regression analysis to establish the relationship between asphalt microstructure evolution and macroscopic indicators. The RAI at 64 °C and CAI at −12 °C were selected from the macroscopic indicators and fitted with the carbonyl growth rate (HIR₁₇₀₀) and sulfoxide growth rate (HIR₁₀₃₀). The results are shown in Figure 12.

From Figure 13, the R² values of RAI and CAI with respect to HIR₁₇₀₀ and HIR₁₀₃₀ exceeded 0.8, indicating a good correlation among them. It suggested that both HIR₁₇₀₀ and HIR₁₀₃₀ could effectively characterize the aging degree of SBSMA and CRMA. RAI showed a positive correlation with HIR₁₇₀₀ and HIR₁₀₃₀, while CAI exhibited a negative correlation with HIR₁₇₀₀ and HIR₁₀₃₀. It was attributed to the sulfur (S) in asphalt molecules being oxidized to form S=O under the aging process, while C=C bonds underwent cleavage and absorbed oxygen (O₂) from the air to form C=O, S=O and C=O were polar functional groups, which easily enhanced intermolecular interactions. Thus, the permanent resistance of asphalt was strengthened, leading to an increase in the rutting factor. However, the increase in polar molecules could lead to a decrease in the stress relaxation ability of asphalt, namely the CAI, which decreased as HIR₁₇₀₀ and HIR₁₀₃₀ increased.

4. Conclusions

In this work, after undergoing varying temperatures of short-term aging, the high- and low-temperature rheological characteristics of CRMA and SBSMA were comparatively analyzed. Subsequently, the evolution mechanism of asphalt properties was examined using fluorescence microscopy and Fourier infrared spectroscopy tests. A correlation study was developed between the macro indices and the microscopic evolution of asphalt. The following conclusions were drawn:

• With the increase in RTFOT temperature, the aging of SBSMA and CRMA was aggravated under the same aging conditions, with improved high-temperature performance and decreased low-temperature performance. Compared to CRMA, SBSMA was more sensitive to the temperature of short-term aging; thus, the temperature during paving should be controlled reasonably to avoid the aging of SBSMA.
• Short-term aging of SBSMA resulted in disruption of the internal original network structure, and rubber powder degradation was seen in CRMA. However, a new gel film was formed by interface interaction between base asphalt and crumb rubber, which enhanced the overall resilience and deformation resistance.
• Compared to SBSMA, CRMA showed better low-temperature crack resistance, a smaller phase angle, a bigger complex modulus, and a reduced rutting factor. It demonstrated greater elastic properties, suggesting better resistance to high temperatures and anti-aging capabilities.
• Rubber powder possessed an uneven distribution as huge block “islands” in asphalt, whereas SBS was consistently dispersed as little particles in asphalt. Temperatures had less effect on rubber powder in CRMA, while they had a great impact on the modifier in SBSMA. As a result, SBSMA is more susceptible to temperatures during short-term aging.
• The amounts of carbonyl and sulfoxide groups in the molecules of CRMA and SBSMA both rose with aging temperature. The HIR1700 and HIR1030 in SBSMA were higher than those in CRMA at the same aging temperature, indicating that CRMA had better anti-aging properties than SBSMA.
• Through correlation analysis, it was found that the HIR₁₇₀₀ and HIR₁₀₃₀ had a good linear correlation with the rutting factor and the creep parameter. The RAI was positively correlated with the carbonyl and sulfoxide growth rates, while the CAI was negatively correlated with the carbonyl and sulfoxide growth rates. The correlation between the HIR₁₇₀₀ and macroscopic indicators was greater than that of the HIR₁₀₃₀.