Rheological Properties and Microstructural Analysis of Asphalt Binder Doped with Anti-Stripping Agent after Aging

Abstract

In order to comprehensively explore the long-term durability of asphalt mixed with an anti-stripping agent, laboratory tests were carried out on base asphalt, SBS modified asphalt and base asphalt mixed with an anti-stripping agent after long-term aging. The rheological properties, chemical composition and microscopic morphology of the three asphalts before and after aging were compared and analyzed from macroscopic and microscopic angles. The results show that the modulus of asphalt is more affected by temperature than aging, and the anti-aging ability of the asphalt with the anti-stripping agent is better than that of base asphalt and slightly worse than that of SBS modified asphalt. The change in the absorption peak intensity before and after the aging of asphalt with the anti-stripping agent is the same as those of the base asphalt and SBS modified asphalt at 2920 cm⁻¹ and 2850 cm⁻¹, while the opposite pattern is observed in other four places. The honeycomb structures of the base asphalt and SBS modified asphalt became longer and the number of peaks increased slightly after aging. However, the asphalt surface with the anti-stripping agent had no obvious honeycomb structures before aging, and there were obvious honeycomb structures after aging, the number of which was between those of the base asphalt and SBS modified asphalt after aging; moreover, the lengths of honeycomb structures were still significantly smaller than those of the base asphalt after aging. This study investigated the performance changes of the anti-stripping-agent asphalt before and after long-term aging at multiple scales and compared the asphalt mixed with the anti-stripping agent with base asphalt and SBS modified asphalt to fully evaluate its long-term durability.

1. Introduction

At present, in the construction of highways in China, acidic stones with tight structures, wear resistance and slip resistance are generally used, such as granite and quartzite. Because the surface of an acidic stone has strong hydrophilicity, it does not adhere well to oily asphalt. Therefore, under the action of rainwater and groundwater, the asphalt is easily peeled off from the surface of the stone, resulting in water damage to the highway. Water damage has become one of the main reasons for early damage to high-grade highways [1,2,3]. For asphalt pavements that are prone to water damage, commonly used repair measures include selecting alkaline aggregates, adding anti-stripping agents to improve the adhesion between the aggregates and asphalt and selecting densely graded mixtures to reduce porosity in order to reduce water infiltration. Anti-stripping agents have become a common means to improve the water stability of asphalt mixtures due to their simple production process, low dosage and remarkable effects [4,5].

Currently, commonly used anti-stripping agents are mainly divided into amines and non-amines. The heat resistance and long-term stability of amine anti-stripping agents are weaker than those of non-amines, and amine substances are not friendly to the environment. Therefore, non-amine anti-stripping agents will become the main target of future research and development [6]. The addition of anti-stripping agents to the base asphalt can be regarded as a modification, and its purpose is to improve the anti-stripping ability of the asphalt after adhering to the aggregates [7,8]. Iskender et al.’s research found that after the addition of anti-stripping agents, the adhesion between the asphalt and aggregates is enhanced and the anti-fatigue performance is improved. Moreover, after multiple dynamic water scour-freeze-thaw cycles, the increase in porosity is significantly reduced and the splitting tensile strength is significantly increased [9,10,11,12,13,14]. Most of the above studies focused on the evaluation of the adhesion between the asphalt and aggregates after adding anti-stripping agents but ignored the change in the overall performance of the asphalt after adding anti-stripping agents. In recent years, Zhu investigated the cohesive energy density and solubility parameters of five anti-stripping agents with asphalt using molecular dynamics, and the results showed that the differences in the solubility parameters of each component were within 2.0 and that the compatibility was good [15]. Heng et al. conducted macroscopic physical and rheological tests on asphalt mixed with different anti-stripping agents and found that anti-stripping agents can improve the anti-aging performance of asphalt but reduce the shear resistance of asphalt at high temperatures [16,17,18]. Mansourkhaki et al. found that adding anti-stripping agents can improve the low temperature performance of asphalt and analyzed the chemical composition of asphalt before and after aging using infrared spectroscopy, finding that the number of macromolecules and oxygen-containing functional groups of asphalt after aging increased [19,20]. Hou et al. used AFM to study the surface elastic modulus and nanohardness of anti-stripping agents in asphalts, and the results showed that the effects of anti-stripping agents on a modified asphalt binder were greater than those of an unmodified asphalt binder. After adding the anti-stripping agent, the elastic modulus and nanohardness of the unmodified asphalt binder decreased by 16.51–20.67% and 22.24–25.78%, respectively [21]. Like other commonly used asphalts on pavements, asphalt pavement with anti-stripping agents also experiences aging during operation. Zheng studied the effects of two types of anti-stripping agents on the microscopic strength of asphalt after short-term and long-term aging, and the results showed that anti-stripping agents may influence the adhesive and cohesive strengths of asphalt [22]. However, most of the previous studies focused on amine anti-stripping agents, limes and metal saponification, and there were few reports on the long-term durability of asphalt mixed with new non-amine anti-stripping agents, especially on the joint analysis of macrorheological properties with chemical composition changes and surface micromorphological structures [23,24,25,26,27,28,29,30,31,32]. Moreover, relative to the macro, the micro perspective can more fundamentally evaluate the performance change principle before and after aging.

Based on this, a non-amine anti-stripping agent was selected to prepare an anti-stripping-agent asphalt binder and compared with the base asphalt and SBS modified asphalt without anti-stripping agents. The three asphalts were subjected to long-term aging by means of pressure aging vessel (PAV) accelerated asphalt aging tests to simulate the aging of an asphalt binder in long-term pavement service. Then, a dynamic shear rheometer (DSR), infrared spectrometer (IR) and atomic force microscope (AFM) were used to reveal the changes in the overall properties of the asphalt mixed with the anti-stripping agent before and after aging with respect to macroscopic rheological properties, chemical composition and microstructure in order to provide help for the research on the performance of anti-stripping-agent asphalt binders after long-term aging.

2. Materials and Methods

2.1. Materials

The asphalts used in the tests are Panjin 90# base asphalt and I-C type SBS modified asphalt. The basic performance indicators are shown in

Table 1. Three major indicators of asphalt.

Type	Penetration (0.1 mm)	Ductility	Soft Point (°C)
90# base asphalt	89.4	>100(15 °C)	49.2
SBS modified asphalt	69.6	50.3 (5 °C)	67.1

. The anti-stripping agent is a fourth-generation non-amine anti-stripping agent from a company. Its appearance is shown in Figure 1, and its technical indicators are shown in

Table 2. Indicators of anti-stripping agent.

Test Items	Technical Indicators	Test Results
Appearance	—	Dark brown viscous liquid
Main ingredient	—	fatty acids
Density (20 °C, g/mL)	0.900~0.980	0.945
Water content (105 °C, 1 h) %	≤0.3	0.05
Soluble (with hot asphalt binder)	Soluble	Soluble

. The dosage of anti-stripping agent is 0.3% of the mass of Panjin 90# base asphalt. The anti-stripping-agent asphalt can be prepared by directly mixing the anti-stripping agent into the hot asphalt and stirring it evenly with a mixer.

2.2. Methods

2.2.1. Long-Term Aging Test

Pressure aging vessel (PAV, Changji Geology Instrument, Shanghai, China) accelerated asphalt aging tests were used for the three asphalts according to AASHTO R 28 [33]. For the long-term aging test process, the asphalt samples were placed in a pressure aging oven at a temperature of 100 °C and a pressure of 2.1 MPa for 20 h after a rotary film oven aging test (RTFOT). Then, the samples were removed and placed under normal temperature and pressure to decompress until all air bubbles were removed.

2.2.2. Rheological Property Test

The rheological properties of the three asphalts before and after aging were tested with frequency scan tests using an Anton Paar MCR302 Dynamic Shear Rheometer (DSR, Anton Paar, Graz, Austria) according to the method of AASHTO T315 [34]. The test temperature was 10 °C~30 °C (normal temperature) and 40 °C~60 °C (medium and high temperature), and there were eight scanning frequencies: 0.01 Hz, 0.05 Hz, 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz, 60 Hz. At 10 °C~30 °C, a parallel plate fixture with a diameter of 8mm and a spacing of 2mm was used, and at 40 °C~60 °C, a parallel plate fixture with a diameter of 25mm and a spacing of 1mm was selected, as shown in Figure 2.

2.2.3. Chemical Composition Analysis Test

A Niconet IS5 infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used to analyze the changes in chemical composition of the three asphalts before and after aging. The spectral measurement range is from 4000 cm⁻¹ to 650 cm⁻¹, and the resolution is 1 cm⁻¹. A total of 16 scanning tests were carried out. During the tests, the asphalt samples were clamped by two rectangular silica gel plates with a side length of 1.5 cm and sealed in an aluminum box to prevent contamination.

2.2.4. Surface Microtopography Analysis Test

Atomic force microscopy (AFM) is a technique that can analyze nanoscale material surface structures and can test the microstructure of asphalt material surfaces. In the tests, a dimension icon atomic force microscope (Bruker, Billerica, MA, USA) was used, and the tapping mode was selected to test the morphological changes in the three asphalts before and after aging. The test frequency was 1Hz, the elastic constant of the probe was 0.4 N/m, the scanning range was 20 µm × 20 µm, and the resolution was 512 × 512.

3. Results and Discussion

3.1. Rheological Analysis

Asphalt is a typical viscoelastic material. Its mechanical behavior is affected by temperature and frequency, and it has equivalent viscoelastic properties under high temperatures and low frequencies and under low temperatures and high frequencies, which conforms to the time–temperature superposition principle (TTSP). Based on the idea of TTSP, this paper combines the (1) CAM equation with the (3) WLF equation and moves the results of the frequency scanning tests at different temperatures to the reference temperatures of 10 °C and 60 °C in order to obtain the viscoelastic mechanical behavior of asphalt at a wider range of temperatures and frequencies. The main curves of the complex shear moduli of the three bitumen are shown in Figure 3. In the figure: B stands for base asphalt; SBS stands for SBS modified asphalt; AN stands for base asphalt with the anti-stripping agent added; the suffix A stands for long-term aging; and UA stands for unaged. This marking method is used uniformly in the subsequent charts.

$$\left|G^{*}\right|=\frac{\left|G_g^{*}\right|}{{\left[1+(f_c/f\prime {)}^k\right]}^{m/k}}$$

(1)

In the formula: $\left|G_g^{*}\right|$ is the asphalt glass shear modulus; K and m are the main curve fitting parameters; $f_c$ is the main curve fitting parameter of loading frequency; and $f\prime$ is the conversion loading frequency according to Equation (2).

$$f\prime ={\alpha }_{\mathrm{T}}•f$$

(2)

In the formula: $f$ is the actual loading frequency, and ${\alpha }_{\mathrm{T}}$ is the temperature shift factor.

From the main curves of the complex moduli, it can be seen that regardless of the kind of asphalt, whether base asphalt, modified asphalt or asphalt with the anti-stripping agent, the complex moduli increase after aging; moreover, the increases are more obvious in the low frequency range, and the increases in the high frequency range basically coincide with those before aging. When the temperatures are the same, the loading frequency increases, and the complex moduli of the asphalts increase accordingly; when the temperatures are different, under the same loading frequency, the complex moduli at low temperature are higher than those at high temperature. Compared with 10 °C, the change degree of the asphalt moduli before and after aging at 60 °C is smaller, indicating that the asphalt modulus is more affected by temperature than aging.

In Figure 3b, the increase in the complex modulus of the SBS modified asphalt after aging is significantly lower than that of the base asphalt, indicating that long-term aging has a greater impact on the base asphalt than SBS modified asphalt. In Figure 3c, the asphalt with the anti-stripping agent is similar to the SBS asphalt. At 10 °C, the increase in the complex modulus is slightly higher than that of the SBS modified asphalt and significantly smaller than that of the base asphalt. This shows that the anti-aging ability of the anti-stripping-agent asphalt is better than that of the base asphalt and slightly worse than that of the SBS modified asphalt. At 60 °C, the main curves of the complex modulus before and after the aging of the asphalt with the anti-stripping agent basically overlap. This indicates that the improvement of the anti-stripping agent on asphalt aging resistance is also affected by temperature, and the higher the temperature the more obvious the improvement of the anti-stripping agent on asphalt aging resistance.

In addition, based on the time–temperature equivalence principle, the temperature displacement factor equation WLF (3) can be used to obtain the shift factor of the complex modulus of asphalt in a wide frequency range at 10 °C and 60 °C, which can better illustrate the variation in the dynamic rheology of asphalt materials at different temperatures. The calculation results are shown in Table 3.

$$\log \alpha T=-\frac{C_1(T-T_g)}{C_2+T-T_g}$$

(3)

In the formula: αT is the temperature shift factor; t is the test temperature, °C; T_g is the reference temperature; and °C; C₁, and C₂ are constants.

Table 3. WLF formula parameters table.

Temperature/°C	Parameter			Type
		B-A	B-UA	SBS-A	SBS-UA	AN-A	AN-UA
10	C1	−18.5	−20.5	−19.5	−20.6	−20.5	−20.5
	C2	142	140.2	144.3	144.8	142	142
60	C1	−10.3	−10.5	−9.5	−9.8	−10.02	−10.22
	C2	149.6	152.3	144.3	144.7	150.97	152.01

Table 3. WLF formula parameters table.

Temperature/°C	Parameter			Type
		B-A	B-UA	SBS-A	SBS-UA	AN-A	AN-UA
10	C1	−18.5	−20.5	−19.5	−20.6	−20.5	−20.5
	C2	142	140.2	144.3	144.8	142	142
60	C1	−10.3	−10.5	−9.5	−9.8	−10.02	−10.22
	C2	149.6	152.3	144.3	144.7	150.97	152.01

Analysis of shift factors C₁ and C₂ in Table 3 shows that the shift factors C₁ and C₂ of the base asphalt before and after aging at 10 °C changed by 10.81% and 1.28%, respectively, and shows that the C₁ and C₂ values of the SBS modified asphalt changed by 5.64% and 0.35%, respectively. However, adding the anti-stripping agent to the asphalt produced basically no changes to C₁ and C₂. At 60 °C, the differences between the three asphalts before and after aging are very small, indicating that the effect of temperature on the modulus is greater than that of aging. When the temperature reaches a certain level, aging has little effect on the modulus of asphalt.

3.2. Chemical Composition Analysis

Figure 4 shows the infrared spectra of the three asphalts before and after aging, and

Table 4. Characteristic peak transmittance (%).

Wave Number/cm−1	B-UA	B-A	SBS-UA	SBS-A	AN-UA	AN-A
2920	52.82	52.45	55.75	53.69	56.90	54.60
2850	63.90	63.02	66.44	64.81	66.00	65.40
1600	95.95	93.39	95.83	95.73	92.30	93.62
1456	76.05	73.05	77.62	76.92	74.10	75.82
1376	85.31	81.18	86.40	86.05	81.70	84.81
1030	95.26	88.18	95.30	95.07	89.00	89.90

shows the transmittance values of some characteristic peaks. The base asphalt, SBS modified asphalt and asphalt mixed with the anti-stripping agent all showed strong absorption peaks at 2920 cm⁻¹, 2850 cm⁻¹, 1456 cm⁻¹ and 1376 cm⁻¹, among which there are two distinct absorption peaks in the range of 2800 cm⁻¹~3000 cm⁻¹. Different characteristic peaks correspond to different functional groups, which indicate that C-H is the host in the three asphalts. These strong absorption peaks are caused by the C-H bond vibrations in the alkanes and cycloalkanes and the -CH₂-stretching vibrations at wave numbers 2920 cm⁻¹ and 2850 cm⁻¹. The absorption peak at wave number 1600 cm⁻¹ represents the C=O double bond in the carbonyl group, and the content here can reflect the aging degree of the asphalt to a certain extent [35]. At the same time, when the benzene ring skeleton vibrates, the conjugated double bond C=C also affects the absorption peak there. The absorption peaks at 1456 cm⁻¹ and 1376 cm⁻¹ are caused by the in-plane stretching vibration of C-H and C-CH₃ in –CH₂⁻, respectively. The absorption peak at wave number 1030 cm⁻¹ is the sulfoxide-based S=O double bond. Like the C=O double bond, the intensity of the absorption peak here can also reflect the degree of aging. Wave numbers from 910 cm⁻¹ to 650 cm⁻¹ are benzene ring substitution regions, and the C-H out-of-plane rocking vibrations form several absorption peaks.

Studies have shown that aging is mainly caused by high temperature action and oxidation reaction, which is manifested as an increase in the number of oxygen-containing functional groups, such as carbonyl and sulfoxide [36]. Among the three asphalts, there is a clear absorption peak at wavenumber 965 cm⁻¹ for the SBS modified asphalt in Figure 4 (b), which does not appear in either Figure 4a or Figure 4c and which corresponds to butadiene in the SBS modifier.

As shown in Figure 4a, the overall trends of decreasing transmittance and increasing intensity of the absorption peaks after aging of the base asphalt indicate that the content of the functional groups increases, with increases of 2.56% and 7.08% at wavenumbers 1600 cm⁻¹ and 1030 cm⁻¹, respectively, being particularly obvious. This shows that the content of the S=O double bonds and C=O double bonds increases after the aging of the base asphalt, and the chemical composition becomes larger, which is consistent with the dynamic rheological test results. Compared with the base asphalt, in Figure 4b, the SBS modified asphalt shows little change in infrared spectral transmittance after aging, indicating that its internal structure is stable and that it has a strong anti-aging ability. At wave numbers 1600 cm⁻¹ and 1030 cm⁻¹, the enhancements were 0.1% and 0.23%, respectively, indicating that the SBS modifier weakened the oxidation reaction during the aging process of the asphalt.

As shown in Figure 4c, the content of the functional groups in the asphalt mixed with the anti-stripping agent changes to a certain extent before and after aging, and the degree of change is between those of the base asphalt and the SBS modified asphalt. The changes in its absorption peak intensities are relatively complex. Combined with the analysis of the functional group content in Table 4, all absorption peak intensities are enhanced after the aging of the base asphalt and SBS modified asphalt. However, the absorption peaks at 2920 cm⁻¹ and 2850 cm⁻¹ for the asphalt mixed with the anti-stripping agent are enhanced, which is the same as those of base asphalt and SBS modified asphalt; moreover, the absorption peaks at 1600 cm⁻¹, 1456 cm⁻¹, 1376 cm⁻¹ and 1030 cm⁻¹ weakened, contrary to the other two at 1030 cm⁻¹ and 1600 cm⁻¹, which weakened by 1.01% and 1.43%, respectively. That is, the content of the C=O double bond in the carbonyl group and the S=O double bond in the sulfoxide group decreased, indicating that the addition of the anti-stripping agent makes the C=O double bond and the S=O double bond in the asphalt molecule unstable and that an aging reaction occurred during the process resulting in a reduction in the content.

In order to distinguish the anti-aging properties of the three asphalts more clearly, the absorption peak areas around 1600 cm⁻¹ and 1030 cm⁻¹ were taken as the absorption peaks of the carbonyl and sulfoxide groups, respectively, and the normalized carbonyl index (NCI) and normalized sulfoxide index (NSI) were used to quantitatively analyze the anti-aging properties of the three asphalts. Compared with the carbonyl index (CI) and sulfoxide index (SI), the NCI and NSI overcome the drift of the initial values of the CI and SI and can reflect the changes in the asphalt with aging time more clearly [37]. The specific calculation methods of the aging indices are shown in

Table 5. Specific calculation methods of aging indices.

Structural Indices	Area Ratio
NCI	CIt—CI0/ CI0
NSI	SIt—SI0/ SI0
CI	AC = O/ΣA
SI	AS = O/ΣA

. In the table: CI₀ and CI_t represent the carbonyl indices before and after long-term aging, respectively; SI₀ and SI_t represent the sulfoxide indices before and after long-term aging, respectively; A_C=O is the carbonyl absorption peak area; A_S=O is the sulfoxide absorption peak area; and ΣA is a total peak area between 2000 cm⁻¹ and 600 cm⁻¹. The calculation results of the NCI and NSI of each asphalt after long-term aging are shown in

Table 6. Indices before and after long-term aging.

Type of Asphalt	CI0	CIt	NCI	SI0	SIt	NSI
Base Asphalt	0.00483	0.00862	0.78	0.12712	0.23579	0.85
SBS	0.01729	0.02516	0.46	0.08719	0.13714	0.57
Anti-Stripping Agent Asphalt	0.00381	0.00653	0.71	0.10341	0.17948	0.74

.

It can be seen from Table 6 that the NCI and NSI of the base asphalt are significantly higher than those of the SBS modified asphalt, indicating that the carbonyl and sulfoxide groups generated by the SBS modified asphalt during the long-term aging process are less than those of the base asphalt, which is consistent with the qualitative analysis results of the infrared spectrum in this section. The addition of the anti-stripping agent slightly improves the aging resistance of the base asphalt probably because the fatty acids in the anti-stripping agent play a role similar to that of active antioxidants, inhibiting the reaction between polar groups in the asphalt and thus slowing down the aging of the asphalt.

3.3. Surface Micromorphology Analysis

Figure 5 and Figure 6 show the two-dimensional surface topography and three-dimensional elevation maps, respectively, of the three asphalts before and after aging.

Table 7. Morphological parameters of different asphalts before and after aging.

Parameter	B-A	B-UA	SBS-A	SBS-UA	AN-A	AN-UA
Rq	8.57	7.76	9.57	8.9	3.03	0.84
Ra	6.09	5.50	6.46	4.99	1.8	0.623
Height differences of bee structures (nm)	62.11	41.36	65.3	50.0	40.03	6.98
Maximum length of bee structures (μm)	14.82	10.33	6.74	7.09	6.90	—

shows the topographic parameters obtained using NanoScope Analysis software. The root mean square roughness, Rq, and the arithmetic mean difference, Ra, of the surface profile were selected to characterize the three-dimensional topographic features, and the height differences and the maximum honeycomb structure length are used for auxiliary analysis.

Observing Figure 5, we can see a large number of honeycomb structures. Relevant studies have shown that honeycomb structures on the microscopic surface of asphalt are crystals formed by the eutectic interaction between wax molecules and the side chain alkyl groups of macromolecular substances, such as asphaltenes and colloids [38]. From the comparison of the two-dimensional morphology, it is obvious that the anti-stripping-agent doped asphalt is different from the other two. The honeycomb structures of the base asphalt (a) and SBS (b) modified asphalt before aging are relatively scattered and mostly short and coarse, while the honeycomb structures become longer and the number of peaks increases slightly after long-term aging (as shown in Figure 5d,e); however, the anti-stripping-agent doped asphalt (c) does not see any honeycomb structures before aging, although after aging (f) there appeared some, presumably because the asphalt internal asphaltene is increasing due to aging, which provides crystalline cores for wax molecules and thus produces honeycomb structures.

In order to more clearly show the evolution of the microstructures of the anti-stripping-agent asphalt before and after aging, AFM three-dimensional images were introduced to assist in the analysis, as shown in Figure 6. Observing the three-dimensional morphology of Figure 6, it can be seen that the heights and undulations of each region of the base asphalt and SBS modified asphalt tend to decrease with aging, presumably due to the weakening of the physical or chemical interactions between the parts due to long-term aging; moreover, the convergence of chemically similar components and the weakening of the connections between the regions, which in turn leads to the distribution of each region becoming independent and dispersed, and the macroscopic aspects are manifested as increased stability, such as high temperature stability and softening point enhancement [39].

From Figure 6 and Table 7, it can be concluded that the relative height difference of the base asphalt increases by 50.17% after aging, and the Rq and Ra increased by 10.44% and 10.73%, respectively, indicating that the roughness of the base asphalt increases after long-term aging. This may be because in the long-term aging process, the molecular chain of the base asphalt is damaged and reorganized, the relative molecular mass of the asphalt increases and the light components in the asphalt volatilize, so long-term aging makes the asphalt appear rough [40]. The number of honeycomb structures increased after the long-term aging of the SBS modified asphalt, and the Rq and Ra increased by 7.53% and 29.46%, respectively. After the anti-stripping agent was added to the base asphalt, the honeycomb structures on the surface of the asphalt disappeared, and only two highly different phases could be distinguished with a three-dimensional topography map moreover, the Rq and Ra of the honeycomb structures were much smaller than those of the base asphalt and SBS modified asphalt. While the honeycomb structures reappeared on the surface of the anti-stripping-agent asphalt after long-term aging, the Rq and Ra increased about 3.5 and 2.9 times, respectively; however, the value of the honeycomb structures is still significantly smaller than those of the aged base asphalt and SBS modified asphalt, and the amount of honeycomb structures is more than that of the aged base asphalt and less than that of the aged SBS modified asphalt, which is between the two. Through the above analysis quantitatively and qualitatively, the effects of aging on the micromorphology of asphalt were explored, and the results show that aging causes dramatic changes in the microsurface structures and honeycomb structures of asphalt, which in turn affected its macroscopic properties.

4. Conclusions

In this study, the macrorheological properties, chemical composition and micromorphological changes in the three bitumen before and after long-term aging were analyzed on a multi-scale basis, which fully revealed the macro- and microproperties of the asphalt mixed with the anti-stripping agent after long-term aging. The following are the main conclusions:

(1)

The complex moduli of the base asphalt, SBS modified asphalt and asphalt with the anti-stripping agent increase after long-term aging, and the asphalt modulus is more affected by temperature than aging. Among them, the anti-aging ability of the asphalt mixed with the anti-stripping agent is better than that of base asphalt and slightly worse than that of the SBS modified asphalt.

(2)

After aging, the transmittance of the base asphalt decreases and the intensity of the absorption peak increases. The SBS modified asphalt has little change in infrared spectrum transmittance after aging, and its internal structure is stable and its anti-aging ability is strong. However, the change in the absorption peak intensity of the asphalt mixed with the anti-stripping agent is more complex, and the absorption peak intensities were at 2920 cm⁻¹ and 2850 cm⁻¹, which is the same as the change law of the base asphalt and SBS modified asphalt. However, the absorption peaks at 1600 cm⁻¹, 1456 cm⁻¹, 1376 cm⁻¹ and 1030 cm⁻¹ were weakened, which was contrary to the other two.

(3)

The order of the Ra and Rq after the long-term aging of the three types of asphalt is SBS modified asphalt > base asphalt > anti-stripping-agent asphalt. The overall trend of AFM images of the asphalt surface before and after aging is that when not aging, the honeycomb structures are mostly short, but after long-term aging, the honeycomb structures become longer, surface roughness increases and the ups and downs of each area gradually decreases with aging. The SBS modifier and anti-stripping agent have different mechanisms of influence on asphalt properties and microstructures.