Study on the Aging Behavior of Asphalt Binder Exposed to Different Environmental Factors

Abstract

Accelerated aging methods commonly used in laboratories struggle to replicate the outdoor aging process of asphalt binder. The aim of this study is to elucidate the impact of different environmental factors on the aging of asphalt binder and recreate the exposure process of asphalt binder. To achieve the study’s objectives, the asphalt binder was subjected to various environmental conditions through different aging modes. Three exposure modes (all environmental factors, the effects of light, temperature, oxygen, the effects of temperature, oxygen, and others) were established to assess the impact of various environmental factors on asphalt binder aging behavior. This mode was labeled O+UV-aging, earning it the name O-aging. The aging behaviors were assessed across multiple dimensions, considering apparent morphology, rheological properties, and chemical composition. The study’s findings highlight that factors such as ultraviolet radiation are primarily responsible for the formation of micro-cracks and increased surface roughness in aged asphalt binder. Ultraviolet radiation significantly affected the aging of asphalt binder during outdoor exposure. SBS modifiers increased the risk of fatigue cracking in the virgin asphalt binder but enhanced its aging resistance. After All-aging, the G-R parameter increase of virgin asphalt binder was 2.6 times that of SBS-modified asphalt binder. Throughout the exposure process, the broken molecular chains and the original molecular chains in the asphalt binder underwent polymerization reactions, resulting in longer carbon chains and cycloalkane aromatization. It was discovered that exposure showed less effect on the characteristic functional groups of SBS-modified binder than on virgin binder. After All-aging, the carbonyl index of SBS-modified asphalt binder was 56.4% higher than that of virgin asphalt binder.

1. Introduction

Asphalt pavement stands as the preferred choice for road construction due to its exceptional leveling, driving comfort, low noise, resistance to salt decay, and ease of maintenance. Nevertheless, asphalt pavements inevitably confront adverse factors like sunlight, oxygen, varying temperatures, and moisture over prolonged periods, leading to aging-related challenges. This results in decreased durability and a shortened service life for asphalt pavements [1]. The primary cause of asphalt pavement deterioration lies in the aging of the asphalt binder, which binds aggregates, mineral powders, and additives together [2]. Effectively enhancing asphalt pavement durability and implementing precise pavement maintenance measures constitute urgent challenges to address. Solving these problems necessitates a more precise and profound comprehension of the principles governing asphalt binder aging.

Aging typically manifests in macroscopic properties, marked by hardening, reduced penetration, decreased ductility, irreversible creep flexibility, and phase angle, as well as an increase in the softening point and zero shear viscosity. Zhang et al. [3] observed a decrease in the ductility with increasing aging, albeit with an elevated critical low-temperature point. SBS-modified asphalt binder displayed superior plastic deformation capability and robust resistance to aging. Liu et al. [4] noted that UV aging led to higher softening point and viscosity values, accompanied by decreased ductility and penetration of asphalt binder. Layered double hydroxides enhanced asphalt binder’s aging resistance. Valtorta et al. [5] revealed an increase in complex modulus and temperature sensitivity of aged asphalt binder. Asphalt pavement exhibited signs of hardening and loosening due to aging. On the microscopic front, aging is typified by the thermal evaporation of light components, the transformation of light fractions into heavier ones, an uptick in polar functional groups, and a rise in average molecular weight. Traxler [6] concluded that asphalt binder aging primarily stems from light component volatilization, oxidation, structural changes, photopolymerization, and thermal condensation. Zhao et al. [7] observed an augmentation in the macromolecular structure and average molecular weight following aging. Zhang et al. [8] detected an increase in straight chain length, a higher number of large molecules, and heightened internal friction in asphalt binder molecules during motion after aging.

Presently, much of the research on asphalt binder aging relies on laboratory-accelerated simulated aging [9]. The Thin Film Oven Test (TFOT) or Rolling Thin Film Oven Test (RTFOT) simulates asphalt binder aging during production, transportation, and paving. Pressure Aging Vessels (PAV) simulate aging during service life [10]. Researchers also have developed UV aging equipment. Asphalt binder UV aging tests are now increasingly becoming standardized [11,12,13,14,15]. Ozone, water, and multifactorial coupled aging are also garnering increased attention [10,16,17,18]. Qian [19] delved into the effects of UV radiation and various water environments on asphalt binder, uncovering that water environments and UV aging exacerbated asphalt binder aging and diminished its low-temperature crack resistance. Interactions and synergistic effects between diverse aging factors are evident. Multifactorial effects on asphalt binder are substantial. Part of the multifactorial effects can induce notable deviations between the actual environment and the simulated environment.

However, asphalt binder undergoes forced aging through accelerated laboratory simulations. This process aims to reduce the aging time of the binder by subjecting it to higher temperatures, greater pressures, or more intense radiation than it experiences in its natural environment. Theoretically, the parameters for this acceleration simulation are scientifically grounded in energy conversion principles. Nevertheless, the environment of pavements is shaped by climate, geography, and other factors. Consequently, replicating the aging of various binders in such complex conditions through laboratory simulations remains challenging [20]. Moreover, the study of asphalt binder exposed to real-world conditions presents difficulties due to extended test durations and associated costs. In many areas of Northwest China, including the focus of this study, there are approximately 3360 h of sunshine annually, with extreme temperatures reaching as high as 45.1 °C. Additionally, these regions contend with various deteriorating factors, such as strong winds, saline soils, and intense light-oxygen radiation. These conditions accelerate the aging of binders exposed to the elements, leading to highly intricate aging behaviors.

The primary aim of this study is to elucidate the impact of diverse environmental factors on asphalt binder aging and uncover the aging processes experienced by asphalt binders exposed to real-world conditions. Initially, various aging modes were established to age asphalt binder. Subsequently, high-magnification cameras were employed to capture the apparent morphology of samples exposed to outdoor conditions, facilitating an analysis of aged asphalt binders. Finally, it characterized the effects of various environmental factors on the chemical composition and rheological parameters of these two types of asphalt binders at multiple scales, revealing the mechanisms behind asphalt binder aging. The results of this investigation provide a deeper understanding of the behavior of asphalt binder in actual service conditions, aiding in the prediction of performance changes during service.

2. Materials and Methods

2.1. Materials

The exposure test site is situated in Dunhuang City, Gansu Province. The type of asphalt binder commonly used in the area was selected, taking into the local climatic characteristics. The test utilized SK 90# virgin asphalt binder and SBS modified asphalt binder, obtained through the modification of SK 90# virgin asphalt binder.

Table 1. Fundamental properties of virgin asphalt binder.

Indicators		Results	Standard
Penetration (25 °C, 100 g, 5 s)/0.1 mm		81.5	80~100	JTG F40
Wax content/%		0.6	≤2.2
Ductility (5 cm/min, 10 °C)/cm		100	≥30
Softening point/°C		45	≥44
Flashpoint/°C		305	≥245
Solubility (trichloroethylene)/%		99.8	≥99.5
Aging test (163 °C, 5 h)	Mass change/%	−0.13	≤±0.8
	Residual penetration ratio/%	64	≥57
	Residual ductility Ratio/%	11	≥8

and

Table 2. Fundamental properties of SBS modified asphalt binder.

Indicators		Results	Standard
Penetration (25 °C, 100 g, 5 s)/0.1 mm		66	60~80	JTG F40
Ductility (5 cm/min, 5 °C)/cm		42	≥30
Softening point/°C		87.5	≥55
Flashpoint/°C		280	≥230
Solubility (trichloroethylene)/%		99.5	≥99
Elastic recovery (25 °C)		80	≥65
Storage stability (Softening point difference, 48 h)		1.0	≤±2.5
Aging test (163 °C, 5 h)	Mass change/%	−0.4	≤±1.0
	Residual penetration ratio/%	75	≥60
	Residual ductility Ratio/%	30	≥20

present the fundamental properties of these materials.

2.2. Outdoor Exposure Test

A comprehensive asphalt binder exposure test was conducted. The test site is situated in Dunhuang City, Gansu Province, characterized by a dry climate, minimal rainfall, significant day-night temperature fluctuations, prolonged sunshine hours, intense UV radiation, and radiation levels second only to Tibet. This region is famously referred to as the “World Wind Farm”, with maximum wind speeds reaching 25.2 m/s at 3 m, 24.16 m/s at 2 m, 18.76 m/s at 1 m, and a surface maximum of 12.85 m/s. Summers here scorch with temperatures exceeding 40 °C, and the average day-night temperature difference stands at 25.6 °C, peaking at 30 °C. Annual precipitation averages 39.9 mm, evaporation hits 2486 mm, and the yearly dryness index registers at 21.67, classifying it as an extremely arid climate zone. Sunshine hours exceed 3200 annually, with a cumulative annual radiation amount of 6800 MJ/m², earning it Class I status and a daily cumulative ultraviolet radiation value of 1.2347 MJ/m². These unique natural conditions, including extreme drought, frigid cold, strong winds, substantial temperature variations, and potent ultraviolet radiation, contribute to the intricate aging of asphalt binder, significantly impacting asphalt pavement performance. The exposure test involved positioning the samples 0.5 m above the ground, as depicted in Figure 1. After 3 months of outdoor exposure, the aged samples were recycled. The dimension of the samples exposed to outdoor was 16 cm × 11.5 cm × 0.32 cm.

As illustrated in Figure 2, three distinct exposure modes were established. Mode 1 entailed a complete exposure test, where the asphalt binder samples remained unobstructed, fully exposed to the natural environment, and subjected to all environmental factors, including temperature, oxygen, light, wind, rain, dust, and more. This mode was also referred to as All-aging. Mode 2 involved covering the top of the samples by using a glass, exposing them to the effects of light, temperature, oxygen, and similar factors. This mode was labeled O+UV-aging. Mode 3 introduced a shade in the form of an impermeable plate above the asphalt binder samples, allowing them to experience only the effects of temperature, oxygen, and related factors, earning it the name O-aging. It is worth noting that the UV-transparent glass and the impermeable plate are not in direct contact with the binder. After aging from outdoor exposure, the surface of the aging sample was first rinsed with water. Then, a cleaning cloth was used to wipe it down. Finally, it was heated to a fluid state in an oven at 135 °C. It was mixed well and a sample was taken for testing.

2.3. Indoor Accelerated Aging Methods

According to JTG E20 T 0609, the TFOT aging test duration was set at 5 h, with a temperature of 163 °C. According to JTG E20 T 0630, a pressure aging vessel (PAV) was used to simulate the long-term aging of asphalt binder. The samples underwent 5 h of short-term aging, followed by 20 h in PAV. The temperature was set at 100 °C. The air pressure was set at 2.1 MPa.

2.4. Rheological Characterization Methods

Dynamic shear rheometer tests were conducted on asphalt binder samples using a Dynamic Shear Rheometer DHR-2. An aluminum fixture with a 25 mm diameter and a parallel plate spacing of 1 mm was selected. The angular frequency ranged from 0.1 to 100 rad/s, and testing was performed at temperatures of 35 °C, 45 °C, and 55 °C. The dynamic shear rheology test employed a 1% strain control mode. Subsequently, the modulus-frequency curves at each temperature were adjusted using the time-temperature equivalence principle, with 45 °C as the reference, to derive the complex modulus master curve covering the entire frequency range. This process characterized the viscoelastic properties of the asphalt binder. The master curve was constructed with 15 °C as the reference, and the complex modulus and phase angle were determined at a temperature of 15 °C and a loading frequency of 0.005 rad/s. The Glover-Rowe (G-R) parameter was calculated according to Equation (1).

$$\mathrm{G}-\mathrm{R}=\frac{\left|G^{*}\right|\cdot {\left(\cos \delta \right)}^2}{\sin \delta }$$

(1)

where G* = Complex modulus of asphalt binder at 15 °C, 0.005 rad/s (kPa); δ = Phase angle of asphalt binder at 15 °C, 0.005 rad/s (°).

2.5. Chemical Composition Characterization Methods

For chemical composition characterization, an ASCEndTM400 (AVANCE HD) III NMR spectrometer, manufactured by Bruker in Germany, was utilized. The spectrometer boasted a magnetic field strength of ≥9.39 Tesla (400 MHz) and exhibited a minimal magnetic field drift of ≤8 Hz per hour. To conduct the test, 10 mg of asphalt binder sample was dissolved in 0.5 mL CDCl₃. The resulting spectrum was analyzed using Topspin software 2.1.

3. Results

3.1. Analysis of Outdoor Exposure on the Apparent Morphology of Asphalt Binder

High-magnification cameras were employed to capture the apparent morphology of asphalt binder samples exposed to outdoor conditions. Figure 3, Figure 4, Figure 5 and Figure 6 depict the apparent morphology under different aging modes.

Figure 4 illustrates that asphalt binder experienced noticeable surface changes due to all-weather exposure. These changes included the adhesion of dust and other impurities, a significant increase in surface roughness, and the appearance of micro-cracks. All-weather coupled aging affected both virgin asphalt binder and SBS-modified asphalt binder similarly. In Figure 5, surface changes were more pronounced after asphalt binder aging. The presence of UV-transparent glass prevented impurities from adhering to the surface, but direct UV radiation led to evident surface cracks and significantly greater roughness compared to all-factor coupled aging. The impact of O+UV-aging was more pronounced on SBS-modified asphalt binder than on virgin asphalt binder. SBS-modified asphalt binder exhibited a network of interlaced surface cracks and considerably greater roughness. O-aging had a lesser impact on the surface morphology of both virgin asphalt binder and SBS-modified asphalt binder. After six months of O-aging, the apparent morphology resembled that of non-aged samples. In summary, it becomes apparent that the number of microcracks on the aged asphalt binder’s surface and the increase in roughness were primarily related to factors such as UV radiation, with O-aging having a minimal effect on apparent morphology.

3.2. Analysis of Outdoor Exposure on the Macroscopic Properties of Asphalt Binder

3.2.1. Effect of Outdoor Exposure on the Complex Modulus of Different Asphalt Binder

To accurately characterize these viscoelastic properties, rheological methods are commonly employed. A dynamic shear rheometer was used to evaluate asphalt binder’s rheological properties, which can objectively reflect high-temperature performance, low-temperature performance, and fatigue performance. The complex modulus is indicative of the asphalt binder’s resistance to deformation. Frequency scanning tests were performed on samples. Complex modulus principal curves were obtained with a reference temperature, representing the rheological characteristics of asphalt binder over a broader frequency range (or temperature domain), as illustrated in the figure below.

Figure 7 illustrates that TFOT aging resulted in an increased complex modulus of asphalt binder. A higher complex modulus indicates a lower proportion of viscous and more elastic components, rendering the asphalt binder harder, more brittle, and resistant to deformation with reduced viscoelasticity. The complex modulus after O-aging closely approximated that after TFOT aging, indicating that O-aging had little impact on the complex modulus. Notably, outdoor exposure at room temperature and atmospheric pressure did not induce significant aging in virgin asphalt binder. O+UV-aging and All-aging had a substantial effect on the complex modulus of virgin asphalt binder, signifying that exposure to light expedited its aging. As shown in Figure 8, the TFOT aging had an insignificant effect, while O-aging led to a notable increase in complex modulus. The effects of O+UV-aging and All-aging were significant on SBS-modified asphalt binder, with All-aging exerting the greatest influence, followed by O+UV-aging. The disparity between All-aging, O+UV-aging, and O-aging hinged on the impact of light as a factor, revealing the significant role of light in asphalt binder’s rheological properties.

The increase in the complex modulus of SBS-modified asphalt binder was less significant than that observed in virgin asphalt binder under the same aging conditions. This suggests that the addition of the SBS modifier enhances the aging resistance of virgin asphalt binder, with a notable improvement in the aging resistance of the asphalt binder, consequently enhancing its road performance [21]. Specifically, the complex modulus of SBS-modified asphalt binder exhibited a substantial increase following O-aging, whereas O-aging had a comparatively minor impact on the complex modulus of the virgin asphalt binder. This discrepancy highlights that thermo-oxygenation exerts a more pronounced influence on SBS-modified asphalt binder. When subjected to O+UV-aging, the impact on the SBS-modified asphalt binder was less severe than that on All-aging, while O+UV-aging had the most substantial impact on the virgin asphalt binder. The primary distinction between All-aging and O+UV-aging lies in their susceptibility to external factors such as wind, rain, and dust. Generally, it is believed that dust accumulation diminishes the asphalt binder’s exposure to outdoor elements, resulting in reduced aging. Notably, All-aging had the most pronounced effect on SBS-modified asphalt binder, suggesting that it is less influenced by environmental factors like wind, rain, and dust.

3.2.2. Effect of Outdoor Exposure on the G-R Parameter of Different Asphalt Binder

Notably, G′(η′/G′) exhibited a strong correlation with 15 °C ductility, effectively characterizing the hardening and fatigue cracking tendencies of the asphalt binder. Furthermore, the correlation between the fracture energy of asphalt mixtures and G′(η′/G′) exceeded 90%, making it a valuable index for evaluating the cracking resistance of asphalt binder [22]. In this study, the G-R parameter for various asphalt binder samples was obtained by using Equation (1).

Figure 9 reveals a significant increase in the risk of fatigue cracking upon the addition of the SBS modifier to virgin asphalt binder. The G-R parameter for unaged virgin asphalt binder measured a mere 0.60 kPa, while that for SBS-modified asphalt binder was already at 4.41 kPa. The SBS modifier also demonstrated improved anti-aging properties during the TFOT aging process. The G-R parameter for SBS-modified asphalt binder increased by 10% after TFOT aging, in contrast to a 101.7% increase for virgin asphalt binder. However, during the exposure process, the SBS modifier did not exhibit anti-aging effects. The G-R parameter for SBS-modified asphalt binder increased by 156.9% following O-aging, whereas the G-R parameter for virgin asphalt binder increased by 94.2% after O-aging. Outdoor exposure significantly impacted the fatigue resistance of both virgin asphalt binder and SBS-modified asphalt binder. It is noteworthy that SBS modifiers mitigated the adverse effects of UV radiation on asphalt binder properties. The G-R parameter for O+UV-aged virgin asphalt binder increased by 49.4% compared to O-aged virgin asphalt binder, while the G-R parameter for O+UV-aged SBS-modified asphalt binder increased by only 6.0% compared to O-aged SBS-modified asphalt binder. In summary, it is evident that temperature and oxygen have a more pronounced effect on the performance of SBS-modified asphalt binder, while ultraviolet light has a more significant impact on virgin asphalt binder.

3.3. Analysis of Outdoor Exposure on the Chemical Composition of Asphalt Binder

3.3.1. Effect of Outdoor Exposure on the Structure Composition of Different Asphalt Binder

Nuclear Magnetic Resonance (NMR) is a phenomenon in which, when a sample is exposed to a radio-frequency electromagnetic field perpendicular to a strong external magnetic field, magnetic nuclei meeting resonance conditions absorb electromagnetic waves, resulting in a jump in nuclear energy levels. This process generates electromagnetic resonance signals corresponding to magnetism and provides information about characteristics and chemical structures in the induction coil perpendicular to the external magnetic field. Nuclear Magnetic Resonance Hydrogen Spectroscopy allows us to assess the chemical environments of different hydrogen atoms in asphalt binders. As hydrogen atoms occupy distinct chemical environments, they produce resonance peaks at varying positions, creating a chemical shift relative to a reference peak. We can deduce the position of hydrogen atoms within the carbon chain and analyze their molecular structure. This study classifies hydrogen types in the spectra into four categories: H_A, H_α, H_β, and H_γ, based on their chemical shifts, as presented in

Table 3. ¹H-NMR spectral region delineation and attribution.

Hydrogen	Attribution	Chemical Shift, ppm
HA	Hydrogen is directly linked to aromatic carbon	6.3~9.3
Hα	Hydrogen is linked to the α-carbon of the aromatic ring	2.0~4.0
Hβ	Hydrogen on the β-carbon of the aromatic ring and the CH2 and CH groups farther from β-carbon	1.0~2.0
Hγ	Hydrogen in the γ-position of the aromatic ring and the CH3 group far from γ	0.5~1.0

and Figure 10.

The ¹H-NMR spectra of virgin asphalt binder are shown in Figure 11.

Segmental integration was performed using the four attributed hydrogens: H_A, H_α, H_β, and H_γ in the spectra. The content of these four hydrogens in the virgin asphalt binder was normalized, as illustrated in

Table 4. Hydrogen content of virgin asphalt binder before and after aging.

	HA	Hα	Hβ	Hγ
Unaged	0.058	0.204	0.583	0.154
TFOT	0.060	0.197	0.586	0.157
O-aging	0.051	0.148	0.603	0.198
O+UV-aging	0.067	0.149	0.608	0.176
All-aging	0.066	0.153	0.612	0.169
PAV	0.067	0.201	0.202	0.155

.

From the ¹H-NMR spectra and the hydrogen content analysis results in Table 4, it is evident that among the four attributed hydrogens, Hβ comprises the highest content, accounting for approximately 60% of the total. This indicates that hydrocarbon elements in virgin asphalt are primarily structured as carbon chains. The hydrogen content at the γ and α positions of the aromatic ring is relatively similar, ranging from 10% to 20%, suggesting a limited number of chain hydrocarbons in the asphalt binder. Hydrogen linked to the aromatic ring exhibits the lowest content. H_A content increased moderately in all asphalt binders after short-term aging, signifying a rise in the number of aromatic ring structures. Following O+UV-aging, there was a significant upsurge in H_A content, particularly in the γ-position of the aromatic ring compared to the α-position. This suggests a notable increase in unbound carbon chains in the asphalt binder. The hydrogen distribution in All-aged asphalt binder closely resembled that of O+UV-aged asphalt binder, indicating that outdoor exposure leads to an increase in aromatic ring structures within the asphalt binder. Molecular chains undergo breakage and reorganization, resulting in longer carbon chains. Conversely, the impact of aromatic ring structure due to O-aging was less pronounced. The introduction of UV radiation increased the number of aromatic rings, reduced the presence of branched chains on the aromatic rings, and elevated the number of carbon chains not linked to the aromatic rings. During exposure, broken molecular chains and the original chains in the asphalt binder underwent additional polymerization reactions, transforming their structural forms into aromatic rings, cycloalkane aromatization, and ultimately forming more intricate ring structures.

The ¹H-NMR spectra of SBS-modified asphalt binder are presented in Figure 12.

Segmental integration was conducted according to the four attributed hydrogens of H_A, H_α, H_β, and H_γ in the spectra. The content of the four attributed hydrogens, H_A, H_α, H_β, and H_γ, was obtained after normalization, as shown in

Table 5. Hydrogen content of SBS-modified asphalt binder before and after aging.

	HA	Hα	Hβ	Hγ
Unaged	0.058	0.286	0.583	0.133
TFOT	0.059	0.289	0.515	0.138
O-aging	0.060	0.175	0.607	0.158
O+UV-aging	0.061	0.153	0.611	0.175
All-aging	0.066	0.159	0.615	0.159
PAV	0.074	0.279	0.499	0.148

.

The ¹H-NMR spectra and the hydrogen content analysis results in Table 5 reveal that among the four attributed hydrogens, H_β exhibits the highest concentration, accounting for more than 50% of the total. This indicates that hydrocarbon elements in SBS-modified asphalt binder still predominantly manifest as carbon chains. The H_A content in SBS-modified asphalt binder experienced a relatively modest increase following short-term aging, signifying a rise in the presence of aromatic ring structures. Upon asphalt binder exposure, the pattern of hydrogen distribution mirrored that of virgin asphalt binder. The H_A content in the asphalt binder increased post-exposure, suggesting an increase in the aromatic ring structure after binder aging. The distribution of hydrogen elements in O-aged asphalt binder closely resembled that of O+UV-aged asphalt binder, with all-weather aging exerting a more pronounced impact on SBS-modified asphalt binder compared to virgin asphalt binder.

3.3.2. Effect of Outdoor Exposure on the Functional Groups of Different Asphalt Binder

Asphalt binder is composed of numerous molecules, rendering its chemical composition exceptionally intricate. Precisely extracting the composition of asphalt binder remains challenging. Fourier Transform Infrared Spectroscopy (FTIR) can evaluate the type and content of certain functional groups in asphalt binder based on wave numbers and peak shapes in the spectrum. The analysis of functional groups in asphalt binder subjected to various aging modes was conducted using FTIR, and the results are depicted in Figure 13 and Figure 14.

As evident in Figure 13 and Figure 14, unaged asphalt binder exhibits a multitude of distinctive peaks. The peak at 2920 cm⁻¹ corresponds to the antisymmetric stretching vibration of methylene (-CH₂-), while the symmetric stretching vibration of methylene -CH₂- appears at 2850 cm⁻¹. The absorption peak near 1600 cm⁻¹ is attributed to both the backbone vibration of the benzene ring and the carbon-oxygen double bond. Carbonyl group absorption peaks lie between 1800 cm⁻¹ and 1650 cm⁻¹, while sulfoxide group absorption peaks are near 1030 cm⁻¹. Most absorption peaks below 600 cm⁻¹ correspond to compounds comprising carbon and other elements.

Notable alterations in the intensity of absorption peaks, particularly in the vicinity of 1700 cm⁻¹ and 1030 cm⁻¹, were observed before and after asphalt binder aging. The carbonyl and sulfoxide indices serve as common metrics for quantifying the extent of asphalt binder aging.

$$CI=\frac{CA}{Cref}$$

(2)

$$SI=\frac{SA}{Sref}$$

(3)

where CI = Carbonyl index; CA = Carbonyl characteristic peak area; SI = Sulfoxide index; SA = Sulfenyl characteristic peak area; Cref and Sref = Reference peak area.

Typically, peak areas around 1700 cm⁻¹ were chosen to represent the carbonyl group and around 1031 cm⁻¹ for the sulfoxide group. The carbonyl and sulfoxide indices of the unaged asphalt binder and the three naturally aged asphalt binders were calculated using Equations (2) and (3), with the results presented in

Table 6. Calculation Results of Carbonyl and Sulfoxide Index.

Binder	Modules	AC=O	AS=O	AC-H	IC=O	IS=O
Virgin asphalt binder	Unaged	0.289	0.754	18.244	0.0158	0.0413
	TFOT	0.254	0.751	17.616	0.0144	0.0426
	O	0.287	1.853	18.096	0.0159	0.1024
	O+UV	0.296	2.490	16.760	0.0177	0.1486
	All	0.360	2.590	17.792	0.0202	0.1456
	PAV	0.499	2.154	16.086	0.0310	0.1339
SBS-modified asphalt binder	Unaged	0.540	0.138	17.933	0.0301	0.0077
	TFOT	0.061	1.140	18.201	0.0034	0.0626
	O	0.227	2.244	17.666	0.0128	0.1270
	O+UV	0.241	1.936	17.709	0.0136	0.1093
	All	0.2000	2.226	17.620	0.0114	0.1263
	PAV	0.612	3.455	17.644	0.0347	0.1958

.

As observed in Table 6, both the carbonyl index (I_C=O) and sulfoxide index (I_S=O) of the asphalt binder increased following aging in various modes. Generally, the carbonyl index exhibited relatively small changes before and after natural aging, while a notable increase was evident in the sulfoxide index. Notably, O+UV-aging and All-aging had the most significant impact on the sulfoxide index of virgin asphalt binder, surpassing even the effects of PAV aging. All-aging on the carbonyl index of virgin asphalt binder was slightly less than that of PAV aging, accounting for approximately 65.2% of the carbonyl index observed after PAV aging. Conversely, O-aging had a minor impact on virgin asphalt binder. Natural aging had a lesser effect on the characteristic functional groups of SBS-modified asphalt binder. Among these, All-aging exerted the most substantial influence on the sulfoxide index of SBS-modified asphalt binder, though it represented only 64.5% of the effect of PAV aging on SBS-modified asphalt binder. Additionally, O+UV-aging had the most profound impact on the carbonyl index of SBS-modified asphalt binder, accounting for only 39.2% of that of SBS-modified asphalt binder.

4. Conclusions

This study procured various types of aged asphalt binder samples through outdoor exposure tests. The impacts of diverse environmental factors on the two asphalt binders were assessed across multiple scales, encompassing apparent morphology, rheological properties, and chemical composition. The primary conclusions can be summarized as follows:

(1)

Following exposure aging, micro-cracks manifested on the surfaces of asphalt binder. The prevalence of these microcracks and the escalation in surface roughness were chiefly associated with UV radiation, while the influence of O-aging on apparent morphology remained minimal.

(2)

Thermo-oxidative aging’s effects on the asphalt binder primarily manifested during the mixing, transportation, and paving stages of asphalt mixture production. It becomes evident that light exposure significantly impacted the rheological properties.

(3)

The incorporation of SBS modifiers markedly heightened the risk of fatigue cracking in the virgin asphalt binder. However, it concurrently bolstered the aging resistance of asphalt binder. Moreover, the SBS-modified asphalt binder displayed a higher tolerance to external factors like wind, rain, and dust.

(4)

Outdoor exposure exerted a substantial influence on the fatigue resistance of asphalt binder. Among the various aging scenarios, the most pronounced impact was observed with All-aging for both types of asphalt binder.

(5)

Throughout the exposure process, the molecular chains within the asphalt binder underwent fracture, followed by an additional polymerization reaction involving the fractured and original molecular chains. This transformation resulted in elongated carbon chain lengths, cycloalkane aromatization, and an alteration in structural configuration toward macromolecules.

(6)

In comparison to virgin asphalt binder, natural aging had a lesser effect on the characteristic functional groups of SBS-modified asphalt binder. O+UV-aging and All-aging had the most significant impact on the sulfoxide index of virgin asphalt binder, surpassing even the effects of PAV aging.

(7)

This study focused only on samples aged for 3 months by outdoor exposure. The characterization of aging samples at different times will be a focus of future research. A dynamic relationship between the time of outdoor aging and the service condition of asphalt pavements will be established.