4.1. Functional Group
Figure 6 illustrates the evolution of functional groups in SBS-modified asphalt under various ageing conditions, as analysed by FTIR spectroscopy.
As illustrated in
Figure 6, distinct absorption peaks are evident in the FTIR spectra of asphalt subjected to various ageing conditions at 2919 cm⁻
1, 2851 cm⁻
1, 1601 cm⁻
1, 1455 cm⁻
1, 1375 cm⁻
1, 1030 cm⁻
1, 966 cm⁻
1, and 725 cm⁻
1. The strong peaks at 2919 cm⁻
1 and 2851 cm⁻
1 are attributed to the asymmetric and symmetric stretching vibrations of C-H bonds in methylene (-CH
2-) groups, respectively. Additionally, the absorption peaks detected at 1601 cm⁻
1, 1455 cm⁻
1, 1375 cm⁻
1, and 1030 cm⁻
1 are assigned to the stretching vibrations of C=C bonds with in condensed aromatic rings, the asymmetric bending vibrations of methyl (-CH
3) groups, the symmetric bending vibrations of methyl (-CH
3) groups, and the stretching vibrations of sulfoxide (S=O) functional groups, respectively. Notably, the peaks at 966 cm⁻
1 and 725 cm⁻
1 are attributed to the bending vibrations of C=C bonds in polybutadiene and the wagging vibrations of methylene (-CH
2-) groups in the aromatic rings of polystyrene, respectively, serving as characteristic absorption peaks for SBS modifiers. The intensity of the absorption peaks varied, while their positions remained relatively stable. This indicates that the types of functional groups in the asphalt did not change, only their concentrations did. Under different ageing conditions, the decrease in intensity at 966 cm⁻
1 indicated the scission or crosslinking of butadiene within the asphalt. Meanwhile, changes in peak intensity at 1030 cm⁻
1 and 1600 cm⁻
1 indicate an increased degree of oxidation and enhanced aromatic conjugation, respectively. Previous studies have established a correlation between the areas of these absorption peaks and the concentrations of the respective functional groups [
40]. Given the relatively small absorption peak area of carbonyl groups, which can introduce significant measurement errors, this study employs three indices to characterise the ageing and regeneration mechanisms of SBS-modified asphalt: the Butadiene Index (
), which quantifies SBS content; the Sulfoxide Index (
), which measures S=O groups; and the Aromaticity Index (
), which assesses C=C bonds.
,
,
, and
represent the absorption peak areas at 966 cm⁻
1, 1030 cm⁻
1, 1600 cm⁻
1, and 1375 cm⁻
1, respectively. The calculated values of BI, SI, and CI under various ageing conditions are presented in
Table 8 and
Figure 7.
As illustrated in
Figure 7, the asphalt subjected to various ageing conditions exhibited notable changes in its chemical properties compared to the unaged asphalt. Specifically, the butadiene index (BI) of the aged asphalt decreased significantly, while the sulfoxide index (SI) and aromaticity index (CI) showed increasing trends. These findings suggest a reduction in the concentration of butadiene functional groups in the SBS-modified asphalt during the ageing process, accompanied by an increase in sulfoxide and oxygen-containing groups, as well as aromatic components. This observation highlights the molecular degradation of the SBS polymers and the progression of oxidation reactions during asphalt ageing. Thermal-oxidative ageing resulted in a 24.56% decrease in the BI, an 18.18% increase in the SI, and an 11.06% increase in the CI. These changes provide clear evidence of the pronounced oxidation reactions occurring in asphalt under thermal-oxidative conditions. The substantial decrease in the BI was accompanied by a marked reduction in the softening point, as discussed in
Section 3.1. The migration of lighter components during this process is insufficient to mitigate the deterioration of asphalt’s high-temperature performance, resulting in a pronounced decline in its high-temperature characteristics. Following thermal-ultraviolet-humidness coupling ageing treatment, BEC, HEC, and SHEC exhibited additional BI reductions of 25.57%, 37.31%, and 30.00%, respectively, compared to thermal-oxidative ageing. Furthermore, increases in SI of 19.17%, 30.35%, and 23.62% were observed, along with increases in cohesion CI of 22.85%, 33.35%, and 27.63%, respectively. The severity of butadiene degradation and the increase in sulfoxide and aromatic groups followed the order: high temperature and humidness environment conditions > slightly high temperature and humidness environment conditions > benchmark environment conditions. This trend aligns with the ageing intensity hierarchy established in
Section 3.1. These findings indicate that thermal-ultraviolet-humidness coupling ageing predominantly accelerates the thermal-oxidative degradation of butadiene, leading to the disruption of the cross-linked network structure within the asphalt. Concurrently, this process promotes the oxidation of thioether compounds into sulfoxide groups, enhancing intermolecular interactions and inducing the migration of lighter components, As a result, the hardness of the asphalt increases. Consequently, these chemical transformations lead to elevated softening points and reduced ductility in asphalt performance metrics.
A comparative analysis between HEC and BEC revealed that HEC exhibited a 15.78% lower BI, 9.39% higher SI, and 8.54% higher CI. These results indicate that the combined effects of Guangzhou have a greater impact on asphalt ageing than Beijing, which are characterised by high ultraviolet radiation. A high-temperature and high-humidness environment accelerates the degradation of butadiene and the oxidation reactions within the asphalt matrix. Compared to the findings of Ma et al. [
41], who demonstrated the prompting effects of a coupled humidness and thermal environment on asphalt ageing, the above results indicate that a similarly detrimental effect is also exerted by highly humidness and thermal conditions present in coupled thermal-ultraviolet-humidness ageing. A comparison between SHEC and BEC revealed that, SHEC exhibited a 5.96% decrease in BI, accompanied by increases of 3.74% and 3.89% in SI and CI, respectively. These findings indicate that Chengdu exert stronger coupling effects than Beijing, thereby confirming the role of humidness in accelerating the degradation and oxidation of butadiene. Mechanistically, under high-humidness conditions, the temperature-induced formation of abundant polar groups within the asphalt leads to the formation of hydrogen bonds with water molecules. This interaction facilitates molecular migration within the asphalt and intensifies oxidation reactions. Notably, the ageing of asphalt in Beijing was constrained by the surface hardening of the aged materials, which impeded the penetration of ultraviolet radiation into the deeper layers. However, this constraint had a limited effect on the coupling interactions between temperature and humidness, as liquid water or temperature-induced water vapour could still diffuse into the interior of the asphalt, disrupting its colloidal structures. Collectively, these findings underscore that the coupled influence of temperature and humidness exerts a more substantial effect on butadiene degradation and oxidation than ultraviolet radiation, with elevated humidness levels significantly enhancing the ageing mechanisms.
FTIR analysis of rejuvenated asphalt was conducted to calculate the corresponding BI, SI, and CI indices, as illustrated in
Figure 8,
Table 9, and
Figure 9, respectively.
As illustrated in
Figure 8, a new peak emerges at 1725 cm⁻
1, indicating chemical interactions between industrial animal oil and aged asphalt. This peak was attributed to the stretching vibration of the carbonyl groups (C=O) present in the fatty acids of industrial animal oils.
Figure 9 illustrates that, with increasing industrial animal oil content, all groups exhibited significant upward trends in BI, accompanied by notable reductions in SI and CI, consistent with the observed variations, aligning in pavement properties. This indicates that the incorporation of industrial animal oil not only increases the butadiene content but also decreases the concentrations of both polar and non-polar functional groups, further validating its effectiveness in rejuvenation. Compared to asphalt subjected to coupling ageing, the incorporation of a 6% content of industrial animal oil resulted in increases in the BI of 10.64%, 7.66%, and 9.96% for BEC, HEC, and SHEC, respectively. This modification was accompanied by reductions in the SI by 38.88%, 36.74%, and 37.74%, and decreases in the CI by 63.77%, 62.54%, and 63.11%, respectively. The relatively modest improvements in BI suggest that the restorative effects of industrial animal oil on SBS polymers are limited. In contrast, the significant reductions in SI and CI indicate that the primary rejuvenation mechanism involves the dissolution and dilution of both polar and non-polar molecules generated during the ageing process, along with the replenishment of lighter components lost over time. When the concentration of industrial animal oil was increased to 9%, the BI, SI, and CI values initially exhibited slight increases before subsequently decreasing under all ageing conditions. Although these indices approach the levels observed in the unaged asphalt, they remain below the benchmarks established for unaged materials. This observation suggests that, while higher contents of industrial animal oil can further enhance asphalt performance, the overall efficacy of the rejuvenation process remains limited. Specifically, the lower index levels observed in SHEC and HEC at a 9% content indicate an intensified interaction between humidness and other ageing factors under their respective environmental conditions. The persistent performance gap between regenerated asphalt and unaged asphalt underscores the inherent limitations of industrial animal oil in fully restoring aged asphalt to its original state, particularly in response to complex ageing mechanisms driven by interrelated environmental factors.
4.2. Microscopic Morphology
The distribution characteristics and variations in SBS in the asphalt binders are illustrated in
Figure 10 and
Figure 11, respectively. These observations were made under different ageing conditions and following industrial animal oil regeneration, using a microscope with 400× magnification.
As illustrated in
Figure 10, unaged asphalt
Figure 10a displays a continuous cross-linked network structure of SBS polymers, which substantially enhances the viscosity and cohesion of the asphalt. This structure configuration serves as a fundamental mechanism for maintaining stability and ensuring superior performance. However, following thermal-oxidative ageing
Figure 10b, a significant portion of the cross-linked network undergoes thermal-oxidative degradation, resulting in the formation of flocculent and fragmented chain structures. This structural evolution results from two primary mechanisms: (i) the volatilisation of light components (e.g., saturates and aromatics), which reduces the compatibility of asphalt with SBS and weakens structural support, and (ii) the thermal-oxidative degradation of butadiene segments, which disrupts the molecular architecture of SBS. This observation aligns with the decline in the butadiene index reported in
Section 4.1. Notably, the substantial reduction in the softening point of thermal-oxidative aged asphalt, as documented in
Section 3.1, further underscores the critical importance of cross-linked network integrity in maintaining high-temperature performance.
Under coupled environmental ageing conditions, BEC (
Figure 10c), HEC (
Figure 10d), and SHEC (
Figure 10e) exhibited distinct patterns of structural degradation. As depicted in panels (
Figure 10c–e), the aged asphalt exhibit a complete disintegration of the original cross-linked network into smaller, dispersed, and fragmented chains, accompanied by a noticeable reduction in SBS fluorescence intensity. Comparative analysis revealed the following: (i) high-temperature and humidness environment conditions (Guangzhou) resulted in the most dispersed SBS fragments, characterised by the smallest particle dimensions and the weakest fluorescence intensity; (ii) benchmark environment conditions (Beijing) exhibited the largest particle dimensions and the most pronounced fluorescence intensity among the fragmented chains; and (iii) the moderately high-temperature and humidness environment conditions (Chengdu) displayed intermediate characteristics. These findings indicate that high-temperature and humid environments exert the most significant degradation effects on SBS. The humid conditions in the SHEC promote chain scission decomposition, while the SBS degradation caused by the high ultraviolet radiation environment in the BEC is comparatively less pronounced. Under ultraviolet radiation, water undergoes photolysis, generating abundant hydroxyl radicals that act as strong oxidants. These radicals attack the C=C double bonds in the butadiene segments of SBS oxidation reactions. The decomposition process leads to the formation of numerous polar groups, ultimately resulting in the breakdown of the SBS structure. Due to the lower humidness conditions in the BEC, this process was leading to relatively improved sizes and connectivity of SBS chain scission structures. The extent of the aforementioned degradation effects was correlated with the changes in the butadiene index described in
Section 4.1. This correlation that the combined effects of high temperature and humidness have a more pronounced impact on the structural degradation of SBS than the effects of increased ultraviolet radiation.
The incorporation of industrial animal oil significantly influenced the microstructural and performance characteristics of aged asphalt. As illustrated in
Figure 11, the fragmented SBS chains become miscible with the light components introduced by the industrial animal oil, leading to a gradual attenuation of the fluorescence intensity. Simultaneously, the residual SBS fragments swell and coalesce upon absorbing light components, forming prominent fluorescent light spots. These microstructural transformations were macroscopically manifested as reduced softening points and enhanced ductility, indicating improved material plasticity. With increasing industrial animal oil content, a significant proliferation of fluorescent, enlarged SBS aggregates was observed. This phenomenon reflects two primary mechanisms: (i) the replenishment of light components, which enhances asphalt-SBS compatibility, and (ii) the facilitation of SBS fragment coalescence and swelling. These mechanisms are consistent with the trend of BI elevation documented in
Section 4.1.
Comparative analysis of the SBS distribution under various ageing conditions revealed sparse and unevenly distributed fluorescent aggregates under Guangzhou, whereas Beijing produced the most abundant and uniformly distributed fluorescent aggregates. Notably, industrial animal oil was unable to reconstruct fractured SBS molecular chains or restore the cross-linked network architecture. As a result, rejuvenated asphalt exhibits significant performance recovery disparities between high- and low-temperature properties. While low-temperature performance improves, the addition of industrial animal oil fails to compensate for losses in high-temperature performance and, in some cases, even accelerates their decline. These findings underscore the necessity of maintaining strict control over the high-temperature stability of industrial animal oil-rejuvenated asphalt in practical pavement engineering applications to ensure satisfactory road service performance.