A commercial SBS (I-D) modified asphalt and rutile-type nano-TiO₂ were selected. The nano-TiO₂ was incorporated at a dosage of 1% by weight of the SBS modified asphalt. The composite modified asphalt was prepared by shearing the mixture at 800 r/min for 30 min at 150 °C, as shown in Figure 1. According to the specifications in the Test Methods of Asphalt and Asphalt Mixtures for Highway Engineering (JTG E20, 2011), the original asphalt underwent short-term thermal-oxidative aging using the thin film oven test (TFOT, T0609), followed by long-term aging using the pressure aging vessel (PAV, T0630), to produce the primary aged asphalt samples.

Asphalt mixture rejuvenators are usually classified by their mechanism into rejuvenating and fluxing effects (Loise et al., 2021). A commercial rejuvenator was selected, meeting the RA5 grade requirements specified in the Technical Specification for Asphalt Pavement Recycling (JTG/T 5521, 2019); its performance indices are presented in Table 1. The rejuvenator was added to the primary aged asphalt at 150 °C at a dosage of 5% by weight of the aged asphalt, and the mixture was stirred at 300 r/min for 6 min. Since new asphalt is typically added during plant-mixed or in-situ hot recycling of asphalt pavements, an equal amount of SBS (I-D) modified asphalt without nano-TiO₂ was added to the aged asphalt to simulate this process. The mixture was then stirred at 500 r/min for an additional 10 min to obtain the primary regenerated asphalt, as illustrated in Figure 2.

The primary regenerated asphalt was subjected to short-term and long-term aging once again to obtain the secondary aged asphalt samples. The secondary regenerated asphalt was prepared following the same regeneration procedure as that used for the primary aged asphalt.

Using the previously prepared asphalt samples—original asphalt, primary aged asphalt, primary regenerated asphalt, secondary aged asphalt, and secondary regenerated asphalt—AC-13 asphalt mixtures were produced (Figure 3) shows some samples of asphalt mixture prepared from original asphalt. The mix design gradation is presented in Figure 4, which also indicates the upper and lower gradation limits for AC-13 asphalt mixtures as specified in the Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40, 2004). Basalt was used as the coarse aggregate, and limestone was used for both the fine aggregate and mineral filler. The asphalt–aggregate ratio was uniformly set at 4.9%.

In accordance with the Test Methods of Asphalt and Asphalt Mixtures for Highway Engineering (JTG E20-2011), various macro-scale performance tests were conducted on the original, aged, and regenerated asphalt and asphalt mixtures. In addition, Gel Permeation Chromatography (GPC) and Scanning Electron Microscopy (SEM) tests were performed to analyze the molecular weight distribution and microscopic morphology of asphalt samples, respectively.

For the SEM test, an FEI Inspect F50 scanning electron microscope was used to observe the microstructural characteristics of asphalt samples. In the GPC test, an Agilent 1260 Infinity high-performance liquid chromatography system was employed. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1 mL/min. The injection volume was 100 μL, with a sample concentration of 5 mg/mL. GPC analysis was performed on five types of asphalt samples: original, aged, and regenerated.

Under the elution effect of the mobile phase, SBS macromolecules in the composite modified asphalt exhibit shorter retention times in the gel columns compared to asphalt molecules. Therefore, based on the differences in elution peak times on the chromatogram, the SBS phase and asphalt phase can be distinguished. For each identified elution peak, the corresponding molecular phase was selected, and the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) were calculated using Equations 1–3 to characterize the molecular weight distribution of the original, aged, and regenerated asphalt. In the equations, w_i represents the number of molecules with molecular weight M_i, w is the total number of molecules, n_i is the molar amount of molecules with molecular weight M_i, and n is the total molar amount of all molecular types (Qin et al., 2021; Ding et al. 2023). M W = ∑ i = 1 n w i × M i w (1) M n = ∑ i = 1 n n i × M i n (2) P D I = M W M n (3)

The elution curves obtained from the Gel Permeation Chromatography (GPC) tests for the original asphalt, primary aged asphalt, primary regenerated asphalt, secondary aged asphalt, and secondary regenerated asphalt samples are shown in Figure 5. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) for both the asphalt phase and the SBS phase of the five asphalt types are presented in Figures 6, 7.

It can be observed that all elution curves of the original, aged, and regenerated SBS modified asphalt samples exhibit two prominent peaks. The elution peak between 14 and 18 min corresponds to the earlier eluting SBS phase, while the peak between 20 and 27 min corresponds to the later eluting asphalt phase. After aging, the peak intensity of the SBS phase significantly decreases. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the SBS phase decrease by 48.3% and 41.6%, respectively, after primary aging, and by 36.3% and 31.5%, respectively, after secondary aging. It is evident that in both aging stages, the reduction in Mn is consistently greater than that of Mw, indicating that thermal-oxidative aging causes chain scission and oxidative degradation of the SBS polymer into smaller polymer fragments. This leads to a larger proportion of low-molecular-weight species in the SBS phase. Furthermore, the polydispersity index (PDI) of the SBS phase increases after both primary and secondary aging. This is also attributed to the degradation of large molecules, which increases the disparity among molecular weights, thereby widening the gap between Mn and Mw. In contrast, the Mn and Mw of the asphalt phase both increase after primary and secondary aging, reflecting the oxidative mass gain of asphalt molecules during the aging process.

After primary or secondary regeneration, the introduction of the rejuvenator brings a significant number of small molecules into the asphalt phase, leading to a reduction in both Mn and Mw. However, these values remain higher than those of the original (unaged) asphalt, and the asphalt-phase elution peak shifts slightly to the right compared to the original asphalt. The addition of fresh SBS modified asphalt during the regeneration process introduces a large number of high-molecular-weight SBS chains, which contributes to an increase in Mn and Mw of the SBS phase by 29.7%–43.2%.

Among the five asphalt types (original, aged, and regenerated), the PDI of the asphalt phase ranges from 3.3 to 3.7, while that of the SBS phase ranges from 1.3 to 1.6. The significantly lower PDI of the SBS phase indicates that the variation in molecular weight among SBS macromolecules is relatively small. This observation is further supported by the narrower elution peak of the SBS phase compared to the much broader peak of the asphalt phase. In terms of Mn and Mw of the SBS phase, the order from highest to lowest is: original asphalt > primary regenerated asphalt > secondary regenerated asphalt > primary aged asphalt > secondary aged asphalt. For the asphalt phase, the order is: secondary aged asphalt > primary aged asphalt > secondary regenerated asphalt > primary regenerated asphalt > original asphalt.

Further SEM imaging was performed on the original asphalt, secondary aged asphalt, and secondary regenerated asphalt samples, as shown in Figure 8. In the original asphalt, the SBS chains form an interwoven network structure, and the polymer chains appear intact with smooth surfaces. In contrast, the SBS chains in the secondary aged asphalt exhibit a large number of surface cracks, indicating that molecular degradation of SBS due to aging also weakens its network structure. In the secondary regenerated asphalt, both damaged/cracked and intact/interwoven SBS chains are observed. The intact chains originate from the addition of fresh SBS modified asphalt during repeated regeneration. As an inorganic material, nano-TiO₂ remains essentially unchanged during both aging and regeneration processes.

The conventional physical properties (penetration, softening point, and ductility) of the five asphalt samples were tested, and the results are summarized in Table 2. It can be observed that after both primary and secondary aging, the penetration and ductility values of the asphalt significantly decrease, while the softening point slightly increases. The magnitude of changes after secondary aging is greater than after primary aging. After regeneration, due to the introduction of the rejuvenator and fresh SBS modified asphalt, the lost light components (from asphalt oxidation and volatilization) and degraded SBS macromolecules are replenished (Zhao et al., 2016; Shafabakhsh et al. 2015). As a result, the penetration, ductility, and softening point of the regenerated asphalt are partially restored.

The Dynamic Shear Rheometer (DSR) test, as shown in Figure 9, was conducted using a Smart Pave 102 dynamic shear rheometer at four different temperatures: 58 °C, 64 °C, 70 °C, and 76 °C. The complex modulus G and phase angle δ of the five asphalt samples were measured, and the rutting factor G/sinδ was calculated. The results are shown in Figures 10–12.

Among the five asphalt types, at all test temperatures, the original asphalt exhibits the lowest complex modulus and rutting factor and the highest phase angle. This is followed by the primary regenerated asphalt, then the secondary regenerated asphalt and the primary aged asphalt, while the secondary aged asphalt exhibits the highest complex modulus and rutting factor and the lowest phase angle. The relative values of complex modulus, phase angle, and rutting factor between secondary regenerated asphalt and primary aged asphalt vary with temperature.

These results indicate that both primary and secondary aging cause asphalt hardening, leading to an increase in complex modulus. The increase in elasticity of the asphalt phase due to aging outweighs the reduction in elasticity caused by partial degradation of the SBS network, resulting in a decrease in phase angle. A higher rutting factor reflects better high-temperature performance. The increase in rutting factor after aging confirms the enhancement of asphalt’s high-temperature performance. At 58 °C–76 °C, the rutting factor of secondary aged asphalt is 29%–46% higher than that of the primary aged asphalt.

As the number of regeneration cycles increases, even though rejuvenator and fresh modified asphalt are added to the aged asphalt, the rutting factor of primary regenerated asphalt increases by 35%–110%, and that of secondary regenerated asphalt increases by 46%–130%, compared to the original asphalt at various temperatures. This indicates that, although SBS degradation occurs during aging, aging of the asphalt phase is more dominant, and the high-temperature performance of the secondary regenerated asphalt remains better than that of the original asphalt. Studies have shown that the addition of nano-TiO₂ improves the high-temperature performance of asphalt but adversely affects its low-temperature performance (Han et al., 2023; Cao et al. 2023; Zhang et al., 2023). Given that aging enhances high-temperature performance while significantly deteriorating low-temperature performance, regeneration should focus on restoring low-temperature properties. Therefore, in this study, SBS modified asphalt and rejuvenator were used to supplement the degraded SBS component and light fractions lost during aging. The use of SBS–nano-TiO₂ composite modified asphalt in regeneration is not recommended, in order to avoid further degradation of low-temperature performance.

Tables The Bending Beam Rheometer (BBR) test was performed using a PAVETEST B216 device at three temperatures: 24 °C, −18 °C, and −12 °C, as shown in Figure 13. The creep stiffness and creep rate of the five asphalt samples were measured, and the results are shown in Figures 14, 15.

Lower creep stiffness and higher creep rate indicate better low-temperature performance of asphalt. Within the temperature range of −24 °C to −12 °C, the order of creep stiffness from largest to smallest or creep rate from smallest to largest for the five asphalt samples is: secondary aged asphalt, primary aged asphalt, secondary regenerated asphalt, primary regenerated asphalt, and original asphalt. This shows that the low-temperature performance of these five types of asphalt increases in the stated sequence. The reason is that under repeated aging, the light components in the asphalt phase evaporate, asphalt molecules oxidize and become harder and more brittle, while macromolecules in the SBS phase degrade, and the crosslinked structure is partially destroyed. Compared with the original asphalt, at different temperatures, the creep stiffness of primary and secondary regenerated asphalt increases by 13%–23% and 50%–116%, respectively, while the creep rate decreases by 7%–20% and 13%–28%, respectively. Therefore, the low-temperature performance of asphalt after primary and secondary regeneration remains significantly lower than that of the original asphalt.

Rutting tests were conducted at 60 °C, as shown in Figure 16. The 45-min deformation and dynamic stability of asphalt mixtures prepared with original, primary aged, primary regenerated, secondary aged, and secondary regenerated asphalt were tested to characterize their high-temperature rutting resistance. The results are presented in Figure 17. Compared with the original asphalt mixture, the 45-min deformation of the secondary regenerated asphalt mixture was still 24.0% lower, while its dynamic stability was 32.8% higher, indicating that repeated aging significantly improved the high-temperature rutting resistance of the mixture.

Bending tests at 0 °C, as shown in Figure 18 were performed to measure bending stiffness modulus and maximum tensile strain of the five asphalt mixtures, as shown on Figure 19. A larger maximum tensile strain indicates better low-temperature crack resistance. After repetitive aging, the bending stiffness modulus of the mixtures increases, and the maximum tensile strain significantly decreases, indicating that aging greatly reduces the low-temperature performance of the composite modified asphalt mixtures. The maximum tensile strain after primary and secondary aging decreases by 52.5% and 73.4%, respectively, compared with the original asphalt.

After primary and secondary regeneration, the maximum tensile strains are 2734 με and 2512 με, slightly above the lower limit of 2500 με for modified asphalt in cold regions specified in the “Technical Specification for Construction of Highway Asphalt Pavements” (JTG F40, 2004), but still 30.1% and 38.6% lower than the original asphalt. This indicates that low-temperature crack resistance is a challenging property to maintain for SBS-nano TiO₂ composite modified asphalt mixtures during aging. Without supplementation of original composite modified asphalt during secondary regeneration, and only adding rejuvenator, it is difficult to restore the low-temperature crack resistance of the secondary aged asphalt mixture.

Two groups of Marshall specimens were prepared (with no fewer than four specimens in each group) and then placed in a water bath maintained at 60 °C. One group was immersed in water for 48 h, while the other was immersed for no less than 30 min. The Marshall stability of the five asphalt mixtures was determined, and the residual stability index after water immersion was calculated, as shown in Figure 20.

The entire freeze-thaw splitting test procedure is illustrated in Figure 21. First, the Marshall specimens were subjected to vacuum treatment (vacuum pressure of 97.3–98.7 kPa for 15 min). After vacuum extraction, the specimens were soaked in water for 30 min. Subsequently, the specimens were immediately placed in a low-temperature test chamber at −18 °C and stored for 16 h to simulate freeze-thaw conditions. Prior to freezing, specimens were sealed in plastic bags containing 10 mL of water. After freezing, the specimens were immediately transferred to a constant-temperature water bath at 60 °C for 24 h of curing. Finally, the first and second groups of specimens were separately soaked in a 25 °C water bath for 2 h. After cooling, splitting strength tests were performed to obtain the load values. The freeze-thaw splitting strength ratio was calculated, as shown in Figure 22. Higher residual stability and freeze-thaw splitting strength ratio indicate better water stability of the mixtures. Larger residual stability and freeze-thaw splitting strength ratio indicate better moisture resistance of the mixtures.

It is observed that Marshall stability before and after immersion and splitting strength before and after freeze-thaw significantly increase after aging but decrease after regeneration. This is because aging enhances the tensile and compressive properties of composite modified asphalt, but residual stability after immersion and freeze-thaw splitting strength ratio decline after aging. Aging reduces the adhesion performance at the asphalt-aggregate interface, making the interface more susceptible to various microscopic damages after water immersion or freeze-thaw cycles, leading to decreased moisture resistance of the mixtures.

After primary or secondary aging, the decrease in residual stability compared with original asphalt is within 1%, while the decrease in freeze-thaw splitting strength ratio is between 4% and 6%, but both remain above the requirements of the “Technical Specification for Construction of Highway Asphalt Pavements” (JTG F40, 2004), which requires residual stability above 85% and freeze-thaw splitting strength ratio above 80% for modified asphalt mixtures in wet regions. After primary or secondary regeneration, residual stability remains basically stable compared with original asphalt, while freeze-thaw splitting strength ratio decreases by 3.3% and 4.1%, respectively.

According to China’s “Specifications for Design of Highway Asphalt Pavements” (JTG D50–2017), the dynamic modulus at 20 °C and 5 Hz is adopted in asphalt pavement design. Therefore, for a certain degree of simplicity, the uniaxial compression dynamic modulus tests were conducted on five types of asphalt mixtures at 20 °C under six frequencies: 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz, and 25 Hz. The test procedure is illustrated in Figure 23, and the resulting dynamic modulus curves are presented in Figure 24.

The dynamic modulus of the composite modified asphalt mixtures increases with frequency. At different frequencies, aging significantly increases the dynamic modulus of composite modified asphalt mixtures, indicating repetitive aging benefits the compressive performance of mixtures. Except for a few points, at the same frequency between 0 and 25 Hz at 20 °C, the order of dynamic modulus from highest to lowest is secondary aged, primary aged, secondary regenerated, primary regenerated, and original asphalt mixtures. At 20 °C and 10 Hz, dynamic modulus is a key indicator for characterizing mechanical performance of asphalt surface layers in structural design. After primary and secondary regeneration, it remains 22.0% and 33.2% higher than that of the original asphalt mixture, respectively.

Four-point bending fatigue tests were performed at 15 °C and 10 Hz to measure the number of loading cycles corresponding to the bending stiffness modulus decreasing to 50% of its initial value, defined as fatigue life. The test procedure is illustrated in Figure 25. The results are shown in Figure 26.

Fatigue life of composite modified asphalt mixtures significantly decreases with aging and partially recovers after regeneration. However, fatigue life after primary and secondary regeneration remains 15.0% and 26.2% lower than the original asphalt mixture, respectively. Although fresh modified asphalt is added during regeneration, degraded SBS chains cannot be restored, and new and old asphalt molecules cannot fully integrate, increasing internal micro-damage. Compared with the original asphalt, the asphalt-aggregate interface bonding weakens after regeneration, and numerous weak areas exist internally and at bonding interfaces under repeated loading. Thus, fatigue resistance of the five mixtures ranks from highest to lowest as: original, primary regenerated, secondary regenerated, primary aged, and secondary aged asphalt mixtures.