*3.5. AFM Analysis*

Figure 15 shows the two-dimensional and three-dimensional morphology maps of the matrix asphalt. As seen in Figure 15, the microscopic surface of the asphalt was widely distributed with a bee-like structure, which was called a "bee-like structure" in the current study [25]. The colors in the asphalt morphology diagram only represent the height of the microscopic morphology and do not represent the actual color of the asphalt sample. A bee-like structure in the matrix asphalt was randomly selected using NanoScope analysis software to extract its height profile information. As shown in Figure 16, the height of this bee-like structure had a "wavy" shape along the horizontal axis. The peaks correspond to lighter areas of the morphology, while the valleys correspond to the darker areas of the morphology. Numerous studies have shown that the bee-like structures on the microscopic surface of asphalt are wax crystals formed by the eutectic of wax molecules and long-chain alkyl groups of polar macromolecules, and the molecular motility of asphalt significantly affects its aggregation state [26–28].

**Figure 15.** Two-dimensional (**a**) and three-dimensional (**b**) maps of the substrate asphalt morphology.

**Figure 16.** Bee-type structure cross-section.

Figure 17 shows the 2D morphologies of the different asphalts. We can see that all asphalts showed a bee-like structure. There was no essential difference between the microstructures of the matrix asphalt, aged asphalt, and regenerated asphalt; the main difference was the size and number of the bee-like structures. The aged asphalt had a smaller number of bee-like structures and a larger size compared to the matrix asphalt. The addition of the regenerating agent increased the number of bee-like structures and decreased the size of bee-like structures, which was an improvement compared to those of the aged asphalt. The presence of macromolecules was the main reason for the formation of bee-like structures. The addition of regenerating agent regulated the asphalt components and reduced the content of macromolecules in the aged asphalt, which is consistent with the results of GPC analysis. Table 5 and Figure 16 show the number of bee-like structures, the area of individual bee-like structures, and the roughness parameters.

**Figure 17.** AFM morphologies of different asphalts.

(**e**) RA6


**Table 5.** Results of parameters for different asphalts.

As seen in Figure 18, the number of bee-like structures of the matrix asphalt decreased by 50% after short-term aging. The number of bee-like structures increased with the dose of the revertant. The aging process increased the area of the individual bee-like structures of the matrix asphalt. The area of the individual bee-like structures of the asphalt decreased after the addition of the rejuvenation agent. The more the dose of the rejuvenation agent, the more the area of the individual bee-like structures of the asphalt decreased. The asphalt surface roughness *Rq* and *Ra* increased gradually with the increase of the rejuvenation agent dose. The results showed that aging led to the increased asphaltene content, providing a crystalline core for wax molecules, which helped reduce the number of bee-like structures and increase their area. The addition of a regenerating agent had a degrading and diluting effect on the internal macromolecules of the asphalt, thus reducing the viscosity of the asphalt. Consistent with the results of the GPC and FTIR tests described above, the rejuvenator helped to reduce the proportion of heavy components in the aged asphalt and to convert the heavy components to light components by physical or chemical action, thus reducing the asphalt viscosity and softening the aged asphalt. The microscopic level analysis of the rejuvenation agent can improve the construction and ease of the aged asphalt.

**Figure 18.** *Cont*.

**Figure 18.** Variations of different asphalt parameters. (**a**) Trends of different asphalt bee-like structure indicators. (**b**) Trends of different asphalt roughness indices.

#### **4. Conclusions**

The significance of this study was to analyze the effect of the recycler dosing on the high-temperature performance and viscosity of recycled asphalt at the macroscopic and microscopic levels, to provide a scientific basis for the dosing of the recycled asphalt pavement, the mixing temperature of the recycled asphalt mixture, and the recycler dosing, to maximize the reuse rate of recycled asphalt pavement and to optimize the overall performance of the recycled asphalt mixture. In this paper, the changes of the relative molecular weight, the chemical composition, the microstructure, and the viscosity properties of asphalt before and after regeneration were analyzed by GPC, FTIR, AFM tests, and the asphalt activation energy. This paper also explored the microscopic change mechanism of the regenerated asphalt. The following main conclusions were obtained in this paper.


**Author Contributions:** Conceptualization, J.G. and J.Y.; methodology, J.X.; investigation, C.H. and L.L.; writing—original draft preparation, L.L.; writing—review and editing, J.G.; funding acquisition, J.G. and J.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** Natural Science Foundation of Jiangxi Province (Grant No. 20202BABL214046); Youth fund of Jiangxi Provincial Department of Education (Grant No. GJJ190361); Central University Basic Scientific Research Fund (Grant No. 300102210501); Key R & D projects of Xinjiang Uygur Autonomous Region (Grant No. 2021B01005).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The experimental and modeling data used to support the findings of this study are available from the corresponding author upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.
