## 3 Nano-Morphological Parameters and Technical Indexes of Aged Bitumen
### Relation Between Nano-Morphological Parameters and Technical Indexes of Bitumen After Ageing  
Ageing also affects the pavement performances of bitumen. In this paper, the relationship between nano-morphological parameters and pavement indexes of A-90 bitumen before and after ageing was established based on the test results. The bee-like structure and penetration, ductility, softening point, and viscosity of A-90 bitumen are shown in Figure 11.  
Figure 11: The relationships between the area ratio of the bee-like structure and technical indexes. (**a**) penetration at 25 \({}^{\circ}\)C; (**b**) softening point; (**c**) ductility at 15 \({}^{\circ}\)C; (**d**) viscosity at 60 \({}^{\circ}\)C.  
Figure 11 shows that the area ratio of the bee-like structure has a certain correlation with penetration, ductility, softening point, and viscosity of A-90 bitumen. With the increases in the area ratio of bee-like structure, the penetration of bitumen decreases, the softening point increases, and the viscosity at 60 \({}^{\circ}\)C increases. Penetration and softening point are the evaluation indexes of bitumen viscosity under specific conditions, and viscosity at 60 \({}^{\circ}\)C is a direct evaluation index of bitumen viscosity, therefore, the area ratio of the bee-like structure is bound to be related to the viscosity of asphalt, i.e., the bigger the area ratio of the bee-like structure, the bigger the viscosity of the bitumen. The area ratio of the bee-like structure is linear with the bitumen ductility (correlation coefficient is 87.02%), i.e., the bigger area ratio of the bee-like structure, the smaller the ductility at 15 \({}^{\circ}\)C of bitumen.  
The relationships between the roughness and technical indexes of A-90 bitumen are shown in Figure 12.  
It can be seen from Figure 12 that with the increases of roughness, the viscosity at 60 \({}^{\circ}\)C and the softening point of A-90 bitumen increase obviously, while the penetration at 25 \({}^{\circ}\)C and the ductility at 15 \({}^{\circ}\)C decrease. However, compared with the correlation between the area ratio of the bee-like structure and the bitumen technical indexes, the relationship between roughness and technical indexes is relatively discrete. The reason for this phenomenon is probably due to the change in bitumen  
Figure 12: The relationships between the roughness and technical indexes. (**a**) penetration at 25 \({}^{\circ}\)C; (**b**) softening point; (**c**) ductility at 15 \({}^{\circ}\)C; (**d**) viscosity at 60 \({}^{\circ}\)C.  
composition. The increase in asphaltene content is likely to be the main reason. During the ageing of the bitumen, the composition continuous changes. Saturate and aromatic are continuously volatilized and partially converted into the resin, some of which are converted into asphaltenes. Also, asphaltenes aggregate with each other.  
The relationships between the maximum amplitude and technical indexes of A-90 bitumen are shown in Figure 13.  
The maximum amplitude is intrinsically related to the development of the bee-like structure. The larger the single volume of the honeycomb structure, the larger the maximum amplitude. The bee-like structure of bitumen after ageing is well developed and the larger the area ratio of the bee-like structure, the larger the maximum amplitude. As shown in Figure 13, the relationship between the maximum amplitude and bitumen-related technical indexes are similar to that between the area ratio of the bee-like structure and bitumen-related technical indexes. With the increase of the maximum amplitude, the penetration at 25 \({}^{\circ}\)C decreases, the softening point increases, the ductility at 15 \({}^{\circ}\)C decreases, and the viscosity at 60 \({}^{\circ}\)C increases.  
Figure 13: The relationships between the maximum amplitude and technical indexes. (**a**) penetration at 25 \({}^{\circ}\)C; (**b**) softening point; (**c**) ductility at 15 \({}^{\circ}\)C; (**d**) viscosity at 60 \({}^{\circ}\)C.