## 4 Results and Discussion
### Gradual Microscopic Contact Behaviour of the Main Skeleton
#### 4.1.2 Gradual Development of Contact Point Characteristics  
Under the action of load, the asphalt mixture macroscopically manifests deformation and microscopically manifests an increase in the number of contact points and a decrease in the number of'suspended' coarse aggregates. Figure 7 shows the total number of contact points in the three types of asphalt mixtures at each loading stage. In order to study the quantity of contact points and the development trend of putting when the material is subjected to loading, the relationship between the quantity of contact points and rut depth (RD) at each loading stage is shown in Figure 8.  
Figure 6: Gradual change in mesoscopic contact characteristics of SMA-13 main skeleton. (**a**) Before loading; (**b**) after 1-h loading; (**c**) after 2-h loading; and (**d**) after 3-h loading. Figure 7: Gradual development of contact point. The following can be observed in Figures 7 and 8:  
(1) Under the load action, the asphalt mixture is continuously compacted, and the macroscopic rut depth increases as the number of mesoscopic contact points increases. The rut depth and number of contact points increase rapidly at the initial loading stage and tend to be steady at the end of loading. The quantity of contact points formed by coarse aggregates larger than 2.36 mm has a good linear correlation with the RD. (2) The quantity range of contact points in the two-dimensional cross-sectional images is determined by gradation; it has no relationship with the type of asphalt. A large number of contact points do not indicate that the mixture has a better rutting resistance performance. For instance, as shown in Figure 8, although the quantity of contact points in SMA-13 is less than that in AC-13C, the rutting resistance of the former is better than the latter; this resistance may be related to the coarse aggregate content. By comparing the passing rates of 13.2-, 9.5-, and 4.75-mm coarse aggregates, it is found that the amount of coarse aggregates with particle sizes larger than 4.75 mm in SMA-13 is considerably higher than that in AC-13C, whereas the amount of aggregates that are 2.36-4.75 mm in AC-13C is considerably higher than that in SMA-13. Although the aggregates in the size range 2.36-4.75 mm can provide more contact points, the smaller particle size could result in an unsteady skeleton structure. For this reason, when the material is subjected to load, relative displacements are easily generated; this leads to poor rutting resistance. The main skeleton structure constituted by a moderately large quantity of coarse aggregates with particle sizes larger than 4.75 mm is steadier and has a better ability to resist external loads. Therefore, the quantity of contact points cannot be used to evaluate the rutting resistance of asphalt mixture; the stability of contact points in the main skeleton is the key factor that affects rutting resistance. (3) The asphalt performance has an insignificant influence on the initial quantity of contact points; however, it has a certain influence on the increase in the number of contact points. In Figure 7, it can be noted that there is only a slight difference in the number of initial contact points between 70# asphalt AC-13C and rock asphalt AC-13C. However, with the increase in loading time, the quantity of contact points in 70# asphalt AC-13C slightly increases faster than that in rock asphalt AC-13C although both mixtures have the same gradations. The harder the asphalt, the more it can restrain the movement of aggregates and ensure the stability of contact points in the main skeleton; consequently, the rutting deformation is relatively small. In summary, the internal friction is larger in the main skeleton that contains more coarse aggregates with particle sizes larger than 4.75 mm, whereas the cohesion of asphalt with a higher peformance grade (PG) is greater.