# Effect of Gradation Segregation on Mechanical Properties of an Asphalt Mixture
## 3 Results  
### Effect of Gradation Segregation on the Aggregate Skeleton  
Aggregate skeleton plays a critical role in terms of asphalt mixture's service performance [26; 27]. In this study, aggregates composing the aggregate skeleton in asphalt mixture were determined according to their role under loading. Figure 2 presents the contact points and contact forces of D-type asphalt mixture with the deformation of 2 mm. In Figure 1(a), dark and light gray spheres indicate aggregates with diameters greater and less than 2.36 mm, respectively. Green dots between two simplified aggregate spheres are contact points. Black lines with different widens inside spheres representing contact force with magnitudes.  
Based on the data presented in Figure 1(a), it can be seen that vertical load is transferred through aggregates with diameters larger than 2.36 mm. Thus, the aggregate in the asphalt mixture with a diameter larger than 2.36 mm consists of the aggregate skeleton and the remaining fine aggregates mainly play a role of filling. In Figure 1(b), green dots indicate the contact points between aggregates with diameter less than 2.36 mm, yellow dots indicate contact points between the aggregates with diameter greater than 2.36 mm, and the orange dots indicate the contact points (transition contact points) formed due to aggregate (\(<\)2.36 mm) sandwiched by two aggregates (\(>\)2.36 mm). As shown in the figure, the contact points on the main force chains are basically the yellow contact points and the orange contact points, while the other contact points (green) are hardly on the main force chain. This demonstrates that the skeleton of mixture mainly consists of aggregates with diameters greater than 2.36 mm.  
To further verify the above conclusions, according to the particle size of the aggregates at both ends of the contact, the contacts between the aggregates are divided into four types: blue, red, black and green, as shown in Figure 2(a). The maximum, minimum and average values of the forces of four kinds of contact are shown in Table 6. Aggregates with a particle size greater than 2.36 mm are defined as coarse aggregates, and aggregates with a particle size less than 2.36 mm are defined as fine aggregates. It can be seen from Table 6 that the contact force at the blue contact is about 8 times that of the black contact, which means that the contact force between the coarse aggregates is much larger than the contact force between the fine aggregates. When the fine aggregates are in contact with the coarser aggregate, although the contact force increases, the value is still small. When the fine aggregates are sandwiched between two coarse aggregates, the value of the contact force is large, which is also due to the interaction between the coarse aggregates. Therefore, it can be concluded that in the D-type mixture, the coarse aggregates interact to form a skeleton which plays a major role in res  
Figure 2: Distribution of contact points and contact force at larger (**a**) and smaller (**b**) scales.  
In order to further analyze the impact of gradation segregation on the aggregate skeleton in the asphalt mixture, the HC and HF-types were studied as example. Figure 4 illustrates the distribution of contact points and contact stress. In Figure 4, the dark and light grey disks represent aggregates with diameter greater and less than 4.75 mm, respectively. While, in Figure 4, the dark and light grey disks represent aggregates with diameter greater and less than 1.18 mm, respectively. In both figures, green dots represent the contact points between aggregates. Combining Figures 2 and 4, it is easy to see that the gradation segregation significantly affects the aggregate skeleton in the asphalt mixture. Specifically, with the increase of segregation, the aggregate skeleton in the asphalt mixture changes accordingly. Given this, it can be proposed that the mechanical properties of the asphalt mixture can be affected by the gradation segregation as the aggregate skeleton undergoes the main load.