# Effect of Gradation Segregation on Mechanical Properties of an Asphalt Mixture
## 3 Results
### Effect of Gradation Segregation on the Asphalt Mixture  
#### 3.2.1 Mechanical Characteristics of the Asphalt Mixture  
In this study, VMA, the number of contact points between the aggregates and the contact force at the contact points are involved as the mechanical characteristics of asphalt mixture. Based upon these \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Contact Force/N**} & \multicolumn{4}{c}{**The Contact Types**} \\ \cline{2-5}  & **Blue** & **Green** & **Red** & **Black** \\ \hline maximum values & 35203 & 28837 & 13738 & 14471 \\ minimum values & 154 & 0.15 & 0.003 & \(2.01\times 10^{-6}\) \\ average values & 10079 & 6162 & 2098 & 1340 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The contact force of four contact types. Figure 4: Distribution of contact points and contact force with different segregation levels: HC (**a**) and HC (**b**). Figure 3: Distribution of contact force. parameters, the influence of gradation segregation on the mechanical properties of the asphalt mixture is analyzed and compared. The VMA and contact points are listed in Table 7.  
After the model was established, a certain initial speed was given to the upper wall to make it load downwards, and the servo control system was used to control the load process until the loading stress was maintained at 2.5 MPa. After that, the PFC2D 5.0 subroutine was used to calculate the VMA, the total number of contact points, the average contact force, and the maximum contact force. The results are shown in Table 7.  
In Table 7, it can be seen that with decreasing gradation segregation level, the VMA gradually decreases and the number of contact points increases dramatically. To further analyze the change in the contact points, the aggregate was divided into three grades according to the particle size: \(>\)13.2 mm, 2.36\(\sim\)13.2 mm, and \(<\)2.36 mm. The number of contact points around every grade of aggregate particles was counted, as shown in Figure 5. It can be seen that with the gradation from coarse to fine, although there are vibrations on the number of contact points around the aggregates with diameters larger than 13.2 mm, there is no obvious difference. Conversely, it should be noted that the number of contact points around aggregates with the diameter of 2.36\(\sim\)13.2 mm and smaller than 2.36 mm show continually growing trend. This indicates that with the increase of fine aggregates in the mixture, the contact points around the fine aggregates increase sharply, while the contact points around the coarser aggregates change slightly. When the loading stress was maintained at 2.5 MPa, the contact force distributing in the mixture was shown in Figure 6. In this figure, the thicker the black lines, the greater the contact forces. It can be seen from Figure 6 and Table 7 that with the gradation from coarse to fine, the contact force chains in the mixture gradually become thinner. It reveals that the average contact force and the maximum contact force gradually decrease. The average contact force of the HC-type mixture is about 5 times of that of the HF-type, and the maximum contact force is about 4 times.. Since aggregates are more susceptible to be crushed under high stress, when coarse aggregate segregation occurs, coarse aggregates in the mixture are more likely to be crushed under the action of vehicle loads, directly affecting the stability of skeleton structure and further affecting the road performance of the mixture.