## 5 Analysis of Test Results
### Influence of Vertical Load and Interlayer Characteristics between the Asphalt Layer and Base Layer  
In the preceding sections, the vertical load is applied at 0.7 MPa, which simulates the stress situation between the upper layer and the middle layer of asphalt. Owing to the diffusion of force, the vertical load between the asphalt surface layer and the lower layer, the lower layer and the base, is not 0.7 MPa, but reduced by a certain degree. Therefore, the vertical compressive stress of three asphalt layers is calculated by means of mechanical calculation, and this is used as the basis for the vertical load in the laboratory. Based on the results of experiment and calculation, the best value for the spreading amount is recommended. The vertical compressive stress between AC-20 and AC-25 is calculated by ANSYS software to be 0.16 MPa, and the vertical compressive stress between AC-25 and the cement-stabilized base layer is 0.07 MPa. Specific experimental parameters are shown in Table 10. According to the above description, it can be found that the interlayer bonding behavior of oil A is superior to that of oil B under comprehensive conditions. The corresponding interlayer shear test was carried out as described in the previous section. The adhesion coefficient \(K_{\text{max}}\) between the AC-20 and AC-25 layers is shown in Figure 17.  
As can be seen from Figure 17, the maximum adhesion coefficient between layers is not regular when the temperature is \(-10\)\({}^{\circ}\)C, and the adhesion coefficient is not only affected by the structural depth on the surface of the two specimens, but also has a strong relationship with the adhesion between the asphalt film and the cohesion of the viscosity-level oil. Therefore, the influence of the amount of viscosity oil on the interlayer interface is not obvious under such low temperature conditions. When the ambient temperature is 0, 25, and 50 \({}^{\circ}\)C, then \(K_{\text{max}}\) increases first and then decreases in the upper and middle layers. In general, the spreading amount of viscosity oil has a certain effect on the adhesion coefficient between layers in asphalt. From 0 to 1.5 kg/m\({}^{2}\), \(K_{\text{max}}\) gradually increases first and then decreases, basically presenting a parabolic form with downward opening. Therefore, there is an optimal spreading amount. By fitting the curve of Figure 17, the polynomial is obtained, as shown in Table 11; thus, the optimal spreading amount of viscosity oil can be obtained under different working conditions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Experimental Variable** & **Experiment with the Selected Variables** \\ \hline Upper layer & AC-20, AC-25 \\ Following layer & AC-25, Cement-stabilized crushed stone \\ Viscosity-level & Viscosity-level oil A, Site sampling of \\  & transparent layer of oil A \\ Experimental temperature & \(-10\), 0, 25, 50 \({}^{\circ}\)C \\ Amount of viscosity-level oil spread & 0, 0.5, 1.0, 1.5 kg/m\({}^{2}\) \\ Vertical compression stress & 0.16, 0.07 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Experimental parameters to verify the effect of vertical load. It can be seen from Table 11 that the interlayer adhesion coefficient reaches its maximum value when the spreading amount is controlled in the range 0.4-0.7 kg/m\({}^{2}\) at the ambient temperature 0-50 \({}^{\circ}\)C.  
The characteristics of the interlayer adhesion coefficient between AC-25 and the cement-stabilized macadam base are also tested and analyzed, and the test results are shown in Figure 18.  
Owing to the large void ratio of AC-25 and cement-stabilized macadam base mixture, a large amount of viscosity-level oil is absorbed into the specimen. At low temperature, the interlayer adhesion coefficient \(K_{\text{max}}\) increases obviously when the amount of spread increases from 0 to 1.0 kg/m\({}^{2}\). However, \(K_{\text{max}}\) decreases about 25% when the amount of spread increases to 1.5 kg/m\({}^{2}\).