## 3 Interface Model and Shear Test Fixture of Asphalt Pavement  
### Interlayer Interface Model  
(1) Theoretical model  
Under vehicle load, there is not only friction resistance but also relative horizontal displacement on the interlayer contact surface. The Goodman model is used in classical pavement mechanics to measure the mechanical state of interlayer bonding, as shown in Figure 2. When the relative horizontal displacement of the upper and lower layers of the interlayer interface is \(\Delta u\), the shear stress can be expressed as Equation (1):  
\[\tau=K\Delta u \tag{1}\]  
where \(K\) is the interlayer bonding coefficient (N/cm\({}^{3}\)), \(\tau\) is the shear stress at the interlayer interface (N/cm\({}^{2}\)), and \(\Delta u\) is the relative displacement at the interlayer interface (cm). \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & & \multicolumn{6}{c}{**Mass Percentage through the Square Hole Screen**} \\ \hline Specifications & 26.5 & 19 & 16 & 13.2 & 9.5 & 4.75 & 2.36 & 1.18 & 0.6 & 0.3 & 0.15 & 0.075 \\ AC-13 & 100 & 100 & 99.7 & 95.1 & 76.8 & 38.4 & 29.1 & 23.0 & 17.3 & 12.2 & 10.4 & 7.9 \\ AC-20 & 100 & 99.1 & 91.1 & 73.3 & 54.9 & 30.3 & 23.6 & 18.7 & 14.2 & 10.0 & 8.6 & 6.6 \\ AC-25 & 99.1 & 79.0 & 73.0 & 65.7 & 52.7 & 36.7 & 32.0 & 23.7 & 15.0 & 10.2 & 8.2 & 6.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Gradation of the selected asphalt mixture. Figure 2: Goodman mechanical model. After introducing the Goodman model, the contact conditions between arbitrary \(i\)-layers of multilayer elastomers are shown in Equation (2):  
\[\tau_{Zri}=\tau_{Zri+1}=K_{i}(u_{i+1}-u_{i}) \tag{2}\]  
where \(\tau_{Zri}\), \(\tau_{Zri+1}\) are the shear stresses (N/cm\({}^{2}\)) between the upper and lower layers of the \(i\)-th interlayer interface; \(u_{i}\), \(u_{i+1}\) are the displacement values of the upper and lower layers at the same point (cm); and \(K_{i}\) is the \(i\)-layer bonding coefficient (N/cm\({}^{3}\)). According to Equation (1), the value of the shear stress at the interface is the value of the interlayer bonding coefficient when unit-relative displacement occurs. The larger the value of the interlayer bonding coefficient, the better the adhesion between layers, and the more it tends to be completely continuous. Therefore, it is reasonable and feasible to evaluate the interlayer bonding state of the pavement by using the interlayer adhesion coefficient according to the Goodman model. By determining the average shear stress at the interlayer interface and the relative horizontal displacement at this stress value, the value of the interlayer bonding coefficient can be measured and the interlayer bonding state of the pavement can be evaluated. (2) Experimental model  
The curve of the relationship between shear load and relative horizontal displacement at the interlayer interface of specimens is shown in Figure 3. The relationship curve between the interlayer bond coefficient \(K\) and the relative horizontal displacement is shown in Figure 4.  
It can be seen from Figure 3 that with the increase in relative shear displacement, the shear load on the interlayer interface increases at first, and then decreases gradually to a stable state. According to this phenomenon, we can know that when the interlaminar shear load increases to the ultimate shear strength of the specimen, the interlayer structure is damaged. However, residual shear strength still exists between the layers after interlayer failure. Figure 4 shows that the interlayer bonding coefficient increases to a limit value at first, and then decreases and tends to be stable. Currently, most researchers assume that the stress-strain curve of the pavement interlayer specimen before failure rises in a straight  
Figure 3: The relationship between interlayer shear load and relative shear displacement. line at the initial stage. The slope of the straight line is defined as the initial interlayer bonding coefficient \(K_{0}\). With the rise of the curve, the specimen reaches the ultimate shear strength.