# Interface Shear Performance between Porous Polyurethane Mixture and Asphalt Sublayer
## 5 Results and Discussion  
### Shear Strength of Composite Specim without Adhesive Material  
Figure 3 illustrates shear stress-deformation curves of three composite specimens without adhesive material at \(25\)\({}^{\circ}\)C. The shear deformation was the projection of total vertical deformation in the direction of interface, as shown in Figure 2c. It can be seen that the interface shear stress first increases to the peak value and then decreases with the increase of shear displacement. The composite specimen PPM-10 + AC-20 has the lowest interface shear strength among three composite specimens without adhesive material. This indicates that the bonding between PPM and asphalt mixture is insufficient compared to the interface between two asphalt layers, which may cause delamination between PPM and its underlying asphalt layer.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & **Freeze-Thaw Cycle** & **Thickness of Adhesive Layer (mm)** & **Temperature of Shearing Test (\({}^{\circ}\)C)** \\ \hline Shear test & No & \(0.3,0.5,1.0\) & \(-18,25,60\) \\ Shear test & Yes & \(0.5\) & \(25\) \\ Shear fatigue test & No & \(0.5\) & \(25\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Test matrix of composite specimens.  
The average shear strengths of three replicates of composite specimens without adhesive material are showed in Figure 4. The standard deviation values were shown as the error bar from the averages in the figure. The results show that the interface shear strength of PPM-10 + AC-20 composite specimen is the lowest at both 25 \({}^{\circ}\)C and 60 \({}^{\circ}\)C. This can be caused by two reasons. The first reason is the inter-penetration between the upper and lower layer. For AC-13 + AC-20 and OGFC-13 + AC-20 specimen, the asphalt binder of AC-20 penetrated into the upper layer when AC-13 and AC-20 specimens were compacted at high temperatures, which was different from the case of PPM that was mixed at room temperature. Another reason is that the greater air void of PPM-10 causes the interface contact area smaller than those of the other two composite specimens, resulting in the smaller shear strength. This is similar to the finding reported by previous work, in which it was found that the contact area was an important factor affecting the interface shear strength between OGFC and dense-graded asphalt mixture [32]. The effect of air void on shear strength can be proved by the observation that the interface shear strength between AC-13 and AC-20 is greater than the one between OGFC-13 and AC-20.  
Figure 5 shows the appearance of fractured surfaces after shear testing of composite specimen. It can be noticed that the fractured surface of PPM-10 + AC-20 composite specimen is smooth, which indicates the pure interfacial failure. However, the fractured surfaces of AC-13 + AC-20 or OGFC-13 + AC-20 composite specimen were found much rougher, which indicated that the failure was  
Figure 4: Shear strengths of composite specimen without adhesive materials.  
Figure 3: Shear stress-deformation curves of three composite specimens without adhesive materials.  
not pure interfacial sliding due to interlocking of aggregates and inter-penetration of asphalt binder across the interface. The texture profile of interface can be measured using laser sensor or microscopy if more accurate quantification is needed. The insufficient bond between PPM-10 and AC-20 may cause earlier failure of pavement due to slippage cracking or shoveling. Thus, it is necessary to apply adhesive materials to increase the bonding of PPM with the sublayer.