# Research on Performance of a Dense Graded Ultra-Thin Wearing Course Mixture
## 2 Experimental
### Test Procedures
#### 2.2.2 Texture Depth Evaluation  
To guarantee the driving safety, high-quality skidding resistance is an important characteristic for asphalt mixtures used in the top surface layer or the wearing course layer of pavement. Thus, the surface texture depth (TD) of asphalt mixture and pavement is usually used to describe the skidding resistance. However, the texture depth (TD) test that followed the standard protocol of Chinese specification cannot reveal the wearing resistance of asphalt mixtures, which is another important characteristic for asphalt mixtures used in the top surface layer or the wearing course layer of pavement. Thus, to reveal the degradation behaviour of surface texture of different asphalt mixtures that can represent their wearing resistance, the texture depth test was combined with the wheel tracking test in this study. The designed test processes were summarized as follows:  
1. After the rectangular specimen with dimensions of \(300\times 300\times 50\) mm was prepared for the wheel tracking test, its original texture depth (TD) named \(K_{0}\) was measured, as shown in Figure 2a.
2. After the silica sands on the specimen surface were washed away, the specimen was submitted to the wheel tracking test for 180 minutes.
3. After the wheel tracking test, the rutting area right underneath the wheel loading positions was cut from the tested specimen and then weighed to get its mass \(m_{1}\), as shown in Figure 2b.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mixture Type** & \begin{tabular}{c} **Asphalt** \\ **Content (\%)** \\ \end{tabular} & **VV (\%)** & **VMA (\%)** & **VFA (\%)** & \
\begin{tabular}{c} **Marshall** \\ **Stability (kN)** \\ \end{tabular} &
\begin{tabular}{c} **Flow Value** \\ **(0.1 mm)** \\ \end{tabular} \\ \hline SMA13 & 6.0 & 4.3 & 18.28 & 76.7 & 8.9 & 26.8 \\ AC13 & 5.1 & 4.8 & 19.90 & 75.9 & 8.52 & 27.4 \\ SMA10 & 6.3 & 4.2 & 18.53 & 77.5 & 10.54 & 28.9 \\ UWM10 & 5.2 & 5.1 & 16.0 & 68.0 & 9.83 & 26 \\ M-UWM10 & 5.2 & 4.7 & 15.6 & 70.1 & 12.00 & 24 \\ \hline \hline \end{tabular}  
* VV: percent air voids in bituminous mixtures; VMA: percent voids in mineral aggregate in bituminous mixtures; VFA: percent voids in mineral aggregate that are filled with asphalt in bituminous mixtures.  
\end{table}
Table 1: Marshall design parameters for different asphalt mixtures.  
4. According to the texture depth test, sands were laid on the surface of the cut strip specimen, as shown in Figure 2\(c\), and then the total mass m\({}^{2}\) for the sands and strip specimen was weighed, as shown in Figure 2d.
5. The texture depth \(K_{1}\) for the cut strip specimen, which represents the rutting area under the wheel loading positions after the wheel tracking test, was determined by Equation (1): \[K_{1}=\frac{(m_{2}-m_{1})}{\rho_{G}\cdot s},\] (1) where \(s\) is the top surface area of the cut strip specimen, and \(\rho_{G}\) is the density of the silica sands.
6. By comparing \(K_{0}\) and \(K_{1}\), the degradation degree of the surface texture depth caused by wheel loading can be obtained.