# Effect of Mixture Design Parameters of Stone Mastic Asphalt Pavement on Its Skid Resistance
## 2 Materials and Methods
### Mixture Design  
There were four types of SMA with different nominal maximum sizes (NMAS) used in this paper, namely SMA-16, SMA-13, SMA-10, and SMA-5. The Marshall mix design procedure was used for mixture design. The target air voids (VV) were 3.5% and the optimal asphalt content (OAC) could be calculated according to Chinese standard JTG F40-2004 [31]. The OAC of SMA-16, SMA-13, SMA-10, and SMA-5 were 5.7%, 6.3%, 6.5%, and 6.6% respectively. For SMA pavement, the percent air void in coarse aggregates (VCA) must be verified. There is a controlled sieve size (P\({}_{\text{CS}}\)) in this process. According to Chinese standard JTG F40-2004, P\({}_{\text{CS}}\) with both SMA-16 and SMA-13 are 4.75 mm, P\({}_{\text{CS}}\) with SMA-10 and SMA-5 are 2.36 mm and 1.18 mm respectively. P\({}_{\text{NMSA}}\) and P\({}_{\text{CS}}\) were key factors for SMA gradation design [32]. Moreover, surface texture is affected by AC [8]. Therefore, P\({}_{\text{NMSA}}\), P\({}_{\text{NMSA-1}}\), P\({}_{\text{CS}}\), P\({}_{\text{CS-1}}\), and AC were selected for mixture design factors. Each factor had four levels. The mixture design is multi-factorial and multi-leveled. When there are more than three factors, there will be many experiments, thereby making our objectives hard to achieve. Thus, the orthogonal experimental design (OED) was chosen. OED selects some representative points from the comprehensive experiments based on the orthogonality. These representative points have the characteristics of \"uniform and dispersion, neat and comparable\". Moreover, OED is the main method of the factorial design, and is a highly efficient, fast, and economical experimental design method. In this paper, OED was applied to analyze the effect of mixture design parameters on the skid resistance of SMA pavement, and orthogonal design table L\({}_{16}\)(4\({}^{5}\)) was selected. The value of four parameters (P\({}_{\text{NMSA}}\), P\({}_{\text{NMSA-1}}\), P\({}_{\text{CS}}\), and P\({}_{\text{CS-1}}\)) associated with gradations was determined according to gradations ranges in Chinese standard JTG F40-2004. The values of AC were selected according to OAC. Tables 5-8 show the factors and levels of different SMA types. Table 9 is the orthogonal experimental design table with five factors and four levels for each factor. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Items** & **Unit** & **Value** & **Technical Requirements [30]** & **Specification [31]** \\ \hline Apparent relative & — & 2.719 & \(\geq\)2.50 & T0352 \\ density & \% & 0.2 & \(\leq\)1.0 & T0352 \\ \hline Grain sizes \textless{}0.6 mm & \% & 100.0 & 100 & \\ \textless{}0.15 mm & \% & 96.0 & 90–100 & T0351 \\ \textless{}0.075 mm & \% & 89.3 & 75–100 & \\ \hline Hydrophilic coefficient & — & 0.69 & \(\leq\)1 & T0354 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Physical properties of mineral filler. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Level i** & **A** & **B** & **C** & **D** & **E** \\ \cline{2-5}  & **P\({}_{\text{NMSA}}\)/\%** & **P\({}_{\text{NMSA-1}}\)/\%** & **P\({}_{\text{CS}}\)/\%** & **P\({}_{\text{CS-1}}\)/\%** & **AC/\%** \\ \cline{2-5}  & **16 mm** & **13.2 mm** & **4.75 mm** & **2.36 mm** & \\ \hline
**1** & 90 & 65 & 20 & 15 & 5.6 \\ \hline
2 & 93 & 71 & 24 & 18 & 5.9 \\
3 & 96 & 78 & 28 & 21 & 6.2 \\
4 & 100 & 85 & 32 & 24 & 6.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The factors and levels of stone music asphalt (SMA)-16. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Level i**} & **A** & **B** & **C** & **D** & **E** \\ \cline{2-5}  & **P\({}_{\text{NMSA}}\)/\%** & **P\({}_{\text{NMSA-i}}\)/\%** & **P\({}_{\text{CS}}\)/\%** & **P\({}_{\text{CS-1}}\)/\%** & **AC/\%** \\ \cline{2-5}  & **13.2 mm** & **9.5 mm** & **4.75 mm** & **2.36 mm** & \\ \hline
**1** & 90 & 50 & 20 & 15 & 5.6 \\ \hline
**2** & 93 & 58 & 24 & 18 & 5.9 \\
**3** & 96 & 66 & 29 & 22 & 6.2 \\
**4** & 100 & 75 & 34 & 26 & 6.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The factors and levels of SMA-13.