## 4 Mix Design of SPS Asphalt Mixture
### Determination of Optimal Fiber Content and Optimal Asphalt Content
#### 4.2.2 The Influence of Filler-Asphalt Ratio on the Performance of SMA-II  
Mineral filler is of vital importance to asphalt mixture. The cohesion between asphalt and aggregate is generated by an asphalt absorbing mineral filler based on a thin asphalt film, that is, the combination of asphalt and mineral filler is the real binder. The interaction between the asphalt and mineral filler was the determining factor of the formation of the asphalt mixture structure, which is directly related to the strength, temperature stability and aging rate of the asphalt mixture [40]. Therefore, it is necessary to study asphalt and mineral filler as a group. In this section, the freeze-thaw splitting test and the low-temperature bend test of SMA-II with a different filler-asphalt ratio were conducted to compare the influence of different filler-asphalt ratios on the water stability and low temperature properties of the mixture. SMA-II-3 without fiber was adopted in this test. Gradations are shown in Table 12:  
Specimens of five gradations in Table 12 were made with the optimal asphalt content, the rest of the results of the freeze-thaw splitting test and the low temperature bend test are shown in Tables 13 and 14. Original splitting strength (\(R_{\rm TI}\)), freeze-thaw splitting strength (\(R_{\rm T2}\)) and Tensile strength ratio (TSR) were calculated based on Equations (A2)-(A4) shown in Appendix B. Flexural tensile strength \(R_{\rm B}\), ultimate tensile strain \(\varepsilon_{\rm B}\) and flexural stiffness modulus \(S_{\rm B}\) were calculated based on Equations (A6)-(A8) shown in Appendix B.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Gradations**} & **Passing Rate of** & **Passing Rate of** & **Effective** & **Filler-Asphalt** \\  & **1.18-mm Sieve** & **0.075-mm Sieve** & **Asphalt Content** & **Ratio** \\ \hline
3-1 & 47.1 & 10.5 & 6.5 & 1.62 \\
3-2 & 47.1 & 10.5 & 7.5 & 1.40 \\
3-3 & 47.1 & 10.5 & 8.5 & 1.24 \\
3-4 & 47.1 & 10.5 & 9.5 & 1.11 \\
3-5 & 47.1 & 10.5 & 10.5 & 1.00 \\ \hline \hline \end{tabular}
\end{table}
Table 12: Gradations with different mineral filler contents.  
Figure 9: The influence of mineral filler content on air voids.  
In Table 13, all TSRs of SMA-II-3 with different filler-asphalt ratios can meet the requirements of Chinese specification JTG F40-2004 [39]. The splitting strength and TSR of all mixtures increased gradually with the decrease of the filler-asphalt ratio. The splitting strength decreases rapidly when the filler-asphalt ratio is 1.6, which indicates a rapid increase of air voids. TSR increases rapidly when the filler-asphalt ratio is 1.0 and all values of TSR are more than 95%, which indicates that the air voids are very small. Hence, the recommended range of filler-asphalt ratio should be from 1.1% to 1.4%.  
In Figures 10 and 11, with the increase of the filler-asphalt ratio, the maximum flexural strain curve of the mixture is a concave decreasing curve, while the flexural stiffness modulus curve is a convex increasing curve. All maximum tensile strains in this test are more than 10,000, which is much larger than the 2800 that is required by Chinese specification JTG F40-2004 [39]. As a matter of fact, when the filler-asphalt ratio is small, because of the existence of too much free asphalt, the flexibility of the mixture enhances, then the bending tensile strain becomes smaller. The increase in the filler-asphalt ratio will result in the decrease of free asphalt, combined with the absorption and reinforcement of fiber, the flexural stiffness modulus increases while the bending tensile strain decreases. To sum up, the recommended filler-asphalt ratio should be from 1.0% to 1.6% from the perspective of low-temperature performance.