## 2 Influential Factors on the Performance of GSP
### Raw Materials and Admixtures
#### 2.2.1 Aggregate Gradation  
Open-grade aggregate is designed primarily as a skeleton of OAC for GSP. It mainly maintains compressive pressures from traffic loads and forms inter-air voids of OAC. Therefore, a good gradation can not only provide a good compressive strength but also construct an even and interconnective space of void. An optimal gradation mostly can be realized by two design methods: an experiential method and a volumetric method called the main aggregate filling (MAF) method. The latter assumes coarse aggregate is supporting the main structure of OAC and can achieve the required void volume by filling fine aggregate and binder into the compacted coarse aggregate. Due to back-calculation of the void volume, this method is concise to control the final void rate of OMA [43; 44]. However, interconnectivity of void is overlooked in this method, which is another important factor for permeability of OAC equally to void volume [45]. Therefore, factors affecting particle sizes and consecutiveness of aggregate gradation are considered to establish the relationships between the given characters above, illustrated in Figure 2, Tables 2-4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gradation** & **Density [g/cm\({}^{3}\)]** & **Relative Density** & **Void [\%]** & **Permeability Rate** \\ \hline Gradation A & 1.83 & 2.46 & 25.57 & 1.34(10\({}^{-3}\) [m/s]) \\ Gradation B & 1.85 & 2.46 & 24.85 & 1.11(10\({}^{-3}\) [m/s]) \\ Gradation C & 1.87 & 2.46 & 23.78 & 0.94(10\({}^{-3}\) [m/s]) \\ BSI-4\% & N/A & N/A & 33.05 & 362.9(m/day) \\ Densiphalt12-4.5\% & N/A & N/A & 32.08 & 247.68(m/day) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effects of gradations on permeability of OAC [46; 47]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Gradation** & **Cantabro Loss [\%]** & **Indirect Tensile Strength Test [kPa]** \\ \hline BSI-4\% & 33.2 & 113.35 \\ Densiphalt12-4.5\% & 34.91 & 136.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Effects of gradations on physical properties of OAC [47]. The main coarse aggregate used in GSP normally falls in a range of 4.75 mm-13.2 mm particle size. Several combinations of different tiers in this range can form OMA with the requested void rate. Under the similar void rate, gradation with a large particle size will benefit the whole structure. Moreover, the water permeability of samples which have a large proportion in larger particles (14\(\sim\)10 mm) is significantly higher than that of average samples (Table 2) [46]. A lower residual void rate of GSP will be obtained after grouting [50; 51]. Overall, a coarser aggregate gradation can be more conducive to forming a larger void space, which has better interconnection, to make the grouted cement mortar more easily able to penetrate the OMA mat [46; 52]. Subsequently, flexural tensile strength and compressive strength of the following GSP are also be improved, which are positively linked with low-temperature performance and high-temperature performance, respectively [49]. On the contrary, some other properties may be slightly harmed with the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Gradation**} & \multicolumn{3}{c}{**Compressive Rebound**} & \multicolumn{3}{c}{**Low Temperature Bending Test**} \\ \cline{2-7}  & **Compressive** & **Robound** & **Maximum** & **Flexural** & **Maximum** & **Stiffness** \\  & **Strength [Mpa]** & **Modulus [Mpa]** & **Load [N]** & **Strength [Mpa]** & **Strain [\(\mu\)\(\varepsilon\)]** & **Modules [Mpa]** \\ \hline Continuous & 4.66 & 1913.48 & 802.30 & 6.26 & 1140 & 5483.77 \\ Uniform & 5.23 & 2812.60 & 866.48 & 6.73 & 1040 & 6486.53 \\ Composite & N/A & N/A & N/A & 6.95 & 1838 & 3780.6 \\ gradation & & & & & & \\ Upper limit & N/A & N/A & N/A & 6.73 & 1943 & 3467 \\ Lower limit & N/A & N/A & N/A & 7.57 & 1523 & 4970.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Effects of gradations on physical properties of GSP [48; 49]. Figure 2: Some used gradations with different particle sizes and proportions. increase of particle size.