## 2 Materials and Methods  
The aim of this study is to evaluate the role of the aggregate skeleton gradation on the cracking resistance of asphalt concrete mixtures for thin overlays. For this purpose, the experimental work of this research was performed and discussed according to two main stages: (1) evaluation of the influence of the fine fraction content and (2) assessment of the NMAS. Additionally, the mix design included two different types of asphalt binder. In order to evaluate the effect of the fine fraction content, three aggregate gradations were designed, (see Table 1 below). All gradations were designed with a NMAS of 8 mm. As observed in Figure 1, the designed gradations cover from continuously graded mixes to gap-graded mixes. The same filler content and nature (calcium carbonate) was used in all mixes to reduce variables in this study. In the case of the NMAS assessment four different aggregate gradations were designed as shown in Table 2. Mixture specimens with four different NMAS, ranging from 11.2 mm to 4 mm, and the same fine fraction content were fabricated, Figure 2. Again, the same filler content was used in all mixes. The used coarse aggregates consisted of porfidic aggregates (specific gravity of 2.841 kg/m\({}^{3}\)) while fine aggregates had a calcareous nature (specific gravity of 2.697 kg/m\({}^{3}\)). All aggregates were collected from a local quarry and are being used for paving constructions in many locations. As mentioned, two different types of asphalt binders were selected to manufacture all mixtures: a conventional binder, B35/50, and a polymer modified binder, PMB 45/80-65. Asphalt binder specifications are shown in Table 3. Both asphalt binders are frequently used in the manufacture of mixtures. In the case of study of the fine fraction effect, the bitumen content was 5.0% of the total mixture weight for both types of asphalt binders, while for the NMAS study the bitumen content was 5.5% of the total mixture weight, (see Table 4 below). \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**\% Passing**} & \multicolumn{5}{c}{**Sieve Size (mm)**} \\ \cline{2-7}  & **10** & **8** & **4** & **2** & **0.5** & **0.063** \\ \hline Gradation 1 (F1) & 100 & 91 & 70 & 50 & 22 & 5 \\ \hline Gradation 2 (F2) & 100 & 82 & 40 & 29 & 15 & 5 \\ \hline Gradation 3 (F3) & 100 & 78 & 20 & 16 & 10 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Passing percentage for the designed aggregate gradations for stage 1. Figure 1: Aggregate gradation for stage 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gradation** & **Asphalt Binder** & **Binder Content (\%)** & **Mean Density (kg/m\({}^{3}\))** & **Average Air Void Content (\%)** \\ \hline F1 & PMB45/80-65 & 5.0 & 2.310 & 9.7 \\  & B35/50 & 5.0 & 2.292 & 10.4 \\ \hline F2 & PMB45/80-65 & 5.0 & 2.249 & 11.5 \\  & B35/50 & 5.0 & 2.261 & 11.0 \\ \hline F3 & PMB45/80-65 & 5.0 & 2.074 & 17.2 \\  & B35/50 & 5.0 & 2.079 & 17.0 \\ \hline S1 & PMB45/80-65 & 5.5 & 2.347 & 7.9 \\  & B35/50 & 5.5 & 2.346 & 7.9 \\ \hline S2 & PMB45/80-65 & 5.5 & 2.322 & 8.0 \\  & B35/50 & 5.5 & 2.315 & 8.3 \\ \hline S3 & PMB45/80-65 & 5.5 & 2.259 & 11.0 \\  & B35/50 & 5.5 & 2.265 & 10.8 \\ \hline S4 & PMB45/80-65 & 5.5 & 2.147 & 16.6 \\  & B35/50 & 5.5 & 2.149 & 16.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Properties of asphalt concrete mixtures. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**\% Passing** & \multicolumn{6}{c}{**Sieve Size (mm)**} \\ \cline{2-9}  & **16** & **11.2** & **8** & **5** & **4** & **2** & **0.5** & **0.063** \\ \hline Gradation 1 (S1) & 100 & 80 & 68 & - & 40 & 29 & 15 & 5 \\ \hline Gradation 2 (S2) & & 100 & 78 & - & 40 & 29 & 15 & 5 \\ \hline Gradation 3 (S3) & & 100 & - & 40 & 29 & 15 & 5 \\ \hline Gradation 4 (S4) & & & 100 & 40 & 29 & 15 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Passing percentage for the designed aggregate gradations for stage 2. Figure 2: Aggregate gradation for stage 2. Marshall specimens were manufactured with 50 blows per side.