## 2 Experimental Part
### Aggregates Materials: Gradation and Main Properties  
Limestone aggregate with a nominal aggregate size of 12.5 mm was used for both mixtures. The main aggregate properties are listed in Table 2.  
Weighed amounts of various sieve passages were used for the preparation of all the asphalt concretes. This procedure allowed rendering the size distribution as invariant in all our experiments. The particle size distribution is reported in Figure 1 (solid circles): it should be noted that the design aggregate gradation is an average distribution between the upper and lower specification limits (crosses) for surface courses, according to the Italian Standard Specifications [45]. This aggregate \begin{table}
\begin{tabular}{c c c} \hline \hline
**BITUMEN** & **Penetration (dmm)** & **Ring and Ball (\({}^{\circ}\)C)** \\  & **(\(\pm\)1)** & **(\(\pm\)1)** \\ \hline Neat & 66 & 48 \\ RTFOT-Aged & 27 & 59 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Penetration grade and ring and ball test results. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Aggregate Type** & **Test/Index** & **Value** & **Standard** \\ \hline \multirow{4}{*}{Coarse Aggregate} & Aggregate nature & Limestone & - \\  & Los Angeles Abrasion Test [\%] & 21 & EN 1097-2 (2010) \\  & Shape Index [\%] & 26 & EN 933-4 (2008) \\  & Aggregate density [g/cm\({}^{3}\)] & 2.772 & EN 15326 (2009) \\  & Aggregate nature & Limestone & - \\  & Sand Equivalent Test [\%] & 94 & EN 933-8 (2012) \\ Fine Aggregate & Fine Aggregate Angularity Index [\%] & 46 & ASTM C1252 (2006) \\  & Filler Stiffness Power (filler/binder = 1.5) & 8 \({}^{\circ}\)C (\({}^{\circ}\)) & EN 13179-1 (2013) \\  & (\({}^{\circ}\)) AR\&B = Delta ring and ball test (EN 1427:2015) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main properties of coarse and fine aggregates. gradation also meets the limits imposed by the Superpave SHRP [46] which identifies a restricted zone for the rutting problem (gray area): it can be noted that sand content in our mixture falls conveniently below such area. The process of grinding the limestone coarse aggregates produced fine powder which was sieved to select those particles with size <75 \(\upmu\)m. They were used as the filler for asphalt concretes preparation. Preparation of Asphalt Concretes Specimens: Volumetric Analysis by Gyratory Compactor (GC) and Indirect Tensile Strength (ITS)  
Asphalt concrete specimens were prepared by mixing the bitumens described in Section 2.1 with the mineral aggregates described in Section 2.2 (bitumen 4 w/w%; filler 7 w/w%, aggregates 89 w/w%) and compacting the mixtures following the Gyratory compaction method according to the EN 12697-31 (UNI EN 2007), under a constant pressure of 600 kPa, with a rotational speed of 30 rpm, a nominally constant angle of gyration 1.25\({}^{\circ}\) and a temperature of 155 \({}^{\circ}\)C.  
Note that GC is an established practice of dense-volumetric based analysis of asphalt mixes [46]. In the procedure, the volume of the sample within the compactor is continuously measured as a function of time during compaction. From this, knowing the volumetric properties of the individual components (aggregate and bitumen) makes it possible to trace the density variation % (%Gmm) of the mix, as well as other quantities like Void Air (Va), Void in Mineral Aggregates (VMA), voids filled with asphalt (VFA) and other indicators, which are shown in Appendix A together with their physical meaning (CDI, TDI, CFI, TFI). Generally, the compaction criteria by GC focus on the volumetric parameters above mentioned in correspondence to three characteristic points during the compaction effort: an initial (Nini), design (Ndes), and maximum (Nmax) number of gyrations. In all specimens, the bitumen amount was 4 w/w% because this composition maximizes the stability of the final product as found by routine Marshall stability tests (compaction energy of 75 pestle blows for each side of the specimen--EN 12697-30) carried out at various compositions.