## 3 Results and Discussion
### Volumetric Analysis by Gyratory Compactor  
In a gyratory compactor, the bitumen, the filler, and the macro-sized particles are homogenized and compacted through continuous stirring of the mixture allowing elimination of voids. In the procedure, the volume of the sample within the compactor and the shear resistance are continuously measured as a function of time (or number of gyrations). The volumetric data, in function of gyrations allowing the reconstruction of curves for the following main parameters:  
- Density variation (%Gmm);  
- Air voids content (%Va);  
- Voids in mineral aggregate (%VMA);  
- Voids filled with asphalt (%VFA). They are reported in Tables 4 and 5 and Figure 4a for the preparation of asphalt concrete specimens with neat and RTFOT-aged bitumens. Apparently, the values shown in Tables 4 and 5 describe the two concrete asphalt mixes as overlapping. In the real sense, the focus of this study is concentrated between Nini and Ndes where the real workability of the mixes is manifested and has a major effect on compaction attitude. From these curves, and also from the shear resistance values as a function of gyrations, important indicators can be derived. They are described in Appendix A. Among these, in this study, the locking point (LP) is of particular interest and is defined as the number of gyrations beyond which asphalt mixture compaction and the aggregate structure become stable. Beyond this point, further compaction does not contribute much to the increase in mixture density and can even damage aggregate particles [60]. In fact, in terms of air voids (Va) for the two mixtures, Tables 4 and 5 show that from N\({}_{\text{ini}}\) to N\({}_{\text{LP}}\) there is a decrease of about 8 percentage units corresponding to an increase in the number of gyrations of 53 (for Neat Bitumen) and 44 (for Aged Bitumen), respectively; while from N\({}_{\text{LP}}\) to N\({}_{\text{max}}\), it is evident that the number of gyrations approximately triples to obtain the same decrease of Va (approximately 8 percentage units). In this study, the locking point is assumed to be the number of gyrations required to obtain 92% (%Gmm) of the \"ideal\" compaction maximum. The different compaction grade (as a result of a possible different workability) of each system allows the reaching of LP at a different number of gyrations so the LP is system-specific (see Figure 4a showing graphically the derivation of locking point). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Neat Bitumen**} \\ \hline
**Number of** & \multirow{2}{*}{**\%Gmm**} & \multirow{2}{*}{**\%VMA**} & \multirow{2}{*}{**\%VFA**} & \multirow{2}{*}{**\%Va**} & **\%Va** \\
**Gyrations** & & & & & **Acceptance Thresholds [45]** \\ \hline Nini = 10 & 84.0 & 26 & 38 & 16 & 11–15 \\ N\({}_{\text{LP}}\) = 63 & 92.0 & 19 & 57 & 8 & - \\ Ndes = 120 & 94.3 & 17 & 66 & 6 & 3–6 \\ Nmax = 210 & 96.1 & 15 & 74 & 4 & \(\geq\)2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Density variation and air voids content at different gyration levels for neat bitumen asphalt. Figure 4: Gmm% as a function of the number of gyrations. Panel (**b**) shows he same data of panel (a) with scaled x-values. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**RTFOT-Aged Bitumen**} \\ \hline
**Number of** & \multirow{2}{*}{**\%Gmm**} & \multirow{2}{*}{**\%VMA**} & \multirow{2}{*}{**\%VFA**} & \multirow{2}{*}{**\%Va**} & **\%Va** \\
**Gyrations** & & & & & **Acceptance Thresholds [45]** \\ \hline Nini = 10 & 84.9 & 25 & 39 & 15 & 11–15 \\ N\({}_{\text{LP}}\) = 54 & 92.0 & 19 & 57 & 8 & - \\ Ndes = 120 & 94.9 & 16 & 68 & 5 & 3–6 \\ Nmax = 210 & 96.5 & 15 & 76 & 3 & \(\geq\)2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Density variation and air voids content at different gyration levels for bitumen RTFOT asphalt. It must be noted that a lower LP value means better workability since a lower number of gyrations are required to obtain the same degree of compaction.