## 3 Results and Discussion
### Volumetric Analysis by Gyratory Compactor
To take into account for the \"way\" in which this degree of compaction is reached during press rotation, another parameter is considered, the compaction densification index (CDI) whose derivation and meaning is reported in Appendix A. The derived LP and CDI values are reported in Table 6.  
The different behaviors of the two mixtures can be rationalized in terms of differences in efficiency of mechanical energy transfer from the press to the inner structure of the mixture. If this perception is correct, then the number of revolutions of each sample could be scaled down for a factor accounting for this system-specific efficiency in energy transfer to give a unique common master-curve. The coincidence of the two curves is actually found if the abscissa values for the mixture containing the RTFOT bitumen is multiplied by a factor 1.16 (see Figure 4b). This means that each revolution of the gyratory press compacting the RTFOT bitumen/filler/particles has a 1.16 times more efficient compaction effect as compared to the neat bitumen/filler/particles. Interestingly, the same ratio (1.16) is found if the compaction densification indexes and the locking points are considered:  
* being CDI for the neat bitumen = 5,528 and CDI for the RTFOT-aged bitumen = 4,747, their ratio is 5,528/4,747 = 1.16;
* being LP for the neat bitumen = 63 and LP for the RTFOT-aged bitumen = 54, their ratio is 63/54 = 1.16. All these clues however are a geometrical consequence of the fact that the two curves have inherently the same shape as evidenced in Figure 4b where the scaling factor is considered, which further suggests that the physical principle behind the process must be universal. Another important consequence reflects on the so called \"traffic densification index\" (TDI, see Appendix A for its derivation). It is an integral of the curve between LP and a maximum number of revolutions (Nmax), in this study taken as Nmax = 210 [45], so it is considered as indicator of the stability of the compacted mixture under traffic load [61]. However, taking into account that the compaction curves can be x-scaled according to the efficiency of compaction to give a unique curve, also the \"maximum\" number of revolutions should be scaled if comparisons are to be made. Some samples can be more efficient in the compaction process than others so a unique maximum number of revolutions for them means that at the end of the process, the two mixtures will be at different degrees of compaction. With this observation, it can be argued that keeping Nmax as constant renders the derived TDI meaningless from the physical point of view. Indeed, this is not the case for our samples as the ratio of the TDI for bitumen and aged bitumen is 378/451 = 0.84 i.e., markedly different from the value of 1.16. It must be also noted that the TDI value for the neat bitumen is lower than that for RTFOT-aged bitumen, and is an obvious consequence of the fact that the compaction curve of the former is \"in delay\" as compared to that for the latter. Furthermore, these considerations probably constitute the physical basis why, in ref [46], different Nmax (or Nini and Ndes) are considered depending on the level of traffic and the maximum air temperature in operating condition. From this viewpoint, the same considerations can be made about the values indicated of Ni (I = ini, des, max) by [46] which are increased by 10 and 20 revolutions if a modified bitumen by polymers with soft (\(\approx\)3%) or hard (\(\approx\)5%) modification is used, respectively [62]. \begin{table}
\begin{tabular}{c c c c} \hline
**Index** & **Neat Bitumen Mix** & **RTFOT-Aged Bitumen Mix** & **Ratio** \\ \hline Locking point (LP) & 63 & 54 & 1.16 \\ CDI & 5528 & 4747 & 1.16 \\ TDI & 378 & 451 & 0.84 \\ CFI [kN m\({}^{-2}\)] & 24,930 & 21,540 & 1.16 \\ TFI [kN m\({}^{-2}\)] & 65,370 & 70,590 & 0.93 \\ \hline \end{tabular}
\end{table} 
Table 6: Compaction Densification and Compaction Force Indexes.