## 3 Results and Discussion  
### Rheological Properties  
In the first stage of this research, as an essential procedure, the effectiveness of the recycling agents in recovering the changed physical/rheological properties of the aged bitumen was assessed by means of conventional bitumen testing methods. According to the results, which are summarized in the Table 4, as expected, every cycle of aging increased the stiffness of the bitumen, and each recycling action brought about the recovery of the mechanical properties of the aged bitumen. Based on the results, it can also be inferred that all of the tested recycling agents were capable of recovering the fundamental physical properties of the aged bitumen, and no significant difference was recorded between the obtained results. In other words, at this stage, both recycling agents could be classified as fluxing (softening) agents. Bitumen, upon aging, becomes stiffer and more rigid, thereby increasing its transition temperature. In this study, from the rheological point of view, the complex modulus G* which is bitumen's overall resistance to deformation is evaluated in relation to the loss tangent (tan delta). Tan delta is the relationship between the storage (viscous) modulus (G\") and the loss (elastic) modulus (G') of the material. G* is a direct measure of the rigidity of bitumen's soft solid structure when exposed to stresses below the yield stress, and this makes it a good indicator of the flexibility or stiffness of bitumen. With increasing temperature, G' decreases at a faster rate than G\", and so a crossover temperature occurs. This temperature is the temperature that was earlier referred to as the transition temperature [17]. The DSR profiles shown below depict G* in relation to the crossover temperatures of the samples. Higher transition temperature values imply greater stiffness of a sample. The reference samples for this study were virgin bitumen and three other samples that were subjected to multiple aging. This, on one hand, was conducted in order to simulate the realistic multiple aging cycles, and, on the other hand, to compare them to the samples containing recycling agents that were also aged multiple times. As can be seen in Figure 1, with each increasing aging cycle, the transition temperature, which corresponds to the loss tangent, increases significantly. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Penetration** & **Softening** & **Dynamic Viscosity** \\  & **(dmm)** & **Point (°C)** & **(Pa-s)** \\ \hline ID & 25 \({}^{\circ}\)C & - & 100 \({}^{\circ}\)C & 135 \({}^{\circ}\)C \\ \hline Virgin bitumen & 76 & 44.8 & 11.4 & 1.2 \\ \hline RTFO + PAV Bitumen & 51 & 57.5 & 38.8 & 2.6 \\ \hline (RTFO + PAV Bitumen) \(\times\) 2 nd PAV & 38 & 66.5 & 68.5 & 4.0 \\ \hline (RTFO + PAV Bitumen) \(\times\) 3 rd PAV & 28 & 75.8 & 141.6 & 5.7 \\ \hline (RTFO + PAV Bitumen) + R1 & 74 & 46.0 & - & - \\ \hline (RTFO + PAV Bitumen) + R2 & 75 & 44.2 & 6.4 & 0.8 \\ \hline (RTFO + PAV Bitumen + R1) + PAV & 70 & 52.8 & - & - \\ \hline (RTFO + PAV Bitumen + R2) + PAV & 74 & 44.6 & - & - \\ \hline (RTFO + PAV Bitumen + R1 + PAV) + R1 & 72 & 43.3 & - & - \\ \hline (RTFO + PAV Bitumen + R2 + PAV) + R2 & 80 & 41.8 & 8.0 & 0.8 \\ \hline (RTFO + PAV Bitumen + R1 + PAV + R1) + PAV & 78 & 41.4 & 7.3 & 0.8 \\ \hline (RTFO + PAV Bitumen + R2 + PAV + R2) + PAV & 68 & 52.6 & 11.5 & 1.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The physical/rheological properties of test bitumen with and without the recycling agents. Figure 2 shows that both recycling agents are effective, and were able to recover the physical properties by reducing the stiffness of the aged binder. It is interesting to note that R1 even after two aging cycles was able to restore the bitumen almost exactly to its initial unaged state. The R1-modified sample remained the same even after another subsequent final aging cycle. According to Figure 2, R2 was also able to reduce the stiffness of the aged bitumen, even though it did not do this as effectively as R1.