## 3 Results and Discussion
### Results of Advanced Performance-Related Characterization Tests
#### 3.3.2 MSCR Tests  
Results obtained from MSCR tests carried out with a stress of 3.2 kPa are plotted in Figures 9 and 10, which show the values of percent recovery (R\({}_{3,2}\)) and non-recoverable creep compliance (J\({}_{\text{nr3.2}}\)), respectively, as a function of temperature. Represented data refer to binders considered in RTFO-aged conditions.  
As expected, polymer modification enhanced the elastic response of the base bitumen with a remarkable increase in R\({}_{3,2}\) and with a significant reduction in J\({}_{\text{nr3.2}}\). These effects increased with  
Figure 8: Ageing indexes.  
Figure 10: Results of MSCR tests—non-recoverable creep compliance at 3.2 kPa (RTFO-aged binders):  
Figure 9: Results of Multiple Stress Creep Recovery (MSCR) tests—percent recovery at 3.2 kPa (RTFO-aged binders): (a) SBR-modified binders; (b) PBR-modified binders.  
polymer dosage and were also found to be dependent upon polymer type and quality. When considering the R\({}_{3.2}\) plots, no specific ranking could be identified with respect to these last two factors. However, analysis of the Jnr3.2 data suggests that, in the case of the SBR polymers, the premium quality and waste products were almost equivalent, while a non-negligible difference occurred when comparing the two PBR products. In particular, the w-PBR polymer proved to be more effective than the corresponding premium quality product, with lower values of permanent strain occurring under repeated loading.  
Obtained results were consistent with the findings of other researchers, who reported a significant enhancement of the high-temperature characteristics of SBR-modified binders [9; 12]. Nevertheless, it should be mentioned that such effects can be strongly dependent upon the specific characteristics of employed polymers and productions technologies, since other investigators found only marginal improvements in similar conditions [8; 10; 11].  
A synthetic assessment of MSCR test results was carried out by referring to the grading criteria described in AASHTO M332-14, which identify limiting values of Jnr3.2 for different traffic levels. These values are fixed at 4.5 kPa\({}^{-1}\), 2.0 kPa\({}^{-1}\), 1.0 kPa\({}^{-1}\), and 0.5 kPa\({}^{-1}\) for traffic levels indicated as \"S\" (standard), \"H\" (heavy), \"V\" (very heavy), and \"E\" (extremely heavy), respectively. Thus, temperatures at which these values are reached, referred to as high critical temperatures (T\({}_{\text{c}}\)), were calculated for all binders. Corresponding results are listed in Table 6. For comparative purposes, Table 6 also contains the values of test temperatures at which the G*/sin\(\delta\) parameter was found to be equal to 1.0 kPa for samples tested in their unaged state (as per AASHTO M332-14). Such an assessment was based on the results of the SAOS tests discussed in Section 3.3.1.  
Evaluation of the temperature values reported in Table 6 confirmed the comments made when discussing the plots presented in Figures 9 and 10. The two types of polymer yielded similar variations of the high critical temperatures, which were found to be between 1.7 \({}^{\circ}\)C and 5.4 \({}^{\circ}\)C. Greater changes were recorded when considering the T\({}_{\text{c}}\) values associated to the threshold value of G*/sin\(\delta\).