# Impact of Preheating Temperatures and RAP Characteristics on the Activation of RAP Binder
## 4 Results
### Influence of RA Binder Aging
#### 4.2.2 Evaluation of Binder Activation  
Due to the results shown in the previous section, two preheating temperatures (120 \({}^{\circ}\)C and 180 \({}^{\circ}\)C) were chosen to activate the RA binder, representing the low and high preheating temperature conditions, respectively. The activation of two types (SK70 and SBS) of asphalt binder under three aging conditions (unaged, with 4 h (short-term) aging and with 5 day (long-term) aging) were considered in this section. Considering that the aging of RA binder may have significant impacts on the low-temperature performance of mixture, the Marshall stability test was replaced by the splitting strength test at \(-10\)\({}^{\circ}\)C.  
The air voids as a function of aging and preheating temperature for the two types of mixture (SK70 and SBS) are shown in Figure 10. It can be seen from the results that a higher level of oxidation leads to an increase of air voids regardless of the preheating temperature and binder type. This is because the RA binder with higher stiffness is difficult to soften and contributes less to compaction.  
Figure 6: Gel permeation chromatography (GPC) chromatograms for recovered styrene–butadiene–styrene (SBS) binder based on elution time.  
Figure 7: Percent of large molecules for recovered SBS binder with different aging times.  
Figure 8: Area under carbonyl and saturated C-C peaks in FTIR spectra for recovered SBS binder with different aging times.  
Figure 10: Air voids of specimens with different aging conditions.  
Figure 9: Aging index for recovered SBS binder with different aging times.  
Figure 11a presents the splitting tensile strength results of specimens tested at 15 \({}^{\circ}\)C. It can be seen from the results that when the preheating temperature is 180 \({}^{\circ}\)C, the splitting tensile strength increases with the increase of oxidation level regardless of the binder type. It seems that aging does not show any negative influence on the activation of RA binder preheated at 180 \({}^{\circ}\)C, which also means that preheating temperature has a significant impact on the activation of RA binder.  
The strength of the two types of specimens at 120 \({}^{\circ}\)C showed different trends, where the strength of SK70 increased while the strength of SBS decreased. It seems that the aging of RA binder not only indirectly affects the strength by affecting activation, but also directly affects the strength. However, it was found that the difference in splitting tensile strength between 120 \({}^{\circ}\)C and 180 \({}^{\circ}\)C had a tendency to become larger when the degree of aging of the RA binder increased. For the SK70 binder with no aging (unaged), short-term aging and long-term aging, the splitting tensile strength at 120 \({}^{\circ}\)C was 16.5%, 26.9% and 39.2% lower, respectively, than that at 180 \({}^{\circ}\)C. For the SBS-modified binder, the corresponding percentage decreases were 14.2%, 25.4% and 49.5%, respectively. This would imply that aging has an adverse effect on the activation of RA binder preheated at 120 \({}^{\circ}\)C.  
Similar tendencies were observed from the results of splitting tensile strength tested at -10 \({}^{\circ}\)C and TSR, as shown in Figure 11b,c. Regardless of the asphalt type, the TSR values of the specimens preheated at 180 \({}^{\circ}\)C were relatively high and stable. However, the TSR values at 120 \({}^{\circ}\)C decreased with the increase of oxidation level, indicating the decrease of binder activation.  
Figure 11: Test results of different asphalt types and different aging conditions. (**a**) Splitting tensile strength (15 \({}^{\circ}\)C); (**b**) Splitting tensile strength (\(-\)10 \({}^{\circ}\)C); (**c**) Tensile strength ratio (TSR).