# Impact of Preheating Temperatures and RAP Characteristics on the Activation of RAP Binder
## 4 Results  
### Influence of Preheating Temperature  
Figure 3 shows the specimens of three sources of RAP preheated at different temperatures and their air voids. The results show that the air voids of specimens increase significantly with the decrease of preheating temperature regardless of the RAP source. Figure 3a-f shows the specimens of RAP-A preheated at six temperatures from 180 \({}^{\circ}\)C to 30 \({}^{\circ}\)C. As the preheating temperature decreased from 180 \({}^{\circ}\)C to 30 \({}^{\circ}\)C, the appearance of the specimens changed from good to bad. Looseness and spalling appeared in specimens with low preheating temperatures (i.e., 90 \({}^{\circ}\)C, 60 \({}^{\circ}\)C and 30 \({}^{\circ}\)C), which means that the activation effect of RA binder is poor in these conditions. In addition, due to the poor activation effect, specimens of RAP-B and RAP-C cannot be formed at a preheating temperature lower than 120 \({}^{\circ}\)C.  
Figure 3: Specimens of three sources of RAP preheated at different temperatures. (**a**) RAP-A preheated at 180 \({}^{\circ}\)C; (**b**) RAP-A preheated at 150 \({}^{\circ}\)C; (**c**) RAP-A preheated at 120 \({}^{\circ}\)C; (**d**) RAP-A preheated at 90 \({}^{\circ}\)C; (**e**) RAP-A preheated at 60 \({}^{\circ}\)C; (**f**) RAP-A preheated at 30 \({}^{\circ}\)C; (**g**) RAP-B preheated at 180 \({}^{\circ}\)C; (**h**) RAP-B preheated at 150 \({}^{\circ}\)C; (i) RAP-B preheated at 120 \({}^{\circ}\)C; (j) RAP-C preheated at 180 \({}^{\circ}\)C; (**k**) RAP-C preheated at 150 \({}^{\circ}\)C; (**l**) RAP-C preheated at 120 \({}^{\circ}\)C.  
Regarding the preheating temperatures of 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, for all three sources of RAP, no obvious difference was found in the appearance of the specimens at the two temperatures. However, compared with the compaction effect at 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, the compaction effect of all the RAP-B specimens decreased significantly at 120 \({}^{\circ}\)C (Figure 3i) and the increase in air voids was greater than that of RAP-A and RAP-C. Figure 4a-c shows the results for Marshall stability (MS), splitting tensile strength at 15 \({}^{\circ}\)C and tensile strength ratio (TSR) of specimens with 100% RAP. According to the results, it is clear that increasing preheating temperature improves the mechanical properties of specimens. Moreover, the influence of preheating temperature is significant for all the sources of RAP. Regarding the splitting tensile strength, the RAP preheated at 180 \({}^{\circ}\)C presented a high and similar value for all three RAP sources, where the RA binder seemed to be highly activated regardless of the RAP source. Compared to the results at 150 \({}^{\circ}\)C, all the RAP preheated at 120 \({}^{\circ}\)C presented a sharp decrease in MS values and splitting tensile strength, indicating a poor activation effect. It can be seen from this comparison that a large amount of RA binder is still not activated at the preheating temperature of 120 \({}^{\circ}\)C.  
The effect of RA binder activation can be explained in terms of workability and cohesion. On the one hand, high preheating temperature reduces the viscosity of RA binder, resulting in better compaction effect (low air voids). On the other hand, more RA binder is activated by high temperature during the preheating process, increasing the effect of coating and cohesion. Other factors such as the binder stiffness and asphalt content also seem to have an impact on the results. Regarding the TSR results,  
Figure 4: Test results of three RAP sources and three preheating temperatures. (**a**) Marshall stability (MS); (**b**) Splitting tensile strength (15 \({}^{\circ}\)C); (**c**) Tensile strength ratio (TSR). although increasing the preheating temperature improved the activation of RA binder, the TSR value of RAP-B was lower than those for RAP-A and RAP-C. Moreover, large differences in MS values can also be observed when comparing RAP-B and the other two RAP sources.