# Impact of Preheating Temperatures and RAP Characteristics on the Activation of RAP Binder
## 1 Introduction
The efficient utilization of RAP has become a key concern in recent years, and questions mainly revolve around how much RA binder can be activated and participate in the blending between RA binder and virgin binder during the recycling process. The degree of binder activity (DoA) and degree of blending (DoB) are two indicators used by many scholars to quantify the utilization effect of RA binder [18; 19; 28; 29; 30; 31; 32; 33]. The concept of DoA proposed by [PERSON] et al. [29] is an intrinsic property of RAP and is only affected by RAP components and processing conditions. It is detailed as \"an index indicating the minimum percentage of active RA binder that a designer can consider for a selected RAP and a selected asphalt manufacturing process\", and should be determined before incorporating virgin binder, virgin aggregate and rejuvenator. Previous studies mainly focused on the DoB [34; 35], but in recent years, scholars have turned their attention to DoA, trying to better understand the activation mechanism of RA binder, assessing DoA through mechanical blending and numerical simulation techniques [36] and evaluating the potential environmental impacts of improving the activation of RA binder. [PERSON] et al. [37] developed a procedure called coating study to estimate how much RA binder will transfer to virgin aggregates during the mixing process. Without addition of a rejuvenator, the RA fine particles were blended with virgin coarse aggregate particles at the mixing temperature of 190 \({}^{\circ}\)C for 3 min. Then, the fine particles and coarse particles were separated using a \"threshold\" sieve, and the amount of RA binder transferred from RA to virgin aggregate could be determined. It was found that the DoA of the RA binder was approximately 11% when only using mechanical action. [PERSON] developed the cohesion test [38] to study the activation of RA binder from different sources. In the cohesion test, the RAP was preheated at different temperatures (20 \({}^{\circ}\)C, 75 \({}^{\circ}\)C and 100 \({}^{\circ}\)C) for 4 h and then mixed at the same temperature. Finally, the Marshall compaction method was used to compact specimens with 100% RAP. The indirect tensile strength (ITS) results of specimens tested at 25 \({}^{\circ}\)C were used as an indicator to evaluate the activation of RA binder. Compared with the inactivated RA binder, it is believed that the activated RA binder will result in higher ITS values. In addition, the preheating process softens the RA binder and results in lower air voids, which also improves the strength of specimens. This procedure provides a useful method of assessing the activation of RA binder, which is also easy to perform. [PERSON] et al. [28] proposed a method to quantify DoA based on the cohesion test. In this method, 100% RAP specimens were produced following the procedure of the cohesion test. Meanwhile, an artificial RA binder with similar characteristics to the original RA binder was prepared using the high-shear mixer aging method [39]. The artificial RA binder was then mixed with virgin aggregate to produce a control mixture, assuming a situation where the RA binder was 100% activated. By comparing the ITS results of the 100% RAP mixture and the control mixture, it was possible to estimate the DoA. Different preheating temperatures (70 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, 140 \({}^{\circ}\)C and 170 \({}^{\circ}\)C) and mixing times (30 s, 60 s, 90 s and 180 s) were considered, and the results showed that the ITS value and energy pre-peak had good correlations with temperature. In addition, the DoA analysis was found to be a useful tool for quantifying the degree of RA binder activation and improving the mix design. [PERSON] et al. [40] conducted laboratory experiments and discrete element method (DEM) simulations to investigate the mixing behavior between superheated virgin aggregate and RAP materials.