# Impact of Preheating Temperatures and RAP Characteristics on the Activation of RAP Binder
## 1 Introduction  
With increasing emphasis on the sustainable development of resources, various techniques are attracting much attention in road construction and maintenance, such as hot central plant recycling [1; 2; 3; 4] and full-depth reclamation (FDR) [5; 6]. Full-depth reclamation is a cold technique that blends the existing asphalt layer with underlying base material and stabilizing agent to produce a new base layer, while hot recycling can activate the reclaimed asphalt (RA) and apply RA pavement (RAP) to the production of surface layers. Since the less expensive RA binder can replace a portion of the more expensive virgin binder, the use of RA binder is economical and may also help to reduce a set of environmental burdens [2; 7; 8]. With the rapid rise of asphalt prices [9], there have been several research efforts made towards maximizing the quantity of RAP used in the production of recycled asphalt mixtures (RAM) [10; 11; 12; 13]. Many successful applications incorporating high RAP content have proved the great potential of RAP in road construction [1; 14; 15; 16]. However, there are still some problems associated with the activation of RA binder, namely the treatment of RAP and mixture design. The problem of treatment of RAP is mainly related to the preheating temperature. Considering that the processes of binder aging and binder activation occur simultaneously during the preheating process of RAP, it is generally believed that higher preheating temperature and longer preheating time may cause excessive aging of RA binder. In the production process of RAM, when using a continuous asphalt mixing plant, RAP materials are typically dried and mixed with superheated virgin aggregate in double drums for 60-90 s without preheating [17; 18; 19; 20]. When using an intermittent asphalt mixing plant, a second drying drum is usually used to preheat the RAP before the mixing process. The preheating temperature and time are generally restricted by the current Chinese standards [21], where the preheating temperature should not be higher than 130 \({}^{\circ}\)C and the preheating time should not exceed 2 h. Similar limitations can also be found in the National Cooperative Highway Research Program (NCHRP) Report 452 [16], which states that the preheating temperature should not be higher than 110 \({}^{\circ}\)C. However, it is also found that the preheating conditions are of great significance for the performance and workability of RAM [22]. It is necessary to determine an appropriate preheating temperature during the recycling process. At present, the activation degree of RA binder is not considered in the mixture design, and the state of RAP is generally assumed to be \"black rock\" in cold recycling or \"full blending\" in hot recycling, where 0% or 100% of the RA binder, respectively, is considered to be combined with new materials. Under the \"black rock\" assumption, the RAP is considered to be a kind of aggregate, while full blending assumes that all of the RA binder is activated and can be included in the total binder content of the recycled asphalt mixture. In fact, the RA binder is not completely inactivated or fully activated, but in a state between 0% and 100% activated, as affected by the property of reclaimed materials and processing conditions. In this condition, the inaccurate estimation of RA binder activation may lead to insufficient or excessive amounts of asphalt in the mixture design. Rutting may occur when the asphalt content is higher than expected, while insufficient binder content may lead to fatigue and thermal cracking [23]. When the content of RAP is relatively high, the effect of adding softer virgin binder becomes less evident, and measures must be taken to improve the activation of RA binder, such as using a rejuvenator or warm-mix technology [24; 25; 26; 27]. Therefore, it is necessary to make reasonable assumptions about the degree of RA binder activation and improve the activation of RA binder, so as to improve the blending and obtain reclaimed asphalt mixtures with higher quality.