## 1 Introduction  
The use of reclaimed asphalt pavement has contributed to saving a great amount of asphalt and aggregates and alleviating the problem of resource waste [1], land occupation and secondary pollution, which allowed this technology to achieve rapid development in last two decades [2]. As early as the end of last century, reclaimed asphalt pavement (RAP) was utilized for weaker pavement layers such as the base or sub base [3]. Generally, RAP contents from 10% to 30% by mass of the mixture are considered to be adequate to produce asphalt mixtures with performance similar to conventional mixtures [4; 5; 6]. Rejuvenators, additives and warm mix asphalt technology are usually employed to improve the effective contribution of RAP binder towards the total binder of the mixture [7; 8; 9]. However, the use of RAP is at a very low level in many developing countries, for there are no relevant and unified recycling guidelines. Since the key to recycling old asphalt pavement is the rejuvenation and rembilization of asphalt binder [10; 11], it would be of great significance to establish a RAP evaluation guideline based on the condition of the reclaimed asphalt binder (RAB). The classification of RAP materials is the basis of its efficient utilization. Today however, arbitrary stockpiling of RAP is still a common practice from the lack of standards or specifications [12; 13], especially in Asia and Africa. Different pavement layers are paved with different asphalt binders, such as conventional base asphalt, lake or rock asphalt, polymer modified asphalt (PMA), fiber asphalt and so on [14]. Recycling of unclassified RAP may lead to many difficulties, depending on the aging characteristics of different RAP binders. The types of binders, pavement layers, service life and position of lanes should be taken into account in milling and recycling. The aging of asphalt binder is a long-term oxidation process. According to SARA (saturate, aromatic, resin and asphaltene) analysis for binder aging [15; 16], oxidative polymerization reaction causes mutual transformation of different components. The lighter aromatics in the binder are transformed into colloids while the polar aromatics into asphaltenes. This chemical process finally leads to a stiffer asphalt binder [17; 18]. Most researchers have used accelerated laboratory aging to study the composition and property changes of asphalt binder during aging process. [PERSON] et al. proposed critical aging viscosity as the aging grade index. They divided aging grades according to the relationship between the viscosity of the aged binder and the critical aging viscosity [19]. [PERSON] divided aging asphalt into three grades by penetration, namely, mild aging, moderate aging and severe aging [20]. It was also found that the penetration of asphalt with severe aging could be restored by adding a rejuvenator and modified asphalt, moderately aged asphalt by adding just a rejuvenator and mildly aged asphalt by only adding some virgin asphalt. [PERSON] et al. put forward that the short-term aging simulation should conducted at the equiviscous temperature instead of the standard temperature of 163 \({}^{\circ}\)C to overcome the non-homogeneous distribution of the binder on the glass [21]. A number of studies have been conducted to evaluate the aging degree of recycled virgin asphalt and modified asphalt, including SARA analysis, softening point, penetration, ductility, Brookfield viscosity and so on. However, this work has not been put towards developing of quantitative aging grades of RAP binder. Most studies have simulated aging virgin asphalt binder, but not field sourced reclaimed asphalt binder (RAB). In China, as in most other places, there are no uniform standards for using RAP. PMA has been widely used in asphalt pavement [22; 23; 24], but scholars mainly focused on the study of aging base asphalt RAP.