## 1 Introduction  
With the aging of asphalt pavement, large quantities of reclaimed asphalt pavement (RAP) are produced. Virgin bitumen and aggregate materials are non-renewable, and their costs have significantly increased over the past few years; as a result, RAP needs to be recycled more effectively to reduce mixture costs and negative environmental impacts. At present, most agencies permit the use of only 10-25% RAP in recycled asphalt mixtures (RAM), which greatly contributes to a low RAP utilization rate and directly diminishes the economic benefits of RAM. Hence, increasing the RAP ratio in RAM would be significantly helpful for promoting the application of RAP; moreover, satisfying the performance requirements of RAM with a high RAP content is a critical issue that needs to be addressed [1; 2; 3; 4; 5]. Most highway agencies assume that the blending state of virgin and RAP binders in RAM is complete, but this assumption is not applicable to RAM with high RAP contents. To date, many studies have explored the effects of high RAP contents on RAM [6; 7; 8; 9; 10; 11]. [PERSON] et al. evaluated the cracking resistance of RAM, the results of which indicated that the inclusion of RAP generally increased the stiffness and indirect tensile strength of the mixture. Moreover, the mixture properties changed significantly at a RAP content of 30% compared with those at RAP contents of 10% and 20% [12]. The studies indicated that RAM were more susceptible to various modes of cracking (i.e., fatigue, thermal, and reflection cracking) with an increase in the RAP content, likely resulting in a lower resistance to cracking and the occurrence of moisture damage. The primary reason is that the RAP binder is stiffer than virgin bitumen [11], and the different viscosities of their compositions leads to poor blending between virgin and RAP binders during the mixing process. Because the virgin and RAP binders do not blend fully, relatively weak regions are present in the final product; as a result, the RAM exhibits a reduced water stability, a low-temperature crack resistance, and a short fatigue life [13; 14]. Therefore, to improve the performance of RAM and increase the RAP content of it, it is necessary to evaluate the blending degree of virgin and RAP binders. Many studies have explored the blending state of virgin and RAP binders in pure bitumen; these investigations were conducive to promoting research on the blending degree of virgin and RAP binders [15; 16; 17; 18]. For example, [PERSON] et al. quantitatively explored the blending degree of virgin and RAP binders in each recycled binder layer; the blending degree was extracted from RAM using Fourier transform infrared (FTIR) spectroscopy and gel-permeation chromatography (GPC) [19]. However, due to the influences of aggregates, fibers and other materials, the analysis of blending state of virgin and RAP binders in RAM is difficult compared to that of pure bitumen [20; 21]; hence, the use of tracer is a viable approach to distinguish virgin bitumen from the RAP binder, and to explore the blending degree of virgin and RAP binders in RAM. [PERSON] et al. used X-ray computed tomography to qualitatively analyze the blending state of virgin bitumen containing metal powder in the simulated asphalt mixture and found that the virgin and RAP binders were only partially mixed [22]. Carboxyl-terminated butadiene acrylonitrile (CTBN) was used as the tracer in a previous study to mark virgin bitumen and track its movements [23], and the blending degree of virgin and RAP binders in RAM was investigated qualitatively using FTIR. The results indicated that the blending states of virgin and RAP binders depends on the location of RAP and is not uniform hierarchically across the three sample layers; however, the blending degree was not clearly understood. Thus, a quantitative analysis is necessary to determine the appropriate virgin bitumen content and mixing parameters for the production of RAM.