# The Evaluation Method of Particle Clustering Phenomena in RAP
## 1 Introduction  
The asphalt RAP mixture has an enhanced ductility and a higher strain capacity when compared to the control asphalt mixture. These improved properties are useful in the construction of the road bases and sub-bases [1; 2]. The average RAP content added in the new asphalt mixture has steadily increased in recent years [3]. From an environmental perspective, higher percentages of RAP (e.g., >50%) are being utilized to reduce costs and save natural resources. The demolished old asphalt pavement and waste asphalt mixture will not only occupy a large amount of land resources, but also cause environmental pollution and ecological damage [4]. Currently, the asphalt pavement recycling technology provides an effective way for the disposal of waste asphalt mixture [5; 6; 7]. According to the construction site and temperature, asphalt pavement regeneration technology generally could be divided into four categories: in-plant hot recycling technology, hot in-place recycling technology, in-plant cold recycling technology and cold in-place recycling technology [8; 9; 10]. However, these recycling methods are faced with a common problem. The utilization rate of RAP is low, the performance variability of recycled materials is large, and the road performance thus cannot be fully guaranteed. So as to improve the utilization rate of old materials, a large number of studies have been carried out. [PERSON] et al. used two methods to evaluate asphalt mixtures with high RAP contents based on the performance of fatigue-cracking resistance. It was found that faster fatiguedegradation was observed for the 40% RAP binder and RAP mixture when subjected to repeated loading [11]. [PERSON] studied the glass fiber modified warm mix asphalt mixtures with high RAP content, and improved performance of the WMA mixture was shown due to the glass fiber and higher RAP percent [12]. [PERSON] et al., proposed that an accurate mix design and the selection of adequate binder could overcome the potential drawbacks related to the use of high RAP percentage, given the possibility to produce suitable recycled mixtures [13]. The performance variability of recycled materials cannot be solved fully, and the reason is because there is a great variability of RAP [14; 15; 16; 17]. The particle cluster phenomenon in RAP is one of the most important factors to affect the gradation variability and road performance of regenerated mixtures. Many studies had been carried out to investigate the clustering phenomena in regenerated asphalt mixtures. [PERSON] (2015) contended that the rheology, and in particular the complex modulus, could be used to study the cluster phenomenon in mixtures containing RAP [18]. In [PERSON]'s more recent research, he concluded that the quantity and the quality of virgin aggregates play a significant role in the clusters' formation [19].  
According to related studies, the cluster level of different sources of RAP materials was quite different [20; 21; 22]. It could be affected by the pavement aging degree, material composition, gradation characteristics, milling equipment, milling temperature, pavement humidity, pavement layer and many other factors [23; 24; 25]. If the old asphalt mixtures were not broken fully, it would seriously affect the performance of the regenerated mixture and reduce the utilization rate of the RAP [26; 27; 28]. Therefore, to fully reduce the clustering degree of old asphalt mixtures and improve the utilization rate of old materials, it is necessary to study the clustering status, the fragmentation behavior and the disposal measures of RAP mixtures.  
The objectives of this study are to study the cluster phenomenon in RAP mixtures, including the clustering degree, particle composition in RAP, law of particle breakage, and the stability of the cluster. These quantitative evaluation indexes are suggested with a purpose to put forward some efforts to reduce the cluster phenomenon in RAP materials.