# The Evaluation Method of Particle Clustering Phenomena in RAP
## 5 Conclusions  
The particle clustering phenomena in RAP was one of the most important factors to limit its utilization rate in pavement engineering. In this paper, the clustering characteristics of RAP, such as  
Figure 14: Comparison of edge angle between new and RAP materials.  
Figure 15: Comparison of Sphericity between new and RAP materials.  
the particle composition of each particle size, the clustering degree and the clustering stability, were investigated by the extraction test and crushing test. The following conclusions can be drawn:  
1. The cluster phenomenon in RAP occurred mainly in the coarse part (>4.75 mm), and the clustering degree was more serious as the particle size increased. In terms of the RAP particle composition, the cluster of coarse aggregate (>4.75 mm) was mainly composed of its next particle size and 4.75 mm particle size aggregate.
2. The RAP materials can be divided into three types of structures, namely weak cluster structure, strong cluster structure and old aggregate. The crushing process of RAP can be divided into three stages. The first stage was dominated by weak cluster structure, which was the crushing of the flat, needle-shaped particles; the second stage was dominated by strong cluster structure, which was the crushing of blocky-shaped particles; and the last stage was the crushing of aggregates. It is important to reduce the content of the weak cluster structure, because the clustering degree of the weak cluster structure accounts for the largest part and is more likely to have an adverse effect on gradation variation.
3. The quantitative indexes, loss rate (_PL_ and _CL_) and stability index (_w_), were proved to evaluate the clustering degree, the crushing characteristics and the clustering stability of RAP materials. They provided meaningful references for the comparison of different RAP materials and different crushing processes. Recommendations for reducing the particle clustering phenomena in RAP:  
1. Due to the high content of 4.75 mm particles, as well as the large clustering degree of particles above 4.75 mm, it is suggested that the coarse aggregate, especially particles of 4.75 mm key sieve size, be fully crushed and broken.
2. The weak cluster structure in RAP is very fragile, and had a serious impact on the RAP variability. It is suggested that the formation of weak cluster structure should be avoided and the content of the old aggregate particles should be increased in the process of milling and planning. Separating the weak cluster structure, or subject to secondary breaking, could also reduce the variability of RAP.
3. The method of measuring the proportion of weak cluster, strong cluster and old aggregate in RAP could be used to analyze the advantages and disadvantages of crushing processes. The stability index (_w_) can be used for quantitative comparison.  
Conceptualization, T.M.; methodology, G.X. and T.M.; validation, G.X. and Z.F.; formal analysis and investigation, T.M and G.X.; resources and data curation, G.X. and W.Z.; writing--original draft preparation, Z.F.; writing--review and editing, G.X.; supervision, T.M and X.H.; project administration, T.M.  
This research was funded by the National Natural Science Foundation of China (grant number 51808116 and 51878164), Natural Science Foundation of Jiangsu Province with grant number BK20180404, and Fundamental Research Funds for the Central Universities, grant number 2242018K40003.  
The authors declare no conflict of interest.