# The Evaluation Method of Particle Clustering Phenomena in RAP
## 4 Results and Discussion
### Comparison and Analysis of Weak Cluster Structure, Strong Cluster Structure and Old Aggregate  
In this paper, the particle size of 16-19 mm in RAP 2 was used to further analyze the three kinds of structures in RAP, including the proportion, the clustering stability and the clustering degree. The proportion of three kinds of structures is shown in Table 2. The results of the crushing test and extraction test are shown in Figures 9 and 10 respectively.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Revolutions** & \(w\) \\ \hline  & 50 r & 53.59\% \\  & 100 r & 42.54\% \\
**RAP 1** & 200 r & 36.64\% \\  & 300 r & 31.85\% \\ \hline  & 50 r & 65.09\% \\  & 100 r & 57.01\% \\
**RAP 2** & 200 r & 47.98\% \\  & 300 r & 41.23\% \\ \hline \end{tabular}
\end{table}
Table 1: The Stability Index (\(w\)) of Two Kinds of RAP Materials.  
Figure 8: Three Different Cluster Structures of RAP.  
As shown in Table 2, it can be found that the content of strong cluster structure was the highest, followed by the weak cluster, and the content of old stones was the least. Figure 9 shows that the loss rate of the weak cluster structure increases fastest, and the loss rate was the largest; followed by the strong cluster structure; the loss rate of the old aggregate particles was least. This indicates that the stability of the weak cluster structure was worst, easily broken. In contrast, the strong cluster structure was relatively stable, but would experience a greater loss under a full effect of external forces. The old aggregate particles were most stable, mainly corner damage could be expected.  
Figure 10 shows that when the sieve size was above 16 mm, the percentage retained of weak cluster structures was 0. The weak cluster structure was mainly composed of 4.75 mm and below fines, and the content of mineral powder was high. This indicated that the clustering degree of the weak cluster structure was the largest, and this is likely to have an adverse effect on gradation variation. According to the result of extraction test, the loss rate of the strong cluster structure was larger, and it mainly affected the content of coarse particles. For the old aggregate, there is no loss and it thus had little effect on gradation variation.  
Therefore, from the comparative analysis of these three kinds of structures, it can be found that the order of influence on the gradation variability of the RAP was: weak cluster structure \(>\) strong cluster  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Number** & **1** & **2** & **3** & **4** & **5** & **Average** \\ \hline Weak cluster structure & 36.06\% & 36.61\% & 33.10\% & 38.13\% & 34.03\% & 35.58\% \\ Strong cluster structure & 45.29\% & 42.41\% & 48.53\% & 41.43\% & 43.66\% & 44.27\% \\ Old aggregate & 18.65\% & 20.98\% & 18.37\% & 20.44\% & 22.32\% & 20.15\% \\ \hline \end{tabular}
\end{table}
Table 2: The Proportion of Three Kinds of Structures (16–19 mm).  
Figure 10: The Percentage Retained of Three Kinds of Structures after the Extraction Test.  
Figure 9: The Crushing Loss Rate of the Three Kinds of Structures.  
structure > old aggregate. Therefore, to reduce the cluster phenomenon and reduce the variability of RAP, the formation of the weak cluster structure should be avoided as much as possible during crushing process at first. It is important to reduce the content of strong cluster structures, and to increase the content of old aggregates. It is suggested that the nonlinear viscoelastic response of RAP mixtures at high temperatures should be considered in the future investigations [31]. The particle packing ability before and after crushing for different types of RAP should be considered to provide a better understanding of the effect of particle clustering on the overall performance of the mixture in the future study [32; 33; 34].