# Durability Evaluation Study for Crumb Rubber-Asphalt Pavement
## 3 Results and Discussion  
The durability evaluation method proposed herein was verified using the following laboratory tests. In order to verify the effectiveness of the evaluation method mentioned above, this study used the standard method and the method proposed in this study to investigate crumb rubber-asphalt mixture (CR-reinforced SMA-16) [52] mixed with five different asphalts [36; 53]. The following test results mainly refer to the three aspects of durability evaluation mentioned in Section 2.2. (1) The results of the water-boiling test for adhesion evaluation of asphalt and coarse aggregates are shown in Table 4 and Figure 5.  
Table 4 shows that 1-5 characterize the adhesion grade corresponding to the degree of peeling off of the bitumen film from the aggregate surface after boiling. The determination of the adhesion grade was carried out by referring to the T 0616 method [46] in \"Specifications and Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering \". As summarized in Table 4, when the boiling time reaches 10 min, the adhesion grade of rubber and stone particles begins to differ. Extending the boiling time to 20 min causes little change in the asphalt film on the surface of the stone material, but causes significant detachment of the asphalt film from the surface of the rubber crumbs. Figure 5 shows that the adhesion of stone and rubber particles to all of the asphalts can reach grade 4 or higher at 3 min of boiling. Therefore, the method in the specification (boiling for 3 min) cannot effectively evaluate the adhesion of rubber particles. The test results prove that the proposed adhesion evaluation method for asphalt and crumb rubber coarse aggregate is intuitive and effective. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Aggregate** \\ **Type** \\ \end{tabular} } & \multirow{2}{*}{**Asphalt Type**} & \multirow{2}{*}{**First-Time**} & \multicolumn{2}{c}{**3 min (T 0616)**} & \multicolumn{2}{c}{**10 min (D 3625)**} & \multicolumn{2}{c}{**20 min**} \\ \cline{3-8}  & & & **Allhesion** & **Peeling** & **Adhesion** & **Peeling** & **Adhesion** & **Peeling** \\  & & **Time (min)** & **Grade [46]** & **Ratio (\%)** & **Grade** & **Ratio (\%)** & **Grade** & **Ratio (\%)** \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} Stone \\ aggregate \\ \end{tabular} } & SK-70 & 2 & 4 & 7 & 3 & 18 & 2 & 48 \\ \cline{2-8}  & SBS & 4 & 5 & 1 & 4 & 7 & 3 & 15 \\ \cline{2-8}  & TPS (12\%) & 7 & 5 & 0 & 5 & 2 & 4 & 9 \\ \cline{2-8}  & TPS (13.5\%) & 9 & 5 & 0 & 5 & 2 & 4 & 8 \\ \cline{2-8}  & TPS (15\%) & 9 & 5 & 0 & 5 & 1 & 4 & 8 \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Rubber \\ particles \\ \end{tabular} } & SK-70 & 2 & 4 & 8 & 3 & 25 & 2 & 63 \\ \cline{2-8}  & SBS & 2 & 4 & 6 & 3 & 19 & 2 & 41 \\ \cline{2-8}  & TPS (12\%) & 5 & 5 & 0 & 4 & 9 & 3 & 19 \\ \cline{2-8}  & TPS (13.5\%) & 5 & 5 & 0 & 4 & 9 & 3 & 17 \\ \cline{2-8}  & TPS (15\%) & 6 & 5 & 0 & 4 & 8 & 3 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of water-boiling test. Figure 5: _Cont._  
(2) Asphalt and fine aggregate adhesion evaluation by the boiling-sieving method:  
The values recorded in the boiling-sieving test are listed in Table 5. Referring to the blended proportions of raw materials, \(a_{1}\) is 80% and \(a_{2}\) is 10%. The crumb rubber-asphalt mixture using SBS-modified asphalt (and SK-70 asphalt) has a test result of 52.5% (and 71.9%), which is higher than the results with TPS-modified asphalt. The high-viscosity TPS-modified asphalt can effectively improve the adhesion of the aggregate to the bitumen, and has good compatibility with crumb rubbers [54; 55]. The test results in Table 5 and Figure 6 show that the proposed method can be used to effectively distinguish the adhesion properties of asphalt and fine aggregates in crumb rubber-asphalt mixture. Figure 5: Adhesion test results of aggregate with different asphalt: (**a**) boiled for 10 min; (**b**) boiled for 20 min.