# Durability Evaluation Study for Crumb Rubber-Asphalt Pavement
## Materials and Methods
### Experimental Methods Setup  
#### 2.2.1 Adhesion Evaluation for Asphalt and Crumb Rubber Coarse Aggregate  
The net adsorption test developed by the Strategic Highway Research Program in the USA is based on the bitumen-water-mineral system [44]. This test was developed to evaluate the affinity of bitumen for aggregate and to determine the water sensitivity of a given bitumen-aggregate pair. Briefly, the aggregate is placed in a flowing bitumen-toluene solution, and the absorbance of the solution is measured using a photometer to calculate the amount of bitumen being stripped off. The static immersion test (AASHTO T 182) uses water osmosis to simulate the water-related damage after an actual road surface comes in contact with water [45]. Nonetheless, the above-mentioned mainstream foreign methods include subjective analyses and have long test durations and insufficient reliability. China's \"Testing Procedures for the Mixing of Asphalt and Asphalt Mixtures for Highway Engineering\" (JTG E 20-2011) adopts a water-boiling method in which the adhesion grade is determined according to the degree of peeling of bitumen membrane from the surface of a coarse aggregate particle in boiling \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material Type** & **Coarse** & **Machine-Made** & **Mineral** & **Rubber** \\  & **Aggregate** & **Sand** & **Powder** & **Particles** \\ \hline (Apparent) Density (g/cm\({}^{3}\)) & 3.085 & 2.960 & 2.646 & 1.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The density of raw materials. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Asphalt** & **Penetration at** & **Ductility at 10 \({}^{\circ}\)C** & **Softening Point** & **Viscosity at 60 \({}^{\circ}\)C** \\  & **25 \({}^{\circ}\)C (0.1 mm)** & **(cm)** & **(\({}^{\circ}\)C)** & **(Pa. s)** \\ \hline SK-70 & 70.9 & 158.9 & 48.4 & 249 \\ \hline SBS & 53 & 115 & 87 & 1213 \\ \hline
12\% TPS & 49 & 69 & 81.5 & 58,175 \\ \hline
13.5\% TPS & 43.9 & 72 & 88.5 & 78,460 \\ \hline
15\% TPS & 37.5 & 82 & 95 & 194,600 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The properties of different asphalts. SBS: Styrene-Butadiene-Styrene; TPS: Thermo Plastic Styrene. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Sieve (mm)** & **19** & **16** & **13.2** & **9.5** & **4.75** & **2.36** & **1.18** & **0.6** & **0.3** & **0.15** & **0.075** \\ \hline Passing rate (\(\%\)) & 100 & 93.4 & 79.9 & 54.8 & 23.9 & 22.8 & 18.2 & 13.7 & 11.7 & 10.7 & 8.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Design Synthesis Grading of SMA-16. water (grades range from 1 to 5) [46]. After a comprehensive analysis, this study proposed an improved evaluation method based on the existing standard boiling method. The improved method is based on the method of \"Adhesion test of asphalt and coarse aggregate (T 0616)\" [46] and \"Standard Practice for Effect of Water on Bituminous-Coated Aggregate Using Boiling Water\" (ASTM D 3625) [47]. It can be used for both crumb rubber-aggregate and rock aggregate by making the following specific modifications:  
(a) The rubber crumbs have low density. To prevent the bitumen-covered crumbs from failing to be immersed in water, which would otherwise affect the test results, stone and rubber crumb are tied together with a cotton thread before testing so that the rubber crumbs will be weighed down by the mineral materials. However, in the boiling water, they are separated by 5 cm, with the mineral material beneath the rubber crumb (Figure 2a), and both become completely immersed in boiling water. (b) In the actual production of the crumb rubber-asphalt mixture, the rubber crumbs are directly dispersed in the mixing tank without heating. Therefore, the rubber crumbs are required to be immersed in hot asphalt at a normal temperature, and the mineral aggregate is required to be immersed in hot asphalt at a higher temperature (175 \({}^{\circ}\)C).