# Nano-Interaction Mechanism Between Crumb Rubber and Asphal Components: A Molecular Dynamic Study
## 3 Results and Discussion
### Mechanical Properties
#### 3.3.1 Mechanical Modulus
With the increase in temperature, the intermolecular movement is more intense and the system energy increases (temperature increased from 250 K to 400 K, the total energy of the asphalt model increased by 60.355 kcal/mol). This causes the asphalt to gradually move from the elastic material to the plastic body, reducing the modulus. The bulk modulus of rubber asphalt showed a significant decreasing trend under each CR dosage condition, and the shear modulus of 20% dosage CRMA was the largest. Figure 16: Calculation results of model modulus at different temperatures. After CR doping, the bulk modulus change rule with temperature was more consistent. Furthermore, the bulk modulus reduction trend showed a significant two-stage change under different dosages. Compared with the base asphalt, at 250 K-350 K, bulk modulus reduction of small dosage (10%) CRMA was in the temperature range of 4-31%. A 0.4% to 19.2% enhancement was achieved with a 20% dosage, and a 0.3% to 24.4% decrease was achieved with a large dosage (30%). In terms of the change in bulk modulus, the 20% dosage of CRMA had the best performance. It can also be found that when the temperature rises to 400 K (about 127 \({}^{\circ}\)C), the three dosage models of the bulk modulus do not significantly increase after CR dosing. This is due to asphalt being a molten liquid state at this temperature, which contributes less to the modulus of elasticity. Meanwhile, the higher melting point of the CR still has a high compressibility, which in turn causes an increase in the bulk modulus. The larger the dosage increase, the more significant it is. The shear modulus showed different patterns of change with the change of CR dosage. Compared with the base asphalt, the shear modulus mostly showed a decreasing trend under 10% dosage. This may also be one of the reasons why the CRMA requirements of the CR dosage should not be too small (the relevant specification requires the CR dosage to be not less than 15% [35]). However, the shear modulus showed two change intervals with temperature under 20% dosage. The shear modulus decreased when the temperature was before 298 K, while it increased when it reached 325 K (Figure 17). Among them, the decrease in the shear modulus is slight when the temperature is 250 K. The simulation mechanism is more consistent with dry CR-modified asphalt because this model did not consider the swelling and dissolving reactions of CR in asphalt. Therefore, from the simulation mechanism, it is assumed that when using dry CR blending, CR will have a certain adverse effect on shear performance at room temperature due to stress concentration and other effects. However, it will significantly improve the shear performance in high-temperature service conditions. Compared with the service loss of asphalt pavement at room temperature conditions, the deterioration of the mechanical properties of the pavement under high-temperature conditions has a more significant impact. Therefore, in a comprehensive analysis, even if the dry process is used, CR will improve shear resistance, which in turn will enhance the service life of asphalt pavement. At high CR dosage (30%), the shear modulus before 325 K was the same, indicating that there were no significant changes in the viscoelastic properties of CRMA. The excess CR played a dominant role in the shear modulus at this point. An optimal modified dosage and temperature range existed when comparing modulus change under each dosage. Among them, the small dosage (10%) of the mechanical properties' improvement is limited. Increasing the dosage (20%) can significantly increase the asphalt bulk modulus. Although it would adversely affect the shear modulus at low and room temperature, the shear modulus at the middle and high temperature can be  
Figure 17: Shear modulus changes the magnitude of CRMA with 20% CR. significantly improved. When the dosage was too large (30%), the bulk modulus appeared to reduce, and the shear modulus was also smaller than the 20% dosage in the high-temperature zone.