# Adhesion Evaluation of Asphalt-Aggregate Interface Using Surface Free Energy Method
## 4 Adhesion of Asphalt-Aggregate Interface
### Adhesion of the Asphal-Aggregate Interface  
The results of energy ratios (\(\mathit{EP}_{1}\) and \(\mathit{EP}_{2}\)) are presented in Figure 6. As mentioned above, the \(\mathit{EP}_{1}\) value is an indication of how water impacts the adhesion of an asphalt-aggregate or organic additive-asphalt-aggregate interface. All the \(\mathit{EP}_{1}\) values were greater than 1, indicating that the adhesive properties of asphalt-aggregate or organic additive-asphalt-aggregate reduce with residual water in the asphalt mixture, and thus the adhesion energy of asphalt-aggregate or organic additive-asphalt-aggregate in a wet condition declines. In addition, the adhesion energy of asphalt-aggregate or organic additive-asphalt-aggregate in a wet condition decreases more rapidly, when the \(\mathit{EP}_{1}\) value becomes higher. It can be found that the \(\mathit{EP}_{1}\) values of organic additive-asphalt-aggregate were always higher than that of asphalt-aggregate. It indicates that the adhesion of organic additive-asphalt-aggregate in the wet condition is far lower than that of asphalt-aggregate with water. This means that the organic additive has significantly negative influences on asphalt-aggregate interface adhesion. Because the two organic additives are also organic waxes,  
Figure 5: Adhesion of the Asphalt-Aggregate Interface in Different Conditions (mJ/m\({}^{2}\)).  
it can form an isolating layer to prevent aggregate from absorbing asphalt when the aggregate interface contains some residual water.  
Therefore, the organic additive lowers the adhesiveness of asphalt-aggregate sharply in a wet condition. The \(EP_{1}\) value of asphalt-basalt in a dry condition is less than that of asphalt-limestone, which indicates that the adhesiveness of asphalt-basalt in a dry condition is relatively better. However, the \(EP_{1}\) value of asphalt-basalt in a wet condition is greater than that of asphalt-limestone, indicating that limestone has a positive influence on the adhesiveness of asphalt-aggregate in a wet condition; or limestone is less sensitive to water; or that limestone has a good compatibility with asphalt. The compatibility between asphalt and aggregate should therefore be studied and improved to reach a high adhesion energy of the asphalt-aggregate interface.  
The \(EP_{2}\) value indicates that organic additives impacted the adhesiveness of asphalt-aggregate. All the \(EP_{2}\) values of asphalt-aggregate or organic additive-asphalt-aggregate in dry conditions were lower than 1. However, all the \(EP_{2}\) values of asphalt-aggregate or organic additive-asphalt-aggregate in wet conditions were higher than 1. Once there is some residual water in the asphalt mixture, the organic additive has a negative influence on the adhesion of asphalt-aggregate. It can also be concluded that the organic additive is very sensitive to water and has hydrophobic characteristics. The adhesion of organic additive-asphalt-aggregate in wet conditions can be reduced due to the dual action of water and organic additive. For preventing water damage to WMA, the residual water in aggregate should be excluded. The \(EP_{2}\) value of Sasobit-asphalt-aggregate is higher than that of RH-asphalt-aggregate, in either the dry or wet condition, which indicates that Sasobit is more sensitive to water and decreases the adhesiveness of asphalt-aggregate. For ensuring the high adhesion of the asphalt-aggregate interface, the compatibility of asphalt, aggregate and organic additive should be strictly observed.