## 3 Results and Discussion
### The Contact Angle Moisture Susceptibility Results  
In this study, the moisture susceptibility of COBA was evaluated by observing the change in the contact angle of COBA before and after water conditioning on the slide surface. Typically, the smaller the contact angle, the better the adhesion of the asphalt to the solid surface [26]. The contact angle of COBA is shown in Table 11.  
As observed in Table 11, for the virgin samples, the contact angle between COBA and the glass slide decreased with the addition of bio-oil before water conditioning. It demonstrated the increase in adhesion between asphalt and siliceous aggregates by bio-oil. With the process of water conditioning, the contact angle between COBA and the glass slide increased, indicating the decrease in adhesivity. This is because the affinity of COBA and water for SiO\({}_{2}\) is different under chemically driven moisture damage [27]. Initially, COBA covers the surface of the dry slide with a low contact angle. But water gradually penetrates into the interface of COBA-slide and replaces COBA on the aggregates, increasing the contact angle between COBA and the glass slide. After aging, the contact angle between COBA and the slide increased gradually. Under thermal oxidation, the light components in COBA decreased, accompanied by the increase in colloidal components, asphaltene, and other heavy components. As a result, the diffusion of COBA on the surface of the slide decreased and the contact angle increased. Meanwhile, the contact angle of COBA changed more after water conditioning, and the moisture damage resistance also changed significantly. Therefore, the moisture damage resistance of COBA was evaluated by calculating the CAMSI of COBA before and after thermal oxidation. The results are shown in Figure 6.  
The increase in CAMSI indicates a decrease in the adhesivity of asphalt with silica when it is exposed to water [12]. As shown in Figure 6, for the virgin samples, the CAMSI of mixtures with 5% and 10% COBA decreased by 7.5% and 4.4%, respectively, compared to base asphalt, while the CAMSI of mixtures with 15% COBA increased by 58.5%. This is consistent with the water-boiling method, demonstrating that low bio-oil dosage can improve the moisture damage resistance of COBA. This is due to the fact that low bio-oil dosage can better interact with asphalt and prevent moisture sub-penetration into the binder, reducing its moisture susceptibly [28]. In fact, while bio-oil changes the rheological properties of asphalt, it also weakens the durability of its mixture. The presence of a large amount of bio-oil in 15% COBA will cause the acidic compounds inside to diffuse rapidly and interact with the siliceous aggregate to produce carboxylic acid, etc. They will accumulate and crystallize at the interface of the aggregates, and then hydrolyze upon  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Content of Bio-Oil\%**}} & \multicolumn{5}{c}{**Aging Condition**} \\ \cline{2-7}  & **Virgin** & **RTFOT 85** & **RTFOT 385** \\ \hline  & Dry & 9.1\({}^{\circ}\) Wet & 23.6\({}^{\circ}\) & Dry & 10.6\({}^{\circ}\) Wet & 26.8\({}^{\circ}\) & Dry & 18.1\({}^{\circ}\) Wet & 56  
contact with water, which is also the reason for the weakening adhesivity between the aggregates and COBA with a high dose of bio-oil ([PERSON] et al., 2018; [PERSON] et al., 2018). Under the effect of thermal oxidation, the CAMSI after short-term aging was the smallest, while the CAMSI after long-term aging was even smaller than the original status. It demonstrated that COBA still has good adhesion property to silica when exposed to water and the short-term thermal oxidation effect has good advantage to improve the moisture damage resistance of COBA. [PERSON] et al. found that the improved moisture damage resistance of bio-asphalt can be attributed to the ability of bio-oil to delay the formation of acidic compounds ([PERSON] et al., 2018). [PERSON] et al.