## 3 Results and Discussion
### The Contact Angle Test Results  
In this research, the contact angle, polar component, dispersion component, and surface free energy of the COBA and aggregates were measured using the sessile drop method. The results of cohesion work, the adhesion work, the exfoliation work, and the compatibility ratio of COBA-aggregate are shown in Figure 5.  
Figure 4: Adsorption of phenol saffron solution. (**a**) Limestone–phenol safranin solution. (**b**) Residual phenolic saffron solution after limestone–COBA absorption. (**c**) Steel slag–phenol safranin solution. (**d**) Residual phenolic saffron solution after steel slag–COBA absorption. (1) Surface free energy and cohesion work of COBA  
As shown in Table 8, the surface energy and cohesion work of COBA increased with the increase in bio-oil addition and short-term thermal oxidation. For the virgin samples, the cohesion work of COBA increased by 12.9%, 25.1%, and 31.4%, respectively, compared  
Figure 5: COBA–aggregate adhesion under different aging states. (**a**) Work of adhesion (limestone–COBA). (**b**) Work of adhesion (steel slag–COBA). (**c**) Work of exfoliation (limestone–COBA). (**d**) Work of exfoliation (steel slag–COBA). (**e**) Compatibility ratio (limestone–COBA). (**f**) Compatibility ratio (steel slag–COBA). with the base asphalt when the bio-oil content was increased from 5% to 15%. After short-term aging, the surface energy and cohesion work of COBA increased by 4.0% and 16.4%, respectively, compared with the virgin sample. This indicated that the addition of bio-oil and short-term thermal oxidation could increase the cohesive properties of COBA. (2) Work of the adhesion of COBA-aggregate  
The work of adhesion characterizes the adhesion performance between asphalt and aggregates under anhydrous conditions. The larger the work of adhesion value, the better the adhesion of asphalt to aggregates. According to Figure 5a,b, for the virgin samples, the work of adhesion between COBA and aggregates had an average increase rate of 5% with the increase in bio-oil content, among which, the adhesion work increased up to 15.7% when the bio-oil content was 15% compared with the base asphalt. This indicated that the bio-oil had a positive effect on the adhesion work between COBA and aggregates. After short-term aging, the effect of bio-oil content on the adhesion of COBA to aggregates was reduced. In addition, the work of the adhesion of base asphalt and 5% COBA to aggregates increased compared with virgin samples. This was because, after short-term aging, the light components in COBA gradually volatilized, and the macromolecular substances began to migrate qualitatively, which increased the polar components. The polar components adhered to aggregates via chemical adsorption, with high strength, which was not easy to be peeled off, so the adhesion was improved. However, the high content of bio-oil made COBA more prone to aging, and more resin were converted into asphaltene under thermal oxidation. Therefore, when the bio-oil content exceeds 10%, the adhesion between COBA and aggregate decreases. This is consistent with the decrease in adhesion grade via the boiling method. After long-term aging, the adhesion work between COBA and aggregates was lower than that of base asphalt, and decreased by 12.6% on average compared to the virgin samples. This is due to the rapid increase in heavy components such as asphaltene and polar functional groups in COBA caused by long-term thermal oxygen, which makes the asphalt molecules harden. In addition, these components are easily converted into carboxylic acid and other substances under these conditions, making the asphalt brittle, thus leading to a decrease in the adhesion between COBA and aggregates. In terms of the effect of aggregate type, the adhesion work between different aggregates and COBA increased at a similar rate with the increase in bio-oil content. However, the adhesion work between COBA and steel slag was higher.