## 3 Results and Discussion  
### The Boiling Results  
The results of the boiling test are shown in Table 9. The stripping section of the asphalt film has been marked with a red frame, and the corresponding adhesion grades are shown in Table 10.  
It can be seen from Tables 9 and 10 that with the increase in bio-oil dosage, the overall peeling area of the asphalt film from the aggregates was slightly increased. The low content of bio-oil could improve the adhesion of COBA and aggregate, while the opposite was true for the high content of bio-oil. For instance, when the content of bio-oil was 5%, there was no obvious asphalt film shedding on the surface of the aggregates. When the content of bio-oil was 10%, the slag surface started to show patchy peeling and a small quantity of the asphalt film peeled off from the surface of the limestone, but this peeling area was still smaller than that of base asphalt. While the content of bio-oil was 15%, the peeling area of asphalt film on the surface of limestone was further increased. This is due to the fact that the added bio-oil increases the acid value of asphalt, and the acid molecules in asphalt will bind to the active site on the surface of the aggregate silicate to form alkyl acids, and then crystallize to form adhesions [23]. However, this acidic compound is easy to hydrolyze in water, which will weaken the moisture damage resistance of the COBA-aggregate. Therefore, during the boiling process, the more serious the hydrolysis process of the accumulated alkyl acids on the surface of the aggregates coated by COBA with high bio-oil dosage, the larger the area of asphalt-film peeling.  
was aggravated after long-term aging, which led the peeling area to exceed 10%. This is due to the fact that COBA becomes hard and brittle under thermal oxidation, and 15% COBA with a higher bio-oil dosage is more prone to aging. Secondly, the polarity of water is much larger than COBA, and its diffusion ability on the surface of the aggregates is stronger. Therefore, the water can displace the asphalt film from the surface of aggregates more easily, thus reducing the adhesion grade of COBA-aggregate.  
Under the same conditions, the adhesion grade of COBA and limestone was the same as that of steel slag, but the peeling positions of asphalt film were slightly different. The steel slag is more hygroscopic and has a rough surface, so the aged COBA is more likely to be peeled off from the surface of the steel slag. Meanwhile, the main mineral phase of limestone is alkaline CaCO\({}_{3}\), which can react with acidic compounds in COBA to produce some water-insoluble sticky substances. Therefore, the asphalt film on the surface of limestone is more likely to peel off at the corners and uneven edges.  
Since the results of the boiling test are affected by several factors such as the angle of the aggregate, the surface roughness, and the thickness of the asphalt film, only a rough qualitative evaluation of the adhesivity of the COBA and aggregates can be made. To accurately distinguish the relationship of the moisture damage resistance of COBA and aggregates under the same adhesion grade, it needs to be further determined through other tests.