Adhesion Mechanism and Interfacial Failure Behavior of Castor Oil–Based Bioasphalt and Aggregate under Thermal Oxidation

The objective of this research is to investigate the degradation mechanism of adhesion and interfacial failure behavior between bioasphalt and aggregate under the action of thermal oxidation. This study investigates castor oil–based bioasphalts with varying bio-oil contents in conjunction with two types of aggregates: limestone and steel slag. The pullout and cohesion tests were conducted to explore the pullout strength, cohesion, and interface failure behavior of bioasphalt and aggregate under thermal oxidation. Meanwhile, molecular dynamics simulations were employed to investigate the changes in cohesive energy density, diffusion coefficient to aggregate, and interface energy between bioasphalt and aggregate under thermal oxidation aging conditions. It was found that the results of the tensile strength and cohesion are highly consistent, thereby enabling mutual validation and providing a basis for evaluating the adhesion of asphalt-aggregate. The tensile strength and cohesion of bioasphalt-aggregate increased with the increase of bio-oil content, first increasing and then decreasing with the aging degree. However, after long-term aging, the tensile strength of bioasphalt limestone is still 5.2% higher than that of unaged bioasphalt limestone. The interface failure type between bioasphalt and aggregate was mainly adhesion failure. The results of molecular dynamics simulation showed that the increase in the bio-oil amount promoted the increase of interface energy between the bioasphalt and aggregate, and also explains the variation trend of tensile strength. As the extent of thermal oxidation increased, the cohesive energy density of bioasphalt increased, consequently resulting in a reduced diffusion capacity. After long-term aging, the diffusion coefficient of bioasphalt on quartz and calcite decreased by 36.8% and 38.2%. Bioasphalt exhibits superior adsorption with alkaline aggregates. Also, the interface energy between bioasphalt and calcite is 11 to 20 times greater than that between bioasphalt and quartz.