Effect of esterification on the adhesion between WCO rejuvenated asphalt and aggregate: performance evaluation and mechanism

The high content of hydrophilic free fatty acids in waste cooking oil (WCO), when utilised as an asphalt rejuvenator, typically compromises adhesion performance in the rejuvenated asphalt. Esterification was employed to reduce the free fatty acid content in WCO, and its degree was evaluated for its effects on asphalt-aggregate interfacial adhesion and moisture resistance via boiling tests, contact angle measurements, and freeze‒thaw cycling. Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) force-curve analysis were further used to support the interpretation of microstructural restoration and interfacial adhesion improvement. Deep esterification of WCO reduced its acid value by 90.62%, and the deeply esterified asphalt exhibited 42.69% less mass loss and 56.35% lower stripping rate than the unesterified asphalt in the boiling test. The adhesion work of the deeply esterified asphalt increased by 11.35% relative to the aged asphalt, exceeded that of the virgin asphalt by 1.15%, and was approximately 2% higher than that of the unesterified rejuvenated asphalt. The deeply esterified WCO reduced the freeze‒thaw modulus degradation index of the asphalt mixture by 68.16% compared to the unesterified WCO. FTIR and AFM force-curve results indicate that esterification improves the microstructural state and microscopic adhesion characteristics of aged asphalt. Esterification produces hydrophobic ester functionalities and long-chain alkyl groups that effectively inhibit moisture penetration and enhance asphalt-aggregate adhesion and moisture resistance.