Decoupling oxidation and hardening in ageing: evaluation of bitumen-stone adhesion using different experimental methods

The adhesion at the bitumen-aggregate interface critically affects asphalt pavement resistance to moisture damage and cracking and is significantly influenced by the bitumen ageing process. Coupled oxidation and hardening during ageing lead to varying results across evaluation methods. This study employs direct tensile pull-off tests at iso-temperature and iso-stiffness conditions, contact angle tests, and boiling water tests to assess bitumen-stone adhesion at different ageing levels. Results indicate oxidation and hardening have dual impacts: short-term ageing initially enhances adhesion, while prolonged ageing reduces it significantly. Polymer-modified bitumen shows improved ageing resistance due to delayed polymer degradation. Iso-temperature tests fail to clearly differentiate oxidation from hardening effects, whereas iso-stiffness tests offer more accurate assessments by excluding physical hardening. Due to interactions between chemical and physical changes in bitumen, ageing evaluations vary by indices and binder type. A comprehensive assessment combining pull-off tensile tests, surface free energy analysis, and mixture-scale evaluations is recommended.