Film-thickness-mediated ageing pathways in bitumen: New evidence under photo-oxidative and thermo-oxidative exposure

Thermo-oxidative ageing (TOA) is widely used to characterise bitumen ageing in the laboratory; however, more realistic approaches should also consider photo-oxidative ageing (POA) driven by ultraviolet (UV) radiation, particularly with respect to thin binder films representative of field exposure. To address this, bitumen films of controlled thickness (20–500 µm) were prepared using a precision thin-film applicator, enabling consistent, uniform films across a range of practical thicknesses. These films were subjected to TOA in an oven and POA in a commercial UV accelerated ageing chamber under identical temperatures following standard short-term ageing. This approach isolates bulk-dominated thermal oxidation from surface-driven UV effects, enabling direct comparison across realistic film thicknesses. Ageing-induced changes were evaluated via infrared spectroscopy, SARA fractionation, and rheological testing across high-, intermediate-, and low-temperatures. TOA produced largely thickness-independent oxidation and uniform stiffening, whereas POA caused severe thickness-dependent effects, including accelerated chemical oxidation, depletion of molecules within the aromatic fraction, and disproportionate deterioration in fatigue resistance and low-temperature stress relaxation in thin films. To capture maltene transformations masked by conventional indices, a Maltenes Imbalance Index (‘ I MI ’ ) was proposed, which demonstrated superior sensitivity to UV-induced compositional changes and strong coupling with chemo-mechanical performance. These results reveal that POA follows a fundamentally distinct, surface-dominated ageing pathway not represented by conventional TOA protocols.