Viscoelastic responses reveal thickness- and UV-controlled photo-oxidative ageing in bitumen

Thermo-oxidative ageing (TOA) is the standard approach for simulating bitumen ageing at the laboratory scale; however, it does not capture surface-controlled, ultraviolet (UV)-driven degradation governing near-surface pavement performance. This limitation leads to underestimation of ageing severity and associated cracking susceptibility under field conditions. This study quantifies the role of film thickness and UV exposure in controlling photo-oxidative ageing (POA) of unmodified and styrene-butadiene-styrene (SBS)-modified binders and evaluates its equivalence with conventional TOA and field ageing. Films of 30, 100, and 500 µm were prepared using a precision applicator and subjected to TOA in a Pressure Ageing Vessel (PAV) or POA in a QUV chamber following short-term ageing. Rheological master-curve analysis, Fourier Transform Infrared Spectroscopy (FTIR), and Linear Amplitude Sweep (LAS) testing characterised ageing evolution. POA produced thickness-dependent, surface-controlled ageing with enhanced carbonyl and sulfoxide formation, and FTIR evidence consistent with progressive SBS network degradation, whereas TOA resulted in moderate bulk stiffening. Comparison with UV-aged conditions shows that PAV ageing represents only moderate severity. In contrast, 30 µm films aged for 8 days under UV produced viscoelastic and cracking responses comparable to the selected field-aged RAP references, with Glover-Rowe parameters exceeding PAV-aged binders by over an order of magnitude. A viscoelastic ageing index ( VEAI ) derived from master curves showed greater sensitivity to ageing-induced changes, enabling clearer identification of critical cracking states than conventional indices. These results demonstrate that controlled thin-film UV ageing is essential for realistic simulation of binder degradation and improved predictive capability of laboratory protocols.