Mitigating ageing-induced stiffening in asphalt mixtures using zinc diethyldithiocarbamate: mechanical performance, digital image correlation, and inter-laboratory validation

Abstract While antioxidant additives have shown promising results to mitigate oxidative ageing at the binder scale, their effectiveness at the asphalt mixture level and their reproducibility across different laboratories remain insufficiently understood. This study evaluates the potential of zinc diethyldithiocarbamate (ZDC) to mitigate ageing-induced stiffening in asphalt mixtures through a comprehensive mechanical characterisation and an inter-laboratory validation campaign. Asphalt mixtures containing 3% and 5% ZDC by binder weight, together with an unmodified reference mixture, were subjected to unaged, short-term (ST) aged, and long-term (LT) aged conditions. Mechanical performance was assessed using dynamic modulus, semi-circular bending (SCB), indirect tensile strength (ITS), and digital image correlation (DIC). To account for differences in mixture composition among laboratories, normalised ITS Ageing Indices (IAI) were introduced and statistically analysed. The results showed that ZDC effectively mitigated ageing-induced stiffening. Dynamic modulus testing indicated that 5% ZDC reduced the ageing susceptibility of the mixtures by approximately 30% compared with the control mixture, while SCB, ITS, and DIC analyses demonstrated improved cracking resistance, enhanced ductility, and reduced strain localisation after ageing. Despite differences in mixture composition and gradation, the inter-laboratory analysis confirmed a statistically consistent antioxidant effect ( p > 0.05), with the most pronounced benefits observed at low temperatures for mixtures containing 5% ZDC. Overall, ZDC proved effective in mitigating ageing-induced stiffening and cracking susceptibility, supporting its potential as a practical strategy to improve the long-term durability of asphalt pavements.