Systematic comparison of nano metal oxides as tracers for assessing blending degree in recycled asphalt

With the increasing application of recycled asphalt mixtures, accurately evaluating the blending degree between aged and virgin asphalt has become a key concern in pavement performance analysis. This study investigated the use of nanoscale metal oxides as tracers to evaluate the blending behaviour between aged and virgin asphalt binders through both microscopic and macroscopic approaches. Three representative nanoparticles, Fe3O4, TiO2 and ZnO, were selected. Their performance as tracers was systematically compared through a multiscale framework incorporating microscopic characteristics, spectroscopic response and macroscopic rheological properties. The particle size distribution, dispersion uniformity, and Fourier transform infrared spectroscopy (FTIR) spectral features were analysed to determine tracer identifiability and compatibility. Blending behaviour under various types of rejuvenators, blending temperatures and blending times was quantified through linear amplitude sweep (LAS) and FTIR testing. To establish a robust analytical system, the blending degree index (BDI), fatigue life (N t ), and coefficient of variation (CV) were introduced for comprehensive evaluation. The results demonstrated that Fe3O4 exhibited the highest tracer sensitivity and stability. It showed the strongest correlation between FTIR and LAS across the blending temperature and blending time gradients, with Pearson coefficients of 0.931 and 0.965, respectively, and R2 values of 0.886 and 0.932, respectively. Moreover, Fe3O4 exhibited a stronger central tendency within the 95% confidence and prediction intervals, outperforming TiO2 and ZnO. These findings confirm that Fe3O4 as the suitable tracer for asphalt binder blending analysis. The proposed ‘micro-macro’ integrated methodology offers a novel and effective approach to quantifying the blending degree between aged and virgin asphalt.