Rejuvenation mechanisms in bituminous RAP mastics: insights from FTIR spectroscopy and multivariate discriminant analysis

This study examines the rejuvenation mechanisms in bituminous RAP mastics using Fourier transform infrared (FTIR) spectroscopy and multivariate discriminant analysis. The chemical interactions between RAP binders, virgin bitumen, warm mix additives, and two commercial recycling agents (RJs) were evaluated to determine their effectiveness in restoring aged binders. A hybrid approach combining partial least squares regression and linear discriminant analysis (PLSR-LDA) was applied to extract latent variables, classify samples, and identify critical wavenumbers associated with rejuvenation. The findings indicate that Sylvaroad mitigates oxidation effects, particularly around ~ 1758–1715 cm^− 1, while Storflux primarily influences methyl bending near 1370–1360 cm^− 1. Despite these effects, irreversible oxidative aging remains evident in the 1300–1160 cm^− 1 range, linked to oxygen-containing compounds, suggesting that complete restoration of binder properties is unattainable. Additionally, mineral composition markers, particularly in the 489–446 cm^− 1 and 590–555 cm^− 1 ranges, persist as key indicators for distinguishing varying RAP contents after rejuvenation.