Continuous retardation spectrum-based methodology for quantifying the rutting potential of asphalt mixtures

Rutting is a major distress in hot-mix asphalt (HMA) pavements, mainly caused by accumulated permanent deformation under repeated traffic loading. This study develops a continuous retardation spectrum-based method to characterize the rutting-related viscoelastic response of asphalt mixtures. Storage modulus master curves were fitted using a generalized sigmoidal model, and loss moduli were obtained using an approximate Kramers–Kronig relation. The complex modulus was then transformed into complex compliance, from which an analytical expression for the continuous retardation spectrum, L(ρ), was derived. The validity of the derived spectrum was checked by reconstructing storage and loss compliances through numerical integration and comparing them with the corresponding compliance master curves. Spectrum parameters, including peak intensity, peak position, and full width at half maximum, were further used to construct a preliminary Spectrum Rutting Index (SRI). The reconstructed compliances showed excellent agreement with the compliance master curves, with R² close to 1. The proposed SRI also showed a strong correlation with repeated load permanent deformation results (R²=0.8243). These findings indicate that the derived retardation spectrum can reasonably reproduce the linear viscoelastic compliance response and that the SRI has potential for preliminary interpretation of rutting-related behavior.