Advanced viscoelastic characterization of modified bitumen at low temperatures using 4-mm DSR testing

In this study, we combine low-temperature 4-mm DSR sweeps with (LVE) modeling to investigate broadening of the glass-transition region in multicomponent asphalt binders modified with crumb rubber (CR), SBS, and a (HV) additive. Two PG 64–22 base binders were tested as neat binders and as modified formulations containing CR (8%), SBS (2%), and HV (8%) in different combinations after TFOT and PAV aging. Increasing modification level produced a progressively smoother and more gradual liquid-to-glassy transition, clearly expressed as broadening of the long-time relaxation spectrum. A central contribution of this work is extending the broadened power-law (BPL) relaxation spectrum approach to CR/SBS/HV-modified binders and demonstrating that the stretching parameter β serves as a quantitative indicator of long-time relaxation broadening. β increased from 0.16 (unmodified) to 0.56 (CR+SBS+HV), indicating the broadest and most gradual long-time tail of the relaxation spectrum in the most modified formulation. Time–temperature superposition was used to construct G* and δ master curves; among the evaluated models, the generalized sigmoidal form provided the most consistent description of both G* and δ. To build a consistent low-temperature assessment framework, β was interpreted alongside independent LVE descriptors near the glass transition, including smoother Booij–Palmen trends, progressively broader reduced G′ and G″ transitions, and wider δ(T) isochrone spans (ΔTδ). Apparent molecular-weight distributions derived from the δ-method further supported the activation of additional relaxation processes in modified binders. Overall, the integrated rheological framework provides screening metrics to identify binders with improved stress-relaxation capacity during cooling, thus critical for low-temperature durability. • An integrated rheological–spectral workflow is proposed for reliable low-temperature binder evaluation. • 4-mm DSR testing combined with generalized sigmoidal modeling enabled robust assessment of low-temperature rheology. • Parameter β from the BPL model increased with modification, providing a quantitative indicator of long-time relaxation broadening. • Modified binders exhibited a broader viscoelastic transition, evidenced by smoother Booij–Palmen slopes, broader dynamic moduli shapes, and wider thermal spans (ΔTδ).