Using multi-wave technique to characterize the rheological response of bitumen

The rheological characterization of bitumen commonly assumes that, within the linear viscoelastic (LVE) regime, the measured response is independent of the loading protocol. While this assumption is generally valid for simple polymeric systems, its applicability to bitumen, which exhibits a heterogeneous microstructure and a broad relaxation spectrum, remains unclear. The present study investigates the influence of loading history on the rheological response of an unmodified bitumen using multi-wave and frequency sweep experiments conducted at 0, 20, 40, and 60 ° C . Both experiments employ identical amplitudes and frequencies (0.1–2.5 Hz) but differ in the temporal sequence of loading. Full waveform data obtained from stress- and strain-controlled experiments are analyzed using Fast Fourier Transform (FFT) techniques to determine linear viscoelastic parameters and the corresponding relaxation spectrum. The results show measurable deviations between multi-wave and frequency sweep responses, particularly at lower temperatures, despite satisfying conventional criteria for linear viscoelasticity. These differences are interpreted in terms of the distinct strain-rate histories imposed by the two loading protocols and their interaction with the material’s relaxation behavior. The study demonstrates that, for bitumen, linear viscoelasticity does not necessarily imply protocol independence. The findings have implications for the interpretation of rheological measurements and the application of multi-frequency techniques to materials with complex relaxation characteristics. The investigation is limited to an unmodified bitumen and the experimentally accessible frequency range.