Viscoelastic transitions exhibited by modified and unmodified bitumen

The working temperature of a bituminous pavement can typically range from 75∘C to −20∘C. Bitumen shows a wide spectrum of mechanical behaviour in this temperature range and these include those of a viscoelastic fluid, a viscoelastic solid and an elastic solid. Due to the amorphous nature of the material, the transitions between such mechanical responses are gradual. This investigation focuses on the transition between the viscoelastic solid and the viscoelastic fluid state of a wide class of bitumens with regard to unaged and short-term aged conditions. An unmodified binder and three modified binders were subjected to temperature sweep and frequency sweep in the viscoelastic regime across a wide range of temperatures. Depending on the material and the post-processing method followed, one could discern a viscoelastic fluid and a viscoelastic solid regime and a frequency-dependent mixture regime consisting of a viscoelastic solid and a viscoelastic fluid, across the tested temperatures and frequencies. It is shown in this investigation that parameters such as G′–G′′ crossover and frequency independence of tan δ cannot be used to estimate the temperatures corresponding to the onset of either fluid-like or solid-like behaviour of the body as the temperature varies. It is also shown that the time–temperature superposition principle does not hold good in the temperature regime tested (75–25∘C) due to the viscoelastic transitions exhibited by bitumen and modified bitumen.