Establishing the limits of linear viscoelastic behaviour for bitumen

Bitumen exhibits linear viscoelastic behaviour only within some limits, beyond which the behaviour can be highly nonlinear. This study emphasises the importance of determining the linear viscoelastic (LVE) limit of bitumen by considering its viscoelastic fluid-like nature. Unlike conventional methods of identifying LVE limit in terms of strain, this study investigates the LVE limit in terms of strain rate and stress. In this context, experiments involving oscillatory strain and stress sweeps were carried out at different frequencies across a temperature range of 30 to 70∘C. Additionally, rotational strain rate sweep tests were carried out at 50 and 60∘C. The tests were conducted on two polymer-modified binders- a plastomer-modified binder and an elastomer-modified binder. The findings demonstrate that while the LVE limit based on strain is frequency-dependent, the limits derived from strain rate and stress remain relatively unaffected by frequency. Moreover, strain rate sweep tests in rotational mode produce significantly lower LVE limits than those from oscillatory tests, indicating the influence of the testing method. This work emphasises the importance of defining LVE limits using strain rate or stress for improved material characterisation.