Flow Number Testing as an Alternative Approach for Cohesion Evaluation in Bitumen Stabilised Materials

Cold recycling is gaining popularity as a sustainable and cost-efficient pavement rehabilitation technique. With its growing adoption, a more comprehensive understanding of the mechanical response of cold recycled mixtures (CRMs) is required. Owing to their partially bonded nature, the performance of Bitumen Stabilised Materials with bitumen emulsion (BSM-Es) can be described using both granular- and viscoelastic-material approaches. This study aims to characterise the cohesion of BSM-Es by analysing their permanent deformation response using Flow Number (FN) test originally developed for HMA. Dynamic creep testing was performed under a 600-kPa cyclic deviatoric stress at 50 °C on BSM-Es containing 0–90% RAP, 4% bitumen emulsion, and 1% cement, as well as on asphalt concrete mixture used as a viscoelastic reference. Analysis of the FN parameter and the non-standard indicators ε(FN) and FNindex showed that Flow Number testing effectively differentiates BSM-Es in terms of cohesion, simultaneously revealing notable differences in strain-accumulation mechanisms in BSM-Es and HMA. It was shown that low-RAP BSM-Es resist deformation primarily through intergranular friction, whereas higher RAP contents lead to a more binder-dependent and viscoelastic response of the material. The findings support the development of performance-based design approaches for CRMs by providing new insight into their mechanical behaviour.