A coupled diffusion–flow model by phase-field method is proposed in this paper with the goal of predicting the storage stability of polymer-modified bitumen (PMB). In this study, the incompressible Navier–Stokes equations were coupled with a previously developed phase-field model for PMB phase separation. The coupled model was implemented in a finite element software package with experimentally calibrated parameters and reported data in the literature. Effects of the parameters (bitumen density and dynamic viscosity) that affected the gravity-driven flow and phase separation in PMB were evaluated at 180°C with the simulation results. The results indicate that the coupled diffusion–flow model can predict the storage stability (and instability) of PMBs. A good correlation between the simulation results and the previously reported experimental results (storage stability tube test) was observed. The different gravity-driven phase separation behaviors of PMBs might have resulted from the different composition of the equilibrium phases in the PMBs as well as the different densities and dynamic viscosities of the individual components (polymer and bitumen). A bigger polymer–bitumen density difference, a lower bitumen dynamic viscosity, or both caused a faster flow and separation in the PMB at storage temperature. The investigated variation of bitumen dynamic viscosity had a more significant influence than the investigated variation of bitumen density in this study, but this finding might depend on the specific values of the model parameters. With this study as a foundation, further experimental and numerical studies will be conducted to increase understanding of storage-stable PMB binders and to develop a more efficient test method for determining PMB storage stability.
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