Advanced Mixing Rheometry for Dynamic Shear Rheometer Testing of Highly Heterogeneous Bituminous Binders

In the era of asphalt mixtures designed with any type of admixture and secondary materials, it is not rare for bitumen technologists to deal with issues in the testing setup of new materials and/or in the interpretation of non-conventional trends of results. Within this family, it happens to have multi-phase bituminous binders with a tendency to phase separation, here defined as highly heterogeneous bituminous binders (HHBBs). Amongst the HHBBs there is certainly the crumb rubber modified bitumen which is being used worldwide even though presenting technical drawbacks, limiting further implementation, such as its stability during storage at high temperature at the asphalt plants. A similar scenario might be encountered also at laboratory scale during high-temperature rotational viscosity testing where the use of conventional methods, developed mainly for the characterization of homogeneous binders, may not be suitable for HHBBs. This study wants to address this technical issue by introducing enhanced mixing rheometry for cup-and-bob systems as an optimal solution tailored for dynamic shear rheometers rotational testing of HHBBs. The investigation evaluates two types of HHBBs, an 18% Crumb Rubber Modified Binder and a 6% Polymer Modified Binder, by testing them through four different impellers to record possible differences in viscosity measurement trends. The laboratory experiments were augmented by a numerical analysis to visually assess the stability of HHBBs during rotational testing. The results obtained from this combined empirical and numerical approach indicate that the current rotational measurement standards are inadequate for HHBBs due to their poor mixing efficiency. However, the use of the dual helical ribbon (ST-26.5-LPG) stirrer, which exhibits improved mixing performance, appears to resolve this issue.