Exploring physical hardening in bitumen based on 4 mm DSR measurements

Physical hardening (PH) is an important factor affecting the low-temperature performance of bitumen. At present, most PH characterizations are performed by the bending beam rheometer. To present an alternative method based on the Dynamic Shear Rheometer (DSR) equipment, this paper explores the possibilities of the 4 mm plate-plate test to investigate PH in bitumen. A variety of binders was selected, differing in their content of natural waxes, in crude origin, and production procedure. An aged sample was also included. The results show that a 20 min equilibrium period is sufficient to stabilize the sample temperature. To evaluate the PH, a time follow-up of 4 h was selected, as after 4 h the change in slope of complex modulus versus time drops below 1%. Further conclusions show that the physical hardening index (PHI) of bitumen is temperature- and frequency-dependent. This PHI is largest at 0 °C and increases when tested at lower frequencies. The non-waxy binder had almost no hardening, while the waxy binders showed evident hardening. Although the presence of wax is an important factor, the PHI of these binders is not directly related to the total wax content, which was determined by differential scanning calorimetry. In conclusion, the findings show that 4 mm DSR tests provide an effective approach to characterize PH of bitumen.

Graphical abstract