Rheology rethink

A proposed new method for studying the rheology of bitumen and binders at low temperatures proves promising. Hilde Soenen explains the method’s potential.

The objective is to obtain a consistent method for determining the rheological behaviour of bitumen binders at the lowest service temperatures,” says Hilde Soenen, Bitumen Research Manager and responsible for Nynas lab at the University of Antwerp in Belgium, after evaluating a new method together with two PhD students.

Presently, two standard methods are used to determine the rheological behaviour of bitumen - at medium to high temperatures a Dynamic Shear Rheometer (DSR) is used, and at low temperatures a Bending Beam Rheometer (BBR). The DSR method, which involves samples being placed between parallel plates, is more versatile and enables tests to be performed using smaller samples than the BBR.

Current DSR methods are based on 25 and 8 mm diameter plates, and for bitumen this allows testing to around 0° C, due to rheometer equipment limitations. By using plates of a diameter of only 4 mm, it seems possible to extend the temperature range to -20° C or even -30° C. A smaller sample size is also an advantage when testing recovered and aged binders, as these are difficult to obtain in larger quantities.

“A student from Aalto University in Helsinki, Olli-Ville Laukkanen, looked into the 4 mm DSR method during his Master’s and PhD study. He’s been optimising the testing, for instance the sample loading procedure and the necessary calibrations, using equipment at the University of Massachusetts in Amherst,” says Hilde Soenen.

Another visiting research student, Judita Gražulytė from the Road Research Institute at Vilnius University in Lithuania, developed the 4 mm DSR method for use with equipment at the Nynas lab, producing test results comparable with those obtained in the US lab.

“That suggests a level of repeatability and reproducibility, which is a prerequisite for any test measurement used in a performance-based selection system for bituminous binders for road applications.”

However, the 4 mm DSR method also presents a number of challenges, for example with regard to temperature control and the preparation and placement of samples.

“We are working to solve these issues and there is still a lot more to be done.”

The relevance of rheological information is also widening. Low temperature rheological properties can, for instance, provide insight into the durability or ageing of a binder. A specific parameter (delta Tc), used for such evaluations, is calculated from rheological data.

“We believe that the new 4 mm method will have an important role to play, also here.”

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