Influence of Specimen Geometries on the Viscoelastic Properties of Bituminous Mixtures in Torsion

Abstract

A standard test method for determining the linear viscoelastic properties of bituminous mixtures in torsion prescribes the use of a dynamic shear rheometer. The method uses a prismoidal specimen (rectangular cross section) sliced from a Superpave gyratory compacted sample. However, the geometry utilized in the standard is a non-axisymmetric cross section, which leads to the development of warping stress when the specimen is tested in torsion using a dynamic shear rheometer. On the other hand, axisymmetric geometry, such as cylinder (circular cross section), does not warp when subjected to torsion. This study investigates the influence of the two specimen geometries, namely prismoidal and cylindrical, on the linear viscoelastic properties of bituminous mixtures in torsion. The specimens are obtained from a bituminous mixture with a nominal maximum aggregate size of 13.2 mm, and statistical analysis is carried out to determine whether the material constitution in the specimen geometries is identical. A frequency sweep experiment is conducted at 10, 20, and 30°C with a constant strain amplitude of 0.001% in the frequency range of 0.01 Hz to 20 Hz. The absolute shear modulus of the prismoidal specimens is found to be higher than that of cylindrical specimens because of warping stresses in prismoidal specimens. It was seen that the effect of warping stress is greater at higher temperatures and lower frequencies. Correction factors are used to correct the absolute shear modulus of prismoidal specimens. These findings highlight the importance of considering correction factors for warping when characterizing prismoidal bituminous mixtures in torsion.