Influence of specimen geometry on torsion of bituminous mixtures

Abstract

This investigation focuses on the measurement of linear viscoelastic properties of the bituminous mixture using different specimen geometries in Dynamic Shear Rheometer (DSR). Two different specimen geometries, cylindrical and prismoidal (rectangular cross-section) specimens are used for testing in torsion. The cylindrical specimen of 12 mm diameter and prismoidal specimen with width to thickness ratio of 1.3 are used in the study. The length of both specimen geometries is restricted to 50 mm. The linear viscoelastic limit for the bituminous mixtures was estimated by performing an amplitude sweep experiment. The linear viscoelastic parameters are measured using a strain amplitude of 0.001 % and at frequencies ranging from 45 to 0.1 Hz. The results show the difference in the linear viscoelastic parameters measured using cylindrical and rectangular specimens i.e., the absolute shear modulus of the prismoidal specimen is greater than the cylindrical specimen. Such an increase in absolute shear modulus is because of the end restraint in torsion for prismoidal specimen i.e., warping. This study has documented such results for a range of temperatures and frequencies. The difference in the linear viscoelastic properties is found to be insignificant at 15 °C for the frequencies above 0.5 Hz and predominant at 35 and 55 °C for all the frequencies. The absolute shear modulus of bituminous mixtures for prismoidal specimens is computed and it is seen that the difference in absolute shear modulus is contributed predominantly due to the restriction of the warping in the prismoidal specimen due to the end restraint. This study shows the necessity for the correction of the absolute shear modulus of the bituminous mixture measured using prismoidal specimen for a given aspect ratio.