Viscoelastic linearity limits for bituminous materials

As bituminous materials are viscoelastic in nature, their performance must be characterised using test methods and analytical techniques that account for time (or rate) of loading and temperature. In addition, it is usually advisable to confine the characterisation of a bitumen to its linear viscoelastic response (small strains) to simplify the mathematical modelling of the material, as non-linear response, particularly for viscoelastic materials, is extremely difficult to characterise in the laboratory and model in practical engineering problems. This paper describes an investigation of the linearity limits of a range of unmodified and modified bituminous binders and mixtures using a dynamic shear rheometer and a purpose-built dynamic, direct tension-compression, servo-hydraulic testing apparatus. The results show that there are strain dependent linearity criteria for both binders and asphalt mixtures at high stiffness values (low temperatures for binders and low to intermediate temperatures for mixtures) as well as a high temperature strain dependent linearity criterion for elastomeric modified binders. The linearity strain criterion for the mixtures was found to be in the order of 100 microstrain with the criterion for the binders being at least 100 times greater at just over 10,000 microstrain and the polymer network strain criterion at 1,000,000 microstrain.