The dynamic shear rheometer is currently being used by the asphalt community for determination of the viscoelastic properties of asphalts following the recommendations of the Strategic Highway Research Program. The Superpave specification parameter |G*|/sinδ was identified as the term to be used for high temperature performance grading of paving asphalts in rating the binders for their rutting resistance. However, this term was found to be inadequate in describing the rutting performance of certain binders, particularly, the polymer-modified ones. A refinement of the Superpave specification parameter for performance grading of asphalt led to the evolution of the term |G*|/(1 − (1/tanδ sinδ)). This performance-based specification term was shown to be more sensitive to the variations in the phase angle δ than the Superpave specification parameter and thus was found to describe the unrecovered strain in the binders more accurately, especially in the case of polymer-modified asphalts.
The high specification temperature THS (°C) is specified as the temperature at which the term |G*|/(1 − (1/tanδ sinδ)) takes a value of 1 kPa for the original unaged binder and a value of 2.2 kPa for the RTFOT aged binder. Though this refinement of the Superpave specification parameter has led to a better discrimination between the rutting resistances of various asphalts, it is found that not all asphalts that have been performance-graded as being the same behave identically in their rutting performance. Thus, there is a need to create a rut-controlling term that will distinguish between asphalts that are graded the same but prepared by different modification routes. Such a rut-controlling term can then be used as a performance-related specification.
The present work extracts the rut-controlling term by model-fitting the master curve formed from the |G*|/(1 − (1/tanδ sinδ)) or |G*|/(sinδ)9 versus frequency data at various temperatures for each binder. Each master curve is fitted with a constitutive equation from which model parameters are evaluated. The slope B1 in the low frequency region of the master curve is normalized with the term (T/THS), resulting in the rut-controlling term that relates to the permanent strain after 5000 cycles in the repeated shear at constant height experiment on the Superpave shear tester at temperature T.