Mark G Bouldin

Proposed Refinement of Superpave High-Temperature Specification Parameter for Performance-Graded Binders

A semiempirical approach developed to predict the viscoelastic response of a binder in repeated creep recovery tests is described. This model provides an avenue to predict the rut resistance (R) as a function of loading (time and load) and temperature from data at a single frequency or frequency sweeps when needed. Thus, it can be used to develop a grading procedure for asphalt binders that not only accurately captures the delayed elasticity of modified binders but also accounts for the effect of traffic speed and traffic loading. The current Superpave binder specification attempts to capture the relative high-temperature performance (i.e., resistance to rut) of a binder via the inverse shear loss compliance, 1/J” or G*/sin δ, at 10 rad/s. This parameter represents an improvement over the absolute viscosity because it is measured at a defined rate of deformation and accounts to some degree for the viscoelasticity of the binder via the phase angle. The parameter would correctly predict the relative R for an ideally viscous (R ∞ η) material or an ideally elastic material (R = ∞). However, there is mounting evidence that at phase angles between 40° and 75°, the parameter may not fully capture the viscoelastic nature of many modified binders. Various authors have shown that the R of mixtures can be well described by models using data from dynamic creep experiments, in which the mix is subjected to a load followed by a relaxation period. More recently, Bahia proposed to capture their high-temperature performance by using a similar technique on neat binders.

Field Validation of New Superpave Low-Temperature Binder Specification Procedure: Performance Data from Pennsylvania Test Sections

A new specification procedure was proposed recently to determine the low-temperature performance grade of asphalt binders. This new procedure uses bending beam rheometer (BBR) and direct tension test (DTT) data at two temperatures to determine the low-temperature grade of an asphalt binder. A study was conducted to validate this procedure by using asphalt binder retained samples from the widely published test road in Pennsylvania. This road was constructed in 1976 in Elk County, and performance—including low-temperature transverse cracking index—was monitored over 6 years. In this study, the retained binder samples from test sections T-1 to T-6 were tested with the BBR and the new Superpave® DTT. The data from these two tests were analyzed to obtain critical cracking temperatures and low-temperature performance grades. Results indicate that the new procedure correctly predicts the temperature at which cracking was observed in the field.