Rheological Properties of Asphalts IV Observations Concerning the Anomalous Flow Characteristics of Air-Blown Asphalts

Abstract

A great many of the asphaltic bitumens used commercially possess anomalous flow properties to some degree. Although all asphalts are probably elastic in some measure, detection and measurement of this property are difficult in the cases of certain bitumens, especially soft ones where the permanent deformation obscures the elastic effects. Data obtained by means of the rotating cylinder type of viscometer at a low shearing stress show the presence in an asphalt of elastic fore- and after-effects. Frequently, when an asphalt is subjected to increasing shearing forces, a ``threshold'' value is reached above which the ``viscosity'' at a particular stress decreases with time until a constant value is obtained. If an asphalt is subjected to a high shearing stress (above the ``threshold'' value) in a rotating cylinder viscometer and the stress is reduced suddenly to a low value, the viscosity will increase gradually even though the asphalt is being worked constantly. Another manifestation of thixotropy is the age-hardening of asphalts. This increase of viscosity with time, which varies greatly with different asphalts, can be overcome by heating or mechanical working. Upon standing, the viscosity again increases with time. Also, it has been found that for many asphalts the ``viscosity'' diminishes with increased shearing stress. The extent of these anomalous flow characteristics is dependent upon the source of material, the method and extent of processing to which it is subjected, and the temperature and rate of shear at which the measurements are made. Using numerous kinds and consistencies of asphalt, correlations of the Bingham-Stephens alternating stress, the falling coaxial cylinder, and the conicylindrical rotation viscometers have been made. Comparisons were made at essentially the same mean shearing stresses in all three instruments for a particular asphalt. Good agreement was found when the asphalts were essentially viscous. Deviations of the results obtained for the three instruments became greater as the anomalous flow properties became more pronounced.