Rheological Properties of Asphalts III. A Viscosity Index

A review is given of the work which has been done on the temperature‐viscosity relations of bituminous materials and it is pointed out that the methods commonly used for evaluating the changes of consistency with temperature are empirical and unsatisfactory. Viscosity measurements at temperatures ranging from 15° to 35°C have been made on fourteen asphalts of paving consistency by means of the alternating stress method described by Bingham and Stephens. It has been found that for these asphalts and temperatures a plot of the logarithm of viscosity against temperature gives essentially a straight line. An asphalt viscosity index is established which is defined by the relationship: $$\mathrm{A.V.I.}{\text{=100(10}}^a\text{−1),}$$ where ``a'' is the slope of the log viscosity versus temperature plot. To obtain numerical values for the A.V.I. it is only necessary to determine experimentally the viscosity at two temperatures (in the range where the straight line relation holds) and substitute in the expression: $$\mathrm{A.V.I.}\text{=100}\left[{\left(\frac{{\eta }_c}{{\eta }_d}\right)}^{{\text{1/(t}}_c{\mathrm{-t}}_d)}\text{−1}\right],$$ where η_c and η_d are the viscosities (in poises) at t_c and t_d (°C), respectively. This gives the percentage decrease in viscosity of the asphalt for 1°C rise in temperature. The values obtained for the viscosity index for the asphalts studied varied from 19.2 for Venezuelan oil asphalt prepared in a batch steam still to 24.7 for a vacuum reduced Californian oil asphalt.