Six viscosity-graded asphalts from different sources were used in the construction of the pavements studied and reported in this paper. Tests to determine the viscosity-related properties of the original asphalts, as well as the asphalts recovered from time to time from the pavements in service, have been conducted. Evaluation of the performance of these test pavements has been carried out by a team of five engineers.
The aging index based on viscosity at 77 F (25 C) has been found to conform to the pavement performance rating and thus seems to be more meaningful to indicate comparative aging and life expectancy of the test pavements. Shear susceptibility of the asphalt cements seems to be one of the major factors affecting the pavement performance. Present viscosity-graded specifications should be supplemented with a maximum permissible shear-susceptibility value at 77 F so as to control the durability of the asphalts. The maximum amount of shear susceptibility that may develop in a paving-grade asphalt prior to pavement failure is indicated in the data obtained.