Rheological Properties of Asphalt II. Discussion of Penetration-Viscosity Relationships

Although a simple exponential type of equation such as that given by Saal and Koens may give an approximation suitable for many practical purposes, it does not relate the viscosity and penetration of asphalts with the accuracy warranted by the experimental data. An accurate correlation would require a much more complicated equation which would be of doubtful practical value. Asphalts obtained from different sources or those prepared or processed differently require distinctly characteristic constants in the equation proposed by Saal and Koens. This is true no matter whether the change in consistency is due to variations in the temperature of testing or in the method or extent of processing. An asphalt brought to a given penetration by regulating the temperature does not necessarily have a viscosity identical with that of the same type of asphalt having that penetration at a different temperature.