Bitumens are viscoelastic materials without sharply defined melting points; they gradually become softer and less viscous as the temperature rises.
The softening point of bitumen can be determined through the use of a ring-and-ball apparatus immersed in distilled water (30 to 80°C) or USP glycerin (above 80 to 157°C). The softening point is reported as the mean of the temperatures at which the two disks soften enough to allow each ball, enveloped in bitumen, to fall a distance of 25 mm (1.0 in.).
The softening point is useful in the classification of bitumens, as one element in establishing the uniformity of shipments or sources of supply, and is indicative of the tendency of the material to flow at elevated temperatures encountered in service.