Theory & Mehod
Softening Point Tester for Bitumen – QualiSP™ 1000
The ring-and-ball softening point test is the standard empirical method for characterizing the temperature-dependent consistency transition of bituminous and thermoplastic materials. Because bitumen and asphalt are viscoelastic materials with no sharp, well-defined melting point, the softening point is defined operationally as the temperature at which a bitumen specimen, confined in a brass ring and loaded with a standardized steel ball, softens sufficiently to allow the ball to descend a defined distance of 1.0 inch (25.4 mm) under its own weight as the bath temperature is raised at a controlled rate.
The test procedure specified in ASTM D36 begins with sample preparation: molten bitumen is poured into two brass rings (shoulder rings), allowed to cool and solidify, and then the excess bitumen is trimmed flush with the top of the ring to produce two flat, level specimens. The prepared ring specimens are positioned on a ring holder support inside the bath, and a steel ball of defined mass (3.5 g) is centered on each specimen. The entire assembly is submerged in the bath liquid — distilled water for the 5 °C to 80 °C softening point range, or glycerol for the 32 °C to 160 °C range — which is then heated at the controlled rate of 5 °C/min ± 0.5 °C/min specified in ASTM D36.
As the bath temperature rises, the bitumen specimen progressively softens and deforms under the weight of the steel ball. The softening point is recorded as the temperature — read from the bath thermometer at the instant — at which the steel ball, surrounded by the softened bitumen, descends through the ring and touches the bottom plate of the ring holder assembly, completing the 1.0-inch (25.4 mm) drop. The test is typically run in duplicate (two rings and balls simultaneously), and the mean of the two values is reported as the softening point of the sample, provided the two results agree within the precision limits of ASTM D36.
The choice of bath medium is critical to test validity: distilled water is used for conventional paving bitumens (softening points typically 40–70 °C) because glycerol at these temperatures introduces viscosity effects on the specimen surface, while glycerol is mandatory for higher-softening-point materials (oxidized bitumens, polymer-modified asphalts, tar, and pitch) because water cannot be maintained in the liquid phase above 100 °C. The QualiSP™ 1000 supports both media with a single 1,000 mL glass bath, making it straightforward to switch between test ranges as required by the sample type.
