Softening point test of bitumen

Principle

The softening point of bitumen is the temperature at which a bitumen disc held in a brass ring softens enough for a standard steel ball placed on it to sink through and fall a distance of 25 mm, when heated at a specified rate in a water or glycerine bath (ring and ball test, IS:1205). It shows the temperature at which bitumen changes from a solid to a flowing state, and so indicates its temperature susceptibility.

Apparatus

• Brass ring (inside diameter 15.9 mm, height 6.4 mm), steel balls (9.5 mm diameter, 3.5 g),
• Ball guides and a support frame holding the rings horizontal, with a bottom plate 25 mm below the rings,
• Beaker (800 ml) containing distilled water (for softening points below 80 degrees C; glycerine for higher), thermometer (0 to 100 degrees C), stirrer and heater.

   thermometer
      |   ball
   ===|===(o)===  ring + bitumen
      |    |
      |    | 25 mm
   ___|____v_____  bottom plate
   water bath, heated 5 C/min

Procedure

1. Heat the bitumen until fluid and pour it into the heated ring placed on a metal plate (smeared with glycerine); cool for 30 minutes and trim the surface flat with a hot knife.
2. Fix the rings in the holder, place a steel ball on each, and place the assembly in the beaker of distilled water at 5 degrees C for 15 minutes.
3. Heat the bath so that the temperature rises at a uniform rate of 5 +/- 0.5 degrees C per minute, with stirring.
4. Note the temperature at which the bitumen, carrying the ball, touches the bottom plate. The average for the two balls (if they differ by not more than 1 degree C) is the softening point.

Significance

• A higher softening point indicates a bitumen with less temperature susceptibility, suitable for hot climates, where it resists bleeding and flushing in summer.
• It is used for grading (IS:73: 40 to 55 degrees C for 60/70, 35 to 50 degrees C for 80/100; at least 47 degrees C for VG-30) and for finding the penetration index.
• It is also a rough guide to the heating temperature for mixing and compaction (the mixing temperature is very roughly 100 degrees C above the softening point).