The softening point is the temperature at which the substance attains a particular degree of softening under specified condition of test. The softening point of bitumen is usually determined by Ring and Ball test. The concept of softening point test and the test set-up is shown in the following figure.
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Generally higher softening point indicates lower temperature susceptibility and is preferred in warm climates.
A brass ring containing test sample of bitumen is suspended in a beaker with liquid bath: water is used as the bath if the softening point is less than 80°C and glycerine is used for temperatures exceeding 80°C. A steel ball is placed upon the bitumen sample and the liquid medium is then heated at a rate of 5°C per minute. The temperature at which the softened bitumen touches the metal plate placed at a specified distance below the ring is recorded as the softening point of the bitumen. Harder grades of bitumen possess higher softening point than soft grade bitumen.
The softening point of various bitumen grades used in paving jobs vary between 350 to 70°C.
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