Softening Point Test
Bitumen is a material that doesn't have a well-defined melting point, instead it gradually softens and its viscosity decreases with increasing temperature. The point at which this softening occurs is known as the softening point, and can be determined using the softening point test.
The softening point test is a laboratory testing method that is used to determine the temperature at which a bitumen material transitions from a solid to a semi-solid or liquid state. During the test, a sample of the material is heated at a controlled rate in a special apparatus called a ring and ball apparatus. The temperature at which the material begins to soften and flow is recorded, and this temperature is referred to as the softening point of the material.
The softening point test is used to determine the temperature resistance of bitumen, which helps in selecting the appropriate bitumen binder for road construction depending on the weather conditions. For hotter areas, bitumen with a higher softening point is needed to withstand high temperatures, whereas for moderate to colder climates, bitumen with low temperature resistance is used. It is important to ensure that the softening point value is higher than the pavement temperature to prevent the bitumen from becoming soft and sticky.
International standards provide several methods for determining the softening point of bitumen, including ASTM D36, ASTM E28-67/E28-99, ASTM D6493-11, IS 1205, EN 1427, IP 58, ISO 4625, and JIS K 6863. ASTM D36 is the most widely used method.
Test method
The ASTM D36 method is commonly used to determine the softening point of bitumen. The ring and ball apparatus is used in this method, which consists of two steel balls, two brass rings, a glass beaker, a thermometer, ring holding bases, and a heater. The steps involved in the test are as follows:
Prepare the sample by heating the bitumen to a temperature of 75-100 °C until it becomes completely liquid.
Coat the surface of a metal or glass plate with a mixture of glycerin and dextrin to prevent the bitumen from sticking to the plate.
Heat the rings to approximately the same temperature as the molten bitumen and place them on the plate.
Pour the heated bitumen a little over the volume of the rings inside each ring and let them cool at room temperature.
Remove the excess bitumen with a slightly heated knife once the test pieces have cooled.
Fill a water bath with boiled distilled water and maintain a temperature of 5 °C.
Fix the rings filled with bitumen in the support frame and place them in the water bath at 5 °C for 15 minutes. Cool the steel balls to a temperature of 5 °C.
Place a steel ball on the top of the rings attached to the support. Place this assembly in a beaker filled with distilled water. The water level must be at least 50 mm higher than the ball's top surface.
Put the beaker on the heater and adjust the stirrer and thermometer. Allow the beaker to be heated at a uniform rate of 5 ± 0.5 °C/min.
Continue heating until the bitumen becomes soft and the ball starts to move down because of its weight.
Note the temperature from the thermometer when each ball touches the plate bottom.
The average value of the softening point of two balls (to the nearest 0.5 °C) is the softening point of Bitumen.
Conclusion
The softening point of bitumen is measured using the ring and ball apparatus since bitumen lacks a distinct melting point. This test is helpful in determining which type and grade of bitumen is suitable for various weather conditions.