Softening point of Bituminous material
The softening point of Bituminous material is the temperature at which the material attains a particular degree of softening.
As per IS: 334-1982, it is the temperature (in ⁰C ) at which a standard ball passes through a sample of bitumen in a mold and falls through a height of 2.5 cm, when heated underwater or glycerine at specified conditions of test given below.
For the purpose of use in the road, the binder should have sufficient fluidity. And for various road use applications, the determination of the softening point helps to know the temperature up to which a bituminous binder should be heated.
Softening point of bituminous materials is determined by Ring and Ball apparatus.
Purpose of Ring ball test
To determine the softening point of bitumen/tar.
- The ring and ball apparatus consist of following:
- Steel balls – two numbers each of 9.5 mm dia. And weighting 3.5 ± 0.05 g.
- Brass rings – two numbers of the ring each having depth of 6.4 mm. The inside diameter at the bottom is 15.9 mm and the top is 17.5 mm.
- Ball guides – to guide the movement of steel balls keeping it in the center.
- Support– that can hold rings in position and this also helps for suspension of a thermometer. The distance between the bottom of the rings and top surface of the bottom plate of the support is 25 mm.
- A thermometer that can read up to 100 ⁰C with an accuracy of 0.2 ⁰C.
- Bath – A heat resistance glass beaker not less than 85 mm in diameter and 1220 mm in depth.
Procedure Of Ring ball test
- Preparation of test sample: Heat the material to a temperature between 75 ⁰C – 100 ⁰C above its softening point; stir until it is completely fluid and free from air bubbles and water.
- If necessary filter it through IS sieve 30. On a metal plate, place the rings, previously heated to a temperature approximating to that of the molten material. This metal plate has been coated with a mixture of equal parts of glycerine and dextrin.
- level the material in the ring by removing the excess with a warmed, sharp knife after cooling for 30 minutes in the air.
- Assemble the apparatus as shown in fig with the rings, thermometer and ball guides in position.
- Fill the bath with distilled water to a height of 50 mm above the upper surface of the rings. The staring temperature should be 5 ⁰C.
Note: Use Glycerine in place of water if the softening point is expected to be above 80 ⁰c; the starting temperature may be kept 35 ⁰c.
- Apply heat to the bath and stir the liquid so that the temperature rises at a uniform rate of 5 ±0.5 ⁰C per minute.
- As the temperature increases, the bituminous material softens and the ball sinks through the ring, carrying a portion of the material with it.
- Note down the temperature when any of the steel ball with bituminous coating touches the bottom plate.
- And carefully record the temperature when the second ball just touches the bottom plate. Then calculate the average of the two readings to the nearest 0.5 ⁰C, that value is the softening point.
- For the heating medium, the distilled water should be used.
- During the conduct of the test, the apparatus should not be subjected to vibration.
- Take care that the bulb of the thermometer should be at about the same level as the rings.
Record of observation
|observation no.1||Observation no. 2|
|Temperature when the ball touches bottom, ⁰C|
|Softening point of bitumen/tar =|
Interpretation of results
The temperature at which binders possess the same viscosity is an indication of the Softening point. It is observed that bituminous materials do not have a definite melting point. Rather the change of state from solid to liquid is gradual and over a wide range of temperatures.
The softening point has a very important function for the determination of the materials that are to be used as joint and crack fillers. A higher softening point indicates and protects that they will not flow during service.
In general, the higher the softening point, the lesser the temperature susceptibility.
For the construction in warm places, bitumen with a higher softening point may be preferred.