Softening Point Test of Bitumen
The softening point shows the temperature at which the bitumen gains a certain degree of softening under the specifications of the test. This test is carried out by using the Ring and Ball apparatus. The softening point helps to determine the temperature up to which bitumen can be heated for different road use applications.
It is also known as the Ring and ball Test. This is because ring and ball apparatus is major equipment used for determining the softening point of bitumen.
The significance, apparatus and materials, precautions, observation table, result, and conclusion for the softening point test of bitumen are given below:
Aim
To determine the softening point of asphaltic bitumen, fluxed native asphalt, road tar, coal tar pitch, or blown type bitumen in the laboratory.
Significance
The softening point helps in the determination of the temperature beyond which the bitumen is softened beyond a pre-specified softness.
Hence, the softening point test of bitumen helps in knowing the maximum temperature to which the given bitumen can be exposed.
This is utilized while using bitumen as a binder for road construction. Softening point value gives the temperature at which the bitumen should be heated before using it for pavement construction.
If the climatic temperature exceeds the softening point, the bitumen used as a binder starts to melt and the pavement becomes sticky.
Read More: Float Test of Bitumen | Apparatus, Procedure & Result
Important Terms related to Softening Point Bitumen Test
Softening Point: It is the degree of temperature at which a solid substance starts to convert into a liquid state under certain specific conditions.
Alternatively, the softening point test determines the temperature at which the bitumen attains a particular degree of softening under specified conditions.
In the case of bitumen, the softening is the temperature at which the steel ball passes through the bitumen specimen kept in a ball and travels the height of 2.5 centimeters when heated underwater in the specified conditions.
Apparatus
apparatus used in ring and ball test of bitumen,
2 Balls: Each ball is 9.5 mm in diameter and its weight is 3.5 ± 0.5 gram.
2 Brass rings :
The rings are tapered from inside- the outside diameter of the ring is constant while the inner diameter decreases from top to bottom.
The rings have three pins that act as a centering guide and they form an imaginary diameter of about 7.5 mm. this arrangement enables the movement of the steel ball during the test.
Dimensions of each ring are mentioned below:
Depth of ring = 6.4 0.1 mm
Inside diameter at Top = 17.5 0.1 mm
Inside diameter at Bottom = 15.9 0.1 mm
Outside diameter = 20.6 mm
Support: A metallic frame is used to support the rings. Its requirements are specified below:
The topmost plate: It should have a hole in the topmost plate from which a thermometer can be suspended so that it can remain in level with the bottom of the rings. The thermometer should remain within 10 mm distance of the rings but it should not touch them.
The middle plate: It has three holes. Two outer holes support the rings in a horizontal position such that the upper surface of the ring is 50 mm below the water level. The thermometer passes from the central hole.
The bottom plate: It is a solid plate with no holes. The distance should be 25 mm exact between the bottom of the rings and the top of the bottom plate.
Beaker: It should be heat resistant and its capacity should be 600 ml.
Stirrer: It can be manual or mechanical. It is used to ensure a uniform heat distribution at all times when the sample is heated in the water bath. The stirrer should be so placed that when it is operated, the molds are not disturbed.
Thermometer: It is of mercury in glass type fille with nitrogen. Its back is enameled black and its top is provided with an expansion chamber and a glass ring. Its temperature range should be from -2 to 80 °C with a least count of 0.2 °C.
Water bath: A heat-resistant glass vessel is taken for the water bath having a minimum diameter of 85 mm and is 120 mm deep.
Other accessory apparatus includes,
- Hot plate with energy regulator
- Vessel for heating bitumen
- Forceps
- Sharp-edged knife
Materials for Softening Point Test
Following materials used in softening point test of bitumen-
- Bitumen to be tested
- Glycerine
- Dextrine
- Distilled water- for heating medium
Precaution
Following precautions should be taken while performing the test obtain accurate results:
- The water used in the practical should be freshly boiled distilled water. Else there is a possibility that air bubbles may form in the specimen which will affect the accuracy of the results
- To obtain accurate results, the rate of heating should be strictly followed as mentioned
- To prevent the material from sticking to the glass vessel, a conveniently weighed sheet of filter paper or thin amalgamated sheet can be placed on the bottom of the glass vessel. This will save time and would provide ease in cleaning
Softening Point Test of Bitumen Procedure
Sample Preparation
- Take the bitumen sample in a vessel and heat it to a temperature of 75-100°C- above its approximate softening point. Allow the bitumen to melt until it is converted completely into a liquid state. Stir the bitumen if necessary so that it is melted completely and is free from air bubbles and water. If the need arises, filter it through IS sieve 30.
- Prepare a mixture of glycerine and dextrine in equal proportions. Mix it well. Then coat the surface of the metal or glass plate. This prevents the bitumen from sticking to the plate.
- Heat the rings to approximately same temperature as that of the molten bitumen and place them on the glass or metal plate coated with the mixture of glycerine and dextrine.
- Pour the molten bitumen in the rings carefully till the rings are slightly filled above the top level.
- Allow the rings to cool at room temperature in air for 30 minutes.
- Cut the excess bitumen on the surface with the help of hot, straight-edged putty knife or spatula and level the top surface.
Testing of the Specimen
- Fill the water bath with freshly boiled distilled water to such a height that the water level remains 50 mm above the upper surface of the rings. Maintain this water bath at 5 °C temperature.
- Fix the rings filled with bitumen to the ball guide. Assemble the rings in the support frame.
- Place the rings fixed in the support frame 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 ring guide of the rings attached to the support with the help of forceps.
- Place this assembly in a glass beaker filled with distilled water to such a height that the water level is at least 50 mm above the top surface of the ball.
- Put the beaker on the hotplate and adjust the stirrer. Insert the thermometer too.
- Allow the beaker to be heated at a uniform rate of 5 ± 0.5 °C/min. The rate can be adjusted with the help of the energy regulator.
- Stir the water continuously to ensure that the water is heated uniformly.
- Continue the heating till the bitumen filled in the ring softens and the ball placed on it starts to move down owing to its own weight.
- Note the temperature when each of the balls touches the bottom plate while sinking from the thermometer.
Observation
Following observation taken during ring and ball test of bitumen,
- As the water is heated continuously, the bitumen in the rings starts to melt.
- As a result, the steel balls placed over it starts to descend in the water due to its own weight.
The temperature at which the steel ball touches the bottom plate is observed and its value is recorded. This is done for both the balls.
Observation Table
A sample observation table for softening point test is drawn below
| Description | Ball No. – 1 | Ball No. – 1 |
|
The temperature when the ball touches the bottom plate °C |
 |  |
The softening point of Bitumen = Average value of softening point of 1 and 2 (to the nearest 0.5 °C).
Result
The average of the softening point values to the nearest 0.5 °C obtained for the two steel balls is the softening point of the given bitumen specimen.
The softening point value of the two readings should not exceed 1.0 °C for the softening point of bitumen range 40-60 °C and 1.5 °C for the softening point between 61-80 °C. Else the test is considered invalid and it is repeated.
| Softening Point °C | Repeatability °C |
| 40-60 | 1.0 |
| 61-80 | 1.5 |
Minimum softening point values standardized by Bureau of Indian Standard for Bitumen to be used for pavement for different grades are tabulated below:
Also softening point of various bitumen grades,
| Paving Grades | Softening Point |
| VG 10 | 40 °C |
| VG 20 | 45 °C |
| VG 30 | 47 °C |
| VG 40 | 50 °C |
Conclusion
By performing the softening point test of bitumen, we can determine whether the given bitumen sample meets the specification requirements for softening point as given in the standards.
The softening value found out by the practical is compared to the standard value and thus it can be concluded whether the sample meets the softening point criteria.
Softening point is one of the measures of the quality of bitumen along with other measures such as penetration value, ductility value, and so on.
Summary
Above softening point test of bitumen summaries below,
Procedure | |
 |
The bitumen sample is heated in a vessel at 75-100 °C- above the approximate softening point of bitumen till it is completely melted into a fluid. It is stirred if needed to prevent the formation of air bubbles. |
 | Glycerine and dextrin are mixed together in equal proportions. |
 |
Glycerine and dextrin mixed together are coated on a metal or glass plate to prevent the bitumen from sticking to it. |
 | Bitumen is poured into a convenient vessel so that it can be poured into the rings properly. |
 |
Rings are preheated to approximate the same temperature as that of the molten bitumen and then placed on the glass plate. Melted bitumen is poured into it. |
 |
The rings are allowed to cool at room temperature in air for 30 minutes. |
 | Excess bitumen above the ring surface is cut off by a hot, sharp-edged knife and a leveled surface is obtained at the top. |
 | The ring are fixed to the support frame |
 |
The ring is fixed to the support frame. |
 |
Place the assembly of rings attached to the support frame in the water bath maintained at 5 ° for 5 minutes. Freshly boiled distilled water is filled in the water bath. |
 | The steel balls are precooled to 5 ° C and are placed on top of the ring guide with the help of forceps. |
 |
This assembly is kept in a beaker filled with distilled water such that the distance between the water level and the top of the steel ball is 50 mm. |
 |
The beaker is put on a hotplate and is allowed to heat at the rate of 5 ± 0.5 °C/min. Stirring is done continuously so that water is uniformly heated. |
 | The bitumen starts to melt and the steel ball starts to sink due to its own weight. |
 | The temperatures at which each ball touches the bottom plate are noted. |
Result | |
| The softening point of the sample | The average of the softening point value for both the balls |
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