The Ductility Test of Bitumen is one of the important tests of bitumen which is essential before using it in road construction. The Ductility Test of Bitumen used to measure the adhesive and elastic properties of any bitumen sample.
To determine the ductility of cutback bitumen, blown type bitumen, or any other bituminous products.
In this ductility test of bitumen, the amount by which the bitumen will stretch at a temperature below the softening point is determined.
Significance of Ductility Test of Bitumen:
- The ductility test of bitumen sample is one of the important tests of bitumen to be conducted before road construction.
- Ductility measures the adhesive property of the bitumen too along with its elasticity.
The bituminous binder used in the pavement should be ductile enough to take up the deformations acting upon it without exceeding the deflection limit. Ductile bitumen can form a thin ductile film around the aggregate particles. Thus, it does not crack at low temperatures. However, the films of brittle bitumen are not ductile and crack may easily develop in lower temperatures.
Important Terms related to Ductility Test of Bitumen:
Ductility: It is the property of a material by virtue of which a material is able to undergo plastic deformation when pull force is applied without rupture or breaking.
Ductility of Bitumen: The amount in centimetre by which the bitumen can stretch before breaking is called ductility of bitumen.
Grade of Bitumen: There are five grades of bitumen on the basis of penetration value. They are demonstrated as S-35, S-45, S-55, S-65, S-90, and S-200.
Principle of Ductility Test of Bitumen:
The practical works on the below principle:
When a standard pulling force is applied to a bitumen specimen at a specific speed and specific temperature, the specimen will elongate before breaking. The limit up to which the specimen can elongate is recorded which is termed as ductility of the bitumen sample.
- Ductility apparatus: It is the testing machine for bitumen. The various parts of the testing machine are described below:
- A rectangular tank with copper or steel lining which is filled with water so as the specimen remains submerged in the water
- Built-in heater which is used in a cooler climate
- Pump to maintain a uniform temperature by circulating water
- Clutch for regulating and adjusting suitable speed while testing
- Briquette mould- 3 in no.: The material is brass and it is 8-shaped. Its parts are described below:
- Two Clips- There are two holes in the clips. These holes are used to fix the mould in the testing machine. One clip is attached to the fixed end while the other to the moving end.
- Two Sides
Dimensions of the Briquette mould are mentioned below:
Total length = 75 mm ± 0.5 mm
Distance between the 2 clips = 30 mm ± 0.3 mm
Thickness of the clip = 10 mm ± 0.1 mm
Width of the clip = 20 mm ± 0.2 mm
Water bath: It should contain a minimum of 10 litres of water. Bitumen specimen should be immersed to at least 100 mm depth. Moreover, the specimen should be supported over a perforated shelf which should be above 50 mm from the bottom of the bath.
Thermometer or Temperature indicator– To measure the temperature of water in the ductility apparatus. Its range should be from 0 to 44 °C.
Other accessory apparatus include:
- Heating oven
- Bitumen of known grade
Precaution in Ductility Test of Bitumen:
Following precautions should be taken while performing the ductility practical to obtain accurate results:
- The briquette mould should not have a crack at any side
- The base plate of the briquette mould on which the sample is to be laid should be completely flat
- The sample should not be overheated
- While filling the mould with the sample, care should be exercised so that the parts of the briquette are not displaced
- Care should be taken that no air bubble is entrapped within the sample while preparing the mould
- The temperature of the water bath should not exceed 27 °C
- The pull rate of the ductility testing machine should not exceed 50 mm/min
- It should be ensured that the specimen is completely immersed in the water during the time of testing. The water depth above and below the specimen should be at least 25 mm.
- Initial readings should also be noted carefully if the pointer is not at zero
- Final reading should be carefully noted for all the three sample specimens at the time of the breaking of the bitumen thread
The Procedure of Ductility Test of Bitumen:
The ductility test of bitumen is carried out in two steps:
- Sample Preparation: In this step, the bitumen sample is filled in the briquette mould after melting it.
- Testing of the Specimen: In this step, the bitumen sample taken in the mould is tested for its ductility in the ductility testing machine.
- Take the bitumen sample in a beaker 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.
- Prepare a mixture of glycerine and dextrin in equal proportions. Mix it well. Then coat the surface of the brass plate and the interior of the sides of the briquette mould. This prevents the bitumen from sticking to the mould.
- Fix the sides, clip over the base plate, and tighten the screw of the clip with the help of a screwdriver.
- Prepare all the three briquette moulds in a similar manner.
- Pour the melted bitumen in the briquette mould until they are full.
- Allow the mould to cool at room temperature in air for about 30 to 40 minutes.
- Place the whole assembly with brass plate and mould in a water bath, which is maintained at 27 °C for about 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. The dimension of the briquette thus formed is exactly 1 sq cm.
Testing of the Specimen:
- Place the brass plate along with mould containing the specimen again in the water bath at 27 °C for 85-95 minutes.
- Remove the briquette from the base plate, unscrew the clips, and remove the sides of the briquette mould.
- Attach the assembly of bitumen sample with a base plate with the help of rings of the clips to the pins or hooks in the ductility machine. One clip of the mould is attached to the moving part while the other clip is attached to the fixed part of the ductility machine.
- Tighten the screw of the clips of the mould with the help of a screwdriver.
- Check the pointer of the ductility machine is at zero. (Or note down the initial reading of the ductility machine).
- Switch on the ductility machine and adjust the gear so that the movable part moves at the speed of 50 mm/minute. (The pull rate of the machine is to be maintained 50 mm/min).
- Thus, the two clips are pulled apart horizontally at uniform speed until the briquette specimen ruptures.
- Measure the distance between the clips at the time of rupture of the specimen.
As the ductility machine is allowed to run, one end of the machine remains fixed, while the other end is pulled apart.
The distance at which the thread of the bitumen breaks is to be noted down for all the three samples.
If the specimen does not break even at 75 centimetres, the machine is stopped and the ductility value is then mentioned as greater than 75 cm.
Observation Table for Ductility Test of Bitumen:
(A sample observation table is drawn below)
|Description||Sample- 1||Sample- 1||Sample- 1|
|Initial Reading (a) in mm|
|Final Reading (b) in mm|
|Ductility (b-a) in mm|
Ductility of the sample = Average value of ductility of 1, 2, 3.
The average of the ductility values to the nearest whole value obtained from the three moulds is the ductility value of the given bitumen specimen.
If the ductility value of the three samples does not lie within ±5 % of the mean value of the three, then the average of the higher two values is calculated. If the higher two ductility values lie within ±5 % of their mean value, then this means the value is recorded as ductility value.
If the bitumen specimen comes in contact with the bottom of the testing apparatus, then the test is not considered normal. To prevent this, methyl alcohol or sodium chloride is added to the water bath to increase the specific gravity of the water.
If on three consecutive results, a normal test is not obtained, then the ductility value is mentioned as unobtainable in the report.
The ductility test of bitumen specimen is dependent on the grade of the bitumen. Hence, ductility requirements vary with the grade of the bitumen.
Minimum ductility values standardised by Bureau of Indian Standard for Bitumen Type1 for different grades are tabulated below:
|Grade of Bitumen Type-I||Ductility (in cm)|
|S – 35||50|
|S – 55||75|
|S – 65||75|
|S – 90||75|
|S – 200||75|
By performing the ductility test of bitumen, we can determine whether the given bitumen sample meets the specification requirements for ductility as given in the standards.
The ductility value found out by the practical is compared to the standard value and thus It can be concluded whether the sample meets the ductility criteria.
Ductility is one of the measures of the quality of bitumen along with other measures such as penetration value and so on.
Ductility Test of Bitumen Procedure
1. The bitumen sample is heated in a beaker at 75-100 °C- above
the softening point of bitumen till it is completely melted into a fluid
2. Glycerine and dextrin are mixed together in equal proportions and applied to all the parts of the mould that will come in contact with bitumen to prevent the bitumen from sticking to itThe clips and sides are then fitted to the base plate and the screw is tightened
3. Melted bitumen is poured into the mould till it is completely filled
4. 3 such briquette moulds are preparedAll the moulds are allowed to cool at room temperature for 30 minutes
5. The whole assembly is placed in the water bath at 27 °C for 30 minutes
|7. Excess bitumen is cut off by a pointed sharp-edged knife to obtain a leveled surface at the top|
|8. Briquette is separate from the base plate|
|9. The whole assembly is placed in the water bath at 27 °C for 85-95 minutes|
10. Then it is attached to the ductility machine with the help of rings of the clip and hook of the machineThe screws of the clip are then tightened
11. The specimen is so placed that it is submerged in the water both for a minimum depth of at least 25 mm on both upper and lower part of the plate
12. The screw of the clip is tightened and the sides are removed
13. The pointer is set to zero
14. The machine is switched on and the gear is so adjusted that the movable part pulls the sample at the rate of 50 mm per minute
15. The 2 clips are pulled apart and the distance at which the specimen breaks is noted down for all the three samples
The distance is measured in centimetre from the indicator given on the machine
16. Care should be taken that the bitumen thread does not touch the bottom of the machine, else the test is not considered normal
17. The average ductility value of 3 samples
(The values should not differ by ±5 % of the mean value)