Specific Gravity Test of Bitumen is determining the specific gravity of bitumen which one of the fundamental properties of bitumen. It can therefore be used in the classification of bitumen binders used for pavement construction.
It can also help in identifying the source of the bitumen binder.
It can also help in pointing out the mineral impurities present in the bitumen specimen. The value of specific gravity will increase if the quantity of mineral impurities is more. This is important during the extraction of impurities from the bitumen binder.
Presenting asphalt concrete mix design, sometimes the volume of bitumen is specified instead of weight. For the conversion of weight into volume, a specific gravity test of bitumen is important.
To determine the specific gravity of bitumen, asphalt, coal tar, road tar, and other bituminous products.
Here, the specific gravity of bitumen sample is determined by Pyknometer method.
Important Terms related to Specific Gravity Test of Bitumen:
Specific gravity: It is the ratio of the mass of the substance in consideration to the mass of an equal volume of water at a specified temperature.
(It is also calculated by taking the ratio of the density of a substance to the density of water)
The Specific gravity test of bitumen works on the below principle:
Archimedes Principle: If a solid material is first weighed in air and then weighed after immersing it in water, then the difference in the two weights gives the volume of water displaced by the solid material.
Following are the apparatus and instruments used for specific gravity test of bitumen,
- Specific Gravity Bottles: Wide-mouthed capillary type specific gravity bottle is used.
(Note: There are two types of specific gravity bottles with a capacity of 50 ml are used.
- Ordinary capillary type-specific gravity bottle whose neck diameter is 6 mm. It is used for materials that remain absolute fluid at 25 °C temperature.
- Wide-mouthed capillary type-specific gravity bottle whose neck diameter is 25 mm. It is used for materials that remain semi-solid or highly viscous at 25 °C temperature)
- Stopper: It is centrally located with reference to the vertical axis. Its diameter is 1-2 mm. it is smooth and plain at the top while the lower surface is kept concave to allow the air to escape from the bore. The height of the concave surface at the center is 4.0-6.0 mm.
- Constant temperature bath: It should be maintained at the given temperature with 0.2 °C accuracies. Its depth should be greater than that of pycnometer bottles.
- Bath Thermometer – To measure the temperature of water in the ductility apparatus. Its range should be from 0 to 44 °C. Its least count should be 0.1 °C
Other accessory apparatus include:
- Electronic balance
- Heating oven
- A clean and dry cloth
Materials for Specific Gravity Test of Bitumen:
- Bituminous product
- Distilled water
- Carbon disulphide, benzol, or any other solvent for cleaning apparatus
Following precautions should be taken while performing the specific gravity test of bitumen to obtain accurate results:
- The specific gravity bottles should be cleaned first before use
- The distilled water should necessarily be freshly boiled and used after cooling
- The bottles should be cleaned with a clean and dry cloth after removing from the water bath and before weighing
- While taking the weight of the sample, the temperature should never be allowed to exceed the specified one by 0.1 °C
- Expansion and over-flow of the contents should be prevented during heating
- While filling the bottle and also while inserting the stopper, care should be ensured that no air bubbles are entrapped
- As the bottle is filled, weighing should be followed quickly and should be weighed up to 0.1 mg with accuracy
- Instruments like a thermometer and weighing balance should be calibrated well
- If the testing material possesses high viscosity, then the apparatus should be warmed in the oven up to 100 before cleaning. Heat it until most of the material is melted and then swab it with a cloth or cotton. After the apparatus is cooled, rinse it with carbon disulphide, benzol, or any other solvent.
Following steps to be follow while performing the specific gravity test of bitumen-
- Clean the specific gravity bottles and weigh them after they are dried along with the stopper. Note down this weight as a.
- Fill distilled water in one of the bottles and put it on the stopper. The water should be freshly boiled and cooled before filling.
- Place the bottle submerged up to its neck in a beaker of distilled water for at least 30 minutes at 27 ± 0.1 °C. or any other temperature at which specific gravity is to be determined can be maintained.
- Remove the bottles from the water bath and clean the surplus moisture on the outside of the bottle with a dry cloth.
- Measure the weight of the bottle again. Note down this weight as b.
- Dry the bottle again after taking the weight.
- Bring the bitumen specimen to a fluid state by the application of heat carefully so that the material does not evaporate.
- Warm the specific gravity bottle slightly.
- Pour the bituminous sample in a clean and dry specific gravity bottle until half. Ensure ht material does not come in contact with the sides of the bottle. A small funnel can be used for pouring to prevent the material from coming in contact with the neck of the bottle.
- Allow the partly filled bottle to stand at 60-70 temperature so that the entrapped air bubbles can escape if any.
- Allow the bottle to cool up to the specified temperature and weigh it. Note down this weight as c.
- Fill the bottle containing asphalt with freshly boiled distilled water and loosely place the stopper. Prevent air bubbles in the bottle.
- Place the specific gravity bottle in the water bath and attach the stopper firmly for at least 30 minutes.
- Remove the bottle from the water bath and wipe the surplus moisture deposited on the outer surface of the bottle with a dry cloth.
- Measure the weight of the bottle again. Note down this weight as d.
Specific gravity of the specimen can be determined by:
a = weight of the specific gravity bottle
b = weight of the specific gravity bottle + weight of distilled water
c = weight of the specific gravity bottle + weight of the specimen half-filled in the bottle
d = weight of the specific gravity bottle + weight of the specimen half-filled in the bottle + weight of the water filled in the remaining half of the bottle
(Specific gravity test of bitumen sample observation table is drawn below)
|Sample 1||Sample 2||Sample 3|
|Weight of Empty Bottle (a)|
|Weight of Bottle filled + Weight of Distilled Water (b)|
|Weight of bottle + Weight of Bitumen filled till half (c)|
|Weight of bottle + Weight of Bitumen filled till half + Weight of Distilled Water filled till half d|
|Specific gravity = (c-a) / (b-a) – (d-c)|
Specific gravity of the sample = Average value of specific gravity of 1, 2, 3.
The specific gravity of the specimen is recorded as the ratio of the mass of specimen to the mass of an equal volume of distilled water.
Specific gravity of bitumen falls within the range- 0.97 to 1.02 at 27 °C.
Minimum specific gravity values standardized by Bureau of Indian Standard for Paving Bitumen at 27 °C for different grades are tabulated below:
|Grade of Bitumen||Specific Gravity|
|A – 25||0.99|
|A – 35||0.99|
|A – 45||0.99|
|A – 65||0.99|
|S – 35||0.99|
|S – 65||0.99|
|A – 90||0.98|
|S – 90||0.98|
|A – 200||0.97|
|S – 200||0.97|
By performing the specific gravity test of bitumen practical, we can determine whether the given bitumen sample meets the specification requirements for specific gravity as given in the standards.
The specific gravity found out by the practical is compared to the standard value and thus it can be concluded whether the sample meets the specific gravity criteria for the given grade of bitumen.
Following table show step by step procedure of specific gravity test of bitumen with picture-
|The specific gravity bottle is cleaned from inside with a cotton swab or a dry cloth.|
|Heat the bottle in the oven.|
Weigh the bottle along with the stopper-
Note this weight as a.
|The bottle is filled with the freshly boiled distilled water after cooling.|
|The stopper is placed at the top.|
The bottle and stopper assembly are placed in a water bath for 30 minutes which is maintained at 27 ± 0.1 °C temperature.
The bottle is immersed in the water bath till the neck.
The outer surface of the bottle is cleaned with a dry cloth.
|The weight of the bottle along with the stopper and distilled water is noted as b.|
|The bitumen is heated up to boiling temperature.|
|The liquid bitumen is poured into a clean and dry specific gravity bottle up to half.|
|The stopper is placed and the bottle is allowed to cool down at room temperature for 30 minutes.|
|The bottle is immersed till neck in the water bath maintained at 27 ± 0.1 °C temperature for 30 minutes.|
|The outer surface of the bottle is wiped with a dry cloth to clean it.|
The specific gravity bottle along with the bitumen is weighed.
The weight is noted as c.
|The remaining half-space of the bottle is filled with distilled water.|
|The stopper is placed at the top of the bottle.|
|The bottle is immersed till the neck in the water bath maintained at 27 ± 0.1 °C temperature for 30 minutes.|
|The outer surface of the bottle is wiped with a dry cloth to clean it.|
The specific gravity bottle filled half with bitumen and half with distilled water is weighed.
The weight is noted as d.
At least 3 readings should be taken following a similar procedure.
And their average is taken as the specific gravity of the specimen.
|The specific gravity bottles are cleaned and dried before using|
|Specific gravity bottle should be wiped clean with a cloth after taking out from water bath and before weighing.|
The test temperature is to be maintained at 27 °C.
Tolerance of 0.1 °C is allowed.
|It should be ensured that instruments like the balance and thermometer are well-calibrated.|
= (Weight of the bitumen sample)/(Weight of equal amount of distilled water)
|Specific Gravity of the Bitumen Specimen||Average of values of specific gravity taken through a minimum of 3 tests|