The specific gravity of bitumen, which is also known as asphalt, varies depending on the type and grade of bitumen. Generally, the specific gravity of bitumen at room temperature (25 degree Celsius or 77 degree Fahrenheit) is in the range of 1.01 to 1.06. Bitumen is less dense than water which has a specific gravity of 1.0. The specific gravity of bitumen is an important feature to consider when determining a bitumen sample for its suitability for various applications. Different grades and sources of bitumen may have different specific gravity values, so its important to refer to the specific product specifications for precise information.
This test is conducted to determine the density (specific gravity) of bituminous materials. This test is needed for quality control and grading of bitumen in construction and paving applications.
Scope & Significance
- Specific gravity of a bitumen binder is a fundamental property frequently required as an aid in classing binders for use in paving jobs.
- Bitumen weights sometimes have to be converted into volumes for asphalt concrete mix design calculations for which a knowledge of specific gravity is essential.
- Specific gravity is also used in identifying the source of bitumen binder.
- Bitumen binder has specific gravity in the range of 0.97 to 1.02.
- In case bitumen contains mineral impurities the specific gravity will be higher. Thus it is possible for a quantitative extraction of mineral impurity in bitumen.
How specific gravity test is conducted?
Apparatus:
A list of instruments and apparatuses used for testing bitumen’s specific gravity follows,
- Specific Gravity Bottles: Wide-mouthed capillary type specific gravity bottle is used.
- 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
- Beaker
Materials for Specific Gravity Test of Bitumen:
- Bituminous product
- Distilled water
- Carbon disulphide, benzol, or any other solvent for cleaning apparatus
Precaution:
When performing a specific gravity test on bitumen, the following precautions should be observed:
- 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.
Procedure:
- 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.
Calculation:
where,
- 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