The specific gravity of fine aggregate, commonly referred to as sand, is a crucial physical property used in civil engineering and construction. It is defined as the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. This dimensionless quantity helps determine the density and quality of the material, which in turn affects the strength and durability of concrete and other construction mixtures.
The specific gravity of sand typically ranges around 2.65 , although this value may vary slightly depending on the source and composition of the sand. Accurate determination of specific gravity is essential for mix design calculations and quality control in construction projects.
Apparatus Required for Specific Gravity Test
To perform the specific gravity test of fine aggregates, the following equipment is required:
- Balance
- Capacity: At least 3 kg
- Accuracy: Readable and accurate to 0.5 grams
- Purpose: Used to weigh the pycnometer along with the sample and water
- Oven
- Temperature range: 100°C to 110°C
- Purpose: To dry the aggregate sample completely before testing
- Pycnometer
- Capacity: Approximately 1 liter
- Features: A metal conical screw top with a 6 mm hole at its apex; watertight seal
- Purpose: Precise measurement of volume by displacement
- Warm Air Supply
- Purpose: Helps in drying or removing surface moisture from the sample
- Tray
- Minimum area: 32 cm²
- Purpose: For transferring and handling the aggregate sample
- Airtight Container
- Purpose: To store the sample prior to testing and prevent moisture absorption
- Filter Paper and Funnel
- Purpose: To drain water from the soaked aggregate after testing
Procedure for Specific Gravity Test
Follow these steps to accurately determine the specific gravity of fine aggregate:
- Sample Preparation:
Take approximately 500 grams of the fine aggregate sample and place it into the clean, dry pycnometer. - Add Distilled Water:
Fill the pycnometer with distilled water until it is full, ensuring that the sample is completely submerged. - Remove Entrapped Air:
Rotate the pycnometer gently on its side while covering the 6 mm hole at the apex of the cone with your finger. This action helps release any air bubbles trapped within the mixture. - Weigh the Pycnometer with Sample and Water:
Wipe off any excess moisture from the outer surface of the pycnometer and weigh it. Record this weight as W . - Transfer the Contents:
Carefully transfer all the contents of the pycnometer into a tray, making sure no aggregate is left behind. - Refill the Pycnometer with Water Only:
Refill the pycnometer with distilled water up to the same level as before and weigh it again. Record this weight as W₁ . - Drain the Water from the Sample:
Use a funnel and filter paper to drain the water from the aggregate sample. - Dry the Sample:
Place the wet sample in a tray and put it in the oven at a temperature of 100–110°C for 24 ± 0.5 hours . Stir occasionally to ensure uniform drying. - Cool and Weigh the Dry Sample:
After drying, allow the sample to cool to room temperature and weigh it. Record this final dry weight as W₂ .
Calculations
Using the recorded weights, calculate the apparent specific gravity using the following formula:
Where:
- W2 = Weight of the dry sample
- W = Weight of the pycnometer with sample and water
- W1 = Weight of the pycnometer filled only with water (optional for alternate calculation methods)
This formula calculates the ratio of the weight of the dry sample to the weight of an equal volume of water, giving the apparent specific gravity of the fine aggregate.
Question Section
What is the unit of specific gravity of fine aggregate?
Since specific gravity is a dimensionless quantity , it has no unit . It is simply expressed as a numerical value , such as 2.65. The result depends on the density of the aggregate relative to the density of water.
Conclusion
Determining the specific gravity of fine aggregate is a fundamental step in assessing the quality and suitability of sand for use in construction materials like concrete and mortar. The test involves precise weighing and volume measurements using standard laboratory apparatus such as the pycnometer and balance.