The specific gravity of aggregates is a measure of the density of the material compared to the density of water. It is an essential property in determining the weight-to-volume relationship for aggregates used in concrete or asphalt. The specific gravity of both coarse aggregate and fine aggregate can vary depending on the type of material.
Specific Gravity of Coarse Aggregate and Fine Aggregate as Per IS:2386 (Part-3)
Specific gravity is defined as the ratio of the weight of an aggregate to the weight of an equal volume of water. It serves as an indicator of the strength or quality of the material, with aggregates having a lower specific gravity generally being weaker than those with a higher specific gravity. This property is useful for the general identification and evaluation of aggregates.
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| Specific Gravity Balance |
Specific Gravity of Coarse Aggregate
Coarse aggregates are the larger particles, such as gravel and crushed stone, used in construction materials like concrete. The specific gravity of coarse aggregates typically ranges from 2.50 to 2.90. For instance, gravel usually has a specific gravity between 2.60 and 2.70, while crushed stone generally falls within a range of 2.60 to 2.80. This property helps in determining the weight-to-volume relationship, which is very important for the correct proportioning of materials in construction.
Specific Gravity of Fine Aggregate
Fine aggregates are the smaller particles, such as sand, used in construction materials like concrete. The specific gravity of fine aggregates generally ranges from 2.60 to 2.70. For example, natural sand typically has a specific gravity between 2.60 and 2.70, while manufactured sand usually falls within a range of 2.60 to 2.80. This property is important for determining the appropriate proportions of materials in construction projects, ensuring the desired strength and durability of the final product.
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| Pycnometer |
Determination of Specific Gravity of Aggregate
Determination of specific gravity of aggregate as per IS:2386 (Part-3):1963 (Reaffirmed Dec 2016) - Methods of Test for Aggregates for Concrete.
Apparatus Required
Wire Mesh Basket: A wire basket with a mesh size not exceeding 6.3 mm or a perforated container of convenient size, with thin wire hangers for suspending it from a balance.
Setup for Specific Gravity Test (for aggregates > 6.3 mm): The setup consists of a container for filling water, a wire basket suspended in the water, an airtight container of similar capacity to the basket, a shallow tray, and two dry absorbent cloths.
Pycnometer: A 1000 ml pycnometer is used for aggregates finer than 6.3 mm.
Test Procedure
For Aggregates Coarser Than 6.3 mm
Sample Preparation: Take about 2 kg of the aggregate sample, wash it to remove fines, and place it in the wire basket.
Immersion: Immerse the basket with the sample in water at a temperature between 22°C to 32°C.
Air Removal: Immediately after immersion, remove any entrapped air by lifting the basket 25 mm above the water's base and allowing it to drop 25 times, with a rate of about one drop per second.
Immersion Duration: Keep the basket and aggregate fully immersed for 24 ± 0.5 hours.
Weighing in Water: Weigh the basket and aggregates while suspended in water at 22°C to 32°C.
Drying the Aggregate: Remove the basket and aggregates from the water, dry them with absorbent cloths, and weigh again.
Oven Drying: Place the dried aggregates in a shallow tray, heat in an oven at 110°C for 24 hours, cool in an airtight container, and weigh.
For Aggregates Finer Than 6.3 mm
Pycnometer Preparation: Take a clean, dry pycnometer and weigh it.
Sample and Initial Weighing: Add about 1000 g of the clean sample into the pycnometer and weigh it again.
Water Immersion: Fill the pycnometer with water at 27°C, just enough to immerse the sample.
Air Removal: Shake the pycnometer and cover the hole at the top to remove any entrapped air.
Complete Filling and Weighing: Fill the pycnometer completely with water, ensuring no air is trapped, and weigh it again.
Cleaning: Discharge the contents of the pycnometer and clean it.
Final Weighing: Refill the pycnometer with water and weigh it again.
For Mineral Filler: A specific gravity bottle is used, filled up to one-third of its capacity, and the process follows similarly as described for aggregates finer than 6.3 mm.
Observation and Recording
Observation Table for Aggregates Coarser Than 6.3 mm
| S. No. | Description | Observed Value |
|---|---|---|
| 1 | Weight of saturated aggregate and basket in water; W1 g | |
| 2 | Weight of basket in water; W2 g | |
| 3 | Weight of saturated aggregates in air; W3 g | |
| 4 | Weight of oven-dried aggregates in air; W4 g | |
| 5 | Apparent Specific Gravity; W4 / [W4 – (W1 - W2)] | |
| 6 | Bulk Specific Gravity; W4 / [W3 – (W1 - W2)] |
Observation Table for Aggregates Finer Than 6.3 mm
| S. No. | Description | Observed Value |
|---|---|---|
| 1 | Weight of Pycnometer in air; W1 g | |
| 2 | Weight of aggregates and pycnometer; W2 g | |
| 3 | Weight of aggregates, pycnometer and water; W3 g | |
| 4 | Weight of water and pycnometer in air; W4 g | |
| 5 | Apparent Specific Gravity; (W2 – W1) / [(W4 – W1) - (W3 - W2)] |
Remarks
- The specific gravity of most aggregates used in construction typically ranges from 2.5 to 3.0, with an average value of around 2.68.
- Specific gravity is considered an indicator of an aggregate's strength, with higher specific gravity generally corresponding to higher strength.
- The water absorption of aggregates provides insight into their porosity and is used as a measure of resistance to frost action and weathering.
