Short Answer:

The specific gravity of aggregates is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. It shows how dense the aggregate is compared to water. Higher specific gravity means denser and usually stronger aggregates, which are preferred for making concrete.

It is measured by using a special test where the aggregate is first soaked, weighed in water, and then in air. These weights are used to calculate the specific gravity. This test helps in selecting quality aggregates that give strength and stability to concrete structures.

Detailed Explanation:

Specific gravity of aggregates

The specific gravity of aggregates is an important physical property that helps in understanding the quality and strength of the material. It is defined as the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water at the same temperature. This property is useful in mix design, weight calculations, and quality control in construction work.

In simple terms, specific gravity tells us how heavy the aggregate is in comparison to water. If the specific gravity is higher, it means the aggregate is denser and typically stronger. In civil engineering, aggregates with a specific gravity between 2.6 and 2.9 are considered suitable for most construction works.

There are two main types of specific gravity used in construction:

• Apparent specific gravity – calculated by excluding the pores inside the aggregate.
• Bulk specific gravity – calculated by including the pores that can absorb water.

Both values are useful for different types of concrete design, depending on whether water absorption is a concern.

How is it measured

The specific gravity of aggregates is measured in a laboratory using a standardized test procedure. The common method followed is as per IS 2386 (Part III) or ASTM C127 for coarse aggregates and ASTM C128 for fine aggregates. The process involves the following steps:

1. Sample Preparation:
   Take a clean, dry sample of the aggregate. It is first soaked in water for 24 hours to fill up the pores and then surface dried using a cloth.
2. Weighing in Air:
   The aggregate is weighed in air using a digital balance. This weight is recorded as W₁.
3. Weighing in Water:
   The same aggregate is then placed in water, and its weight is recorded while it is submerged. This is recorded as W₂.
4. Weighing after Oven Drying:
   The sample is then dried in an oven at 100–110°C for 24 hours and weighed again. This weight is noted as W₃.
5. Calculation:
   Using these three weights, the specific gravity is calculated using the formula:

Specific Gravity = W₁ / (W₁ – W₂)

For more accurate results, other forms of the formula may be used depending on whether bulk or apparent specific gravity is being calculated.

This test helps engineers select aggregates that are suitable for producing strong and durable concrete. A low specific gravity may indicate the presence of porous or weak material, which is not good for construction.

Importance of knowing specific gravity:

• Helps determine the weight-to-volume relationship of materials in mix design.
• Indicates the quality and strength of aggregates.
• Essential in calculating the volume occupied by aggregates in concrete.
• Used in checking conformity to standards and quality specifications.

Conclusion:

The specific gravity of aggregates shows how dense and strong the material is in comparison to water. It is measured through a simple lab test that involves soaking, weighing, and drying the aggregate. This value helps engineers choose high-quality aggregates for concrete that will last long and perform well under loads.