# Specific Gravity/Relative density of Coarse Aggregates- ASTM Standards

Specific gravity of an aggregate is defined as the mass of the aggregate in air divided by the mass of an equal volume of water. For example, an aggregate with a specific gravity of 2.50 means, aggregate is two and one-half time heavier than water.

Each construction aggregate is made out of some solid matter and voids, that may or may not contain water. As aggregates mass varies with the moisture content, we determine the specific gravity of aggregate at a fixed moisture content. There are four moisture contents defined for aggregates, which is dependent on the amount of water that is held within the pores on the surface of the particles.

1. Damp/Wet
2. Saturated surface-dry
3. Air-dry
4. Oven-dry
Aggregates with pores connected to the surface are filled with water and free water on surface comes under damp/wet condition. Saturated surface dry aggregates have pores that are connected to the surface filled with water and no free water on surface. Air-dry aggregates has dry surface with some water in the pores. Oven-dry aggregates have no water on the pores.

The volume of the aggregate particle is considered as the volume of the solid matter and internal pores. Based on whether choose a oven-dry or saturated surface dry mass, the specific gravity values vary.

### Bulk Specific Gravity and Saturated Surface-Dry (SSD) Specific Gravity

Bulk specific gravity is the oven-dry mass of aggregate divided by the mass of a volume of water equal to the saturated surface dry mass. While SSD bulk specific gravity is the saturated surface dry mass of aggregate divided by the mass of volume of water equal to the SSD aggregate volume. Most normal weight aggregates have a bulk specific gravity SSD between 2.4 and 2.9.

### Determination of Specific Gravity of Coarse Aggregates

The test method to determine specific gravity of aggregates are explained in ASTM C 127, "Specific Gravity and Absorption of Coarse Aggregate," and ASTM 128, "Specific Gravity and Absorption of Fine Aggregate."

In the test method, the coarse aggregate is thoroughly washed, dried to constant mass at 100 to 110 degree celsius , cooled in air and immersed in water for 24 hours. The sample is then removed from water and dried to saturated surface-dry (SSD) state with a large absorption cloth.

 Drying Coarse Aggregates Using Cloth - SSD Determination

Note: Take care to avoid evaporation of the water from the aggregate pores during this operation.

1. Determine the mass of the sample in air ( B)
2.  Specific Gravity Frame For Coarse Aggregates
3. Place the sample of coarse aggregate in a wire basket suspended in water for determination of its apparent mass in water (C). Before adding aggregates into wire bucket immersed in water, the whole arrangement with wire basket inside water weight must be made to zero reading.
4. After the mass in water is determined, the sample is oven dried and the oven dry mass is determined (A)
NOTE: The apparent mass of the sample in water is less than that in air. The loss in mass is equal to the mass of the water displaced. Hence, the loss in mass is the mass of a volume of water equal to the aggregate volume lost.

Then bulk specific gravity and bulk specific gravity SSD is determined as follows,

Apparent Specific Gravity = A/(A-C)

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