Apparent Specific Gravity:

The specific gravity of an aggregate in its absolute sense is the ratio of the weight of the solid in a given volume of sample to the weight of an equal void-free volume of water at the same temperature. Due to the presence of internal voids in the aggregates specific gravity of aggregate is more conveniently designated as apparent specific gravity. It is defined as ratio of the weight of oven dried aggregate to the weight of water occupying the volume of aggregates (including the volume of internal voids). It is determined in the laboratory by the following formula:

Apparent Specific Gravity = c/(c - b)

Where, c = Weight of aggregates dried in an oven at 100º-110ºC for 24 hours, and

b = Weight of saturated surface dry aggregates in water.

Saturated Surface Dry (SSD) Condition

Saturated surface dry condition means the aggregate is saturated and there is no water film (moisture) on the surface of aggregate. The specific gravity based on the SSD condition is most frequently used in concrete technology. Because the water present in the internal pores of the aggregate does not contribute in the hydration process so it can be considered as part of the aggregate.

Comments

The bulk density is used to convert the weight proportion into the volume proportion or vice-versa and to judge the quality and grading of aggregates by comparing it with the normal density of that type of aggregates. It is also used for determining the void ratio.

Void Ratio = Bulk density/Apparent specific gravity

The range of average specific gravity for many commonly available rocks is 2.6 to 2.7. Any variation in the specific gravity indicates the variation in shape/grading of aggregate. It is useful in determining the percentage voids of aggregates. It also helps in converting the weight of aggregates into solid volume and also used for determining the compacting factor in workability test.