Specific Gravity and water absorption
The specific gravity and water absorption of
aggregates are important properties that are required for the design of
concrete and bituminous mixes. The specific gravity of a solid is the ratio of
its mass to that of an equal volume of distilled water at a specified
temperature. Because the aggregates may contain water-permeable voids, so two
measures of specific gravity of aggregates are used:
specific gravity and bulk specific gravity.
Apparent Specific Gravity, Gapp
, is computed on the
basis of the
net volume of aggregates i.e the volume excluding water-permeable voids.Thus
where, MD is the dry mass of the aggregate, VN is the net
volume of the aggregates excluding the volume of the absorbed matter, W is the
density of water.
Specific Gravity, Gbulk , is computed on the basis of the total volume of
aggregates including water permeable voids. Thus
is the total volume of the aggregates including the volume of absorbed water.
Water absorption, The difference between the apparent and bulk
specific gravities is nothing but the water-permeable voids of the aggregates.
We can measure the volume of such voids by weighing the aggregates
in a saturated, surface dry condition, with all permeable voids filled with
water. The difference of the above two is MW . MW is the weight of dry
aggregates minus weight of aggregates saturated surface dry condition. Thus
The specific gravity of aggregates normally used in road
construction ranges from about 2.5 to 2.9. Water absorption values ranges from
0.1 to about 2.0 percent for aggregates normally used in road surfacing.