The aggregates are generally classified based on size, as
- Fine Aggregates
- Coarse Aggregates
The Size of Aggregates used in concrete range from few centimetres or more down to a few microns
|Fig.2: Coarse Aggregate|
It is the aggregate most of which passes through a 4.75mm IS sieve and contains only that much coarser materials as it is permitted by the specifications. Sand is generally considered to have a lower size limit of about 0.07mm. The fine aggregate may be one of the following types:
Natural Sand : This are fine aggregate that is obtained from natural disintegration of rock and or that which has been deposited by stream and glacial agencies.
Crushed Stone Sand : Fine aggregates produced by crushing hard stone.
Crushed Gravel Sand: Produced by crushing natural gravel
The fine aggregates can be classified as coarse, medium and fine sands based on the fineness modulus
- Fine Sand - Fineness Modulus of 2.2 to 2.6
- Medium Sand - Fineness Modulus of 2.67 to 2.9
- Coarse Sand - Fineness Modulus of 2.91 to 3.2
According to particle size distribution, IS: 383 -1970 has divided the fine aggregate in to 4 grading zones. The grading zone become progressively finer from grading zone 1 to grading zone IV.
The aggregate that is most retained on 4.75mm IS sieve . This contains only finer material as permitted by the specification. Based on the source, the coarse aggregate can be classified as:
- Uncrushed Gravel or Stone : It is obtained from the natural disintegration of rock
- Crushed Gravel or Stone : It results from crushing of gravel or hard stone
- Partially crushed gravel or stone : It is the product of blending of the above two aggregate
According to size, the coarse aggregate is described as graded aggregate of its nominal size .i.e 40mm, 20mm, 16mm and 12.5mm etc. For example a graded aggregate of nominal size 20mm means and aggregate most of which passes 20mm IS sieve. A coarse aggregate that has sizes of particulars mainly belonging to a single sieve size is known as single size aggregate. For example 20mm single size aggregate mean an aggregate most of which passes through 20mm IS sieve and its major portion of which is retained in 10mm IS sieve.
All in Aggregate
The all in aggregate composes of both fine aggregate and coarse aggregate. According to size , the all-in-aggregate is categorised as all in aggregates of its nominal size i.e. 40mm, 20mm etc.
For example all in aggregate of nominal size of 20mm means an aggregate in which most of them passes through the 20mm IS sieve and contains fine aggregates also.