Classification of Aggregates

Depending upon the size of their particles aggregates are classified as:

  1. Fine aggregates.
  2. Coarse aggregates.
Fine Aggregates:

Aggregates whose particles pass through 4.75mm IS sieve are termed as fine aggregates. Most commonly used fine aggregates are sand (pit or quarry sand, river sand and sea sand) and crushed stone in powdered form, however some times sukhi and ash or cinder are also used.

a. Sand:

It consists of small angular or rounded grains of silica depending upon the source from which it is obtained. It is classified as:

i. Pit or quarry sand:

It is found as deposited in soil and is to be excavated out. Its grains are generally sharp or angular. It should be free from organic matter and clay. It is usually considered to be the best fine aggregate for use in mortar and concrete.

ii. River Sand:

It is obtained from the banks and beds of rivers. It may be fine or coarse. Fine sand obtained from beds and banks of rivers is often found mixed with silt and clay so it should be washed before use. But coarse sand is generally clean and excellent for use especially for plastering.

iii. Sea Sand:

It consists of fine rounded grains of brown colour and it is collected from sea shores or sea beaches. Sea sand usually contains salts and while using that in mortar, etc., causes disintegration of the work in which it is used. In R.C.C work these salts will attack reinforcement if salt content is high. These salts may cause efflorescence. It should be used locally after thorough washing.

b. Crushed stone:

It is obtained by crushing the waste stones of quarries to the particular size of sand. Sand obtained from by crushing a good quality stone is excellent fine aggregate. Mortar made with this sand is usually used in ashlar work (good quality of work).

Coarse Aggregates:

Aggregates whose particles do not pass through 4.75mm IS are termed as coarse aggregates. Most commonly used coarse aggregates are crushed stone, gravel; broken pieces of burnt bricks, etc.

a. Crushed stone:

It is an excellent coarse aggregate and is obtained by crushing granite, sand stone or grained lime stone and all types of stones. Crushed stones are used for the construction of roads and railway tracks, etc.

b. Gravel:

It is another very good coarse aggregate. It is obtained from river beds, quarries and seashores. The gravel obtained from sea shores should be well washed with fresh water before use in order to remove the impurities which may be clay, salts, silt, etc. It is commonly used in the preparation of concrete.

c. Broken pieces of bricks:

It is also a good artificial source of coarse aggregates. It is obtained by breaking well burnt bricks. It is generally used in lime concrete at places where aggregates from natural sources are either not available or are expensive. It can be used at places where low strength is required. It should be watered well before using it in the preparation of concrete. It is commonly used for mass concrete in foundations and under floors.

 

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