Aggregates are a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates.
Aggregates are the inert i.e. chemically inactive materials which are bonded together by means of cement paste to form a solid mass. Nearly of about 75% of volume of concrete is composed of aggregates and hence the properties of concrete such as workability. strength, durability and economy are greatly influenced by them.
The aggregates are classified as fine aggregates and coarse aggregates.
The aggregate which passes I.S. sieve of size 4.75 mm is called as fine aggregates. Natural sand is the commonly used fine aggregate. Sand is obtained from sea, river, lake or pit. Sand to be used in concrete mix should be clean and should have uniform size of particles. It should not contain particles of silt, clay, salt or any other organic matter. As a good practice sand should be washed with water thoroughly before it is used in work. While using sea sand in RCC works proper care must be exercised to remove the salt content. Otherwise it will lead to corrosion of reinforcement. River sand usually contains fine particles of silt and hence it should be washed thoroughly to remove the fine matter.
Coarse aggregates are those aggregates which are retained on I.S. sieve of size 4.75mm. The size of aggregates to be used depends upon the nature of work, size of member, mix proportion etc. Coarse aggregates are generally obtained by crushing stones like granite, basalt, quartz, lime stone, gniess etc. Light weight aggregates such as expanded blast furnace slag, cinders, pumice stone, slate etc are used to make light weight concrete, but such mixes are unsuitable for RCC works. Heavy weight aggregates like scrap iron, iron shots, magnetite barytes are used to make heavy weight concrete. Such concrete is mainly used for specified purposes like radiation shielding etc. Segregation is the usual problem when magnetite, byrites iron shots etc. are used as coarse aggregates since they are not properly graded and are heavy in weight. Over burnt brick bats are sometimes used to make lean concretes which are generally used for foundation bed, matting below RCC footings, base for ground floors etc. Such brick bats should be free from lime content and should be soaked in water immediately before use. Under burnt brick bats should not be used for making up of concrete.
General Requirements of Aggregates
They should be hard, strong, and durable, should have uniform texture and should be closely grained.
They should be free from loose matter, silt, grit, loam. (Particles of size between 0.060101 to 0.0020101 is called as silt. Such a very tine matter affects the bond between the aggregates and the cement paste.)
Fine aggregates should pass 1.S. sieve of size 4.750101. At the same time it should not contain more than 10% of very fine particles which may pass through 1.S. sieve of size 150 micron (no. 15) They should be free from vegetable matter. This usually happens in sand. By washing thoroughly it can be removed.
They should be free from adherent coatings, oil etc. Such coatings will not allow aggregates to bond properly with the cement paste and cause weak planes. Their surface should be rough. They should have good crushing strength.
Their specific gravity should be between 2.6 to 2.7.
They should be porous. Porous aggregates make porous concrete.
They should not contain harmful impurities since they will affect the process of hydration. They should not contain salt. Sand obtained from sea shore usually contain salt by 4 to 6% of its weight. Such sand must be washed I thoroughly with clean water before use, otherwise the presence of salt will cause efflorescence and corrosion of reinforcement.
They should be free from gypsum and other sulphates.
They should be sound. Unsound aggregates will lead to deterioration of concrete due to expansion.
Too angular, flaky and elongated aggregates should be avoided. They affect the process of handling and cause segregation. (Flaky material is that whose least dimension i.e. thickness is less than the other two dimensions. Ex-Laminated rocks. Elongated aggregates are those dimension is greater than the other two dimensions.)