Building Materials

Aggregates

The fine and coarse aggregates occupy about 60 to 75% of the concrete volume and hence they strongly influence the properties of fresh and hardened concrete, its mixture proportions and economy.

Aggregates used in the concrete should comply with IS 383:1970.

Classification:

  • Based on Size
  1. Fine Aggregates: These generally consist of natural sand or crushed stone  with particle size  smaller than 5mm (materials passing through 4.75mm IS sieve).
  2. Coarse Aggregates: These consist of a combination of gravels or crushed stone with particle size larger than 5mm (usually betweeen 10mm and 40mm).
  • Based on Source
  1. Naturally Occurring: These are gravel, pebbles, sand etc.
  2. Synthetically manufactured: These include bloated clay aggregates, sintered fly ash aggregates etc.
  • Based on density
  1. Light weight aggregates: Aggregates with densities less than 1220 kg/m³. Examples of these aggregates are shale, clay, slate, slag, pumice, perlite, vermiculite and diatomite.
  2. Normal weight aggregates: Aggregates with densities between 1520 to 1680 kg/m³. Examples of these are sand, gravel, crushed rock, and blast furnace slag.
  3. Heavy weight aggregates: Aggregates with densities above 2000 kg/m³. Examples of these aggregates are hematite, steel. These are used in special applications such as providing radiation shielding and abrasion resistance.

6

The above figure shows the Different shapes and sizes of the aggregates.

Facts: In several countries including India, natural coarse aggregates and river sand are scarce, and at the same time, the waste from demolition of building and industries is increasing. In such places recycles aggregates and by-product from industries can be used effectively.

Requirements of good Aggregates:

Aggregates must be clean, hard, strong and durable. Aggregates should be free from clay, chemicals and fine materials. They should be washed to remove impurities.

Grading or particle size distribution of aggregates is a major factor determining the workability, segregation, bleeding, placing and finishing characteristics of concrete.

 

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