Wednesday, February 08, 2017

Classification and Properties of Aggregates as per IS 456:2000

Classification and Properties of Aggregates as per  IS 456:2000

Aggregates constitute nearly 70 to 75% of the total volume of concrete and are essentially inert in nature. Aggregates have two prime functions, namely, to provide concrete with a rigid skeletal structure and reduce the void space to be filled by the cement paste. Most natural rocks, whether massive or broken down by nature, are suitable for making concrete. 

Aggregates are commonly classified into two sizes, fine and coarse:; the dividing line being the 4.75-mm IS sieve. Aggregates can also be classified in tow more ways. 

Depending on the source, they could either be naturally occurring (gravel,sand, etc) or synthetically manufactured (bloated clay aggregates, etc.). 

Further, depending on the bulk density, aggregates can either be normal weight, lightweight, or heavyweight

IS:456- 2000 specifies that aggregates shall comply with the requirements of IS 383. The nominal maximum size of coarse aggregates should not be greater than one- fourth of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form. 

                                The aggregates used to make concrete must be clean, dense, hard, durable, structurally sound, capable of developing good bond with cement, weather-resisting, and unaffected by water. The properties of the concrete depend upon the quality of the aggregates- their strength, water absorption, shape and texture, the maximum size of aggregate, etc. Impurities in aggregates are undesirable as they may hinder the hydration of cement and prevent adhesion of the aggregates with the cement paste, reducing strength and lowering durability. Sand containing an excess amount of silt or organic matter should be washed. Sand sometimes contains moisture, which cause a film of water on the surface of the particles, fluffing them apart. This is called bulking of sand, which should be taken into account while batching the mix. Storage of materials shall be as described in IS 4082. Storing on dusty, muddy, and grassy spots should be avoided. Aggregates should be placed in stockpiles in individual units not larger than a truckload and in suitable layers to prevent segregation.