[Kavita Jain]

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and portland cement, are an essential ingredient in concrete.

For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories--fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inch, but generally range between 3/8 and 1.5 inches in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.



Characteristics of Aggregates

Grading
Durability
Particle shape and surface texture
Abrasion and skid resistance
Absorption and surface moisture
Unit weights and voids

[shreyansh jain]

Well shaped aggregates have following advantages:-
1) Better shape reduces the water demand for a given mix, hence decides the
cement consumption for a given workability.
2) It improves the compactability and cohesion characteristics of green concrete.
3) It also reduces the mortar content of concrete for making concrete mixes
pumpable.
4) It reduces segregation and bleeding characteristics of green concrete.
5) It improves physical characteristics of hardened concrete including compressive
strength, flexural strength, abrasion and impact resistance of concrete.
6) It also reduces the permeability and improves the durability of concrete.

[AnishKrishnan]

The Effect of Aggregate Properties on Concrete


The compressive aggregate strength is an important factor in the selection of aggregate. When determining the strength of normal concrete, most concrete aggregates are several times stronger than the other components in concrete and therefore not a factor in the strength of normal strength concrete. Lightweight aggregate concrete may be more influenced by the compressive strength of the aggregates.

Other physical and mineralogical properties of aggregate must be known before mixing concrete to obtain a desirable mixture. These properties include shape and texture, size gradation, moisture content, specific gravity, reactivity, soundness and bulk unit weight.
These properties along with the water/cementitious material ratio determine

• strength of concrete.
  
• workability of concrete.
  
• durability of concrete.