Aggregates are one of the most important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 60 to 80 % of total volume of concrete. So, we can say that one should know definitely about the aggregates in depth to study more about concrete.
Aggregate may be natural, manufactured or recycled. It includes gravel, crushed stone, sand, slag, recycled concrete and geosynthetic aggregates.
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.
SIZES OF AGGREGATE
At the most general level, aggregates come in two varieties:
 Fine Aggregate
 Coarse aggregate
 FINE AGGREGATE
Fine Aggregates are usually sand or crushed stone that are less than 9.55mm in diameter. When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it. Natural sand is generally used as fine aggregate, silt, stone dust and clay also come under this category. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.
Below are List of Different Fine Aggregate with their Size Variation:
Stone Dust: 0.5mm – 5mm
Coarse Sand: 0.5mm – 2mm
Medium Sand: 0.25mm – 0.5mm
Fine Sand: 0.06mm – 0.25mm
Silt: 0.002mm – 0.06mm
Clay: < 0.002mm
 COARSE AGGREGATE
Coarse aggregates are particulates that are greater than 9.5mm. The usual range employed is between 9.5mm and 37.5mm in diameter. Typically the most common size of aggregate used in construction is 20mm. A larger size, 40mm, is more common in mass concrete. Larger aggregate diameters reduce the quantity of cement and water needed because of its lower Voids.
When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete.
The size range of various coarse aggregates given below:
Fine gravel: 4mm – 8mm
Medium gravel: 8mm – 16mm
Coarse gravel: 16mm – 64mm
Cobbles: 64mm – 256mm
Boulders: > 256mm
TYPES OF AGGREGATE
Gravel or Graded stone, is another coarse aggregate suitable for concrete mixes. Gravel is formed of rocks that are unconnected to each other. Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments.
Sand is the finest aggregate. It comes in different gradations: sharp sand, builder’s sand, and kiln-dried silver sand, which vary from coarsest to finest, respectively. Finer sands are a good aggregate for mortars and grouts. Coarser sands are also a popular aggregate for concrete mixes.
Granite is a types of felsic intrusive igneous rock that is granular and phaneritic in texture. Granite is being Crush into Different Diameters for the purpose of Aggregate in Concrete.
 CRUSHED STONE
Crushed Stone is the coarsest aggregate. A mixture of all sorts of aggregates, it is appropriate as a base for concrete in projects that do not require precise aggregate-to-cement ratios.
 RECYCLE CONCRETE
Recycled concrete is created by breaking, removing, and crushing existing concrete to a preferred size. It is commonly used as a base layer for other construction materials.
Recycled concrete can be used as aggregate in new concrete, particularly the coarse portion. When using the recycled concrete as aggregate, the following should be taken into consideration:
Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
 VERMICULITE AGGREGATE
Vermiculite Concrete Aggregate is used to create a lightweight and insulating concrete that can be used on various forms of bases suitable to a variety of engineering designs. This concrete aggregate is often used for roof deck systems and works with most concrete, wood, and metal structural decks.
 GLASS AGGREGATE
Glass aggregate can replace part or all of the sand and gravel in concrete, for effects that range from colorful terrazzo, to granite- or marble-like finishes, to concrete that reflects light like a mirror. Glass aggregate can even be used to produce concrete that literally glows.
 SLAG AGGREGATE
The use of slag aggregates from iron and steel production in construction dates back to the Romans who used crushed slag from the crude iron production of that time to build their roads. Nowadays, slag is still used to build roads.
However, slag use is not limited to roads anymore, but slag aggregates are widely used in all kinds of civil works.