Aggregates

Aggregates are materials used as filler with binding material in production of mortar and concrete.

¤ They are derived from rocks.
¤ They form the body of concrete.
¤ They reduce shrinkage.
¤ 70-80% it contained of concrete
¤ They have considerable effect on properties of concrete.

Characteristic of good aggregates.

* Clean
* Hard
* Strong
* Durable
* Graded in size

To increase bulk density, two types of aggregates are used;

Coarse:- grit (main)

Fine:- sand (filler)

Types of Aggregates

Natural Aggregates.
.'. Obtained from quarries of igneous, sedimentary and metamorphic rocks.
.'. They are usually sieved and washed before used in concrete.

Artificial Aggregates.
.'. It obtained from broken bricks, blast furnace slag, synthetic aggregates.
.'. Broken bricks are used for mass concrete and foundation bases.
.'. Blast furnace slag is not suitable for reinforced concrete works. It is used for precast.

Classification of aggregates

Based on size aggregates is classified into:

- Coarse aggregate.
- All-in aggregate.
- Fine aggregate.
- Grounded aggregate.

Coarse aggregate

mix of fine and coarse ; not recommended for quality concrete.

Fine aggregates

.'. Passing through 4.75mm sieve, smallest- 0.06mm.
.'. Could be fine, medium or coarse.

Graded aggregates

Aggregate of a particular size e.g graded aggregate of nominal size of 20mm.

Classification of Aggregates based on shape

* Rounded aggregate; which has poor interlocking.
* Irregular aggregate; which develops good bond.
* Angular aggregate; which has good bond (most suitable)
* Flaky

Fine aggregate

Sand (greater than 0.07mm) is used as fine aggregate in concrete and mortar.

It is a granular form of silica.

Grade of sand is important in its workability; very fine and very coarse sand are unsatisfactory for making concrete or mortar.

Impurities such as clay, dust and organic materials adversely affect mortar and concrete. They can be removed by washing.

Coarse aggregates

crushed, in crushed or partially crushed gravel or stone which is retained on 4.75mm sieve.

Function of aggregates

~ Prevent shrinkage and development of cracks on a building or any related structures.

~ Furnish strength to mortar and concrete.