Aggregate ( concerete technology)

 Agrregates 

These are chemically inert material which are bounded by cement paste form concerete .

Aggregate is  derived from igneous, sedimentary and metamorphic rocks . Aggregate are generally cheaper than cement and impart greater volume stability and durability to concerete. The aggregate provide about 75% of the body of concerete and hence its influence is extremely important. 

Classification of aggregate 

1- Classification of Aggregates According to source.

2- Classification of Aggregate According to size 

3- Classification of Aggregate According to shape 

4- Classification of Aggregate According to unit weight. 

1- Classification of Aggregates According to source. 

According to sources, the aggregate are classified into two categories: 

1) Natural Aggregate 

2) Artificial Aggregate. 

1) Natural Aggregate 

These aggregate are generally obtained from natural deposition of sand and gravel or from quarries by cutting rocks. The cheapest among them are the natural sand and gravel which have been reduced their present size by natural agents, such as water wind and snow etc. The most common natural aggregate is river deposit which is good in quality. 

2) Artificial Aggregate 

Generally artificial aggregate is obtain from broken bricks or blast furnace slag.The bricks should be free from lime mortar and lime sulphate plaster.   

2- Classification of Aggregate According to size 

According to size the aggregate is classified as 

1) Fine Aggregate 

2) Coarse Aggregate 

3) All in Aggregate 

4) Same size Aggregate 

Fine aggregate 

The aggregate which 90% pass through a 4.75 mm IS sieve. 

The fine aggregate may be one of the following types 

1) Natural sand i.e the fine aggregate resulting from natural disintegration of rock and/ or that which has been deposited by stream and glacial agencies.

2) Crushed stone sand i.e the fine aggregate produced by crushing hard stone.

3) Crushed gravel sand i.e the fine aggregate produced by crushing natural gravel. 

Coarse Aggregate 

The aggregate most of which retained on 4.75mm IS sieve and contain only that much of fine material as in permitted by the specifications are termed as coarse aggregate .The coarse aggregates may be one of the following types. 

i) Crushed gravel or stone obtained by the crushing of gravel or hard stone.

 ii)  un crushed gravel or stone resulting from the natural disintegration of rock.

iii) Partially crushed gravel or stone obtained as a product of the blending of the above two types.

 All in Aggregate 

Sometimes combine aggregates are available in nature comprising different fractions of fine and coarse aggregate, which are known as all in aggregate the all in aggregate is also described  by its nominal size. The all in aggregate are not generally used for making high quality concrete. 

Classification of aggregate according to shape . 

1) Rounded aggregate 

The aggregate with rounded particles has minimum voids ranging from 32 to 33 percentage.its gives minimum ratio of surface area to the volume, thus requiring minimum cement paste to make good concrete. The only disadvantage is that the interlocking between the particles is less and hence the development of the bond is poor, making it unsuitable for high strength concerete and pavement.

2) Irregular Aggregate 

The aggregate having partly rounded particles has higher percentage of voids ranging from 35-38 . It requires more cement paste for a given workability. The interlocking between the particles is less and hence development of the bond is poor. 

3) Angular aggregate 

The aggregate with sharp, Angular and rough particles has a maximum Percentage of voids ranging from 38 to 40 .the interlocking between the particles us good, thereby providing a good bond. The aggregate required more cement paste to make workable concerete of high strength than that required by rounded particles .the angular aggregate is suitable for high strength concerete. 

4) Flaky and elongated aggregate 

The aggregate is termed as flaky when its least dimension ( thickness) is less than three fifth of its mean dimension. The mean dimension of the aggregate is the average of the sieve size through which the particles pass and are retained respectively.

The particles is said to be elongated when it's greater dimension ( Length) is greater than nine fifth of it's mean dimension.

Classification according to unit weight 

1) Normal weight Aggregate 

The commonly used aggregate i.e sands and gravels, crushed rocks such as granite ,basalt ,quartz,sand stone and limestone and brick ballast etc. Concerete with the unit weight ranging from 23 to 26 kN/m3 crushing strength at 28 days between 15 to 40 MPa are termed as normal weight concerete.

2) Heavy weight or High density Aggregate 

Concrete having unit weight of about 30, 31 ,35,38, 40, 47, and 57 kN/m3 can be produced by using typical limonite ,baryte,magnetite , hematite and srap iron respectively. 

3) light weight Aggregate 

The light weight Aggregate having unit weight upto 12 kN/m3 are used to manufacture the structural concerete and masonry blocks for reduction of the self weight of the structure.