Aggregates and its classification

 

Aggregates

Aggregates are raw materials that are produced from natural sources and extracted from pits and quarries, crushed stone and recycled concrete, used in construction.

Aggregates are the major and important constituent of concrete. They form the body of the concrete as it occupies 70–80% of the volume of concrete. Although aggregates were originally considered to be inert material, they have recently been found to be chemically active to some extent. Cement is the only factory-made component, whereas aggregates (both coarse and fine) and water are naturally available materials.

Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage.

 

 Classification of Aggregates

I. Classification of Aggregates Based on Size:

Aggregates are available in nature in different sizes. The size of aggregate used may be related to the mix proportions, type of work etc. The size distribution of aggregates is called grading of aggregates.

Aggregates are classified into 2 types according to size :

l  Fine aggregate

l  Coarse aggregate

 

Fine Aggregate:

When the aggregate is sieved through a 4.75mm sieve, the aggregate passed through it called fine aggregate.

Fine aggregate is sand, which is usually obtained from rivers or lakes. Sometimes beach sand is also used. Natural sand is generally used as fine aggregate, silt and clay also come under this category. The soft deposit consisting of sand, silt, and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.

In places where sand is not available or a large quantity of sand is needed, crushed stone dust is used.

 

Fine aggregate	Size variation(mm)
Coarse sand	2.0mm - 0.5 mm
Medium sand	0.5mm - 0.25mm
Fine sand	0.25mm - 0.06mm
Silt	0.06mm - 0.002mm
Clay	< 0.002mm

Coarse Aggregate :

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Coarse aggregates are produced by the disintegration and crushing of rock. Gravel, cobble and boulders come under this category. The aggregates should be absolutely clean, free from organic matter and other impurities. The aggregate must be capable of resisting weather.

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.

 

Coarse aggregate	Size variation (mm)
Fine gravel	4mm – 8mm
Medium gravel	8mm – 16mm
Coarse gravel	16mm – 64mm
Cobbles	64mm – 256mm
Boulders	>256mm

II.    Classification of Aggregates Based on Shape:

We know that aggregate is derived from naturally occurring rocks by blasting or crushing etc., so, it is difficult to attain required shape of aggregate. But, the shape of aggregate will affect the workability of concrete. So, we should take care about the shape of aggregate. This care is not only applicable to parent rock but also to the crushing machine used.

Aggregates are classified according to shape into the following types

l   Rounded aggregates

l   Irregular or partly rounded aggregates

l  Angular aggregates

l   Flaky aggregates

l   Elongated aggregates

l   Flaky and elongated aggregates

Ø  Rounded Aggregate :

The rounded aggregates are completely shaped by attrition (the resistance of a granular material to wear) and available in the form of seashore gravel. Rounded aggregates result in the minimum percentage of voids (32 – 33%) hence gives more workability. They require a lesser amount of water-cement ratio. They are not considered for high-strength concrete because of poor interlocking behavior and weak bond strength.

Fig: Rounded Aggregates

 

Ø  Irregular Aggregates :

The irregular or partly rounded aggregates are partly shaped by attrition and these are available in the form of pit sands and gravel. Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete.

Fig: Irregular Aggregates

Ø  Angular Aggregates:

The angular aggregates consist well defined edges formed at the intersection of roughly planar surfaces and these are obtained by crushing the rocks. Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability. They give 10-20% more compressive strength due to development of stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.

Fig: Irregular Aggregates

Ø  Flaky Aggregates:

When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate, or on the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate.

Ø  Elongated Aggregates:

When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.

Ø  Flaky and Elongated Aggregates:

When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above 3 types of aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks.