Classification of Aggregates

Classification of Aggregates with Respect to Size

The aggregates can be classified based on size, according to ASTM as:

1. Coarse Aggregate (CA)

Size: 4.75 mm (3/16 in.) to 50 mm (2 in.) (Retained on No. 4 sieve)

2. Fine Aggregate (FA)

Size: <4.75 mm; >75 µm (0.003 in.) (Retained on No. 200 sieve)

Mass concrete may contain up to ≈150-mm (6 in.) coarse aggregate.

Classification of Aggregates with Respect to Shape

British Standard classification of aggregates according to shape is as:

1. Rounded:

Fully water-worn or completely shaped by abrasion (physical wear) e.g. river or sea shore gravel, desert and wind-blown sand.

2. Irregular:

Naturally irregular or partly shaped by abrasion and having rounded edges. e.g hard grey siliceous stone.

3. Flaky:

Materials of which the thickness is small relative to the other two dimensions. e.g laminated rock.

4. Angular:

Possessing well defined edges formed at the intersection of roughly planar faces. e.g crushed rocks of all types

5. Elongated:

Materials usually angular in which the length is considerably larger than the other two dimensions.

6. Flaky and Elongated:

Materials having the length considerably larger than the width and the width considerably larger than the thickness.

Flakiness Index:

The mass of flaky particles expressed as a %age of the mass of the sample is called flakiness index.

Elongation Index:

The mass of elongated particles expressed as a %age of the mass of the sample is called elongation index. 

Irregular/Elongated Aggregates 

Rounded and Flaky Aggregates 

Smooth and Rounded Aggregates

Classification of Aggregates with Respect to Weight

1. Normal Weight Aggregates

·      The aggregates having the bulk density from 1520-1680 Kg/m³ are called normal weight aggregates.

·      Normal weight aggregates are further classified as natural aggregates and artificial aggregates.

·      The natural aggregates are; Sand, gravel, crush rock such as granite, quartzite, and sand stone etc. Natural sands and gravels are the product of weathering and the action of wind or water, while manufactured crushed fine aggregate and crushed stone coarse and fine aggregate are produced by crushing natural stone. Crushing, screening, and washing may be used to process aggregates from either sand and gravel deposits or stone quarries. Aggregates may be produced from igneous, sedimentary, or metamorphic rocks, but geological type does not by itself make an aggregate suitable or unsuitable for use in concrete.

§  The artificial aggregates are; broken bricks, air-cool slag, fly ash, bloated clay etc.

2. Light Weight Aggregates

The aggregates having the bulk density less than 1100 kg/m³, are called light weight aggregates.

Light weight aggregates can be;

§  Processed natural materials (e.g expanded clay or expanded shale)

§  processed by-products (e.g foamed slag)

§  unprocessed materials (e.g pumice)

Light weight concrete results in significant benefits in terms of load bearing elements of smaller x-section and also give better thermal insulation than ordinary concrete.

3. Heavy Weight Aggregates

The aggregates having the bulk density more than 2080 kg/m³, are called heavy weight aggregates.

§  Heavy weight aggregates include Iron, Steel, Limonite etc.

§  Heavy weight aggregates produce heavy weight concrete which is used in the construction of radiation shields.

o  Radiation shields protect the operating personnel against the biological hazards in the nuclear energy industry or x-ray, gamma ray at therapy centers.

Classification of Aggregates with Respect to Surface Texture

Aggregates can be classified on the basis of surface texture into the following (British Standard classification);

§  Glassy

§  Smooth

§  Granular

§  Rough

§  Crystalline

§  Honeycombed

The surface texture of the aggregates affects its bond to the cement paste and also influence the water demand of the mix, especially in the case of fine aggregates.