Different types of aggregates are used in making concrete mixtures & there is wide range of aggregates with some common desired characteristics of concrete.
All aggregate are not the same or as expected for concrete, due to local availability and acceptance.
Desirable characteristics depend on the purpose of the concrete as follows:
- A strong, uniform clean aggregate with no component that will adversely affect the cement.
- A sharp-edged surface that will bond the concrete together.
- Any material or salts which will not dissolve or corrode rebar.
Classification of Aggregates:
aggregate types based on Grain size:
1. Fine Aggregate:
The aggregate with grain size below 4.75 mm is termed as fine aggregate.
The percentage of all type deleterious materials in fine aggregate should not exceed 5 %.
2. Coarse Aggregate:
This types of Aggregates whose particles are larger than 4.75 mm are termed as coarse aggregate.
For RCC work, generally 20 mm aggregate is used.
aggregate types based on its unit weight:
1. Lightweight aggregate:
Its Unit weight up to 121 kN/m3.
Lightweight aggregates are either natural such as pumice, diatomite, sawdust, rice husk, volcanic cinders, scoria, etc. or artificial such as foamed slag, sintered fly ash, bloated clay, coke breeze, expanded perlite, etc.
It is used to reduce the weight of the structure with better thermal insulation and fire resistance.
2. Normal Weight Aggregate:
Produces concrete with unit loads ranging from 23 to 26 kN / m3 with a specific gravity between 2.5 and 2.7.
It has crushing strength at 28 days between 15 to 40 MPa.
Commonly used aggregates i.e. sand and gravel; crushed rocks such as granite, basalt, quartz, sandstone and limestone.
3. Heavy Weight Aggregate:
The aggregates having specific gravities ranging from 2.8 to 2.9 and unit weights from 28 to 29 kN/m3 are termed as heavyweight aggregate.
The magnetite (Fe304) and Barytes (BaSO4) and scrap iron are used in the manufacturing of heavyweight concrete.
aggregate types based on Geological Origin:
1. Natural Aggregate:
These aggregate are obtained from natural deposits of sand and gravel or from quarries by cutting rocks.
The river deposits are the most common and of good quality.
Aggregates from igneous rocks are normally hard, tough and dense with glassy texture.
Aggregate from sedimentary rocks may vary from soft to hard, porous to dense and light to heavy.
They may produce flaky aggregate, metamorphic rocks show a foliated structure.
2. Artificial Aggregate:
The most commonly used artificial types of aggregates are cleanly broken bricks and air-cooled fresh blast-furnace slag.
The bricks should be free from lime mortar and lime sulphate plaster.
Blast-furnace slag aggregate has good fire resisting qualities.
aggregate types based on shape:
1. Rounded Aggregate:
The aggregate having rounded particles with minimum voids ranging from 32 to 33 per cent.
It gives the minimum ratio of surface area to the volume thus requiring minimum cement paste to make good concrete.
The interlocking of its particles is less hence the development of bonds is impaired.
It is unsuitable for high strength concrete and pavement.
2. Irregular Aggregate:
This types of aggregates is the mixture of rounded aggregate and angular aggregate.
More cement paste is required for a given workability.
Interlocking between particles is preferable so it is used for high strength concrete.
3. Angular Aggregate:
A set of sharp, angular and rough particles (crush rock) is called an angular aggregate.
It requires more cement paste to produce high strength concrete.
The maximum range percentage of voids is between 38 and 40.
4. Flaky and Elongated Aggregates:
The aggregates having its least dimension i.e. thickness is less than 3/5th of its mean dimension are termed as flaky aggregate.
The aggregates having its greatest dimension i.e. length is greater than 9/ 5th of its mean dimension is termed as elongated particles/ aggregates.
Properties of Aggregates:
Composition: For use in concrete, aggregates should not be included in materials that can react with alkali in cement.
Size: The largest possible size of aggregate results in a reduction in water requirement, cement content and reduction of drying shrinkage.
The maximum size of coarse aggregate should be less than:
- One-fourth of the minimum dimension of concrete member and
- One-fifth of the minimum dimension of reinforced concrete cement.
Shape: The shape of the aggregate types affects the work ability of the concrete and its strength.
These are in four types-
1. Rounded: It has 33-35% voids, it is not suitable for concrete.
2. Irregular or partly rounded: it has 35-37% voids, it useful for medium quality concrete.
3. Angular: It has 38-41% voids, it is best for concrete.
4. Flaky: It has a high percentage of voids, it is suitable for lower grade concrete.
Texture: The surface texture is the property that defines whether a particular surface is polished or dull, smooth or rough.
For good concreting, coarse-textured aggregate are used because they develop higher bond strength under stress than smooth-textured aggregates.
Strength: For making strong concrete, strong aggregate is an essential requirement.
Naturally available aggregate are strong enough for making normal strength concrete.
Bulk density: For coarse aggregate, it is the ratio of net weight of aggregate to its volume and is expressed in kg/litre.
In the case of fine aggregates, the density of aggregate considered along with the volume of voids or empty spaces between the particles.
Specific gravity: specific gravity of aggregate is the ratio of its density to the density of water, it is between 2.6 and 2.8.
Water absorption or moisture content: It is the difference between the weight of very dry aggregates and weight of saturated aggregate types with dry surface conditions.
Water absorption of aggregates effects on its w/c ratio and durability.
Bulking of aggregates: The free moisture content of the fine aggregate causes the volume.
The free moisture forms a thin film around each particle, these moisture films keep neighbouring particles away from it.
Therefore, no point of contact between the particles is possible, the volume increases.
Cleanliness: Aggregates must be free of impurities and deleterious substances that are likely to interfere with the process of hydration, preventing affection bonding between the aggregates and the matrix.
Soundness: The aggregates should be sound that means it should have the ability to resist excessive change in volume due to change in physical conditions.
Thermal expansion: This affects the durability of concrete, especially under rapid temperature changes.