What are Aggregates?
Aggregates are the main ingredient in making materials of Concrete and Mortar which occupies 70-80% of Concrete Volume. Aggregate as a filler with binding material is the derived product of Sedimentary, Igneous and Metamorphic rocks and thus it is hard, strong and durable. It can also be made from blast furnace slag. As aggregates are used in large quantity in manufacturing of concrete, they have a significant effect on the properties of the Concrete. Therefore, it is vitally important to choose the right variety and quality before constructing. Unlike other making materials like cement and water, this does not require any special storage place and has negligible effects when exposed to various climatic conditions.
Classification of Aggregates:
1) Based on Geological Origin: They can be divided as
Natural Aggregates: These are one of the types of aggregates and are obtained from the naturally obtained rocks like Sedimentary, Igneous and Metamorphic rocks. Sand and gravel are also fallen into this type of aggregates. As these Aggregates are naturally obtained which may have a lot of impurities, so it should be sieved and washed before used in Concrete.
Artificial Aggregates: Blast Furnace Slag, Broken Bricks and Synthetic aggregates are Artificial Aggregates. Blast furnace slag aggregate is formed from slow cooling of the slag followed by crushing and are used for making precast concrete products by obtaining strong, denser particles. Broken bricks are used in Foundation for mass concerning and are not recommended for reinforced concrete works. Synthetic aggregates are made by Thermally processed materials like expanded clay and shale (finely-grained sedimentary rock, composed of mud)
2) Based on Size: They can be classified as
Coarse Aggregate: Aggregate retained on 4.75 mm sieve are known as Coarse Aggregates. They are obtained by natural fragmentation or by artificially crushing the rocks. The maximum size of coarse aggregate can be 80 mm.
Fine Aggregate: Aggregates that are passed through 4.75 mm sieve are classified as Fine Aggregates. They are natural sand, crushed stone and crushed gravel stone. The minimum size of fine aggregates can be 0.06 mm.
Graded Aggregate: Aggregate which passes through a particular size of the sieve is
known as graded aggregate. For example, a 12mm aggregate which passes through 12 mm size of the sieve.
All-in-Aggregate: These aggregates are naturally available which consist of various fractions of coarse and fine sizes of aggregates are known as All-in- Aggregates.
3) Based on Shape: They can be classified as
Rounded Aggregates: These aggregates are round due to the friction of water. So, they are obtained from river, seashore. These have minimum voids of about 32% in the concrete. These aggregates have a poor interlocking bond and require minimum cement paste.
Irregular Aggregates: As these aggregates are irregular in shape and thus have a strong locking bond. So, these are used for making ordinary concrete. These aggregates have 36% voids and require more cement paste than rounded aggregates.
Angular Aggregates: These aggregates are rough, sharp, and angular which provides a solid bond. So, these can be recommended to use in the construction of pavements. These aggregates have about 40% voids and require comparatively more cement paste.
Flaky Aggregates: These aggregates are generally rough and very sharp. It can be better explained using an example. The mean sieve size for an aggregate passing through 30mm and retained on 40 mm sieve is (30+40)/2=35 mm. The minimal lateral dimension is less than 0.6 x 35 mm=21 mm, the aggregate is known as Flaky Aggregate. Generally, If the thickness of aggregate should be less than the width and length of the aggregate, it can be identified as Flaky Aggregates.
The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension.
Flaky Aggregates are not Elongated Aggregates (Its length should be 1.8 times its mean dimension).
4) Based on Unit Weight: Based on Unit weight, aggregates can be divided into three. They are Normal Weight, Heavy Weight and Light Weight.
Sand, Gravel, Granite Sandstone, Limestone can be considered as Normal Weight.
Scarp iron, Magnetite and Baryte are considered as Heavy Weight.
Cinder, Clay, Dolomite and Pumice are considered as Light Weight.