Aggregate are the important part of concrete. Aggregates contribute to a majority of proportion (70- 80 %) in concrete and thus responsible for the increase in weight of concrete. They give body of the concrete, reduce shrinkage and effect economy. Earlier researchers consider aggregate as an inert material but now its not the case. Aggregates can also have chemically active materials in it.
Without in depth knowledge in aggregates, the study of concrete is incomplete. In the most simplest form concrete is the mixture of coarse aggregate, fine aggregate, water and cement. The mixture of water and cement called as paste act as the binding agent between the coarse and fine aggregates. The cement paste fills the pores between the particles of aggregates and through the chemical reaction called as hydration, the cement paste hardens and helps in strength gain.
To find the best quality of aggregate for concrete mix, we check the following properties. They are:
Size of aggregate
Size is a very important property, it can be a limiting factor for
- thickness of section
- spacing of reinforcement
- clear cover
- mixing handling and placing techniques
Shape
It affects the workability of concrete. Some of the common shape of aggregate available are:
- Rounded
- Irregular
- Angular
- Flaky
Texture
It is a measure of relative degree to which particle surface are
- Polished
- dull
- smooth
- rough
Strength of aggregate
This is a very important property as the strength of concrete will depend on the strength of aggregates. The test for strength of aggregate is important when we aim a ultra high strength concrete or aggregates from industrial process. These are the following test :
- Crushing value test
- Impact value test
- Abrasion value test
- Deval attrition test
- Dorry abrasion test
- Los angeles test
Specific gravity of aggregate
In concrete technology, we use specific gravity of aggregates in design calculations of concrete mixes.
Bulk density
Gives important information about the shape and grading of aggregate.
Moisture content
Some of the aggregates are porous and absorptive. Porosity and absorption of aggregate will affect the water/ cement ration and hence the workability of concrete.
Bulking factor
Free moisture forms a film around each particle. This film of moisture exerts what is known as surface tension which keeps the neighbouring particles away from it. This cause bulking of aggregate.
Cleanliness
Aggregates should be free from impurities and deleterious substances. As it can hinder the chemical composition of concrete.
Soundness
Soundness refers to the ability of aggregate to resist excessive changes in volume as a result of change in physical conditions.
Sieve analysis of aggregates
The sieve analysis is to determine the particles size distribution in a sample of aggregate, which we call gradation.
The aggregates used for making concrete are normally of the maximum size 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 600 micron, 300 micron and 150 micron.
The aggregate fraction from 80 mm to 4.75 mm are termed as coarse aggregates.
And aggregates from 4.75 mm to 150 micron we term it as fine aggregates.
Procedure of sieve analysis of aggregate:
- Select a test sample for aggregates.
- Dry the test sample to a constant weight at a temperature of 110 + 5oC and weighed.
- Then sieve the sample using a set of IS Sieves.
- On completion of sieving, the material on each sieve is weighed.
- Cumulative weight passing through each sieve is calculated as a percentage of the total sample weight.
- We should also consider the shaking time. As this will accelerates the particles to pass , from each sieve.
- Calculate the fineness modulus by adding cumulative percentage of aggregates retained on each sieve and dividing the sum by 100.
For detail concrete mix design method, check this post.
Some of the examples of concrete mix design are M10 M20 M25 M30 M40 M50
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