What Is Coarse Aggregate?
Aggregate is one of the most important component parts of the concrete. Coarse Aggregates gives volume to the Concrete.
Coarse Aggregates in concrete provides body and strength to the concrete and acts as a filler material which will give the homogeneous mass of the concrete.
Coarse aggregates are used in every Construction projects which includes the construction of roads, Buildings, Railway Tracks etc.
Coarse Aggregate in concrete was considered as chemically inactive and acts as a filler material only. The coarse aggregates are used for the manufacturing of mortar and concrete.
In this article, you will get to know all about coarse aggregates, classification of coarse aggregate, properties of the coarse aggregates and the different tests which are performed on the Coarse Aggregates.
Coarse Aggregate Definition-
The Aggregates which will get retained on the 4.75 mm sieve or the aggregates which have size more than 4.75 mm are known as Coarse aggregate.
Origin of Aggregates-
Aggregates are commonly obtained by crushing the naturally occurring rocks. The properties of the rocks are mainly depended upon the type of rock whether it is sedimentary rocks, igneous rock or metamorphic rocks.
Requirements of Good Coarse Aggregates:
The Aggregates which are used in the construction must have the following properties
- The coarse aggregate should be durable.
- The coarse aggregate should be hard and strong.
- It should be clean and free from the dust and organic materials otherwise it will reduce the bonding of the aggregate with concrete.
- The aggregates should not react with the cement after mixing.
- Coarse aggregates should not be soft and porous.
- Coarse Aggregates should not absorb water by more than 5%.
- Aggregates should be chemically inert.
- The shape is of the aggregate preferably cubical or spherical.
Uses of Coarse Aggregates in the Construction Works:
The coarse aggregates have many advantages which are listed are as follows
- In the construction of railway tracks, coarse aggregates are widely used in the railway ballast which will help to uniformly distribute the load.
- It will help to increase the volume of the concrete and also reduces the cost of the project.
- Coarse aggregates are also used in the construction of Roads.
- The Coarse Aggregates are also used as the upper layer on the rainwater harvesting to drain off the water into the ground.
Classification of the Aggregates:
The coarse Aggregates are mainly classified are as follows
- According to the source or the nature of the formation of the aggregate.
- According to the size of the aggregate
- According to the shape of the aggregate
The description of the classification of the Aggregates are as follows
Classification According to the Source or Nature of Formation
- Naturally Occurring Aggregates.
- Artificially Manufactured Aggregates.
#1 Naturally Occurring Aggregates-
Coarse Aggregates are obtained from the stone quarries and the stone crushers. Natural aggregate materials originate from bedrocks.
#2 Artificially Manufactured Aggregates-
The broken bricks or blast furnace slag are the artificial aggregates which are also used for the various concreting work.
The Air-cooled slag is also used as a coarse aggregate as well as fine aggregates. It has fire resistance Property.
Classification of Aggregates According to the Size
The Aggregates are mainly classified into two types which are given below
- Fine Aggregates
- Coarse Aggregates
#1 Fine Aggregates-
The Aggregates which has a size less than 4.75 mm is known as fine aggregate.
#2 Coarse Aggregates-
The Aggregates which has a size of more than 4.75 mm is known as Coarse aggregate.
The Aggregate sizes which range from 4.75 mm to 80 mm are the coarse Aggregates and the 150 microns to 4.75 mm aggregates are known as Fine Aggregates.
The size 4.75 mm is common for both fine Aggregates as well as coarse Aggregates.
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Classification According to the Shape of the Aggregates
- Rounded Aggregates.
- Angular Aggregates.
- Flaky Aggregates.
- Irregular Aggregates.
#1 Rounded Aggregates-
Natural aggregates smoothed by weathering, erosion and attrition. Rocks, stone, sand and gravel found in riverbeds are your most common rounded aggregates.
Rounded aggregates are the main factor behind workability.
#2 Angular Aggregates-
Angular aggregates have a higher specific surface area than the smooth rounded aggregate. With a greater specific surface area, the angular aggregate may show higher bond strength than rounded aggregates.
Also, angular aggregates exhibit a better interlocking effect in concrete that contributes to the strength of concrete.
#3 Flaky Aggregates-
an aggregate is termed flaky when its least dimension (thickness) is less than three-fifths of its mean dimension.
The mean dimension of aggregate is the average of the sieve sizes through which the particles pass and are required, respectively.
#4 Irregular Aggregates-
These are also shaped by attrition but are not fully rounded. These consist of small stones and gravel and offer reduced workability to rounded aggregates.
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Properties of Coarse Aggregates
The coarse aggregates are widely used in the construction so it is very important that we should know about the properties of the Coarse Aggregates.
The properties of the coarse aggregates are as follows
- Surface Texture.
- Water Absorption.
- Specific Gravity.
- Bulk Density.
The size of the coarse aggregate depends upon the purpose for which the concrete is used in the construction. For the mass concreting work the larger aggregates of 80 mm,40 mm and 20 mm sizes are used.
For the construction of residential building generally, 20 mm size aggregate is used.
Generally, the angular and Irregular shape of aggregates is used because they have more Interlocking effect. The total surface area of the angular aggregate is more in the angular aggregates as compared to the rounded Aggregates.
#3 Surface Texture-
The surface texture of the aggregate deals with the roughness and the smoothness of the coarse aggregates.
The surface texture of the rounded aggregate with a smooth surface will require less amount of cement paste and hence increased the yield per bag.
#4 Water Absorption-
Water absorption is one of the most important properties of the coarse aggregate which is measured in the percentage.
Water absorption of the coarse aggregate depends upon the porosity of the aggregate. If the water absorption is more then it will affect the workability and the durability of the concrete.
The soundness of the aggregate is defined as the resistance which is offered by the aggregates to any type of volume change.
Porous aggregates are unsound and more liable to the attacks by the chemicals. The foundation of the aggregates is measured by putting the aggregates in the sodium or magnesium sulphate and then oven drying it.
#6 Specific Gravity-
Specific gravity is defined as the ratio of the dry weight of the aggregate to the weight of an equal volume of water.
The specific gravity of the aggregate is the indication of the Strength. Generally, the aggregates with the specific gravity range from 2.5 to 3.0 are used in the construction work. Greater the specific gravity more will be the strength.
#7 Bulk Density-
The bulk density of the aggregate defined as the ratio of the net weight of the aggregate to the volume of the aggregate.
Grading of Coarse Aggregate:
The grading of coarse aggregate for coarse material is the art of doing particle size distribution of the aggregate.
The grading of the coarse aggregate affects the workability, strength and durability of the concrete which is used in the construction.
It is very necessary to do the proper grading of the aggregates in order to make the structure dense and prevent the problem of honeycombing.
Grading of aggregate is done to achieve proper finishing and workability of the concrete.
Different Types of Test Which Are Carried Out on the Coarse Aggregates
#1 Crushing value of Aggregate ( IS 2386 Part IV -1963)
The crushing value of the coarse aggregate is defined as the relative measure of the resistance which is offered by the aggregate to the crushing under the gradually applied load.
#2 Impact value of Aggregate ( IS 2386 Part IV -1963)
Impact value of the coarse aggregate is the test which gives a relative measure of the resistance offered by the aggregate to the suddenly applied load which is also known as impact load.
#3 Abrasion Value Test ( IS 2386 Part IV -1963)
Abrasion value test of the coarse aggregate will give the relative resistance of the aggregate to the wearing. Abrasion value of the coarse aggregate help to know whether the aggregates are suitable to use for wearing surfaces.
#4 Flakiness Index of coarse Aggregates ( IS 2386 Part I -1963)
The flakiness index of the aggregate is the percentage by the weight of the particles whose least dimension or the thickness is less than 3/5th of their mean dimension.
The flakiness index of the aggregate will give the idea about the thickness of the aggregate.
#5 Elongation Index of Coarse Aggregate ( IS 2386 Part I -1963)
The elongation index of the aggregate is the percentage by the weight of the particles whose greatest dimension or the length is greater than 1.8 times of their mean dimensions.