Coarse aggregates are irregular broken stones or naturally occurring round gravels that are used to make concrete, coarse aggregates for structural concrete consist of broken stones of hard rock like granite and limestone (angular aggregates) or river gravels (round aggregates).
Aggregates larger than 4.75 mm in size are termed as coarse aggregates.
These aggregates are obtained from stone quarries and stone crushers, the size between 4.75 mm to 80 mm.
Good qualities of an ideal aggregate:
- These aggregate used for the construction of concrete and mortar must meet the following requirements.
- This should include natural stones, gravel, sand, or various mixtures of those materials.
- It should be inflexible, strong, and sturdy.
- It should be dense, clear, and free of any coating.
- Also, free from harmful vegetable issues.
- It shouldn’t have flaky (angular) and long items.
- In the case of reinforced concrete, no materials should be liable to attack the reinforcement of steel.
Types of coarse aggregates:
Classification based on shape:
Their size is 1.8 times or nine-fifths of its mean dimension.
At least the lateral dimension should be lower than 0.6 times or three-fifths of the common dimension, in a plane oriented downward with water and air.
Conversely have an effect on the durability and are restricted to a maximum of 15%.
Classification based on geological origin:
Natural coarse aggregates:
They are obtained by crushing rocks and weathering motion / natural agencies of rocks.
They are broadly igneous rocks.
They are not used for R.C.C functions.
Artificial aggregates are damaged bricks, blast furnace slag and artificial aggregates.
Classification based on size:
Totally different fractions of fine and coarse sizes.
They are not advisable for high-quality concrete.
A mixture that passes through a particular shape of the sieve are often known as hierarchical aggregates.
Properties of Coarse Aggregates:
1.Size of Coarse Aggregates:
The size of the coarse aggregate depends on the use of the concrete is to be poured.
For large-scale concreting operations without complex reinforcement, large aggregates of size 80 mm, 40 mm, 20 mm are used.
For the simple construction of residential or other buildings, 20 mm is the maximum size will be used.
Aggregates are graded in a concrete mix design and the ratio of each size is determined using grading curves, charts, tables, or other methods.
2.Shape of Coarse Aggregates:
The shape of the aggregate is a crucial characteristic because it impacts the workability & strength of the concrete.
In general, the angular aggregate is desirable for round and smooth sets for the following reasons:
- The angular aggregate shows a better interlocking effect which gives a better concrete.
- The total surface area of a rough angular is set greater than that of a smooth circular set, so bond formation is provided with greater bond strength.
- The surface of the aggregate can be smooth, polished, rough or dull.
- A rough surface provides more area for bonding with cement paste and gives more strength, but where workability is required a roughly textured aggregate will be less practical and rigid.
- A round aggregate with a smooth surface will require less cement paste therefore an increase in yield per bag.
- Overall rough textured aggregates should be preferred over textured textures, but according to need the use of smooth textured aggregates for unimportant work.
3.Soundness of Coarse Aggregates:
Soundness refers to the resistance offered by the coarse aggregates to any type of volume change.
The change may be in temperature, alternate wetting and drying potable water or alternate wetting and drying in seawater.
Aggregates which are porous are unsound and more liable to attack by chemicals.
Also, these aggregates will get broken if subjected to alternate cold and heat.
The soundness is measured by immersing the aggregate in both sodium or magnesium sulphate and oven drying it below specified situations.
The average loss of weight after 10 such cycles should not exceed 12% if tested with sodium sulphate and 18% if tested with magnesium sulphate respectively.
Water absorption of coarse aggregates is measured by the percentage improve in weight of an oven-dried pattern after being immersed in water for 24 hours.
It does not give an accurate picture and many times, the absorbance is measured for a period equal to the final setting time of the cement.
Water absorption will reduce the amount of water content in the concrete, it will require a greater water/ cement ratio.
Water absorption will depend on the porosity of the aggregate, the greater water absorption will affect both the workability and durability of concrete.
5.Specific Gravity of Coarse Aggregates:
It is the ratio of the dry weight of aggregate to the weight of an equal volume of water.
The specific gravity of aggregate is determined along with the water absorption as specified in IS 2386 part III 1963.
Sizes of Coarse aggregate:
|Fine gravel||4mm – 8mm|
|Medium gravel||8mm – 16mm|
|Coarse gravel||16mm – 64mm|
|Cobbles||64mm – 256mm|
Uses of Coarse Aggregates:
- They are also used in the preparation of breaking moisture under slabs and vapor barriers.
- Also, they are part of base preparation for driveways and roadways.
- These types of aggregates are used to facilitate drainage to maintain perimeter drains, septic leach fields, and retaining walls.
- Also, used in temporary road surfaces (think of gravel roads) and to create tire knock areas for trucks leaving construction sites.
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