AGGREGATES
Aggregates are fundamental components, primarily consisting of materials such as sand, gravel, and crushed stones. These granular materials play a vital role in determining the strength, durability, and workability . One of the key ways aggregates are classified is by their shape, which can significantly impact the performance . Below are the primary shapes
Aggregate Accor ding
to Shape :
• rounded aggregate
• Irregular aggregate
• angular aggregate
• flaky and elongated aggregate
The aggregates of rounded shape (river or sea shore gravel) have minimum voids ranging from 32 to 33%.
It gives minimum cement paste to make good concrete.
The aggregates of irregular shape (pit sand and gravel) have higher percentage of voids ranging from 35 to 38.
It requires more cement paste for a given workability.
The aggregates of sharp, angular and rough shape (crushed rock) have maximum percentage of voids ranging from 38 to 40.
The aggregate requires more cement paste to make workable concrete of high strength than that required by rounded shape.
The aggregate is said to be flaky when its least dimension (thickness) is three-fifth of its mean dimension.
The mean dimension of the aggregate is the average of the sieve sizes through which the particles pass and retained respectively.
The aggregate is said to be elongated when its greatest dimension (length) is greater than 1.8 times its mean dimension.
Aggregate According
to Size :
Aggregate are divided into two categories from the consideration of size
Size Needed:
Coarse Aggregate which is greater than 4.75 mm Fine aggregate which is less than 4.75 mm.
The aggregate which pass through 4.75 mm IS sieve and entirely retain on 75 micron IS sieve is called fine aggregate.
It may be natural sand, crushed stone sand or crushed gravel sand.
The minimum particle size of fine aggregate is
0.075 mm and the maximum particle size is 4.75 mm.
Thematerial having particle size varying from 0.002 to 0.06 mm is termed as silt and still smaller particles are called clay.
Generally the maximum size of aggregate should be as large as possible within the limits specified, but in any case not greater than one-fourth of the minimum thickness of
the members.
Reinforced Effect:
For heavy reinforced concrete member the nominal size of aggregate should usually be restricted to 5 mm less than minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement, whichever is smaller.
But in practically for reinforced concrete work, aggregate having maximum sizes of 20 mm are generally considered satisfactory.
Uses:
• Ingredients used in manufacturing of mortar and concrete
• Includes sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates.
• They are major ingredients of concrete which occupy 70% to 80% of the volume of concrete
• Aggregates are the most mined materials in the world.
• They provide strength to concrete and act as a filler material
• Also increases density of concrete
