Classifications of Aggregates According to Shape
Classification of Aggregates Based on Shape
The practical shapes of
aggregates influence the properties of fresh concrete more than those of
hardened concrete. Depending upon the particle shape the aggregate may be
classified as rounded, irregular or partly rounded, angular or flaky.
We know that aggregate is derived from
naturally occurring rocks by blasting or crushing etc., so, it is difficult to
attain required shape of aggregate. But, the shape of aggregate will affect the
workability of concrete. So, we should take care about the shape of aggregate.
This care is not only applicable to parent rock but also to the crushing
machine used.
Aggregates are classified according to shape into the following types
- Rounded aggregates
- Irregular or partly rounded aggregates
- Angular aggregates
- Flaky aggregates
- Elongated aggregates
- Flaky and elongated aggregates
Rounded Aggregate
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| rounded aggregate |
The
aggregate with rounded particles (river or seashore gravel) has minimum wide
ranging from 32 to 33 percent. It gives minimum ratio of surface area to the
volume; the requiring minimum cement paste to make good concrete. The only
disadvantages that the interlocking between its particles is less and hence the
development of the bond is poor, making it unsuitable for high strength
concrete and pavement.
Irregular Aggregate
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| irregular aggregate |
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The aggregate having
partly rounded particles (pitsand and gravel) has higher percentage of voids
ranging from 35 to 38. It requires more cement paste for a given workability.
The interlocking between particles, through better than that obtained with the
rounded aggregate is inadequate for high strength concrete.
Angular Aggregate
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| angular aggregate |
Aggregate
with sharp, Angular and rough particles (crushed rock) has a maximum percentage
of voids ranging from 30 to 40. The interlocking between the particles is good,
thereby providing a good Bond. The aggregate requires more cement paste to make
workable concrete of high strength than that required by rounded particles. The
angular aggregate is suitable for high strength concrete and pavement subjected
to tension.
Flaky and And Elongated Aggregates
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| Flaky and And Elongated Aggregates |
An aggregate
termed flaky when its least dimension(thickness) is less than three- fifth of
its mean dimension. The mean dimension of aggregate is the average of sieve
size through which the particles pass and are retained, respectively. The
particle is said to be elongated when it greatest dimension (length) is greater
than nine-fifth of its mean dimension.
The
angularity of aggregate effects of workability or stability of the mix which
depend on the interlocking of the particles. The elongated and flaky particles
also adversely affect the durability of concrete as they tend to be oriented in
one plane with water and air wire forming underneath. The presence of these
particles should be restricted to 10 to 15%. This requirement is particularly
important for or crushed fine aggregate since the material made this content
flatter and more elongated particle. The angularity of the aggregate can be
estimated from the proportion of voids in a sample compacted as prescribed in
IS: 2386(part-1)-1963. The higher the angularity number the more angular is the
aggregate. The elongation index of an aggregate is defined as the percentage by
weight of particles present in whose greatest dimension(length) is greater than
nine-fifth of their mean dimension.
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