Apparatus
1.
Set of sieves
2.
Weighing balance
3.
Thickness gauge
4.
Length gauge
Purpose
The test is used to
determine particle shape of an aggregate. Usual shapes are:
1.
Rounded (river gravel)
2.
Flaky (laminated rock)
3.
Elongated
4.
Angular (crushed rock)
Significance
1.
The degree of packing of particles of
one size depends upon their shape.
2.
Flaky and elongated particles lower the
workability of concrete mixes due to high ratio of surface area to volume.
3.
For concrete, the presence of flaky and
elongated particles in excess of 10-15 % is generally considered undesirable.
4.
For construction of base coarse, the
presence of flaky and elongated particles are considered undesirable as they
may cause inherent weakness. BS 1241 specifies a flakiness index not exceeding
30 % irrespective of the aggregate size.
5.
Maximum permitted elongated index is 35,
40 and 45 % for aggregate sizes 2.5″-2″, 1.5″- 0.75″ and 0.5″ – 0.375″
respectively.
Flaky Particle
Flaky
particle is one whose least dimension (thickness) is less than 0.6 times the
mean size. These are the materials of which the thickness is small as compared
to other two dimensions.
Flakiness Index
Flakiness
index is the percentage by weight of flaky particles in a sample.
Elongated Particle
It
is a particle having length considerably larger than the other two dimensions.
Its greatest dimension is more than 1.8 times its mean size.
Elongation Index
It
is the percentage by weight of elongated particles in a sample.
Procedure
The
procedure is given below:
Flakiness Index
The
aggregate is first sorted on test sieves into a number of closely limited
particle size groups. Each group (faction) is weighed and tested for thickness
on appropriate opening of the thickness gauge by passing each particle through
slot of the specified thickness along the least dimension. This procedure is
repeated for all the fractions. The weight of particles passing the thickness
gauge is recorded for each fraction. This is the weight of flaky particles.
Elongation
Index
The
given sample is divided into different fractions by sieve analysis as before
and weight of each fraction is recorded. In order to separate elongated
particles, each fraction is gauged individually for length in a length
gauge. The pieces of aggregate from each
fraction which do not pass and termed as elongated particles. The process is
repeated for all fractions.
Precautions
1.
Sieve analysis should be done carefully.
2.
No particle should be forced to pass
through the thickness gauge.
Observations and Calculations
a.
Flakiness
Index
Sieve Size
|
Weight Retained (g)
|
% age Retained (%)
|
Wt. of Flaky Particles (g)
|
Individual F.I. (%)
|
b.
Elongation
Index
Sieve Size
|
Weight Retained (g)
|
% age Retained (%)
|
Wt. of Elongated Particles (g)
|
Individual E.I. (%)
|
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