What is the Flakiness and Elongation Index of Aggregate?
The flakiness index of aggregate is defined as the percentage by
mass of particles (stones) in it whose least dimension (thickness) is less than
three-fifths of their average dimension.
The elongation index of
aggregate is defined as the percentage by mass of particles (stones) in it
whose greatest dimension (thickness) is greater than one and four-fifths of
their average dimension.
Why is Flakiness and Elongation Index Test on Aggregate Done?
Flakiness index of aggregate test is conducted on coarse
aggregates sample to estimate the shape of aggregates. For cement concrete
types and base coarse and bituminous-construction, the presentence
of flaky and elongated aggregate particles are considered undesirable
as they may cause inherent weakness with possibility of breaking down under
heavy loads.
Thus, evaluation of shape of the aggregate particles, particularly
with reference to flakiness and elongation is necessary.
The flakiness index of an aggregate sample is found by
separating the flaky aggregates by sieving. Flakiness index is expressed
as their weight as a percentage of the weight of the sample tested.
The Elongation index of an aggregate sample is found by
separating the elongated aggregates by sieving. Elongation index is
expressed as their weight as a percentage of the weight of the sample tested.
The test flakiness index of aggregate is not applicable to the
materials passing through the 6.30 mm IS test sieve or retained on the 63.00 mm
IS test sieve while the test elongation index of aggregate is measured on
particles passing through IS sieve size of 63mm and retained on IS sieve size
of 6.3 mm.
Flakiness and
Elongation Index Test, Shape Test on Coarse Aggregate, IS: 2386-Part 1-(1963)
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Allied Gyan. Here I have explained about the flakiness and elongation index
test of coarse aggregate. Read definition, apparatus, IS code, procedure,
observations, formula, result, lab report about flakiness and elongation
index test of coarse aggregate. Save in pdf.
- Thickness gauge
- Length Gauge
- Weighing balance
- Gauging trowel
- Sieves
- IS: 2386 (Part 1)–1963, Method of test of aggregates for formation of concrete
- IS: 383–1970, Specification for coarse and fine aggregate from natural source for formation of concrete
Procedure For the Test Flakiness Index of Coarse Aggregate:-
- Sieve the aggregate sample with sieves specified in Table 1.
- Take sufficient quantity of aggregate to provide the minimum number of 200 pieces of any fraction to be tested.
- Then to separate the flaky materials, gauge each fraction for thickness on a thickness gauge of the pattern shown in Fig 1 or in bulk on sieves having elongated slots.
- The width of the slot used in the thickness gauge or sieve should be of the dimensions specified in column 4 of Table 1 for the appropriate size of material.
- Weigh the total amount of flaky material passing the gauge to an accuracy of at least 0.1 percent of the weight of the test sample.
 |
Fig. 1 Thickness gauge
|
Procedure For Elongation Index Test of Aggregate:-
- Sieve the aggregate sample with sieves specified in the given Table 2.
- Take sufficient quantity of aggregate to provide the minimum number of 200 pieces of any fraction to be tested.
- Then to separate the elongated materials, gauge each fraction in tum for length on a length gauge of the pattern shown in Fig 2 or in bulk on sieves having flaky slots.
- The length gauge, used in the test, should be of the dimensions specified in column 4 of Table 2 for the appropriate size of material.
- Weigh the total amount of elongated aggregate retained by the length gauge to an accuracy of at least 0.1 percent of the weight of the test sample.
 |
Fig. 2 Length gauge
|
Calculations
and Observations:-
Formula
for flakiness index of
aggregate:
Where,
X = The weight of material passing through the
various thickness gauges and
W = The
weight of aggregate passing and retained on the specified sieves.
Formula
for elongation index of
aggregate:
Where,
Y = The weight of materials
retained on specified length gauges and
W =
The weight of aggregate passing and retained on the specified sieves.
Table 1: Dimensions of Thickness gauge
Table
2:
Dimensions of Length gauge
Results:
Flakiness Index of Aggregate Particles =
................. %
Note:-
Surface texture and aggregate shape influence the properties of
freshly mixed concrete more than the properties of hardened concrete.
Rough-textured, angular and elongated aggregates require more water to produce
workable concrete than smooth, rounded compact aggregates.
Consequently, the cement contents must also be increased to
maintain the water-cement (W/C) ratio. Generally, flat and elongated aggregate particles
are avoided or are limited to about 15 % by weight of the total aggregate.
Recommended
Values of Flakiness Index and Elongation Index
The aggregate shape tests give only a rough
idea of the relative shapes of aggregates. Flaky aggregate particles and
elongated aggregate particles should be avoided in pavement construction,
particularly in surface course.
If the flaky aggregate particles and elongated aggregate
particles are present in appreciable proportions, then the strength of pavement
construction’s layer would be adversely affected due to probability of breaking
down under heavy loads. Workability of cement concrete is reduced. IRC
recommendations for maximum limits of flakiness index are as given below.
|
Sl. No: |
Type of pavement |
Maximum limits of flakiness index, (%) |
|
1 |
Bituminous carpet |
30 |
|
2 (i) |
Bituminous / Asphaltic concrete |
25 |
|
(ii) |
Bituminous Penetration macadam |
|
|
(iii) |
Bituminous surface dressing (single coat, double
coats and precoated) |
|
|
(iv) |
Built up spray grout |
15 |
|
3 (i) |
Bituminous macadam |
Read also -
Angularity number test
of coarse aggregate (shape test of aggregate) |
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