Flakiness index and Elongation Index of Coarse Aggregates
i. to determine the elongation index of the given aggregates
ii. to determine the flakiness index of the given aggregates
APPARATUS:
The apparatus for the shape tests consists of the following:
(i) A standard thickness gauge
(ii) A standard length gauge
(iii) IS sieves of sizes 63, 50 40, 31.5, 25, 20, 16, 12.5,10 and 6.3mm
(iv) A balance of capacity 5kg, readable and accurate up to 1 gm.
THEORY:
The
particle shape of aggregates is determined by the percentages of flaky
and elongated particles contained in it. For base course and
construction of bituminous and cement concrete types, the presence of
flaky and elongated particles are considered undesirable as these cause
inherent weakness with possibilities of breaking down under heavy loads.
Thus, evaluation of shape of the particles, particularly with reference
to flakiness and elongation is necessary.
The Flakiness index of
aggregates is the percentage by weight of particles whose least
dimension (thickness) is less than three- fifths (0.6times) of their
mean dimension. This test is not applicable to sizes smaller than 6.3mm.
The Elongation index of
an aggregate is the percentage by weight of particles whose greatest
dimension (length) is greater than nine-fifths (1.8times) their mean
dimension. This test is not applicable for sizes smaller than 6.3mm.
PROCEDURE:
i) Sieve the sample through the IS sieves (as specified in the table).
ii)Take a minimum of 200 pieces of each fraction to be tested and weigh them.
(iii) In
order to separate the flaky materials, gauge each fraction for
thickness on a thickness gauge. The width of the slot used should be of
the dimensions specified in column (4) of the table for the appropriate
size of the material.
(iv) Weigh the flaky material passing the gauge to an accuracy of at least 0.1 per cent of the test sample.
(v) In
order to separate the elongated materials, gauge each fraction for
length on a length gauge. The width of the slot used should be of the
dimensions specified in column (6) of the table for the appropriate size
of the material.
(vi) Weigh the elongated material retained on the gauge to an accuracy of at least 0.1 per cent of the test sample.
| Size of aggregates |
Weight of
fraction consisting of at least 200 pieces,g |
Thickness gauge size, mm
|
Weight of aggregates in each fraction passing thickness gauge,mm
|
Length gauge size, mm
|
Weight of aggregates in each fraction retained on length gauge,mm
| |
Passing through IS Sieve, mm
|
Retained on IS Sieve,
mm
| |||||
1
|
2
|
3
|
4
|
5
|
6
|
7
|
63
|
50
|
W1
|
23.90
|
X1
|
–
|
–
|
50
|
40
|
W2
|
27.00
|
X2
|
81.00
|
Y1
|
40
|
31.5
|
W3
|
19.50
|
X3
|
58.00
|
Y2
|
31.5
|
25
|
W4
|
16.95
|
X4
|
–
|
–
|
25
|
20
|
W5
|
13.50
|
X5
|
40.5
|
Y3
|
20
|
16
|
W6
|
10.80
|
X6
|
32.4
|
Y4
|
16
|
12.5
|
W7
|
8.55
|
X7
|
25.5
|
Y5
|
12.5
|
10
|
W8
|
6.75
|
X8
|
20.2
|
Y6
|
10
|
6.3
|
W9
|
4.89
|
X9
|
14.7
|
Y7
|
Total
|
W =
|
X =
|
Y =
| |||
OBSERVATIONS:
Flakiness Index = (X1+ X2+…..) / (W1 + W2 + ….) X 100
Elongation Index = (Y1 + Y2 + …) / (W1 + W2 + ….) X 100
RESULT:
i) Flakiness Index =
ii) Elongation Index =
RECOMMENDED VALUE:
The
shape tests give only a rough idea of the relative shapes of
aggregates. Flaky and elongated particles should be avoided in pavement
construction, particularly in surface course. If such particles are
present in appreciable proportions, the strength of pavement layer would
be adversely affected due to possibility of breaking under loads.
Workability is reduced for cement concrete. IRC recommendations for
maximum limits of flakiness index are as given.
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, two coats & precoated)
| |
(iv)
|
Built up spray grout
| |
3 (i)
|
Bituminous macadam
|
15
|
(ii)
|
WBM base course and surface course
|
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