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. It
is measured on particles passing through IS sieve size of 63mm and retained on IS
sieve size of 6.3mm.
Elongation
index Test of aggregate is conducted on coarse
aggregates sample to estimate the shape of aggregates. For cement
concrete types and base coarse construction 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 Elongation and elongation is necessary.
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 Elongation index of aggregate is
not applicable to the materials of sizes smaller than 6.3 mm.
Elongation is the measure of the
ductility of a material that is determined by a tension test. It is the
increase in the length gauge of a test specimen after fracture
divided by its original length gauge. Higher elongation means higher
ductility.
Elongation Index Test Lab Report, Elongation Index of Aggregate, IS: 2386 (Part 1)-1963
Hi friends, you are welcomed in the world
of Civil Allied Gyan. Here
I have explained about elongation index test of coarse aggregate.
By the help of this, you can easily find the Elongation
Index of Aggregate. So please continue to the end & keep your love and
support on me.
- Metal 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 Elongation Index Test of Aggregate:-
- Sieve the aggregate sample with sieves specified in the given Table.
- 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 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 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.
Calculations
and Observations:-
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: Dimensions
of Length gauge
Results:
Elongation
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 and elongated aggregate particles
should be avoided in pavement construction, particularly in surface course.
If the flaky and elongated aggregate particles
are present in appreciable proportions, 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
|
|
(ii)
|
WBM
base course and surface course
|
The
Significance of Elongation
Index Test of Aggregate
The test elongation
index of aggregate is used to determine the particle shape of the
aggregate specimen and each particle shape being preferred under specific
condition.
The significance
of flakiness and elongation index of aggregate are following:
- The degree of packing of the aggregate particles of one size depends upon their shape.
- Due to high surface area to volume ratio, the flaky and elongated aggregate particles lower the workability of concrete mixes.
- For cement concrete types and base coarse construction
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.
- According to BS-1241, a flakiness index should not exceed 30% irrespective of the aggregate size.
- Maximum permitted elongated index is 35%, 40% or 45% for aggregate size 21/2” – 2”, 11/2” - ¾” and 1/2” - ⅜”.
- Both Flakiness index Elongation Index tests are not applicable to the materials of sizes smaller than 6.3 mm i.e. ¼” sieve.
Read
also:
