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

Elongation Index Test Lab Report, Elongation Index of Aggregate, IS: 2386 (Part 1)-1963


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  • Metal gauge (Length Gauge)
  • Weighing balance
  • Gauging trowel
  • Sieves

Elongation Index Test Lab Report, Elongation Index of Aggregate

  • 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:-

  1. 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.
  3. 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.
  4. The length gauge, used in the test, should be of the dimensions specified in column 4 of Table for the appropriate size of material.
  5. 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.

Elongation Index Test Lab Report, Elongation Index of Aggregate

Calculations and Observations:-
Formula for elongation index of aggregate:

Elongation Index Test Lab Report, 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
Elongation Index Test Lab Report, Elongation Index of Aggregate

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.

  1. The degree of packing of the aggregate particles of one size depends upon their shape.
  2. Due to high surface area to volume ratio, the flaky and elongated aggregate particles lower the workability of concrete mixes.
  3. 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.
  4. According to BS-1241, a flakiness index should not exceed 30% irrespective of the aggregate size.
  5. Maximum permitted elongated index is 35%, 40% or 45% for aggregate size 21/2” – 2”, 11/2” - ¾” and 1/2” - ⅜”.
  6. Both Flakiness index Elongation Index tests are not applicable to the materials of sizes smaller than 6.3 mm i.e. ¼” sieve.


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