Shape test of Aggregate - Flakiness index - Elongation index :-
This particles shape of the aggregate mass is determined by the percentage of the flaky and elongated particles in it. Indian standard explain only the method of calculating Flakiness Index and Elongation Index. But the specifications do not specify the limits. British Standard BS 882 of 1992 limits the flakiness index of the coarse aggregate to 50 for natural gravel and to 40 for crushed corase aggregate. However, for wearing surfaces a lower values of flakiness index are required.
In this Article :-
Q. Shape test of Aggregate
Q. What is Flakiness index ?
Q. What is Elongation index ?
Q. Test for determination of Flakiness index ?
Q. Test for determination of Elongation index ?
What is Flakiness index ?
It is defined as the percentage by weight of aggregate particles whose least dimension is less than 0.6 times there mean size.
Apparatus required for the Flakiness index test of Aggregate :-
- Thickness gauge
- Sieves
- Weighing mechine
- Gauging trowel
Gauging trowel
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| Gauging trowel |
Test for Determination of Flakiness Index :-
The flakiness index of aggregate is the percentage by weight of particles in it whose least dimension is less than three-fifths of their mean dimension. This test is conducted by using a metal thickness gauge.
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| Thickness gauge Flakiness index |
Procedure :-
- 1. A minimum number of 200 pieces 0f aggregate in each fraction is taken and weighted.
- 2. In order to separate flaky particles, each fraction is then gauged individually for thickness on the metal gauge.
- 3. The total amount of flaky material passing in the guage is weighed to an accuracy of 0.1 percent of the weight of the samples.
| Sieve size in mm | Thickness gauge mean X0.6 | |
| Passing through IS-Sieve | Retained on IS-Sieve | |
| 63 | 50 | 33.9 |
| 50 | 40 | 27.0 |
| 40 | 31.5 | 21.45 |
| 31.5 | 25 | 16.95 |
| 25 | 20 | 13.5 |
| 20 | 16 | 10.8 |
| 16 | 12.5 | 8.55 |
| 12.5 | 10 | 6.75 |
| 10 | 6.3 | 4.84 |
The flakiness index is taken as the total weight of the material passing the various thickness gauges expressed as a percentage of the total weight of the sample taken.
What is Elongation index ?
It is defined is the percentage by weight of particles whose greatest dimension is 1.8 times there mean dimension.
Apparatus required for the Elongation index test of Aggregate :-
- Thickness gauge
- Sieves
- Weighing mechine
- Gauging trowel
Test for Determination of Elongation Index :-
The Elongation index on an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension. This test is also conducted by using metal length guage.
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| Elongation Thickness gauge |
Procedure :-
- 1. A minimum number of 200 pieces 0f aggregate in each fraction is taken and weighted.
- 2. In order to separate elongated particles, each fraction is then gauged individually for dimension (length) on the metal gauge.
- 3. The total amount retained by the guage length shall be weighed to an accuracy of at least 0.1 percent of the weight of the test sample.
| Sieve size in mm | Length gauge mean X 1.8 | |
| Passing through IS-Sieve | Retained on IS-Sieve | |
| 63 | 50 | 101.7 |
| 50 | 40 | 81.0 |
| 40 | 31.5 | 64.35 |
| 31.5 | 25 | 50.85 |
| 25 | 20 | 40.5 |
| 20 | 16 | 32.4 |
| 16 | 12.5 | 25.65 |
| 12.5 | 10 | 20.25 |
| 10 | 6.3 | 14.52 |
The elongation index is the total weight of the material retained on the various length gauges expressed as a percentage of the total weight of the sample gauged. The presence of elongated particles in excess of 10 to 15 percent is generally considered undesirable.
NOTE - The elongation index is not applicable to sizes smaller than 6.3 mm.
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