Flakiness and Elongation Index
EXPERIMENT NO. 1:
Determination of the flakiness and the elongation index for the given aggregate. sample.
SCOPE & SIGNIFICANCE:
This test is used to determine the particle shape of the aggregate and each particle shape being preferred under specific conditions.
The significance of flakiness & elongation index is as follows;
- The degree of packing of the particles of one size depends upon their shape.
- Due to high surface area to volume ratio, the flaky and elongated particles lower the workability of concrete mixes.
- Flaky and elongated particles are considered undesirable for base coarse construction as they may cause weakness with possibilities of braking down under heavy loads.
- BS-1241 specifies a Flakiness index not exceeding 30% irrespective of the aggregate size.
- Maximum permitted Elongated index is 35, 40 or 45% for aggregate sizes 2 ½’’ – 2’’, 1 ½’’ – ¾’’ & ½’’ – 3/8’’.
- Both Flakiness and Elongation tests are not applicable to sizes smaller then 6.3mm i.e. ¼’’ sieve.
SHAPES OF PARTICLES
The usual shapes of the particles are;
- Rounded (river gravel) Flakiness and Elongation Index
- Flaky (laminated rock) Flakiness and Elongation Index
- Elongated Flakiness and Elongation Index
- Angular( crushed rock) Flakiness and Elongation Index
PREFERRED USE OF EACH SHAPE
Rounded aggregates are preferred in concrete roads (rigid pavements) as the workability of concrete increases due to the less friction between the surfaces.
Angular shape of the particles is desirable in granular base coarse (flexible pavement) due to better interlocking and increased stability.
Flaky and Elongated particles are considered as a source of weakness.
- Thickness/Flakiness Index Gauge
- Length/Elongation Index Gauge
- Aggregate sample to be tested
- A flaky particle is the one whose least dimension (thickness) is than 0.6 times the mean size.
- These are the materials of which the thickness is small as compared to the other two dimensions.
- Limit of flaky particles in the mixes is 30%. If the flaky particles are greater than 30% then the aggregate is considered undesirable for the intended use.
FLAKINESS INDEX Flakiness and Elongation Index
It is the percentage by weight of flaky particles in a sample.
- Perform the sieve analysis on the given aggregate sample
- The aggregates are then arranged in the into a number of closely limited particle size groups -stored orted on the test sieves into a number of closely limited particle size groups – 2 ½’’ – 2’’, 1 ½’’ – ¾’’ & ½’’ – 3/8’’
- Each group (fraction) is weighed and tested for thickness on appropriate opening of the thickness gauge by passing each particle through slot of specified thickness along least dimension.
- The weight of particles passing the thickness gauge is recorded for each fraction. This is the weight of flaky particles.
- The flakiness index is calculated by expressing the weight of flaky particles as a percentage of total weight of the sample.
Calculation for Flaky Particles:
|Sieve Size||Weight Retained||Percentage Retained||Weight of Flaky Particles||Individual Flakiness Index||Weighted Flakiness Index|
|Σ= 8913.5||Σ = 23.07|
These are the particles having length considerably larger than the other two dimensions and it is the particle whose greater dimension is 1.8 times its mean size.
Limit of elongated particles in the mixes is 45%. Thus, if the elongated particles are greater than 45%, then the aggregate is considered undesirable for the intended use.
It is the percentage by weight of elongated particles in a sample. The Elongated index is calculated by expressing the weight of Elongated particles as percentage of total weight of the sample.
Calculation for Elongated Particles:
|Sieve Size||Weight Retained (gm)||Percentage Retained||Weight of Elongated Particles||Individual Elongation Index||Weighted Elongation Index|
|Σ=8913.5||Σ = 48.91|
- While sieving, care must be taken that the particles that are chocked in the sieve must no be forced down into the next sieve. Such particles should be pushed back into the same sieve.
- While placing different fractions on the table, place them some distance apart so that no two fractions may get mixed.
- Be careful while selecting the opening of the flakiness and elongation gauges for any particular fraction.Flakiness and Elongation Index Flakiness and Elongation Index
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