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Flakiness & elongation
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FLAKINESS AND ELONGATION INDEX 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 flackiness & elongation index is as follows :
The degree of packing of the particle of one size depen upon their shape Due to high surface area to volume ration, the flaky and elongated particle 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 size 2½” – 2” , 1½”-¾” & ½”-3/8” . Both Flakiness and Elongation tests are not applicable to sizes smaller than 6.3mm i.e. ¼” sieve
RELATED THEORY: SHAPES OF PARTICLES
The usual shapes of the particles are: 1. 2. 3. 4.
Rounded aggregate are preferred in concrete road (rigid pavement) as the workability of concrete increases due to the less friction between the surfaces. Angular shape of particle is desirable in granular base course(flexible pavement) due to better interlocking and increased stability Flaky and Elongated particles are considered as a source of weakness
APPARATUS:
Thickness/flackiness index gauge Length/Elongation index gauge Aggregate sample to be tested
FLAKY A flaky paticle is the one whose least dimension (thickness) is than 0.6 times the mean size These are the material of which the thickness is small as compared to the other two dimension 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 It is the percentage by weight of flaky particles in a sample.
PROCEDURE Page 2 | 4
1. The sieve analysis on the given aggregate sample is perform 2. The aggregates are arranged in the into a number of closely limited particle size groups – store orted on the test sieves into a number of closely limited particle size groups - 2½”2”. 1½” - ¾” & ½” -3/8” 3. 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 4. The weight of particle passing the thickness gauge is recoreded for each fraction. This is the weight of flaky particles. 5. The flakiness index is calculated by expressing the weight of flaky particle as a percentage of total weight of the sample
Calculation for flaky particles:
Sieve Size Passing 2 1½ 1 ¾ ½ 8-Mar ¼
Retained 1½ 1 ¾ ½ 8-Mar ¼ Pan
Weight Retained
Percentage Retained
(gm)
(%)
Weight of Flaky Particles (gm)
Individual Flakiness Index (%)
Weight Flakiness index (%)
ELONGATED PARTICLE 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 if elongated particle in the mixes is 45%. Thus , if the elongated particles are greater than 45%, then the aggregate is considered undesirable for the intended use.
ELONGATION INDEX Page 3 | 4
It is the percentage by weight of elongated particle in sample. The elongated index is calculated by expressing the weight of Elongated particle as percentage of total weight of the sample
Sieve Size Passing 2 1½ 1 ¾ ½ 41341 ¼
Retained 1½ 1 ¾ ½ 41341 ¼ Pan
Weight Retained
Percentage Retained
(gm)
(%)
Weight of Elongated Particles (gm)
Individual Elongated Index (%)
Weight Elongated index (%)
PRECAUTION 1. While sieving , care must be taken that the particles that are checked in the sieve must not be forced down into next sieve. Such particles should be pushed back into the same sieve. 2. While placing different fraction on the table, place them some distance apart so that no two fractions may get mixed. 3. Be careful while selecting the opening of the flakiness and elongation gauges for any particular fraction.
