Combined Flakiness and Elongation Test Procedure Index of Coarse Aggregates

Flakiness and Elongation Test Procedure

Aim: To determine the combined Flakiness and Elongation Index of Coarse Aggregates. (According to MORTH)
Apparatus required: Thickness gauge, Length gauge, IS sieves of sizes 63, 50, 40, 31.5, 25, 2a 16, 12.5,10 and 6.3mm, Weighing Balance Material required: Coarse Aggregate.
Learning objective: 
The students will learn how to check the quality of aggregates at site and how to segregate them.

The usual shapes of the particles are;

  •     Rounded (river gravel)
  •     Flaky (laminated rock)
  •     Angular( crushed rock)

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.
Thickness Gauge

Theory: 

  • Aggregates which happen to fall in a particular size range may have rounded cubical, angular flaky or elongated shape of particles.
  • It is evident that the flaky and elongated particles will have less strength and durability when compared with cubical, rounded or angular particles of the same stone. Hence too flaky and too much elongated aggregates should be avoided as far as possible.
  • The particle shape of aggregate mass is determined by the percentage of elongated and flaky particles contained in it and by its angularity is of importance in view of its effect on the design of mixes and their ultimate strength.
  • The evaluation of shape of the particles made in terms of flakiness index, elongation index and angularity number. Elongated and flaky aggregates are less workable and they are also likely to break under smaller loads than the aggregates which are spherical or cubical.
  • This test covers the procedure for determining the flakiness index and elongation index
  • The Flakiness index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three- fifths (0.6times) of their mean dimension.
  • This test is not applicable to sizes smaller than 6.3mm.The Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than ninefifths (1.8times) their mean dimension.
  • This test is not applicable for sizes smaller than 6.3mm.The permissible limit of combine flakiness and elongation index of aggregates are 30% of total weight of aggregate.
Gauge Marking

 

Procedure: 

1. Sieve the sample through the IS sieves (as specified in the table).
2. Take a minimum of 200 pieces of each fraction to be tested and weigh them.

3. In order to separate the flaky materials, gauge each fraction for thickness on a thickness gauge. The width of the slot used should be of the dimensions specified in column (4) of the table for the appropriate size of the material.
4. Weigh the flaky material passing the gauge to an accuracy of at least 0.1 percent of the test sample.
5. In order to separate elongated particle, now the elongated particle is separated from remaining non flaky material by using length gauge.
6. Separation of Elongated Material- each fraction shall be gauged individually for length on a metal length gauge.
7. Weighing of Elongated Material – the total amount retained by the length gauge shall be weighed to an accuracy of at least 0.1 percent of the weight of the test sample.

Formulae:

Flakiness Index = (X1+ X2+…..) / (W1 + W2 + ….) X 100
Elongation Index = (Y1 + Y2 + …) / (W1 + W2 + ….) X 100

Precautions: 

1. Weigh the aggregate carefully.
2. Sieving of aggregates needs to be done uniformly.

Dimension of Thickness and Length Gauges


Important Notes to Read

ELONGATED PARTICLES

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.

ELONGATION INDEX

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.

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.


FLAKY

  •     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.

Please follow and like us:

Add a Comment

Your email address will not be published. Required fields are marked *

error: Content is protected !!