What is The Flakiness Index of Aggregate?


The flakiness index of aggregate is defined as the percentage by mass of particles (stones) in it whose least dimension (thickness) is less than three-fifths of their average dimension.


Flakiness index of aggregate test is conducted on coarse aggregates sample to estimate the shape of aggregates. For cement concrete types and base coarse 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 flakiness and elongation is necessary.


The flakiness index of an aggregate sample is found by separating the flaky aggregates by sieving. Flakiness index is expressed as their weight as a percentage of the weight of the sample tested.

The test flakiness index of aggregate is not applicable to the materials passing through the 6.30 mm IS test sieve or retained on the 63.00 mm IS test sieve.

Flakiness Index Test on Aggregate, Shape Test of Aggregate, IS 2386 Part 1-1963

Flakiness Index of Aggregate, Shape Test of Aggregates, IS: 2386 (Part 1)-1963


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Apparatus Required For The Test Flakiness Index of Aggregate:
  • Thickness gauge
  • Weighing balance
  • Gauging trowel
  • Sieves


  • 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


  1. Sieve the aggregate sample with sieves specified in 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 flaky materials, gauge each fraction for thickness on a thickness gauge of the pattern shown in Fig or in bulk on sieves having elongated slots.
  4. The width of the slot used in the thickness gauge or sieve should be of the dimensions specified in column 4 of Table for the appropriate size of material.
  5. Weigh the total amount of flaky material passing the gauge to an accuracy of at least 0.1 percent of the weight of the test sample. 

Flakiness Index Test of Aggregate, Shape Test on Aggregate, IS 2386 Part 1-1963

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

Flakiness Index Test of Aggregate, Shape Test o Aggregate, IS 2386 Part 1-1963

Where,
X = The weight of material passing through the various thickness gauges and
W =  The weight of aggregate passing and retained on the specified sieves.


Table: Dimensions of Thickness
Flakiness Index Test on Aggregate, Shape Test of Aggregate, IS 2386 Part 1-1963


Results:
Flakiness index of aggregate = ................. 

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 test flakiness 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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