What is the Flakiness and Elongation 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.

The elongation index of aggregate is defined as the percentage by mass of particles (stones) in it whose greatest dimension (thickness) is greater than one and four-fifths of their average dimension.


Why is Flakiness and Elongation Index Test on Aggregate Done?
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 Elongation index of an aggregate sample is found by separating the elongated aggregates by sieving. Elongation 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 while the test elongation index of aggregate is measured on particles passing through IS sieve size of 63mm and retained on IS sieve size of 6.3 mm.

Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate

Flakiness and Elongation Index Test, Shape Test on Coarse Aggregate, IS: 2386-Part 1-(1963)


Hi friends, you are welcomed in the world of Civil Allied Gyan. Here I have explained about the flakiness and elongation index test of coarse aggregate. Read definition, apparatus, IS code, procedure, observations, formula, result, lab report about flakiness and elongation index test of coarse aggregate. Save in pdf.


  • Thickness gauge
  • Length 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


Procedure For the Test Flakiness Index of Coarse Aggregate:-

  1. Sieve the aggregate sample with sieves specified in Table 1.
  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 1 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 1 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 and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate
Fig. 1 Thickness gauge

Procedure For Elongation Index Test of Aggregate:-

  1. Sieve the aggregate sample with sieves specified in the given Table 2.
  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 elongated materials, gauge each fraction in tum for length on a length gauge of the pattern shown in Fig 2 or in bulk on sieves having flaky slots.
  4. The length gauge, used in the test, should be of the dimensions specified in column 4 of Table 2 for the appropriate size of material.
  5. Weigh the total amount of elongated aggregate retained by the length gauge to an accuracy of at least 0.1 percent of the weight of the test sample. 

Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate
Fig. 2 Length gauge

Calculations and Observations:-
Formula for flakiness index of aggregate: 
Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate
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.


Formula for elongation index of aggregate:
Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate
Where,
Y =  The weight of materials retained on specified length gauges and
W =  The weight of aggregate passing and retained on the specified sieves.


Table 1: Dimensions of Thickness gauge
Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate


Table 2: Dimensions of Length gauge
Flakiness Index and Elongation Index Test on Coarse Aggregate, Shape Test of Aggregate


Results:


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 aggregate particles and elongated aggregate particles should be avoided in pavement construction, particularly in surface course.

If the flaky aggregate particles and elongated aggregate particles are present in appreciable proportions, then 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


Read also -
Angularity number test of coarse aggregate (shape test of aggregate) 



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