Flakiness Index and Elongation Index Test on Coarse Aggregates

The particle shape of aggregate is determined by the percentages of flaky and elongated particles contained in it. In case of gravel, it is determined by its Angularity Number. Flaky and Elongation is conducted on coarse aggregates to assess the shape of aggregates. Aggregates which are Flaky and elongated are detrimental to the higher workability and stability of mixes. They are not conducive to good interlocking and hence the mixes with an excess of such particles are difficult to compact to the required degree.

1. Flakiness Index Test on Aggregates

AIM :

To determine the flakiness Index of a given aggregates sample.

APPARATUS REQUIRED:

  1. The apparatus consists of a standard thickness gauge, I.S. sieves of sizes 63, 50, 40, 31.5, 25, 20, 16, 12.5, 10 and 6.3 mm. 
  2. Balance to weigh the samples.

FLAKINESS INDEX:

The flakiness index of aggregates is the percentage by particles whose least dimension (thickness) is less than 3/5th (0.6) of their mean dimension. The test is not applicable to sizes smaller than 6.3 mm.

procedure:

  1. The sample is sieved with the sieves mentioned in the table.
  2. A minimum of 200 pieces of each fraction to be tested are taken and weighed (w1 gm).
  3. In order to separate flaky materials, each fraction is then gauged for thickness on thickness gauge, or in bulk on sieve having elongated slots as specified in the table.
  4. Then the amount of flaky material passing the gauge is weighed to an accuracy of at least 0.1% of test sample.
  5. Let the weight of the flaky materials passing the gauge be w1gm. Similarly the weights of the fractions passing and retained on the specified sieves be w1, w2, w3, etc. are weighed and the total weight w1+w2+w3+………. = w gm is found.
  6. Also the weights of the materials passing each of the specified thickness gauges are found = W1, W2, W3… and the total weight of the material passing the different thickness gauges = W1+W2+W3+………. =W gm is found.
  7. Then the flakiness index is the total weight of the flaky material passing the various thickness gauges expressed as a percentage of the total weight of the sample gauged.

OBSERVATION AND CALCULATION:

Table 1:- Observation table of Flakiness Index Test on Coarse Aggregates

REPORTING OF RESULT:

The flakiness index of the given sample of aggregates is _______________%.

RECOMMENDED VALUES OF FLAKINESS INDEX:

Table 2:- Recommended Values of Flakiness Index

 

2. Elongation Index Test on Coarse Aggregates

AIM :

To determine the Elongation Index of a given aggregates sample.

APPARATUS REQUIRED:

  1. Length gauge, I.S-sieves as given in the table
  2. The balance of accuracy 0.01 gm.

ELONGATION INDEX:

The elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1 and 4/5th times (1.8 times) their mean dimensions. The elongation test is not applicable to sizes smaller than 6.3 mm.

PROCEDURE:

  1. The sample is sieved with the sieves mentioned in the table.
  2. A minimum of 200 pieces of each fraction to be tested is taken and weighed (w1 gm).
  3. Each fraction is thus gauged individually for length in a length gauge. The gauge length is used should be those specified in the table for the appropriate material.
  4. The pieces of aggregates from each fraction tested which could not pass through the specified gauge length with its long side are elongated particles and they are collected separately to find the total weight of aggregate retained on the length gauge from each fraction.
  5. The total amount of elongated material retained by the length gauge is weighed to an accuracy of at least 0.1% of the weight of the test sample.
  6. The weight of each fraction of aggregate passing and retained on specified sieves sizes are found – W1, W2, W3,…………. And the total weight of the sample determined = W1 +W2 +W3+……………. = W gm.
  7. Also the weights of material from each fraction retained on the specified gauge length are found = x1, x2, x3 … and the total weight retained determined = x1 +x2 +x3 +…… = x gm.
  8. The elongation index is the total weight of the material retained on the various length gauges, expressed as a percentage of the total weight of the sample gauged.

OBSERVATION AND CALCULATION:

Table 1:- Observation table of Elongation Index Test on Coarse Aggregates

REPORTING OF RESULT:

The elongation index of the given sample of aggregates is _______________%.

DISCUSSION:

For base coarse and construction of bituminous and cement concrete types, the presence of flaky and elongated particles are considered undesirable as they may cause inherent weakness with probabilities of breaking down under heavy loads. Rounded aggregates are preferred in cement concrete road construction as the workability of concrete improves.

Angular shape of particles are desirable for granular base coarse due to increased stability derived from the better interlocking when the shape of aggregates deviates more from the spherical shape, as in the case of angular, flaky and  elongated aggregates, the void content in an aggregate of any specified size increases and hence the grain size distribution of the graded aggregates has to be suitably altered in order to obtain minimum voids in the dry mix or the highest dry density. It is determined according to the procedure laid down in IS-2386 (PART- I).

 

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