Shape tests on Aggregates( Flakiness Index and Elongation Index Test)
- The shape test of aggregates determines the shape and size of particle.
- Size & shape of aggregates directly has impact on strength of concrete, bond strength, workability of concrete, cement requirement and cost required for concrete.
- Rounded, elongated and flaky aggregates offer good workability but less bond strength and mechanical strength.
- Angular & irregular aggregates offers lesser workability but good bond strength and mechanical strength.
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- There two terminologies related to the shape of aggregates and its test.
- Flakiness Index
- Elongation index
- Aggregates are called Flaky & Elongated aggregates when its length is larger than its width and width is larger than its thickness.
- i.e. length>width>thickness.
- Flaky & elongated aggregates are not suitable for concreting. It does not form a very good bond with concrete. These are generally obtained from poorly crushed rocks.
Flakiness Index:
- The flakiness index is defined as the, percentage of aggregates by weight whose least dimension is less than 0.6 times than mean size.
- The procedure to find out flakiness index is as per IS: 2386 part 1.
- The apparatus used for this test is as shown below,

Elongation Index:
- The Elongation index is defined as the, percentage of aggregates by weight whose larger dimension (generally length) is 1.8 times more than mean size.
- This test is application to aggregates lager than the 6.3mm size.
- The procedure to find out Elongation index is as per IS: 2386 part 1.
- The apparatus (Elongation gauge) used for this test is as shown below,

Apparatus & Equipment:
- A standard thickness gauge
- A standard length gauge
- IS sieves of sizes 63, 50 40, 31.5, 25, 20, 16, 12.5,10 and 6.3mm
- A weighing balance of capacity 5kg, readable and accurate up to 1 gm.
Procedure:
- 1. Sieve all the sample through the IS sieves as mentioned in the table.
- 2. Take the minimum of 200 pieces of each fraction for testing and weigh them.
- 3. For Flaky materials, gauge the fraction for thickness on thickness gauge.
- 4. Weigh the flaky material passing through the thickness gauge.
- 5. For elongated materials, gauge the fraction for length gauge.
- 6. Weigh the elongated materials retained on length gauge.
Data Recording and Calculation:
Size of aggregates | Weight of fraction consisting of at least 200 pieces,g | Thickness gauge size, mm | Weight of aggregates in each fraction passing thickness gauge,mm | Length gauge size, mm | Weight of aggregates in each fraction retained on length gauge,mm | |
Passing through IS Sieve, mm | Retained on IS Sieve, mm | |||||
1 | 2 | 3 | 4 | 5 | 6 | 7 |
63 | 50 | W1 | 23.90 | X1 | – | – |
50 | 40 | W2 | 27.00 | X2 | 81.00 | Y1 |
40 | 31.5 | W3 | 19.50 | X3 | 58.00 | Y2 |
31.5 | 25 | W4 | 16.95 | X4 | – | – |
25 | 20 | W5 | 13.50 | X5 | 40.5 | Y3 |
20 | 16 | W6 | 10.80 | X6 | 32.4 | Y4 |
16 | 12.5 | W7 | 8.55 | X7 | 25.5 | Y5 |
12.5 | 10 | W8 | 6.75 | X8 | 20.2 | Y6 |
10 | 6.3 | W9 | 4.89 | X9 | 14.7 | Y7 |
Total | W = | X = | Y = |
Record of Shape Test:
Flakiness Index = (X1+ X2+…..) / (W1 + W2 + ….) X 100
Elongation Index = (Y1 + Y2 + …) / (W1 + W2 + ….) X 100
Results:
- Flakiness Index =
- Elongation Index =
Recommended Values of Flakiness Index and Elongation Index
- Flaky & elongated aggregates are not suitable for concreting. It does not form a very good bond with concrete. These are generally obtained from poorly crushed rocks.
- Thus avoid this kind of aggregates in pavement as well.
- IRC recommendations for maximum limits of flakiness index are as given.
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, two coats & precoated) | |
(iv) | Built up spray grout | |
3 (i) | Bituminous macadam | 15 |
(ii) | WBM base course and surface course |