OBJECT
Assess aggregate particle shape quality using the Flakiness & Elongation Index Test as per IS 2386 (Part I). This essential aggregate shape test determines the percentage of flaky and elongated particles that can adversely affect compaction, interlocking, and overall pavement performance.Excessive flaky and elongated aggregates reduce load-bearing capacity, increase voids, and lead to premature failures such as rutting, cracking, and surface deformation in flexible pavements. Therefore, strict compliance with IS specifications is critical for ensuring structural durability and long service life.This practical, site-ready guide covers:
- Required apparatus and gauge dimensions
- Step-by-step laboratory procedure
- Calculation formulas for Flakiness Index (FI) and Elongation Index (EI)
- Permissible limits as per MoRTH and IS standards
- Interpretation tips for QA/QC engineers
APPARATUS
The apparatus for the shape tests consists of the following:
- A standard thickness gauge
- A standard length gauge
- IS sieves of sizes: 63, 50, 40, 31.5, 25, 20, 16, 12.5, 10 and 6.3 mm
- A balance of capacity 5 kg, readable and accurate up to 1 g
THEORY
The particle shape of aggregates is determined by the percentages of flaky and elongated particles contained in it. For base course and construction of bituminous and cement concrete types, the presence of flaky and elongated particles is considered undesirable as these cause inherent weakness with possibilities of breaking down under heavy loads. Thus, evaluation of shape of the particles, particularly with reference to flakiness and elongation is necessary.
In highway projects, excessive flaky and elongated particles increase void content in bituminous mixes, leading to poor compaction, reduced Marshall Stability, and higher susceptibility to rutting under traffic loads.
The Flakiness Index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three-fifths (0.6 times) of their mean dimension. This test is not applicable to sizes smaller than 6.3 mm.
The Elongation Index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than nine-fifths (1.8 times) their mean dimension. This test is not applicable for sizes smaller than 6.3 mm.
PROCEDURE
- Sieve the sample through the IS sieves (as specified in the table below).
- Take a minimum of 200 pieces of each fraction to be tested and weigh them, or take the maximum number available up to 200 pieces.
- 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.
- Weigh the flaky material passing the gauge to an accuracy of at least 0.1% of the test sample.
- To separate the elongated materials, gauge the non-flaky material for length on a length gauge. The width of the slot used should be of the dimensions specified in column (6) of the table for the appropriate size of the material.
- Weigh the elongated material retained on the gauge to an accuracy of at least 0.1% of the test sample.
IS SIEVE & GAUGE TABLE
| Passing through IS Sieve, mm | Retained on IS Sieve, mm | Weight of fraction (200 pieces), g | Thickness gauge size, mm | Weight passing thickness gauge (Xi) | Length gauge size, mm | Weight retained on length gauge (Yi) |
|---|---|---|---|---|---|---|
| 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 = |
OBSERVATIONS and: FORMULAE
Record every fraction's weights clearly. Use at least two significant figures for percentages and record sample piece counts.
Flakiness Index = ((X1 + X2 + …) / (W1 + W2 + …)) × 100
Elongation Index = ((Y1 + Y2 + …) / (W1 + W2 + …)) × 100
| Fraction | Total pieces taken (Wi) | Flaky weight (Xi) | Elongated weight (Yi) | Remarks |
|---|---|---|---|---|
| 63–50 mm | ||||
| 50–40 mm | ||||
| 40–31.5 mm | ||||
| 31.5–25 mm | ||||
| 25–20 mm | ||||
| 20–16 mm | ||||
| 16–12.5 mm | ||||
| 12.5–10 mm | ||||
| 10–6.3 mm | ||||
| Total |
RESULT
I. Flakiness Index = X
II. Elongation Index = Y
NOTES & REFERENCES
- This document preserves the original technical content. Do not alter the definitions if your contract specification references a specific IS edition.
- Reference: IS 2386 Part 1 – Methods of Test for Aggregates for Concrete (Particle Shape Tests).
- Record environmental conditions and the balance calibration status with every test batch for traceability.
Permissible Limits as per MoRTH
| Application | Combined FI + EI Limit |
|---|---|
| Bituminous Concrete (BC) | ≤ 30% |
| Dense Bituminous Macadam (DBM) | ≤ 35% |
| Wet Mix Macadam (WMM) | ≤ 35% |
Note: Always verify latest MoRTH revision applicable to your contract.