Flakiness and elongation index tests

The shape of aggregate particles is judged by two indices (IS:2386 Part I).

• Flakiness Index (FI) - percentage by weight of particles whose least dimension (thickness) is less than 0.6 times their mean dimension (flaky).
• Elongation Index (EI) - percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension (elongated).

Flakiness index test

Apparatus: thickness gauge (a metal plate with slots of different widths for each size fraction), IS sieves (63, 50, 40, 31.5, 25, 20, 16, 12.5, 10 and 6.3 mm), balance.

1. Take a sample large enough to give at least 200 pieces of each size fraction to be tested, and sieve it into the standard fractions (a fraction is tested only if it is more than 5% of the sample).
2. Weigh each fraction ($W$).
3. Pass each particle of the fraction through the slot of the corresponding width in the thickness gauge (slot width = 0.6 x mean size of the fraction; for example 0.6 x 22.5 = 13.5 mm for the 25-20 mm fraction).
4. Weigh the particles passing the slot ($w$).

$$\text{FI} = \frac{\sum w}{\sum W}\times 100\%$$

Elongation index test

Apparatus: length gauge (a metal gauge with pins at spacings of 1.8 times the mean size of each fraction).

1. Use the same sieved fractions (at least 200 pieces each).
2. Try each particle in the length gauge; those that cannot pass through the gap between the pins (length greater than the gap) are elongated. Weigh them ($w$).

$$\text{EI} = \frac{\sum w}{\sum W}\times 100\%$$

Limits

Combined FI + EI should not exceed about 30% for wearing courses and bituminous layers, and 35% for base layers (NRS 2070/IRC/MoRTH). Flaky and elongated particles lower the stability, break during compaction and need more binder.