---
title: "Flakiness and elongation index tests"
subject: "Transportation Engineering I"
chapter: 6
key: "semester_6/transportation_engineering_i:fi-ei-test"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/fi-ei-test
site: BCT Engineering
---

# Flakiness and elongation index tests

Transportation Engineering I · Chapter 6: Highway Materials

- Asked in: 2081 Chaitra, 4 marks
- Video: [Flakiness and Elongation Test of Aggregate | Shape test](https://www.youtube.com/watch?v=V_KpeEem0kM) — Civil Site visit
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/fi-ei-test
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-6

## Question

How are the Flakiness Index (FI) and Elongation Index (EI) values determined in the laboratory?

## Short answer

For FI, each sieved fraction is weighed, passed through a thickness gauge slot of width 0.6 times mean size; particles passing are weighed. FI = (total passing weight/total weight) × 100%. For EI, particles retained on a length gauge with gap 1.8 times mean size are weighed; EI = (retained/total) × 100%.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A particle is flaky if its least dimension is less than
   A. 0.6 times its mean dimension
   B. 1.8 times its mean dimension
   C. 0.18 times
   D. 6 times
   Answer: A. Flakiness is tested with a thickness gauge. (Key point: Flakiness definition)

2. In the flakiness test, which particles are weighed (w)?
   A. Those retained on the slot
   B. Those passing the slot of the thickness gauge
   C. Those passing 2.36 mm
   D. Those broken by hammer
   Answer: B. FI = sum w/sum W x 100. (Key point: Flakiness test)

3. Which particles are elongated in the length gauge test?
   A. Those that pass through easily
   B. Those smaller than 6.3 mm
   C. Those that cannot pass the gauge opening
   D. Those passing the thickness gauge
   Answer: C. Elongated particles are longer than 1.8 times mean size. (Key point: Elongation test)

4. The combined FI + EI limit for wearing courses is about
   A. 3%
   B. 80%
   C. 100%
   D. 30%
   Answer: D. About 30% for wearing courses, 35% for base. (Key point: Limits)

## Related in Transportation Engineering I

- [AIV, AAV, ACV and flakiness/elongation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaggregate-impact-abrasion-crushing) — Transportation Engineering I, chapter 6
- [Tests on road aggregates](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaggregate-tests-list) — Transportation Engineering I, chapter 6, asked 8×

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