An aggregate particle is said to be flaky if its minimum dimension is less than
Correct Answer: C. 3/5th of mean length
📚 Detailed Explanation: Flakiness Definition (IS 2386 Part I)
The flakiness criterion is one of the most frequently tested aggregate definitions in civil engineering examinations. The threshold of 3/5th (0.6 times) the mean sieve size is fixed by IS 2386 (Part I) and does not change regardless of the size fraction being tested.
Why C (3/5th = 0.6) is correct: By IS 2386 Part I, a particle is “flaky” if its least dimension (thickness) is less than 0.6 × the mean of the upper and lower sieve sizes defining its fraction. This 3/5 = 0.6 factor is a fixed standard value. Option A (1/5 = 0.2) is too small; Option B (2/5 = 0.4) is incorrect; Option D (4/5 = 0.8) would include too many particles. The standard uses exactly 3/5.
Flakiness vs. Elongation Criteria
| Shape Test | Criterion | Factor |
|---|---|---|
| Flakiness Index | Min dimension < 0.6 × mean sieve size | 3/5 = 0.6 |
| Elongation Index | Max dimension > 1.8 × mean sieve size | 9/5 = 1.8 |
Key Concepts for Students
- Flakiness factor = 3/5 = 0.6; Elongation factor = 9/5 = 1.8 — these two numbers define the shape tests.
- Flakiness index should be ≤35% by mass for aggregate used in concrete (IS 383).
- High flakiness index causes segregation, poor workability, and lower concrete strength due to weak aggregate orientation under load.