If the least lateral dimension of aggregate is less than 0.6 times of its mean dimension, the aggregate is classified as
Correct Answer: B. flaky
📚 Detailed Explanation: Flakiness — Definition and Threshold
The shape of an aggregate particle significantly affects its packing, workability, and bond with cement paste. IS 2386 (Part I) and IS 383 define specific geometric criteria to classify aggregates as flaky, elongated, or neither. Flakiness refers to particles that are disproportionately thin relative to their average cross-sectional size.
Why B (flaky) is correct: An aggregate is termed flaky when its least dimension (thickness, t) is less than 0.6 times (= 3/5ths) of its mean sieve size (dmean). Mathematically: t < 0.6 × dmean. This 0.6 factor — or equivalently 3/5 — is the standard criterion used in IS 2386 (Part I) for the flakiness index test. Angular and rounded describe overall shape, not the thickness-to-width ratio.
Flakiness Index Test (IS 2386 Part I)
| Parameter | Details |
|---|---|
| Definition | % by weight of particles whose least dimension < 0.6 × mean size |
| Apparatus | Slot gauge (thickness gauge) |
| Limit (IS 383) | Flakiness index ≤35% for general concrete |
Key Concepts for Students
- 0.6 (or 3/5) is the magic number for flakiness — the most commonly tested threshold in exams.
- Flaky aggregates reduce concrete workability, increase water demand, and create planes of weakness, reducing strength and durability.
- Elongated aggregate: a separate criterion — length > 1.8 × mean dimension. A particle can be both flaky and elongated.
- The combined flakiness + elongation index should not exceed 40% per IS 383.