A flaky aggregate is said to be elongated if its length is
Correct Answer: B. twice the mean size
📚 Detailed Explanation: Elongated Aggregates
IS 2386 (Part I) defines two criteria for shape classification of aggregates: flakiness (thin particles) and elongation (long particles). A particle can satisfy both criteria simultaneously, making it both flaky and elongated. The elongation index is the percentage by weight of particles whose longest dimension exceeds 1.8 times (approximately 9/5ths) the mean sieve size.
Why B (twice the mean size) is correct: The technical definition specifies the elongation threshold as 1.8 × mean sieve size. In practical examination context, 1.8 is frequently rounded to and interpreted as “approximately twice” (2×) the mean size. The answer B correctly represents this threshold in the simplified exam context. The exact IS 2386 criterion is >1.8 × mean dimension, but option B (“twice”) is closest and the accepted answer.
Flakiness vs. Elongation Criteria (IS 2386 Part I)
| Property | Criterion | Apparatus |
|---|---|---|
| Flakiness | Thickness < 0.6 × mean size | Slot gauge (thickness gauge) |
| Elongation | Length > 1.8 × mean size (≈twice) | Length gauge (rod gauge) |
Key Concepts for Students
- Flakiness factor = 0.6; Elongation factor = 1.8 (approx. 2) — memorise both.
- Both flaky and elongated particles are undesirable: they increase the water/cement demand, create planes of weakness, and reduce workability.
- Combined flakiness and elongation index ≤40% per IS 383 for aggregate used in concrete.