An aggregate which passes through 25 mm I.S. sieve and is retained on 20 mm sieve, is said to be flaky if its least dimension is less than :
Correct Answer: D. 13.5 mm
📚 Detailed Explanation: Flakiness Threshold Calculation
The flakiness criterion per IS 2386 (Part I) states: an aggregate particle is classified as flaky if its least dimension (thickness) is less than 0.6 times its mean sieve size. The mean sieve size is the average of the two sieve sizes between which the particle falls.
Step-by-step solution:
1. Upper sieve = 25 mm, Lower sieve = 20 mm
2. Mean dimension = (25 + 20) ÷ 2 = 22.5 mm
3. Flakiness threshold = 0.6 × 22.5 = 13.5 mm
So if the least dimension (thickness) of the particle is less than 13.5 mm, it is flaky. Option D (13.5 mm) is correct. Note: 0.6 × 22.5 = 13.5, not 13.35 — this is a straightforward multiplication so watch for rounding traps in options.
1. Upper sieve = 25 mm, Lower sieve = 20 mm
2. Mean dimension = (25 + 20) ÷ 2 = 22.5 mm
3. Flakiness threshold = 0.6 × 22.5 = 13.5 mm
So if the least dimension (thickness) of the particle is less than 13.5 mm, it is flaky. Option D (13.5 mm) is correct. Note: 0.6 × 22.5 = 13.5, not 13.35 — this is a straightforward multiplication so watch for rounding traps in options.
Flakiness Threshold Examples
| Sieve Range | Mean Dim (mm) | Flakiness Limit = 0.6 × Mean (mm) |
|---|---|---|
| 12.5 mm / 10 mm | 11.25 | 6.75 |
| 20 mm / 16 mm | 18.0 | 10.8 |
| 25 mm / 20 mm | 22.5 | 13.5 |
| 40 mm / 31.5 mm | 35.75 | 21.45 |
| 75 mm / 63 mm | 69.0 | 41.4 |
Key Concepts for Students
- Formula to memorise: Flakiness limit = 0.6 × (sum of two sieve sizes / 2)
- The flakiness index test uses slot gauges whose slot widths correspond to 0.6 × mean sieve size for each size fraction.
- The flakiness index should not exceed 35% as per IS 383 for use in concrete.