If aggregates completely pass through a sieve of size 75 mm and are retained on a sieve of size 60 mm, the particular aggregate will be flaky if its minimum dimension is less than ……….
Correct Answer: C. 40.5 mm
📚 Detailed Explanation: Flakiness Limit Calculation for 75mm/60mm Fraction
The flakiness criterion (IS 2386 Part I): an aggregate particle is 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 consecutive IS sieves between which the particle falls.
Step-by-step:
1. Upper sieve = 75 mm, Lower sieve = 60 mm
2. Mean dimension = (75 + 60) ÷ 2 = 67.5 mm
3. Flakiness threshold = 0.6 × 67.5 = 40.5 mm
So any particle in this fraction whose minimum dimension is less than 40.5 mm is classified as flaky. Option C (40.5 mm) is correct.
1. Upper sieve = 75 mm, Lower sieve = 60 mm
2. Mean dimension = (75 + 60) ÷ 2 = 67.5 mm
3. Flakiness threshold = 0.6 × 67.5 = 40.5 mm
So any particle in this fraction whose minimum dimension is less than 40.5 mm is classified as flaky. Option C (40.5 mm) is correct.
Flakiness Limits for Various Sieve Fractions
| Sieve Fraction | Mean Dim (mm) | Flakiness Limit (mm) |
|---|---|---|
| 25 mm / 20 mm | 22.5 | 13.5 |
| 40 mm / 31.5 mm | 35.75 | 21.45 |
| 75 mm / 60 mm | 67.5 | 40.5 |
Key Concepts for Students
- Memorise the formula: Flakiness limit = 0.6 × (Upper sieve + Lower sieve) / 2
- Compare Q36: for 25/20 mm fraction, limit = 13.5 mm. For 75/60 mm fraction, limit = 40.5 mm.
- The same 0.6 factor applies regardless of the size fraction being tested.