The fineness modulus of coarse sand is
Correct Answer: D. 2.9 to 3.2
📚 Detailed Explanation: Fineness Modulus of Coarse Sand
The fineness modulus (FM) is a single index number representing the average particle size distribution of a fine aggregate. For coarse sand (the coarsest sub-category of fine aggregate), the FM falls in a specific range that distinguishes it from medium and fine sand.
Why D (2.9 to 3.2) is correct: IS 383 classifies fine aggregate by FM range: Fine sand = 2.2–2.6; Medium sand = 2.6–2.9; Coarse sand = 2.9–3.2. Note the subtle difference: Option C (2.9–3.5) is slightly broader than the standard IS 383 range; Option D (2.9–3.2) is the precise IS 383 range for coarse sand. In the standard 4-zone system, Zone I sand (coarsest) corresponds to FM around 2.9–3.5 in some texts, but the IS 383 specific table for coarse sand classification gives 2.9–3.2. The question's answer key selects D as the more precise answer.
Sand Classification by Fineness Modulus (IS 383)
| Sand Type | FM Range | IS Grading Zone |
|---|---|---|
| Very fine sand | <2.0 | Below Zone IV |
| Fine sand | 2.2–2.6 | Zone IV |
| Medium sand | 2.6–2.9 | Zone II / III |
| Coarse sand | 2.9–3.2 | Zone I |
Key Concepts for Students
- If both C (2.9–3.5) and D (2.9–3.2) appear in an exam, D is the more specific IS 383-aligned answer.
- Coarse sand (high FM) is preferred for mass concrete and pavements where maximum aggregate size permits coarser FA.
- For structural concrete, medium sand (FM 2.6–2.9) is generally preferred for workability-strength balance.