If fineness modulus of sand is 2.5 it is graded as ..
Correct Answer: B. fine sand
📚 Detailed Explanation: Sand Classification by Fineness Modulus
The fineness modulus (FM) of fine aggregate is a numerical index that summarises the particle size distribution in a single number. It is calculated by summing the cumulative percentages retained on a standard set of sieves and dividing by 100. Higher FM = coarser sand; lower FM = finer sand.
Why B (fine sand) is correct: The IS classification for fine aggregate based on FM is: Fine sand FM = 2.2–2.6; Medium sand FM = 2.6–2.9; Coarse sand FM = 2.9–3.2. An FM of 2.5 falls within the range 2.2–2.6, classifying it as fine sand. It is not “very fine sand” (which would be FM <2.0) — that category is too fine for most concrete mixes and is generally unsuitable.
Sand Classification by Fineness Modulus
| Sand Type | FM Range | Typical Use |
|---|---|---|
| Very fine sand | <2.0 | Not recommended for concrete |
| Fine sand | 2.2–2.6 | Plastering, thin sections |
| Medium sand | 2.6–2.9 | General concrete (most preferred) |
| Coarse sand | 2.9–3.2 | Mass concrete, pavements |
Key Concepts for Students
- Optimum FM for concrete is typically 2.6–2.9 (medium sand); too fine (low FM) increases water demand, too coarse (high FM) reduces workability.
- For any sieve analysis: FM = ∑(cumulative % retained on 150µm, 300µm, 600µm, 1.18mm, 2.36mm, 4.75mm sieves) / 100.
- An FM of 2.5 is slightly below ideal — in mix design you'd reduce water content slightly or blend with coarser sand.