The fineness modulus of fine aggregate is
Correct Answer: C. indicator of average size of particles in aggregate
📚 Detailed Explanation: Fineness Modulus as an Indicator
The fineness modulus (FM) of an aggregate is calculated by summing the cumulative percentages retained on a standard set of sieves (150 µm, 300 µm, 600 µm, 1.18 mm, 2.36 mm, 4.75 mm for fine aggregate) and dividing by 100. It is a single number that characterises the particle size distribution without specifying the full grading curve.
Why C (indicator) is more precise than A (proportional): The FM does reflect average particle size — a higher FM means the distribution is skewed toward larger particles. However, saying it is simply “proportional” to average size (Option A) is not accurate because the relationship is not linear and two aggregates with identical FM can have completely different grading curves (bi-modal vs. uniform). The FM is therefore correctly described as an indicator of average particle size rather than a direct proportional measure. Option B (inversely proportional) is clearly wrong — higher FM always = coarser aggregate.
Fineness Modulus Ranges
| Material | FM Range | Average Size Indication |
|---|---|---|
| Fine sand | 2.2–2.6 | Small average particle size |
| Medium sand | 2.6–2.9 | Moderate average size |
| Coarse sand | 2.9–3.2 | Larger average size |
| Coarse aggregate | 5.5–8.0 | Much larger average size |
Key Concepts for Students
- FM is an indicator, not a complete descriptor of grading — two different gradation curves can give the same FM.
- IS 10262 uses FM in the mix design procedure to calculate the proportion of fine aggregate in the combined aggregate.
- Higher FM = coarser aggregate (never the reverse): a helpful mnemomic is “Fine Modulus goes fine when Fine.”