The aggregates are classified as fine aggregates if their size is ________ and less :
Correct Answer: A. 4.75 mm
📚 Detailed Explanation: Classification Boundary Between Fine and Coarse Aggregate
Indian Standard codes (IS 383 and IS 456) define the boundary between fine aggregate (FA) and coarse aggregate (CA) at the 4.75 mm IS sieve size. Material passing through a 4.75 mm sieve is fine aggregate; material retained on it is coarse aggregate. This boundary is universally adopted in Indian practice for all structural concrete applications.
Why A (4.75 mm) is correct: The 4.75 mm sieve corresponds to No. 4 in the US sieve system and is the internationally recognised division between fine and coarse fractions. Aggregates in the 4.75 mm–80 mm range are coarse aggregates; those below 4.75 mm (down to 75 microns, below which material is classified as silt or clay) are fine aggregates.
Aggregate Size Classification
| Fraction | Size Range | Examples |
|---|---|---|
| Coarse Aggregate | 4.75 mm — 80 mm | Crushed stone, gravel, pebbles |
| Fine Aggregate (Sand) | 75 µm — 4.75 mm | River sand, crushed stone powder, M-sand |
| Silt / Clay | <75 µm | Deleterious material in concrete |
Key Concepts for Students
- 4.75 mm is the universal dividing sieve for fine vs. coarse aggregate in Indian Standards.
- Fine aggregate passes 4.75 mm but is essentially retained on 75 µm; material passing 75 µm is silt/clay and is limited to ≤3% in FA for concrete.
- Cyclopean aggregate (>75 mm) is a special category used in mass concrete structures like dams.