As per IS (383-1970), in how many zones are the Grading limits of fine aggregates distributed?
Correct Answer: B. 4
📚 Detailed Explanation: Grading Zones of Fine Aggregate per IS 383:1970
IS 383:1970 is the Indian Standard specification for coarse and fine aggregates from natural sources for concrete. For fine aggregates, the standard introduced a four-zone grading classification to help engineers select the correct sand and adjust mix proportions accordingly.
Why B (4) is correct: The four zones — Zone I through Zone IV — represent a range from coarsest (Zone I) to finest (Zone IV). Each zone is defined by permissible percentage-passing values at specific sieve sizes (4.75 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm). The zones guide water-cement ratio adjustments: Zone IV (finest) typically needs more water to achieve the same workability as Zone I.
Zone Summary — IS 383:1970
| Zone | Description | % Passing 600 µm sieve |
|---|---|---|
| Zone I | Coarsest sand | 15–34% |
| Zone II | Medium-coarse | 35–59% |
| Zone III | Medium-fine | 60–79% |
| Zone IV | Finest sand | 80–100% |
Key Concepts for Students
- Zone I = Coarsest, Zone IV = Finest — remember this order; many exam questions reverse it as a distractor.
- Zone IV sands require a higher water content and yield lower strength at the same w/c ratio; IS 456 recommends caution when using Zone IV in high-strength mixes.
- IS 383:2016 (the revised edition) retained the same four-zone classification.