As per IS 383 (1970), the percentage passing of fine aggregates under a 2.36 mm IS sieve during grading Zone I is
Correct Answer: D. 60 – 95%
🧱 Detailed Explanation: Zone I Fine Aggregate Grading at 2.36 mm (IS 383:1970)
Zone I is the coarsest grading zone for fine aggregates under IS 383:1970. It represents sands with a higher proportion of larger particles (2–4.75 mm range), which translate into higher fineness modulus values and better suitability for high-strength concrete mixes where a lower surface area minimises cement paste demand.
Why D (60–95%) is the correct answer for Zone I at 2.36 mm:
A wide range of 60–95% passing at 2.36 mm is specified for Zone I because this coarse zone accommodates natural variation in river sand and crushed rock sand while keeping the sand classification “coarse.” Up to 40% of the material can be retained on the 2.36 mm sieve (i.e., particles between 2.36 mm and 4.75 mm), which is what makes Zone I the coarsest among the four zones.
A wide range of 60–95% passing at 2.36 mm is specified for Zone I because this coarse zone accommodates natural variation in river sand and crushed rock sand while keeping the sand classification “coarse.” Up to 40% of the material can be retained on the 2.36 mm sieve (i.e., particles between 2.36 mm and 4.75 mm), which is what makes Zone I the coarsest among the four zones.
• Option B (95–100%) corresponds to Zone I’s own range at the 4.75 mm sieve — not 2.36 mm.
• Option C (0–10%) matches the 150 μm sieve range for Zone I.
• Option A (25–40%) has no direct match in IS 383 grading tables.
Zone I Grading Limits (IS 383:1970) — % Passing
| IS Sieve Size | Zone I % Passing | Notes |
|---|---|---|
| 10 mm | 100 | All fine aggregate passes 10 mm |
| 4.75 mm | 90–100 | Upper boundary of fine agg. |
| 2.36 mm | 60–95 | Tested limit — coarse zone has more retained |
| 1.18 mm | 30–70 | Wide range in Zone I |
| 600 μm | 15–34 | Low % passing = coarser sand |
| 300 μm | 5–20 | |
| 150 μm | 0–10 |
Key Concepts for Students
- Zone I = Coarsest; Zone IV = Finest. As zone number increases, more material passes each sieve at a given size.
- At 2.36 mm: Zone I allows 60–95% passing, while Zone IV allows 95–100% passing — showing Zone IV has negligibly coarse particles.
- Zone I sand is preferred for high-strength concrete because coarser sand has lower surface area, requiring less cement paste for coating.
- Memorise the 600 μm landmark: Zone I = 15–34%, Zone II = 35–59%, Zone III = 60–79%, Zone IV = 80–100%. This column discriminates the zones best.