Fine aggregate is classified into four grading zones by sieve analysis. Zone I is the coarsest and Zone IV the finest. The zone affects how much sand the mix needs and how workable and strong the concrete will be.
Grading Limits: Percentage Passing by Weight
| IS sieve | Zone I | Zone II | Zone III | Zone IV |
|---|---|---|---|---|
| 10 mm | 100 | 100 | 100 | 100 |
| 4.75 mm | 90 – 100 | 90 – 100 | 90 – 100 | 95 – 100 |
| 2.36 mm | 60 – 95 | 75 – 100 | 85 – 100 | 95 – 100 |
| 1.18 mm | 30 – 70 | 55 – 90 | 75 – 100 | 90 – 100 |
| 600 micron | 15 – 34 | 35 – 59 | 60 – 79 | 80 – 100 |
| 300 micron | 5 – 20 | 8 – 30 | 12 – 40 | 15 – 50 |
| 150 micron | 0 – 10 | 0 – 10 | 0 – 10 | 0 – 15 |
The 600 micron sieve is the quickest way to tell the zones apart.
What It Means for Your Concrete
- Zone II is the usual target for structural concrete.
- As sand gets finer (towards Zone IV), the mix needs less fine aggregate relative to coarse aggregate. The mix design must be adjusted, not just the brand of sand.
- IS 383 advises against Zone IV sand in reinforced concrete unless tests confirm the mix is suitable.
- Very fine sand is often fine for plaster, but it is not a default choice for RCC.
Silt Check on Site
A quick field check: fill a measuring jar with sand to the 100 ml mark, add salt water to about 150 ml, shake well and let it settle for around three hours. The silt settles as a layer on top of the sand. Many sites reject or wash sand when the silt layer exceeds about 8% of the sand volume, and send doubtful lots for a lab test of material finer than 75 micron under IS 383.
What We Check
- A sieve analysis for each new source, and whenever the sand visibly changes
- A field silt test on every lorry for structural work
- Separate stacking for concrete sand and plaster sand, so they don’t get mixed up
Reference
- IS 383:2016 Coarse and fine aggregate for concrete, Specification
Summary for site use. Always work from the current edition of the standard and your project specification. Where they differ, the structural engineer’s specification governs.