Cover protects reinforcement from corrosion and fire. Too little and the steel rusts, the concrete spalls, and the member loses strength within years. The right cover depends on the exposure condition, which is also what fixes the minimum concrete grade and the maximum water-cement ratio.
Step 1: Pick the Exposure Condition (IS 456, Table 3)
| Exposure | Typical situations |
|---|---|
| Mild | Concrete protected against weather or aggressive conditions, except in coastal areas |
| Moderate | Sheltered from severe rain; exposed to condensation and rain; completely immersed in water; buried in non-aggressive soil |
| Severe | Exposed to severe rain, alternate wetting and drying, or severe condensation; exposed to a coastal environment |
| Very severe | Exposed to sea water spray, corrosive fumes or severe freezing whilst wet; in contact with aggressive subsoil or groundwater |
| Extreme | Tidal zone; members in direct contact with liquid or solid aggressive chemicals |
In Kolkata, exterior members exposed to heavy monsoon rain and wetting and drying are often classed as severe. Foundations in non-aggressive soil are commonly moderate. Where a soil test shows aggressive sulphates or chlorides, the classification goes up. The structural engineer decides.
Step 2: Nominal Cover (IS 456, Table 16)
| Exposure | Nominal cover, not less than (mm) |
|---|---|
| Mild | 20 |
| Moderate | 30 |
| Severe | 45 |
| Very severe | 50 |
| Extreme | 75 |
- For mild exposure, cover may be reduced by 5 mm for main bars up to 12 mm diameter.
- Columns: cover to longitudinal bars not less than 40 mm, nor less than the bar diameter (clause 26.4.2.1).
- Footings: minimum cover 50 mm (clause 26.4.2.2).
Step 3: Mix Limits for RCC (IS 456, Table 5)
| Exposure | Min. cement content (kg/m³) | Max. free water-cement ratio | Min. grade |
|---|---|---|---|
| Mild | 300 | 0.55 | M20 |
| Moderate | 300 | 0.50 | M25 |
| Severe | 320 | 0.45 | M30 |
| Very severe | 340 | 0.45 | M35 |
| Extreme | 360 | 0.40 | M40 |
Cement content is inclusive of mineral admixtures and applies to 20 mm nominal maximum aggregate.
What We Check on Site
- Cover blocks of the right thickness at the spacing shown, especially at slab bottoms, beam sides and column corners
- No stones, broken bricks or timber used as cover blocks
- Cover still correct after workers walk on the mesh, before the pour
- No water added to the mix on site to “make it flow”, because that breaks the water-cement ratio limit
Reference
- IS 456:2000 Plain and Reinforced Concrete, Code of Practice: clauses 8.2, 26.4 and Tables 3, 5, 16
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.