Under which condition highest water cement ratio is used?
Correct Answer: D. Light structural members exposed to alternate wetting and drying
📚 Detailed Explanation: W/C Ratio for Different Exposure Conditions
IS 456:2000 specifies maximum water-cement ratios based on the exposure condition and type of structural element. The principle is: more severe exposure or greater structural importance = lower maximum w/c.
Why D (Light structural members, wetting/drying) is correct: According to IS 456 durability requirements, light structural members in moderate exposure (alternate wetting and drying) have the highest permissible w/c — typically 0.50–0.55. Option A (heavy sections exposed to wetting/drying) would require lower w/c due to the heavier loading. Option B (hydraulic structures in rain/snow) require very low w/c (0.45 or lower) due to aggressive water exposure. Option C (heavy sections protected from rain/frost) would use moderate w/c — not the highest. Light members in moderate exposure = least restrictive condition = highest w/c.
IS 456 Maximum W/C by Exposure (Approximate)
| Exposure Class | Max W/C | Typical Application |
|---|---|---|
| Mild | 0.55 | Interior members, protected |
| Moderate | 0.50 | Light members, wetting/drying |
| Severe | 0.45 | Coastal, chemical exposure |
| Very severe | 0.45 | Marine splash zone |
| Extreme | 0.40 | Tidal/submerged/aggressive soils |
Key Concepts for Students
- Light structural members in moderate conditions = highest permissible w/c (least demanding exposure).
- Hydraulic structures and sea-exposed elements = lowest w/c (most demanding exposure).
- As exposure severity increases, maximum w/c decreases and minimum cement content increases — IS 456 Table 5.