As per IS 7861 (Part 2), concreting done at atmospheric temperature ________ or below is termed as cold weather concreting.
Correct Answer: B. 5°C
📚 Detailed Explanation: IS 7861 Part 2 — Cold Weather Concreting Threshold
Why B (5°C) is correct: IS 7861 (Part 2) sets the cold weather concreting threshold at 5°C atmospheric temperature or below. Below this temperature, hydration rate drops dramatically and water in the mix risks freezing before the concrete gains adequate strength. If water freezes before sufficient hydration, the expansion (~9% volume increase on freezing) damages the concrete matrix permanently.
Why 5°C Is Critical
| Temperature | Effect on Hydration |
|---|---|
| 20°C (reference) | Normal hydration; standard strength development |
| 10°C | Hydration rate slowed by ~50%; curing period extended |
| 5°C | Hydration nearly stops; cold weather precautions mandatory (IS 7861 Part 2) |
| 0°C (freezing) | Free water freezes; expansion causes micro-cracking; irreversible damage |
| <0°C | Severe damage; concrete may be permanently weakened |
IS 7861 Part 2: Precautions for Cold Weather Concreting
| Precaution | Detail |
|---|---|
| Minimum placement temperature | Not less than 4.5°C |
| Minimum temperature during hardening | Maintain above 2°C until concrete achieves design strength |
| Accelerating admixture | Calcium chloride (CaCl2) up to 2% by weight of cement (OPC only) |
| Cement type preferred | High C3S content (e.g., OPC 53 grade) for faster early strength |
| Avoid salts/anti-freeze chemicals | Generally not recommended as they degrade concrete quality |
| Heated water / aggregates | Warm the mix to ensure adequate reaction temperature |
- Cold weather threshold per IS 7861 (Part 2) = 5°C or below.
- Cement preference: high C3S, low C2S for faster early strength in cold.
- Freezing water expands ~9% by volume — this is why preventing freezing before adequate strength is paramount.