Which of the following correctly explains the effects of cold weather on concrete? (i) Delay in setting and hardening (ii) Freezing of concrete at early age (iii) Alternate Freezing and Thawing
Correct Answer: D. (i), (ii), and (iii)
📚 Detailed Explanation: Cold Weather Causes All Three Effects on Concrete
Why D [(i), (ii), and (iii)] is correct: Cold weather adversely affects concrete through multiple mechanisms simultaneously. Each of the three listed effects is real, distinct, and well-documented in standards and practice. Together, they explain why cold weather concreting requires special precautions.
Analysis of Each Effect
| Statement | Effect | Mechanism | Correct? |
|---|---|---|---|
| (i) Delay in setting and hardening | Concrete takes much longer to achieve structural strength | Cement hydration rate is strongly temperature-dependent. Below 10°C it slows significantly; below 5°C it becomes very slow; near 0°C almost stops. Van't Hoff rule: reaction rate approximately halves for every 10°C decrease. | ✓ TRUE |
| (ii) Freezing of concrete at early age | Fresh or early-age concrete water freezes; structure destroyed | If ambient temperature drops below 0°C before concrete develops sufficient strength (≈3.5 MPa minimum per ACI 306), water in capillary pores freezes and expands ~9%. The nascent C-S-H gel matrix, not yet strong enough to resist, is disrupted. Permanent strength loss of up to 50% can result. | ✓ TRUE |
| (iii) Alternate Freezing and Thawing | Cyclic damage accumulates; concrete progressively deteriorates | Each freeze-thaw cycle creates ice in capillary pores (expansion) then melts (contraction). Repeated cycles cause fatigue cracking, surface scaling, and internal microcracking. This is a long-term durability effect, distinct from early-age freezing. | ✓ TRUE |
Temperature Thresholds for Cold Weather Concrete Effects
| Temperature | Effect on Fresh/Hardening Concrete |
|---|---|
| 10°C to 15°C | Hydration slows; setting delayed; extended curing needed |
| 5°C to 10°C | Significant delay; minimum recommended placement temperature |
| 0°C to 5°C | Severe delay; risk of freezing if freshly placed; cold weather concreting precautions mandatory |
| Below 0°C | Water freezes; early-age damage; hydration essentially stops without admixtures |
| Cyclic 0°C | Freeze-thaw cycling; long-term surface deterioration and scaling |
- Cold weather causes all three effects: delayed setting, early-age freezing damage, and freeze-thaw cycling deterioration.
- Air-entraining admixtures specifically combat effect (iii) by providing void space for ice expansion.
- IS 7861 Part 1 defines cold weather as ambient temperature below 5°C or expected to fall below 5°C within 24 hours of placement.