A project in winter has ambient temperatures around – 5°C. Which measure is most critical to ensure early strength and prevent freezing?
- AAdd retarder to the mix
- BPreheat water and aggregates, use accelerators and cover concreteCorrect
- CUse air-entraining admixtures only
- DReduce cement content
Solution
Concept
Thermal and chemical precautions in cold weather concreting
Formula
NONE
Step-by-step solution
1. Concrete exposed to sub-zero temperatures (-5°C) risks freezing of free mixing water, causing irreparable disruption of the matrix. 2. Preheating water and aggregates ensures fresh concrete is placed well above critical threshold temperatures (typically > 5°C to 10°C). 3. Accelerators speed up initial setting and accelerate the early liberation of hydration heat. 4. Insulating covers retain heat to protect early-age concrete from freezing until it reaches 5 MPa frost resistance. Answer: B
Explanation
When concreting at sub-zero temperatures (around -5°C), fresh mixing water risks freezing before gaining critical frost-resistance strength (5 MPa), which can cause permanent 50% strength loss. Heating the mix water and aggregates ensures adequate placement temperature, accelerating admixtures hasten heat evolution, and insulating blankets prevent thermal dissipation.
Why the other options are wrong
A) Retarders delay hydration, leaving fresh concrete vulnerable to freezing for longer periods. C) Air-entraining admixtures protect hardened concrete, not fresh concrete against early freezing. D) Reducing cement content reduces hydration heat and slows strength development.
Shortcut
Sub-zero concreting = Preheat materials + Accelerator + Thermal insulating covers.
Common mistake
Relying solely on air-entraining agents, which protect hardened concrete from freeze-thaw cycles but cannot prevent fresh paste from freezing.
Correct answer: B) Preheat water and aggregates, use accelerators and cover concrete