Cold weather concreting

Effects of cold weather (below about 5 °C)

• Slower hydration, so setting and strength gain are delayed and the formwork must stay in place longer.
• If fresh concrete freezes before reaching about 3.5 MPa, the water turns to ice and expands by about 9%, causing a permanent loss of strength (up to 50% or more) and poor durability.
• Freeze-thaw damage of young concrete, and scaling of the surface.
• Thermal stress cracking from the temperature difference between the warm core and the cold surface.

Precautions (IS 456 Cl. 14.1, which refers to IS 7861 Part 2; ACI 306)

1. Heat the mixing water (up to about 60 °C) and, if needed, the aggregates; do not let cement come into contact with water hotter than about 60 °C (flash set). The concrete temperature at placing should be not less than 5 °C (IS 7861 Part 2), and 10 to 20 °C is recommended.
2. Use rapid-hardening cement, a higher cement content, a lower W/C, accelerating admixtures (non-chloride for RCC) and air-entraining agents.
3. Remove snow, ice and frost from forms, reinforcement and subgrade; never place concrete on frozen ground.
4. Cover with insulating blankets, polythene and straw, or use heated enclosures.
5. Keep the concrete above 5 °C for at least 48 hours and protect it for about 7 days; delay form stripping, and judge strength from test cubes cured under site conditions.
6. Avoid sudden cooling when the protection is removed.