Pouring Concrete in Cold Weather (ACI 306)
When temperatures drop, standard concrete casting risks freezing, which can permanently reduce its strength by over 50%. Following ACI 306 guidelines ensures cold-weather pours achieve full load capacity.
1. The Danger of Freezing
Concrete must never be allowed to freeze before it reaches a compressive strength of at least 500 PSI (3.5 MPa), which typically takes 24 to 48 hours under normal conditions. If water inside the wet concrete freezes, it expands by 9% in volume, shattering the developing cement-silicate matrix.
2. Temperature Protection Rules
According to ACI 306, cold weather is defined as a period when the average daily temperature drops below 40°F (4°C) for three consecutive days. During this period, follow these shielding steps:
- Heated Aggregates: Batch plants must heat mixing water and sand/gravel to ensure concrete arrives on site at a minimum temperature of 55°F (13°C).
- Insulation Blankets: Cover all slabs with insulated curing blankets to trap the concrete's natural heat of hydration.
- Hydronic Heaters: For large structures, run heated pipes under insulation wraps to maintain temperatures above 50°F (10°C) for the first 3 to 7 days.
3. Using Accelerators
Adding Calcium Chloride (up to 2% by weight of cement) accelerates the set time, reducing the duration concrete must be insulated. However, do not use calcium chloride in reinforced structures, as it accelerates rebar corrosion.