Cold Weather Concrete
Whether your pour is a cold-weather pour, the protection temperature and hours by section size, the mix changes, the time to 500°F·h, and the blankets and enclosure it takes.
A cold-weather pour is 5 consecutive days with the air below 40°F. A 40°F pour on an 8 in section needs 50°F protection for 24 hr, and a 40°F pour on a 4 in slab needs 55°F for 24 hr, with the blankets on before the sheen goes.
How the math works
- The classification (ACI 306.1): cold weather is 5 consecutive days with the mean air temperature below 40°F, and freezing weather is 3 consecutive hours with the air below 32°F for more than half of each 24 hr period. A 5-day forecast under 40°F is the trigger, and a single cold night on a warm stretch is a frost job.
- Protection (ACI 306 Table): keep the concrete at 50°F for 24 hr on a section over 4 in placed at 40 to 50°F, 55°F for 24 hr on a thin section under 4 in placed at 40 to 50°F, 50°F for 24 hr at 40 to 55°F placement on anything over 8 in, and 72 hr of protection where mass concrete, a low section, or a freezing forecast is in play.
- The mix (ACI 306 Table 2.3.2.1): minimum 6 sacks of cement (7 sacks on a near-freezing pour), 12% entrained air where freezing and thawing are in the plan, 5 to 7 lb of a non-chloride accelerator per sack, mixing water heated to 60 to 90°F, and a placement temperature at 50 to 90°F with the subgrade free of ice and frost.
- 500°F·h (ACI 306 and ACI 503): fresh concrete reaches 500°F·h of maturity before it can be damaged by freezing, and freezing damage is permanent on a slab that freezes in the first 24 hr. A slab placed at 55°F reaches 500°F·h in about 16 hr, and the blanket stays on past that.
- Heating and enclosure (ACI 306.1): a heated enclosure needs a heater sized to the enclosure volume, ventilation for a fuel-fired heater, the concrete at 50 to 70°F, a temperature log at 4 hr intervals for 7 days, and the enclosure cooled slowly (about 25°F per 24 hr) before it comes off.
When does concrete count as a cold weather pour? ACI 306 says 5 consecutive days with the mean air temperature below 40°F, and freezing weather is 3 hours or more per day with the air below 32°F for 2 of the 3 days. A forecast that dips to 28°F tonight on an otherwise mild week is a frost protection call, not a cold-weather program.
A winter pour with a 5-day forecast under 40°F needs the whole ACI 306 program: mix changes, heated materials, blankets, and a temperature log.
How long do concrete blankets stay on? 24 hr at 50°F minimum on a normal section, 48 hr on a thin section, and 72 hr where the low goes under 32°F, with the blankets on before the bleed sheen goes and off only when the temperature is stable. Past 500°F·h of maturity the concrete can survive a freeze, which is the practical clock, not the calendar.
Blankets on a slab that has already frozen in the first few hours is a repair conversation: the top scales, the air voids are gone, and the cover is delaminated.
Do you need heated water for winter concrete? Yes, ACI 306 wants the mixing water heated and the placement temperature between 50 and 90°F with the aggregates above 60°F and free of ice. Chute wash water is not the reason a 45°F delivery is short of spec, and a hot-water batch with 6 sacks of cement is the usual winter answer.
Never add dry ice to a winter load, and a 95°F placement on a blanket-sealed slab is a flash-set risk in its own right.