Curing Concrete in Cold Weather

Cold weather curing has two jobs: keep the slab from freezing in its first days, and keep it warm enough to gain strength after that. NRMCA’s cold weather concreting guide defines cold weather as more than 3 days in a row averaging below 40°F (5°C), with the air at or below 50°F (10°C) for more than half of any day.

Can concrete freeze while curing?

Yes. Fresh concrete freezes below about 25°F (minus 4°C), and NRMCA says “the potential strength of frozen concrete can be reduced by more than 50% and it will not be durable.” The fix is protection until the slab reaches 500 psi (3.5 MPa), which NRMCA puts at “about two days after placement.”

After that first hurdle there is a second one. Concrete exposed to water and repeated freezing and thawing should be air-entrained, and NRMCA says to protect it from freeze and thaw cycles until it reaches at least 3,500 psi (24 MPa). On a driveway, that usually means the first winter.

Does cold weather affect concrete curing?

Yes, it slows everything. NRMCA’s rule of thumb is that “a drop in concrete temperature by 20°F [10°C] will approximately double the setting time.” Strength gain slows too: at 50°F (10°C), ordinary Type I concrete needs about 21 days to reach 85% of its 28-day strength, compared with 16 days at 70°F (21°C).

Those day counts come from ACI 306R, as reproduced by the Federal Highway Administration. For the plain timeline and when you can use the slab, see how long concrete takes to cure.

How do you cure concrete in cold weather?

Seal the surface first, then insulate it. NRMCA says “water curing is not recommended when freezing temperatures are imminent” and to use “membrane-forming curing compounds or impervious paper and plastic sheets” on slabs. Then lay insulating blankets over the top, lapped well past the edges.

For the pros and cons of each sealing method, see wet curing vs curing compound, plastic and blankets. On spec work, ACI 308.1 sets the curing period at 7 days with the concrete above 50°F (10°C), so a cold slab that dips below that is not counting curing days.

How do concrete curing blankets work?

Insulating blankets hold in the heat the cement gives off as it hydrates. NRMCA says “insulating concrete retains heat and maintains favorable curing temperatures,” and that blankets or insulated forms should be used in extreme cold, “especially for thin sections.” A 4 in. slab is a thin section. It has little mass to keep itself warm.

Edges and corners lose heat first, so run blankets well past them. The Indiana Ready Mixed Concrete Association says that from mid-September to mid-April, insulating blankets should keep the curing temperature above 55°F (13°C) for at least 7 days. ACI 306R includes tables of insulation values to hold concrete at 50°F (10°C) for 3 or 7 days.

When can you take the blankets off?

After the curing period, and slowly. The NRMCA curing guide says not to let concrete cool faster than 5°F (3°C) per hour for the first 24 hours. NRMCA’s cold weather guide adds that cracking can start when the surface and core differ by about 35°F (20°C), so remove insulation gradually.

What about heaters and enclosures?

Use them when blankets alone cannot hold the temperature, such as a hard freeze in the forecast. NRMCA says fossil-fueled heaters in enclosures “should be vented” for safety and to prevent carbonation of the new surface, “which causes dusting.” ACI 306R notes heated enclosures dry concrete out, so keep the surface sealed.

Can you use curing compound in cold weather?

Yes, above the product’s minimum temperature. Water-based compounds are the ones to watch. The W.R. Meadows CC-309-1WS data sheet, a typical water-based product, says “KEEP FROM FREEZING” and “do not apply if concrete temperature is less than 40 F (4 C).” Store the pails indoors overnight, not in the truck.

State agencies take this seriously too. Iowa DOT Section 4105 requires compounds to be applied at a material temperature above 40°F and bars carryover lots that have frozen. For how these products behave, see water-based curing compound.

On an interior slab that will later get tile, epoxy or glue-down flooring, the compound under the blankets still has to suit the finish. Read the curing compound and flooring guide before the pour.

How do you set up a cold weather pour?

Most of the work happens before the truck arrives. NRMCA’s cold weather guidelines come down to a short list, and every item is something you can check that morning.

1. Clear snow and ice. Forms, subgrade and steel touching the concrete should be above freezing, which may mean thawing the base with blankets or heaters first.
2. Order air-entrained concrete at the lowest practical slump, and ask the supplier for warm concrete.
3. Speed up strength gain with Type III cement, more cement or an accelerator. NRMCA notes the supplier may also cut fly ash or slag in cold weather.
4. Have blankets, plastic and compound on site before the pour, not on order.
5. Keep test cylinders in an insulated box at 60 to 80°F (16 to 27°C) for the first 24 to 48 hours.

Accelerators have limits. NRMCA says they “do not prevent concrete from freezing,” and calcium chloride should not exceed 2% by weight of cement. Use non-chloride accelerators where steel or metal embedments could corrode.

If your slab froze or you skipped the blankets

If the concrete froze within its first day or two, before it reached 500 psi, assume it lost strength and durability. Look for a flaky, weak surface that scales off, and have the slab checked by your ready mix supplier or a testing lab before you put weight on it. Do not seal over the damage.

If it only got cold and never froze, it is still gaining strength, just slowly. Cover it now, keep it above 50°F (10°C) and add the cold days to your curing period. Keep cars off longer than the usual 7 days.