Pouring Concrete in Cold Weather

How cold is too cold to pour concrete?

ACI 306R-16 defines cold weather as when the air temperature "has fallen to, or is expected to fall below, 40°F (4°C) during the protection period," per ACI's free preview of the guide. NRMCA's older CIP 27 uses an average daily temperature below 40°F. Either way, it is a trigger for protection, not a ban.

The limit that matters is the temperature of the concrete itself, and it depends on how thick the pour is. Thin sections lose heat fastest, so they need to start warmer:

CIP 27 adds that the concrete should not exceed those temperatures by more than 20°F, so overheating the mix is not the answer either. Almost every residential pour is under 12 inches thick, so the number to hold is 55°F.

Two hard limits sit underneath that. Fresh concrete freezes below about 25°F, per CIP 27. And it must be protected from freezing until it reaches 500 psi, which ACI's guide says most mixes reach "within 48 hours" at 50°F. If it freezes before then, ACI's FAQ on early freezing says it can "disrupt the mass and permanently impair the strength," and the NIH's facilities bulletin on cold weather concreting puts the loss at up to 50 percent of ultimate strength.

How long to protect it

The full protection table in ACI 306 is paywalled, but the USDA's Natural Resources Conservation Service reproduces the ACI-style figures in its Pennsylvania cold weather fact sheet:

A building department in Idaho uses the same numbers in plainer language: Post Falls' cold weather concrete SOP says regular concrete "shall be protected from freezing for at least 72 hours (three days)," and accelerated or high early strength mixes for 48 hours. For a driveway or patio, plan on three days of protection.

When the protection comes off, take it off slowly. ACI's guide says to remove insulation so the surface temperature drops gradually over the following 24 hours, and CIP 27 warns of cracking when the temperature difference between the concrete and the air exceeds about 35°F. Pulling the blankets off a warm slab on a 10°F morning is exactly that.

Concrete curing temperature: what cold does to the timeline

Cold does not stop concrete curing, but it slows it hard. CIP 27's rule of thumb is that every 20°F drop in concrete temperature roughly doubles the setting time. Penn State's civil engineering course material gives the full curve:

Source: Penn State, setting time of concrete at various temperatures. Strength gain slows the same way. Quikrete's data sheets ask for 5 days of curing at 70°F or above and 7 days between 50°F and 70°F. Engineers track this with the maturity method, which adds up time and temperature together. FHWA's maturity tech brief generally uses 14°F as the temperature at which strength gain stops, which is why a slab kept at 40°F still gains strength, just slowly.

For the full strength-by-age picture at normal temperatures, see how long concrete takes to cure.

Before the pour

• Never pour on frozen ground. The NRCS fact sheet: "Concrete will not be placed on frozen subgrade." Illinois DOT's cold weather memo says the same and requires the base be thawed to at least 40°F. Frozen ground settles when it thaws and chills the bottom of the slab below the temperatures in the table above. Cover the subgrade with insulating blankets for a few days beforehand to thaw it, as the NRCS suggests.
• Clear the snow and ice. CIP 27 wants forms, rebar and anything else the concrete touches above freezing and free of ice.
• Order the concrete warm. Ready-mix plants heat the water and sometimes the aggregate. South Carolina DOT's cold weather specification caps heated water at 170°F at the mixer and has the hot water mixed with the aggregate before the cement goes in, which prevents flash set.
• Order it air-entrained. CIP 27: use air-entrained concrete wherever it will see moisture and freezing. For outdoor flatwork that is every pour.
• Do not add water at the truck. Cold concrete looks stiff because it is cold. CIP 27 notes that adding water for slump delays setting, which is the last thing you want in cold weather.

Accelerators and calcium chloride

An accelerator speeds the set and early strength so the concrete reaches 500 psi sooner. It does not make concrete freeze-proof. CIP 27 is direct: "Accelerating admixtures do not prevent concrete from freezing."

Calcium chloride is the cheap, effective option, and CIP 27 caps it at 2 percent by weight of cement. The trouble is steel. The NRCS fact sheet says calcium chloride "should not be used to accelerate setting because of increased chances of corrosion" of reinforcing, and NRMCA notes it can also discolor concrete. For anything with rebar or wire mesh, ask for a non-chloride accelerator instead. These are sold under ASTM C494 as Type C (accelerating) or Type E (water-reducing and accelerating). Euclid's Accelguard 80 data sheet, for example, says it "contains no added chlorides" and still tells you to keep the concrete from freezing until 500 psi.

The other way to buy speed is a high early strength mix: Type III cement, or extra cement per yard. Both cut the protection period in the table above by a day.

Curing blankets, enclosures and heat

CIP 27 lists the usual protection: "Insulated blankets and tarps, as well as straw covered with plastic sheets." Enclosures with heaters are the next step up for serious cold. For a residential slab, insulated curing blankets are the practical tool, and they work by holding in the heat the cement produces as it hydrates.

Three things decide whether a blanket works:

• Contact. The NRCS fact sheet says insulation must be kept in close contact with the concrete or form. A blanket draped loosely with a gap underneath lets the wind take the heat.
• Edges and corners. These lose heat from two or three sides at once. The NRCS recommends insulation about three times as thick at corners and edges. Overlap blankets and weight the edges down.
• R-value against the cold. Thicker slabs and colder nights need more insulation. Illinois DOT requires the R-value be matched to slab thickness in a written plan. No free source we found publishes a simple chart for residential slabs, so if the forecast is well below freezing, double up or ask your ready-mix supplier.

Do not water-cure in the cold. CIP 27: "Water curing is not recommended when freezing temperatures are imminent," and ACI's guide warns it leaves the concrete saturated just as protection ends. Keep the moisture in with plastic under the blankets instead. And Quikrete's data sheets say not to use a curing compound if temperatures below 50°F are expected within 24 hours. How to cure concrete covers the methods in normal weather.

Bagged concrete in cold weather

The bag makers' instructions are short. Quikrete's 5000 data sheet says to protect the concrete from freezing for the first 48 hours, and that "Plastic sheeting and insulation blankets should be used if temperatures are expected to fall below 32 °F." It describes Quikrete 5000's high early strength as ideal "for colder weather applications," with 1,500 psi at one day and 2,500 psi at three. Sakrete's Canadian data sheet asks for mix and substrate temperatures between 40°F and 86°F for 24 hours before and 48 hours after.

Two practical tips for bagged mix: store the bags somewhere warm, and mix with warm tap water. Bagged mix poured at 35°F has no heat to spare, and a small pour loses what it has quickly. How long Quikrete takes to cure has the product-by-product times.

The first winter: keep the salt off

The cold weather risk that catches the most homeowners is not the pour. It is the first salt. New concrete is still saturated and has not reached full strength, and that is when de-icers do the most damage. NRMCA's CIP 2 on scaling says to "Avoid the use of deicing chemicals in the first winter." The American Concrete Pavement Association's FAQ sets the minimum: no de-icers earlier than 30 days after the curing period. The Pennsylvania Aggregates and Concrete Association goes further and says no de-icers on concrete less than a year old.

So a late fall pour has two clocks running: the protection period for the first few days, and the drying period before salt. Work out the second one here.

Short on days? Use sand for traction that winter and keep slushy tires off the slab where you can. Today's Type IL cement leaves a slimmer margin here, which our piece on Type IL cement and driveways that flake explains.

And the other end: hot weather

Heat is the opposite problem. Concrete sets faster, needs more water for the same slump, and ends up weaker if it cures hot. ACI 305.1 limits concrete to 95°F at discharge. NRMCA's CIP 12 covers the fixes: chilled water or ice in the mix, a dampened subgrade, and at least three days of curing. The mixing side of both hot and cold weather is in how to mix concrete.