Cold weather concrete in Minnesota: what counts as cold and how pours are protected

Minneapolis Public Works sums up the problem: in Minnesota, concrete placed in the fall and winter needs protection so it can develop enough strength and durability for its intended service life. Several groups publish guidance on what that protection looks like. Their numbers don't all match, so this post keeps each figure with the source that states it.

The three sources here are the National Ready Mixed Concrete Association's (NRMCA) guide "CIP 27, Cold Weather Concreting," the City of Minneapolis Public Works cold weather concrete protection plan, and the City of Blaine building inspections page on cold weather concrete and masonry work.

What counts as "cold weather"

NRMCA defines cold weather as a period of more than 3 consecutive days when the average daily temperature is below 40°F and the air temperature is not above 50°F for more than half of any 24-hour period.
Minneapolis Public Works says cold weather conditions exist when the air temperature has fallen to, or is expected to fall below, 40°F. It also uses October 1 as a consistent date for cold weather protection to begin, regardless of air temperature.
Blaine says its cold weather concrete requirements apply when the air temperature has fallen, or is expected to fall, below 40°F during the protection period. The city says its guidance is based on ACI 306R and on cold weather practices used in Minnesota and in MnDOT specifications.

Why the cold matters

NRMCA's guide gives the reasons in plain numbers:

Fresh concrete freezes if its temperature falls below about 25°F.
Concrete that freezes can lose more than 50 percent of its potential strength, and it won't be durable.
Concrete should be protected from freezing until it reaches a compressive strength of 500 psi, which the guide puts at about two days after placement.
As a rule of thumb, a 20°F drop in concrete temperature roughly doubles the setting time.
Concrete that will be in contact with water and exposed to freeze-thaw cycles should be air-entrained, and protected from those cycles until it reaches at least 3,500 psi.

Before the pour

NRMCA says snow and ice should be removed, and the surfaces and metal embedments the concrete will touch should be above freezing, which may mean insulating or heating the subgrade first. Minneapolis puts it more bluntly: fresh concrete should never be placed on frozen soils or snow. Blaine's page says the subgrade, forms, reinforcement and other embedments must be completely free of frost, snow and ice, with no placement during snow or freezing rain.

The temperature of the concrete itself is the other half. NRMCA recommends these minimum concrete temperatures at the time of placement, based on the smallest dimension of the section:

Section size, minimum dimension	Concrete temperature as placed (NRMCA)
Less than 12 inches	55°F
12 to 36 inches	50°F
36 to 72 inches	45°F

NRMCA adds that concrete shouldn't be more than 20°F above those temperatures, and that the ready mixed concrete producer can control the temperature of the concrete it delivers. Blaine's page sets its own, different minimums by temperature range: concrete at 60°F or more at placement in the 30 to 50°F range, and 65°F or more in the 0 to 30°F range.

Concrete running down the chute of a ready mix truck — Ready mixed concrete coming off the truck chute.

NRMCA also covers the mix:

Accelerating admixtures are commonly used to offset slow setting. Calcium chloride works but shouldn't exceed 2 percent by weight of cement, and non-chloride accelerators should be used where corrosion of reinforcing steel is a concern.
Accelerators don't keep concrete from freezing, so protection is still needed.
Concrete should be placed at the lowest practical slump, because adding water can delay setting.

Protecting the concrete after it's placed

According to NRMCA, the common tools are insulated blankets and tarps, and straw covered with plastic sheets, with enclosures and insulated forms added depending on conditions. A few of its specific points:

Corners and edges lose heat fastest.
Fuel-burning heaters inside enclosures should be vented, both for safety and to prevent carbonation of the fresh surface, which causes dusting.
Water curing isn't recommended when freezing temperatures are close. Curing compounds, impervious paper or plastic sheets are used on slabs instead.
Cracking can occur when the difference between the surface and interior temperature exceeds about 35°F, so protection should come off gradually.
Forms shouldn't be stripped for 1 to 7 days, depending on strength gain, conditions and loading, and in-place strength should be estimated before stripping or loading.

Blaine's page gives minimum protection after placement for footings, walls and slabs by temperature range, with the concrete kept above its minimum curing temperature, which it says is typically above 50°F:

30 to 40°F: insulated blankets, 1 to 2 layers, for 24 to 48 hours
20 to 30°F: insulated blankets, 1 to 2 layers, plus wind breaks, for 48 hours
0 to 20°F: insulated blankets, 2 layers, with a heated enclosure preferred, for 48 to 72 hours
Colder still: heated enclosures or hydronic ground heat, with continuous monitoring in the coldest conditions

For structural members, Blaine calls for the same protection for 72 hours or until design strength meets the specification.

Minneapolis says the ideal concrete temperature during the protection period is a constant 50°F, and that concrete should stay protected from cold until it reaches its design compressive strength. After that, it calls for a membrane sealer to protect against the deicers the city uses on snow and ice. Minneapolis also asks permit applicants to submit a written plan describing how they will protect fresh concrete.

Keeping the numbers straight

None of these sources is a single statewide rule for every pour. NRMCA's guide is industry guidance, and the Minneapolis and Blaine documents are written for work in those cities. They share the basics: all three use 40°F as the reference point for cold weather, all three say fresh concrete must be kept from freezing, and all three want frozen ground and snow dealt with before the pour. They differ on details such as placement temperatures. If your project needs a permit, ask your local building department which requirements it uses.

Planning a slab, footing or other concrete work in or around Montgomery as the weather turns? Estevez Construction does concrete work for residential and commercial customers. Call us at (612) 601-3995 or request a free estimate.