How Cold Is Too Cold to Pour Concrete? Temperatures, Blankets and Cure Time
There is no single cutoff, but once the air is at or headed below 40°F, concrete needs cold-weather protection: placed warm, never on frozen ground, and kept from freezing until it reaches 500 psi. Here is how that works for a patio, walk or driveway.
The answer to how cold is too cold to pour concrete isn’t one number: the American Concrete Institute says you are pouring in cold weather any time the air has fallen to, or is expected to fall below, 40°F while the new concrete is being protected. Below that line the pour can still go ahead if the concrete goes in warm, never lands on frozen ground, and is kept from freezing until it reaches 500 psi, which takes about two days.
Quick answer
Plan cold-weather steps whenever the forecast drops to 40°F or lower in the days after the pour. For slabs under 12 inches thick, the concrete itself should be at least 55°F when it’s placed. Fresh concrete freezes once its own temperature falls below about 25°F, and it has to be kept from freezing until it reaches 500 psi, roughly two days.
How cold is too cold to pour concrete?
Concrete is in cold weather once the air is at or forecast below 40°F, per ACI 306R-16, but there’s no single air temperature that’s too cold. A slab can still be poured if the concrete is placed at 55°F or warmer, never on frozen ground, and kept from freezing until it reaches 500 psi, about two days.
What ACI counts as cold weather
ACI 306R-16, the Guide to Cold Weather Concreting, defines cold weather as any time the air temperature has fallen to, or is expected to fall below, 40°F during the protection period, as ACI explains in its cold-weather summary. The protection period is the time the concrete needs to be shielded from cold damage.
That definition updates an older one in the ACI 306.1-90 specification, which only applied after more than three days in a row averaging under 40°F. The old version could miss the nights that matter most: 50°F at noon, 27°F before sunrise.
So the real test is keeping the concrete warm enough for long enough. With planning, concrete has been placed safely in air well below -4°F, and ACI notes that makeshift protection is where cold pours go wrong.
What happens if fresh concrete freezes
Fresh concrete is a paste full of water. When that water freezes it expands and breaks the paste apart, and ACI says the damage to strength and to the air-void system can be permanent.
NRMCA’s CIP 27 on cold weather concreting puts numbers on it: fresh concrete freezes once its own temperature drops below about 25°F, and frozen concrete can lose more than 50 percent of its potential strength.
Two strength targets matter:
- 500 psi. Protect the concrete from even a single freeze until it reaches 500 psi. NRMCA says that’s about two days after placement.
- 3,500 psi. A new slab that’s saturated and will see repeated freezing and thawing should be protected from those cycles until it reaches at least 3,500 psi.
Cold also slows everything down. NRMCA’s rule of thumb is that a 20°F drop in concrete temperature roughly doubles the setting time, so finishing, jointing and cleanup all run later than they would in spring.
Reading the forecast before a cold pour
Watch the lows for the days after the pour, not just the day of. Here is how we read a forecast before a flatwork pour.
| Forecast for the days after the pour | What the pour needs |
|---|---|
| Lows stay above 40°F | Normal practice. Cure it and keep it from drying out. |
| Lows between 32 and 40°F | Cold weather by ACI’s definition. Order concrete at 55°F or warmer, have blankets on site, expect slow finishing. |
| Lows in the 20s | All of the above, plus an accelerator, extra cover on edges and corners, and blankets left on until the slab reaches 500 psi. |
| Highs below freezing, frozen or snowy ground | Thaw the ground, enclose and heat the work. For most home projects, wait. |
What counts is the concrete’s own temperature. NRMCA notes the ready-mix plant can control the load’s temperature to meet a target. Its recommended minimums as placed:
| Thinnest dimension of the pour | Minimum concrete temperature as placed |
|---|---|
| Less than 12 in. (slabs, walks, driveways, pads) | 55°F |
| 12 to 36 in. | 50°F |
| 36 to 72 in. | 45°F |
Our practice for a late-season flatwork pour is warm, air-entrained concrete, a covered base, and blankets on the slab before the crew leaves. When a run of below-freezing highs is coming, we move the date.
Never pour on frozen ground
ACI’s guidance is plain: do not place concrete on frozen ground, because the ground settles as it thaws and the slab cracks. Snow, ice and standing water come out of the forms and off the base, and every surface the concrete touches (base stone, forms, rebar or mesh) should be above freezing.
NRMCA adds that this may mean insulating or heating the subgrade before the pour. On a cold job we cover the compacted base with insulated blankets ahead of pour day so the stone isn’t frozen when the truck backs in. If the soil under it is frozen, we thaw it or move the date. Base depth still matters in winter; our guide to how thick a concrete driveway should be covers slab and base thickness.
Curing blankets, heat and cure time in the cold
Once the slab is finished, the job is to hold its own heat in. Cement gives off heat as it hydrates. ACI says that heat can protect the concrete by itself only if the concrete is placed above 50°F and the forms are insulated, with exposed surfaces, corners and edges doubly insulated. Otherwise you need outside heat.
- Curing blankets. Insulated blankets, tarps, or straw under plastic sheets. Corners and edges lose heat first, so double them up.
- Heaters. Fuel-burning heaters in an enclosure must be vented, for the crew’s safety and the slab’s. Unvented exhaust carbonates the fresh surface and leaves it soft and dusty.
- Moisture. Don’t water-cure when freezing is coming; use a curing compound or plastic sheeting. Heated enclosures also dry concrete fast, because winter air is so dry.
- Removal. Take blankets and heat away gradually. NRMCA says thermal cracking can start when the surface and the inside differ by more than about 35°F.
Cure time in cold weather runs longer at every step. Two days gets you to 500 psi with protection in place, and strength keeps building more slowly after that. Our article on how long before you can drive on new concrete covers the later milestones, which cold pushes back too.
See how much a cold week pushes back your dates with our free concrete cure time calculator: pour date and average temperatures in, walk, drive and seal dates out.
Builder’s note: Have the blankets on site and unrolled before the first truck arrives. A finished slab sitting bare for an hour on a 25°F evening while someone hunts for tarps is how cold-weather pours fail.
Accelerators, and why calcium chloride needs care
Accelerating admixtures speed up setting and early strength, which shortens the time the slab is at risk. NRMCA lists ASTM C494 Types C and E as the common choices.
Calcium chloride is the most common accelerator. ACI says to keep it at no more than 2 percent of the cementitious material by weight (preferably 1 percent), add it only in solution, and, to minimize corrosion, not to use it in any steel-reinforced concrete. For any slab with rebar or wire mesh, ask for a non-chloride accelerator.
An accelerator does not stop concrete from freezing, and NRMCA is clear that it doesn’t replace protection. More cement, Type III (high early strength) cement, or less fly ash or slag cement also speed things up. Adding water at the truck does the opposite and delays the set.
The first winter: air, strength and no salt
New outdoor concrete also has to survive its first winter of freeze-thaw. NRMCA’s CIP 2 on scaling says exterior concrete that will stay moist, freeze and see deicers should be air-entrained, with a recommended total air content of 6 percent for concrete made with 3/4 or 1 inch aggregate, and specified at 4,000 psi.
The same document says to avoid deicing salts like calcium or sodium chloride in the first winter after placing. Use clean sand for traction, hose off the salty slush cars carry onto a new driveway or garage floor, and never use fertilizer (ammonium sulfate or ammonium nitrate) or magnesium-based salts as a deicer. The same salt questions come up with pavers; see our guide to salt on pavers.
Already seeing flakes or pop-outs on older concrete? Our guide to concrete spalling covers what causes it and when it can be repaired.
If your project slides to spring, use the wait: our online patio design plans set the size, pitch and base so you can line up a contractor early.
DIY or hire a pro
A small bagged-mix pad is a fair DIY job when the lows stay above 40°F for several days. Hire a pro when:
- Lows are near or below freezing and you don’t own curing blankets
- The base or soil is frozen and has to be thawed first
- The slab has steel and needs a non-chloride accelerator
- The pour is big enough that finishing will run past dark
For contractors: a spec citing ACI 306.1-90 or ACI 301-20 carries the older cold-weather definition, and ACI suggests an RFI. NRMCA also notes acceptance cylinders must cure at 60 to 80°F for the first 24 to 48 hours under ASTM C31.
Related: All our concrete guides in one place
Frequently Asked Questions
Can you pour concrete in winter?
Yes, if the concrete is placed warm, the ground isn’t frozen, and the slab is kept from freezing until it reaches 500 psi, about two days.
What happens if fresh concrete freezes?
Ice breaks up the paste. NRMCA says frozen fresh concrete can lose more than half its potential strength and won’t be durable, so have an early-frozen slab evaluated before you accept it.
How long do you leave curing blankets on new concrete?
At least until it reaches 500 psi, about two days with protection, and longer if it will see freeze-thaw while still saturated. Remove them gradually.
What is the lowest temperature you can pour concrete in?
There’s no hard air minimum. Place slabs at a concrete temperature of 55°F or warmer and keep them from freezing, since fresh concrete freezes once its own temperature falls below about 25°F.
Can I put salt on new concrete?
Not in the first winter. Use clean sand for traction instead of calcium or sodium chloride.
Can you pour concrete if it freezes at night?
Yes, with protection. ACI treats air at or forecast below 40°F during the protection period as cold weather, so order slab concrete at 55°F or warmer, get insulated blankets on before dark, and keep the slab from freezing until it reaches 500 psi, about two days.
How long does concrete take to set in cold weather?
Longer than you’d expect. NRMCA’s rule of thumb is that a 20°F drop in concrete temperature roughly doubles the setting time, and cold concrete gains strength more slowly too, so finishing and form removal run later.
Does an accelerator keep concrete from freezing?
No. Accelerators speed up setting and early strength, but NRMCA says they do not prevent concrete from freezing, so you still need blankets or heat. For slabs with rebar or mesh, ask for a non-chloride accelerator; ACI says calcium chloride should not be used in steel-reinforced concrete, to minimize corrosion.
Should you cover new concrete with plastic in cold weather?
Yes. NRMCA recommends curing compound or plastic sheets for slabs, and says water curing isn’t recommended when freezing is coming. For frost protection, it lists insulated blankets and tarps, or straw covered with plastic sheets.
Sources: ACI, Mastering Cold Weather Concrete Placement, summary of ACI 306R-16; ACI, What is the effect of freezing on concrete?; ACI, What methods are used for protecting concrete against low temperatures?; ACI, Can the heat liberated by the hydrating cement be relied on in cold weather?; ACI, What is the most commonly used accelerating admixture?; NRMCA, CIP 27 Cold Weather Concreting; NRMCA, CIP 2 Scaling Concrete Surfaces
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Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.