Will Concrete Freeze at 28°F Overnight? The 500 psi Rule
Fresh concrete can freeze on a 28°F night if it's unprotected. ACI 306R-16 says protect it until 500 psi, about 48 hours at 50°F. Here's what to cover.
Short answer: Yes, it can. A 28°F night can freeze fresh concrete that isn't protected, especially thin slabs, edges and corners. ACI 306R-16 says protect concrete from freezing until it reaches about 500 psi, which most mixes reach within 48 hours at 50°F. Cover it before dark and keep it covered.
Air temperature alone doesn't decide it. Fresh concrete makes its own heat, and a thick footing holds that heat far better than a 4-inch slab. What matters is whether the concrete itself drops below freezing before it has gained enough strength.
What happens if fresh concrete freezes?
The water in the paste turns to ice and disrupts it. ACI 306R-16 defines freezing as "the development of solid water ice within the paste that disrupts the paste." NRMCA's CIP 27 says the potential strength of concrete that freezes fresh "can be reduced by more than 50%" and it will not be durable. The NIH Office of Research Facilities' cold-weather bulletin puts it at "up to 50%" of ultimate strength. Either way, you could lose half the strength you paid for.
At what temperature does concrete actually freeze?
NRMCA says fresh concrete freezes if its temperature falls below about 25°F. ACI 306R-16 section 8.10 uses a more conservative assumption, that freezing damage "may take place when the concrete temperature drops below 32°F." The safer reading is the ACI one: once the concrete surface is near 32°F, you are at risk.
On a 28°F night the air is already four degrees below freezing. A thick footing may stay above 32°F on its own heat for a while. A thin slab with an exposed top surface may not. Wind and a clear sky make it worse.
How long is new concrete at risk?
The danger window runs until the concrete reaches about 500 psi. After that, ACI 306R-16 section 3.2 says it "will not be damaged by exposure to a single freezing-and-thawing cycle."
| Concrete age and strength | Risk on a freezing night | What ACI 306R-16 and NRMCA say |
|---|---|---|
| First 24 hours | Highest. Still saturated, may not have set | ACI 306R-16 §4.2: protect for at least the first 24 hours even outside the cold-weather season |
| Up to about 500 psi (about 48 hours at 50°F for most mixes) | One freeze can cause permanent damage | ACI 306R-16 §3.1: degree of saturation falls below critical at about 500 psi |
| Above 500 psi, still saturated | Survives one freeze; repeated freeze-thaw cycles can still damage it | NRMCA CIP 27: protect from repeated cycles until about 3,500 psi |
| Concrete below 40°F | Strength gain largely stalls | ACI 306R-16 §8.10 assumes early-age hydration is negligible below 40°F |
Cold slows everything down. NRMCA's rule of thumb is that a 20°F drop in concrete temperature roughly doubles setting time. Concrete poured at 2 p.m. in late October may still be plastic when the temperature falls after dark.
What protection do specs require for a 28°F forecast?
State DOT specs give the clearest answer, because they tie the cover to the forecast. These rows are for pavement and flatwork. Structures have separate rules in each spec.
| Forecast low | WisDOT 2025 (§415.3.13.2) | Iowa DOT Series 2023 (Table 2301.03-2) | IDOT 2022 (§1020.13(c)) | MDOT 2020 (§602.03.T.3) |
|---|---|---|---|---|
| 32°F to 35°F | Thermal protection if freezing is forecast | One layer of burlap (35–32°F) | Protection required at 32°F or below | Plastic sheeting allowed only if forecast stays above 32°F; R-7 blankets if below 40°F for more than 8 hours |
| 28°F (the night in question) | Freezing forecast: minimum protection applies; table starts at 22 to below 28°F with one layer of polyethylene | Two layers of burlap, or one layer of plastic on burlap (31–25°F) | Two layers of polyethylene, one of polyethylene and one of burlap, or two layers of waterproof paper (32–25°F) | Insulating blankets or polystyrene, minimum R-7.0 (required when below 40°F is forecast for more than 8 hours) |
| 17°F to below 22°F | Two layers of polyethylene | Four layers of burlap between 4-mil plastic, or insulation blankets (below 25°F) | Insulation with minimum R-6, covered with polyethylene (24–17°F) | R-7.0 blankets |
| Below 17°F | 6 in of loose, dry straw or hay between two layers of polyethylene | As above | Cold weather protection plan required | R-7.0 blankets |
Where those columns differ, use the heavier cover. ACI 306R-16 section 9.2 does not recommend straw, because it is flammable and stops insulating when wet. Insulated blankets with a waterproof shell are the safer default.
For Michigan DOT pavement, the spec also says don't place concrete until the air is at least 25°F and rising, and protect it from freezing until it reaches 1,000 psi. Our Michigan DOT cold-weather answer quotes the section.
Should you pour today if it's 28°F tonight?
You can, if you plan for it. ACI 306R-16 treats any pour where the air is expected to fall below 40°F during the protection period as cold weather concreting. That means a plan, not a cancellation:
- Order warm concrete. ACI 306R-16 Table 5.1 recommends at least 55°F as placed for sections under 12 inches and 50°F for 12 to 36 inches. At 0 to 30°F air, it recommends 65°F and 60°F as mixed.
- Have covers on site before the truck arrives. ACI 306R-16 section 4.1 and ACI 306.1 section 2.1 both say protection materials should be there first.
- Don't pour on frozen ground. Clear frost, snow and ice from the subgrade and forms. See can footings be poured on frozen ground.
- Pour early. Give the concrete the warmest hours of the day to set.
- Cover as soon as the surface won't mar. Lap and weight the edges. Corners and edges lose heat first.
- Put a thermometer under the cover at an edge. ACI 306R-16 section 4.4 says the surface temperature, not the air, shows whether protection is working.
- Leave the cover on through the protection period. See how long to keep blankets on new concrete for the day counts.
Accelerators help but don't replace covers. NRMCA says accelerating admixtures "do not prevent concrete from freezing." WisDOT 502.3.9.2 forbids adding salt or other chemicals to prevent freezing.
What if the concrete already froze overnight?
Don't assume it's fine because it looks fine. Call the engineer of record. ACI 306R-16 section 8.3 lists in-place methods for estimating strength, including pullout testing (ASTM C900), penetration resistance (ASTM C803) and pulse velocity (ASTM C597), when correlated to the mix. On DOT work the rule is blunt. WisDOT 502.3.9.1 says "remove and replace frozen or frost damaged concrete," and MDOT 602.03.T.2 says the same for pavement slabs damaged by cold weather.
Asking on site
A finisher can ask Verika "we're pouring the garage slab at noon and it's 28 tonight, what do we need?" It answers from ACI 306R-16, the DOT spec loaded for that state and your company's own cold-weather procedure, and cites each one. The Wisconsin rules are on our Wisconsin cold-weather answer, and Iowa's are on the Iowa DOT cold-weather answer. Your project spec and local building official have the final say on private work.
Sources
- ACI 306R-16, Guide to Cold Weather Concreting (preview: sections 1.1, 2.2, 3.1–4.4) — American Concrete Institute, 2016
- ACI 306R-16 excerpt (sections 8.3, 8.10, 9.2; Table 7.2) — American Concrete Institute, 2016
- ACI 306.1-90 (Reapproved 2002), Standard Specification for Cold Weather Concreting, preview — American Concrete Institute
- Cold Weather Concrete, reproducing ACI 306R-16 Table 5.1 — Cemstone, 2024
- CIP 27, Cold Weather Concreting — NRMCA
- Technical Bulletin: Cold Weather Concreting, April 2021 — NIH Office of Research Facilities
- WisDOT Standard Specifications 2025, Section 415 (415.3.13) and Section 502 (502.3.9) — Wisconsin DOT
- Iowa DOT Standard Specifications, Series 2023 (Table 2301.03-2) — Iowa DOT
- IDOT Standard Specifications for Road and Bridge Construction, adopted January 1, 2022 (Art. 1020.13) — Illinois DOT
- MDOT 2020 Standard Specifications for Construction (602.03.T) — Michigan DOT
Frequently asked questions
›At what temperature does fresh concrete freeze?
NRMCA's CIP 27 says fresh concrete freezes if its own temperature falls below about 25°F. ACI 306R-16 uses 32°F as a conservative modeling assumption for when freezing damage may start. The concrete's temperature matters, not just the air's.
›Is concrete safe from freezing after 24 hours?
Not necessarily. ACI 306R-16 says concrete needs about 500 psi to survive a single freeze, and most mixes reach that within 48 hours at 50°F. Colder concrete takes longer. Twenty-four hours is ACI's minimum protection for unexpected frost, not a guarantee.
›Will plastic sheeting stop concrete freezing at 28°F?
Some DOT specs accept plastic at that forecast, but only as part of a rule set. WisDOT 415.3.13.2 calls for one layer of polyethylene at 22 to below 28°F on pavement; MDOT 602.03.T only allows plain plastic when the forecast stays above 32°F.
›Does calcium chloride stop concrete freezing?
No. NRMCA says accelerating admixtures, including calcium chloride, speed up setting but do not prevent freezing, and do not replace curing and protection. NRMCA limits calcium chloride to 2% by weight of cement.