How Long to Keep Blankets on New Concrete (ACI 306)

Short answer: Keep blankets on until the concrete has reached at least 500 psi and the protection period is over. ACI 306R-16 Table 7.2 recommends 2 days for unloaded concrete that won't be exposed to weather, 3 days if it will be exposed, and 6 days if it carries partial load. Elevated structural work goes by in-place strength, not days.

Those days assume the concrete is held at the minimum temperatures in ACI 306R-16 Table 5.1 the whole time. If the blankets can't hold that temperature, the clock doesn't run at full speed. On DOT and many commercial jobs, the project spec sets the rule, and it is often longer than ACI's minimum.

How many days does ACI 306 say to protect concrete?

ACI 306R-16 gives a minimum protection period by service condition and by whether the mix is normal-set or accelerated. The table below is ACI's Table 7.2. The examples column is our plain-English reading; your engineer of record decides which row your pour falls in.

A "day" in that table is a 24-hour period at the Table 5.1 minimum temperature. "Accelerated" means a mix designed to set and gain strength faster, typically with an accelerating admixture (ASTM C494 Type C or Type E).

For elevated slabs, beams and girders, ACI 306R-16 section 8.1 says the Table 7.2 minimums "do not allow adequate strength gain." Base removal of protection, forms and shores on in-place strength, measured by maturity or other in-place tests.

Why is 500 psi the number?

At about 500 psi, the concrete has used up enough of its mix water that a single freeze can no longer damage it. ACI 306R-16 section 3.1 explains that fresh concrete is saturated with water. As cement hydrates, that saturation drops below the critical level at about the time the concrete reaches 500 psi. At 50°F, most well-proportioned mixes get there within 48 hours.

Two limits apply:

• 500 psi covers one freeze, not a winter. ACI 306.1's checklist notes that 500 psi is "only adequate to protect the concrete against damage from one cycle of freezing and thawing." NRMCA's CIP 27 says concrete that will see repeated freeze-thaw cycles while saturated should reach about 3,500 psi first.
• Freezing early does lasting damage. NRMCA says the potential strength of concrete that freezes fresh "can be reduced by more than 50%" and it will not be durable.

The full rule and what a 28°F night does to a fresh pour are covered in will concrete freeze overnight at 28 degrees.

How warm does the concrete have to stay under the blankets?

The blankets have to keep the concrete at or above the Table 5.1 minimum for the section size. These are concrete temperatures, not air temperatures.

A 4-inch slab sits in the top row. It has little mass and a large exposed surface, so it loses heat fast. A 30-inch footing holds its own heat much better. NRMCA also notes that concrete placed well above these values is not better: it needs more water, loses slump faster and is more prone to cracking.

Do state DOT specs require longer than ACI?

Often, yes. DOT specs tie removal to opening strength, age or the forecast, not just to ACI's minimum days.

The Wisconsin requirements are summarised on our Wisconsin cold-weather concrete answer and the Iowa DOT cold-weather answer.

Which blankets actually work?

Use blankets that stay dry inside and sit tight against the concrete or the forms. ACI 306R-16 section 9.2 recommends blankets with a moisture-proof shell over closed-cell foam or air pockets. It does not recommend blankets filled with mineral wool, fiberglass, cellulose or open-cell foam, because they stop insulating when wet.

Some specs set a minimum R-value:

• MDOT 602.03.T.3 requires insulating blankets or polystyrene with a minimum R-value of 7.0 on pavement when the forecast is below 40°F for more than 8 consecutive hours. Plain plastic sheeting is allowed if the forecast stays above 32°F.
• IDOT 1020.13(d)(1) sets R-values by pour thickness and forecast. A 6-inch pour needs R=4 for a 44–35°F forecast, R=6 for 34–25°F and R=8 for 24–17°F. Below 17°F, it requires Protection Method II under 1020.13(d)(2) instead.
• Iowa DOT Table 2301.03-2 steps pavement protection from one layer of burlap (35–32°F forecast) up to four layers of burlap between plastic, or insulation blankets, below 25°F.

Sources disagree on straw. ACI 306R-16 says straw is "not recommended" because it is bulky, flammable, ineffective when wet and hard to keep in place. WisDOT 415.3.13.2 still requires 6 inches of loose, dry straw or hay between two layers of polyethylene on pavement below 17°F, or an approved equivalent. On a DOT job, follow the spec. Elsewhere, blankets are the safer choice.

How do you take blankets off without cracking the concrete?

Take them off gradually. A warm slab hit by cold air cracks at the surface. ACI 306R-16 Table 5.1 limits the drop in the first 24 hours after protection ends to 50°F for sections under 12 inches and 40°F for 12 to 36 inches. Specs set their own rates:

• WisDOT 502.3.9.5: no more than 5°F per hour or 40°F in 24 hours.
• MnDOT 2401.3G.5: no more than 20°F in any 12 hours.
• IDOT 1020.13(c): if the concrete is more than 35°F warmer than the air when protection ends, cool it gradually, for example by removing the plastic first and the insulation later.

NRMCA notes thermal cracking can start once the surface-to-core difference exceeds about 35°F.

Can you lift the blankets to saw joints?

Yes, briefly, where the spec allows it. WisDOT 415.3.13.2(4) lets you remove pavement covering for the minimum time needed to saw joints or do other required work, with the engineer's approval. Put it straight back. Corners and edges lose heat first, so cover them first.

Blanket checklist for a cold-weather pour

1. Have blankets, plastic and tie-downs on site before the pour. ACI 306R-16 section 4.1 and ACI 306.1 section 2.1 both say protection materials should be there before cold weather.
2. Check the forecast lows for the whole protection period, not just pour day.
3. Cover as soon as the surface won't mar. Lap the edges and weight or tie them down.
4. Put temperature sensors at corners and edges, the coldest spots. ACI 306R-16 section 4.4 says to monitor surface temperature there.
5. Log the concrete temperature at least twice every 24 hours. ACI 306.1's checklist tells specifiers not to require less often than that.
6. Hold the blankets until the protection period, strength or spec criterion is met, whichever comes last.
7. Remove gradually, within the temperature-drop limit above.

Asking on site

A foreman can ask Verika "slab poured Tuesday, lows of 25 all week, when can the blankets come off?" Verika answers from ACI 306R-16 and the state DOT spec loaded for that region. If your company has its own cold-weather procedure, it answers from that too and cites each source. If nothing on file covers the question, it says so. Minnesota's structure rules are summarised on our MnDOT cold-weather answer. Pavement curing is covered on the WisDOT pavement curing answer. For the full set of cold-weather rules, see our cold-weather concreting requirements guide. To try it on your own procedures, start a free trial.

Project specs and your local building official have the final say. ACI guides are recommendations unless your contract makes them mandatory.

Sources

1. ACI 306R-16, Guide to Cold Weather Concreting (excerpt: sections 3.1, 8.1, 8.8, 9.2; Table 7.2) — American Concrete Institute, 2016
2. ACI 306R-16 preview (sections 1.1, 2.2, 3.1–4.4) — American Concrete Institute, 2016
3. ACI 306.1-90 (Reapproved 2002), Standard Specification for Cold Weather Concreting, preview and checklists — American Concrete Institute
4. CIP 27, Cold Weather Concreting — NRMCA, 1998/2014
5. Cold Weather Concrete, reproducing ACI 306R-16 Tables 5.1 and 7.2 — Cemstone, 2024
6. WisDOT Standard Specifications 2025, Section 415 (415.3.13, 415.3.15) and Section 502 (502.3.9, 502.3.10) — Wisconsin DOT
7. Iowa DOT Standard Specifications, Series 2023 (2301.03, 2403.03 F) — Iowa DOT
8. IDOT Standard Specifications for Road and Bridge Construction, adopted January 1, 2022 (Art. 1020.13) — Illinois DOT
9. MDOT 2020 Standard Specifications for Construction (602.03.T, 706.03.J) — Michigan DOT
10. MnDOT Standard Specifications for Construction, 2025 Edition (2401.3G.5) — Minnesota DOT