How to Protect a Small Concrete Pour From a Freeze

Why this matters

A small pour, a few post footings for a deck, a pad for a condenser or a generator, a step patch, is the job where cold-weather practice gets skipped, because it is small. Nobody orders a ready-mix truck with an accelerator for four footings; somebody mixes bags in a wheelbarrow and goes home. Then the first night drops to 24 F, the top few inches of fresh concrete freeze before they have gained strength, and in spring the tops of the footings crumble under a post base or the pad scales and cracks.

What cold does to fresh concrete, the ACI 306 cold-weather trigger and the higher minimum concrete temperature it sets for thin sections, is covered in Concrete for the Small Pour in Cold and Hot Weather. The larger-pour standard is Cold Weather Pour Standard. This card is the procedure for a pad or a set of footings: what to do, in order, and what each step prevents.

Hazards in the work

Wet concrete is caustic. It causes alkaline burns that do not hurt until the damage is done, and kneeling in it or letting it fill a boot is how most of them happen. Wear waterproof gloves, boots with pant legs over them, and eye protection, and wash any concrete off skin promptly with clean water. Concrete in the eye gets flushed with clean water for at least 15 minutes and then a clinician.

Dry mix is a dust. Dumping and mixing bags releases cement and sand dust, and the sand is crystalline silica. Open bags slowly and low, stand upwind, add the water early in the mix, and do not dump bags in a closed space. Where dust cannot be kept down, a respirator is used under a written program per 29 CFR 1910.134, which 29 CFR 1926.103 applies to construction work, and a tech outside the program does not do the dusty part.

Bags and holes. An 80-pound bag is a two-person lift into a high mixer. Open footing holes are a leg-breaking trip hazard on a site where people walk at dusk; they stay covered whenever nobody is working them (which also matters in Step 2).

Step 1: Read the forecast for the protection period, not pour day

Before the pour is confirmed, look at the overnight lows for every night the concrete will need protection, which is several days for a small pour in the cold. The question is not whether today is warm enough to work. It is whether the next few nights can be covered.

Why: pour-day weather decides how comfortable the crew is. The nights decide whether the concrete holds. What breaks if you skip it: a pleasant 46 F pour followed by two nights in the low 20s with no blankets on the truck. Reading the hourly forecast for the work is owned by Reading a Forecast for the Work, Not the Weekend.

Step 2: Never place on frozen ground

Look at and probe the bottom of every hole or the full base of a pad. Frost, ice or snow in the hole, or soil that is hard and crunchy, is frozen. Either thaw it (an insulated blanket or ground-thaw blanket left over it overnight) or dig the frozen material out down to unfrozen soil. Frozen material dug out does not go back in the hole.

Why: concrete on frozen soil loses heat downward and freezes from the bottom, and when the soil thaws it settles out from under the footing. What breaks if you skip it: a footing that was right on the day and is out of level by spring. Holes dug the day before a cold night should be covered with plywood and a blanket overnight; that one move usually means nothing is frozen in the morning.

If digging out frost takes a footing deeper, that is fine: a deeper footing is still below the frost line. The depth the permit or the building department requires (deck footings fall under IRC R507 as adopted by your jurisdiction, with the local frost depth set by the jurisdiction) is a minimum.

Step 3: Use the supplier's cold-weather mix, never homemade antifreeze

Ask the supplier for a mix suited to the cold: for bags, a high-early-strength or cold-weather product; for ready-mix, warm water in the batch and an accelerator at the plant's dose. Get the supplier's cold-weather sheet for that mix, because it states how long the concrete needs protection.

Never add salt, automotive antifreeze, windshield fluid or a hand-measured accelerator. There is no antifreeze for concrete: an accelerator makes it gain strength faster so it is vulnerable for less time, it does not stop it freezing. Chloride accelerators in particular corrode embedded steel, which on a small pour means anchor bolts, post bases and rebar, so many suppliers offer non-chloride versions for exactly that case. What breaks if you skip it: a homemade additive changes the set unpredictably, weakens the concrete, and attacks the steel set in it.

Step 4: Keep the materials warm

Bags spend the night in the shop, not in the truck or on the ground. Mixing water is warm, within whatever limit the bag states. Tools and mixer start the day dry, because a frozen film in a wheelbarrow chills the first batch.

Why: a thin section, under 12 inches in its least dimension, which covers every footing tube and most pads, loses heat fast, and the concrete starts its protection period at whatever temperature you placed it. What breaks if you skip it: a placed temperature already below what the cold-weather guidance asks to be held.

Step 5: Place and finish early, so the warmest hours are the concrete's

Pour in late morning on a cold day, once the air has come up, and be finished and covered before mid-afternoon, when the temperature starts down. Do not pour at 3 p.m. because that is when the crew is free.

Step 6: Insulate before the temperature falls

As soon as the surface can take it without marking, cover it: plastic sheeting first to hold moisture, then insulated concrete blankets. Lap the blankets well past the edges of a pad, and for footings, past the tube onto the surrounding ground. Where there are not enough blankets, a thick layer of straw under poly works; the poly keeps the straw dry, and wet straw insulates poorly.

Why the lap: the edges and the top few inches lose heat first, and the ground around a footing top is frozen to some depth and pulls heat sideways. What breaks if you skip it: a pad that is sound in the middle with scaled edges, and footings whose tops are weak exactly where the post base anchors.

Step 7: Watch the nights, and hold protection for the time the supplier gives

Put a min/max thermometer with its probe against the concrete surface under the blanket on the most exposed piece, and read it every morning against the minimum concrete temperature the supplier's sheet or ACI 306 names for a thin section. If the minimum comes in below that, add a second blanket or an electric curing blanket that night rather than hoping the next one is milder. Keep the protection on for the full period the supplier's sheet gives for that mix at those temperatures, and longer if a night comes in colder than forecast. Do not take it off because the concrete feels hard; it gets hard well before it is safe to freeze.

Take blankets off on a mild morning, not into a hard freeze, so the surface does not go from warm to frozen in an hour. Do not load the concrete (set framing, a unit, a post) until the supplier's time for that load has passed.

Worked example: four deck footings in November

A deck crew is setting four footings for a ground-level deck. Holes are 10 inches in diameter in tube forms; the permit calls for 42 inches deep. The holes were dug and inspected yesterday afternoon. Each footing gets a galvanized post base set in the top.

The forecast (Step 1). Pour day high 44 F; then nightly lows of 24 F, 28 F and 35 F, with day highs in the low 40s. Three nights of protection, two of them below freezing. The crew loads six insulated blankets.

The holes (Step 2). Two holes were left open overnight and have a 2-inch crust of frozen soil at the bottom; the two that were covered are not frozen. The crew digs the frozen crust out of the two uncovered holes and probes again: soft soil. Those two are now 44 inches deep.

The volume. A 10-inch tube has a cross-section of 3.1416 times 5 inches squared, 78.5 square inches.

• Two holes at 42 inches: 78.5 times 42 is 3,297 cubic inches, divided by 1,728 is 1.91 cubic feet each.
• Two holes at 44 inches: 78.5 times 44 is 3,454 cubic inches, 2.00 cubic feet each.
• Total: 1.91 plus 1.91 plus 2.00 plus 2.00 is 7.82 cubic feet.

At about 0.60 cubic feet per 80-pound bag, 7.82 divided by 0.60 is 13.03 bags, so 14 to be sure the last tube fills. The crew brought 15 bags of a high-early mix from the supplier, one spare for spillage.

The mix (Step 3). The supplier's mix is non-chloride, which matters because galvanized post bases go in the tops. The supplier's cold-weather sheet for it, read at the counter, gave a protection period that, for these forecast lows, worked out to keeping it covered and above freezing for three days. (That figure is illustrative; read your own supplier's sheet.)

Placing (Steps 4 and 5). Bags came from the shop at about 55 F. The pour starts at 10:45 a.m. with the air at 39 F, warm water in the mixer. The last footing is struck off and its post base set by 12:30 p.m.

Covering (Step 6). By 1:15 p.m. each footing top has poly over it and a folded blanket lapping about 3 feet out onto the ground in every direction, weighted at the corners. A min/max thermometer probe goes against the concrete under the blanket on the northwest footing, the most exposed one. The target is the 55 F thin-section minimum, since a 10-inch footing is under 12 inches in its least dimension.

The nights (Step 7).

• Morning 1, after the 24 F night: concrete minimum 57 F, held by the blanket, the warm ground below and the mix's own heat. Covered.
• Morning 2, after the 28 F night: minimum 55 F, right at the target. The crew adds a second blanket on the two northwest footings for night 3.
• Morning 3, after the 35 F night: minimum 58 F. Three nights are done, the supplier's three days have passed, and this morning is 38 F and rising toward a 48 F afternoon.

The blankets come off at 11 a.m. on day 4, when the air is above 40 F. Framing on the post bases was scheduled for day 5 per the supplier's time before loading.

What would change the answer:

• A third night forecast at 20 F instead of 35 F. The protection stays on through it; the supplier's period is a minimum, and a hard freeze inside or just after it extends it.
• A ready-mix pour for a larger pad. The plant controls the mix temperature and accelerator, and the slab's least dimension may push it past the thin-section line. The procedure then follows the larger-pour standard named above.
• No blankets available. The pour moves. Pouring with a plan to find blankets by evening is not a plan.

How to know it held

In spring, look at the tops of the footings and the edges of any pad. A sound surface rings when tapped with a hammer; a frozen-early surface sounds dull and flakes. Scaling or crumbling at the top inch, or a post base that has loosened, means the top froze before it gained strength, and the ticket's thermometer readings tell you whether protection or placement was the cause.

References

• ACI 306, Guide to Cold Weather Concreting (edition your job or local practice follows)
• IRC R507, Exterior decks (footings), as adopted by your jurisdiction
• 29 CFR 1910.134, Respiratory protection
• 29 CFR 1926.103, Respiratory protection (applies 1910.134 to construction work)
• See related: Concrete for the Small Pour in Cold and Hot Weather
• See related: Cold Weather Pour Standard