What the standard actually requires
CSA A23.1 is the Canadian standard for concrete materials and construction, and it is specific about cold weather. When the air temperature is likely to fall below 5°C within twenty four hours of placing, every material and piece of equipment needed to protect and cure the concrete has to be on hand and ready before the pour starts.
That wording matters. Not ordered, not on the way. On site, before the truck backs in. A crew that shows up to a December pour and then goes looking for blankets has already failed the standard.
The concrete then has to be kept warm, generally a minimum of 10°C, through the curing period, which is typically around three days for this purpose.
Why cold is dangerous rather than just slow
Concrete gains strength through hydration, a chemical reaction that slows down as it gets colder and effectively stops near freezing. Cold concrete is not just curing slowly, it may not be curing at all.
The real danger is freezing early. If concrete freezes before it has developed enough strength, the water in it expands and disrupts the structure permanently. The damage is internal, it does not undo when it thaws, and the slab never reaches the strength it was specified for. The commonly used threshold is that concrete has enough strength to resist early frost damage once it reaches about 7 MPa.
This is why a cheap winter pour is the worst thing you can buy. The failure is invisible on day one and permanent.
What protection looks like on a real job
Insulating blankets over flatwork. Hoarding and heat around foundation walls. Heated enclosures for anything that has to hold temperature for days. Warm mix from the plant, and sometimes accelerators, though those are a tool and not a substitute for protection.
Ground that is frozen has to be thawed before you place on it, because concrete placed on frozen ground will settle when the ground thaws underneath it.
All of that costs money and days. On our quotes winter protection is a stated line rather than a surprise, because the alternative is a contractor discovering winter in February and handing you a change order.
Frozen ground has to be dealt with first
You cannot place concrete on frozen ground. The ground under it thaws eventually, and when it does it settles, and the slab settles with it. Everything you paid for in the base is undone.
So a winter pour starts by thawing what you are placing on, usually with ground heaters and blankets, and that happens before the truck arrives. It is a real line item and a real day or two on the schedule, and it is the first thing a cheap winter quote leaves out.
What winter costs you in schedule, not just money
Concrete gains strength more slowly in the cold even when it is protected, so everything downstream moves with it. Stripping forms takes longer. Loading the slab takes longer. If somebody needs to frame on it, that date moves too.
Finishing windows also get strange. In summer the concrete sets predictably and the finishers work to a rhythm. In the cold the set can crawl for hours and then come on quickly once the heat of hydration builds up under a blanket. That is why decorative work is harder in winter: the stamping window is narrower and less predictable, and a crew that misjudges it cannot go back and do it again.
What to ask before you sign a winter pour
Is winter protection in the price, and what specifically does it include. Blankets, hoarding, heat, ground thawing: which of those are carried and which are extra.
What happens if the forecast turns. A pour scheduled for a Tuesday that drops to minus fifteen should move, and the contract should say who makes that call and who wears the cost.
How long will the protection stay on. Pulling blankets on day two to get them onto the next job is a real thing that happens, and it is exactly the failure that does not show up for years.
Is the mix specified for the conditions, and does the plant know it is going into a cold weather pour.
When we would tell you to wait
If it is a back yard patio and there is no reason it cannot happen in April, wait. You will pay less, the finishing window is more forgiving, and decorative work in particular is better done in reasonable weather because the stamping window shortens in the cold.
If it is a foundation holding up a build schedule, an interior slab, a garage floor, or anything where waiting for spring costs you more than protection does, pour it. That is exactly the work winter protection exists for, and it is a large part of what we do while most residential crews are parked.
Written by the crew at Ironground Construction Ltd., a concrete contractor in Kitchener. We self-perform our own forming, reinforcing steel, placing and finishing. If you want a second opinion on a job, the site visit is free and we will tell you straight if you do not need the work.