Ask five contractors when concrete season ends in Ontario and you’ll get five answers, most of them some version of “October-ish.” The actual requirements are written down, they’re specific, and knowing them tells you whether a late pour is a bad idea or just a more expensive one.
What the standard says
Cold-weather concreting in Canada is governed by CSA A23.1. The Ready Mixed Concrete Association of Ontario publishes a technical specification summarising it, and the numbers that matter are these:
Protection kicks in at 5 °C. Cold-weather measures are required when there’s a probability of air temperature falling below 5 °C within 24 hours of placing. Not when it’s freezing — when it might drop below five.
Minimum placing temperature is 10 °C for sections under 1 m thick, which covers every driveway, patio, walkway and set of steps.
That 10 °C has to be maintained for the whole curing period, not just at the truck.
Basic curing is three days at 10 °C or more, or until the concrete reaches 40% of its specified strength. Where additional curing is required it’s seven days at 10 °C and 70% strength.
Why freezing is permanent
Concrete gains strength through hydration, a chemical reaction that slows as it cools and effectively stops near freezing. That part is recoverable — warm it back up and it carries on.
What isn’t recoverable is fresh concrete actually freezing. The water in it expands, disrupts the structure before it has strength to resist, and permanently costs the slab a large share of its final strength. You can’t see it on the surface and you can’t fix it later.
Season by season
April to early June. The best stretch and the busiest. Ground has thawed, temperatures are moderate. Book it in late winter or you won’t get it. Early spring subgrade is saturated, so base prep often needs more excavation than the same job in August.
Late June to August. Hot-weather measures instead of cold. Above roughly 28 °C concrete sets fast, which cuts finishing time and can trap a weak surface layer. Crews pour early, use retarders, and get curing protection on quickly.
September to late October. Our favourite window. Cool, steady, less rush than spring, ground still workable, and curing is slow and even — which makes a good surface. Watch the first frost.
November to March. Possible, not free. Heated mixes, accelerators, insulated blankets, sometimes hoarding and heat. Done routinely on commercial work where a schedule demands it.
So should you pour in November?
If it’s a residential driveway and nothing forces the date: wait for spring. The cold-weather measures cost real money, the window for things to go wrong is wider, and you’ll get a better job in April for less.
If the date is forced — a closing, a commercial schedule, an accessibility deadline, a foundation that has to be backfilled before winter — it can be done properly. It needs to be priced properly: heated mix, blankets, monitoring, and a protection period measured in days rather than hours. A contractor who quotes a November pour at the same price as a June one has not planned to protect it.
Curing times, adjusted for the season
Foot traffic after 24–48 hours. Light vehicles after about a week. Full specified strength at 28 days. In cold weather all of those stretch, because the clock effectively only runs while the concrete is warm enough to cure.
And for any slab poured within a few months of winter: no rock salt the first season. Sand for traction. First-winter concrete is at its most vulnerable and de-icer is the fastest way to ruin a new surface — which is the subject of the next article.