My name’s Doug Fraser. I’ve spent the better part of eighteen years pouring and finishing concrete around Dunedin and out into the hill suburbs, and my own brother is one of the people who rings me first when a job goes sideways. He lives up in Mornington, and last winter he had a concrete pad poured for a new garden shed. The crew turned up on a still, clear Otago morning, laid it beautifully, and by the following week the surface was crazed and dusty and flaking under a broom. He rang me assuming the concrete itself was dodgy. It wasn’t — it was the cold. Fresh concrete and an Otago frost are a bad combination, and once you understand why, you can time and protect a pour so it actually lasts. I’ve kept notes on this since, because it’s the sort of thing that gets glossed over until it goes wrong.
Why cold ruins a fresh pour
Concrete doesn’t just dry — it cures, through hydration, a chemical reaction between cement and water that keeps working for weeks but does most of its early strength gain in the first few days. That reaction slows right down as the temperature drops and pretty much stalls near freezing. Pour a slab right before a cold snap and it simply won’t reach the strength it should on schedule.
It gets worse if the water in the fresh mix actually freezes. Water expands as it turns to ice, and if that happens before the concrete has set, it tears the structure apart from the inside. That’s exactly what my brother ended up with: a weak, dusty, scaling surface that never hardened properly. Up on the hill in Mornington, where the frost settles and sits, that risk runs from roughly late autumn through to spring.
The numbers I work to when it’s cold
| What | Rule of thumb |
|---|---|
| Minimum curing temperature | Keep concrete above about 5°C while it cures |
| Freeze protection window | First 24–48 hours minimum, longer if you can manage it |
| Strength loss in the cold | Roughly halves for every 10°C drop in temperature |
| Ground condition | Never pour onto frozen ground — the base has to be thawed and firm first |
The daytime figure isn’t the one that catches people out — it’s the overnight low, which in an Otago winter can sit several degrees below whatever’s forecast for the flat. A cold week can double how long you need to wait before loading a slab. None of this is guesswork on my part either; the proper technical detail sits in the concrete standards, and products and methods should meet the relevant Standards New Zealand requirements rather than being left to whatever the crew feels like doing that day.
How a good crew handles a winter pour
None of this means you can’t pour in winter — plenty of Dunedin driveways go down in the cold, and concrete contractors around Terrace End handle winter work as a matter of course. It just means the crew has to plan for it rather than treat a winter pour like a summer one. The tools I rely on:
- Timing: pour early enough in the day that the concrete gets hours of above-5°C curing before the night frost sets in.
- Warm-mix measures: warm water or accelerating admixtures at the plant to speed early strength gain, plus a mix design adjusted for cold conditions.
- Insulation: curing blankets, or straw under polythene, trapping the heat the reaction itself gives off and keeping the surface above freezing overnight.
- Patience: formwork stays on longer, and traffic and loads stay off until the slab has genuinely cured, not just until it looks set.
My brother’s pad had none of that — it went down late in the day and sat bare through a hard frost overnight. A single layer of curing blankets would have saved the whole job.
Blanket now, or breakout and repour later
Cold-weather protection is cheap insurance next to the alternative:
- Curing blankets and the extra labour to place them: roughly $200 to $600 including GST on a typical domestic pour.
- Redoing a failed slab: breaking out and carting away the old concrete, then pouring again — which can easily double what the job cost the first time round.
For a driveway running into the thousands, that’s money you didn’t need to spend. If you’re weighing up a winter driveway pour in the south, it’s worth a read of will your Dunedin driveway survive the frost before you lock in a date.
Questions I get asked about winter pours
Can you actually pour concrete in winter in Dunedin or Otago? Yes — with cold-weather precautions. Keep the concrete above about 5°C while it cures, protect it from freezing for at least the first day or two, and use warm mix, insulating blankets, and early-enough timing so it cures before the overnight frost hits.
What happens if fresh concrete freezes — can it be repaired, or does it need replacing? If the water in fresh concrete freezes before it sets, the expanding ice disrupts the structure from within and the slab never reaches full strength. What you’re left with is a weak, dusty, flaking surface, and in most cases that means breaking it out and re-pouring rather than patching it.
How long before a cold-weather slab is strong enough to use? Strength develops much more slowly in the cold — roughly halving for every 10°C drop — so you might be waiting a week or more before light foot traffic and considerably longer before you drive on a driveway. Get specific guidance from your contractor for the mix and conditions on your job.