Got to pour outdoor concrete next week, but forecast is cold. What are my options?

Question

I have to pour outdoor concrete next week. The pour is in two main parts - a 100 mm cover / 2nd pour over a 6 x 3 m area of existing concrete from a 1st rough pour, and the bottom stub part of a retaining concrete wall profile about 500 mm wide x 750 mm high x 4 m long.

Unfortunately the weather outlook is quite cold, getting up to 6-8°C (43-46°F) late next week daytime, but generally otherwise around 2-4°C (36-41°F) in daytime, which is below the 5°C minimum for concrete. The concrete can be poured in the morning so it'll have about 6 hours of daylight to start setting, but nights are forecast to be cold - close to zero (0 - 1°C / 32-34°F). Both are exposed outdoor pours, there isn't an issue with wind or rain, just temperature.

I can't really afford to put off the pour (previous pour date didn't happen for unrelated reasons) especially with no guarantee that the week after (or the one after that) will be any better,but I do have several days to prepare, if preparations are needed to ensure success.

If nothing else comes up, I might have the capability, as a last resort, of "tenting" the entire poured areas under a lightweight wood frame overlaid with heavy polythene (DPM) and running some air /space heaters under it for a couple of days before or after, if there isn't a better option. But that's only a layperson's guess.

What are my options for ensuring the concrete sets properly despite the forecast low temperatures?

Answer 1

The absolutely critical factor is that the concrete does not cool to 0C before it has developed enough strength to resist the expansion if any free water in the mix freezes. If it does freeze before setting, most likely you will have to remove all of it and start again.

At 5C, concrete should develop enough strength to resist freezing after about 48 hours, and faster at higher temperatures.

The temperature of the concrete itself when it is poured is an important factor in this. Your supplier may be able to deliver heated concrete - typically at 10C minimum when it is poured. If your supplier can't guarantee a pour temperature of at least 5C, then whatever you do it going to be risky. If you are mixing the concrete on site, consider using pre-heated water. Water taken straight from the supply pipes will be at a higher temperature than water from an uninsulated tank.

The chemical reaction while the concrete is setting is exothermic, so good insulation against wind chill and radiation cooling at night is probably enough, so long as the ambient temperature stays above freezing.

Note that if you do supply artificial heat, there is a risk of cracking the concrete when the heat source is removed, if the ambient temperature is low.

But the good news is that the final strength of the concrete will be higher if it sets at slowly at low temperature.

For small scale work (e.g. setting fence posts in concrete), you can get fast-setting concrete which will set within an hour or so of pouring, but that isn't a sensible option for your 18 square meter slab - unless you really need to build it "right now" even though the climate is permanently below freezing!

Answer 2

There are complete specifications that you could find for your area and read. Department of Transportation specifications are usually available for free. All of these specifications will have some sort of the following (which is what you can reasonably follow yourself):

1. Make sure that there is no snow or frozen material where the concrete will touch, e.g. reinforcing steel or soil.
2. Make sure that concrete temperature stays above freezing. (This means concrete blankets and or plastic to keep wind away and heat in.)
3. Sometimes you will see requirements for the maximum temperature difference between the outside of the concrete and the inside. All this mean is that you don't want to uncover the concrete before it reaches air temperature on its own (a couple of days usually).

Placing concrete is completely doable even when it is really cold, you just need to make sure that the air around the concrete stays above freezing.