Ideally, you want that concrete project of yours to be done in mild to hot weather–anything 50 degrees Fahrenheit or above. That allows the best conditions for the chemical reactions that occur when concrete sets.
But sometimes (heck, many times) you don’t have that luxury. And mother nature be damned, you’re getting that project done.
Pouring a concrete slab in cold temperatures isn’t necessary easy, but it can indeed be done–if you take the right precautions.
But first–define ‘cold weather’.
When it comes to concrete, ‘cold weather’ is actually a strictly defined term. The American Concrete Institute defines it as “when air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period” (ACI Committee 306). The protection timeframe typically can range from a 24-72 hour period.
Experts agree, however, that if the temperature sinks below 20-25°F, you’re better off not trying at all. Hydration can’t occur at that level of cold, and the concrete will most likely freeze.
But if the weather gods have smiled on you and the temperature isn’t too cold for concrete, here are some measures to take:
Before you pour:
- Remove any snow or ice from the designated area where you’ll be pouring the concrete.
- Make sure the ground is at least 32°F before you pour. You may need heaters or electric blankets in order to do so.
- This is important: you should never be pouring concrete on frozen ground. Bill Palmer from the Concrete Network points out that doing so runs a high risk of cracking the concrete as well as crusting–when the top part of the concrete sets but the bottom is still soft.
- Mix your concrete with hot water to keep it above 50°F longer.
- Adding calcium chloride to your concrete mix can also speed up the setting time. But be sure not to exceed a 2% ratio.
- Juan Rodriguez from The Spruce recommends using Portland cement Type III and avoiding fly ash or slag cement.
- Keeping your materials dry is also essential. Cascade Concrete operates in the rain-heavy pacific northwest and knows the risks associated with leaving materials on the jobsite outside in the elements.
When you pour cold weather concrete:
- Heated enclosures and/or insulated blankets may be necessary to keep the concrete temperature where it needs to be in order to set properly. Enclosures could be made of plastic, canvas or wood. Just remember: these materials themselves need to be 32°F or warmer before coming in contact with the concrete.
- Low temperatures aren’t your only risk factor–insulating your concrete from the wind is also important to consider. Even if the temperature is above 40°F, consider the windchill. Cold winds can have the same effect as freezing temperatures. High winds can cause an evaporation process that is too quick. So windbreaks can be an asset if you’re placing concrete during the winter months.
- Cold weather conditions can slow down the curing process and take more time for the concrete to set. So you may want to be aware of that when scheduling out your project.
- Make sure you wait for bleed water to evaporate–which takes longer in cold weather. Neglecting to do so runs the risk of weakening the surface of the concrete slab.
Remember: the temperature of the concrete needs to remain at or above 50°F for the duration of the concrete curing and setting process. If concrete freezes, its durability will weaken and its surface can crack.