Placing Concrete in hot or cold weather

posted by Bob

People who pour concrete for a living can work almost year round in most parts of the country. This is because either through trial and error or spending time reading a lot of technical journals they have figured out how to successfully place concrete even though it is sweltering hot or freezing cold. For almost everyone else I would recommend that you limit your concrete activities to more moderate weather. If it’s so hot out that all you can think about is going for a swim I would suggest you get a cold beverage, turn up the AC and forget the concrete. If it’s so cold that you need gloves, consider spending time in front of the fireplace with a good book.

If this doesn’t quite give you specific enough guidelines, maybe we should define what moderate temperatures are? This is open for debate and involves other factors but in general if the air temperature is between 50° F and 90° F you should be safe. You can safely place concrete outside of these limits but you need to do a few things to make sure your job won’t turn into a nightmare.

The air temperature by itself isn’t the determining factor on whether you should pour concrete. The temperature of the air, the humidity level, and the wind speed, the temperatures of the surface where you are placing the concrete, the water and the dry concrete in the bag all play a huge part and must be taken into consideration. The air, wind and humidity are pretty much beyond your control but some of the others you can influence. It is important to remember that the temperature of the mixed material is as important as the air temperature.

Cold Weather
If the air temperature is below 32° F I would really advise that you wait until warmer weather or call a pro. Unless you want to erect a tent with a space heater or steal your spouse’s electric blanket off the bed, this will only lead to trouble. If it is so cold out that the ground is frozen don’t pour concrete under any circumstances. The biggest issue when pouring concrete when the air temperature is just above freezing is the night time temperatures that will follow. Concrete sets much slower in cold weather. It is critical (I’m going to repeat that- critical) that the concrete sets before it is exposed to freezing temperatures. The problem is that when water freezes it takes up more space in the ice phase than it does in the liquid phase. When all of the water that you used for mixing freezes, it expands, causing your concrete to crack. The key is doing what you can to make sure the concrete sets fast enough to prevent this.

The first thing that pros do in the winter is use hot water. If you use hot water and keep the dry product in a heated area of your house or garage until you are ready to use it this will greatly accelerate the set of the concrete. You can buy products designed to set quick such as Sakrete Fast Setting Concrete. It will not set as fast as the literature says it will if the air temp is anywhere near freezing but it will set much faster than normal concrete. You can also buy additives to accelerate the set. The only concern here is the type of accelerator. If it contains calcium chloride and your concrete will contain rebar or metal wire mesh, the chlorides will attack it and produce rust. This will ultimately crack your concrete. When concrete sets it gives off heat. Not like frying an egg but there is a slightly exothermic reaction (a big word for a reaction that gives off heat-use it to impress your friends). You can use this to your advantage by covering the concrete (after it has stiffened) with a blanket. They sell blankets for this purpose so your kids won’t have to sleep in the cold. You could also put up a tent or lean to and put a space heater inside.

Hot Weather
If the air temperature is above 90° F you need to be careful. Of course what you are doing with the concrete also makes a difference. We will address that later. In addition if the wind is blowing hard and the humidity is low, even 90° might be a problem. The issue with hot weather isn’t really the heat. Neither the cement nor the aggregates have a problem with temperature. This isn’t like a chocolate bar on the front seat of the car in July. The issue is that the top layer of concrete will dry much faster than the bottom layer. As concrete dries it shrinks. This means that the top will be shrinking while the bottom is stationary. At this point you get your own north/south civil war breaking out inside the slab. There will be casualties.

To avoid this aggression you need to keep the top and the bottom curing at the same rate. There are several things you can do before and during mixing and several things you can do after placement. Before mixing store the material in a cool place or at least avoid laying it in the hot direct sun. Then use the coldest water that you find. Ready mixed concrete companies actual use ice to replace all or most of the water to slow down the set. After you have placed the concrete and it has taken a set, you need to keep the slab moist. This can be done in several ways. You can spray the slab periodically with a hose, turn on a sprinkler with a fine mist, cover the slab with wet burlap or with chemicals designed to keep water from evaporating so quickly. In very high temperatures, very low humidity or high winds you may to do this for several days. Almost everything in this discussion on hot weather is directed at someone pouring a slab. If you are mixing up concrete and placing it in a hole to support a fence post of deck, hot weather normally isn’t a problem. If the concrete is setting too fast to place in the hole, then using cold mixing water or ice will help.



Meg, for best results all materials should be stored between 40°F and 80°F 24 hours prior to installation. Air, mix and substrate temperatures should be between 40°F and 90°F with no rain in the forecast within 24 hours of application. Colder temperatures or higher humidity conditions will retard set times. Protect from freezing for 48 hours. These are the parameters for using our concrete materials.
- Lee-Technical Service
Friday, March 3, 2017 at 12:33 PM

We won a federal grant for concrete walkway construction throughout our community. It should have funded in July, but did not fund until November. I have put off the installation of the walkways until the weather reaches a consistently warmer temperature 45 degrees average for a period of 4 or more days. The government is saying that because we have had a milder winter that we should have already started work. I am concerned about durability and strength if we go too soon. Opinions, please? Thanks.
- Meg
Monday, February 27, 2017 at 4:44 PM

Sandra, sorry but I am not able to answer that question. You will need to have either the contractor or an inspector to evaluate the condition of the concrete.
- Lee-Technical Service
Monday, February 6, 2017 at 9:07 AM

people poured our driveway dec. 2016 when it was below freezing and snowing they covered it with blankets but it is already cracking and on top of concrete is white discolor on top do you think it will be ok
- sandra
Saturday, February 4, 2017 at 4:22 PM

R Taylor, if the material freezes then it can crack. The cold weather slows the set time and this will cause issues if your low temperatures are below 40 degrees. If it is too hot then the concrete can set too fast and cause shrinkage cracking. You want to install concrete between a high of 90 degrees and a low of 40 degrees.
- Lee-Technical Service
Thursday, January 12, 2017 at 4:17 PM

is cement more likely to crack if poured in cold weather or warm weather?
- R Taylor
Wednesday, January 11, 2017 at 5:26 PM

Maureen, I would not start it before the weather is within the optimal temperature range. Especially if you plan on closing it in. You want to make sure that it is done right the first time so that none of your finish materials end up getting damaged.
- Lee-Technical Service
Tuesday, November 29, 2016 at 1:21 PM

What about patching think cracks on a screened in back porch? The man who was supposed to do this, check moisture, etc. did not show up. This screened in porch will be turned into a 4 season room but will not get done until about a week before Christmas. Should I wait until warmer weather to do anything? I dread having all this done once enclosed but may have to. I wanted the repairs down early on so that the flooring could be installed before Christmas. Local installer said he would. Now? I am in Chesterton, IN zip 46304- cold and raining today. Guess I don't get to use this for the great grandkids to play in this Christmas. Thank you.
- Maureen
Monday, November 28, 2016 at 6:34 PM

Geno, it is not good to do concrete work of any kind if the weather is too cold. The ground temperature will play a big factor in this as well. If it is too cold the concrete will not set properly.
- Lee-Technical Service
Monday, November 28, 2016 at 4:52 PM

Nazneen, we would need to know what temperature and product that you are talking about and the application.
- Lee-Technical Service
Monday, November 28, 2016 at 4:40 PM



Enter Text From the Image Above: