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.

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96 USER COMMENTS

Sam, we suggest using ice cold water to help slow the setting time due to the extreme temperatures. I also suggest renting a concrete mixer and to also keep the material in a cool dry place 24 hours prior to mixing.
- Chris Technical Services
Tuesday, July 28, 2015 at 8:43 AM

I'm going to be pouring a 7'x7' slab myself, but I live in Las Vegas and the temps are maxing out at 105, with lows around 80. Will I be ok if I pour early in the morning and then keep the slab covered in plastic sheeting and watering a couple times a day? Are there any admixtures I should consider? Thanks.
- Sam
Monday, July 27, 2015 at 2:05 PM

Russ/RJ, yes adding calcium will accelerate the set time and if not kept wet/damp the concrete will flash dry and shrinkage cracking will result.
- Chris Technical Services
Thursday, July 16, 2015 at 4:10 PM

does adding calcium to concrete when its upper 70`s and sun shining make it more susceptible to cracking that night or early morning?
- rj
Thursday, July 16, 2015 at 8:27 AM

my concrete cracked four or five different places which had to be that night or early morning.it was upper 70`s when poured and sunny. they added calcium in cement when they did this,did it set to fast causing it to crack?
- russ
Thursday, July 16, 2015 at 7:58 AM

Janet, while there are many speculations that can be formed as to why it is cracking the most fit explanation is due to the cold temperatures. If the concrete freezes the water that is wicking away during the cure period will expand thus causing the area to crack.
- Chris Technical Services
Monday, April 20, 2015 at 10:54 AM

I had a garage built the end of Oct.-Nov.2014. They poured the concrete after it was up. The temp. got to 32 at night,but we put 2 heaters on inside. also put straw and a tarp on the new sidewalk they poured leading to garage.Daytime temp. 37-39. Now it is beginning of April. Concrete is cracking and pieces are breaking off, even on sidewalk. Do you think the concrete was mixed wrong or just to cold for it. We had a professional pour it.
- Janet
Friday, April 10, 2015 at 12:19 AM

Kathy, pouring hot water on frozen concrete will not cause it to crack. The issue is more likely that during the winter when water freezes and turns to ice the volume changes, thus causing expansion. This winter expansion is the reason why expansion joints are necessary in concrete work and also why water limits are listed for mixing the material.
- Chris Technical Services
Tuesday, March 31, 2015 at 3:53 PM

It was still pretty cold out, at the end of this winter season and I decided to clean some remaining snow and dirt off my concrete driveway with the hose. I used hot water. Could that cause the driveway to crack? It was installed in August/September, 2014. I don't know if the crack was already there because it was covered with snow and dirt. I just wondered if I could have caused it. Thank you
- Kathy
Tuesday, March 31, 2015 at 1:44 PM

Keith, I would have to agree with you. Typically when concrete freezes it stops the cure and hydration process and will not reach the strength that is desired. Ice occupies roughly 9% more volume than water so it is difficult to say how far the cracks penetrate. The integrity of the steps is unfavorable and may cause problems down the line.
- Chris Technical Services
Thursday, March 12, 2015 at 9:58 AM

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