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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107 USER COMMENTS

Hi there. We are casting T-beams and using covers to accelerate curing. We have found that if we cover the bed immediately after casting, 4 hours later the concrete it still wet, whereas if we for example cover 2 hours after casting, after the surface water has dried out, the beams cure much faster - is there science behind this or do we really need to be covering immediately and keeping water in?
- Matt M
Monday, May 23, 2016 at 10:07 AM

By waiting to cover you are loosing a lot of the water to evaporation resulting in lower strengths. By covering immediately you are allowing the material to hang on to the moisture longer allowing the Portland to use the water needed to gain strength. The reason for the appearance of quicker curing again is due to loss of water too quickly.
- Lee-Technical Service
Friday, May 20, 2016 at 4:53 PM

Hi there. We are casting T-beams and using covers to accelerate curing. We have found that if we cover the bed immediately after casting, 4 hours later the concrete it still wet, whereas if we for example cover 2 hours after casting, after the surface water has dried out, the beams cure much faster - is there science behind this or do we really need to be covering immediately and keeping water in?
- Matt M
Friday, May 20, 2016 at 12:21 PM

Shawn, You should always use clean potable water. If the water has a high iron content and you use it in mortar it can change the color of the mortar later. The iron will continue to rust and leech through the mortar staining it. If the areas are isolated to a certain spot you can try routing out the bad areas and repointing with new mortar. But, you need to use clean potable water.
- Lee-Technical Service
Thursday, April 21, 2016 at 10:18 AM

If you are using water that has a high rust content 9.5 ppm, does that affect the color of the mortar as years to by, in other words, does that rust show up in the mortar later and turn it to a brown or orange color?
- shawn
Thursday, April 21, 2016 at 9:41 AM

Shanna, the air, mix, and substrate temperatures should be a minimum of 40°F with no rain in the forecast for 24 hrs. Protect from freezing for 48 hours. You may want to use warm water when mixing and defiantly cover with plastic and heating blankets to help curing. If you have any hesitation, wait until spring.
- Chris Technical Services
Thursday, January 7, 2016 at 10:07 AM

We are building a 8X10 deck. I wanted to get it done before any snow. The high today is 46 and the low is 25. I wanted to dig the holes today and keep a heating blanket on them over night then tomorrow when the high is 46 and low 37. I was going to set the posts. Would this cause problems if I kept a heating blanket and tarp on it while it set?
- Shanna
Thursday, January 7, 2016 at 9:22 AM

Judy, I need a little more information to accurately answer your question. If however you are referring to the expansion joint that runs where the slab and wall meet then we suggest using Sakrete Backer Rod as a filler, and Sakrete Concrete Repair Tube to seal the crack. This will provide a flexible sealant but still allow for movement that occurs between 2 dissimilar substrates.
- Chris Technical Services
Monday, November 30, 2015 at 4:48 PM

Bob, we suggest using Sakrete RipRap for your project. Sakrete RipRap is specially formulated to harden in the bag and used for erosion control.
- Chris Technical Services
Monday, November 30, 2015 at 4:44 PM

I want to patch inside my garage along the wall.. my garage is heated..can I patch it
- judy
Monday, November 23, 2015 at 6:47 PM

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