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Placing Concrete in hot or cold weather

posted by Bob Wednesday, September 29, 2010 at 9:31 AM

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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81 USER COMMENTS

Dori, with snow on the ground and I’m sure a very high frost line we would not recommend setting your posts until temperatures are higher and snow is not present. As stated in our Technical Datasheet “Air, mix and substrate temperatures should be between 40°F (4°C) and 90°F (32°C) with no rain in the forecast within 24 hours of application.” Protect from freezing for 48 hours. I advise against this application until temperatures are significantly higher.
- Chris Technical Services
Tuesday, November 18, 2014 at 9:40 AM

Installing a 6' iron fence in central Colorado (7000' altitude). Using the fast setting version of Sacrete for posts. There's an inch of snow on the ground starting to melt. All 2' holes have been dug. Today's high predicted to be 45 degrees. Tomorrow's high predicted to be 38 and sunny. Wind 17mph. Sacrete is stored in my garage, which is relatively warm. Crew uses warm H2O to mix. Would it be safe to set posts in concrete tomorrow?
- Dori
Friday, November 14, 2014 at 1:14 PM

Madge, If your temperatures were above freezing for the first 24 hrs then you should be fine.
- RLC - Tech Service
Tuesday, March 4, 2014 at 4:22 PM

Claudia, If this was poured by a ready-mix company you should give then a call with your concerns. Our Sakrete Concrete mix should be used between 40-90 degrees.
- RLC - Tech Service
Tuesday, March 4, 2014 at 4:16 PM

We are in arlington tx. Sat it was 80 outside had been 50s for several days i think before, so we set 8 10' fence poles in 30 deep. We finished just about 8pm then Sunday the temp dropped to 30's and then 20's every since. Did the concrete have time to set up ????
- madge
Tuesday, March 4, 2014 at 11:10 AM

We are building a new home in Coppell TX. The temp yesterday was around 78* so the foundation was poured. Today the temp is 25* with some sleet rain. Will this affect the the foundation that was poured yesterday????
- Claudia
Sunday, March 2, 2014 at 10:49 AM

Brian, I do not feel that your concrete was too dry. I think it was just the order in which you placed your materials. You want to set and level the post first then pour the mixed concete around it. It would be extremely hard to shove a post down into properly mixed concrete.
- RLC - Tech Service Team
Tuesday, February 25, 2014 at 11:04 AM

Doing fence post holes 5" round 36" deep. 80° F some light wind. Mixed 80lb bag in harborfreight mixer with 3.5gal of water like the bag says. Filled bottom half of hole thinking I could shove post into half full holes. I could not. So I used post hole diggers to get the concrete out of the hole and then put the post in and then fill with concrete. Took about 30 min to get concrete out of hole and level and brace post. Question is : Is it possible my concrete got too dry? What happens when concrete is too dry? Should I have added water after digging concrete out of hole and mixed some more? I'm a newb to this. I always just pure bag into hole and add water with hose. But these big heavy post. I want to be done right.
- Brian
Tuesday, February 25, 2014 at 9:19 AM

Travis, if you could give us a call at 866-SAKRETE that way we can discuss further and see how we can assist you.
Dean - Tech Service Team
Tuesday, January 7, 2014 at 8:49 AM

Dean I used the basic 80 pound bag with between 3/4 and 1 gallon of hot water mixed very well by hand with a hand trowel to make sure it was mixed well. It was not soupy at all and it set fairly quick but has not yet looked cured. the morning after I poured it there was dew and has been almost every day since. ground around the hole was slightly moist but not wet. was 14 here this morning and have had it covered with a heavy towel with plastic drop cloth on top of that for three days until it got to about 50 each day.
- Travis
Monday, January 6, 2014 at 10:28 PM

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