Construction is still playing catch-up from almost 18 months ago. Because of this, a little cold weather isn’t going to slow any construction company down. To keep up with demand, crews will be working long into the cold winter months when they may have usually taken a brief hiatus.
Projects big and small, both indoor and outside are being completed when the weather dips into the 40s and 30s. Using concrete during this time may not seem like an option. It can be if you have the right information and the right procedures in place to ensure the concrete sets properly.
When is it considered cold?
Just like your friend who wears shorts when it is 10 degrees F outside, while your sister wears a jacket when it’s 75 degrees, concrete has its own comfortable temperature. That means concrete can be too cold OR too warm.
The chemicals in concrete want just the right temperature to set and you want the concrete to set properly to ensure the strongest hold. Cold weather for concrete is when temperatures are below 40 degrees F for more than three days in a row and the air temperature is not higher than 50 degrees for no more than 12 hours during those three days. (Concrete can be very picky.) It is also imperative that concrete never be poured on frozen ground or if freezing temperatures will occur in the next 48 hours. Concrete poured when the ground is hard or covered with snow or ice will shift when it thaws causing cracks and uneven surfaces. Use a heater if necessary to thaw the ground and make sure the ground is well compacted before pouring.
Here is why you need just the right temperature for concrete to set. When water and redi mix concrete are mixed there is a chemical reaction which releases heat. This is called the heat of hydration. (Contractors are a little bit of everything, including chemists.) The heat of hydration puts the setting of the concrete in motion. This reaction also develops the concrete’s strength, also called maturity.
If concrete sets a lower temperature it slows down the hydration process which will result in delayed set times and will also reduce the compressive strength of the concrete at the earliest stages. This can lead to weaker concrete that will break down more easily or quickly. When the temperature drops below 40 degrees, the set time more than doubles from when the concrete is first poured until its final set.
So it’s important to have cement poured at its optimal temperature, which is 70 degrees F. This allows for the correct amount of set time and the proper maturity resulting in a solid clean finish.
How to keep concrete at the right temp
All the above information is fine and good, but how does it help you when it’s 45 degrees outside and a client needs a garage floor poured?
There are steps you can take to make sure your concrete pour is done correctly. One step to take is to pour a drier mix, which is exactly as it sounds. Simply pour less water into the mix. This will reduce the water to cement ratio. A lower ratio will result in a shorter set time which will lead to stronger concrete.
A very easy way to increase the temperature of your concrete is to increase the temperature of your water. Using warm water in the mix will create the cement hydration reaction. However, do not use boiling water. This can often cause a flash set. This is a rapid beginning of the hydration process that cannot be stopped and will result in a loss of workability in the mortar.
If you are working with bagged concrete, choose a brand with a premixed solution of calcium chloride which is a common accelerator used to speed up the hydration process. But do not add any chloride containing accelerators to concrete that will include steel reinforcements or that will meet metal. Chloride will greatly increase the potential for corrosion.
One construction myth to avoid is to add automotive antifreeze or windshield wiper fluid winter formula to concrete. (Who thought that was a good idea?) While having antifreeze abilities for their intended purposes, these products do not give antifreeze properties to concrete. It actually negatively impacts both the strength and durability of the concrete.
What about after the pour?
So you’ve poured your concrete, the weather is cold but not freezing, now what can you do to make sure the concrete sets properly?
Cement hydration and calcium chloride are not enough on their own to keep concrete at the ideal 50 degrees. Especially when temps are in the low 40s. So keep that concrete protected! Ideally concrete should be protected for three days following the pour and kept at 50 degrees F. You want to keep concrete protected until it reaches a minimum compression strength of 500 PSI.
When pouring concrete forms during colder temperature months, leave the forms in place for at least one day but up to seven days depending on how cold it is. Keeping the forms in place will help the heat distribute more evenly throughout the concrete.
For other concrete pours, protect the surface of the concrete with several types of insulating materials. These could include insulating blankets or tarps, rigid board insulation or plastic covered in straw. To keep in heat be sure to have a layer of at least 12 inches of straw and then cover the straw with another layer of plastic.
When covering concrete for insulation purposes it is important to weigh down whatever material you are using (off of the curing concrete of course). This will not only keep excess water out but also keep the material from blowing off the concrete.
Once concrete is fully cured it is important to not remove all insulating material at once. Insulation should be removed slowly to avoid thermal shock. This will cause the surface to cool down much more rapidly than the interior of the concrete which can lead to stress and cracks.
There are challenges when it comes to pouring concrete during colder months but it is not impossible. And if done correctly the concrete may benefit from slower curing times resulting in higher strength and more durability.