Around the time the leaves begin to fall each year, many property owners begin to raise the question: can concrete be poured in the cold weather? A majority of people believe the answer is no. However, we are here to tell you that concrete can be poured in the winter if practices meet standards. The standards we follow are set forth by the American Concrete Institute (ACI). ACI 306R-16 establishes industry standards, best practices, and guidelines for preparation, placement, finishing, and protection of finished concrete slabs in cold weather. ACI states that cold weather concreting procedures need to be followed if it is “a period when for more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees Fahrenheit for more than one-half of any 24 hour period.” The good news is that as long as the guidelines are followed your concrete will be strong and durable.
What Happens When Concrete Freezes?
The process that concrete goes through to build strength and durability has to do with hydration and the curing process. As the concrete mix transforms from a liquid to a solid a chemical reaction is occurring that is fortifying the concrete. One characteristic that greatly affects the rate at which concrete cures is the temperature. For instance, curing can happen too fast during the warmer months and too slow during the colder months. Cold weather slows the process down and can even halt it if it gets too cold.
While concrete is curing, it is important to keep the concrete’s temperature regulated. The concrete needs to be prevented from freezing until it has time to achieve the minimum compressive strength of 500 PSI (pounds per square inch). After it has reached this minimum strength, the concrete can be exposed to cold weather without worrying about damages from freezing temperatures. Concrete is porous and the water in the mix is crucial because it is what starts the hydration process necessary for the concrete to set and reach the minimum strength. The ice created with freezing temperatures warps the binding of concrete and ultimately weakens it. If the water in the concrete freezes and expands before it has time to reach the 500 PSI it can cause cracking, scaling, flaking or dusting.
What Can Prevent Concrete From Freezing?
A number of measures can be implemented to hedge the freezing of concrete. For instance, concrete should not be poured on frozen ground, ice, or snow. When the frozen ground thaws it can shift, crack, and weaken the concrete. Frozen ground can be heated up by placing heated sand on top of it or insulating it for a few days.
Altering the chemical make-up of the concrete is a possible solution. Some changes that can be made to better suit a cold environment are adding more cement or aggregates and reducing the amount of water in the mix. The use of heated water, heated aggregates, and admixtures accelerates the rate of the hydration process increasing the rate at which the concrete builds heat as well as strength.
Temporary structures are a popular choice as well to maintain appropriate temperatures for the concrete to cure. Windbreak structures can be placed around the work area to reduce the freezing cold winds that rush through and cause temperatures to drop. There is no perfect height for the windbreak walls, so they need to be built to accommodate the wind’s strength. A heated enclosure can also be constructed. This enclosure can be made from wood, polyethylene, or other sturdy material. If a heater is used in an enclosed area, an indirect-fired heater is suggested. If there is good ventilation to reduce carbon monoxide, a direct-fired heater can be used.
The third and most common solution to protecting concrete from freezing temperatures is a curing blanket. This curing blanket is placed over the concrete after the finishing is done. As concrete cures, it releases heat. The blanket will trap the heat in, keeping the concrete at an optimal temperature to cure even in the cold weather.
Once the concrete has cured long enough and it is okay to expose it to cold temperatures, it needs to be done gradually. Too quick of a temperature change can harm the concrete. Start by removing the framework while keeping the warming blanket on. If the concrete is inside a heated enclosure, simply turn the heater off and let the space equalize with the outside temperature.
If correct procedures are followed, concrete can be poured during the winter. You do not have to halt your dream driveway or patio because it is cold outside. Actually, according to the American Concrete Institute, you should “Take advantage of the opportunity provided by cold weather to place low-temperature concrete. Concrete placed during cold weather, protected against freezing, and properly cured for a sufficient length of time, has the potential to develop higher ultimate strength (Klieger 1958) and greater durability than concrete placed at higher temperatures. It is susceptible to less thermal cracking than similar concrete placed at higher temperatures.”