Pouring concrete in cold weather is a necessity, but our team has to take special precautions when doing so. When Concrete freezes it causes immediate and sometimes permanent damage to fresh concrete, and the strength and compression of concrete won’t return as it cures.
How Does Freezing Affect Concrete?
We mix concrete with a well-calibrated ratio of water and cement. But if the water in this mixture freezes, it expands 9% in volume. This freezing can happen long before the concrete gains strength, and the resulting expansion can increase the porosity of the finished product and reduce the strength by up to 50%.
All it takes is for the freezing to start in the first few hours after concrete placement, or before the concrete attains a compressive strength of approximately 500 psi. But it doesn’t have to be below freezing for the weather to affect newly-poured mixtures.
Concrete And Cold Weather
Any crew pouring concrete in cold weather must protect it from freezing for the first 24 to 48 hours, or until the concrete attains a strength of approximately 500 psi. This is called the protection period. “Cold weather” is usually defined when the air temperature has fallen to below 5° C while the concrete is still in the protection period.
This means it doesn’t have to dip below 0° for the weather to take a toll on new concrete. When it’s cold, our crew needs to work quickly to protect the concrete until it’s ready to handle the cold. We can do two things to help it along: change the mix to help it set more quickly and protect the concrete from the cold.
Protecting Concrete In Cold Weather: Accelerators
Because cold weather delays the setting time of concrete, we can use an accelerator to keep it on track. Adding 2% calcium chloride by weight of cement accelerates the hydration reaction, and it does this reasonably cheap. However, that much chloride can corrode any steel rebar embedded in the concrete and affect the surface appearance of the concrete. While pricier, sometimes non-chloride accelerators are more appropriate for the project.
When the weather gets cold, mixers will use hot water in the concrete. Depending on air temperature and thickness of the concrete element, it’s best to get the concrete to roughly 18°C when the concrete leaves the site. If any admixtures have frozen when left outside, don’t use them – the chemicals could be separated.
Protecting Concrete Against The Cold: Insulation
The other way is to directly protect the pour. Insulation with blankets is simple and economical, and it works using science. The insulation traps in the heat generated by hydration, which is the chemical reaction of the water and cement. Insulation by itself can maintain the right cure temperatures through hydration heat capture, though it depends on the size of the job, the air temperatures, and the mixture.
To insulate, cover the concrete with blankets as soon as possible to keep in the heat of hydration as soon as possible. By catching it early, the blanket promotes further hydration, which in turn produces more heat. Our crew must protect the corners and surfaces of the pour as these areas are the most susceptible to early-age freezing and damage.
When it’s time to remove the insulation, the blanket must come off gradually to allow the surface temperatures to cool slowly during the subsequent 24 hours. If the blanket comes off too quickly, the concrete surface will rapidly cool. This can create thermal gradients between the surface of the concrete and its interior portions, resulting in thermal stresses that will cause the surface to crack.