Taking Precautions to Protect Your Fresh Concrete
Just like during hot weather, cold weather effects the setting characteristics and strength of concrete. Once average daily temperatures fall below 40˚F on more than three consecutive days, contractors and suppliers should be taking the necessary precautions to prevent their fresh concrete placement from being affected.
American Concrete Institute Guidelines
There are ACI guidelines that specify what minimum and maximum temperatures concrete should reach based on the thickness of the structural element. During cold weather, a placement should be monitored to make sure it doesn't slip below those minimum temperatures before the concrete mixture starts to generate its own internal heat from the chemical reactions taking place within the mix.
Per Knights Companies VP of Technical Sales, concrete needs to reach an equilibrium with the environment before the tri-calcium silicate in the cement can generate heat. So on days where 55˚F concrete mixture is placed on the 35˚F subgrade (or, "ground" for the lay-person), the concrete mixture temperature will drop to around 45˚F before it will start generating its own internal heat. This drop in temperature will result in extended setting characteristics, where it may take 10 or 11 hours before the concrete is ready to be finished. That's in contrast to a perfect 70˚F day where you can finish the concrete in around 6 hours after placement.
But, why is freezing that big of a deal?
Freeze-thaw cycling can be a big problem for MANY construction materials, but concrete has its own issues that require trouble-shooting.
Scaling: Surface scaling can occur, leaving the surface of the concrete pock-marked and exposing the underlying concrete aggregate. Over time, these conditions can be perpetuated by on-going exposure to freeze-thaw cycles leading to additional cracking or scaling. To mitigate the potential for scaling, air-entrained concrete can be specified for an exterior slab. Visit the Portland Cement Association for more information on the prevention of scaling.
Carbonation: On interior slabs, where temporary heating is typically used during winter-weather construction, the upper surface of the slab can carbonate. If you have every run your hand over an aged surface and it has left a chalky residue on your hand, that is what carbonation is like. See page 3 of this Portland Cement Association white paper for additional information on carbonation.
Loss of Strength: This is a big one. Or it can be. If a concrete mix ratio is unintentionally augmented by ice accumulation that then thaws and mixes with the concrete, it can lower the ultimate strength of the concrete. High-slump concrete (or concrete that has more water in it to make it more flowable) faces similar challenges in winter conditions.
What can be done?
Typical construction projects typically rely on their suppliers and contractors to meet tight deadlines and schedule requirements. When things that were schedule for summer (when the concrete would only take 6 hours to set up) slip into winter (when the setting time might be double), what can be done to help the concrete set up faster?
Hot Water: Utilizing hot water in a concrete mix helps to better control the temperature of the concrete before it starts producing its own heat, helps to prevent early age freezing, and decreases setting time because the mixture temperature is higher.
Chloride and Non-Chloride Accelerators: A chloride-based accelerator can be very effective but shouldn't be utilized when there is steel reinforcement within the concrete member, due to its corrosive nature. Additionally, chloride-based accelerators will retard the iron oxide compound in the cement, leaving the surface with a more mottled look, which is something to consider if the finished surface will be utilized in a decorative capacity.
Air Entrainment: Air entrained concrete has microscope air bubbles that act as tiny pressure-relief pockets within the concrete, allowing the water within the mixture to expand without damaging the placement. The amount of air entrainment is related to the frequency of freeze-thaw cycling - the more cycles, the more air entrainment. In South Carolina, we typically see about 4.5% while in North Carolina, where the winters can be colder, we typically see around 6%.
Protecting the Concrete: Utilizing plastic sheeting, thermal blankets and straw can help reduce the effects of cold weather placement and speed up the concrete's setting time.
Slump: In cold weather it is generally recommended that the concrete mixture be specified with the lowest practical slump. This means that the concrete being placed is as stiff as is practical for the application and the addition of residual water resulting from iced forms or other winter conditions may not have the same effect on the final concrete material.
Keeping the above tips in mind, remember that winter weather conditions don't mean your pour schedule is necessarily shot. Have a discussion with your concrete supplier and specifier on what can be done to limit the impact of those chilly temps!
Thank you to Knights Companies for providing the basis of this post. Knight’s Redi-Mix focuses on commercial, residential or utility use concrete that is environmentally conscious and carefully mixed for each project in our South Carolina batching plants.
Contact Sharon Benzenberg at sbenzenberg@knightscompanies.com with your next concrete order.