Concrete and Cold Weather
Did you know that the performance of many building and remodeling materials is deeply rooted in basic chemistry and physics? Now hold on there, this isn't going to be a reenactment of your boring high school chemistry class. Give me a chance here. OK, take roof shingles and flashings. Many people look at me funny when I tell them that the only reason they work is because of gravity. If rain water goes backwards up a roof, you will have a leak!
In my opinion, concrete is probably one of most fascinating building materials. While it appears to be a very basic material, nothing could be further from the truth. High quality concrete doesn't happen by accident. It is a highly sophisticated chemical compound.
Think Jello®
Have you ever made instant gelatin or pudding? Well, you take a powdered mixture, mix it with water, stir it up, and before you know it, you have a semi-solid compound. You are an amateur chemist and you didn't even realize it!
There is a difference, however, between concrete and Jello®. The cold temperatures in your refrigerator speed up the transition of the liquid mixture into a semi-solid material. With concrete, cold temperatures slow this transition. In the case of concrete, it can be disastrous.
Concrete is a strong material because of its chemistry. When you mix water with the cement powder, you start an irreversible chemical reaction. Tiny crystals begin to grow. These crystals attach to one another, the sand and the gravel in the mixture. When everything goes right, you basically have created a compound hard as a rock!
Ice Crystal Pressures
Speaking of crystals, let's digress and talk about ice crystals. When water freezes it turns into ice crystals. This transition would be no problem except for one thing. As the ice forms, the volume of the water grows by nine percent! The ice tends to push or break things that get in its way. The force can be enormous.
In the case of freshly poured concrete, ice can destroy your slab. Enough cement crystals must be allowed to grow within the concrete to withstand the forces of growing ice crystals. It can be a race against time. Most concrete chemists and engineers agree that if the concrete can attain a minimum strength of 500 pounds per square inch (PSI), it can resist ice damage.
Did you know that some chemical reactions require heat to complete, while other reactions create heat? Guess what? The chemical reaction of concrete formation creates heat! It is called the heat of hydration. This heat can be trapped and used to fight off the threat of ice. All you or your concrete contractor has to do is to cover the fresh concrete with insulated blankets. Nothing to it!
Common Surface Defects
Concrete which has been damaged by cold weather commonly has surface defects. The most common defect is spalling. Spalling is simply the peeling or loss of the top finished layer of the concrete. It usually happens because the upper surface of the concrete froze before enough crystals grew to give this layer a strength of 500 PSI.
Popouts are another common surface defect. A popout is created when a piece of aggregate (rock) in the upper surface of the concrete explodes as a result of freezing. Some pieces of aggregate have a tendency to absorb water. This water freezes, expands and BOOM! Gravel companies go to great lengths to try to remove this type of rock from the gravel. However, they don't always catch every piece.
Deicing salt damage is another common surface defect related to cold weather. Scaling of your concrete can happen even if everything was done correctly. As concrete is exposed to air, it gets harder. This process is called carbonation. The carbon dioxide in the air reacts with the concrete and creates limestone (calcium carbonate). Some forms of limestone are very hard!
This carbonation process, however, usually takes one year to produce any substantial differences in strength. So, if you use deicing salts or they drip from the under side of your car onto your new slab, you may have a problem.
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Other Cold Problems
Let's say you do everything right. You order heated concrete (either the mix water is heated and/or the sand and gravel is heated by the supplier), you even put extra cement in the mix, you finish the concrete correctly, and you insulate it for three to five days. You still can have problems!
The cold temperatures slooooow down the crystal formation in concrete that gives it its strength. Sure, you hit the 500 PSI mark okay, but your slab or foundation wall may need higher strengths to resist cracking.
So, in colder weather, you must try to keep your concrete protected from loading as long as possible. Avoid backfilling foundations. Don't allow heavy trucks to drive across sidewalks or driveways. Protect your investment!