Concrete and Cold Weather

By
©1993-2012 Tim Carter

        
Summary: Jello® and concrete - you both mix a solid with a liquid in order to get the final product, right? But while Jello® uses the coldness of the refrigerator to harden, concrete actually slows hardening in cold temperatures. This can be disastrous if you don't take the right precautions. Read about why this happens in the following column.

Related Articles:   Cold Weather Concrete Installation, Cold Weather Concrete Tips, Cold Weather Concrete Checklist

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!



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Comments:

Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you.
Richard
13 Feb 2008, 09:45
Tim, we have just poured 650 yards of concrete. Several places have appeared on the surface where the concret is scaling and chipping away about the size of a frisbee.When we poured it was about 40 degrees here in houston, what should i do. PS we had samples taken through out the pour but have not seen the results yet. Thanks
AsktheBuilder
14 Feb 2008, 16:42
Richard,
I doubt it was freezing. It sounds like a finishing error. You need to patch it with silica sand and cement - assuming that is acceptable to the owner and engineer.
Lance Harris
01 Apr 2008, 20:41
Hi, I recently had my driveway demo'd and reinstalled. The temperatures were just close to freezing for two sections of the drive. Two out of the three sections now have dark grey spotting at the edges and even some yellowing as if bruised. I was told that the mix was heated and had added chemicals. Blankets were used for the first night only for each section. The subsequent nights were close to freezing or warmer. Am I seeing cold damage or will the drive bleach out as the weather warms and dries it out? I am in Northern Oklahoma.
AsktheBuilder
04 Apr 2008, 15:04
Lance,
The color should become uniform over time. Hindsight: You should have waited another 40 days to do the job.
Denise
30 Apr 2008, 15:24
my client is having trouble with concrete spalling on an interior concrete floor installation in a restaurant, it's doubtful that it is a freezing problem --- but like you suggested to Richard, a finishing error. any suggestions about how to avoid spalling for the same concrete floor in a new restaurant?
Heinz g. Nonnenmacher
13 Aug 2008, 08:32
what is the most light-weight roofing material besides prohibitively expensive aluminum?
Tony
02 Oct 2008, 08:58
We had a new house built and the slab was poured in the winter.
It was a big floor so they thinned the mix to a soup consistency for easy spreading.
Now the floor is "dusting". Ive tried vacuuming up the dust but to no avail.
Ive tried using concrete sealers and two part epoxies. THe epoxy lifted.
What can I do?
kathryn
28 Dec 2008, 21:10
I am starting a building project and it's winter here in Boston. The builders are preparing to dig and then pour a foundation for a basement and then start the shell. They say it's no problem, even though it's cold. What questions and garentees do I ask the concrete guys? in ten years, I don't want a cracked and weak foundation!

Thanks,
kathryn
robert
06 Dec 2009, 12:38
need to know what temp to poor i live in ft worth tx it is about 34 right now
Clint
15 Dec 2009, 19:10
Am thinking about pouring foundation for our new house within the next 2 days in Missouri. However, the long range forecast for that day looks like 40 for a high, 36 the next 32 the next and then down in the 20's all next week. Should a guy wait?

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