Testing residential foundation poured in cold

[fredhead]

We're having a house built. The foundations for the main house and for the attached garage were poured separately. The main house foundation was exposed to cold, but not freezing, temps (40s F day/30s F night) for the first few days, but then freezing temperatures since. The garage foundation was poured in freezing weather and then exposed to freezing temperatures (mid-20s F night) for the first few days as well as having 8" of snow fall on it the day it was poured. No heating or insulation was used in either case.

Now I am concerned about the effect of this weather on the strength of the concrete. I am primarily concerned about the garage foundation since the top surface of the concrete is easily flaked off. The main foundation at least has a much better appearance. A structural engineer I've spoken to said that he would be reluctant to take core samples now while the concrete is frozen for fear that it would yield inaccurate data.

Are there any suggestions as to how to check the quality of the foundations without having to wait for months?

Thanks!

 

[andycivil]

This is by no means a ASTM or ACI recommended method but....

When I first started out of college, I had to check a few slabs to see if they had frozen, much like you are afraid your garage slab did. I had a very good Concrete Engineer (and PE) who worked over me and instructed me on what to look for. He would send me out a day or two after the slab was poured. At his direction, I would take a standard hammer (claw hammer like you would buy at Home Depot) and walk around hitting the top of the slab. Don't swing with all your might, just swing like you would to drive a nail into a 2 x 4. If the concrete flakes off easily under this pressure, you probably have problems. If it doesn't, the concrete may not be up to design strength, but probably won't have any serious problems. If this doesn't put you at ease, wait and take cores.

Let me know how it goes.

 

[dirtsqueezer]

Fredhead,

Two things.
There are established methods for assigning a psi strength to your concrete without having to take cores. My testing company performs this test with a 'rebound hammer'- similiar to the method above, but can assign an approximate psi strength to the concrete. This wouldn't be enough to take to the bank, so to spead. But if the numbers were low enough, it would probably lead your engineer to himself suggest coring.
Second, what has happened to the finish on your concrete is called scaling. It is known to happen when there is standing water on the concrete while it's wet, and is one of the basic things us concrete inspectors are told to watch for. I'd say 8" would be a good reason for scaling to have occurred.
If your foundation is purely structural and won't be visible, then scaling will have nothing to do with the function of your foundation.
The first cheapest thing to do is look for cracks you can see through. This is a debatable issue, but I was taught that all concrete cracks, even strong concrete, so be careful. But if you have very large cracks you can see down into, you should call your engineer. Scaling can also be caused by improperly mixed concrete, and may lead to further damage to the frame, causing your doors and windows to stick, among other problems. If you find large cracks and want to see what you're engineer's made of, I would think seriously about calling your local testing company, probably AMEC or PSI- i'm not sure what you have out there, and request a rebound hammer test. It's a simple thing and is as easy as operating a stapler. They'd be more than willing to accomodate, especially now during the slow season...
I didn't even cover the possible negative effects of cold-weather curing. Fissure-sized cracks can result here, too, so if you've got gumption, digital camera and your engineer's email adress, I'd say for the moment that's all you need.

gl

 

[bluesman]

Dear Fredhead...
I wouldn't want to be in your shoes right now! All you have been told in replies to your post in all the forums (which by the way are technically correct information) must have alarmed you and scare the hell out of you ..... Take this simple advice..... Wait till the concrete is 28 days old and take cores! Then you will know what the situation is really like! Observing the deterioration of the surface and the hammer test won't rest your soul.... the cores will! Which would properly be what I do if I were you!
A. Sadi

http://www.geocities.com/concretesite

 

[MikeDB]

Don't know if you had a contract or any specifications to follow, but you have every right to be concerned and so should the contractor. Was a building permit required and what did the building inspector have to say about it?

ACI 306 (I think that is the right number)is a specification for placing concrete in cold weather. Our specs do not permit placing concrete when the ambient temp. is below 35F. We do not permit concrete to be placed on frozen soil. We require the temperature of the concrete to be kept above 50F for a minimum of 72 hours after placement and above 32F for another 2 days after that. The contractor should have never placed the concrete without taking proper precautions.

If you are already seeing problems, remember that it will only get worse. You didn't say if the garage foundation was a slab or wall or what. I think I would tell the contractor that it is not acceptable and ask him what he plans to do about it. Based on the limited info I have on the case, I would think that it should all be replaced. If the contractor doesn't satisfy you, you may want to seek legal advise.