Our builders have just poured the foundations for our house this past Friday, but did not cover them to protect them from the cold. We experienced temperatures ranging from -2 to -7 overnight during the last two days, with highs of +6 in the day. I am concerned about the integrity of the strength of the foundations, I have read this type of exposure to cold unprotected can ruin the foundations, and they are most at risk of this in the first 24-48 hours. Are my foundations compromised?

He did say he used a low water mix with an accelerator to help it set quickly, he also said because they were pouring below the frost line (in the ground 85cm) it would be fine. However I don't want the build to continue until I am sure they are ok

1) What are some signs I should look for so see if there are issues with the foundations? 2) How long will these signs take to show up? I want to be sure before we continue the build. 3) In your opinion is it likely these foundations would need to be dug up and re-poured? (we plan to do a third story extension in the future so they need to be right)

  • Do you have a building project manager who you can ask?
    – PeteCon
    Commented Jan 22, 2017 at 20:50
  • I would worry about this too; your concrete contractor's story is not obviously untrue, but it is not fully reassuring either. I wonder if there is some test that could be done on a core from the foundation like is done on streets. There are no doubt labs that do this commercially. You could ask the municipal building inspector for your area. Commented Jan 22, 2017 at 23:03
  • 2
    I'd seek out the concrete supplier. Ask them if your builders choice was correct. FYI- most concrete suppliers won't supply a mix that will fail based on forecasted weather, of course the variable is if they call for a blanket, and it's not supplied.
    – Tyson
    Commented Jan 23, 2017 at 0:57
  • I have poured in sub freezing temps in the past. We added additional calcium i think it was 2% to the mix based on the recommendation from the plant. This was on a state job where cores were taken and crushed (the job passed). I have never had to blanket a job but have had to tarp for rain.
    – Ed Beal
    Commented Jan 23, 2017 at 14:02
  • This is degrees C, right? (The "85cm" suggests that it is.) Commented Jan 24, 2017 at 12:18

2 Answers 2


you sound canadian from the metric and the cold in march, so i will answer with some familiarity. i'm a general contractor in southern ontario so i am very familiar with it. you absolutely cannot let concrete freeze in the first 24 hours. never, never, never, never.

generally, lows at night below freezing are highly undesirable for concrete here. even if it gets super hot the next day, it doesn't take much below zero to get frost penetration into the concrete. however, as the anhydrate in the concrete mix depresses the freezing point, it can fare fine below zero for a while. its all site and condition specific. a good wind can drive the concrete into freezing, whereas the same concrete and no wind might be fine.

however, as concrete generates substantial heat as it cures, it might be okay, but at the very least, it would have to be tarped and blanketed, or as we often do here, buried in hay for the first week. sometimes you just tent the whole foundation and just run diesel heaters for the first three days. either way, you have to prevent freezing.

from your description, it sounds like the concrete was poured unprotected into forms that were probably on frozen ground, or at least very cold (bad as it accelerates freezing). if this is the case, and you had no surface protection, you will definitely see problems. unfortunately, as these problems are essentially in how the concrete crystallized internally, you may not see problems for years. it will first show up as microfractures at corners, then spalling at the frost line, and finally, dissolution of the foundation walls, usually at the outside bottoms (where the weeping tile is). you cannot prove it until the concrete has fully cured, 28 days on, and it requires core samples and engineering studies. you cannot rely on the word of the contractor or the concrete supplier. they obviously don't want any liability.

step 1 - start documenting everything. photos, pour dates and weather reports, etc. have it all notarized to prepare for the inevitable court case down the road.

step 2 - get a lawyer to put it all in writing to the contractor. even if its not a statement of claim, it goes on the record. if your home is in ontario, it will be registered with tarion, and you can use it all to fight it down the road

step 3 - you can go so far as to file a statement of claim if you get engineering data back that proves it. better to demo a bad foundation and do it again than repair a bad foundation ten years from now with a house on it and finished landscaping and driveways around it.

good luck and remember, wishful thinking and hoping for the best are recipes for disaster in house construction.

  • The question is from January.
    – Agent_L
    Commented Mar 27, 2017 at 8:17

If the slab froze it likely will not cure correctly. It may appear to be green and frosty and brittle. If within one week the concrete still feels really brittle it may have been damaged. The rule of thumb at the company I work for is: For lows under 28 degrees Fahrenheit requires a poly cover. Below 16 degrees poly and blankets. Frozen concrete is not good. Winter concrete is not cheap to do, keep in mind if you chose the lowest bid, you may be getting what you paid for.

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