Pouring foundations in cold weather, builder didn't protect them - how to tell if it is set right?
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.