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Cold weather raised my outdoor slab

martyrg | Posted in General Discussion on January 24, 2025 09:16pm

I live in southern Michigan, zone 5B.  I have a heated shop building on my property.  It has three sets of  large double doors; each door leaf is 5 ft w x 10 ft h.  The doors are made from insulated structural panels.  Each door panel weighs about 325 lbs.  The doors open out.

The recent polar vortex – multiple day of single digit temps – seems to have caused concrete slab that is just outside these doors, to raise up between a quarter to one half – inch.  This is enough to block the doors from opening.

The slab is 5 inches thick with a 1/2″ rebar grid. The ground underneath it was plate compacted, no vapor barrier between the ground and the concrete. Rubber expansion joint between the building and the outdoor slab.  The concrete was placed in the summer of 2022.

This is the first time the slab has moved.  I can trim the bottom of each door to clear the slab.  My question is how much more should I expect the slab to rise?  Will it go back down as the weather warms back up?  What should I have done?  What should I do now?

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  1. calvin | Jan 25, 2025 08:07am | #1

    Just south of your border.
    Clay “soil”.

    Under your apron, compacted stone?
    Drain “tile” to daylight around the entire bldg perimeter?
    Apron start out as flush with the bldg slab?

  2. primitive@gmx.com | Jan 25, 2025 02:35pm | #2

    Former Detroiter here. Just curious why in a heavy snow region you would only have a 1/4" to 1/2" clearance on an out swinging door? I would have put a slope on the slab that pitched away from the door for snow and water to drain away from the building and to leave a nice gap. I hate to say it but is there anyway to repour the slab with a pitch at the door openings? Or can you grind the slab down? Good luck

    1. martyrg | Jan 27, 2025 03:26pm | #5

      The slab is pitched away from the building, just not steeply. My mistake was not dropping the exterior slab lower than the interior slab. I'm thinking the long term fix will be to take out the part of the slab that is in the path of the door swing - about 5 ft. out from the building. Then use compacted gravel and rigid insulation under the replacement slab - along the lines of a frost protected foundation. In the mean time, I'll probably trim the door bottoms to clear the raised portion of the exterior slab.

  3. User avater
    unclemike42 | Jan 26, 2025 08:23am | #3

    Frost heave happens when there is a path (usually via soil structure that has a lot of "fines") that allows water from the soil to freeze on the bottom surface. This ice expands as it changes from liquid, pushing the slab up. (ice floats on water, remember, this means the same amount of water takes more space as ice)

    Two ways to handle it are to prevent liquid water from getting to the bottom of the slab, or arranging things so you do not care if the slab moves up and down.

    Proper base material is selected so it is not conducive to travel of liquid water. a local quarry will provide suitable choices. (kind of late for this for you now)

    Future movement will depend on the weather and moisture content of the soils below the slab.

    It is likely the slab will settle back down when the ice melts. Time will tell.

    If you do not need to open the door till then, you can just wait.

    Assuming you do not want to tear up the slab and replace, you can adapt the doors to clear, as needed. The difference between the bottom of a trimmed door and the floor below can be made up with some kind of threshold material. Up to you if this will be permanent, or has provisions to be removed when you want to wheel things in and out.

    1. martyrg | Jan 27, 2025 02:55pm | #4

      Thanks uncle mike42, that was helpful. It gives me a better understanding of of how frost heave works. Right now I'm thinking I'll trim the bottom of the doors to clear the raised slab. Then I'll see how things look in the Spring.

  4. vivafenesterbng | Jan 30, 2025 03:01am | #6

    It sounds like the concrete slab may have heaved due to frost or water expansion, common in colder climates. As temperatures rise, the slab could settle back down, but it’s not guaranteed. To prevent future issues, consider adding a waterproof layer under the slab or improving drainage around the foundation. For now, trimming the doors might be a temporary solution while monitoring the slab’s movement.

  5. martyrg | Feb 03, 2025 07:40pm | #7

    Hey everyone, Thanks for your replys. I took advantage of today's temps in the low 40's to work outside. Myself and two friends took the doors down and trimmed the bottom of the doors. I took off between 1/2" and 3/4". In addition, the warmer weather (low 20's to low 30's on avg.) seems to have reduced the heaving of the slab. Plenty of clearance now. I have the gap covered with a flexable vinyl sweep that will accomadate the slab movement.

    I have rubber expansion joint material between the exterior slab and the building. I'm thinking this may have been the path for water to travel down and under the slab. What would be a good material to use to seal up and waterproof this joint?

    1. User avater
      unclemike42 | Feb 21, 2025 05:06pm | #9

      Sikaflex is pretty good stuff. But moisture lives in the ground.

      Any sealant can limit water access from that point, but when the slab moves because of other water sources, it will be hard for any sealant to maintain the seal when pulled apart.

  6. CandG | Feb 21, 2025 01:36pm | #8

    If it is a continuing issue, you can cut the slab and dig a trench footing that is larger than the door swing. Then pour a slab resting on this, it won't heave then.

    If you look at a commercial building that has outswing doors you will see different concrete in front of the door because it has a frost footing. If you are going to go through the process of cutting the slab out, I would install the footing and not rely on just keeping water our of it. Good luck.

  7. martyrg | Feb 22, 2025 10:06am | #10

    Hi CandG,

    Thanks for the info. Would that footing be around the perimeter of the new slab or under the entire new slab?

    1. CandG | Feb 24, 2025 11:27am | #11

      Just where the doors swing depending on how much concrete is out there. For a 5'wx10' door I would foot an area 11' w and 6' off your structure, and then pour a slab on it.

      Once that slab is re-barred into the footing it will not move and your doors will (hopefully) never bind again.

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