"The winter is coming" , so I started bringing in the items that are not supposed to stay outside during the cold season. Today I removed some materials that where sitting in the crawl space under the stairs to the basement and to my surprise I discovered the below

Here is a general view of the area with the issues detailed below enter image description here
It appears that the builder left an opening in the floor slab when they poured the concrete and then it was masked it with a 2x4 and some more cement added in place but not everywhere. The opening goes along the line marked in the schematic representation at the end of the post, it is like a quarter of an inch wide. The white matter that you see there is dust from the drywall that deposited on the floor in that area as the time passed, not sure why.

This is a zoom in for ❶ enter image description here

❷ is where you can see that the opening was left there, it is not a crack. It rather seems to be a construction error or defect that was covered up

This is a zoom in for ❸, here you can see that the cover up is cracked enter image description here

❹ is there just to indicate a spot where you can see that the wood was stained or oiled with something, that is not mold

Here is how it is built. As you can see they put two 2x4 on top of each other. I guess I will have to use adjustable posts to support the floor while I am removing those vertical and horizontal 2x4.

My question is more about the floor, what would be the procedure to clean the place and pour concrete there an make sure it does not crack again
Update: I would alos consider suggestions regarding ways to just clean and seal the cavity with some foam or other materials, I do not insist to pour concrete there
enter image description here

  • The issue here is drainage. No matter how much concrete you pour you won't solve the issue until you redirect the water source. Water under concrete has amazing powers of penetration, it will find a crack, pinhole etc and due to pressure difference it will surface.
    – Tyson
    Oct 31 '16 at 2:32
  • How do you know it is water under concrete ?
    – MiniMe
    Oct 31 '16 at 2:48
  • Why are you worried about this hole such that you feel the need for extensive work to patch it? I would leave it be unless there is some other related problem... Oct 31 '16 at 4:25
  • I think that I would need to at least replace the wood and maybe seal the crack somehow with some foam or something else
    – MiniMe
    Oct 31 '16 at 10:08

This is a very small opening. You would not use normal concrete or need to reinforce it with re-bar by drilling into the surrounding slab since this patch job will not be supporting any weight and is merely for aesthetics and keeping out rodents, reptiles and insects. I see two viable options.

First, Spray foam.

Second, Portland cement. You would lightly spray the existing slap with some water in a spray bottle to wash away any dirt, debris, or loose material. Then mix up some portland cement to pour in there. You won't get a great seal but enough to keep out any critters.

It depends on whether you need a solid surface. Since this is a crawl space and right next to a wall I would assume not and go with the spray foam.

  • Since then I discovered that the floor is slightly raised by a crack under the vertical 2x4 that supports the stairs behind point 1 in the picture. A friend who came to see it suggested that there might be a crack in a pipe that is under the slab and I should cut open the slab fix the pipe and then pour concrete back. The problem with this is that since this is under that 2x4 I will need to demolish that section and it will involve drywalling and painting ....
    – MiniMe
    Dec 11 '16 at 13:49

You cannot "make sure it does not crack again", all you can do is your best effort and hope it does not. I believe a concrete expert would tell you to drill into the sides of the existing slab and epoxy reinforcing bar stock in to "tie-in/pin" your patch to the existing slab.

  • Well I think that you are right, it might crack again, I just don't want it open like it is and the wood to wood to get wet. Right now everything is dry there but I am not comfortable with the way it looks. I might choose not pour concrete if that is an option
    – MiniMe
    Oct 31 '16 at 10:06

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