Basement – Construction defect in basement slab under stairs in crawl space-how to fix this issue

basementcrackcrawlspacefloor

"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
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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 2×4 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
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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 2×4 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 2×4.

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
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Best Answer

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.

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