Q. I have a garage that sits slightly below the level of the surrounding driveway. Is it safe for me to add 5-6" of concrete to the existing concrete floor of the garage in order to raise it up to prevent water from getting in to the garage? I'm wondering if this would overstress the existing concrete and lead to weakening the structure. The garage is an old structure (1946) made from cinderblock walls on a concrete floor.
A. Adding 5 or 6 inches of concrete to your existing garage floor should be just fine if the existing concrete floor is in good condition, meaning it hasn't settled drastically or heaved from freeze/thaw cycles.
If there is noticeable movement in the existing concrete floor, up or down, you probably have air pockets between the bottom of the concrete floor and the sub-grade. I would recommend removing the floor and starting over if this is the case.
If the floor is in good condition with no noticeable movement, maybe just some small cracks, pouring a new floor on top of it will shouldn't lead to any problems.
We do this quite often, I install a layer of typar on top of the old floor before we pour the new floor, to let both floors move independently from each other. Keep in mind you may have to adjust the height of your garage door if you raise the height of the concrete floor.
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