How would I go about replacing a small section of basement slab?

Question

I would like to replace a small section (approx. 2'x3') of the basement slab. The crumbling section is near the foundation wall, which also has some damage from spalling. I have regraded the exterior away from the house and extended the gutter downspouts, so I think the underlying moisture problem is more or less solved.

The slab appears to be original, nearly 100 years old, and is about 1 1/2" thick. I am assuming it doesn’t have much structural value given how thin it is. I am planning on demolishing the concrete in a slightly larger area, add gravel as needed, compact it and then fill it with fresh concrete.

Is it a problem that the section is close/above (I assume) the footer of the foundation wall? Can I use regular mix, or do I need something like polymer modified concrete to help with bonding to the existing slab? I would hire a professional, but they're not interested in small jobs in my area.

Answer 1

Generally speaking, basement slabs can be cut up and repaired with no structural concerns. They're mostly just floor surfaces and have no bearing on the foundation*. With a little knowledge and good preparation this should not be a serious challenge (other than the lifting--concrete and water are hea-vy).

There are a few obvious steps and some tips I can share.

• Rent a diamond-blade concrete saw. For a good outcome you should start with clean edges and a pleasing shape. Keep corners square to avoid a Dr. Seuss aesthetic.
• While you have it, cut a few shallow horizontal slots into the faces of the opening for the new concrete to engage, in lieu of rebar.
• Expect dust. Lots of it. Seal the area and use water on the cut, along with a shop vacuum run by a helper.
• Use ear, eye, and respiration protection.
• Go thicker than your current slab. 1½" is very thin by modern standards. Twice that is more appropriate, especially with a small repair.
• Standard concrete mix is fine. There's really nothing to bond if you've done the slotting I mentioned, and I wouldn't expect additives to replace mechanical reinforcement anyway.
• Mix exactly to the directions. Concrete is thicker than the movies would have you believe. It's not really liquid. Too much water makes finishing much more difficult and weakens the slab.
• Place the concrete and do a rough screed by sliding a long board across the opening, resting on the old slab.
• Let it sit. Finishing doesn't occur right away.
• Watch some videos on finishing technique. You'll need at least one trowel or float to do it well. Timing is critical to avoid over-working. Too soon and you'll push the concrete around. Too late and you won't get a good surface finish.
• Don't forget to put your initials in the finish. It's tradition.

_{* Exceptions include thickened-edge (integrated) footings and very old structures with non-standard foundations.}