Message Boards

I am doing some remodeling work on a bumpout and discovered that there is an air pocket under the concrete slab. I'm not sure how extensive it is. I have felt around with a prybar in the opening between my foundation and the slab, and it appears to be several inches tall. The bumpout used to have a refrigerator on it. It feels solid, but it is not level (pitches down, away from the foundation)

The bumpout is about 6 feet wide by 10 feet long. I was thinking about cutting a couple small holes and pouring a wet mix of cement, with the idea that a runny mix would fill in all the gaps.

What is the right way to fix this?

Thanks for any advice.

I'm not sure you'll get good coverage just pouring the concrete through a series of holes. I'd probably hire this one out to a concrete leveling company. They'd do what you're talking about except they'd pump the concrete into the void. They are pretty common around here for fixing sidewalks and driveways that have unevenly settled.

I kind of wonder what's causing the substrate to move away from the slab. Typically, water does that. Sometimes, it's a footing/foundation issue largely unrelated to water/drainage.

For 60 square feet, it might be worth breaking out that area, to see what's going on. It's not a big area, and at the least, you'll be able to backfill/pour something better structurally, than hunting and pecking for voids to fill. If you open the floor, you might uncover some issue you really need to address.

Of course, maybe you had a drainage issue at some point, which you've already addressed.

Sort of makes you wonder if the slab was not poured on poorly compacted ground and the natural settling has occured over time leaving a void there.

Or...maybe you found Jimmy Hoffa???

I wondered who that guy was in there. :-)

I'm thinking that the substrate was not compacted enough. The pitch seems deliberate (the sleepers were shimmed to account for the pitch). Maybe it was a landing for an exterior door? The slab seems to be made for the explicit purpose of supporting the bumpout, but who knows?

I guess I'll know more after I open it up in a couple places and check it out.