I'm developing a crack in the slab of the garage floor. I notice that if I tap on the slab with a length of heavy pipe, I can detect a hollow void underneath the crack.

I'm thinking about drilling a few holes in that area (about 2' x 3') and filling the void with expanding foam to support the slab.

Has anyone ever tried this or have a better idea?

I don't want to start breaking up the floor and start replacing sections of the slab, the garage is over 30 years old and I don't want to start sinking big bucks into it.

You might try calling your local mud jacker. Check your yellow pages and find someone who injects fresh concrete under slabs to fill voids and raise sunken slabs to fix problems like this.

^^^ +1 .... I really don't think you can find too many garage floors without at least 1 crack in them (no azz jokes!!)

You might try calling your local mud jacker. Check your yellow pages and find someone who injects fresh concrete under slabs to fill voids and raise sunken slabs to fix problems like this.
I've looked around for someone who does concrete leveling and their doesn't seem to be anyone in the area that does that.

I wouldn't use foam though.

I'd inject new concrete if anything...

Can you chip away the part above the void, then just fill it in and level it off.

I'd inject new concrete if anything...

Can you chip away the part above the void, then just fill it in and level it off.
I could be opening a can of worms by doing that. I'm afraid that before I knew it I would have half of the floor broken up.

Our youngest son worked with his best friend for his friend's grandpa. Grandpa put it in plain english: concrete cracks. Even with wire or that fiberglass stuff in it.

However, I too would be concerned with the hollow sound to the floor. Something could be undermining it (like water or the lack of it as in a sink hole). What is the garage on? New fill? Compacted fill? Good old hardpan/clay? Good drainage under it or none?

+1 for getting a concrete specialist in to look at the situatioin. Hopefully, it will be someone who is honest and/or you can trust.

Its an older DIY garage, I can tell by the finish on the slab that whoever poured it wasn't too experienced so I'm sure the base wasn't as compacted as it should have been. We've had a ton of rain here this summer and I think may have caused some of this problem.

Drill a hole in it, insert a clothes hanger with a bent tip on it, catch the bottom of the slab, and push down til you feel the ground. That will tell you how "hollow" it is. If you feel a need to put something in there, you will need 2 holes, one to go in, and one to come out of, a pressure release.

I can crush expanding foam with my fingers...probably won't do anything under ten tons of concrete and cars.

My garage floor is 54 yrs old, cracked right down the center and sloping towards the sides. You can see it along the door seal...little sunlight in both corners. I'd love to re-pour the slab with a good smooth finish and epoxy sealer, but I'd love to move out of the house sometime, too.

I wound up just smashing a big hole in the slab with a sledge hammer, the base dropped a good 6-8 inches so I packed it and filled it with stone and poured a couple of bags of concrete in the hole. I spent half a day trying to seal up where I thought water was getting in.

Hopefully it will hold up.