New garage floor

When my house and garage were constructed, the slab was poured on top of improperly compacted fill. Over the next 40 years, the fill settled and the slab with it. This has caused all sorts of fun problems with the house which I am just about done dealing with. Now it's time to focus on the garage floor which is as much as 3 or 4 inches lower in the middle than at the footers.

I've started getting quotes for a complete tear-out and repour with rebar. This is the first time I've had a big cement project done so I'm not really sure what to check in advance of accepting a quote beyond the basics (thickness, warranty, etc.).

What questions should I be asking?

For a shop floor you will need high pressure concrete and the correct depth.

Just wondering what did you do to overcome the foundation issue with your home?

Paul B

Sounds like you just need to chisel a channel out of the middle and you can easily clean up :)

Are the garage perimeter walls sitting on the slab, or on perimeter foundation walls?

(I had a similar problem and spent some money mud-jacking the garage slab, because I couldn't remove the slab unless I shored up the walls first - and I decided that was too much project with too little benefit. When they started boring holes to mud-jack, they found 12" tall hollow space under some areas of the slab. It took a lot of grout to fill, and the slab cracked quite a bit as it came up).

Donebrokeit wrote: For a shop floor you will need high pressure concrete and the correct depth. Just wondering what did you do to overcome the foundation issue with your home? Paul B

The footers were good and all the exterior walls were fine. The fill in between had compacted such that there were significant voids present under the slab though. The central load bearing wall caused the slab to sink in around the centerpoint of the house which resulted in misaligned door jambs, cracks in the walls, etc. Also as a result of things moving around so much, I had to have my kitchen and bathroom floors opened up to have broken pipes replaced.

Anyway, to answer your question, I had foam/poly-jacking performed throughout the entire house. The expanding foam has 6000 lbs per sq ft of lifting capacity and backfills the voids and lifts the slab back to its original position. It was pretty neat being in the house as it was happening and watching the cracks in the walls close up and the doors start latching properly again. Other benefits of the foam are that it's permanent and won't erode with water and also creates a moisture barrier under the slab. It's been two years since I had the work performed and so far so good!

OHSCrifle wrote: Are the garage perimeter walls sitting on the slab, or on perimeter foundation walls? (I had a similar problem and spent some money mud-jacking the garage slab, because I couldn't remove the slab unless I shored up the walls first - and I decided that was too much project with too little benefit. When they started boring holes to mud-jack, they found 12" tall hollow space under some areas of the slab. It took a lot of grout to fill, and the slab cracked quite a bit as it came up).

The perimeter walls are on the cinderblock footers that go deep enough down that they haven't moved. The garage floor has already cracked in several places as the center has sunk in and the edges remain in their original places. I'm curious to find out how much of a void there is under there when they remove the old concrete. In the worst spot under the house there was a 10" void.

Variables are reinforcing, concrete strength, concrete mix.

What you really need to sort out, is crack criteria:

What is reasonable to the contractor. Versus what is not okay to you.

Got another quote last weekend. Same ballpark, a little more detailed and complete on the work to be performed. A lot cheaper than I feared it'd be so I am going to pull the trigger and get this done. Any recommendations on a sealant that's appropriate for a shop that will see oil spills, paint overspray, etc.?

Garage Journal has a ton of good information about coating a new floor. I think the important thing is to get it done before you drive/work on it. You've only got this one window where it's properly clean. 

Ghost Shield seems to be discussed most frequently right now, but that doesn't mean it's the best option for your application. Follow Mazduece's suggestion and go do some reading in the flooring section of GJ. There are a handful of vendors that post there routinely and can suggest a type of product that will fit your wants and needs, just be prepared for them to pitch their own stuff.

You'll probably have to give the floor time to cure before applying anything which means that you won't be able to use it during that time. Keep that in mind when shopping, as some products require up to 28 days of cure time before they can be applied to a fresh slab.

Donebrokeit said:

For a shop floor you will need high pressure concrete...

Paul B

No such thing

Your biggest issue to watch out for is the compaction of the soil under the slab. I would hire a geotech company to confirm before pouring (since you've already had a problem)

Standard residential floor thickness is 4" and probably adequate, but I would consider 5". Definitely go at least 6" thick under the posts if you are installing a lift. 

A standard mix compressive strength will be adequate- usually 2500 psi

My shop floor has fiber reinforcing only, and I've never had a problem. 6" wire mesh would be better. Rebar is overkill. 

My shop floor is 4" with fiber reinforcing. Its perfectly fine, and I have had a number of big trucks and the neighbors Kubota tractor in there multiple times. Its only got one small crack in one corner and that was from the ground settling. If I did it again I would go with 6" wire mesh, but I wouldn't change anything else. I would only go to 6" under a two post lift, and then I would go probably just pour a deeper footing right under each post which would probably be overkill.

Thanks for the suggestions all. I will do some more research this weekend.

Just got the contract paperwork yesterday. For the garage floor they are specifying 4" of 4000 psi concrete with a 6mm polyethylene vapor barrier, 10 gauge 6x6 wire mesh reinforcement. They also specify "Euclid AquaCure VOX" sealant, so I'll look that up.

No plans for a 2 post lift, my ceiling is too low.