Levelling over concrete slab?

[freedomlives]

Our house has what is sort of a carport (they call it a shopka in Slovak, I guess like "workshop"). I'm converting it to a garage these next weeks, and I've got that time pressure that often helps me get stuff done, since I moved the hotwater heater out there while renovating the bathroom, so its got to be all enclosed before it gets below freezing in November.

The walls, which right now are just random boards nailed to oak beams-- well, the random boards will go, and some reinforcement 2x4ish wood will go in, and then fiberglass insulation and both sides OSB boards. Ceiling I've also got to make, likewise not such a big deal (right now its open to the roof and the attic).

In the front where it is just open, I'll frame with wood and put a garage door and regular door (with dog door-- he'll be a lot happier this winter with a warm room to sleep in).

The floor is a whole different matter. I guess when decades ago they were pouring concrete in there, they poured it a lot thinner to the front, and it has a ridiculous slope. Photo album

The last 6 feet to the front is pretty much nonexistant, and from the back to the front there is an 10" change in height, over 6m / 22 feet. But most of that change is the first third, where there's hardly any concrete. Physically, the rest of the slab seems pretty solid-- it was a PITA to cut through with a diamond wheel on an 10" angle grinder to run pipes. The other thing I wonder is water vapor coming up through the floor, especially as we are in the bottom of a valley.

Would it be a valid approach to level off the really shallow area with compacted gravel, put a plastic sheet over everything, and then put down, say, 3 inches of concrete with welded rebar reinforcement mesh? Or should I clean the old concrete slab and try to bind on to it? And if I do put down a plastic sheet, then does that just end up driving any vapor in to the walls?

 

[Egon]

Should work. Maybe go to four inches of concrete?

 

[Redlands Okie]

I think your idea is a good one. Do not try to bind the new and old concrete. Serves little purpose and cause the new to be more likely to reflect the old concrete problems. Level up what you have with sand and or gravel as needed. A consistent thickness of new concrete will help with a more consistent drying time also. Put down your vapor barrier as you mentioned which is a great idea. Vapor barrier over sand is less likely to get punctures than over gravel. Two layers not a bad idea if your in doubt about keeping it unmanaged. Then install your rebar and pour new concrete as you mentioned with. Wire is better than nothing and both together are great. A thin layer of sand (perhaps a inch) over the old concrete then your vapor barrier will help isolate the new concrete from the old. Existing cracks will want to transfer and continue into the new concrete. The sand and plastic will help prevent this issue. The plastic directly under the new concrete will also help the new concrete to harden up more consistently over the whole area. Other wise the new concrete on top of the old will get hard slower, concrete over the gravel will get hard quicker and thus make it more difficult for you to get a nice even drying time and thus harder to get a even finish. The vapor barrier will also let the new concrete cure slower (different than getting hard) and be stronger. As egon suggested thicker is better. Less than 3 1/2 is kinda pushing it for many concrete mixes. No idea what your using or have available there. Working in that enclosed space your going to want a bit of extra drying time i suspect. Will you need skies or sliders or someway or something to walk or crawl on to allow you to finish the back of the room and move around on the concrete as it hardens? Might look into that.

Concrete Tools - Sliders | Kraft Tool Company

 

[3Ts]

I agree with Redlands and he gave you a more detailed answer than I was going to so i'll just confirm his post. Do you have any headroom height that you have to meet? You will lose 4 or 5 inches of height when you do this. Likewise, if you're going to put wheeled equipment in it, you'll need a ramp up to the new level.

 

[RNeumann]

Should work. Maybe go to four inches of concrete?

Second this. An inch or two of sand between the two layers will help too. The wire mesh is a PTIA and doesn't save much over #3 rebar. That would be my first choice.

 

[Tractor Seabee]

Use 4" of concrete, forget the wire mesh (it never stays where it needs to be) and use fibered concrete instead. Saw cut control joints as soon as you can get on it (next day is too late) on a maximum of 12' X 12' +/- 1-2 feet. The control joints are the most important thing you can do to have a crack free result. If you are painting the concrete wait till next summer and put joint sealant in the cuts to smooth out.

Ron

 

[freedomlives]

When I say wire mesh-- it is actually sheets of 6 x 10 foot grids of rebar, I think 1/4" that are spot welded to each other. There are different spacings, I would be opting for the largest, which is 6" grid.

The overall size of the garage will be 13' x 22'. An extra 4" of height isn't a problem. Right now, at the highest part of the existing slab, there is 9 1/2' to where the ceiling will be going. So if that turns into just 9', no problem.

Concrete mix-- however I want to make it. We live 25 miles from the city where there is a concrete mixing plant and that distance makes having a truck come quite expensive. Anyway, our driveway has a grape vine trellis over 40' of it, and this limits the size of truck that could get back there.

There are several different grades of portland cement sold, and I just buy the strongest grade to compensate for any mistakes-- it isn't much more expensive. I also bought an additive from this summer's work making an underground room for our vermicomposting wastewater system (california red worms now break down our sewage to a liquid) that prevents air pockets in the concrete and blocks out water. The agent of the company (Murexin) selling it suggested that I use a vibratory plate compactor and avoid a watery mix. I actually have a cheap vibratory plate compactor from another project in the past (again due to distance to the city and a project that needed the plate compactor on several different, non contigous days, buying a chinese one was cheaper than renting), so I'm hoping that's going to help here too.

The village has a 1/5 cubic yard mixer, so that's what we borrow when making concrete.

Hmm... I forgot about finishing it. I need to look and see what kind of tools are sold here for that purpose. My father-in-law and uncle-in-law I've never really seen put much of a finish on the concrete, and that is something I wanted to get better for the garage. And more or less level as well. I did see a product sold here that is a self-leveling screed suitable as a final finish for garage floors. While self-leveling screeds are miraculous, obviously better to get as close to level as possible before applying one.

So quite a bit to think about, but it is obvious I'll need 2 cubic meters of gravel, so that can be delivered tomorrow.

One more question then-- has anyone (particularly those living in climates that get decent winters, e.g. not Georgia where I'm from) put insulation under the slab? Does it make a noticeable difference?

 

[Tractor Seabee]

Your rebar scheme is a drastic overkill as you are going down on a stable sub-grade; so, differential settlement is not an issue. If you stay with it be sure to set it up tied to 1.5" high chairs. With that close of grid spacing do not walk on areas that do not have the concrete in place. Again you are wasting money if the re-bar is displaced by big feet. I still stand on the fibered concrete system for what is a minimal loading situation unless you are running heavy tracked equipment over it; then you need 6" of concrete. 4,000# concrete is more than adequate. You only need one 1.5" deep control joint in the center of the 22' length. If your garage is going to be heated then 2-4" of urethane or isocyanurate rigid insulation will definitely help with heat migration to the sub-grade.

 

[Redlands Okie]

Yep a rigid foam preferably closed cell under the rebar works well. Closed cell foam could be used under the vapor barrier or on top. Guess it depends on if you want the foam to protect the vapor barrier from gravel or rebar. If your foam is open cell and thus will absorb or hold water be sure its on top of the vapor barrier. That way it does not hold water from the ground over time and will insulate better. Any water it absorbs from the concrete will eventually be released.

 

[Redlands Okie]

As several have pointed out you can use fiber mesh if it’s aviable in place of wire mesh. Wire mesh that is wanting to roll up, or move up and down is a big problem as pointed out already. Fiber mesh was made to replace wire and all the problems of getting wire correctly located in the slab. Probably not form your description of supplies aviable. Wire mesh and fiber mesh does not replace the use of rebar. I know i know everyone has a opinion on it but if you look up the manufactures of the products their specs and info will show you that rebar serves a different function and wire mesh and fiber does not replace rebar. Both are good products and have their purpose but personally if i had to pick one or the other i always go with rebar. Your 1/4 welded wire grid might be a interesting compromise on the subject. Especially if it’s stiff and not very flexible.