Foundation slab over existing concrete?

[Dancing Bear]

I am planning to build my oven in a spot where there is already a 3-1/2" reinforced patio slab. Should I:

1. Pour an additional slab on top of it (and should that be the full 5-1/2", or would another 3-1/2" be OK),

2. Remove a section of the existing slab and pour the foundation from the ground up,

Or

3. Just leave it alone and build on top of the existing slab?

I'm favoring option 1; does anyone have insight to provide?

Thank you -

Bill Stewart

[dmun]

The worst case scenario is that the uneven pressure on the existing patio could cause it to crack. You could always have a concrete cutting place cut a joint between the oven foundation and the patio, if a patio crack is going to cause agitation.

Also, is the patio poured on a proper bed of crushed rock? A lack of drainage could cause a problem. A lot of patios are just poured on bare soil.

[Dancing Bear]

Bare soil - what are the potential problems?

[Ken524]

DB,

Pouring concrete on top of concrete won't improve the foundation below the patio (that eliminates the first 2 choices). If the slab is on top of dirt, there could be hollowed out areas from drainage and settling; especially if it wasn't compacted. This could lead to some serious cracking.

Your oven will way a ton. More than a car. No way would I put a car on my patio. It's just not designed for the weight.

My first choice would be to remove a section of the patio (no need to remove the whole thing) and put in a compacted gravel base, followed by rebar and 5.5" of concrete.

2nd choice is Dmun's idea (pretty darn clever!). Have someone saw an area around where the oven will be.

[Dancing Bear]

OK - the soil beneath the patio was compacted prior to pouring, so it should be pretty solid. I'm liking the idea of sawing out the foundation area so that it moves independently of the rest of the patio.

[Breven]

I had to cut out a nice portion of an existing slab in order to pour a solid foundation. It was really easy. I bought a $15 diamond blade for my skill saw, grabbed the garden hose to keep the blade cool and cut away. It was worth the effort to know I'd have a solid foundation.

Here's some pics...

[Ken524]

Breven's idea really is the strongest and best way to go.

[Wiley]

Let's say the final weight of the oven is 4000 lbs and for the sake of arguement say the base is only 60 inches by 60 inches (most are bigger) that's 3600 square inches for basically a 11 lbs per square inch load. Now even the cheap crappy "just add water" concrete is rated at 4000 lbs per square inch compressive strength. That's a pretty hefty safety factor IMHO.

Seems like alot of people think a couple of tons is alot of weight, and it is if it's on your foot. But in construction, weight is figured in pounds per square unit surface area, and for concrete in compression the weight we are talking is not out of line for a 3 1/2" slab especially seeing as how the weight is spread over a fairly large area.

Now all this is assuming reasonable construction techniques were used to lay the original slab (compression of the subsoil etc.) and that the slab was made of minimum standards concrete. All bets are off if we're talking inferior materials and workmanship.

So how old is this existing slab? If it's been in place and shown no problems for a couple of years I would go for. If you want to play it safe then saw out an expansion/ "crack along this line" section and build on it.

Just my opinion,
Wiley

[dmun]

Bare soil - what are the potential problems?

The usual problem is frost heave, but I see you are in Charleston SC, where it may not be so much of a problem.

Full disclosure: part of my oven/fireplace combo is supported by an ancient and fairly decrepit garage floor slab, although most of the weight of the two story chimney is over the below frost line footings that I poured. It hasn't caused any problems that I know of, but the slab is covered by furring strips, sub and finish flooring, so I wouldn't be seeing any cracking if it occurred.

[ttriche]

Originally Posted by Breven

I had to cut out a nice portion of an existing slab in order to pour a solid foundation. It was really easy. I bought a $15 diamond blade for my skill saw, grabbed the garden hose to keep the blade cool and cut away. It was worth the effort to know I'd have a solid foundation.

This seems to be the way to go for my installation. I went to Harbor Freight, bought a $20 grinder and $10 worth of cut-off blades, and have an approximate footprint for the oven slab marked off.

However, I'm in a corner, and there is a wall that forms that corner, which has stood for many years and several earthquakes without issue. I think it may serve as a retaining wall, since my neighbor's lot is a bit higher than mine on that side.

Is it still a good idea to cut through the existing slab and joint the new one? I am a tiny bit concerned about the stability of the wall, though the more I think about it, the more I doubt it would actually be an issue. I would be back to having a 9" stand-off or thereabouts, but so what. If I cut the slab footprint 6" in on both sides (from the wall), is that going to be plenty so that the oven can "move" (hopefully "stay put", more like it) independent of the wall?

thx,

-t