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]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

Bare soil - what are the potential problems?

[Ken524]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

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

[Wiley]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

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]

Re: Foundation slab over existing concrete?

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

[Breven]

Re: Foundation slab over existing concrete?

Good question T. First, I don't know if those cut off blades are going to do the trick? I bought a very cheap diamond blade at Home Depot. It cuts right through it. I bought one for my angle grinder too and I've used it a ton throughout this project. Again, these are cheap diamond blades though- not the cutoff blades. So if those don;t do the trick, try the diamond blade.

A wall is sure to have a deep footing. If the slab you are looking to cut, butts up against the wall, my guess is that it's not helping to hold the wall up anyway- so cut away. Also, if you are simply removing the slab, to pour a 6" rebar reinforced slab in it's place- then that would be better for the wall too, right? Of course, if you are 9" away, it won't matter at all.

[Knackers]

Re: Foundation slab over existing concrete?

I always look at it this way....

Would you put a waterbed in a 2 storey bedroom above your lounge room below??????

would you put a fridge in the same room????

Well in actual fact a fridge has more weight per square foot than the waterbed.

....lets face it if you dug the concrete up & left it bare dirt would the dirt be more solid or would the concrete be more solid???

leave the stuff there & build on top & if you are worried this is what I would do........ in the first course I would cut the center web of the blocks out as well as the ends of the block down about 1/2 way, this will alow 1/2 inch reo bar to be laid horizontally I would put 2 bars in there & then lay your normal blocks above it & then corefill......this would tie all the blocks togeather & it wont go anywhere. you are esentually spreading a fair bit of wweight over a large area..

wont be a problem

[christo]

Re: Foundation slab over existing concrete?

This may be total insanity - but here is what I would have done....

if I was sure the ground was compacted well and the concrete was in there for a few years and remained stable....

I would form up 2x8's on top of the pad and pour a rebar reinforced base right on top of the pad. To me that would ensure that the load is equally distributed and supported on the slab. I'd size the pad so I could stack block right on top. I might drill a few 1/2 inch holes into the existing pad to key everyting together, too.

If I did not know what was under the exisiting concrete - I'd cut it out - I've seen some pretty odd things after removing failed concrete patios. (voids from what I think was once a stump, 55 gallon drum, general construction debris, etc)

Christo

[Knackers]

Re: Foundation slab over existing concrete?

Exactly & its not insane..............he seems sure that the slab is fine.... so why not leave the thing there....

Honestly I would just build straight on top of the existing & not even pour a slab on top of the slab...

[Breven]

Re: Foundation slab over existing concrete?

Knackers- A 3 1/2 inch slab, not reinforced with no footings is too weak for this oven. Even if he poured a new 5 1/2 inch slab on top, it's the dirt under the existing slab that might be a problem. Is it comression tested for 3,000 lbs? Who knows what is under the existing 3 1/2 in. slab. Like Cristo said, could be holes under there, or sand.

Plus, how would the new slab not "float" on top of the old slab. It would have to seperate...especially if the earth moves at all. Like a relief crack in a sidewalk- always cracks. Two different pours on top of eachother, they will crack in-between. Right? Any movement from the ground, will transfer up- but worse. Like a tall building in the wind...doesn't move much at the base, moves a ton at the top. If you have a brick dome on top, movement = more cracks.

I still think unless you know what is under the original slab, which most people won't unless they laid it, it's best to cut out a square and pour a nice solid, reinforced slab, with footings. It's like building a house on sand...might hold up, might not....why take a chance? Literally, it's 1 extra day of work on this big project and maybe a few weeks of sneaking concrete scraps into the garbage bin. Totally worth it in my opinion.