Foundation slab over existing concrete?

[Dancing Bear]

Re: Foundation slab over existing concrete?

We've decided on a change of location - we're moving it into the poolhouse that we're building out of what used to be the garage - so it now has a 6" thick, 60 year old slab with no cracks in it, topped with 3" of new reinforced concrete from bringing the floor up to meet the rest of the new deck. I think that's gonna hold OK.

Now we're in the throes of kitchen design. At least I know which corner it goes in so I can start piling up cinderblocks.

Thanks for the help!

[Frances]

Re: Foundation slab over existing concrete?

For all its worth, I believe Dave's foundation moved slightly, causing cracks in his oven and one of the bricks in the entry to fall out... on the other hand it has to be said that his oven still works beautifully, so when all is said and done, maybe even a shifting foundation doesn't matter that much.

Personally though, looking back on all the work you put into building an, I'd go for the safer foundation.

[Knackers]

Re: Foundation slab over existing concrete?

I hear you man...I may go a little over kill, You really have to with all the ground shaking out here in California. One thing about the foundation though...you only get a chance to do it once. I don't think its a negative to make it "too sturdy". It will likely hold up either way... I just always like to err on making sure it holds up, because you can't go back and re-do it.

Yeah fair enough..... I am originally from New Zealand ... I can remember book cases falling on my head in the middle of the night as an earth quake decided to make its presence felt. I have lived in Australia for 30 years & I tend to forget that not all the places in the world are built on stable ground & never shake.

& Yes you are correct there is only one chance to do footings... & I suppose you are right no time like the present to engineer it corectly.

[Breven]

Re: Foundation slab over existing concrete?

I hear you man...I may go a little over kill, You really have to with all the ground shaking out here in California. One thing about the foundation though...you only get a chance to do it once. I don't think its a negative to make it "too sturdy". It will likely hold up either way... I just always like to err on making sure it holds up, because you can't go back and re-do it.

[Knackers]

Re: Foundation slab over existing concrete?

I was only going on what Dancing bear stated originally..

3 1/2 inch reinforced slab....... & they seemed to think that the ground was compacted b4 the original slab went in. (I was guessing that they probably did it.)

honestly I would do what I stated originally. at Most I would do what christo said & build a reinforced slab on top of original.....

the whole weight of this thing isnt going on to a 1 foot x 1 foot square pad..... its probably 3 meters x 3 meters square.... thats a lot of bearing area if the original slab and the blocks are reinforced then there in no problem

[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.

[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...

[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?

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

[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.

[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

[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.

[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

[Ken524]

Re: Foundation slab over existing concrete?

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

[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...