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-   -   Supporting overhanging concrete (http://www.fornobravo.com/forum/f28/supporting-overhanging-concrete-12203.html)

kebwi 03-15-2010 11:26 AM

Supporting overhanging concrete
 
My hearth currently lies 5.5" below the oven floor: 3" InsBlock + 2.5" firebrick. My current plan is to surface the hearth with 1/2" (ish) 12" granite tiles from HD.

Quick side question: is that kind of tile thick enough to withstand an external application or do I need a real granite "slab"?

So, assuming I want the hearth to be a shave lower than the oven floor (so rain doesn't flow in), that's a target height of about 5" which minus the granite is a residual 4.5" of "fill". My plan is to pour concrete up to that base of the granite, so about 4.5" concrete on top of the current hearth. At the same time I will be extending the hearth to one side some to make a staging area. That side area will be supported by a second low wall, just like the original four-block high base.

But, to finally come to my original question, I'm also considering extending the hearth out from the front a bit, i.e., having this additional concrete slab hang over the original hearth "into the air" about eight inches (remember it'll be about 4.5" thick, maybe 4", I don't know).

So I'm thinking I'll just suspend some 3/8" rebar throughout the poured area such that the rebar crosses the edge of the overhang in a few places. Likewise, the rebar will tie the extended side table (and side wall) into the new poured slab in one solid piece.

Does this sound about right or are there more serious concerns? Do I need to brace the concrete from the bottom for example, or should I use 1/2" rebar perhaps.

Thoughts?

Thanks.

kebwi 03-16-2010 09:00 AM

Re: Supporting overhanging concrete
 
Is a lack a response an indication of a dumb question. :)

i.e., bump.

lwalper 03-17-2010 02:59 AM

Re: Supporting overhanging concrete
 
Well, I don't know about a dumb question, but I'm going to have a small (about 12") cantilever on my barrel stove. I've got a building that I'll be using as a kitchen and have built the oven about 6 inches away from the building. It took 4 1/2 courses of block to get me up to the floor level of the building. The cantilever will extend through the wall far enough to give something solid to lay the interior brick work on so it will be flush with the finished interior wall. I'll be pouring the block cores and using 1/2" rebar in the vertical cores next to the building, and then extending the concrete into the building with 1/2" rebar tied horizontally to the vertical supports. Then, the build continues upward with most of the mass being supported on the existing vertical wall.

dmun 03-17-2010 05:49 AM

Re: Supporting overhanging concrete
 
I think the question was about adding a cantilever to an existing slab, and I think the silence was a response to not knowing the answer. Maybe a sketch or drawing would clarify the question?

sjmeff 03-17-2010 06:47 AM

Re: Supporting overhanging concrete
 
Keith - this has been done by a few folks here. Here's the one that I first saw and that I copied on my build: http://www.fornobravo.com/forum/21/b...html#post39885

I raised the existing slab several inches and added a ~3" cantilever. I then put a granite slab on top of that (which overhangs about 1.5"). I arranged my rebar in a similar fashion to the photos above, though I had two pieces across the front rather than one.

S

lwalper 03-17-2010 07:59 AM

Re: Supporting overhanging concrete
 
Oh. The way sjmeff did it looks solid, with the rebar going farther back like that.

kebwi 03-17-2010 08:58 AM

Re: Supporting overhanging concrete
 
Quote:

Originally Posted by dmun (Post 84857)
I think the question was about adding a cantilever to an existing slab, and I think the silence was a response to not knowing the answer. Maybe a sketch or drawing would clarify the question?

Fair enough, I'll get back to you. Thanks.

Wiley 03-17-2010 10:56 AM

Re: Supporting overhanging concrete
 
Keith,
Short answer (well short for me anyway). Easy to do, just keep in mind that concrete is good in compression and poor in tension. So place your rebar in the top 1/4 to 1/3 of the slab as in your cantilever that is where the tension lies. Placing it in the center (the force neutral area) works to hold the cracked/broken pieces together but otherwise is far less efffective in actually preventing a break in the first place. As for quantity of rebar, more is better and if you can, bend a hook in the ends. Or if you have access to a welder weld a piece of rebar across the ends of the extending rebar. This will not only be stronger but easier to orient in the slab and hold in position while pouring the slab.

Hope this helps,
Wiley

kebwi 03-17-2010 11:47 AM

Re: Supporting overhanging concrete
 
Is 3/8" rebar sufficient for a six to eight inch overhang or should I use 1/2"?

Wiley 03-17-2010 12:42 PM

Re: Supporting overhanging concrete
 
You can probably get by with 3/8 inch (especially if you are bending hooks in the ends) More important is how many pieces and their position in the finished slab, personally (as in what I would do if I were doing it) I would not use less than a piece every 12 inches and would think more on the order of every 8 inches would be better. And like I said in the top 1/4 to 1/3 of the slab.

If this was Haiti then you wouldn't use any at all. The Haitian method would work, well sort of, for a while, until it didn't. Rebar is fairly inexpensive and so I would err on the side of "more". I tend to over build. But then in case of earthquake I expect it to be there afterward.

Hope this helps,
Wiley


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