Supporting overhanging concrete

[kebwi]

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]

Re: Supporting overhanging concrete

Is a lack a response an indication of a dumb question.

i.e., bump.

[lwalper]

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]

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]

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]

Re: Supporting overhanging concrete

Oh. The way sjmeff did it looks solid, with the rebar going farther back like that.

[kebwi]

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?

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

[Wiley]

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]

Re: Supporting overhanging concrete

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

[Wiley]

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

[fxpose]

Re: Supporting overhanging concrete

Sorry for the off topic but since you brought up 3/8" rebar, how is it that the 20' length of 3/8" rebar is cheaper than the 10' length at Home Depot? For this reason I've been buying lots of 20' lengths and bending them in half by hand so I can carry them in my vehicle.
I'm considering using only 3/8" rebars on my oven build.

[kebwi]

Re: Supporting overhanging concrete

Yep, that's how I built my stand and hearth. Pipe-bent 20' 1/2" rebar in the HD parking lot, threw it on top of the car, angle-grinded and bent it as needed at home.

[fxpose]

Re: Supporting overhanging concrete

But in the case of 3/8" rebars, 20' lengths cost $2.77 and 10' lengths run $2.97 ea. That's what I was trying to get at.
Half-inch rebars, OTOH, are priced accordingly, 20' lengths more expensive than 10' lengths.

[Wiley]

Re: Supporting overhanging concrete

Keith if you are carrying it on top of your car it would indicate that you have some sort of roof rack. If so, the way I carry 20 ft lengths is to tie a 20 ft 2x4 on my lumber rack at home before I go to the yard. I buy and then tie the 20 ft lengths to the 2x4. Often I simply use many wraps of duct tape if there is only a few pieces of rebar. The 2x4 keeps the rebar from flopping about and makes tie down easy.

As for bending I cheat and use the blade on the excavator. Simply clamp the rebar between a 2x12 and the blade and then slide a pipe section up the unclamped piece. This makes for easy precisely located bends of fairly close radius. Note: FYI there is a minimum radius for bends in rebar (code thing, for inspected construction).

fxpose, I haven't a clue as to how or why they price the way they do at HD. I try to buy as much of stuff like wood at either of two local construction lumber yards (one 15 minutes the other 20 minutes away, close off island). Nails, tools and fasteners etc. are much cheaper at HD but the other stuff including rebar are less expensive at a construction type lumber yard. The lumber sold at HD sucks!

Bests,
Wiley

[kebwi]

Re: Supporting overhanging concrete

20' is simply too long to be hanging off the front and back of a Forester. I fold it once in the parking lot, take it home on the roof rack, do the rest of the cutting and folder at home.