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jmkvet 08-14-2009 02:13 PM

Reinforced concrete
I've just had a delivery from the local builders yard to get started on my oven. Unfortunately i've managed to order 1/4" rebar for the foundation and hearth. Will this be strong enough for my oven stand or do i need to take it back?

ThisOldGarageNJ 08-15-2009 05:18 PM

Re: Reinforced concrete
do you have enough to double it up,, ?

pacoast 08-15-2009 09:24 PM

Re: Reinforced concrete
1/2 or 5/8" would have been better, but you're probably still okay. Double up the rebar if possible. If you can't, then you might be able to add cement reinforcing fiber to your mix to add strength. You could also place the rebar in the upper third of the slab. This will effectively put the lower 2/3 in compression, while the tension is in the upper third. Concrete is strong in compression, not so much in tension. Since most of the force here is compressive from the oven weight, it adds effective strength to your slab.

jmkvet 08-16-2009 01:37 PM

Re: Reinforced concrete
Thank you folks, I've got loads of the stuff. When you say double up do you mean attach the two meshes together or leave an inch or 2 between then?

ThisOldGarageNJ 08-16-2009 07:15 PM

Re: Reinforced concrete
leave an inch or two between... and put in as much as you have, Im pretty sure it cant hurt. Anyone else care to comment...

pacoast 08-16-2009 10:33 PM

Re: Reinforced concrete
Yup, two layers of rebar, more or less in the middle of the slab (vertically) and they should have an inch or two of vertical space between them. The slab will be a lot stronger if you can get some space between the two layers as the concrete can grip the rebar on all sides then. If you can stagger them slightly, even better but don't sweat it if you can't.

More is better within reason, but too much rebar can weaken a slab significantly. Rebar is good in tension, concrete in compression, so you need both, but still more concrete than rebar.


david s 08-18-2009 01:46 AM

Re: Reinforced concrete
The bar is better placed closer to the bottom of the slab so that it is in tension.

pacoast 08-18-2009 04:08 PM

Re: Reinforced concrete
Having the rebar toward the bottom of the slab, maximizing tension would be best if the compressive weight of the oven was evenly distributed or centered on the slab.

But it is my understanding that the weight of the oven is mostly around the perimeter where the walls (dome) sit on the slab. Or at least that is what every oven manufacturer told me when I inquired about this. If true, then the rebar should be high in the slab to maximize compressive slab strength as most of the weight bearing will be on the lips of the slab.


Neil2 08-18-2009 05:25 PM

Re: Reinforced concrete
Put it in the middle.

In a typical 4-5 inch suspended slab, rebar placed in the middle will go into tension well before any concrete gets near its compressive limit. Also you get better cover for the rebar.

It would take an amazingly detailed analysis to determine what the bending moments are throughout the slab and it dosen't really matter that much.

pacoast 08-18-2009 06:16 PM

Re: Reinforced concrete
For anyone trying to make heads & tails of this, you should probably listen to Neil2. The general purpose & conservative advice is to place rebar in the middle of the slab. Recommendations to place it higher or lower are less common & usually relate to disproportionate loading situations.


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