Perlte Concrete 1st layer

[gdest]

Well 1st of all I have tried a number of times to post questions and also responses with each time a message came up that my post had to be reviewed by the system admin or moderator or something like that.

This is my 1st attempt starting a new thread - Hope it works.

I have just ordered the recomended books for building my own outdoor WFO and I am waiting for them to be delivered. However before I get them a stupid question comes to mind - I have gone online and have watched a number of your postings on the construction and one thing is puzzling. In most cases, not all, the 1st pour on the supporting slab is perlite concrete. This is not a structural concrete. I would think that this concrete would be better suited under the refactory or fire brick.

Can anyone shead some light on this matter. If I'm lucky enough to get this thread to post.

[DaveW]

I was worried about the same thing. But the perlite insulating slab (poured over the steel reinforced concrete slab) only has to hold up under compression. Normal concrete is good to 3000psi. The insulating slab only has to support a few pounds per square inch. Practically nothing.
Dave

Edit: There is another design out there where the insulating slab hangs beneath the structual slab via nails set at angles between the two layers

[Fairview WFO]

That's correct, the perlite layer only has to support the weight of the dome, and if you mix it in the proper ratios, it will have no problem doing just that.
If you add a stone veneer around your dome as I did you will probably want to keep the perlite layout out from under that as the weight of all the stone can get up there, or at least if you go as high as I did.

[Tscarborough]

Here is a calculation I did in another thread:

Do the calculations as to actual loading and you will be surprised. Assuming a 36" firebrick dome and a 3000 pound load of firebrick, parging, etc, that gives you a floor loading of :

38.25(center line of firebrick for a 36" diameter dome) * 3.1416 = 120.2 (circumference) x 4.5 (width of firebrick) =540.75 Square Inches of bearing surface, more or less.

3000 / 540.75 = 5.54 pounds per square inch compressive weight on your substrate.

Normal concrete has a compressive strength of around 3000 PSI and pink closed cell foam insulation is around 25 PSI so other than shrinkage issues as it dries, no worries.

[dmun]

You mentioned the purchase of books. One widely read book on oven building has the perlcrete insulation layer under the supporting slab. Avoid this method like the plague, as heating up the reinforced concrete slab will prevent your oven from reaching pizza temperatures without burning forests of wood.

[Neil2]

I estimate that a 5:1 vermiculite:cement mix is good for about 40-50 pounds per square inch.

[nrobert]

I'm also wondering the same thing here. Is this using the reguler vermiculite that you can get from garden centers for growing aplications or is there a certain type used in this aplication which is designed to support the load?

[Tscarborough]

Cement/Vermiculite 28-day PSI
1/8-----------------70-125
1/7-----------------125-140
1/6 -----------------135-175
1/5-----------------175-225
1/4-----------------225-325

Those are lab made and cured numbers, in the field it is probably 70% of those figures.

Regular vermiculite is OK.

[nrobert]

Thanks a lot Tscarborough.

[Neil2]

Thanks Tscarborough.

Even using a 2x safety factor my 50 psi is conservative.