hearth floor construction

[avkroth]

I am building a 36" dome oven. The base is ~50"x70" for the hearth floor with 2" angle iron on 16" center. I intend to use 1/2" concrete board and then a mixture of portland cement, quickcrete concrete, vermiculite with rebar and make that layer ~2" and set my 24x12x3 firebrick slabs on that.

Question is do you think the mixture of concrete, cement and vermiculite is OK?

[Lburou]

Re: hearth floor construction

Welcome to the forum.

In my mind, you need a structural base (my hearth base was 3.5 inches thick with angle iron and rebar), then an insulating layer (I used a two inch insulating board), then the oven floor (2.5 inch firebrick). I recommend you download and read the pompeii oven plans, they lay out the process for a strong base/hearth structure and a proven oven. Quickcrete will suffice for the hearth, but I would not mix the vermiculite in the structural base. Cast the vermicrete layer over the structural base/hearth. HTH

[boerwarrior]

Re: hearth floor construction

I agree with Lee. What you describe will not insulate well and you will likely have difficulty getting the floor temperature up to where you want it. I used 3 inches of insulating board over a concrete base and I am very happy with the results!

[stonecutter]

Re: hearth floor construction

.....and then a mixture of portland cement, quickcrete concrete.............

There is no need to add Portland cement to a bagged concrete product.

[Tscarborough]

Re: hearth floor construction

That depends Stonecutter. A lot of what is sold in big box stores is only rated at 2500 PSI, with actual breaks well below that. If it is their lowest cost product, a shovel per 80# bag is a good thing.

[stonecutter]

Re: hearth floor construction

Yeah, I agree, but for an oven slab its overkill.

[Tscarborough]

Re: hearth floor construction

For the slab, yeah, but I thought he was talking about the hearth.

[stonecutter]

Re: hearth floor construction

Yes that's right, but I saw he is installing angle at 16" oc for the hearth slab. So 2500 is plenty strong with that kind of support. It's possible that his bag mix is the 5000 psi stuff....and more porty is way overkill that too.

[stonecutter]

Re: hearth floor construction

I should just say I agree that a shovel of porty is good insurance for a #80 bag...and cheap. It is possible to add too much Portland in a mix design, but I think I am over thinking this.

[kanoer54]

Re: hearth floor construction

just gotta ask....to clear my own mind...what happens if there is too much portland in a mix?

[stonecutter]

Re: hearth floor construction

Brittle concrete

[Tscarborough]

Re: hearth floor construction

Shrinkage cracking primarily, but ultimately if it is out of proportion it will weaken the mix design in flexural strength.

For the foundation slab, almost anything is strong enough, even the "fence-crete" garbage the big box stores sell as is. For anything with a span or cantilever you need more flexural strength which is tied to compressive, and the thinner the more important it is. If you are pouring a full 4" slab for the hearth, then a 2500 PSI is adequate. If you are slimming it down to 2-3" it is safer to use something with a higher flexural strength.

Flexural strength is not a standard that is used to designate bag mixes though, so you will never see it listed, and must infer it from the compressive.

[kanoer54]

Re: hearth floor construction

what role does the rebar play into the flexural strength of concrete?

[Tscarborough]

Re: hearth floor construction

Almost all of it, provided it is placed correctly.

[kanoer54]

Re: hearth floor construction

so if two layers of bar are used, the top mat is under compression and the bottom mat would be under stress?