Concrete base mix?

[djginivisian]

Hi. This is my first post. I think I have done well to this point, but have hit a roadblock.
I am ready to pour the concrete base which sits on the cinder block stand.
It is a corner build. The base will be cantilevered 18" on the right and 12" on the left side and will curve in the front.
I believe I have enough rebar support throughout.
This project is in Massachusetts and I am hopeful that the weather (falling temperatures)is still going to hold out for a successful pour/cure.
What is the best concrete mix/ratio to use? I intend on using insulating fiberboard above the base/below the hearth so I am not anticipating using vermiculite.
All suggestions/ideas/concerns are helpful.
Thanks in advance.
David G

[JRPizza]

David, I just used quickcrete high strength mix in bags bought from the local hardware store. Pretty much followed the directions on the bag for how much water to add. Lots of threads on mixing concrete for the hearth. I don't see your curved form in the picture for the landing of your oven, have you added it yet? Also, I'm sure it's overkill but if you have any rebar left I'd be tempted to add a diagonal from the rear of your hearth out to the pie shaped area in the front that is kinda hanging unsupported. Since I had no idea what was needed, I tended to make the hearth as supported as I could "just in case".

[Gulf]

What is the best concrete mix/ratio to use?

David,

Are you planning to mix from scratch or wanting to know what bagged concrete to by? I did both. My stand foundation was mixed from scratch. My hearth slab was poured with bagged quikcrete. Can you post some more pics of your hearth slab form? It appears to be formed with 2X8" lumber (approx. 7½”" thickness for the overhang), with about 5½” thickness directly over the stand.

[david s]

If making your own mix the standard 3:2:1(by volume) of aggregate, sand, cement is way cheaper than bagged product. If you have temporary wooden supports for the formwork you should have wedges under them for easier removal less likely to crack your slab. By cantilevering you have the support better placed under the heavier parts of the dome. You might consider casting a few small holes near the centre of the slab that will help removing moisture from the floor insulation.

[djginivisian]

david s: Could you explain "casting a few small holes near the centre?'

Gulf: I will try to get some new pics posted this week. Your estimates are very close. I wasn't sure if this wouldn't be too thick/heavy. I am still considering pouring the top half with a vermiculite/perlite/plicast layer...I am going to use insulating fiberboard regardless. Have seen some builds use a double layer of fiberboard?

JRPizza: Correct. I still have to work on the curved form. Currently looking at 1/4" masonite? , which should bend right for me. Would also like to run some type of curved moulding on the inside of the curve (top & bottom) to create a 'bullnose' effect for the countertop. So far only found a 3/4 " (reverse) round, would like/need a 2" round.

Thanks everyone for your interest and assistance.

David G

[david s]

A few holes near the centre of the supporting slab help to allow any moisture build up in the underfloor insulation layer. This is more important if the under floor insulation has been made from a wet vermicrete mix. If you use insulating board it is already dry, but still porous so having somewhere for the moisture to escape by is not a bad idea. You can drill these holes after the supporting slab has set, but it's easier to cast them. Polystyrene is good because it's easy to knock out.

[djginivisian]

Hi David,
The holes are a good suggestion. It is not something that has come up often in my research etc.. A few questions: How large diameter? How many holes? How far is the spacing? Dead center under the cooking floor?

Thanks
David

[david s]

Hi David,
The holes are a good suggestion. It is not something that has come up often in my research etc.. A few questions: How large diameter? How many holes? How far is the spacing? Dead center under the cooking floor?

Thanks
David

My oven is small (21") so I have only one in the centre, about 1" diam. For a larger oven maybe a few more. As an illustration of why such a weep hole is an effective design feature I include this example. The same principle applies to some kind of vent in the dome shell for igloo style ovens.

The last new electric kiln I broke in required slow firing (empty) before being used. At around 400 C (750 F) water was dripping out at around one/sec creating a pool under the kiln. Moisture being pushed out condensed against the outer sheet steel skin and gravity working to help find its way out at the bottom. What I found interesting was that this occurred at such a high internal temperature. Higher than I would have expected.