Author Topic: A little different  (Read 522 times)

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Offline Whiskyb

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A little different
« on: September 09, 2014, 08:12:50 AM »
I am back on track with my wfo build. The base was built with concrete blocks filled with cement. My wife and I love an outdoor fire so where most people have a wood storage under the hearth I have a zero clearance insert originally intended for indoor residential use. It vents out the back into a stainless lines chimney.

My hearth is 4" thick concrete and rebar. Yesterday I poured my 5:1 vermiculite/concrete pad on top.
My question is should I use normal concrete to skim coat this? I have access to castable at 51/bag and flue set dry mix at 110/bag

These prices seem high to what some have been able to buy, medium density bricks are 1.66 a piece

My plan was to use the brick for the outer ring and then fill with sand and use the castable for the lid.  If I were to go with a brick lid what is the best mortar to use out of what I can access.

Its covered right now to keep it out of the weather but I will load some pics when I open it up

Thanks for any advice


Offline Tscarborough

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Re: A little different
« Reply #1 on: September 11, 2014, 07:46:54 PM »
Technically, I do not see any issue with your plan, and have a similar design with an existing cast oven cap on a sailor course of firebrick.

Offline stonecutter

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Re: A little different
« Reply #2 on: September 11, 2014, 10:13:16 PM »
I am back on track with my wfo build. The base was built with concrete blocks filled with cement. My wife and I love an outdoor fire so where most people have a wood storage under the hearth I have a zero clearance insert originally intended for indoor residential use. It vents out the back into a stainless lines chimney.

My hearth is 4" thick concrete and rebar. Yesterday I poured my 5:1 vermiculite/concrete pad on top.
My question is should I use normal concrete to skim coat this? I have access to castable at 51/bag and flue set dry mix at 110/bag

These prices seem high to what some have been able to buy, medium density bricks are 1.66 a piece

My plan was to use the brick for the outer ring and then fill with sand and use the castable for the lid.  If I were to go with a brick lid what is the best mortar to use out of what I can access.

Its covered right now to keep it out of the weather but I will load some pics when I open it up

Thanks for any advice

You don't need to skim coat the insulating concrete, unless you plan to stucco the whole slab and oven.  What are you doing....enclosure or stucco on the dome?

As for casting the dome, it's been done...mass is mass for the most part.  The trick is to cast, cure and dry it so that the material doesn't develop structural cracking. 

If you build the dome with brick, 'best' depends on your circumstances, because the mortar is fairly forgiving.  You can use a component mixture of sand,Portland,lime and fireclay for fire mortar or buy a refractory mortar ( though it tends to shrink in large joints) or you can pour castable into the voids after building the dome.
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Offline Whiskyb

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Re: A little different
« Reply #3 on: September 13, 2014, 08:25:01 AM »
I am liking the idea of pouring castable between the joints of firebricks. I have found a supplier of whiteacre greer bricks and heatstop/heatcast. Bags are 45, much better than previous quotes.

What would the minimum air temp be in order to still allow the concrete/castable to set up properly?

Offline stonecutter

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Re: A little different
« Reply #4 on: September 13, 2014, 10:44:16 AM »
I don't see any reason why it would be different than standard concrete or mortar, which is anything above freezing.  The lower the tempurature the slower the cure and vice versa.
« Last Edit: September 14, 2014, 09:00:20 AM by stonecutter »
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Offline Whiskyb

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Re: A little different
« Reply #5 on: September 14, 2014, 08:40:32 AM »
I wil be picking up some supplies this week. Is a layer of castable any good for the oven floor? Are bricks better?

Offline stonecutter

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Re: A little different
« Reply #6 on: September 14, 2014, 09:10:44 AM »
I wil be picking up some supplies this week. Is a layer of castable any good for the oven floor? Are bricks better?

A couple things.
You need to consider the fact that a large area of castable, even in two, three or four pieces, will have a larger surface area subjected to heat tensioning during the firing. A smaller unit like brick is able to balance itself out faster, and that greatly reduces the possibility of cracking. The smaller units are able to take up the expansion, and the seams work like expansion and control joints.   And how a castable performs will depend on how it was made and cured.  You will also want to consider what the thermal conductivity of your castable vs your brick. 

So, I can't say one is better than the other, because you can find positives for both materials, both in durability and compatible thermal conductivity numbers.
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Offline Whiskyb

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Re: A little different
« Reply #7 on: September 14, 2014, 10:38:46 AM »
Bricks it is

Offline stonecutter

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Re: A little different
« Reply #8 on: September 14, 2014, 01:17:18 PM »
That was easy.....

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When we build, let us think that we build for ever.
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Offline Whiskyb

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Re: A little different
« Reply #9 on: September 21, 2014, 11:26:24 AM »
does the vermiculite/concrete ever harden? I have it covered to keep it dry but it still seems crumbly. How hard does it need to get before covering it with a layer of refractory cement


Offline stonecutter

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Re: A little different
« Reply #10 on: September 21, 2014, 11:46:35 AM »
That's the nature of it...crumbly or friable.  What was your cement:vermiculite ratio?  Why are you skimming it with refractory mortar?
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When we build, let us think that we build for ever.
John Ruskin

Offline Whiskyb

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Re: A little different
« Reply #11 on: September 21, 2014, 12:33:10 PM »
I did a 5:1 mix. I was skimming it just to hold it together and seal it. Being in the open and the fact that I move slowly I wanted to protect it  from the elements until I get the brickwork done. Which may not be till the spring depending on the weather.

Offline stonecutter

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Re: A little different
« Reply #12 on: September 21, 2014, 01:55:05 PM »
Don't rely on a skim coat for weather protection, think about a tarp. 
http://oldworldstoneandgarden.com/


When we build, let us think that we build for ever.
John Ruskin