#21  
Old 03-22-2007, 08:36 AM
Marcel's Avatar
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Thumbs up Our ovens are structurally quite forgiving.

(dmun) "I think the whole question is how much weight are you going to put on the casting. My guess, and it's just a guess, that a couple of two-foot sections of 8" square flue tile would be no problem for an un-reinforced vent casting, but a couple of stories of full flue tile stack would."

(M) I agree with David, above. I didn't cast a vent but used angle iron to support a horizontal "slab" of bricks with an opening for the vent. I would not do that a second time but rather cast a vent. Still, I have had no trouble with cracking, etc.

(M) I read with great interest the discussions of structural properties of various refractory reinforced concrete vents. I have yet to read on this forum any reports of vent failure with any vent system, so perhaps our ovens have quite a large degree of structural forgiveness inherent in the relatively low (900 F) degrees of heat to which our vents are exposed.

(M) I worried about the weight of flue tiles that David mentioned and opted instead for single wall metal venting beyond the first and only flue tile. I opted to reinforce the steel walled flue with Perlcrete insulation embedded in chicken wire. Possibly I should have chosen refractory mortar instead, in order to maintain more heat in the flue. I have no scientific documentation to support an inference that Perlcrete is more flexible than refractory mortar but it works, and "if it ain't broke, ... don't fix it."

(M) Here is a previously submitted image of the beginning of the Perlcrete reinforced metal flue on top of the single ceramic flue tile:





Ciao,

Marcel
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  #22  
Old 03-22-2007, 11:56 AM
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Default Re: Casting vent

For the record,
The rebar I added in my casting was for the lintel (instead of an angle iron).

I cast both the lintel and vent out of insulating refractory. The insulating refractory concrete is not very structurally strong. It is almost like the perlcrete...

I did put rebar in the lintel and it cracked during curing, but it is installed and holding up the oven entrance. I have chipped several pieces off of the lintel when putting logs in the oven or when using the door. I am concerned that I may have to go back and reinforce that at some point (maybe a custom made upside down U shaped iron frame?). If I had it to do over again, I would have just used angle irons instead of casting the lintels.

On the other hand, the vent, though made of the same weak insulating concrete, does not have any tension on it and all it has to hold up is the duratech flue. It vents very very well and is plenty strong. I guess if I did it over again I would use a stronger refractory concrete, and if the needles were cheap, I might add them, but even my weak vent is (I believe) a vast improvement over flue pipe attached directly to the arch. I get very little smoke out of the front of the oven.

Hope that helps.

Drake
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  #23  
Old 03-22-2007, 01:22 PM
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Default Re: Casting vent

O.K. Feeling a little better.. Are shopping carts made from stainless steel? My vent won't be supporting a heck of a lot of weight.. Maybe.. MAYBE a couple of flue tiles.. As for the wallless design, i figure if I use Marcel's pre heated vent and chimney plan, it should draw like a champ. Not much wind in my backyard most days.
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  #24  
Old 03-22-2007, 02:38 PM
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Default Re: Casting vent

You’ll have to scrounge elsewhere. Shopping carts are made from mild steel that is chrome plated.

J W
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  #25  
Old 03-23-2007, 03:48 AM
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Default Re: Casting vent

Drake, I didn’t mean to infer that you reinforced your vent, merely that a discussion of vent reinforcement was on one of your threads, and was the first I recall seeing on the subject.

I have remembered your oven because it does seem to vent very well, and I’m going to try to emulate your design when my project reaches that stage. I’ve seen very few ovens without any trace of blackening over the oven entrance and yours is one of them! The distance between your oven opening and vent arch seems to be relatively large and I wonder if this also contributes to the good draw?

John, the info on the Ribtec website looks really interesting, and reinforcing a castable refractory with stainless steel fibres certainly appears to be the way to go. I've never heard of them until now, and I don't know if they're available here or at what price.

Cheers, Paul.

Last edited by Hendo; 03-23-2007 at 04:58 AM.
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  #26  
Old 03-23-2007, 05:22 AM
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Default Re: Concrete & Steel Characteristics

David, perhaps I should have said that steel has a similar coefficient of expansion to concrete. I agree that most reinforced concrete sees little temperature variation, and certainly it’s not usually heated in a furnace and cooled again repeatedly!

However, without wishing to enter into a protracted debate on the matter, I think you’d be interested in the following thermal expansion figures from Gere & Timoshenko - “Mechanics of Materials”. Each number is x 10-6/°F (ten to the minus 6 per degree Fahrenheit), which eliminates all the preceding zeros:
Brick 3-4
Cast Iron 5.5-6.6
Concrete 4-8
Steel 5.5-9.9
Stainless 9.6
Structural 6.5
I have included the first two entries to compare them with the Machinery's Handbook figures of 3 and 6.55 (x 10-6/°F) and both fit into the ranges specified above for those materials. Concrete is specified as 8 in MH, which also fits in with the above range of 4-8. I can’t compare data for steel, but it would appear from the above that structural steel fits well within the range specified for concrete. Note the significant difference between structural steel and stainless steel, which is way outside the range for concrete.

I imagine ranges are specified as there are so many different grades of steel, and concrete varies markedly depending on variables such as aggregate size and composition (and even shape), percentage of cement in the mix, addition of plasticizers, and so on. These will all likely affect strength and other characteristics, eg thermal expansion.

So IMO, it should be possible to match up the coefficients of expansion of rebar and concrete pretty much, by specifying a strength of concrete which has a similar coefficient to the rebar used. It is my understanding that when you and I order medium-strength concrete, it will have similar expansion properties to the rebar we’re using.

Cheers, Paul.
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  #27  
Old 03-23-2007, 07:20 AM
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Default Re: Casting vent

You can buy the stainless fibers here;
eBay Store - HIGH-TEMP REFRACTORY STORE: Ceramic Fiber Products, Mortars and Coatings, Insulating Firebrick
They are great to do business with and will sell by the pound if you need less.
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  #28  
Old 03-23-2007, 04:46 PM
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Default Re: Casting vent

Hendo,

I paid $3.75 a pound for stainless fibers... my understanding is you mix in at a rate of 2% - 5% by weight of your castable. My total tab was $15.. It's not expensive to go this route... Only downside is you might have to wait for delivery... rebar you can just run down to HD... I ordered the fibers with the castable at the same time, so it didn't matter.

JB
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  #29  
Old 03-30-2007, 06:31 AM
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Default Re: Casting vent

To all
I was left with the distinct impression, during a previous discussion with a concrete construction engineer that concrete had to be of a minimum thickness before rebar would add significant improvement to the tensile strength of the concrete in which it was embedded. The minimum concrete thickness would seem to be a function of the weight bearing load and its moment. In my case the vent must support not only the terra cotta flue sections but also partially support a field stone facing. I am guessing that the 2”-3” thickness of the castable concrete vent might be too thin for rebar to add any value. Unfortunately, I have do not have the necessary knowledge to make the required calculations.

Any comments on the point that concrete must have a minimum thickness before the addition of rebar is useful at improving tensile strength? Any suggestions, other than substituting stainless steel fibers, on how to solve the problem if the assertion is correct? (The issue of thermal cycling would seem to be yet another concern.)
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  #30  
Old 03-30-2007, 07:17 PM
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Default Re: Casting vent

Can't speak to the rebar value you are asking about, but need to remind that we are talking about castable refractory refractory here. Much different than concrete (is 40-50% alumina depending on the brand).

I bought KS-4 which at 1500 deg F has a crush strength of 2600 lb/in2 and a modulus of rupture of 600 lb/in2. So I think the material is plenty stong enough in compression, which is were all of the weight will be. In your case the tiles are compression, but the stone face will not totally be. Why not build a frame around your flue to help take the load of your stone?

For anyone interested I have attached a picture of my expanding arch entry and my preliminary carving of the vent transition plug.
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Casting vent-140-4032_vent.jpg   Casting vent-140-4041_img.jpg  
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