Casting vent

[wlively]

Re: Casting vent

Hendo

I took what the refractory specialist said for granted, so thanks for the correction, yes the coefficient of expansion for mild steel and concrete is the same.

But I think we are talking about two different things. We are not talking about a weight bearing structure that the only thermal cycle it will experience is the days of the year. The vent is not designed to be a load bearing structure (although I guess you could) and will be experiencing high levels of humidity and very large temperature cylces (10-1400 deg). I guess a better example would be to say the stainless steel fibers or needles form an interlocking matrix in the casting providing both reinforcement and preventing crack propagation.

Though, refractory is much more porous than traditional concrete and oxidation rates increase with temperature, I didn't mean to imply that a big piece of rebar is going to rust away.

Of course that is the beauty of building our own ovens, we are free to do it any way we like, and see what happens.

[Hendo]

Re: Reinforcing a vent casting

Fred, John, Wade et al – on the matter of reinforcing a vent casting, I wonder if it’s worth going back to first principles and discuss concrete design for a moment?

Steel (ie mild steel) is added to concrete (made with Portland Cement) to strengthen whatever is being cast in terms of tension forces. Concrete on its own is weak in tension, but strong in compression. Steel provides the necessary strength at the tension face to prevent failure. So for suspended concrete slabs, the steel should be placed towards the bottom, as this is the tension face of the casting.

Why mild steel? Simply because it has the same coefficient of linear expansion as concrete. Obvious when you think about it – you can not have two dissimilar materials expanding at different rates and ‘competing’ against each other if you want your structure to remain standing. Providing the concrete has been poured and duly compacted (with a mechanical vibrator or by manually rodding), there is no chance of the steel reinforcement oxidising, providing it has an adequate cover of concrete (various codes specify minimum concrete cover for different applications). Deny steel air (oxygen) and oxidation (rust) can not occur. Otherwise buildings would eventually collapse as the steel rusts.

Providing refractory concrete has the same coefficient of expansion as concrete made with Portland cement, steel reinforcement should be OK. If it doesn’t (as Wade suggests), I frankly can’t see the point of reinforcing with steel – mild or stainless - as the steel and concrete will expand at different rates on heating, and likely lead to failure/cracking of the structural element in question.

While stainless steel would probably hold broken elements together for a longer period than mild steel, this seems to be a ‘band-aid’ solution to the problem, rather than designing to prevent failure (cracking) in the first place. After all, the reason for reinforcement is to prevent failure, not to hold things together after the concrete has cracked!

Surely the best solution would be to use a reinforcing material strong in tension with the same coefficient of expansion as the refractory concrete used, or none at all.

Comments?

[fdn1]

Re: Casting vent

Thanks to all who replied on this subject. The information provided was very helpful.

[james]

Re: Casting vent

JB and Wade,
Let me know how this works and take photos. It sounds like this should work its way into the plans.
James

[johnrbek]

Re: Casting vent

Wade,

Thanks for the compliment...

Going with a "KS-4 like" product is the safer way to go in my opinion, but I did want to qualify that the reason I went with that specific product is because thats what the closest refractory supplier had in stock and thats what they recommended given the application. So, I guess what I'm trying to say is, I wouldn't go way out of my way to specifically buy KS-4, I'd buy whatever has similar properties that is within reasonable driving/delivery distance. In my case, because I'm in S Fla, that wasn't very close unfortunately... If you check manufacturer sites, like ANH Refractories , you'll find that product data sheets that detail product properties and material quantity requirements per cubic foot are generally available.

Good luck!

JB

[wlively]

Re: Casting vent

You definitely don't want rebar.

Steel and refractory have very diff coefficients of expansion and the large piece will move alot causing large cracks. Also, the mortar is corrosive and high heat accelerates oxidation (rust). That is why they use stainless steel fibers or needles. Very thin pieces don't cause much expansion pressure, yet grab enough to reinforce and the stainless resists oxidation.

Also, thanks to all and JB I will be buying some KS-4.

[james]

Re: Casting vent

JB,

Good idea. I added zirconia fibers (like glass but they don't melt at high heat) to the mortar around the dome of a precast oven a couple of times to see how it worked, and it came out fine. It felt like you were adding straw to waddle in the good old days.

One other idea. I have also broken into precast oven domes, and there is a wire net (thin), set in the middle of the refractory.

James

[johnrbek]

Re: Casting vent

Rather than experimenting with rebar and similar materials, consider using stainless fibers.. They are cheap, $3-4/lb, used in ratios of 2-5% by weight, and general available on the web... Here's one manufacturers product description: D & C Supply Co., Inc. -- Stainless Steel Fibers

From what I understand, this is the material that is more commonly used when strengthening castable refractory. The detail on the link gives a decent explanation of how they work.

Not sure about the thickness question, but I've seen cast pieces as little as 1" thick used in similar applications. I would imagine the thicker, the better.

Good luck.

JB

[fdn1]

Re: Reinforcing a vent casting

This question may be a bit off the current topic but it is related to the general subject of casting a vent. Hopefully someone with considerably more knowledge about the structural properties of reinforced concrete will be able to provide an answer.

Is it recommended that the vent casting be reinforced with steel to increase its ability to withstand large bending stresses when it must support loading from heavy masonry flue structures?

One concern in this regard previously raised on this forum by Christo (6/17/06) related to the high temperatures the vent structure would experience. He inquired about the advisability of including thick rebar when casting a vent because the significant difference in its thermal coefficient of expansion compared to refractory concrete coupled with its thickness might lead to a strength failure with repeated heating cycles. (I could not find a response in the archives.)

Another concern might be related to the lack of thickness of the concrete structure. Does the concrete have to have a minimum thickness before the addition of steel increases the bending stress it can withstand without breaking?

To summarize: Does the addition of steel (rebar or wire mesh) actually help the vent to support heavy flue structures? If so what thickness is recommended for inclusion?

[james]

Re: Casting vent

One more thing. We have re-named Refrax to Refmix (yes, I know it's a terrible name) for various reasons. It will be back in stock in NorCal in 3-4 weeks.
James

[james]

Re: Casting vent

I think JB has it right. You want the vent to get hot, and create the draw for the chimney. The standard vents with the FB ovens are made from the same refractory as the oven chamber itself -- not an insulating material.

I like the idea of creating a custom refractory with ground up fire brick piece in it. Refmix, plus additional refractory aggregate and a structural steel, wire or rebar could work well.

What did the folks who have cast their own vents use?

How does the cast vent section of the Pompeii Oven plans look to everyone? Is there enough detail?
James

[johnrbek]

Re: Casting vent

By the way.. you want a refractory castable, not an insulating one.. you want your vent hot to promote a better draw... In addition, insulating castables are not as strong structurally as refractory castables... Insulcast, from it's name sounds like it might be an insulating castable...

JB

[johnrbek]

Re: Casting vent

Last time I spoke with james, he said he was out of refrax.. In any case, though, my understanding is that is more of a mortar which is designed for joints.. However, I will say I am not a mason, so who knows.. it may be a good application.

On the other hand, there are refractory products designed specifically for casting. The folks who sold you the needles should be able to provide you with some. I went with KS-4... the data sheet is here: http://www.hwr.com/ci/datasheetsv1/KS-4.pdf I'd check to see whatever is available locally that will provide similar qualities... Just MHO.

JB

[redbricknick]

Re: Casting vent

I used refrax to put my opening arch together and slow cured it with wet linen for a few days.. It's the hardest substance I've ever come across. Well, i encountered some grape nuts stuck to a cereal bowl once which were harder, but I'm unsure of their refractory qualities.. I was grinding my arch to fit some bricks yesterday, and the cured refrax (Refmix?) was extremely difficult to grind. Bloody hard stuff. Of all the refractories i've experimented with.. 1,3,11, Heat Stop, Selleys fireplace mortar, everything but La Farge Fondue, Refrax is by far the hardest. I have one and a half bags left which I'll be casting my vent with. I also have a ton of firebrick offcuts and a ton of dust left which i don't know what I'll be doing with yet. I was thinking of cutting them into small pieces, and mosaicing a refractory layer over my dome..

[wlively]

Hello all

I finally got back to my oven and will be casting the vent transition soon. When I bought my insualtion I also bought a few pounds of stainless steel needles to reinforce the casting.

I would like to ask what materials are giving the best results? I have been considering insulcast or refrax mixed with crumbled brick. Thoughts/experiences??

Thank you,
Wade Lively