#11  
Old 03-18-2007, 06:45 AM
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Default 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
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  #12  
Old 03-18-2007, 07:22 AM
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Default 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
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  #13  
Old 03-20-2007, 08:37 AM
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Default Re: Casting vent

Thanks to all who replied on this subject. The information provided was very helpful.
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  #14  
Old 03-21-2007, 06:44 PM
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Default 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?

Last edited by Hendo; 03-22-2007 at 11:45 PM.
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  #15  
Old 03-21-2007, 08:15 PM
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Default 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.
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  #16  
Old 03-21-2007, 08:16 PM
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Default Re: Casting vent

Uh, major redesign of my hanging vent? I planned on reinforcing it with rebar.. Should I use stainless rods instead? What metal has the came coefficient expansion as Refmix? Scared.
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  #17  
Old 03-22-2007, 05:26 AM
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Default Re: Casting vent

Wade, Thanks for the feedback. I realise that we may be talking about two different scenarios, and (I hasten to add) I have absolutely no experience in refractory concrete – just the normal stuff.

I wasn’t attempting to correct anyone or anything – just put down some thoughts that may assist people (me too!) when they are agonising over whether to incorporate reinforcement in their vent casting – if they are casting a vent – and even when they next pour a structural slab.

I first saw a discussion of vent reinforcement on Drake Remoray’s site (if memory serves me right), and ever since, I’ve wondered, if I was in the same situation, whether I’d add reinforcement or not. I think the load bearing in this case was for the flue. Not a great weight, but many people seem to want to add reinforcement as some sort of safeguard.

I once saw a great piece of graffiti while cycling to work years ago– well before the current self gratuitous tagging stuff – which said “jumping to conclusions avoids the pitfalls of logical thought.” So I’m just endeavouring to rationalise why I’d add reinforcement, and if I did, would I go to something like stainless steel (or even zirconium) fibres, or simply leave it out. After all, there’s more than one stone bridge in the world without a hint of steel that’s still standing!

Nick, I’ve followed your wonderful structure with great interest, and think that your proposed vent is a little different from the mainstream. Nonetheless, it is still a cast refractory which has to support some weight. If it were me, I’d want to add some reinforcement, but I’d have to keep asking myself why? As long as I came up with a reasonable justification either way, I’d go with it and hang the consequences! And you have the advantage of things being exposed, not buried behind a brick wall or other cladding, so you could easily get to it again if you ever needed to.

I note that I’m rambling on again – please forgive me. Too much malt probably!

Cheers, Paul.
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  #18  
Old 03-22-2007, 05:36 AM
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Default Re: Casting vent

Nick,

Hendo brings some good points.. I am not a mason or a specialist in the thermal properties of these materials, so I cannot comment on their validity.
The only thing I can say is that when look at refractory materials suppliers, the only material they sell to reinforce castable refractory is stainless fibers. Admittedly, when we get into researching there properties on the web, we're talking about much higher (e.g., kiln, blast furnaces, etc) temperatures. Here's an example: Refractories

Here's a quote from that site: "The addition of ... steel fibers to castable, gunning and moldable refractories inhibits crack formation (Figure 1) and transforms the conventional refractory from a brittle material to a tough and tenacious composite (Figure 2) exhibiting dramatic improvements in service life."

It's reading statements like these that make me feel really comfortable about going with stainless steel fibers... I understand rust is not an issue either way. It just seems like it would make sense to use a material that the "big boys" use... especially given the cheap price and ease of use.

If you're still up in the air, buy a little extra castable and mock up two pieces.. one with rebar and one with the steel fibers... you'd have more assurance and we'd have a cool science test!!

Good luck.

JB
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  #19  
Old 03-22-2007, 07:32 AM
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Default Re: Casting vent

This morning I woke up and I couldn';t believe I forgot a major point.

I should have also added that refractory mortars, castables, ect have a high alumina content. KS-4 is 45% alumina, so while concrete has the same coeffecient of axpansion refractory materiels are not even close.

Mental note, no more posts right after a 14hr day at work.
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  #20  
Old 03-22-2007, 08:34 AM
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Default Re: Reinforcing a vent casting

Quote:
Originally Posted by Hendo View Post
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.
Here's a posting of mine from Sept., 2005:

Quote:
Originally Posted by DMUN
From Machinery's Handbook 20th edition:

Values are per unit, per degree F:

Wrought iron:
.00000661

Cast iron:
.00000655

(no value given for Stainless Steel)

Granite (and presumably other ignious stones like basalt):
.0000044

Concrete:
.0000080

Brick:
.0000030

Slate:
.0000058

Sandstone:
.0000065

Pine:
.0000028

What do we learn from this? If mortar expands more than twice as much as the bricks it holds together it's no wonder that masonry walls fall apart. On the other hand, there's still a bunch standing from the time of the Romans.

My guess is that freezing water in wet masonry is the cause of most masonry failure.
Steel doesn't have the same thermal expansion as concrete: it's 20% more. The fact that re-bar works at all is due, I think, to the relative low temperature variations that most concrete structures are subject to. I wouldn't want to use it in a vent casting for this reason.

There's a second problem with steel: as it rusts, it expands. This can break apart concrete, and that's the reason we want the rebar well buried in the slab, so it's well away from moisture. But as anyone who has thrown a can in a campfire, heat excellerates corrosion, and this is another reason why I wouldn't want to use it in a high heat application.

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.
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