#11  
Old 02-10-2013, 02:42 AM
david s's Avatar
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Default Re: Vent transition: castable inner arch?

The beauty of casting this piece is that you are not constrained by construction methods (cutting bricks). It is easy to create compound curves and you can funnel the thing towards the flue pipe for really efficient smoke extraction.Because you are not using bricks you can also make the thing quite shallow which makes the oven way easier to work ie. not having so far to reach into the inner oven. I'm sure you'll be happy with the end result.

Last edited by david s; 02-10-2013 at 02:48 AM.
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  #12  
Old 02-10-2013, 05:45 PM
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Join Date: Sep 2012
Location: Portland, Oregon
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Default Re: Vent transition: castable inner arch?

Thanks for the ideas. So with the wet newspaper, it will prevent the sand from adhering, but will the paper adhere to the concrete? Should I spray the paper with oil?

Do you suggest hand-troweling the castable into the mold?
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  #13  
Old 02-11-2013, 02:37 AM
david s's Avatar
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Default Re: Vent transition: castable inner arch?

No I don't bother oiling the newspaper. Nearly all of it just peels away. Anything left will burn. Yes, just hand trowel the castable in place and try to avoid getting any voids. Your previous experience will pay dividends this time. Wriggle the castable into place a bit, this helps.

Last edited by david s; 02-11-2013 at 02:41 AM.
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  #14  
Old 02-23-2013, 04:51 PM
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Default Re: Vent transition: castable inner arch?

Second attempt worked much better. This time I used 1/8" MDF board (super cheap at $3 for 4x4ft sheet) for the form walls, and used the melamine as both inside and outside frame holds. Again packed sand into the wedge shape on the outside angle of the arch, and layered wet newspaper over the top.

Mixed the refractory at the same consistency as before, but used David's method of sprinkling small bits of the SS needles over the surface as I mixed. Also mixed by hand instead of the mixing blade on the drill this time. It seemed to clump a lot less.

Took handfuls of the refractory and slapped it with the trowel as recommended. It certainly is thixotropic, and I used that property after layering the concrete into the mold also to get it to flow more into the corners. I took extra time to mash the refractory down and try to wedge it into shape. This can be done with the handle end of the trowel or with your thumb.

All of this seemed to help get the concrete into those tight spaces. The end result is much improved, though there are some edges at the sharp 34deg angle that didn't fill all the way down. Even though they look a bit ragged in spots they aren't crumbling like the first try. I did try to remove the frame at 24 hours to use the patch method for these spots, but the concrete was too adherent to the MDF, and some of the edges too soft to risk chipping it out. All in all, it looks more than adequate to fit into place and start mortaring brick to it.

Thanks again for your help!
Attached Thumbnails
Vent transition: castable inner arch?-emptyform2.jpg   Vent transition: castable inner arch?-form2-refractory.jpg   Vent transition: castable inner arch?-arch2-finished.jpg   Vent transition: castable inner arch?-arch2-back2.jpg  

Last edited by pdxsds; 02-23-2013 at 07:48 PM.
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  #15  
Old 02-23-2013, 05:51 PM
david s's Avatar
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Default Re: Vent transition: castable inner arch?

Great, I knew you'd get it the second time.
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  #16  
Old 03-11-2013, 09:08 PM
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Location: Portland, Oregon
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Default Re: Vent transition: castable inner arch?

So far, the chains are fitting fairly well into the casted arch. Obviously the first two "soldier" rows fit perfectly. The third row, which is the first angled one, required cutting a brick longer than the usual 4.5" at a taper that fit the angle of cast arch. However I still think the fit is easier than the standard brick arch would have been.

On a secondary note, the back side joints certainly require a huge amount of mortar to fill. The brick shims cut some of that and perhaps add a bit more stability, but still the >1" gaps are a bit nerve wracking.
Attached Thumbnails
Vent transition: castable inner arch?-third-row-arch-intersect.jpg   Vent transition: castable inner arch?-third-row-complete.jpg   Vent transition: castable inner arch?-third-row-inside-left.jpg   Vent transition: castable inner arch?-third-row-inside-right.jpg  
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  #17  
Old 03-11-2013, 11:07 PM
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Join Date: Jul 2012
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Default Re: Vent transition: castable inner arch?

Great job getting that arch done!

Are you aware that you have quite a few linear mortar joints in the first two courses? On the photos I can see at least 4 of the joints that line up perfectly. It would be better if you could correct this early on in your build since these would be the obvious spots where your dome would crack. This issue has been discussed extensively on the forum. Consider correcting this by placing smaller bricks where required to ensure that the joints do not line up.

I hope this helps. Enjoy the rest of your build.
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  #18  
Old 03-13-2013, 08:14 PM
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Join Date: Sep 2012
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Default Re: Vent transition: castable inner arch?

Thanks for the suggestion. I did notice that there are some joints that line up, but probably only 2 of them are "real". The other two look like they are aligned, but in reality it's just the wide mortar at the inverted V at the bottom of the bricks; the actual joint is a few mm over. In any case, I'll pay more attention on the next row and cut some smaller bricks to avoid that issue.

Cheers!
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