Structural Slab for new WFO

[mikku]

Re: Structural Slab for new WFO

Hello again Lawrence,

Like I said, I have a lot of work to finish.. Probably the only work done outside will be "initial firing for drying and curing" then back indoors for everything else. The base will be done after the frost leaves the ground. Why fight the weather when there is a forklift less than 1 km away that I can use just by calling!

Have a full sized gas oven at the house right now--why rush?
Just doesn't get to the temperatures of wood! Haven't tasted pizza baked in a WFO or as a matter of fact---anything else either! So, I cannot miss what I haven't tasted!

No objections from this camp!
What did you think I was objecting to?

[mikku]

Re: Structural Slab for new WFO

Two pieces of good news today!

1) Took the plastic off the "Oya stone" outer landing area and it looks fine, tapped on it lightly with no echo sound to it. The stone still has a damp appearance to it.. Think that is good--to allow the cement in the thinset to set up slowly. (Just covered it back up with plastic--don't see any reason for a rushed dry here!)

2) The thin render coat over the pericrete has begun to show some hairline cracks. To me that is an indication that the layer below of insulation is finally drying out. The entire render area, structural slab, and exposed areas of pericrete all have the same "light gray" color of dry concrete. I am sure there is still plenty of water deep inside the insulation--but not in the quantities originally.

Question for other forum members: For those who have left a permanent vent in their rendered dome, what did you use? What size did you make it? And why?

[Gulf]

Re: Structural Slab for new WFO

Question for other forum members: For those who have left a permanent vent in their rendered dome, what did you use? What size did you make it? And why?
I left a 2" ish opening in the top of my dome because that is the size of a beer can . Since curing I have reduced that to 1" for a breather.

[stonecutter]

Re: Structural Slab for new WFO

Two pieces of good news today!

1) Took the plastic off the "Oya stone" outer landing area and it looks fine, tapped on it lightly with no echo sound to it. The stone still has a damp appearance to it.. Think that is good--to allow the cement in the thinset to set up slowly. (Just covered it back up with plastic--don't see any reason for a rushed dry here!)

2) The thin render coat over the pericrete has begun to show some hairline cracks. To me that is an indication that the layer below of insulation is finally drying out. The entire render area, structural slab, and exposed areas of pericrete all have the same "light gray" color of dry concrete. I am sure there is still plenty of water deep inside the insulation--but not in the quantities originally.

1) Yes, that is a good sign...and a slow cure is best.

2) Hair cracking in the skim coat is due to the drying process and the material shrinking.

[mikku]

Re: Structural Slab for new WFO

Question for other forum members: For those who have left a permanent vent in their rendered dome, what did you use? What size did you make it? And why?
I left a 2" ish opening in the top of my dome because that is the size of a beer can . Since curing I have reduced that to 1" for a breather.

Thanks Gulf!
I understand this is your breather, but what is it? A special item or something you found that meets your needs?

[mikku]

Re: Structural Slab for new WFO

1) Yes, that is a good sign...and a slow cure is best.

2) Hair cracking in the skim coat is due to the drying process and the material shrinking.

That is indeed good news! Thank you for reinforcing my suspicions and pointing me in the right direction!

[brickie in oz]

Re: Structural Slab for new WFO

Since curing I have reduced that to 1" for a breather.

All you need now is the dipstick.....

[mikku]

Re: Structural Slab for new WFO

All you need now is the dipstick.....

If I remember correctly on your own build, you did not have a full render or pericrete insulation layer so you have no issue with the possibility of built-up steam in these areas.

If you did, however, how would you provide for this moisture to escape? Since every build has entry points for water--even those with full roof enclosures.

[brickie in oz]

Re: Structural Slab for new WFO

even those with full roof enclosures.

The only way for moisture to get into my oven is via the moisture in the air, for 90% of the time the air here is relatively dry so I have no issue.

The ambient moisture content of Victorian air (where I live in Au ) is 12%, so its no big issue.

I know its 12% because some time ago I did a lot of hobby furniture making, one issue with furniture is the moisture content of the timber being used.

[mikku]

Re: Structural Slab for new WFO

[QUOTE=brickie in oz;146554]The only way for moisture to get into my oven is via the moisture in the air

I was referring to your style of build: w/o render or pericrete insulation where steam buildup would not be an issue. Your design would work well in probably all locations. But in more damp locations, wind driven rain blown during heavy storms would still go down the chimney and soak a lot of the masonry. Still no steam issue...except if you attempted a roaring fire without some drying first.

But for me and a lot of other people with dome enclosures with full render and pericrete insulation, how would you advise us to cope with the problem of possible steam damage? This is something that I am try to anticipate in the build phase rather than try to correct later!

[mikku]

Re: Structural Slab for new WFO

Materials update:

The spiral SUS304 pipe came today. Ordered a 1 meter section of 150mm and 200mm along with what is called a "150 T Collar". My fabricator friend called, he had already cut a hole in some 1mm thick stainless and spot welded the collar in place.

If I had the budget for regular insulated chimney, I would have bought ready made. This assembly was Y10000 or approx $100. I also got a couple of pieces of .035 stainless to make caps for the space between the tubes. The 150 fits inside the 200 with a 25mm gap. I'll start with a couple inch layer of refractory at the bottom, then fill the balance of the tube with dry perlite. Once I get to the top, another layer of castable and another stainless ring.

Don't know if this thinking is valid or not, I think that it is important for the inner chimney to heat up as hot as possible to maintain draft. The insulation should assist with that. But it will also cool quickly?

Which is better? Fill with castable refractory and it will keep the temperature high longer, but requires longer to heat up?

Or fill all with perlite, using hi-temp gasket cement to hold the stainless caps in place?

Or my first thought?

That is probably something for Sunday or later next week.

[david s]

Re: Structural Slab for new WFO

I used castable refractory around the single walled stainless flue pipes for the second and third ovens, which were identical, that I built. In both cases the refractory cracked in the same place. I concluded that the expanding pipe, which is going to get hotter way faster than the refractory that surrounded it was powerful enough to fracture the refractory. The pipe in both cases was fairly thick at 0.9 mm. I now use 0.55mm which may not be strong enough to stress the refractory to the point that it would crack, but I was not prepared to take the risk so now always make sure that there is a slight gap. You can do this by wrapping thin cardboard around the pipe before casting up to it, then remove the cardboard once the refractory has set. You don't say how thick your pipe is but this may be a consideration. Ready made pipes are usually pretty thin stuff.

[mikku]

Re: Structural Slab for new WFO

Hello david s!
Both the 150 and 200 are .5mm thickness, the T-collar is .8, and the flat piece it is spot welded to is 1mm.

Are you saying that I should try to line the 200mm with a layer of corrigated cardboard? and then slide the 150 inside? And then fill with refractory?

Is there a problem with a crack in the refractory anyway? Some chimneys are double wall with only air in between?

[mikku]

Re: Structural Slab for new WFO

I was planning to have the chimney removable. The 150 simply sides over the T collar. I was going to build up around this chimney with pericrete.. (possibly 30 cm height )..maybe 5:1 to give the whole thing a little support/ but wrapping cardboard around the 200 before stacking up the pericrete. Final coat would be the render and whatever else--maybe the lime plaster or shikui as a weather coat. The cardboard could burn out or pull out with the pipe removal. I am assuming that the 5:1 would keep its shape after several insertions and removal operations. I could fabricate a water collar that would clamp around the 200 --maybe a simple "T" out of 200 stainless to cap things off. At the top of the dome, I still want to leave a breather vent to get rid of steam in the future. Cannot figure out where to locate a "dip stick"--Maybe "Brickie would kindly tell me a good location"--hehe! lookout now!

[david s]

Re: Structural Slab for new WFO

Hello david s!
Both the 150 and 200 are .5mm thickness, the T-collar is .8, and the flat piece it is spot welded to is 1mm.

Are you saying that I should try to line the 200mm with a layer of corrigated cardboard? and then slide the 150 inside? And then fill with refractory?

Is there a problem with a crack in the refractory anyway? Some chimneys are double wall with only air in between?

Corrugated is probably too thick. You need it between the refractory and the inner pipe because that's the one that gets hot first. Maybe it doesn't matter if it cracks as it is only acting as a spacer between the pipes. It is a different story if it is the structural refractory of the entry though. If you want the pipe removable then making it slightly loose will be useful. You only need about 1mm thick for this.