Hi All! FornoBravo 110 Restoration - Expansion concern

[K-Rimes]

Hi FornoBravo community - I've been reading a ton on this forum and really appreciate all the knowledge and information I've gleaned so far. I purchased a FornoBravo 110 on a trailer that was in a serious state of disrepair and I hauled it home from San Diego to Santa Barbara. It had insulation on it but no concrete or stucco, and a ton of dirt and debris inside so I didn't really know its condition upon purchase but felt it was a good deal.

When I got home, the hearth was absolutely covered in chunks of plaster, it seemed like it was shattered, a complete disaster. I took it all apart, vacuumed everything up, and tried to assess redoing it from the ground up. The previous owner used what appears to be plaster or concrete with no sand for the joints so it jumbled apart and was almost laying flat when I got it home.

I have since fully disassembled the "flower petals", levelled the floor (it's ceramic and vermicrete) with sand, re-done all the joints with refractory cement, cut the re-bar off the lid handle and re attached it to just the "Tray" that it sits atop, and basically re done the whole thing. In doing so, I figured it would be smart to back fill the tray with concrete and now I'm second guessing myself. Is this going to crack the oven when I heat it? I could cut some reliefs in the tray? It goes up maybe 1/4" of the oven wall.

There were some joints that had bigger 1/2" gaps and I loosely packed those with refractory then capped, and others were about 1/4" so I just put a refractory cap on top and called it good.

The re-bar is touching the dome in some spots, but easily flexes up 1/4" if you pull on it. The first photo doesn't show the arch re-bar, which I added on later.

Here are some photos of where I'm at:

Before "filling" tray



After filling tray

[Pizzarotic]

Should be fine, the concrete dome and base should expand and contract together, maybe. You could cut some expansion joints in the base, I'd do 4 at 90⁰ around the dome. Main issue would be heat loss because the base of the dome is not insulated now, also you have created a pond for water to fill. Do you have drain holes below the oven floor?

[K-Rimes]

I ended up removing the concrete due to expansion fears, and went with all ceramic blanket. Completed the lath job this evening. I’m going to stucco it next week after completing the curing fires (300f tonight). There are holes drilled for drainage - but I’m also going to spray the stucco after it has dried for 30 days with a waterproof coating.

[Pizzarotic]

That's going to be awesome when your finished!

[K-Rimes]

Pretty darn close now. Just one more coat of brown, 48 hours and then color coat.

[david s]

Mobile ovens do not like being transported and I suspect that is more likely to be the problem of its damage rather than the heat. If your oven trailer does not have shocks it would be prudent to fit them. Some manufacturers offer one piece domes rather than multi sectioned ones, specifically for mobile ovens, because the advantage gained from less susceptibility to bumps and vibration damage outweighs their disadvantages.

If your plan to allow the extended drying of the stucco dome is to enhance its strength then you have it backwards because the strength enhancement comes from holding in the moisture rather than drying. Ferrocement boat builders talk about 28 damp days for maximum strength. As most of the strength is developed in the early stages, a week of holding in the moisture is generally considered sufficient.(see attached chart) If it were mine, after driving out the moisture from the repair work and the insulating blanket, I'd be applying the render, then immediately covering the render layer, sealing in the moisture with cling wrap and damp curing it for a week. The sealing in of the moisture in the render layer is doubly important because the blanket will have a tendency to suck water from the render because of its high absorbency.

Then give it at least 30 days drying with some cooking fires during that time, before adding any waterproofing coating, as any moisture left can cause blistering under the waterproofing layer.

[david s]

[Pizzarotic]

Or add heat with the moisture. It's possible to achieve 50% of the concrete's ultimate strength in just 12 hours by steaming.

[david s]

Or add heat with the moisture. It's possible to achieve 50% of the concrete's ultimate strength in just 12 hours by steaming.

Be careful of this technique as it can easily lead to shrinkage cracking.Much may depend on your weather conditions. I'm in the tropics and have to do any casting or rendering work in the late afternoon during our hot months, so the early curing can proceed slowly during the night time. If done in the morning the heat of the day results in shrinkage cracking.
Here's a good explanation of concrete curing, moisture and temperature in the attached link.

"Points to keep in mind when curing

• Start curing operations as soon as possible after concrete has been placed.
• For proper curing concrete needs moisture.
• Continuity in curing is a must, alternations of wetting and drying promote the development of cracking.
• If during curing the concrete is allowed to dry out, as may happen in hot weather, the chemical change stops right at the point where the concrete loses its moisture.
• The ideal curing temperature is 23°C.
• Cure concrete for at least 7 days.
• Note: 35°C is the maximum concrete temperature for field placing allowed under AS1379.

The vicious cycle in inadequate curing must be obvious. If enough water evaporates from the concrete before it has attained its maximum strength, there will not be sufficient water remaining in the concrete to fully hydrate the cement and so achieve that maximum strength."

https://www.holcim.com.au/products-a...in-hot-weather

[K-Rimes]

Leaving to dry makes more sense than leaving to wet but of course I’m making sure it stays moist. I’m blessed with a rare window of 65f highs and at 2200’ elevation I’ll be in a cloud for the next few days. This said, I cover the fresh render with towels and mist them periodically to keep them damp.

The trailer has leaf springs suspension and each tire can move independently of the other. In my case I don’t intend to use the trailer for catering, it’s just for storing it and moving it to my next home (I’m renting) but certainly whatever the previous owner used for joint material was not correct. It had no particulate or fiber in the mix, like a plaster.

This is not a final coat by any means and I need to add another 1/4” of brown onto this one (hence the roughed finish) and another 1/8” of color coat. You cannot spray a waterproofing sealant till at least 30 days - hence my comment drying for 30 days.

[K-Rimes]

Finished my base coat and wet cured it for about a week. Hopefully the concrete gods will smile on me!

How long should I wait before firing it up?

[david s]

The drying fires are best done before the render is applied but after the insulation, because moisture you are pushing out from the inner oven parts can escape more easily. If it is trapped by the outer rendered layer(s), then steam pressure build up in the insulation layers can crack that outer render. One litre of water makes over 1500 litres of steam. So your drying fires will need to be extra slow and prolonged to avoid this. If it does crack just render over it again because the outer render is only a protective cover

[K-Rimes]

Thanks David! I did about a week of wet curing the refractory joints, a few mild drying fires before insulation, then 10 days of curing fires with insulation on up to 700f. It probably sat another week due to high ambient temps before I rendered, and has since been wet curing for a week.

Is the render good to go for a pizza night now? I felt not heat at all when at 700f (insulation on but no render).

[david s]

Ok that sounds much better.
If the interior of the oven has burnt off all the black soot, then yes, you're ready to go.Don't attempt to waterpoof the outside of the render until you've had some decent cooking fires and exposure to sun and wind. Any moisture left in the render can cause blistering to a waterproof layer applied over it.

[K-Rimes]

I did not have the courage to go up to the temp needed to burn off the soot, only had that in one corner where my largest fire was made. I will do one more hot fire to get there. I had planned to do one more coat of color render and then leave it a solid 30 days (it's very hot and dry here climate wise) and then give it a waterproof latex finish.