My pizza oven exploded!!!

[david s]

Still looks to me like steam spalling. Because your oven was fine when you used it a number of times and the exploding slab only occurred when you used the fireplace suggests that the supporting slab couldn't take the heat. Heat rising from the fire would heat that slab up extremely quickly and if it has been made from regular concrete you're in trouble. Using embedded rebar in that situation is also a no no because it expands more rapidly with extreme heat rise being more conductive than the concrete that surrounds it. That is part of the reason stainless steel needles are used in refractory castables as the preferred method of reinforcement. The other reason of course is that normal steel rebar can rust, particularly if heated which accelerates any corrosion. The supporting slab would be better made from a castable refractory, the same material your oven is made of, that can withstand the extreme heat applied directly to it.It also contains tiny hairlike fibres that burn away leaving a fine network of pipes that steam can find its way through without exploding the material. Because these fibres are so fine they don't alter the strength or density of the material. No doubt your contractor will have a different idea, but you did ask for advice. Good luck.

[deejayoh]

I think David is spot on - a more accurate title for the thread is "the concrete slab on top of my fireplace exploded". That concrete slab is probably not a material that is appropriate for that amount of heat. That really sucks. I think any repair short of of a tear-down where that slab is replaced probably means you shouldn't use the fireplace in the future

[Dave T]

Yes, that's right. It was the slab between the fireplace and the floor of the oven that exploded. It was the concrete itself, it wasn't pressure from escaping steam. I've had that issue too (see earlier threads). Over last winter, rain had gotten into the oven because there was no cap on the chimney. It soaked into the insulation, and I had a period where I had a really hard time getting the oven up to temperature because the heat was going into evaporating the water. But the steam was forced out through the mortar, and no explosions. This was different - between the pizza fire and the fire in the fireplace below, the concrete slab was heated to a point where it exploded. They now admit that they shouldn't have used concrete.

They are working on fixing it now. I just had a conversation with the guy on the crew doing the work. There's a bit of a language barrier, but it sounds like they are going to replace the concrete slab with brick, and use high-temp mortar between the bricks. Then above the brick will be the insulation, and then above that will be the floor of the pizza oven. It sounds good, but I'm wondering if I should be concerned about them getting every last bit of the concrete slab out of there. It's looking to me like they will leave some of it in - the part supporting the walls of the oven itself. If they didn't, the oven would fall when they chipped out all the concrete. I tried to explain my concern about this, but the language barrier reared its ugly head. I have a call into the owner of the business so he can answer that question for me.

[david s]

It's still definitely steam that created the explosion. If water suddenly turns to steam it expands around 1600 times the volume. The fibres I referred to in my previous post were originally added to standard concrete to reduce steam spalling, thereby imparting some fire proofing to concrete. So, if the concrete is not destroyed by excessive heat a building would not have to be demolished. The same technology is used for castable refractory although it uses a different type of cement that is more refractory (fireproof).

[deejayoh]

If your oven floor was wet, then the water likely got down to the concrete as well. A lot of it was absorbed into the concrete through capillary action. Concrete will wick water for up to 3 or more feet! You need to add something like xypex if you want it to not wick.

[Dave T]

They are still working on the repair. Now that the entire floor/ceiling between the oven and the fireplace below has been removed, I can see how it was put together. Below the oven is the firebrick floor. Below that is a 1-2 inch layer of this hard foam like stuff for insulation. But the oven walls themselves are supported by the insulation, the brick is inside the oven and only goes up to the walls of the oven. Is this the way it's supposed to be built? I would have thought the oven should be resting on the brick.

[UtahBeehiver]

The floors on Pompeli builds can go inside the walls or outside the walls, There are dozens of builds that go either way (do a FB search).The FB Casa kits have tiles that fit inside the walls. The most important part is that the oven floor and walls are isolated by insulation from any heat sinks, ie concrete floors or outside enclosure walls.

[Dave T]

Mine's a Casa, so it sounds like they're doing it right (?). That's a relief. I would normally think they know what they're doing, but after the explosion I'm inclined to second guess them.

[Dave T]

Also, there are some dings in the dome that they're going to repair with high temp mortar. Sound good?

[dakzaag]

The contractor is standing tall here, you should be pleased with the repair. They did screw up on the concrete fireplace roof, I can see why they thought it wouldn't get that hot, but the repair you describe is fine. The brick are fired at high temp and they are using refractory mortar, probably heat stop. Refractory casting is complex stuff so as far as I am concerned, the brick and mortar approach is better. There are a few fireplace systems that use refractory panels to build the unit. Many guys mistake the refractory pieces for regular concrete because it looks and weighs like concrete, but is resistant to heat. This leads to the assumption that concrete is a good fireplace material, which is completely wrong. The lucky part is no one was injured, hopefully anyone reading this takes note of the potential disaster of mixing concrete and flames.

As far as the dents in the dome, while they will not hurt anything as long as they are minor, repairing them is the right thing for the contractor to do. The repair patches will not stick very well and eventually fall. (this is the nature of refractory repairs, the soot prevents the new patch from bonding and then the heat cycles create stress that pops the repair off.) If possible plan to have your oven fires below the repairs so any debris ends up in the fire not the pizza. Or nix the repairs if the dents are not more than 1/2 inch deep. Forno Bravo might be willing to help you determine the severity of the dents and recommend a better repair method.

This thread really points out the seriousness of building these units. Seemingly insignificant details can have a huge impact on the performance of the unit. Many guys don't really understand the difference between refractory masonry and regular masonry. Clay brick will stand a lot of heat, cement brick decay at about 600 F. Steam explosions can happen with any type of unit if it gets wet. The initial explanation that they used the wrong mortar didn't jive, because mortar joints are usually too thin to hold that much water. Mortar joints typically just fall apart under high heat. Cement on the other hand has enough mass to hold water and then explode when heated quickly. I think the described repair is proper although I never did see any pics.

[dakzaag]

I am going to back up a bit on the contractor. I went back and found the pictures linked and noticed a few things. First the exterior stone work is nicely done, but the chimney is really short, I would want a better transition. And where does the FP exhaust to? Personal preference here, but the whole layout is awkward with a polished hearth that you basically have to stand on to use the oven. There is no transition or throat in that fireplace so I bet the one fire you did have was prone to smoke before the explosion. Or perhaps it vents into the oven and then out the oven stack? Unlikely as this takes some very intricate negotiation of the smoke chamber and vent.

The interior of the fireplace sure looks like standard mortar with clay brick. You can count on those joints to fail as you use the fireplace. It could be refractory mortar, but the standard FP construction is refractory cement and firebrick which has a distinctive look that you don't have. I have never seen a proper refractory "mortar" used in a fireplace. Yes it exists and yes it works, but only very well informed masons even know the difference between refractory mortar and refractory cement. The fact that the contractor used a concrete ceiling for the fireplace leads me to believe they did not know that Masonry Cement was not appropriate for a fire place. It won't explode, but it won't last either. You're in between a pickle and a hard place with this situation. I would ask about the mortar joints in the fireplace and have them ground out and replaced with Heat Stop unless they can specifically tell you what mortar they used and produce a product specification sheet. Anything Type N or Type S or Portland Lime Mix is going to fail. Heat Stop is about the only commercial refractory mortar readily available for fireplace construction, and normally firebrick with refractory cement is used.

I do think the contractor is trying to stand behind this, probably because he realizes someone could have been seriously injured in the explosion. Unfortunately you have become his learning experience and probably his last fireplace customer, you might want a true expert to look at this. A certified chimney sweep will understand the basic principles well enough to know if the repairs and initial construction are adequate. I would look for one who has been in business for quite a while and a member of the National Chimney Sweep Guild. None of this stuff is rocket science, but as you can see it does have the potential to be a real cluster if the wrong material is used. $200 will usually get a sweep out to do an inspection, its up to you what to do after he reports on what he sees.

[david s]

If your contractor is replacing the concrete supporting slab with bricks, then what supports the bricks? I should think it would be nigh on impossible to replace the concrete underneath the dome and if that is left could still create problems if it gets wet again. There are also likely to be bonding difficulties between the replaced bricks and the remaining concrete slab. Frankly, I can't see an adequate solution short of a tear down and rebuild. Regarding the filling of the holes in the oven casting dakzaag is correct, repairs such as this are rarely successful. The usual method is to wet the area to be repaired well, wait 10 mins and apply a thin slurry of finely sieved refractory mortar. Wait another 10 mins and force refractory mortar, mixed to a peanut butter consistency, into the area to be patched, as hard as possible.

[Dave T]

Ok, the oven and fireplace repairs are complete. They replaced the cement ceiling of the fireplace with brick, and it looks good. The only potential issue is that they didn't go with the FB firebrick floor for the pizza oven. Instead, they went with an alternative firebrick they could source locally. It's Clipper DP. Here's a link:
http://supplies.foundryservice.com/i...ty-/clipper-dp

The thermal properties seem fine, but it's rough and gritty compared to what I had before. Does anyone know anything about this firebrick and its suitability as a WFO floor?

[Dave T]

If I'm going to ask the contractor to replace the tiles, I need to do that soon. Since this is a different topic than the original topic of this thread, I'm just going to start another thread asking about the Clipper DP. I hope that's ok!