36" in Seattle, second oven

[kebwi]

Thanks. I'll see if I can get the boards along the bottom of the sloping chamfer off. Those are the longest, largest surface area, sections of wood-to-concrete contact. I didn't put plastic in that section because I didn't want creases, even though it will be out of sight seeing as it is a downward facing section.

[kebwi]

Well, the good news is that the concrete didn't bond to the wood at all. The instant I released the screws holding the wood sections on, they simply fell away. The bad news is that despite my best efforts, I got some pretty bad air pockets in my concrete. The worst section runs along the underside of a piece of rebar where the concrete obviously never flowed. The rebar is actually exposed in that section, as you can see in a few photos. How do I best fill in or parge this? I don't want to buy a 100lb bag of portland cement to mix up a cup or two of parging mix. I guess I could buy an entire 60lb bag of concrete, sift out the larger aggregate, use a tiny amount of it, and essentially throw 55.9lbs of concrete away, since it only costs about $4 a bag. I mean, I could keep the leftover as long as possible for future use, but humidity will slowly cure and ruin the bag.

[Giovanni Rossi]

Your photos aren't showing on my device. Not sure if the edit window for that post has closed for you to fix them.

You can buy vinyl concrete patch in small quantities. It's easy to work with and can fill voids up to 2" and can be feathered.

You posted your rebar layout photos the same time as those from your finished pour so there wasn't an opportunity to comment. It's best to keep rebar about 3" from any surface of the finished slab. Not doing that increases the chance of the rebar rusting over time. Doing that may have also helped to prevent the voids you experienced.

[kebwi]

Thanks. Wow 3 inches. I mean, that makes it impossible to embed rebar in 4-in concrete at all. It's going to be way less than 3 inches from either the top or the bottom no matter where you put it. As for the edges, I suppose I could have pulled them in some. I was concerned that the 1-1/2 inch front edge counter that I chamfered up to from the 4 inch hearth would be weak so I wanted to rebar pretty close to the edge, but of course I wanted it fully encased.

Giovanni Rossi I corrected the images. I'm not sure why the post looked good after I posted it yesterday but I couldn't see the images this morning either. Does it work for you now?

[david s]

Voids are caused by the mix being too dry and or insufficient vibration. Too late now, you’ll just have to patch it up.

[kebwi]

Yeah, I know. I think I mixed it pretty wet (mixing too dry is a mistake I've made in the past), but I didn't have a good vibration mechanism. You can see in a photo from an earlier post how I used a 4x6, end-on, to pound the concrete a bit, but it obviously was an imperfect process, given the results. I'll just fill the gaps in with some concrete with the larger aggregate filtered out. Hopefully it'll be okay.

Unrelatedly, I'm considering this outdoor counter to a be possible stopping point, at least temporarily. It represents a nice outdoor work area for arranging food for parties and can be used as a platform for either a grill or one of those metal transportable pizza ovens -- which I went ahead and ordered yesterday (Gozney Roccbox, after TONS of review reading and comparison). I can always continue with building a pizza oven on this completed hearth at a later time.

I already have lots of materials sourced, such as a gigantic box of 1900F insulation board and three huge boxes of InsWool (or similar) blanket, and also stainless steel needles and AR glass fiber (I haven't obtained burnout fibers yet, but I know where to get it locally), to say nothing of nine feed of stovepipe already purchased, so I'm in good shape to plunge forward into building the oven. But some third-party thoughts on the project have second-guessed the practical utility of an oven that is really only applicable to one food type, plus perhaps some bread (which could be made in the kitchen for the most part). Basically the person offering this opinion was that a pizza oven is a pretty bespoke and eccentric thing to go after. I suppose that, in all practicality, a 1000F oven is only useful for pizza. Nothing else really requires it, even if cooking bread or other items in it is fun. So, this third-party input (aka other people's opinions) I received was that this was not a good use of my time or money...to say nothing of the fact that it is not very useful for small cooking since it requires 60-120 minutes of manually overseen firing and constant wood-feeding to reach useful temperatures, just to use for a few minutes and then call it a night (yes, bread the next day, I know, but still). I don't tend to throw lots of large parties that would justify even a small-to-medium oven for, say, 10-15 pizzas. I'm just not that social.

I don't know. I'm kind of demoralized about it right now. I wish people with their opinions about whether this is a silly project would keep their thoughts to themselves -- and I wish I didn't find myself agreeing with them for the most part anyway. I kind of want to build it for the hobby of building it more than the hobby of using it. Not that I wouldn't love to have an easy way to cook really high temp pizzas (my kitchen oven can barely hit the mid 400s even when I set it to 500 or 550, whatever the max is, so even with a pizza stone, my kitchen oven pizza just sucks), but feeding wood into an oven for a long time so I can use it to cook one or two 60 second pizzas is the exact opposite of "an easy way to cook pizza" anyway. Hence my decision to buy a gas/wood hybrid off-the-shelf pizza oven to see if it is more practical. It will heat up on its own with gas (it doesn't seem popular to use gas to heat up brick ovens to avoid the tedium of dumping wood in it for an hour or two straight) and then if I really want a wood fire, I can switch the Roccbox over to wood after heat-iup to cook a few pizzas, and then call it a night relatively easily. And the new counter I just created will be a nice place to use it (and dry storage underneath where I can store it so I don't have to haul it to and from the house all the time, along with a propane tank), although obviously a 2000 pound concrete construction is overkill for such a task; it was obviously intended as an oven hearth. Ugh.

So, 2000 pounds of concrete, blocks and rebar in my backyard, and then I get talked out of the project. Shoot.

Anyway, like I said, I have a lot of the materials to start an oven build at a later time already obtained and sitting in my shed. So, maybe I'll start up again next year -- or in a few months -- or hell maybe I'll just go for it in a few weeks anyway. I don't know.

:-(

[Giovanni Rossi]

kebwi Sorry for the panic. Should have been more precise. Rebar 3" from all edge surfaces. 1.5" from the top and bottom of the slab. Lot's of guys do a 4" slab. You'll be fine.

David is correct about too dry a mix and vibration. My limited experience tells me that vibration is the ticket. I did my slabs using delivered redi-mix and the consistency (slump) was perfect. I really worked the concrete into the forms and between the rebar. I used a reciprocating saw without blade to vibrate the forms and still ended up with some voids on the edge. I feel the only thing I could have done differently was to rent a submersible vibrating rod.

Now for these "friends" of yours. This is your project. You had an idea of what you wanted and ran with it. The only opinion that matters is yours. My friends and neighbors have been nothing but supportive. I'm sure some think I'm nuts for building an oven but keep those thoughts to themselves. I'm sure there are some who are envious bc they would never attempt a project of this magnitude.

I believe you are doing a cast build. Those typically have thinner walls than brick builds and don't take as long to heat. So what if you have to take time to get your oven to temp, There's something magical about watching fire. And, it's a different experience doing it solo vs. in a group. Enjoy the ride!

If you decide to forge ahead and finish your build, just insulate it well and enjoy playing with the different things you can do with the retained heat. Take a look at the end of my build thread and you can see how I'm starting that journey.

[kebwi]

Thanks for all your input. Regarding concrete in general, yeah, it's trickier than it looks. :-D I filled the really bad sections yesterday. It looks better now. I'm sure it added nothing to the structural strength, but it should help the rebar from rusting and will fill the deep voids where plant roots and deep moss might otherwise take residence. That's good enough to get me through to next stages. I might seal the concrete to keep rain from soaking in too much.

Good thoughts on the more personal issues too. I agree and appreciate it.

[kebwi]

So I poured the hearth on Sunday and have mostly left it under a layer of plastic covered by a layer of tarp ever since. I folded the sides up a few times, briefly, to remove the side forms (I was worried the wood would bond to the concrete, but it didn't at all; the forms fell right off) and to do a little parging work on the worst voids, but have otherwise left it covered the entire time. So it has been curing under plastic about six and a half days. It has been very hot and very dry this entire time, so I'm glad I did that, but at this point, it has been nearly a week. I'm wondering if there is any practical value in leaving it covered for another week or two. Would uncovering it this weekend undermine the final cured strength in any realistic way? I realize that, chemically speaking, it will still be curing for many weeks, but does it make a real difference whether it is left under plastic to trap condensation underneath for several weeks, or is most of the first week sufficient?

Thanks!

[david s]

Slabs built on grade (sitting on the ground) only have the top surface to lose moisture if formwork is left in place, but suspended slabs will lose moisture from the top, sides and bottom, so keeping them completely sealed is prudent.When house slabs are laid they are rarely covered because it requires revisiting the site after the initial set because covering with plastic immediately is going to leave crease marks in the freshly finished surface. Therefore the slabs are usually just left, even in strong sun and windy conditions. This probably results in a mx strength of 30mpa which is an industry standard, but if you want max strength keep it covered for 28 days. The accompanying chart explains it.

[kebwi]

I took the forms off today.