Saturday, June 06, 2009

Bridge Slab on the WFO Base

Almost to the fun parts!
The countertop height upper slab upon which the actual oven dome will be built is an unsupported "bridge" that spans the CMU base we just finished building. That means all that's there in the end is a heavily reinforced concrete slab supported on the edges with a big open space below. If you followed along with our house build, it's basically a variation on the construction of our front stoop.

Now, because construction of this type of slab presents the possibility of a ton or so of wet concrete on top of a collapsed form in the middle of a pour, constructing a sturdy support structure is critical. It needs to be strong and reliable to hold up all that wet concrete without sagging or moving yet still be able to be removed once the slab has cured.

Here's the preliminary form. On the top you can see the sheet of plywood that fits into the opening of the base and forms the main platform upon which the concrete will be placed. Before we proceeded, this board got split down the middle to allow it to be removed from below once the slab has cured. Underneath is a couple of stout 2x4 frames to hold the whole thing up, built with plenty of attention paid to ease of removal afterward. The outside of the form is 2 x 6's supported, braced. We shimmed the supports to make the top of th form, and thus the surface of the slab, completely level.
top slab formwork

After the form was finished and verifiably sturdy, I cut and tied rebar in a 1' grid and topped that with a sheet of rebar mesh, all of which ends up embedded in the slab. Since concrete on it's own is an extremely brittle material, this is what does the majority of the load bearing, and what allows this bridge to be able to support a load above.
Fortunately, we had our weekend houseguest as helper for this pour. Unlike the foundation slab where we could just dump concrete from the mixer into the wheelbarrow and then into the form, the height of this one meant we had to shovel concrete from the mixer into five gallon pails to be hoisted up and dumped. It slowed the process down considerably, and between that and a hot sunny day, we were thankful for another set of hands to speed things along.

Another few hours worth of waiting on concrete to cure and finishing work, and a few days wait before we removed the supporting formwork, and the last of the boring bits are complete. There's something really exciting about bridge slabs like this, maybe because there's always just a little bit disbelief that it's actually going to hold itself up. So far, so good :)

So fast forward a couple of weeks, and here's the finished oven base, with the ceramic fiber hearth insulation in place on top of the bridge slab:
bridge slab with insulation in place

Almost ready to start laying bricks!
Eventually, the entire base will be covered with some sort of finish material and that opening will get some doors. Exactly what finish material I'll go with remains to be seen, but at this point I'm leaning towards a miniature version of the house, meaning cement board and corrugated steel with exposed faseners...

Big Progress on the Wood Fired Oven

It's FINALLY raining here, for the first time in what seems like at least a month, so I figured I'd take a minute to get you caught up with the wood fired oven project progress.

At my last check-in, we had dug and poured rebar reinforced footings and completed the formwork for the foundation slab. This bottom, structure-supporting slab is thick and reinforced with rebar and mesh, and it's an important yet fairly boring, non-rewarding part of the project.

Since I had calculated this slab was going to require close to a yard of concrete, it presented something of a dilemma. Based on way too much prior experience mixing bagged concrete by hand, I was SO not feeling the idea by-hand mixing the 35+ bags it was going to require, but when I called around to the ready-mix plant to get the price of having what's called a "short load" delivered, and then to the couple of on-site, small batch concrete delivery services I could find, the price proved to be just a little too outrageous to convince me we couldn't do it ourselves.
So, conceeding that we were in for another couple of adventures with bagged concrete mix, my next step was to call the local big box store and arrange to have this and all the other heavy stuff, ~65 80# bags of Quickcrete, 40 or so concrete blocks, and a bundle of 1/2" rebar, delivered to the house. All in all, it amounts to several tons worth of material that, to move by pickup truck and human power would have meant lots of driving and many exhausting trips. For $69, (any size) delivery seemed like a small price to pay, and it meant the driver could pull the pallets off the truck with his forklift and drive them right to my project site in the backyard. It was money VERY well spent.

On our next free Saturday, I towed home the largest gas powered concrete mixer our local equipment rental place had, and Boy and I got to work mixing and pouring:
mixing the foundation slab
This slab ended up taking forty bags of mix. Not the kind of thing you can dilly-dally at, since the clock starts ticking on concrete curing from the minute you mix the first bag and it's critical to get all of the material in place while it's still fluid enough to be screeded and trowled. The mixer I rented could handle three bags at a time and while it was a slightly frantic couple of hours, the two of us managed to get the job done. Huge props to Boy for hoisting all those #80 bags up into the mixer. And props to me for being the only girl I know of that can wheel a full load of wet concrete. Fortunately the farthest I had to go with it was around the corner of the patio.
Here's all of the material in place, screeded and waiting to stiffen up a bit:
foundation slab in place
A couple of hours worth of curing later, all trowled up nice and smooth:
foundation slab form stripped

The next step was stacking the concrete blocks (CMU's) to form the base for the countertop height slab that actually supports the oven. These got "dry stacked" meaning they're set without mortar in the joints, and not having the mortar there to act as a buffer meant getting everything perfectly level was fairly tricky. Even the ~1/16" variation in level of the foundation slab made a difference, and we ended up putting a bit of sand mortar mix under the block in a few spots, (basically concrete mix without any gravel) to get the first course nice and level.
After that, it was back to work mixing concrete, this time two bags at a time by hand in the wheelbarrow, to fill every other core of the CMU's with a stick of rebar embedded in each. The remaining cores we stuffed with empty Quickcrete bags and leftover gravel to prevent concrete from falling into them when we poured the upper slab:
block base completed
block cores filled
The angle iron you can see in the top photo is to bridge the opening in the base that will eventually be covered by a door of some sort. Rather than buy these couple of pieces of steel, I repurposed part of an old metal bedframe. Yay for recycling!

So there you have it. The boring parts are almost done, I promise. Next up...building the form and supports and pouring the upper "bridge" slab.