Base & Concrete Questions

[iijakii]

Hello,

So I'm going to start building my base this weekend out of cinderblocks. I guess called besser blocks for you folks across the pond. First question is wondering if void filling is really necessary? I know that's good practice for retaining walls and what not but for just a relatively small table it seems massively overkill? But I guess for the price of mortar it's not a big deal. Just wondering.

My plan is to lay the blocks for the sides and back, front open. 3 lengths and 4 rows high. So a 48" square roughly. That brings me to my main next question -- Ive never poured a concrete slab before. Do I need to put some bracing in the middle, or can concrete with steel grid span that no problem? I can't find any info on common practice of how far concrete can span unsupported, not to mention holding 600+lbs ontop. Maybe easy route would be to put a 3/4" piece of exterior grade plywood down first?

[SableSprings]

Welcome iijakii ! Where are you in Oregon?

It's not required to fill all the voids in the cinder blocks. Most often, alternate holes are stuffed with concrete sacks and a piece of bent rebar is placed into the open holes. These bent pieces of rebar are wired to the rebar of the top slab-to-be withing the supporting form work. This way the entire structure is secured as a single piece...no movement or shifting possible. Are you talking about the supporting slab on top? If you are, that wet cement 4" deep or so is really heavy (I'd say more than 600 lbs when wet). You will want to support the base plywood with a 4x4 and another square of plywood to spread the support. The slab itself will be self-supporting with the normal addition of rebar. The slab will distribute the weight of the oven fairly evenly and you've got lots of folks on the forum with ovens to prove it's a workable choice..

Make sure that you are planning for the insulation and outside render/facade in your measurements. An additional 18"-20" of width to your planned internal oven size is common. So for a 36" inside oven space, you'll need 54" to 56" of space on top of your supporting slab to build the final oven.

You don't want the weight of the cement to cause your top slab to sag at all...over building support here is not a bad thing. Also you need to add some easily knocked out shims (1"-2" IMHO) underneath the plywood or 4x4 support system. Once that cement hardens, you need to be able to remove the support (and pushing 600+ lbs of concrete slab up will not be something any of us should try ). Removing the shims, gives you the wiggle room needed .

In my corner build, top slab, I had incorporated several support block stacks below the slab that were meant to stay (and still are there), but I had a totally different goal. (I don't store my wood underneath in "the dark space", I've got carts that roll in/out from the base.)

Sorry, I didn't mean to ramble and I hope this helps...

[iijakii]

Sable,

I'm a couple hours north of you in Wilsonville. Thank you for the post, giving me some things to think about.

So for the base I'm going to use this Quickrete Quickwall stuff to skimcoat the outside instead of grouting. I figured the look would fit in with the cast oven nicely, and supposedly it's even stronger than normal grouting methods. Still unsure on if I'll voidfill or not. Probably should not be lazy and just do it.

Good call on playing it safe and make the tabletop bigger. I was thinking I'd have enough room for insulation by having an extra 12" from outside dome diameter, but no point risking it. I'll probably stick with the 48" wide base but extend the top out a few inches on each side. I have a few 4x4s laying around in my garage, I'll definitely brace the plywood base before pouring cement. I'm building this on my covered patio so that breaking and spilling 8 or whatever cubic feet of concrete sure would be disastrous!

Cheers,
John

[SableSprings]

I choose not to grout or mortar my cement base blocks. I don't really see a need to do a skimcoat now. It won't be going anywhere and until you decide how you want to finish it, it seems to be just a waste of time (IMHO). I stuffed empty cement bags in the alternate holes and trusted the bent rebar and overflow into the remaining voids from the top slab pour to "cement" the system together as a unit. The only problem I had is that by skipping the mortar layer between blocks, it changed the final height and length (shorter). Not a big problem, but tiles had to be cut because the masonry/brickwork industry plans on consistent installation methodology...of which I was not aware

I strongly suggest that if you know someone with some experience in building cement forms that you invite them over with the promise of an adult beverage and coupon to your first pizza party. I was very lucky my forms didn't completely come apart when the concrete was poured...I simply did not have a clue to how much weight and outward force would be exerted on my pathetic form construct

Again, don't forget to provide easy knockout shim(s) so you can remove the support pieces underneath when the cement hardens. That will also be something to be aware of when using a form to create your entry arch(es). And yes, I find having some extra width on my top slab gives me a place to set my beer and other "tools of the trade" ...it's easy to do now.

[iijakii]

I choose not to grout or mortar my cement base blocks. I don't really see a need to do a skimcoat now. It won't be going anywhere and until you decide how you want to finish it, it seems to be just a waste of time (IMHO). I stuffed empty cement bags in the alternate holes and trusted the bent rebar and overflow into the remaining voids from the top slab pour to "cement" the system together as a unit.

So you didn't have anything covering the top of the blocks, so when you poured the top slab it flowed down in there? I see. I was thinking of building a base layer of the tabletop out of plywood (henceforth covering the block tops) and casting the concrete slab over that, cementing it in place. My Dad with construction experience was telling me that'd be perfectly fine since it's not going to be on the ground, don't have to worry about water seeping through the concrete and rotting the wood.

Unfortunately I don't really have any handy friends out here and all of my family is back in Texas. I homebrew beer as well. Once I have the pizza oven up and running I'll have an endless supply of friends willing to trade work I reckon. 'free' pizza and beer makes any man jump!

[SableSprings]

Yes, I just put the form sides up on the outside of my blocks and lifted/braced plywood cutouts into the interior spans. I even laid down plastic on the span areas so when the plywood was dropped out, the resulting bottom surface of the exposed slab would be smooth. I put chunks of Styrofoam and my empty concrete sacks to stuff the alternate exposed block holes to reduce the amount of concrete needed. I actually started to mix my own and after two batches, I called for a cement delivery truck. You can see how the pour was done in my build post's pictures 2 and 3 ( https://community.fornobravo.com/for...712#post386712 ).