Concrete Slab Depth and Reinforcement

[Pizza Fermentor]

I read in the FB-provided build instructions that a 3" gravel base is recommended, with a 5.5" concrete slab atop it. Ideally, 2-3" of the slab would be above ground level. The recommended reinforcement is a rebar frame with standard wire screening suspended in mid-slab by brick pieces.

I live in northern Ohio -- is 3" gravel depth good?

Is screen the best concrete reinforcement...I see a lot of people using 100% rebar for base reinforcement.

My slab size will be roughly 68" x 52". It will support a Primavera 70 oven (yeah, I know...I could have bought the FB base instead of building one...but I'm just not a fan of the look).

I have dug the hole (6" deep, rectangular), so I'm either pouring cement on Saturday, or digging a deeper hole.

[dmun]

Re: Concrete Slab Depth and Reinforcement

There's nothing wrong with having more crushed rock. It should be well drained. If you're working at a place in the yard that gets standing water when it rains, you may want more. Otherwise, the recommended amount is fine.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Thank you! I guess I'm doing gravel and framing tonight, and pouring concrete tomorrow.

[Neil2]

Re: Concrete Slab Depth and Reinforcement

Is screen the best concrete reinforcement...I see a lot of people using 100% rebar for base reinforcement.



Go with rebar, about 1.0% or so. Properly placed it is much more effective for structural slab strength than WWF (welded wire fabric). WWF is good for sidewalks or thinner slabs where all you want the steel for is crack control.

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Rebar is definitely worthwhile directly underneath block wall of your stand - the entire weight of the oven, hearth and block wall is transferred into the foundation through the block wall. As for the center of your foundation slab, if you are going with the standard Pompeii block stand, it will not need to take a significant amount of force.

Therefore, in the center of the foundation slab all you need is reo mesh (WWF) to control cracking. Two lengths of 12mm reo bar running on each edge of the foundation to form a reo 'border' directly underneath the block wall will supply the tensile strength in the foundation where it is needed the most.

I've attached a photo of my foundation prep to illustrate, hope what I said makes a grain of sense!

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

I poured on Saturday. I did frame the area that'll be beneath the cinder block with rebar. Thanks for the advice.

My next question is regarding the concrete surface. The surface of my rectangle isn't perfectly smooth. It had areas where the cement pebbles.rock are exposed. I was told "smoothing" it wan't a big deal for this application, since it will be predominantly covered (only the outer 3 inches or so exposed).

I've kept it wet for 2 days, and kept it covered with a plastic sheet.

Should I be concerned about this smooth/rocky surface (it doesn't look beautiful, but is it a sign that I really screwed my mixing up, or does this surface look not have anything to do with the structural integrity?)

Digging the hole myself...dropping 800 lbs pf gravel...and hand-mixing 19 80lb bags of concrete was a ton of work...if this slab isn't sound, I swear I'll jump off a bridge.

Maybe I mixed the concrete too stiff, but it seems like erring on the stiff side is better than on the soupy side?

Can anyone tell me if I need to jump off a bridge, or relax?

[stoveup]

Re: Concrete Slab Depth and Reinforcement

No need to give the bridge any business today. You are in good shape.

The surface quality of your slab will not affect the strength at all. In fact, the less water used in the concrete, the stronger the final result will be (as long as the concrete is well mixed and adequately vibrated or tamped into place). Even if mixing and placement are somewhat haphazard, the weight the slab will be supporting is not all that great when spread out over the perimeter, so you have a very large built-in safety margin.

If your slab is very rough where the block for the foundation walls will sit, you may find a thicker mortar bed is needed to be able to get the blocks aligned and level. Spend the time necessary on the first course of block to get it as level and straight as possible. This will make it much easier to keep subsequent courses level and plumb.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

No need to give the bridge any business today. You are in good shape.

The surface quality of your slab will not affect the strength at all. In fact, the less water used in the concrete, the stronger the final result will be (as long as the concrete is well mixed and adequately vibrated or tamped into place). Even if mixing and placement are somewhat haphazard, the weight the slab will be supporting is not all that great when spread out over the perimeter, so you have a very large built-in safety margin.

If your slab is very rough where the block for the foundation walls will sit, you may find a thicker mortar bed is needed to be able to get the blocks aligned and level. Spend the time necessary on the first course of block to get it as level and straight as possible. This will make it much easier to keep subsequent courses level and plumb.

Thank you!

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Two quick questions as I go to my next step -

1.) Once the upper slab is cured, will the plywood form that supports the upper slab (inside the wood bin) drop after the supporting 2 x 4's are removed? Should I cover it with plastic before I pour (to ease it's removal)? Should it stay stuck to the inside ceiling?

2.) Since I took the easy way out and went with a pre-insulated Primavera 70, is there any problem with simply pouring a 3 1/2" standard concrete upper slab? There should be no issue with leeching heat due to the "standard" concrete if the oven floor is insulated?

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Hi Fermentor,

In answer to question 1, it really depends on the surface finish of the plywood you have used. If you've used 'formply' as we call it, which has a smooth black veneer laminated to it, then you've got a good chance of pulling the ply off afterwards (you can see the ply I'm talking about in the photos attached). If you've gone with standard timber ply with no laminate, you might find it will stick pretty firmly to the underside of your slab, as the grout from the concrete will work its way into the open fibres of the timber. But if you've gone with the normal Pompeii block stand design, you'll never, ever see the underside of your slab anyway! (Unless you're using it as a bomb shelter, and in that case some ply will be the least of your worries)

Q2. In my humble opinion, it doesn't matter HOW you insulate your floor, so long as you DO insulate it. If you use vermiculite concrete, you won't need as much Calcium Silicate board or ceramic fibre board. I've gone with a plain concrete slab for structural strength, and I'm going to use somewhere between 50 to 100mm of calcium silicate board to insulate the cooking floor from the slab.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Thanks, Ben.

I have one additional concrete slab question --

I mixed and poured the base slab on Saturday. I covered it with plastic and occasionally watered it through yesterday. Pending the weather, I want to lay cinder block tonight.

When I removed the plastic tarp today (to let it dry-out in preparation for mortar and blocks tonight) rain water on the surface poured on to the slab. And it poured through the slab somewhat easily...like a sieve.

I use a hoe to tamp the concrete as I poured. It seemed very dense at the time.

Is this slab porosity normal 3-days after the install? Will it diminish as the slab dries? Do I need to apply a sealant? Did I horrifically screw-up the mix?

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Can you describe the consistency of the concrete mix that you used? You said you used bagged mix - so you can't have gone wrong in regards to cement, sand and aggregate content. That just leaves the possibility of a really dry mix, which if properly vibrated during the pour wouldn't be an issue, however if it has just been tamped down then there is a chance the grout or 'slurry' in the mix hasn't worked its way fully around the aggregate in that area, leaving small voids for water to penetrate.

How much water does it absorb? Are we talking buckets? I still wouldn't be concerned about the strength at all. It'll hold up your oven just fine. The problem is, that if there is a path that water can travel through the concrete to reach the reinforcement in the slab, it will eventually rust the reo and cause cracking in the slab. Additionally, if you live in an area that experiences extremely low temperatures, this water will go through cycles of freezing and thawing - on freezing the water in the concrete expands, creating cracks. On thawing, more water fills the new cracks, then it freezes again and you ned up with pretty severe cracking.

For what it costs, I'd seal it and never worry about it again.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

If I pour a quart of water on it, the water disappears like magic. It shocked the heck out of me. In northern Ohio, freeze/thaw is a definite issue. I gotta fix it.

I called Quickcrete and they recommended I place a 3/8" layer of sand topping over the surface. I guess that is what I'll do. Hopefully I can simply raise my slab form 1/2 inch.

They agreed that the concrete strength would be fine.

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Sounds like a plan my friend. When you say 'sand topping' do you mean some kind of cementitious grout? Like a concrete mix without the coarse aggregate? That would do the job well. You could just get 3/8" strips of timber and nail them to the top of your existing formwork to raise it up, PLUS this will mean that you can level your foundation slab and screed it off to a perfectly smooth finish - makes it a bit easier to lay that first course of blocks.

Block work is good fun, it's very satisfying to see the structure come together. Good luck, and it's about time you gave us some pictures!

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

The mix I mentioned is exactly what you thought. I raised the form tonight, and I believe it will make a nice level surface. I expect to pour it tomorrow.

I called the people at Quickcrete (concrete mix manufacturer), and they offered to have a local rep come out and review my base. I need to call them back tomorrow, but if they are sincere, I'm going to take them up on their offer. I know I must have mixed it too dry, but I followed the directions to a T. Over the course of 19 bags, any overage or underage should have averaged-out?

Pictures? I'll post a before/after shot of the base once it's done. I'll also post the rep's thoughts on what went wrong. Maybe it keeps someone else from making the same mistake. Although some extensive Google searches lead me to believe I'm the first person in the world to experience the sieve-base concrete phenomena.

Thanks to everyone for the guidance and tips. I really appreciate it.