Concrete Slab Depth and Reinforcement

[markdavis]

A 3" gravel base is okay, but 4 to 6 inches would be better for a stronger and more stable slab.

For reinforcement:

• Rebar is stronger than wire mesh.
• If you want it to last and hold the oven well, go with rebar.
• You can still use mesh, but rebar is the better choice for support.

If you can, dig a little deeper, add more gravel, and pour a 5–6 inch slab.
It’ll help your slab last longer and stay solid.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Your slab looks OK to me - it will very strong in spite of the voids. In fact, if it were mine, I would not put the grout layer on the top immediately - I would start the block work directly on the slab as it is.

I'm more comfortable with the block adhering directly to the slab rather than the grout layer, so I think I will take your advice. Thanks!

[benguilford]

Re: Concrete Slab Depth and Reinforcement

That's great news Fermentor! PHEW *wipes brow*

[stoveup]

Re: Concrete Slab Depth and Reinforcement

Your slab looks OK to me - it will very strong in spite of the voids. In fact, if it were mine, I would not put the grout layer on the top immediately - I would start the block work directly on the slab as it is. But either way, you will be fine.

I am not concerned about the rebar in your slab, either. If you had left the rebar out completely, it would still be OK. The plans for the Pompeii oven are seriously over-engineered, so there is [U]lots[U] of leeway.

I am neither a professional mason nor a masonry professional. In a previous life, I was a contractor and did a lot of concrete work, primarily for the U.S. Army Corps of Engineers. Those guys taught me more about concrete than I really want to know - including how to bust it out and do it over!

When you are ready to pour the hearth slab, don't add a whole lot more water to the concrete mix. Based on the picture, it will take only a small amount in addition to what you used for the base - add it in small amounts until you have a mixture that is easy to move around in the forms with a shovel but that does not flow of its own accord. Too much water can weaken the final product by allowing the small aggregate particles (sand) to fall out as the concrete is poured and manipulated.

[RTflorida]

Re: Concrete Slab Depth and Reinforcement

That really looks like a dry mix. Its been my experience that the manufacturers grossly recommend under hydration for their premixed bag products. I'm guessing they are aiming for maximum strength, they always say a dryer mix is the strongest, but the average homeowner has no clue that he needs vibrating equipment just to pour a simple slab for a sidewalk, shed, or in this case an oven. I've done a couple of dozen small slabs with premixed bag products over the years and have had to add more water than recommended every time in order to get it flow. I won't say that I have not affected the strength, but I have never had one crack
I think the rep has you headed in the right direction. I also agree with your plan to seal any remaining exposed areas.

RT

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

The rep evaluated the slab and said it will be fine, as long as I coat it with the sand-mix topping. He recommended 1/4".

He said that, if I'd have smoothed the surface, I would have been fine. It would have sealed. By placing the sand-mix atop (and along the side) of the block, it will take care of the process that should have happened if I'd have troweled the top in the first place. The rep mentioned several times that, overall it "looked like a really nice job." The plastic and the gravel base should prevent water from wicking from the underside...and he felt the bottom 90% of the slab was solid anyhow.

I am going to try the topping. I've already bought all the sand-topping and bonding agent, so I don't have much to lose. I also think it will be a good idea to apply a concrete sealer annually.

I figured I'd have problems somewhere, but never with the base slab. Oh well...any project you undertake costs twice as much as you'd expect, and takes twice as long.

As always, I really appreciate everyone's comments and support. Great group of people here. I look forward to the day when I can discuss pizza and bread techniques rather than my masonic ineptitude.

[benguilford]

Re: Concrete Slab Depth and Reinforcement

I can see what you mean - I think I was picturing a small area in the centre of the slab that was affected by this, but it's the entire slab, through the full thickness.

What did the Quickcrete rep have to say? If you have followed their instructions to the letter as you said, then they clearly did not provide you with sufficient information and are thereby at fault - I'd be asking them to pay for the removal of the slab AND to provide you with replacement bagged concrete. As I've said previously, the slab will still be able to take the load, however Neil is right when he says it will eventually fail, due to water ingress and subsequent freeze/thaw cycles. Putting a layer of grout on the top won't seal the slab completely, as you can see the sides are porous as well, and presumably so is the underside. If the rep can come up with a way of properly sealing the slab (such as flooding the entire slab with low viscosity epoxy resin to fill ALL of the voids - an expensive solution) then it's worth considering, but DANG. That sucks Fermentor...

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Here's a pic of the problematic base.

[Pizza Fermentor]

Re: Concrete Slab Depth and Reinforcement

Quickcrete rep is stopping by at 8am tomorrow. No fee. We'll see what he has to say.

I'll try to post some photos tonight.

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Could you put up some photos of your slab so we can get a visual for what you're dealing with?

[Neil2]

Re: Concrete Slab Depth and Reinforcement

" If I pour a quart of water on it, the water disappears like magic"

I suggest you take it all out and do it over. A slab with that kind of voids is likely to fail.

[benguilford]

Re: Concrete Slab Depth and Reinforcement

Getting the rep out there sounds like a great idea, unless they're going to charge you for it!

I would suggest the problem is this; your mix may have been slightly dry when you poured it, but then the really critical part is that the concrete wasn't vibrated thoroughly. Have you ever seen a stiff mix of concrete being vibrated? It behaves like a liquid, and flows into all corners and around the reo. Without some kind of immersed vibration, dryish concrete even if tamped down, will potentially set with voids all through it. Check out this Youtube clip showing some relatively dry concrete being vibrated to see what I mean; Concrete Sample under Vibration

I'm the Precast Concrete engineer for a large freeway construction project here in Melbourne, so I play with concrete all day! The key to working with stiff concrete is to vibrate it into place (among other things we use pencil vibrators - looks like a brush cutter with a big... well.. 'thick metal rod' on the end that vibrates like mad). If you don't use one of these, at least get a trowel or shovel or something and just give the mix a really good 'poking', this will help the cement slurry work its way around the aggregate and allow air bubbles to work their way to the surface. Definitely do this when you pour your hearth slab, and you'll see the difference.

[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.

[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

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.