#1  
Old 06-17-2011, 05:58 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default Concrete Slab Depth and Reinforcement

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.
Reply With Quote
  #2  
Old 06-17-2011, 10:40 AM
dmun's Avatar
Moderator
 
Join Date: Jul 2005
Location: New Jersey USA
Posts: 4,216
Default 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.
__________________
My geodesic oven project:
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
,
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Reply With Quote
  #3  
Old 06-17-2011, 12:36 PM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default Re: Concrete Slab Depth and Reinforcement

Thank you! I guess I'm doing gravel and framing tonight, and pouring concrete tomorrow.
Reply With Quote
  #4  
Old 06-19-2011, 03:23 PM
Neil2's Avatar
Il Pizzaiolo
 
Join Date: Oct 2006
Location: Vancouver Island
Posts: 1,374
Default 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.
Reply With Quote
  #5  
Old 06-20-2011, 04:32 AM
benguilford's Avatar
Laborer
 
Join Date: Dec 2010
Location: Melbourne
Posts: 99
Default 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!
Attached Thumbnails
Concrete Slab Depth and Reinforcement-foundation-formwork-reo.jpeg   Concrete Slab Depth and Reinforcement-12mm-reo-foundation-border.jpeg  
Reply With Quote
  #6  
Old 06-20-2011, 10:37 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default 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?
Reply With Quote
  #7  
Old 06-20-2011, 12:52 PM
stoveup's Avatar
Laborer
 
Join Date: Sep 2010
Location: Winston-Salem, North Carolina, USA
Posts: 86
Default 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.
Reply With Quote
  #8  
Old 06-20-2011, 08:03 PM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default Re: Concrete Slab Depth and Reinforcement

Quote:
Originally Posted by stoveup View Post
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!
Reply With Quote
  #9  
Old 06-21-2011, 11:09 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default 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?
Reply With Quote
  #10  
Old 06-21-2011, 03:06 PM
benguilford's Avatar
Laborer
 
Join Date: Dec 2010
Location: Melbourne
Posts: 99
Default 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.
Attached Thumbnails
Concrete Slab Depth and Reinforcement-hearth_formwork2.jpg   Concrete Slab Depth and Reinforcement-hearth_reo1.jpg  
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Vermiculite depth - sturdy??? ross Getting Started 3 08-30-2007 01:11 AM


All times are GMT -7. The time now is 10:25 PM.

Powered by vBulletin® Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
2006/10 Forno Bravo, LLC