Lightweight (perlite) concrete and rebar
I’m building a wood-fired pizza oven in my kitchen (yeah, well, gotta keep busy 🙂
It’s on a metal stand (which I built out of 2″x2″ steel tubing, welded together). I plan to pour a lightweight concrete base, then assemble the oven on top of the concrete surface. The oven weighs ~800 lb, and has a 40″ diameter. But the weight is not uniformly distributed (most of it is on the circumference, 3″ thick). I think that the highest load would be 3-4 lb/sq inch, which is well below the specs I saw for lightweight concrete
The top of the metal stand is roughly an exagon (5′ by 5′), with a web of 2×2 tubing to support the bottom of the concrete slab. The biggest “span” inbetween the support tubing is 25″x18″. I plan to put durock or hardiboard on the metal stand (1/2″), and pour a 4:1 mix of perlite and concrete on top (aka lightweight, insulating concrete). I need the perlite for insulation, as the floor of the oven can get very hot (700 F).
The slab is 5″ thick. The question is: should I use rebar in the slab? I read mixed opinions on having rebar in lightweight concrete (compressive strenght is much lower)
I can think of 3 solutions (I can ‘t have more than 5″ slab)
– Pour a 5″ lightweight slab, no rebar. The span is small enough to cause no problems
– Pour a 5″ lightweight slab, with rebars
– Pour a 3 1/2″ lightweight slab, lay rebars on top, add 1 1/2″ standard concrete on top, to provide structrural support to the oven and help spread the load more uniformly
Any other option? Anything less than 3″ – 3 1/2″ perlite concrete risks being not enough insulation (or so I was told)
Thanks in advance, Rob
Replies
Or you could use Fiber reinforced concrete. The three main fibers used are nylon, polypropylene, and steel (I'm sure there are others as well). Since I have no actual experience finishing a slab with fiber in it, take this advice with a grain of salt.
I've heard that the polypropylene fibers tend to stick out of the finished slab like little whiskers, the steel fibers are sometimes randomly visible here and there on the surface, and the nylon are invisible and the slab finishes just like regular concrete.
Here's a # (401 596-3955) for a company called Nycon (http://www.nycon.com). They're distributors for all kinds of secondary reinforcement fibers for concrete. There's a guy there named Bob Cruso who can answer any technical questions you might have including if you need rebar as well as the fibers for your application.
Good luck!
Edited 4/18/2005 3:23 pm ET by fingers
Thanks for the suggestion. I'll call them tomorrow (today they are closed already)
My main concern is that steel fibers could speed up the heat transfer, and nylon or poly will melt at 700F (at least close to the surface)
Any idea on how useful a rebar just 1" or 1" 1/2" below the concrete surface (sitting on a lightweight concrete slab) would be? Rebars are cheap :-) so as long as they don't create more problems, I might as well add them
I read mixed opinions on having rebar in lightweight concrete (compressive strenght is much lower)
Ah, but the rebar is to give concrete tensile strength.
Looking at your numbers, the question to answer is does a 25" span allow the LWC to deflect enough to need tensile reinforcement?
My guess is, "no." That and a sawbuck will get you a coffee at the fancy store, too.
If you can get ready-mix with reinforcing fibers, that might be just the trick. Be prepared, though, LW is more expensive than "standard" weight (the cost of adding reinforcing fibers won't dent the budget much after the LWC mark up).
Maybe there's a good mud hand out there reading; there was something on not using fly ash in LW the same as they now do in regular concrete--but the CRS is keeping me from remembering.
Yeah, rebar is for tensile strenght. What I meant is that rebars "create" tensile strenght by using the concrete compressive strenght, and a weaker compressive concrete might not get much benefit from the rebar (the way I understand it, concrete "grips" the rebar and the rebar provides tensile strength. no compressive strength, no tensile either, as the rebar would just slip when the concrete cracks around it). I just shortened a long thought into one (useless :-) sentence
Yep, the real question is: will a mix of 4 parts perlite 1 part portland cement have any problem over a 20" x 25" span? if so, will a 3/8" rebar make any difference?
Using fiber reinforced concrete won't be a big cost issue (it's a very small slab, after all... probably just 1 sack of cement). I just worry that the fiber could melt and make things worse. And when I say "LW concrete" I just mean a simple mix of portland cement and perlite, nothing fancy
Place a layer of CBU (Durock, etc.) Probably only need a piece of visqueen well glued to the 2x2 struts, but I like OVERKILL!.!.!.
Pour LWC.
That's all.
No rebar, fiber, or any other reinforcing because there is no tensile load on this tabletop pad. Pure compression. . . uh. . . unless gravity reverses, that is.
Heck, use brick loose laid on sand on top of the Durock. Even if you forget to put the brick on top of the sand, it'll be ok.
SamT
I say no rebar.
Reason?
If you're concerned that the high temps might melt the poly fibers within the slab, then I'm concerned that the high change in temp will heat the rebar, causing it to expand, possibly cracking the slab.
I hate to respond but yall know I cannot help it. Fiber offer no structural value at all. fiber just control the expandson and contraction of concrete. Fiber controls the cracking of concrete. The different of temp. will crack a slab.Rebar offer no structural value in compression, only tension. The table top or slab, think of as a beam. with weight added to center the slab will curl at ends. Remember rebar only work in tension. so when the slab curls it pulls and push on the rebar in tension. This counteracts the concrete from curling. so although you think as the slab in compression its really as bending the slab. That why compaction is so important under slabs, it reinforce the slab from bending.In conclusion, rebar is cheap its not hurting anything to add it.one more thing fiber and rebar has two diferent jobs, they cannot replace each other.
Edited 4/18/2005 9:46 pm ET by BROWNBAGG
Brownbagg,
As you probably already know, I'm starting a business. Man it seems like I been 'starting' it forever.
I'll be pouring footings and basement walls. These walls may be subjected to a 15*f/ft temprature differential between air and footing and maybe 100*f exterior to interior.
Except where otherwise indicated, I'll be using a single plane of #4, H&V, 12" OC, and that will set 3" from the interior rather than centered.
Would fibermesh be enough to prevent cracking?
If so, which would you reccommend?
Or, do you have a better idea?
Most of my work will be without engineering (Subbing to O/B's) and I'm trying to find a good standard mix to use.
SamT
that much heat to react with the rebar woud destroy the slab without the rebar anyway...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
Brownbag is correct in that fibers in concrete give no strength value, they simply help control excessive cracking. Rebar does help fight stresses due to tension. I think Imerc is also right...the heat (700 degrees) will destroy your concrete surface. If you don't think so, just light a blow torch and hold it against a concrete sidewalk for about a minute...the concrete will start to pop and tiny bits will fracture off.
In heat situations, brick is king. Brick is insulative, and durable, and helps to hold an even heat. Every pizza Oven I ever saw was made from brick...including the hearth.
There are ready made products that you buy in a 5 gallon bucket that will glue brick together...it's similar to a water down mortar, but is very high temp stuff.
Log on to a coooking site...bet they can give advice on how to construct a pizza oven.
Davo
Wouldn't you need to use some sort of refactory(?) cement/motor? You might do well to check at your local masonary yard.
Jim
The awful thing is that beauty is mysterious as well as terrible. God and the devil are fighting there, and the battlefield is the heart of man.- Fyodor Dostoyevski
What I didn't explain in my previous post, is that the oven has a refractory floor already, and it will have another layer of refractory concrete between the floor and the concrete.
The example of a blowtorch to a sidewalk doesn't apply here, really. There are a few critical differences:
- with a blowtorch (hotter than 700 anyway), you have huge temperature differences between adiacent spot, causing significant mechanical stress due to uneven expansion. The refractory floor takes care of evening out the temperature over a big area
- A blowtorch raises temperature very quickly. Once again, the refractory floor takes care of slowing things down
- more importantly: every pizza oven needs to be fully cured and dried before being fired at full heat. It takes a week or so of slow fires to slowly build up temperature and drive water out of the structure (once cured, as long as it's used relatively frequently, say every month, and the oven is in a dry environment, like a kitchen, it can just be fired). When you take a blowtorch to a sidewalk, most of the spalling and concrete destruction is due to the water steaming out explosively, taking pieces of concrete with it.
Pretty much all the oven construction websites suggest using standard concrete under the refractory floor (bricks or refractory concrete), as long as the oven is properly cured. Alas, pretty much everybody builds the oven on a brick stand, and they have no problem with spans. But I can't use the same, due to the load on my kitchen floor
I use 6 to 1 ratio (by volume). 6 perlite 1 cement 1.5 water. It turns out a density of 700- 800 kg / m3. https://youtu.be/wf74URYwaT4
You didn't say how thick the refractory floor will be, but I assume you are following the instructions/ recommendations of the oven kit. Concrete begins to break down at temps over 400 degrees, though that won't be an issue under the other layers. But if you are worried about the heat protection I'd suggest insulation fire brick. You can find them on the internet, often for repairing or building kilns. They are typically 2.5"x 4.5"x9", and could be laid on top of a slab with rebar in it. The bricks will offer only compression strength,and I wouldn't trust concrete board to provide support over the gap you indicate
The bricks could be laid dry as you will be casting another layer over them. They are very easily cut to fit the shapes you need.
I think your idea of a 5" slab of perlite mix is OK with rebar, but it's not the kind of situation where you can redo it, so it's your call. I'm assuming you plan to do a layer of insulation of some kind over the oven when it's finished or your kitchen could get pretty warm.
I've worked on many masonry projects designed to resist heat, and I'm hoping you have a good plan for venting your oven, especially if it will be a wood fired . Have fun!