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Old 03-28-2007, 05:46 PM
carioca's Avatar
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Default Almost ready for the pour...

Now that the formwork for the round hearth slab is in place, I am getting ready for pouring a 4 in (100 mm) slab - just need to shore up the thinnish 'lost' form work of fibre cement boards at critical points and place #72 reo mesh in the form.

Hopefully it will get a bit cooler (31 C expected today) before I pour the hearth!

(More snaps of my slow progress at Picasa Web Albums - carioca - Forno_del_gallo )

Cheers!

Carioca
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  #2  
Old 03-28-2007, 06:29 PM
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Default Re: Almost ready for the pour...

Wow! That's a serious piece of engineering. Your stand is looking great. Do I understand correctly a 1 meter diameter oven? What is your pad diameter? Are you integrating a landing area and vent into the pad?

Last edited by maver; 03-28-2007 at 06:37 PM. Reason: correction
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Old 03-28-2007, 08:02 PM
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Default Re: Almost ready for the pour...

Looking good! I can just picture those red-bellied black snakes cosily coiled between your support walls, sunning themselves on the warm concrete!

Looks like that galv strapping has tensioned the plywood form up very nicely.

How are you going to keep the reo elevated to ensure a 30mm concrete cover between it and the fibre cement sheets? And I wonder if one round of mesh would do the job well enough? If you use two with 30mm cover top and bottom, there will only be 26mm between each sheet of mesh. Or are you placing them both at the same height, and offset to achieve 100mm squares out of the 200mm mesh?

Cheers, Paul.
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Old 03-29-2007, 03:09 AM
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Default Re: Almost ready for the pour...

I like that! You can start to picture this coming together, and the round shape. Nice.

I also really like how you used the tree stump to bend your rebar. I get a real kick out of rigging stuff up to do a job.
James
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Old 03-29-2007, 05:56 AM
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Default Re: Almost ready for the pour...

Quote:
Originally Posted by maver View Post
Wow! That's a serious piece of engineering. Your stand is looking great. Do I understand correctly a 1 meter diameter oven? What is your pad diameter? Are you integrating a landing area and vent into the pad?
Maver, the engineering, like my baking, is a bit haphazard :-) But thanks for the kind words! Yep, the oven interior diameter is 1 metre; the hearth pad will be a touch over 2 m, so that should give me some room to play for landing area and vent...

Cheers,

Carioca
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Old 03-29-2007, 06:07 AM
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Default Re: Almost ready for the pour...

Quote:
Originally Posted by Hendo View Post
Looking good! I can just picture those red-bellied black snakes cosily coiled between your support walls, sunning themselves on the warm concrete!

Looks like that galv strapping has tensioned the plywood form up very nicely.

How are you going to keep the reo elevated to ensure a 30mm concrete cover between it and the fibre cement sheets? And I wonder if one round of mesh would do the job well enough? If you use two with 30mm cover top and bottom, there will only be 26mm between each sheet of mesh. Or are you placing them both at the same height, and offset to achieve 100mm squares out of the 200mm mesh?

Cheers, Paul.
Paul, I still hope to snap the blacksnakes (and not with our new special $55 snake-catching tool!)...

The reo is a problem area: I poured concrete down all those holes up to 200 mm from the top and stuck the damn things in too deeply, mostly! There will be perhaps 15 mm cover for the bars only in most places, and that includes 6 mm for the fibre cement sheet...

However, the sheet itself will help a bit, and I plan to paint the underside so moisture doesn't wick upwards - in any case, the construction should remain fairly dry due to the nice cross-ventilation under the slab! My German textbooks from the 1970s BTW recommend a mere 10 mm cover for the steel, so I don't feel too bad about my mistake (the hearth will outlast me, I expect :-) )

I have since re-arranged the slab form work to give me 5 in (125 mm) thickness. I am still going to use two rounds of mesh, offset against each other, and placed roughly in the middle of the slab thickness.

My NEXT one will be a damn sight better!

Rgds,

Carioca
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Old 03-29-2007, 07:20 PM
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Default Re: Almost ready for the pour...

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Originally Posted by carioca View Post
There will be perhaps 15 mm cover for the bars only in most places, and that includes 6 mm for the fibre cement sheet...

My German textbooks from the 1970s BTW recommend a mere 10 mm cover for the steel
Luis,

Like a recent discussion on expansion coefficients, a lot depends on the source of the information, doesn't it?! Over the years, I've followed the SAA (Standards Association of Australia) Concrete Structures Code, which specifies 30mm for concrete exposed to the weather or ground/fresh water and 20mm if not so exposed. You could argue that the underside is not exposed to the weather or water, but theoretically a cover of 20mm would still be required.

The code also specifies a couple of "general requirements", which may be worth thinking about: "The cover shall not be less than the nominal diameter of the bar ... being protected." and "... in walls and slabs the cover shall also not be less than the nominal maximum size of the aggregate."

I think the second general criterion is the important one here - if you're using standard 12mm aggregate in your concrete, you risk ending up with a porosity problem below the reo, as the aggregate may be too big to fit under it. So you may not achieve any cover at all in places (except for the fibre cement boards of course). You may want to reduce the aggregate size for these areas, or else pay particular attention to thorough rodding along the length of any low reo, to encourage as much smaller aggregate and sand/cement mixture to flow under and around the reo.

Finally, have you considered placing some Fortecon on top of the fibre cement to assist with moist curing the concrete in these critical areas? Just a thought. But yes, it won't fall down if you don't!

Cheers, Paul.
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Old 03-30-2007, 03:03 AM
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Default Re: Almost ready for the pour...

Paul, thanks for your concern and suggestions!

Yes, I've thought long and hadr about the problem. This morning, it occurred to me that I cut slit the Hardiflex, take some 40 mm poly tubing, slit that in have and chisel little half moons into the concrete blocks where the rebar is too low. than that half pipe would fill with concrete and embed the concrete more properly...

Will keep you posted. BTW, are you a professional engineer? I should have consulted you earlier :-)

Cheers
Luis a.k.a Lutz a.k.a. Carioca
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Old 03-30-2007, 03:39 AM
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Default Re: Almost ready for the pour...

Carioca,

I don’t know that I’d worry about doing as you’re now proposing if it were my project! I mean theory is fine (particular in the planning stages), but at the end of the day, the purpose of good concrete cover is simply to prevent the steel reinforcement deteriorating due to oxidation – ie rust.

The reinforcing bars are tying the walls together, but the steel mesh will also add strength to the hearth slab and prevent it cracking due to tensile forces (bending). Most people don’t have anything more than one sheet of mesh, even when they’re spanning a four foot square void.

Plus you’ve got the fibre cement sheet as well to provide cover for the reo, so I’d think that all that is necessary is to ensure good concrete compaction around those spots where the reo is closest to the sheets (either by smaller aggregate size or zealous rodding!). Add some Fortecon plastic membrane if you like – this will reduce the likelihood of the cement sheet sucking the moisture out of the concrete too quickly and will enhance the prospects of a good moist cure, but this too might be overkill.

Your structure looks stronger than Fort Knox to me, so whatever you choose to do, I think it will remain standing for generations to come!

What do others think?

Paul.
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Old 03-30-2007, 04:12 AM
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Default Re: Almost ready for the pour...

Thanks again, Hendo, and apologies for the inane typos in my earlier reply - I blame it on the Ravenshead Rose we drank after I put the first mesh round in the form (see Picasa Web Albums - carioca - Forno_del_gallo ).

I probably will go without the Fortecon, even though I've got at least half a roll still left over from my shed building exercise - since I'll paint the underside of the Hardiflex I think this should prevent moisture from getting in. And I want the concrete to adhere well to the fibre cement, too...

But I take your point about the aggregate, and perhaps I'll manage to get a more cementitious mix with smaller aggregate in the first few batches that I mix in my old tractor PTO mixer!

Re the bending forces: I plan to use some old 6 mm rods that I've left over from the shed to form a ring or two along the outside of the mesh rounds, but well inside the form work - that should also help keep the damn concrete in one piece :-)

Cheers for now (it's our one night a week for the telly, crime stories on the ABC!)

Rgds,

LMH

Last edited by carioca; 03-30-2007 at 04:14 AM. Reason: URL malformed
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