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Post by h12rpo on Aug 27, 2012 13:37:42 GMT
I've looked quite a few builds but can't see how a cast top slab is supported - ie what spans the void? (The side walls) - a sheet of ply which is left in place ? Or a sheet of ply which fits inside the void but which is supported for the cast and then dropped down when the slab is cured
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Post by pete123 on Aug 27, 2012 14:13:44 GMT
Some people use lintels. Others (myself included) just cast the slab out of concrete reinforced with steel rods (rebar) & mesh. The cast slab supported by the walls. Considering the weight on top of mine I'm amazed it hasn't collapsed! if you cast your own slab make sure you use ballast % perhaps a strong mix of concrete? Given the weight on mine I used the thinnest celcon (themolite) blocks I thought I could get away with. Most people seem to use hollow blocks for the walls.
I think it was Terry who suggested that if there is one bit worth overdoing it's the slab across he middle.
I think a reinforced slab is effectively a lintel?
Good luck.
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Post by faz on Aug 27, 2012 14:57:05 GMT
In the picture below you can see the reinforcing bar I put into my slab, and if you look closely you can see three lintels that I put in, two across the doorway and one diagonally across the centre of the structure. The ply is in place to allow the concrete to be poured in and is removed once the concrete has set.  |
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Post by tonyb on Aug 27, 2012 15:40:14 GMT
I used a plywood base to hold the reinforced concrete and it was left in place. My build was into a banking so no void underneath the wfo.
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Post by turkey on Aug 27, 2012 18:13:50 GMT
i used various sheets of wood to make the base of the cement tray for want of a better term, I balanced them on anything to hand but did make sure they could be semi easily removed after the cast cured.
Some use cement tile backer board so they can leave it in there and not worry, in which case I think they lay this over the blocks slightly and support from below.
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Post by spinal on Aug 27, 2012 19:27:40 GMT
I went with a sheet of ply and some scrap wood for the gaps. All supported by whatever wood scraps I had left over and could cobble together. I did make a mistake, in that my sheet of ply was bigger than my wood-storage "door" - so when I removed the supports, the ply was trapped inside the wood storage area. I had to then take a jigsaw to it in order to remove it in 3 chunks. If I had to do it again, I would have pre-cut the ply (or bought smaller/cheaper ply) and made it easier to remove... Pictures here: ukwoodfiredovenforum.proboards.com/index.cgi?board=pompeiiovens&action=display&thread=557 |
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Post by faz on Aug 27, 2012 20:09:59 GMT
I think the other common mistake is to forget to use shims under whatever timbers you've got supporting the plywood base. If you put shims in it is easy to get the timbers out. If you don't, you might find you have to climb under the oven, watching out for nails sticking out of the ply where you've already removed some of the supports, and kick like mad before the timbers will come out.
Not that I did that, of course.
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Post by spinal on Aug 28, 2012 11:56:59 GMT
I think the other common mistake is to forget to use shims under whatever timbers you've got supporting the plywood base. If you put shims in it is easy to get the timbers out. If you don't, you might find you have to climb under the oven, watching out for nails sticking out of the ply where you've already removed some of the supports, and kick like mad before the timbers will come out. Not that I did that, of course. Or tie a rope and pull... somehow I couldn't bring myself to kick out the supports knowing that there was 700kg of concrete above that I didn't know if it would hold or not  |
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Post by cannyfradock on Aug 28, 2012 17:08:49 GMT
I've looked quite a few builds but can't see how a cast top slab is supported - ie what spans the void? (The side walls) - a sheet of ply which is left in place ? Or a sheet of ply which fits inside the void but which is supported for the cast and then dropped down when the slab is cured ..... ....your nearly there on your reckoning. The underside of your slab will be a piece of ply or chipboard. This will be cut to fit in the internal dimensions of your inner wall. If the inner wall is......the external wall, then a shuttering must be put on the outside surface to hold the concrete in place. I build 2 walls.....thermolite interior... to take the the concrete slab, which site on top of it.....and an exterior "face" wall which forms the outside shuttering when pouring the slab. AS....has been mentioned, the re-bar will give the concrete slab the support to span the small dimensions, that we need. Some people build a supporting wall 3/4 of the depth of the wood storage area. I personally don't as by incorperating the re-bar the short span should support itself. The piece of timber that supports the underside of the concrete slab is sometimes better if put in two halves....purely for ease of getting the timber back out again. Always go way overboard with your shuttering supports, and Faz's comment on shims.....or folding wedges, will play a major role in many stages of your build.... Terry |
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Post by h12rpo on Aug 28, 2012 19:27:01 GMT
Aha that makes sense, and thanks for the tips too fellas  |
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Post by DuncanM on Aug 29, 2012 11:37:49 GMT
I agree with Terry's comments re going overboard with the concrete shuttering support... I had part of mine bend out and wish I'd given that side more support - Something simply fixed if I were to build another in future.
In the end it meant there isn't as much space for insulation between my external (facing) red brick wall and the concrete hearth - which results in 4 or 5 bricks getting a little warmer than I'd have hoped (they don't get so hot it causes a problem, but it's obviously some wasted heat).
I can't remember if I posted any pics showing how I did it in my build, but I used concrete backer board (and therefore left in place on purpose), but supported it with lots of 4"x2" that was supported on more lengths of 4"x2" and a few shims/bricks to make the removal easy.
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conic
WFO Team Player
Posts: 186
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Post by conic on Aug 29, 2012 16:35:56 GMT
me being a tight bugger, instead of buying reinforcing bars or mesh, i used next doors front wrought iron gate.
well he was not using it at the time and its not too far away from him. just in 100mm of concrete.
what a solid job though. i wonder what people will think when its uncovered in years to come. hehehe
ps, i used basic shutytering as per the rado plans
conic
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Post by bookemdanno on Aug 29, 2012 20:19:22 GMT
I've got one in the base of my oven too!
It fitted perfectly, is more robust than A142 mesh and i no longer scratch the wing mirrors of the car on it!
I did chuckle in revenge when that first barrow of compo slid onto it!
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Post by h12rpo on Aug 29, 2012 21:14:36 GMT
* scuttles off to look for an abandoned gate :-)
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Post by turkey on Aug 30, 2012 7:46:40 GMT
my initial plan was to use a gage, I even managed to buy one cheap off ebay but was put off using it for the hearth (no idea why I did not sling it in the base). The reason I did not use it was the time it would have taken to prepare.
the metal reinforcement should be throughout the slab, the rebar is ribbed so each part of the cement has something to grip and then is stronger under stretching forces.
the gate will be mostly smooth with horizontal ties, it will offer some support but I think to be very efficient needs grooves cut in you hobbled welded on to offer up the grip the concrete needs, my other idea was to drill it or similar so I could attach bolts for the grip.
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