lightweight concrete pad

[slackr]

I've just finished building a base for an oven using 8" conc.block. I'm about to pour a slab and am considering doing it entirely with vermiculite concrete. I was curious if anyone else has used this method and how it will hold up in use. My pad will be 7'x7' and I plan on pouring a 6-8" thick slab. I will be using 1/2" re-rod on 1' centers and a layer of concrete mesh in the mix and am curious if this method would support a 42" oven which, in effect, will be resting on an 8" wide perimeter wall. Any input would be appreciated.

[DavidApp]

Re: lightweight concrete pad

Hello Slackr

Several thoughts.
Seems that you will have about 5'+ between your block walls will you have internal walls to add support? 5' seems a wide span unsupported.
I do not think that vermiculite concrete has much structural strength.
A regular concrete pad with a layer of vermiculite concrete on top would probable be stronger.

I have decided to go with a 5" thick pad and will probable use 5000lb concrete.

David

[slackr]

Re: lightweight concrete pad

Thanks David,

I sort of answered my own question the other day by casting a few samples. The 1:5 (cement:vermiculite) sample was, as you stated, really too weak for structural support. I was also after a lightweight mix being as thick a slab as I planned. With a 1:2:3 (cement:sand:vermiculite) mix the sample seems much denser and I believe with the added re-rod and wire mesh, should provide enough support with a 6" plus slab. It also has a weight reduction by a factor of approx. 30% over a conventional 1:2:3 (cement:sand:stone) concrete mix. I am going to lose most of the insulating qualities of a straight vermiculite mix, but supporting the oven takes priority here.

Bob

[Greenman]

Re: lightweight concrete pad

Hello Bob - The usual method is to pour the structural slab and then the vermiculite insulation layer above it. I used a keyhole shaped mould to contain the vermiculite layer within the slab structure. The total depth of the slab is 8". It seems to work well and isolates the structure of the dome and entry.

This provided the structural strength and the insulation properties.

I am happy with it.

[Tonyp]

Re: lightweight concrete pad

Definitely don't want to skimp on either the structural support slab or the insulating layer. If you have up to 8" thickness to work with you could easily do a 5" thick support slab and 3" of ceramic board for insulation which would give you the best of both worlds.

[slackr]

Re: lightweight concrete pad

Steve and Tony,

Appreciate both your replies. I believe I'll go ahead with a 6" slab, then add my 2 or 3" insulating layer on top of that.

Bob

[ATK406]

Re: lightweight concrete pad

Thanks David,

It also has a weight reduction by a factor of approx. 30% over a conventional 1:2:3 (cement:sand:stone) concrete mix. I am going to lose most of the insulating qualities of a straight vermiculite mix, but supporting the oven takes priority here.

Bob

So what's the point? Why compromise the strength of the slab to save on weight? If you want to minimize heat loss to the slab, use more insulation on top of the slab (insulate-insulate-insulate! as advised by everyone on this forum), but you don't have to weaken your slab to do it.

Regards,
AT

[david s]

Re: lightweight concrete pad

Hello Bob - The usual method is to pour the structural slab and then the vermiculite insulation layer above it. I used a keyhole shaped mould to contain the vermiculite layer within the slab structure. The total depth of the slab is 8". It seems to work well and isolates the structure of the dome and entry.

This provided the structural strength and the insulation properties.

I am happy with it.

If you do it this way and the oven is out in the weather, it is a good idea to make a few holes through the supporting slab into the vermicrete layer as drain holes. Should the vermicrete get wet they help dry it out again.