Shaping vermiculite concrete dome

[clg763]

I would like to hear some ideas for the easiest way to shape a vermiculite concrete dome over my modular oven. I have blanket insulation and want to finish it with a stucco dome, from my reading, the simplest way to do this would be to shape vermiculite concrete over the top. I have seen several different people make rebar armatures for their oven and place the concrete on the that, I have also heard that all you need is the vermiculite concrete to shape the dome. I am worried about maintaining a decent looking dome shape with this approach.

I know many have done concrete vermiculite domes but I am having trouble finding good builds and photos from my searching.

[Gulf]

Re: Shaping vermiculite concrete dome

Most just shape the vcrete by eye and do great work. Some have used templates. I can't swear that it is easier to use a template but here are some pics of mine.

[Les]

Re: Shaping vermiculite concrete dome

Gulf,

I wished I would have thought of that. I did mine with rebar and wire and it was a bitch. Yours is incredibly symetrical...

[Neil2]

Re: Shaping vermiculite concrete dome

The chicken wire seems to be to hold the blanket in place. If you are using vermacrete alone as insulation for the dome you can skip the chicken wire.

[Les]

Re: Shaping vermiculite concrete dome

The chicken wire seems to be to hold the blanket in place.

Neil,

I used the wire over the rebar to form the dome. I slopped vermiculite / portland in between the wire and the blanket.

[Gulf]

Re: Shaping vermiculite concrete dome

The chicken wire seems to be to hold the blanket in place. If you are using vermacrete as insulation for the dome you can skip the chicken wire.

I probably, could have saved that step (and $) for the render . I am planning to install a layer of stucco wire in the render over the vcrete. I will be laying 1/2 concrete splits over this to finish.

[Gulf]

Re: Shaping vermiculite concrete dome

Gulf,

I wished I would have thought of that. I did mine with rebar and wire and it was a bitch. Yours is incredibly symetrical...

Thanks Les,
In all of my experince with building (born as an endentured servant into a construction family ), I have been taught to rely on measurements, levels, strings, speed poles, templates (the list goes on). I did not trust my "young eyes" so I definately don't trust this old pair that I have now .

[david s]

Re: Shaping vermiculite concrete dome

I place the vermicrete with my hands, one handful at a time, starting from the bottom of the dome work all the way around with one hand on the outside and then one on top to create a ledge for the next row. I press and hold for about two seconds and it stays really well, but you need to get the mix just right. If you have water sitting in the bottom of your mixing bucket then you've added slightly too much water. When you've placed the vermicrete over the whole dome you can tap the surface with the face of a trowel lightly to get a perfect hemisphere.

[clg763]

Re: Shaping vermiculite concrete dome

Ok, I have begun putting the vermiculite concrete on the dome using a 10:1 ratio, I am having a very hard time getting the cement to set up though.

I started off with a 3:1 ratio of water to cement for the slurry, then poured this over my vermiculite and mixed this by hand. The mixture did not have the feel of good concrete, it was still very crumbly despite all the moisture and nothing I could do seemed to help the texture. I just sort of set a course of a few inches around the dome but the mixture was a fairly loose pack and I don't feel good about it. Now, almost two hours later, the mixture has not set at all. I have never seen concrete do this, what am I doing wrong?

[david s]

Re: Shaping vermiculite concrete dome

I find mixing 10 parts vermiculite to 1 part cement (by volume) dry, then adding 3 parts water and mixing it with my hands in a large (20 litre) bucket works pretty well. Replacing the vermiculite portion with 50% vermiculite and 50% perlite improves the workability of the mix.

[clg763]

Re: Shaping vermiculite concrete dome

How soon does the mixture start to cure? How much should I try to compact the mixture with my hands? So far I have seen very little signs of curing, I am used to concrete no longer being workable after less than an hour.

[david s]

Re: Shaping vermiculite concrete dome

It should be firm in 24 hrs. You'll be able to easily crumble it away, but all you need is a surface that is sufficiently strong enough to render/stucco on to. Dry it out well before doing this though.

[rjdealhome]

Re: Shaping vermiculite concrete dome

I came across this thread while looking for information on using vermiculite concrete as a basecoat for stucco on my dome, and thought I would pass on this chart that I found on the Schundler Vermiculite Concrete site. It may be useful for those wondering (as I did) about different mixes and relative strength.

[lwood]

Re: Shaping vermiculite concrete dome

The most important value is missing, the "R" value. Hundreds of ovens have been successfully built on top of the 7 or 8:1 mix, as long as it is properly supported. The dome certainly is not structural. The vermicrete will crumble to the touch 24 hrs after. So be careful when you remove your forms. I gave it 3 or 4 days before messing with it. It firms up nicely and proceed. It's structural properties are not a concern here, insulation is the issue....the more the better.

[Neil2]

Re: Shaping vermiculite concrete dome

Here is the screed I used for shaping the vermicrete layer (12:1 mix):