Concrete blocks in my barrel oven build

[mrjl]

Thanks Dave, you're a fountain of knowledge! I owe you a pizza

[mrjl]

Tomorrow is D-day for the first concrete layer of the dome. Wish me luck

[mrjl]

A good dense mix can be made cheaply from 3 parts sand, 1 part cement 1, part powdered clay, 1 part hydrated lime, by volume

Hi Dave. I'm going to use this mix for my thermal mass concrete later. How much water needs to go in?

Tbanjs

[oblertone]

How much water ? A. As little as possible whilst making it workable, what goes in has to come out.

[downunderdave]

Jun 1, 2018 22:26:17 GMT mrjl said:

Tomorrow is D-day for the first concrete layer of the dome. Wish me luck

Make it up to “ball up” consistency, ie a handful about the size of a cricket ball it should stay in a ball when thrown vertically 2 feet in the air. Too wet and it will slump, too dry and it won’t hold together well and will result in voids in the casting.

[mrjl]

Thanks chaps. Concrete cricket here we go!

[mrjl]

All went well today but I’ve just checked and there are a few cracks over the oven. No more than 1-2mm wide but there are quite a few. Is this normal?

[mrjl]

Arms are aching today! The clay mix was too much for the little mixer I'm borrowing so had to finish off every batch by hand. Hard graft!

The lime got through a torn glove and into a big popped blister. The flesh is black now, but doesn't hurt at all. Looks knarly though!

[mrjl]

Here’s a pic

[downunderdave]

I think you have shrinkage cracks. I forgot about this and should have warned you, apologies. The homebrew does shrink because of the high clay content. Proprietary castable refratories have zero shrinkage. When using this mix over a sand hump it allows some slight shrinkage, but as yours is over the steel dome will not allow for any shrinkage. I’d be trying to drop the mould carefully after 24 hrs then adding some of the same mix into those cracks from the outside. While you’re about it fill any voids that there may be on the inside surface. For good bonding this should be done before the casting has dried. Cover the whole thing in plastic to retain the moisture for damp curing for at least one week.

[mrjl]

Thanks Dave. I’ve run out of clay unfortunately, so I can make up some of the mix minus the clay if that’ll do? If not I’m buggered, nowhere near here sells it and even if somewhere did they’ll probably be closed until Monday.

[mrjl]

Also struggling to get the rear wall piece of the mound out. I screwed a drawer handle to it but it’s not really budging. Worried about using too much force and making the cracks bigger

[mrjl]

I got the rear piece of the mould out but i fear the steel “ribs” of the drum are stuck fast. I think the likelihood is that they're in there permanently, not a lot i can do about it. They're both separate though, so hopefully when they heat up and expand they've got some wiggle room and dont break the oven completely. Bugger

[mrjl]

I've racked my brains and the only way i think i can get the barrel out is to angle grind it in to pieces. Possible but also dangerous and scary as I’ll have to get my head in there with it. So not something i want to do really!

Whats the danger of the oven cracking if the steel expands? That it will cave in or collapse? If so keeping the steel in might stop that and if I have to deal with it rusting out in future then so be it. But if the danger is cracks themselves surely that’s not the end of the world as this layer is for thermal mass right? And will be covered by 2 insulating layers and render.

I’ve filled the cracks with fire cement and will keep it covered for a week.

[downunderdave]

Jun 4, 2018 8:35:30 GMT mrjl said:

I've racked my brains and the only way i think i can get the barrel out is to angle grind it in to pieces. Possible but also dangerous and scary as I’ll have to get my head in there with it. So not something i want to do really!

Whats the danger of the oven cracking if the steel expands? That it will cave in or collapse? If so keeping the steel in might stop that and if I have to deal with it rusting out in future then so be it. But if the danger is cracks themselves surely that’s not the end of the world as this layer is for thermal mass right? And will be covered by 2 insulating layers and render.

I’ve filled the cracks with fire cement and will keep it covered for a week.

Leave it, fire it and cook in it. Most folk find that after building an oven  and using it that the way it performs leads them to want to build a second improved version. This is when you become hooked. The homebrew is such a cheap material to build with that the main cost is really just your labour. Do take your time with the drying fires as expanding steam can cause severe cracks.