First try with bricks and morter

[comuscomp]

Last year I built a new outdoor kitchen and man-cave for my son, his choices, his cash. But the oven part was like 'painting by numbers', so I have decided to build a 'Proper one' for myself.

It will be on a plinth 1200x1000 covered by concrete slabs 50mm thick.

 

will a vermiculite/cement base of 80mm be OK for an insulation on the slabs?

I am going to use class B Red solid Engineering bricks for the fire floor and tunnel arch.

What is the best material to lay the floor on?

I am going to chamfer each side of the arch bricks about one-third of the height of the side to increase the amount of surface area that touch, then 'Butter' the touching areas with fire-cement and filling in the gaps above with home-brew.

How long will the home-brew take to set?

Will it be OK then to cover the brick tunnel with a 40mm coat of vermiculite/cement straight onto the bricks and when it dries, put a 20mm render onto that?

Looking at different suppliers of powdered clay, there seems to be a lot of different types available, is there a specific one that I should be looking for?

all help appreciated as this normally saves money and time.

Images of before and after of last years build.

[downunderdave]

Mar 24, 2017 18:20:31 GMT comuscomp said:

Last year I built a new outdoor kitchen and man-cave for my son, his choices, his cash. But the oven part was like 'painting by numbers', so I have decided to build a 'Proper one' for myself.

It will be on a plinth 1200x1000 covered by concrete slabs 50mm thick.

 

will a vermiculite/cement base of 80mm be OK for an insulation on the slabs?

I am going to use class B Red solid Engineering bricks for the fire floor and tunnel arch.

What is the best material to lay the floor on?

I am going to chamfer each side of the arch bricks about one-third of the height of the side to increase the amount of surface area that touch, then 'Butter' the touching areas with fire-cement and filling in the gaps above with home-brew.

How long will the home-brew take to set?

Will it be OK then to cover the brick tunnel with a 40mm coat of vermiculite/cement straight onto the bricks and when it dries, put a 20mm render onto that?

Looking at different suppliers of powdered clay, there seems to be a lot of different types available, is there a specific one that I should be looking for?

all help appreciated as this normally saves money and time.

The Americans advise nothing thinner than 100 mm for the supporting slab, but they do tend to overbuild and are renowned for building bomb shelters. You can reduce the strength/thickness of it by cantilevering so the supports are further inboard, then 50 mm should be sufficient so long as you're sure they're reinforced.

80 mm insulating slab is plenty, it should be 5:1 vermiculite, cement, but because it uses so much water, will take weeks to dry out, so don't be in a rush and build straight over it.

The engineering bricks should be fine for the dome, but an unknown quantity for the floor. Far better to use fire bricks there. The floor can be laid on a dry mix of 50/50 sand, clay as a leveller. Left dry will allow replacement of any floor bricks (particularly if you stick with the engineering bricks)

Refractory cement is a waste of money IMO as you're not firing the oven even half of the temperature that mortar is designed to withstand. It also has very short working time which increases wastage and mixing time. The homebrew is quite suitable for the whole oven and is easier to use. Because it contains lime it also increase working and setting time.

Any powdered clay should be suitable as you're only going to around 500 C, but avoid bentonite as it has the highest shrinkage rate. I use Bricklayers Clay which is the cheapest, but failing that you could try ball clay from a potters supplier.

I've found there is a problem when using vermicrete directly over the inner brick layer. (ie no blanket between those layers).Unless you are really carful to eliminate all the water from the layer it can swell and crack your outer shell. So dry it out well before rendering over it. A layer of blanket acts as an expansion layer reducing this problem.

When you say "brick tunnel" it sounds like you are building a barrel oven rather than a hemispherical dome. If so, are you aware that because of the inherent instability of the arch, that it requires either buttressing or bracing and that the end walls should be built und the vault rather than beside it to prevent the vault's expansion pushing them out?

[comuscomp]

Thanks for the quick reply, when you say sand/clay for the bedding layer, is that soft/building sand, sharp sand or plasterers sand?

also can the homebrew be mixed in a cement mixer, there seems to be some controversy surrounding lime mortar mixes in a mixer.

[downunderdave]

Mar 25, 2017 13:55:16 GMT comuscomp said:

Thanks for the quick reply, when you say sand/clay for the bedding layer, is that soft/building sand, sharp sand or plasterers sand?

also can the homebrew be mixed in a cement mixer, there seems to be some controversy surrounding lime mortar mixes in a mixer.

Any sand will do, but the size of the largest particles may make the levelling process a little more difficult so you could sieve it to remove the coarse stuff if that's a problem. There's no problem mixing homebrew in a mixer, but as with anything you put in a mixer it must be cleaned out scrupulously. I've found mixing vermicrete in a mixer not very good as it tends to stick to the sides and mixing blades.

[comuscomp]

So, the base construction is complete, now for the vermicrete base insulation, 5 : 1 seems simple enough!!!!

Is that a weight ratio i.e. 5kg vermiculite to 1 kg cement or
a volume ratio, 5 buckets vermiculite to 1 bucket cement or
a bag ratio, 5 bags of vermiculite to 1 bag of cement?

[downunderdave]

The ratio is by volume. It is also a good idea to seal the top of your slab to prevent rising damp getting up into the insulation. I also put a few holes in the centre of the supporting slab to assist water elimination from the vermicrete slab and cover them with some insect screen to prevent the vermicrete falling into them during placement.

Tip: when mixing vermicrete I always add around a small handful of powdered clay for every 2 litres of cement. This helps produce a much more workable mix not so much with a 5:1 brew, but helps a lot when making a lean mix like 10:1

[comuscomp]

Vermicrete slab formed and drying, bit the bullet and bought fire bricks for the floor, now well over budget. Went to Jewsons and they wanted £5.64 per brick + delivery, needless to say I declined their offer.

Is there any sound reason for butting the end wall against the side wall or could the corners be made with a 'stretcher bond'(I believe that is what the corner joint is called), thus making up the end wall under the back arch.

[comuscomp]

The Diary so far, you may notice Day 2 is missing owing to the fact that at the end of Day 1 whilst cleaning up I dropped one of those blocks on my right foot, then whist imitating a native American wedding dance, I tripped over the work bench and ended up in the rosemary bush, using up a very large supply of short expletives and wondering if I should ever have started this project.

[comuscomp]

Anyone any idea about the insulation qualities of vermiculite? i.e. is it more efficient to use it in a dry form as it comes, or as a vermicrete at 10:1 or 5:1?

[oblertone]

Re vermiculite; it really depends upon the application. If used on a flat surface such as top dressing on molten metal or in a loft then it is normally laid dry. If used flat under an oven floor however you need to stop it compressing under the weight of your oven, so it is laid as vermicrete. For covering a dome, again use vermicrete otherwise it'll just fall off unless you contain it somehow.

[comuscomp]

I'm going to have a cavity between the back oven wall and the back structural wall, would it be better to fill it with loose vermiculite or 10:1 vermicrete, which would have the best insulation effect?

[oblertone]

If you're filling a cavity then defo use dry vermiculite; don't be tempted to compress it, just pour it in and ensure it stays dry.

[comuscomp]

Things have proceeded apace, after 2 weeks of reasonable weather and a gazebo/tent, the build started.


Fire bricks for oven floor down of a sand/fire clay dry mix. First layers of engineering solids for the oven.


First arch over the back oven wall.


Second arch


Third arch


Oven front wall from cut fire bricks


Front arch over the oven wall and forming the chimney


Front arch complete.

and in my spare time I started a new sourdough mother last week here it is at day 5.

[downunderdave]

Have you thought about buttressing or bracing? A barrel oven is inherently unstable and the vault produces sideways thrust  at the junction of the base of the vault and the top of your second course of wall bricks . This creates problems in design unlike the spherical dome which is self supporting. There's not much on barrel ovens on this site. I suggest you go to
community.fornobravo.com 
where you'll find more examples of barrel oven builds and the buttressing or bracing that I mentioned.

[comuscomp]

Having trouble uploading images.