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aureole 07-21-2008 03:52 AM

Another UK oven project
Well, this week was the big day: started working on the oven by laying the foundations.

I'm a complete beginner to this all, so I'm quite nervous about pretty much everything, as well as being a bit inexperienced. Luckily, I've got a bunch of friends that do know what they're doing!

Digging out was hard work since it's on an aggravating angle. I was advised to keep going 'till I hit the clay: I wasn't sure when this'd be, but when I hit it - yup, I knew about it, that stuff is pretty bloody solid.

Not sure about curing my base - how long it will be before I can add the blocks on top. Also, I'm not able to be at home wetting it all the time - I left it uncovered last night (finished pour at 5pm), covered it this morning and pooled on top, but I'm worried it'll dry (or has dried already!) too much! Any advice?

Pizza oven pictures - WIP!

Pizza oven costs (so far!)

nissanneill 07-21-2008 04:20 AM

Re: Another UK oven project
Hi aureole,
I hope not to scare you but I am a little concerned over your foundation!
Firstly, I would have dug a beam footing, say 18" deep x 12" wide on the perimeter with 2 or 3 1/2" rebars top and bottom and incorprratinf the 4" reinforced slab on top. I know that digging through clay is hard, especially if it is dry, but also very sticky if wet.
I have dug almost 100 metres around my home for retaining walls so can speak from experience. I even used a pneumatic jack hammer for some foundations.
I say this especially if you are pouring this on a clay base which, if it gets wet and absorbs the moisture, will move. Also the perimeter will be carrying the total weight directly over the cement blocks presumably used for the base construction.
This can be helped by filling the blocks with concrete and some reinforcing bar as you build the layers. Almost like building solid concrete (reinforced) sides.
Also, what was the ratio of gravel (crushed rock), sand and cement?
It looks from your photos, that you mixed it by a mixer. How and in what order did you mix the concrete? I ask this as a much stronger mix is achieved by mixing in a certain way to other even when using the same ingredient ratios.


aureole 07-21-2008 04:29 AM

Re: Another UK oven project
The foundation was 6" all around. Not shown in the photos was a bulk bag (~tonne) of crushed rock foundation compacted underneath the form. We went down about 3" into the clay bed. I was told that when you get down here, the clay bed is essentially permanently wet (more or less), hence why you bother.

The concrete was made as follows:

Water (~1/2 bucket)
Add little aggregate (1 shovel)
Add all mortar slowly (1 bucket), mix
Add rest of the aggregate (6 buckets)
Add water to consistency

nissanneill 07-21-2008 04:55 AM

Re: Another UK oven project
Hi Matt,
that is a very quick reply!
When you mix your concrete, we should always start with sufficient water in the mixer for most of the batch being made in that mix. I usually mix to and beyond the mixer's capability which sets a few other challenges there. I had a 3.5 cubic foot mixer when building the house and now have a 2.5 cubic foot mixer which get a thorough workout occassionally. I like to 'fill' a large builders wheelbarrow with each mix.
Next add the portland cement as this is the 'glue' part of the mix. Add your 'aggregate' which I would call the crushed gravel or rock. This is simply the bulking agent and its surface should be thouroughly coated with the wet cement mix. Then add your sand which fills in all the gaps between the aggregate. Add a little water to achieve the required 'slump'. This will make your maximum strength concrete so long as you stick to the makers formula for the concrete's final use.
I still feel that a 6" slab with a couple of rebars in the perimeter is still a bit light on especially when you are going to be putting over a ton of bricks and concrete which is sitting on reactive clay.


aureole 07-21-2008 05:24 AM

Re: Another UK oven project
Monday morning, still a bit slow at work, hence the speed of reply.

I hope the foundation is enough - sadly, there isn't much I can do about it now I think.

nissanneill 07-21-2008 06:05 AM

Re: Another UK oven project
Yes, but late monday night here in Adelaise, Soith Australia.
You can reinfirce your walls if you are to use the 8" hollow cement blocks which can be filled with conctere forming vertical concrete sides and very strong. You might lay a course, fill with concrete and then lay the next. I have seen the ends and centre bits of the blocks ghipped out to approx 2/3 the height and a couple of reinforcing rods run along the line/ course of blocks. Wuite easy to do, just takes a little extra time. If you have a look at industriasl buildings built with theses blocks, check out the lintels oveer the wider 8 foot doorways. They are concrete filled with reinforced rebar without steel lintels.
Let's see what some of the other forum members think once they wake up to monday morning.


christo 07-21-2008 06:11 AM

Re: Another UK oven project
I'm quater of a world away, but the clay here is not as reactive (swelling due to water absorption) as the some of the clay in Oz. As a result, we get away with pretty much the same foundation you poured. You may find the clays in your region aren't overly reactive as well.

With all that rebar in there and compacted gravel on top of clay, I think you'll be fine. It's too late now anyway - so enjoy.

Loved the pics and sorry about your hands.


aureole 07-21-2008 07:21 AM

Re: Another UK oven project
Thanks for the help and advice: I got a neighbour to go check the forms, and the plastic had blown off so he reset that and watered it for me.

nissanneill: I've got some chunky (220x220x440) hollow blocks so I'll fill the cores of those with concrete and 1/2" rebar.

aureole 07-22-2008 05:36 AM

Re: Another UK oven project
Question about hearth slabs- do people reduce the amount of gravel in the mix for the slab? I'm wondering if I should be making a mix of C35P instead.

I've got half a bag of sand/gravel aggregate left- I need another 1/2 bag, but don't know if I should get all sharp sand instead and do half aggregate/half sand for the slab (I'm going to get calsil boards too).

dmun 07-22-2008 08:40 AM

Re: Another UK oven project
C35P is a new one on me: Google's no help. We're sort of bucket of this and half a bucket of that kind of folks: maybe you could re-phrase your question?

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