Another UK oven project

[aureole]

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]

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.

Neill

[aureole]

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]

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.

Neill

[aureole]

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]

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.

Neill

[christo]

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.

Christo

[aureole]

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]

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]

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?

[aureole]

Re: Another UK oven project

Haha. I'm more of a wikipedia guy - bugger all practical skills but good on theory.

IIRC standard concrete is a 1:2:4 mix or so - 1 cement, 2 sand, 4 gravel, c20p meaning cement, p for portland and 20 is the compressive strength (20 newtons per mm2 I think). This is also the 1:6 standard aggregate mix used by most people here (as I understand it). C35p is a stronger concrete - 1/2/1 mix of cement, sand and gravel (or 1:3 cement:aggregate).

What I'm asking is: most people have a nice smooth hearth slab with no visible lumps in it - I don't know if this is because they polish it nicely, or if they've been putting a lot more (or entirely) sand in the mix they use for the hearth slab. I'm just wondering if I should buy some mixed sand/gravel ballast/aggregate to make it, or get some

[dmun]

Re: Another UK oven project

I mix my concrete with 1 bucket of sand, one bucket of crushed stone, and a scant half bucket of portland. That's 1 : 2.5 : 2.5, so yes, I'm using more sand than your first example. As far as the aggregate, by the time you level the concrete for the forms and smooth it out (a process called floating) all the fine stuff (and a bunch of water) will come to the surface. Overworking a slab is a classic amateur mistake: I've yet to finish a slab to my entire satisfaction. I'm told that it's a help to leave the leveled slab to set about fifteen minutes before you try to float it.

[Les]

Re: Another UK oven project

When I poured all my sidewalks using a mixer I used 3 sand, 2 gravel, 1 portland. Plenty of sand to float for a smooth finish. I thought that was considered 5 sack - pretty common for driveways, sidewalks, etc.

Les...

[aureole]

Re: Another UK oven project

Overworking a slab is a classic amateur mistake: I've yet to finish a slab to my entire satisfaction. I'm told that it's a help to leave the leveled slab to set about fifteen minutes before you try to float it.

I've read that the ideal time is just after the bleed water on the surface gets sucked back in. Patience isn't my strong point though!

[aureole]

Re: Another UK oven project

When I poured all my sidewalks using a mixer I used 3 sand, 2 gravel, 1 portland. Plenty of sand to float for a smooth finish. I thought that was considered 5 sack - pretty common for driveways, sidewalks, etc.

Was this the same mix you used for the hearth slab?