#11  
Old 07-22-2008, 09:06 AM
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Default 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
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Last edited by aureole; 07-22-2008 at 09:36 AM.
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  #12  
Old 07-22-2008, 09:56 AM
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Default 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.
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  #13  
Old 07-22-2008, 10:15 AM
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Default 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.

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  #14  
Old 07-22-2008, 10:38 AM
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Default Re: Another UK oven project

Quote:
Originally Posted by dmun View Post
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!
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Old 07-22-2008, 10:39 AM
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Default Re: Another UK oven project

Quote:
Originally Posted by Les View Post
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?
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Old 07-22-2008, 10:54 AM
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Default Re: Another UK oven project

When I poured my hearth, I brought in a yard of ready mix (5 sack). That required enough shoveling as it was.

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  #17  
Old 07-22-2008, 11:09 AM
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Default Re: Another UK oven project

Matt,

Maybe some help here:
Mixing Concrete, concrete, sand, cement, ballast, Calculator

regards from karl
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  #18  
Old 07-22-2008, 11:38 AM
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Default Re: Another UK oven project

You may want to look at my concrete mixer thread, as well.

http://www.fornobravo.com/forum/f6/c...asics-790.html (Concrete mixer basics)
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Old 07-22-2008, 04:02 PM
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To improve your concrete surface, after you have screeded it and lightly trowelled ot with a long wooden trowel, mix a toppong of 1 part concrete sand (like a washed coarse river sand) with 1 part portland. When your concrete has initially set enough to walk on carefully (2-3 hours after laying), sling handfulls of the topping over the slab and work it up with a wooden trowel. It will draw water out of the cement and go to a thick slurry which can be finished with either a woodern trowel ( a non slip slightly rougher texture) or a flat steel trowel for a smooth finish. You can also use a household or garden brush to produce a non slip surface for paths/driveways. You can also add some colouring in the form of an oxide if you wish to have a coloured finish rather thanuse a lot more to colour the entire concret thickness.

Neill
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  #20  
Old 07-28-2008, 01:27 AM
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Default Re: Another UK oven project

More work done this weekend: I've finished the evil backbreaking parts - another tonne and a half of concrete poured this weekend and the block stand is finished, covered and curing. I borrowed the idea seen elsewhere on a UK build of putting a sleeper in as a lintle on the front: my plan is to render in terracotta mortar, and do a reveal on the lintel.

Minor niggles- I think I've made the block stand slightly too large - it's 1980mm square - I think using 9" blocks rather than 8" ones enlarged everything too much (I figured too big is always better than too small though). Also, I got a nice finish on the concrete, but then covered it too soon, so the plastic has fouled the surface a little.

I also went through 3 tons of ballast, 20 bags of cement and two cement mixers - the first one I borrowed died halfway into forming the hearth slab. Luckily for me, my neighbour has one I was able to beg/borrow/steal to finish off the work. He also gave me six bags of cement to finish the job. Nice guy!


I'm really glad I've got the stand finished, working in 29c (85f+) sunshine pretty much killed me. At least it's mostly fiddly stuff now.
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