Forming a semi-circular channel in concrete slab?

I'm laying a 20 foot square slightly sloping concrete slab onto which a pig sty will be built. (18" block wall topped by several rows of scaffold poles between piers)
At the lower edge I want to in-situ cast a 6" semi circular drainage channel inside the sty to lead run off into a settling tank. If it wasn't a sty I'd set some aco drain before the concrete pour, but this would block too quickly and not be easy to clear.
But how the heck, given 6 cu M of concrete riddled smooth and rapidly setting do you form a channel???? Indeed is it possible?
AWEM
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Andrew Mawson wrote:

20' length of 6" pipe slathered in something concrete will resist sticking to and set halfway into concrete, then removed before it goes off fully?
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Andy Burns wrote:

+1
Use plastic soil pipe (dose sit have to be 6" or would 4" do?) Spray pipe with mixture of diesel and old engine oil (bit carcinogenic, so gloves, but it's in an american "concrete" book I have) or wrap pipe in DPM plastic which is both tough and even PU foam will not stick to it.
If it is half in, no hurry to remove, it will release.
--
Tim Watts

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I believe expanded polystyrene is sometimes used for this, removed after setting with a wire brush and then acetone.
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In message

Bugger of a job to keep that submerged
As someone said, half plastic pipe, but I would fill it with mix to keep it submerged until it had gone off sufficiently
--
geoff

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Fill it with bricks
Robert
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My next door neighbour could help on this one.
--
Adam



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Andrew Mawson wrote:

easy peasy. get a halfdisk of wood or summat - and use it to form the void you want where you want it.
after the concrete sets, pull it off.
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Andrew Mawson wrote:

Out of interest, how do you make concrete slope? Does it just stay where you put it? I'd expect it to sort of level itself out whilst your back is turned.
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which
where
back is

We're only talking a 6" fall over 20 foot. So long as the concrete is no more than 50 slump it tends to stay put. The mesh re-inforcement tends to help hold it, also no damp membrane otherwise it does slide rather.
AWEM
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GB wrote:

Unless the mixture if too wet, you can put a pretty major slope in it IME.
--
Tim Watts

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Tim Watts wrote:

Brickwork and mortar is vertical slope and that doesn't run out :-)
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On Sep 27, 10:22pm, "Andrew Mawson"

If you used an oiled moulding pipe, there would be no way to ensure absence of voids. If you used a scraper, the result would be a pig's ear. If I were going to do it I'd be tempted to use rectangular timber to mould the recess, then cement/sand line it afterwards. But like most of the replies in this thread, unfortunately this is not from experience. Wrapping with polythene makes a great release agent, and gives a nice smooth surface. Thick polythene doesnt crumple and fold like thin.
NT
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very apropriate I would have thought, given the use of the buidling.
Robert
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