Forming a semi-circular channel in concrete slab?

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.

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

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

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.

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.

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

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

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

Brickwork and mortar is vertical slope and that doesn't run out :-)

My next door neighbour could help on this one.

Fill it with bricks

Robert

very apropriate I would have thought, given the use of the buidling.

Robert