Make Formwork for bell shaped concrete coping blocks

[tonyjeffs]

Building a wall, making 8"x8"x9" ornamental exposed aggregate concrete blocks.

Well I've finished my flat-sided tapered blocks and now I'm on to the copings.

They are bell shaped, with quite tight 1.5" dia curves. The mould will be to take 3 blocks, approx 8x8x9 each.

Any thoughts on how I can make the curve? I can use 3 existing blocks in line as a guide.

I tried wrapping them in polythene, boxing them in, and injecting expanding yellow foam, but it didn't match the contours tightly.

I tried soaking hardboard, bending it when soft, and letting it dry, but was surprised how water and bend resistant hardboard is.

I'm trying papier mache. If it works, it mightn't last very long with the wet concrete.

I thought about getting a sheet of thin stiff plastic, heating it, fitting it, and letting it set into the correct shape. But how would I heat it until it is soft? boiling water wouldn't be hot enough, and a blow-lamp would be too unsubtle.

Or maybe any of those approaches'd work if I did 'em more carefully?

Any suggestions welcome.

Thanx Tony

PS, this group has just cost me a fortune ;-) After reading here about SDS drills - they sound a brilliant addidion

- I am absolutely itching to buy a decent cordless one. Not cheap!

I recently bought an AKG 12v cordless screwdriver. Best tool I ever bought. Brilliant. Recommended

[Baz]

Hotair Gun??

Baz

[Malcolm]

If the curve is in one direction only, look in B&Q for bendy MDF. It is about 4mm thick and kerfed on one side to bend to a reasonable radius, but I suspect not tight enough for what you want. It would be easy to waterproof and you may be able to add more cuts to increase the bend radius.

Malcolm

[The Natural Philosopher]

Get some 1/32" or 1/64" ply and cut formers. You can fill the thing with plaster later to stiffen it.

[awtltd]

Just pop it in the oven!

Probably better to start low and slowly raise the temp - I don't know how accurate the temp on the dial is!

For ABS sheet at about 110 deg C

For Polypropylene at about 120 deg C

Time will vary - depending on size and thickness

Kev

[Rod]

Sculptural concrete (e.g. ciment fondu) casting is often done in a mould made of some form of rubber. Some materials are available here:

I'd guess some might be available elsewhere at lower cost - but not necessarily. :-)

[Steve Firth]

Make a plug in lath, chicken wire and plaster of Paris. Coat with release agent and then layup a glass-fibre mould on the plug.

[meow2222]

It sure would be easier if you could get a sample of the shape you want, using that to cast a mould.

Re bending plastic, last time I did it I just dipped it in boiling water. Works with some plastics - unfortunately I dont remember what kind of plastic it was. Making a mould in thin air is going to be hard to get just right.

NT

[tonyjeffs]

Thanks to All Several good ideas to consider. Replies to all...

NT, I do have a sample of the shape in that I have some of the original coping blocks to work from, so we got that covered.

Steve, I think that's the way to go, but I don't completely understand what you're saying - what you're doing with the lath and plaster of paris. I was thinking to line up the existing blocks with shaped plywood separators between them, wrap them in plastic (blue DPC membrane), apply a few layers of glass fibre & resin, hold the fibre tight to the curves with laths screwed to the spacers. Maybe another layer of plastic first to stop the laths sticking to the mould. Maybe a layer of stiff cardboard to help keep the shape. Sound OK?

Rod, I think the rubber would be good for small items, but would be very expensive for this.

Kev, it wont fit in the oven - it's 3' long. I'll use the tip another time though.

TNP, that sounds a good way. I fancy having a try with the resin though.

Baz, I'll remember your hotair gun suggestion for future reference.

Malcolm, I had a look at the bendy MDF. As you say it wouldn't give a small enough radius. It'd be fiddly cutting lots of extra slots, and the pieces they had were too small in the non-bend direction. Good to know it's there though.

Tony

[meow2222]

So whats stopping you covering it in polythene (release agent), then resin, then concrete to make a mould?

NT

[Rod]

The usual way of creating a mould uses just a relatively thin layer of rubber - made up to a usable shape with something like plaster of paris on the outside. So cost, which I have not worked out at all, might not be quite as bad as you initially think.

[tonyjeffs]

It's a possibility, and after reading all the suggestions I'll do something similar along those lines.

Against using concrete is:

- I'll have to flip the mould over when it's set and shake the blocks out. It'd be very heavy. 3'x9"x9" of concrete.

- the concrete wouldn't have any give in it, so it might grip too tightly when I try to shake the blocks out.

I'll do something similar( if it stops raining), but will use something lighter instead of concrete, like strips of wood or hardboard, which won't have to take the exact shape - the resin will do that, - but will hold the shaped resin in place, and flexible enough to bend a little when removing the product.

Tony

[meow2222]

If the end product's sides are tapered and you use polythene, inverting the mould and tapping should work. If its not a shape that can slide out you can add a divider so the mould is in 2 halves and clamp it together with some wood.

Bear in mind with all this though that its a long time since I did this stuff, so there's always room for data errors.

NT