Concrete form

[CrazyModder]

Hello,

I would like to make a few disks of concrete; roughly 45cm in diameter / 5cm thick (but also smaller diameters). So far, I have done this by taking an appropriate sheet of metal, bend it into a circle, support it on a wooden board, and covering everything with some plastic (from a plastic bag). The sheet has to be pressed down onto the board with quite a bit of force (using screws) to avoid the wet concrete spilling below it / pushing it up.

Needless to say, this process is inaccurate, messy and not very fun at all. The plastic will wrinkle and the finished product is then looking quite ugly.

I would like to be able to do it with a good end product (smooth surfaces), easily repeated (i.e., the end product should come out of the mold easily without damaging it, there should be little time spent cleaning up and being ready for the next cast), and with as little effort as possible for making the mold. Needless to say, this is for a hobby/fun project, and cost should not be high.

Do you have any ideas? Is there a material that's easily bent into shape but still stiff enough to hold the weight of the concrete (about 15-20kg), can be glued (etc.) to a floor of the same material, and will easily let go of the concrete after it hardens? Something that's available in your average home depot would be best.

Thanks in advance for any wild speculations. ;)

[efrench]

Try concrete pier forms cut down to the height you need.

[awemawson]

If you persist with smooth wooden moulds, there is propriety 'mould release' for concrete thus avoiding the polythene,  though I confess to using red diesel personally.

To get good mould filling you need to vibrate the mould which will 'liquidise' the concrete and allow air bubble to escape and make a very dense moulding. A simple eccentric weight on an electric motor will do the job on small stuff. I used to have an electric 'vibrating poker' and a similar one worked off a 'road compressor' for large form work.


(sounds like gym  lifting weights to me)

[shipto]

where about are you? we have some quite big offcuts of plastic at work if not the size you want might be able to get a ring welded up for you.
Just pay whatever for the postage.

[Spurry]

An SDS drill set to hammer-only works quite well too, just to rattle the former, and settle the concrete.

[CrazyModder]

Thanks for the advice so far, I'll see what I can find.

@shipto, I live on the other side of the channel, so 'fraid shipping stuff is going to be a bit expensive, I guess. ;) But thanks for the offer.

[Sid_Vicious]

Could you use the bottom of a plastic bucket as a form? There will be a slight outward form so it will be a millimeter or two bigger at the topside than at the bottom but if you use car wax on the plastic it will slip very easy. 

[vtsteam]

Lard works as a release for concrete. You should melt it in a double boiler -- I use a soup can set in a saucepan with water in it. Just brush it on.

For the form, for clean results, I'd go with all sheet metal sides and bottom, tack welded outside to avoid distortion. Run a fillet of some auto body putty (Bondo) along the inside corner. You'll need a slight draft to the sides for easy release, or you'll need to make a slightly more complex 2 part mold, or a spring mold. you can add a fitting to the bottom to take a compressed air hose to aid release. Tape it over on the inside before pouring. Careful -- don't overdo the air if you do! Very low pressure is all that's needed.

Wood molds for fine concrete work can have problems swelling and deteriorating as the concrete sets ---  particularly for multiple copies. For one-offs it's not bad, but a lot of finish work for the mold if it's just a one off.

Depending on the use, a rich 3:1 sand only mix will give you the best detail. Add wire mesh for strength if needed.

[CrazyModder]

Thanks for you detailed instructions, vtsteam. Would you use a quite thick layer of lard on the sheet metal, or is a thin smearing sufficient, in your experience?

Speaking of the mix... the finished product will often be moved around, and while I'd like a somewhat smooth surface, it would be even more important if it were a bit robust (i.e., slightly resistant to chips flying off all the time when someone clumsily sets them down in a not-so-tenderly fashion). In the past, I have used the pure concrete directly from the bag, just mixing it up with water; I have not experiemented with additional sand.

[awemawson]

Fibre reinforced concrete using pea shingle as the aggregate is the way to go, and as Steve says put some chicken mesh in the form.

[Eugene]

Another wild speculation .....  in fairness I've got all the bits and bobs below to hand; you may not.

If you have a woodwork type router, use it with the aid of a trammel to cut appropriate sized holes in MDF sheets of the required thickness.  If you cut the biggest size first, you can use the "waste" bit for the smaller sizes, Stick each of these to a baseboard using wood glue Take a saw and cut the form in two right along the diameter. I have a Japanese pull saw that has a very thin blade and thus an equally thin kerf.

You now have a split mould that can be held together with a strap wrench or one of those cheap ratchet affairs for trailer covers; the ones below are £5.99 off EBay. Paint the parts with a polyurethane type varnish to smooth them up a bit and make them waterproof.

For a release agent I'd use "Renapur" shoe wax available off the 'net. It's a gobby paste of beeswax and oils that you can apply in layers as thick or as thin as you want. It's excellent in fibreglass work, so should be fine for your application. Good for yer boots too!

http://www.renapur.com





Eug

[PekkaNF]

Depending on your climate, you may want to consider additive that will make the concrete frost proof (or some that kind of words). Here pretty much all pavement needs to have that additive, or it will erode and crumble away in couple of winters. I have heard that additive will actually create micropores - how this will make it winter proof I have no idea.

My brother made quite a lot of these, he had a big table made out of film faced ply wood and moulds were assembled top of it. Then prepared concrete (color + other additives) was poured pretty dry (less water there is harder it will be), then rapped (minimum possible, water separation on top is allready bad thing), leveled, sprayed with water, covered with plastic sheet and let to rest until next morning. Fresh concrete is pretty weak and brittle. In few hours the surface can be brushed with wirebrush to make it bit more rough.

Film coated plywood needed very little oil. There are special oils, but I think he used normal vegetable cooking oil, because these are next to herbs and flowers.

He said that economy goes down on drain, unless you use your own sand, work, concrete mixer and need special size/color/shape or cause any waste. He made one or two mixes at the time on concrete mixer, used it all on moulds. Took few practice runs. He made a lot of long/narrow edging pieces, those were special and those payed off.

Pekka

[vtsteam]

Thanks for you detailed instructions, vtsteam. Would you use a quite thick layer of lard on the sheet metal, or is a thin smearing sufficient, in your experience?

Speaking of the mix... the finished product will often be moved around, and while I'd like a somewhat smooth surface, it would be even more important if it were a bit robust (i.e., slightly resistant to chips flying off all the time when someone clumsily sets them down in a not-so-tenderly fashion). In the past, I have used the pure concrete directly from the bag, just mixing it up with water; I have not experiemented with additional sand.

You can paint it on like thin paint once warmed and that will be sufficient on clean sheet metal. Wood will generally show grain unless highly finished and be harder to remove, which is why I suggested the above together. For a really fine finish gravel is out, for obvious reasons. Bagged mix isn't usually optimal for a fine finish. Mix 3 parts clean masonry sand with 1 part Portland cement for a rich sand mix. For durability you'll want to add the mesh. Keep it clear of the exterior -- sometimes tricky to do in a mold. For small batches of sculptural quality mix it's best to hand mix with gloves on in a bucket.

Concrete mold for a biogas generator project I was involved in:



Castings:

[JD]

In the past I have used cheap cooking oil for mold release, if the wood is dry you may need 2 coats.
JD

[Swarfing]

Just to add, in the past i used olive oil for the release agent and an electric sander to vibrate the mix to expel the air.

[vtsteam]

Tallow is the traditional concrete (not modern chemical) release agent. It's a grease so it stays put rather than mixing with and floating in the mix, and apparently also chemically produces a better cure and release. You can either buy it or easily make it by rendering animal fat in boiling water, skimming off the foam/impurities, and then ladeling into a tin can. Bacon grease works. Boiling takes out the salt and tissue impurities. Tallow is traditionally used on shackles on chain  rodes in salt water for moorings and anchor tackle to prevent corrosion in the threads. You can unscrew them after a year of immersion. Oil doesn't work at all for that.

Not sayin' salad oil won't work on wood forms, I'm sure it will. Just if you want the old time best material for the purpose, it's available everywhere and basically free.