Gallery, Projects and General > How do I??
Concrete form
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.
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