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I’m interested in creating an organic rounded shaped mold to pour concrete into. It will be large, a coffee table and end tables. It has to be a pull apart mold, as I want to carve the shape on all sides, so it’ll be like a beach stone. What carvable material is strong enough for that purpose, and how do I hold the two pieces together? I could use strong webbing straps, I’m thinking. And the only material that I can think of is foam, but I’d that strong enough to hold drying concrete?

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  • Why don't you go have a look at insulated concrete forms? That should answer one of your questions.
    – Ecnerwal
    Dec 3, 2022 at 15:20
  • Google: this old house How to Build Custom Concrete Countertops Dec 3, 2022 at 16:38
  • What carvable material is strong enough for the purpose of making a table? Limestone. - A slab of concrete 3" thick, 3' long and 18" wide is 150 pounds. And that's w/o legs. You sure about this? I had some marble end tables. Those were a bitch to move.
    – Mazura
    Dec 3, 2022 at 20:05

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Normal process is to carve the design wanted in plaster, and use the plaster to make a rubber mold to cast the concrete in. If the design repeats you can mold many copies of one plaster-master. The rubber mold is supported by wood or steel backing. This will be rather expensive if making only 1 and 2 copies (as I infer from coffee table and end tables, which are usually a pair,) as the normal process is to make many copies of the design, so the molding cost is spread over many units produced.

Carving foam in a manner that will release from concrete reasonably is likely to be difficult, but you may be able to paint the surface with a thin rubber layer. Foam, properly supported and tied can certainly be strong enough to hold concrete (it can also "blow-out" if not properly supported and tied) but generally that application is one where the foam remains in place for the life of the concrete, and ties through the concrete are used. If you can make it work as a surface to carve, you'd probably want to support with steel or wood as for a rubber mold.

You can also carve wood molds directly, and apply the usual mold-release products before casting. That might be the more affordable way to go for one or two copies.

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The photo above shows one time-proven method for forming concrete walls. Sheets of plywood contain the concrete; 2x4 lumber fixed on the exterior and steel ties passing through the concrete wall reinforce the form so that the weight of liquid concrete won't deform or break the plywood. (photo: https://www.performancepanels.com/case-histories)

A technique that is sometimes used to give texture to a poured concrete wall is to install foam or rubber molds inside the formwork. Done this way the conventional formwork provides all the strength; there's no need to make a structurally strong mold.

You can adapt these concepts to your project. Your furniture is much shorter than a poured concrete wall and they're also not nearly so long. You could carve your shape into foam sheets, then build a plywood box large enough to fit the foam. Reinforce the plywood with two bands of 2x4 lumber (one low and one high) around the circumference of the box. Leave the 2x4s longer than the length of the box so that the 2x4s can be screwed together where they cross at the corners.

Use a foam block or build a second form to sit inside to reduce the volume of concrete required (and weight of the finished piece!).

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