Instructables

Best way to cast concrete?

I need to know how many ways there are to cast concrete. I heard of silicone used in concrete casting but I would like to know if there are other ways. Thank you very much for your help.

rickharris2 years ago
Latex and silicons molds are used but are expensive and don't last long because the concrete is so abrasive.

Anything that will hold the shape can be used.
Polyurethanes make the toughest moulds these days, at the cost of less flexibility.
rimar20002 years ago
If the stones can have different shapes, you can improvise a mold using a wooden frame and plastic sheets. Polyethylene is a good option: easy to get, cheap, durable, etc. Wrinkles can be smoothed after unmolding.
Burf2 years ago
I've made custom stepping stones for a couple of jobs I did in the past. The easiest and most efficient way I found was to make re-usable forms for the sides out of sheet metal.
Figure out the length of the perimeter of the shape you want to make, add two inches for a one inch clamping tab at each end and add one inch to the width (depth) of the form to bend at 90 degrees away from the center so you can drill holes in it and fasten it to a piece of plywood.
Bend the sheet metal into the desired shape and at each corner (if it has corners) cut the metal one inch deep so you can make the 90 degree bend for the flange to screw it to the plywood. If you have access to a welder, its easier to cut the straight lengths and tack weld together leaving one corner open to bend the tabs at the ends.
Fasten the form to the plywood, spray with form oil and pour the concrete in the form.
Form.png
canucksgirl2 years ago
In construction, they use plywood for things like retaining walls, stairs and foundations, (reinforced to hold the sheets in place). For concrete posts, they use strong cardboard tubes.

Almost anything can be used, it just depends on what kind of shape you are trying to cast.

Is this just a general question, or do you have a specific idea in mind (that you can share)?
blkhawk (author)  canucksgirl2 years ago
Yes, I would like to cast landscaping stones with different shapes.
Plastic containers would work well for specific round or rectangular shapes. If you want something more natural and organic, then I would get a few stones in the size and shape you want, and cover them (on three sides) with fiberglass sheets and resin. Then remove the stones from your new mold once they've dried, and fill with concrete to make as many of the new stones as you'd like. One side would be flat, and that would be the side you place down on the ground. Carving out a styrofoam block would also work well to creating a mold.

I hope that helps.
You can make a mold out of anything you want that will hold its shape, keep the concrete from leaking out, and can be removed from the concrete after it has cured. What you use to make your mold and how you make the mold will depend on what you want to make.