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I am looking into casting a few small concrete blocks to build some steps which I will reconfigure over time.

I've found a styrofoam box which gives me the shape I'm looking for, and I'd like to find a way to build a form which I can reuse in order to produce as many blocks as I would need.

Since styrofoam can be broken easily and all the examples I studied for casting concrete with styrofoam forms involve breaking the forms after casting only 1 object, I started thinking about using acrylic sheets to build a small box inside the stryrofoam, with the hope that I can slide out that acrylic form after casting, take it apart to retrieve the block, then put it together again inside the box.

I have 3 questions:

  • Is acrylic a good material to use for concrete casting?

  • What can I do to minimise the chances of the acrylic sheets sticking too much to the concrete object such that I might risk snapping them?

  • Might there be a better way to improve reusability of a styrofoam form?

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Traditional wood forms intended to be reused are oiled so the concrete doesn't stick. There are many ways to do what you want, breakdown wood forms, plastic, whatever. Not much will stick to concrete if the forms are removed when the concrete is cured just enough to stay together. Make sure the insides of your forms have draft angles built in so they can be removed without tearing out some concrete and lube the forms well. This can be done but you'll have to experiment a bit.

One thing absolutely to keep in mind is concrete is very heavy, a 4 ft x 4 ft sidewalk slab weighs about 700 lbs, so the biggest concern is making the forms strong enough to not collapse. Foam foundation forms are extensively cross tied so they don't blow out and are made from strong foam. A thin acrylic sheet without good bracing will bow severely. Based on one of my messy mistakes, marginal forms explode right when you have them just about full!

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