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Concrete Forms for Small Project

849 Views 6 Replies 3 Participants Last post by  ClarenceBauer
I'm planning on doing a small project with concrete, and I don't have much experience with it so I could use some guidance.

Basically I want to make an open-top box out of a lightweight concrete mixture, such as with vermiculite or perlite instead of gravel, or I might even try some cellular concrete mixtures (aircrete). It will basically look like a large rectangular flower planter made of concrete, a box within a box for forming purposes, and it'll nominally be a monolithic pour.

With the lighter concrete mix, I think I'll be able to get away with some form material weaker and thinner than 1/2" plywood if I have a couple braces. Its total volume of lightweight concrete will be about 3.64 cubic feet, with 4" walls, reinforced with some steel mesh hardware cloth or something.

So that's where I'm kinda stuck. I've heard that plywood forms only last so long even if you sand them smooth and add a good release agent. I don't need forming materials with as much structural integrity as plywood, so I might be able to use something that is thinner but will last for many more pours. So my questions are:

1. How long do plywood forms usually last if you're careful about not letting your concrete cure for too long and use a good release agent?

2. Are there any thinner materials you can think of that would work as lightweight concrete forms, that might even be a bit more expensive, that might last for say hundreds of uses?
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Oh yeah it might help to say that the box's exterior dimensions are 32" long and 19" tall. So I might be able to form all of the exterior box with a 4'x8' sheet of something.
Yes it can be done.
Use the formply as per Newly.
Than build your form. I would make the sides with sheets cut to the height desired & two inches longer than the inside desired measurements than attach 2X2's to one corner of each side piece.
To assemble the form clamp each corner making the inside corners square.
Than make your inside box & MAKESURE IT IS SMALLER @ the bottom Slight tapper to the top this will prevent any lock in when the concrete is set.
Coat all inside surfaces with a rubber coating like Smooth-On molding rubber this will insure a clean release. You can also get mold release from Smooth-On.
You can also make a design in the casting using the rubber.
To keep rust from fracturing the concrete use the Fiber glass re-Bar (V-Rod ) or use fiber Mesh Lath like used for Stucco.
Don't know if the light weight aircrete will be strong enough?
3.64 Cu./Ft. of concrete weight is around 500#
That gives me a much better starting point. Thanks for the advice about actually building the form bracing, I probably would've tried something with more wood. Yeah I'm not sure how much foam I'll add to my aircrete yet. I might add a bit of sand, and I was considering using a fibrous additive as well. I might also use some perlite or vermiculite as aggregate. Testing out mixes will be the brunt of the work for this project, I'll mitigate my failures by making little batches to test bricks of different mixes.
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