The basic principle is simple. Cut your fabric to the required size, attach it to something solid which is then braced to hold it in place, add reinforcement and pour the concrete into the flexible form.
Many shapes and uses can be created with this method, but we will focus on a simple column. You can use this system for standalone columns or posts, or for posts that attach to an existing wall. We will describe a standalone post, as this is a little more difficult.
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Step 1: Base
- Dig a hole, about 12 inches in diameter, where you wish your post to stand, whether it be at the base of an existing wall or on its own. The depth of the hole will depend on what type of force will be exerted on the post.
- Place metal reinforcement in the hole, raised up on rocks or plastic (you want concrete under any metal, so that there is no chance it can leach water and rust).
- Level the metal (both east to west and north to south) and hold it in place with a couple of boards between it and the ground. [If the post is against a wall, attach the metal to the wall if possible, with a gap in between them.]
- Pour concrete into the hole to just above ground level. Make sure you tamp it down every once in a while to ensure that the concrete completely fills the hole. We use a 3:1 cement to sand/gravel mix.
Step 2: Form
- Cut a length of vinyl tarp, 8 feet long and 36 inches wide.
- Place two pieces of 8 feet 1x4s, 22 inches apart (the tarp will stretch a little, so make it a little skinnier than you want the post to end up).
- There should be about an extra 7” of tarp on both sides. Double this back on itself, to make it stronger.
- Staple one side of the tarp to one of the 1x4s, close to the inside edge. Put a staple every 2” or so – the concrete will exert quite a force on the tarp, so more is better than less.
- Staple the other side of the tarp to the other 1x4, checking the distance between the two boards (22” in this case) as you go.
- Remove the boards that were holding the metal reinforcement in place.
- Stand the form up and put it around the metal reinforcement of your post, so that the two 1x4s meet, flat against each other.
[If your post is against a wall, just put each board against the wall on either side of the metal.]
- Screw the boards together at the top, bottom, and once or twice in the middle.
- Level the boards in two different plains, and then brace the form using lumber or metal.
- Pour concrete into the plastic, patting it as you go. You want the concrete a little soupy, so that it fills the form better. However, remember that the drier concrete is, the stronger it will turn out, so do not make it soupier than necessary. Patting helps to compact the concrete and release air bubbles that can make the post weaker. If you pat it too much, the post will start to bulge at that point.
- When the concrete is hard to the touch, take down the braces and then release the screws and carefully peel off the form, which you can store for reuse. We have found that concrete sets up far quicker in a plastic form – we don't really know why.