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The best cement form we built for precast work, was made of welded angle iron on the edges, to make the full pentagon shape, and then filled in with concrete to make the flat surfaces.
A “bond-breaker” needs to be used so the fiber cement you intend to pour does not stick to the form. We used diesel fuel, but other chemicals also work, such as automobile wax or floor wax. Test the one you use to be sure the piece can be painted afterwards.
Wait a week for the Portland Cement to cure before using a crane or block-and-tackle to pull the pentagons.
You’ll notice in the pictures we used “blockouts” made of cement for the unusual shape of the edges. You can use cement blockouts too, to form shingle overlaps, etc. for your bolt together domes, so they won’t leak.
The advantages of Natural Fiber Reinforced Concrete, is that a 3/4 inch (2 cm) thick pentagon works well and can be formed on this homemade pentagon form. Such pieces can be used for above-ground construction, or for making beautiful forms that remain as part of the load-bearing structure, and are finished at the factory.
Test what you build to be sure it’s up to snuff. Both flex tests and compression tests should be done.
In many places, evergreen tree needles can be used as the internal fiber, palm leave strips and other natural fibers have been used. These may not be as strong as steel, but more than sufficient for your use. We did put a ring of steel rebar around the outside edge of each pentagon, to be sure that tensile forces from the bolts at the corner vertices spread evenly throughout the pentagons.
The more fiber you use, the stronger the finished product will be, but the more difficult it will be to work, and you don’t want the cement mud to be too sloppy, as it can become weaker. You can use a vibrating screed, we found this to be very useful. It will take some practice to find the best combination for you.
Make sure the bolts all fit, and will hold the pentagons together at the job site. Make it easy to construct. That was Bucky Fuller’s goal in inventing them.
Do as much in the factory, where it’s easier. Then it should go together quickly on site.
For lifters, do not use rebar. It bends only once. Use cable for them. Learn from experience, and send me the pictures—please.
Thanks for your interest.
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