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I have replaced several of these pieces, but they continuted to rot out. Hence the corner brace of concrete. Replacing the piece is simply a matter of unbolting the old piece, cleaning up the area so that the new pieces can be put into place, and installing the new corner braces.
lol, yea, I was trying to figure out why you went to all the trouble to make the concrete corners. Why have corners at all, just nail the wood together with galvanized nails and lay some plastic in there and be done with it. But the concrete is super cool, you could use PVC. Just slit a 4" or so pvc pipe down the center heat it up and lay it on a flat surface. You'd have a flat plastic piece that will never rot and probably cheaper then a bunch of concrete.
The problem is that PVC releases toxic chemicals such as dioxin when heated and even after it's cooled and hardened again it is much more prone to leaching the chemicals into the soil and your edibles. At least concrete mostly just produces CO2 when made-and in many cases it somewhat safely binds/incorporates "fly ash" (a by-product of burning coal) that would otherwise be a pollutant.
Side note: cement production *is* one of the largest contributors to the rise of atmospheric CO2 but when used carefully in small amounts and for durable uses like in this project, it can make up for it.
It looks like you're pretty set on where you would like these things and, if that is this case, why not just make the entire thing out of concrete? Build a 6" tall form, use either fibers or steel reinforcement... maybe 2-3 inches wide... you'll never have to replace a board again!
Side note: cement production *is* one of the largest contributors to the rise of atmospheric CO2 but when used carefully in small amounts and for durable uses like in this project, it can make up for it.