At the urging of a friend I took a class at 3rd Ward, an enclave of artists that teach classes in woodworking, web design, welding, etc.
One of these classes, "Concrete Creations" said it would teach students the basics of formworking and casting concrete.The class description said the goal was to make a small, 16x16" tabletop.
In essence we were all casting nondescript concrete blocks that were flat and wide. We used acrylic to form the top, and laminated plywood for the edges with a bead of silicone caulk at the corners.
The most interesting element of the class for me was that the instructor had a formula for calculating by volume the precise amount material to mix for a given mold. I'd previously only done it by "feel" and "eye".
The class was informative and worth it for that alone, but my aesthetic view differed from my classmates. Everyone was interested in getting their cast top highly polished, and that seems anathema to the idea of concrete. I enjoy its grey imperfect surface. I even cast recessed imperfections into my top.
The recessed panel in the center of the above piece is identical the recessed panel in the concrete flower pot, but it's been cut apart to give it a straight-edged organic look. It's a simple method to add visual complexity to an otherwise straighforward design.