I love projects that raise humble materials to lofty heights, and a bag of concrete, a length of cedar and a couple of bolts are all this one takes. Here's how to put it together.
concrete form and seat slab
The stone-like seat slab consists of a single 50-lb. bag of ready-to-mix concrete cast in a homemade form. To make the form, start with a 2' x 4' sheet of 5⁄8"-thick melamine-coated particleboard, available at large building supply centres. Using a jigsaw, circular saw or table saw, cut the base to size. I made mine 131⁄4" x 371⁄4". Next, cut the short and long side pieces from 3"-wide particleboard using a power mitre saw. To assemble, first attach one long side piece to the base with screws, driving them in every 5" or so (drywall screws work well here; their thin shanks eliminate the need for pre-drilling). No glue is necessary since the form is temporary and will soon be disassembled to release the hardened concrete. Attach the remaining three sides to the base in the same manner, then add a couple of screws at each corner, as shown.
Place the form on a sturdy, flat surface and slide shims under corners if necessary until it's perfectly level. This is crucial in creating a slab of uniform thickness. Next, coat the inside of the form with a thin layer of vegetable oil so the concrete won't stick.
You can mix the concrete in a wheelbarrow with a shovel, but I prefer using a clean plastic garbage can and a paddle mixer spinning on the end of a corded drill. Dump the concrete into the garbage can and add two litres of cool, clean water and two litres of latex bonding agent. (This is a white liquid sold under various brand names at building supply stores.) The bonding agent helps the concrete to settle in the mould a little better and gives the finished slab a nicer feel, while also reducing any tendency
to crack. Mix thoroughly until the concrete is lump-free. Now scoop out the mixture into the form-I used a plastic milk bag container. To settle the mixture, tap all around the sides of the form with a rubber mallet. The concrete should level out at about 1⁄4" below the top. Let it cure for a few days, then remove the sides of the mould to release the slab. Smooth all the sharp edges of the concrete with a small grinder, or rub them with a stone.