Pouring concrete for patios, walkways and edging strips is easier than indoor projects such as countertops; you don’t have to be so concerned with getting the base of your mold perfectly flat. Your big worry will be the top edges of your form.
Start building your concrete mold by marking out the perimeter, using lengths of string or spray paint. Dig out the area to a depth of 4 inches for patios and pathways, or two inches deep for smaller projects like stepping stones.
Set up lengths of 2 x 4s around the edge, supported by small wood stakes every 3 feet or so. For curved edges, you can use strips of 1/ 4 inch benderboard or plywood, staked a little closer. If you are casting steps, you also will need to lay in crossbars of 2 x 4s on end as well, at various distances depending on the grade. The last step is adding a bed of pea gravel or crushed rock to the form, 2 to 4 inches will do.
Countertops and Other Indoor Concrete Projects
These can either be pre-cast and then moved into position, or cast in place, although it is more common to precast the slab in a workshop area and then install it, since you won’t have a big mess in the house, and if something goes wrong with the casting, it will be easier to recast.
It is easy to set up a robust worktable with three sawhorses, one in the middle at one at each end. Take a sheet of ¾ inch plywood and attach four evenly spaced 2 x 4s set on edge, lengthwise, to it using screws. Put the plywood on the sawhorses, 2 x 4s facing down, and check it with a level, shimming if required to get it level. You can build your concrete mold on top of this.
You now have a few choices to make for building the actual mold for your concrete countertop, flooring slab or tabletop. First, you need to decide if you want to mold the piece right side up, smoothing the visible surface manually, or cast upside down, and have the bottom of the mold produce the smooth visible surface. For decorative finishes where you will be adding inlaid materials, right side up makes more sense. For textured surfaces or patterns, upside down is better.
Then there are 2 ways to make the actual mold. Either you build a frame and screw it to the worktable directly, using the worktable top as the base of the mold, or attach framing sidepieces to a base piece that is the size of your slab.
In either case, you will want to use side braces to support the outside of the frame walls so they don’t bow outward from the weight and pressure of the curing concrete. The side braces can be made from two pieces of plywood at right angles to each other supported with a vertical brace member. Attach the brace to the frame walls and the worktable base.
The best material to use for your concrete mold is ¾ inch thick melamine-coated particleboard. The melamine coating prevents the concrete from sticking to the wood, sort of like Teflon, so it is easier to get your project out of the mold. You can buy it in sheets of 4 x 8 size at any lumberyard or home reno store.
If you are building something with complex surfaces, like a sink or ornamentation, it is better to make a mold from Styrofoam (polystyrene foam) or foam board insulation.
In order to prevent cracking when using your mold, consider using concrete reinforcement.