Some of my favourite projects are those that raise lowly materials to lofty heights. Even a close inspection of the stone-like benchtop may not reveal its humble beginnings: a single 20-kg bag of sand-mix concrete. The secret to making the concrete look so good is all in the form. Casting the slab upside-down in a melamine-coated particleboard form gives the concrete a convincing, stone-like finish, and its gently bevelled sides make the slab appear lighter.
| YOU WILL NEED | |||
|---|---|---|---|
| BENCHTOP | MATERIAL | SIZE (TxWxL*) | Qty |
| Form base | melamine-coated particleboard | 5/8" x 16" x 45"** | 1 |
| Long sides | melamine-coated particleboard | 5/8" x 4" x 48" | 2 |
| Short sides | melamine-coated particleboard | 5/8" x 4" x 18"*** | 2 |
| Concrete | sand-mix | 20-kg bag | 1 |
| Bonding agent | acrylic | 2L | 1 |
| SUPPORT STRUCTURE | |||
| Legs | cedar | 2 1/2" x 2 1/2" x 12" | 4 |
| Long rails | cedar | 1 1/2" x 4" x 35" | 2 |
| Short rails | cedar | 1 1/2" x 4" x 8" | 2 |
| *Length indicates grain direction ** Cut at 30° bevel *** Cut to fit | |||
Good form
Start the project by building the form for casting the concrete. Cut the base of the form to size using a tablesaw with the blade tilted 30° from vertical for the cuts on all four of the form base's sides. Make sure the top of the bevel is facing up so that it doesn't slip under the fence as you make the cuts.
Next, tilt the blade back to vertical and rip the sides of the form to width. Cut the long sides to length. Attach them to the base with 15/8"-long drywall screws spaced about 6" apart. Let the sides run long past the ends of the base. Don't use glue here since the form needs to come apart later. Carefully drive the screws straight in-if you get too close to an edge, the screw will bubble the particleboard outward. This kind of imperfection will mirror in the concrete. Also, remember that the concrete top is cast upside down, so position the sides angled in toward the open top of the form.
Complete the form by cutting the short sides at a compound angle that fits into the long sides tightly. Cross cut the sides to length, using a compound mitre saw with the blade tipped to approximately 15° from vertical and the table rotated to 28°. Attach them to the form with more drywall screws. Mix the concrete and fill the form before moving on to the support structure. (See Mix the concrete for tips).
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