This guide will show you how to cast a fiber-reinforced concrete clock using a melamine box as the form and rigid foam as a knockout to recess the clock hands and mechanism. You’ll be introduced to the basics of crafting with concrete.
What You’ll Need
7/16″ Drill Bit
100% Door and Window Silicone Caulk
Sanding Block/Sandpaper (60, 100, 220)
CHENG Corner and Seam-Shaping Tools
What You’ll Need
Diamond Hand Sanding Pad
CHENG Concrete Countertop Sealer
Clock Kit + AA Battery
3/4″ Melamine-Coated Particle Board
1-1/4″ Drywall Screws
Styrofoam, 2″ thick
7/16″ Wood Dowel Rod (2″ long)
3M Super 77 Spray Adhesive
Clear Packing Tape
CHENG D-FRC Casting Blend
Step 1 – Build the Box
Cut the Base and Form Walls from 3/4″ melamine-coated particle board or equivalent. The melamine coating is waterproof and provides a good surface to cast against. Alternatively, you could use plywood that has been sealed or covered with clear packing tape.
(1x) Base: 3-3/4″ x 3-3/4″
(4x) Walls: 2-3/4″ x 4-1/2″
Place walls around base. Pre-drill and assemble with 1-1/4″ drywall screws. Use a ruler/square to keep the form walls at a 90-degree angle to the base.
If any water is absorbed by the form walls, the concrete will have a chalky finish in those areas.
Step 2 – Seal the Box
After the box is assembled, seal all inside edges with 100% silicone. This will make the form water-tight as well as give the edges a nice round-over. Focus on making the vertical edges look good, as the seams at the base will be covered by the knockout and won’t be visible in the finished piece.
Consult the Corner and Seam Shaping Tools Guide for detailed tips about caulking the form.
Work on the foam knockouts while the silicone cures.
Step 3 – Make the Knockouts
Cut two square blocks of foam, a little bit larger than the final size, and sand it down after drilling the hole. This way the hole stays centered. The clock-mechanism-knockout is not visible, so precision isn’t as important.
The front knockout will require the most time. Power tools can help, but I find that shaping foam goes quickly with a rasp and a rough sanding block.
Clock Mechanism Knockout: 2-1/2″ x 2-1/2″ x 3/4″ (actual size) – If the clock mechanism you’re going to use is larger than 2.5″ x 2.5″, change this dimension. If it needs to be deeper than 3/4″, then change the overall depth of the clock.
Face Recess Knockout: 3-3/4″ x 3-3/4″ x 1″ (actual size)
Mark the center of each block by connecting the corners with a pencil line. Drill a 7/16″ hole (or whatever size spindle your clock mechanism uses). Drill both of these holes 1/2″ deep.
If you drill them deeper, you will have to lengthen the wooden dowel. If you drill through the foam completely, you have to start over. If you aren’t using a drill press, drill straight up and down the best you can.
Step 4 – Shape the Face Knockout
Draw guide lines on the large knockout so you know where to remove material. Gradually cut and sand the foam away until you get close to the finished shape. Measure continuously, be precise, and don’t dent or scratch the foam. Any imperfections will show up in the concrete, so take your time.
Work up from 60, 80, 100, 120, to 220 grit sandpaper until the foam is smooth.
Step 5 – Tape the Knockouts
Cover the knockouts with clear packing tape and trim where necessary with a sharp knife, always being as neat as possible. The packing tape will make it easier to remove the knockouts and will give a glossy finish. You could skip this step, but the finished surface will be slightly rough like the foam.
Step 6 – Assemble the Knockout
Cut a 7/16″ dowel 1-1/4″ long and wrap in clear packing tape. If you don’t wrap it in tape, you might have to drill it out of the concrete (it will steal moisture, swell up, and possibly crack the concrete since it’s so thin at the face).
After checking to make sure you have 1/4″ between knockouts with the dowel in place, put a dab of silicone down in each hole and gently slide the knockouts onto the dowel. The silicone will cure and help hold the two pieces of foam together.
Step 7 – Glue the Knockout in the Box
The Face Knockout goes in the box first. Be sure to test fit everything before sticking it in place with spray adhesive or silicone.
Make sure the Clock Mechanism Knockout is not lower than the form walls (or it will get covered over with concrete when you pour).
Step 8 – Cast Concrete
Before mixing and pouring the concrete, screw a plywood ‘bridge’ to the top of the form. This will hold the knockout down (the buoyant force of the liquid concrete will try to float it up). If using a bridge, pre-drill into the form walls for the screws or they might deform and split.
Cover with plastic and leave to cure in a shady place. (D-FRC needs 24 hours to cure; regular concrete can take up to 4 days).
Vibrate the form to remove air bubbles. Gently lift one side and drop it to the casting table repeatedly or use a table vibrator. Wipe air bubbles off the surface and make sure the form is filled to the top.
Alternately, you can use a lead weight or something small and heavy. Either way, leave some room to pour concrete into the form.
Step 9 – Demold the Clock
Unscrew form walls and remove concrete piece.
Remove the wooden dowel. If the dowel is stuck, don’t force it, because you’ll probably just break the concrete. Instead, take a small drill bit and drill through the center of the dowel until you can pinch it together and remove it.
Dig out the foam without prying on the concrete. Stabbing a flat screwdriver in the foam and lifting it up usually works.
Step 10 – Finishing
Sand any sharp edges away with a diamond hand pad. Grind the back side down if you have a variable-speed polisher. Invest in a tool like this if you plan on working with concrete – it really opens up a world of sculpting and finishing techniques.
Make a cover for the back of the clock to hide the mechanism (optional).
Install the battery and set the time.
Insert the mechanism from behind and thread the nut on the threaded shaft.
Re-shape, paint, and bend the clock hands at an angle so they don’t touch the face of the clock.
Seal with CHENG Sealer and wax if desired.
Finished Concrete Clock
A quality concrete sealer will protect the concrete and make it easy to clean.