Introduction: DIY CONCRETE:: Clock

This Instructable will show you how to cast a fiber-reinforced concrete clock in a melamine box, using a foam knockout to recess the clock hands and mechanism.

About Fiber Reinforced Concrete: CHENG D-FRC is a fiber reinforced concrete mix that is ideal for large or small projects where strength and thinness are desired. Fiber reinforced concrete doesn't contain rock aggregate, instead it contains a range of fibers that act as the aggregate and help bind it together. The 1/4" thickness at the face of this clock is less of a concern with D-FRC because it's designed to be cast thin. The aggregate in regular sacked concrete can be up to 5/8" in size, and getting small forms to fill without large voids can be tricky (a 5/8" rock won't fit in a 1/4" gap). Concrete without aggregate (cement by itself) is brittle, but with that said, try this project even if you're not using fiber reinforced concrete, just take out the larger aggregate, don't mix it too dry, and vibrate the form to remove as much air as possible.

It takes some time to shape the foam knockouts and then they can only be used once, but the foam is relatively easy to shape with hand tools, and the total cost of forming materials is pretty cheap.

Because I didn't have so many pictures of this project, I modeled it in sketchup to help explain the process, hope it helps!

Materials + Supplies:
  • Hand Saw / Table Saw
  • Screw Gun + Pre-drill bit
  • Drill + 7/16" Drill Bit
  • Caulking Gun + 100% Silicone
  • Sanding Block + Sandpaper (60,100,220)
  • File / Rasp
  • Acrylic Shaping Rods for tooling silicone beads

Concrete Mixing:
Concrete Finishing:

Step 1: Build Form

Step 1: Build Box
Cut the Base and Form Walls from 3/4" Melamine-coated particle board or equivalent. The melamine coating is water proof and provides a good surface to cast against. Alternatively you could use plywood that has been sealed or covered with clear packing tape, but if any water is absorbed by the form walls, the concrete will have a chalky finish in those areas.
1x - Base: 3-3/4" x 3-3/4"
4x - Walls: 2-3/4" x 4-1/2"

The size of the base and height of the form walls determine how large the clock will be. Any changes in size must have corresponding changes to the size of the Knockout in Step 3.

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.

Step 2: Seal 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. Sloppy silicone work will be obvious in the finished concrete piece. 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.

Tool the silicone (use something like the acrylic shaping tools for best results, not your finger...) and remove excess with a razor blade or wait for the silicone to cure and peel up the remaining lines.

Work on the foam knockouts in the mean time.

Step 2: Make Knockouts

A knockout is what creates a void in the finished concrete piece. It is placed in the form before the pour and removed after the concrete has cured. In this case, the clock has two knockouts - one creates the recess in the face, the other will create a recess for the clock mechanism. The two knockouts are attached together with a 7/16" dowel rod. The dowel makes the hole in the clock face.

Rigid Insulating Foam works best (blue or pink). Regular white styrofoam can work, but since it isn't as dense, the surface finish won't be as nice. The more time you spend to make the foam knockouts perfect, the nicer the end result will be.

Step 3: Cut Foam Knockouts
Cut two square blocks of foam. I like to cut the 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 Larger 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, 220 grit sandpaper until the foam is smooth.

Step 5: Tape Knockouts
Cover the knockouts with clear packing tape, 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 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 piece of foam together.

Step 3: Put Knockout in Box

Step 7: Glue Knockout in 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 4: Mix + Pour Concrete

Step 8: Cast Concrete

1. 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. Alternately you can use a lead weight or something small and heavy. Either way, leave some room to pour concrete into the form.

2. Blow any debris out of the form.

3. Mix concrete and fill form by hand.

4. Vibrate 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.

5. 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).

Step 5: De-mold

Step 9: De-mould
Unscrew form walls and remove concrete piece.

2. Dig out the foam without prying on the concrete. Stabbing a flat screwdriver in the foam and lifting it up usually works.

3. Remove 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.

Step 6: Finish

Step 10: Finish and Fit Clock Mechanism
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.

2. Seal with CHENG Sealer and wax if desired.

3. Re-shape, paint, and bend the clock hands at an angle so they don't touch the face of the clock.

4. Insert the mechanism from behind and thread the nut on the threaded shaft.

5. Install the battery and set the time.

Step 11: Cut Felt Backer
6. Make a cover for the back of the clock to hide the mechanism (optional).

That's it. If you have a chance to make this project, send some photos to
Or if you have any ideas for step-by-step project instructions you'd like to see, let us know! Thanks for following.