Instructables
Picture of 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:
Tools:
  • 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:
 
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DB287047 days ago

Thanks for the share! A classmate and I are making a scale model of the Kimbell Art Museum and wanted to get as close to the real thing as possible. This is really given me some ideas as to how to proceed!

CHENG Concrete (author)  DB287044 days ago

Great! The Kahn building or the new Piano Pavillion?

Love this project! Simple and stunning.

kylecomeaux5 months ago

Dear CHENG,

I've been seriously interested in trying out the D-FRC Casting blend (the one used in this project). Although, according to the instructions on the CHENG website, the all D-FRC blends are to be used with the "Activator" product. Yet, here on 'instructables', the activator is never mentioned.

What is the difference in performance when not using the activator with a casting?

Thank you!

CHENG Concrete (author)  kylecomeaux5 months ago

The liquid activator was used on this project. It is a necessary component of the mix. Not using it, or using less than required, will have negative effects on the strength and quality of the piece. Thank you for pointing this out!

Raitis1 year ago
Looking all over these concrete things I started to wonder - is there any substance to cover the mold, so after curing the clock would just drop out nicely without disassembling the box?

As an example - when working with glass (i.e. slumping bottles) the furnace floor is preferably lined with some kind of special paper to prevent sticking.
justalf Raitis1 year ago
There are commercial products used for helping concrete release from a cast. They do seem to be oil based - here's a start http://www.fuchs.com.au/categories.asp?cid=26&c=10441
Raitis justalf1 year ago
Thanks. Seems like oil will do the trick :)
CHENG Concrete (author)  Raitis1 year ago
A shape with no draft, like a box, isn't ever going to just drop out of a rigid form, no matter how much form release you use. If the shape is more like a pyramid, you might have better luck. A very light coating of vegetable oil can be used as a form release, but it's not usually necessary.
Oh, that sparks an idea or two, thanks! (:
WhiteTech2 years ago
Im really liking the visuals you have there, really makes things clear!
timwikander2 years ago
Very cool! I've been wanting to do some small scale concrete casting for a while now. Great tips in this instructable!
iOskr2 years ago
I like ur design... thanks for sharing
wilgubeast2 years ago
That looks pretty good. And great concrete casting info for folks who may need something similar. Thanks for sharing!
Awesome design! Love the simple, modern look :)