Enginursday: The Look - the Feel - of Concrete

Inspired to try something new for International Week of Making, Mary mixed and poured concrete over electronics in her office.

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In honor of National Week of Making, I wanted to try something new. Something a little more civil if you know what I mean.

I spent about a week mixing various bags of concrete and cement in search of the perfect pourable consistency, strength and smooth finish in which to encase some electronics projects. My favorite mixture is two pounds of Rapid Set® Cement All® with ¾ cup of water. I mixed the cement in a 1-liter plastic measuring cup with a pour spout and used a set of tiny spatulas to spread and smooth. This bag will make hundreds of projects, which means the cost of an individual project enclosure is at most a few cents.

I found a generic plastic wrap and tape works best to protect the circuit from possible water damage. The concrete clock only had electrical tape over the soldered connections between each WS281T breakout, which worked well.

What I did not expect to happen:

  • The electronics to work after the cement cured — every time
  • Feldi to rub the final projects on her face — it’s that smooth

Tips for your projects:

  • Buy Rapid Set cement or countertop concrete so your waterproofed projects are exposed to water for the shortest amount of time and have no aggregate material — or the least amount possible.
  • Use plastic molds for resin, or silicone baking molds. The projects where I used cardboard cutouts — like Pinterest told me to — gave the worst results (see the first two projects).
  • Have your project completely ready, troubleshot, waterproofed and tested again before mixing the cement. This stuff cures in 15 minutes — or less for smaller, thinner projects.
  • Double-sided sticky tape is a great way to add cutouts for LEDS, switches and an opening for wires.

Drawbacks:

  • Once your project is in, forget trying to troubleshoot.
  • It takes awhile to get a good pouring technique and think of circuits in terms of negative space within a mold.

Now I’ve got about 51 pounds of cement left to play with — any suggestions?


Comments 21 comments

  • I would be concerned about encasing a Lithium battery in concrete. Check out the videos on the dangers of lithium batteries then imagine that happening inside a concrete block.

    • I was concerned as well. That’s why you see fewer and fewer pieces of electronics cast in concrete with each iteration. At the suggestion of another user - I’m 3D printing some small boxes with lids (like battery holders in old toys) that will be cast for the microcontrollers and batteries to live - after the cement has cured.

  • ROB-24601 / last month * / 2

    My wife and kids recently found a kit for making cement stepping stones at a hobby store, and for a greatly discounted price. It comes with white cement and a bunch of tumbled, colored glass. Naturally, my first thought was to embed a solar panel, photocell, and LEDs in it, so that it charges during the day, and is illuminated through the colored glass at night. Thanks for letting me know it’s possible!

    • I want to see that when it’s finished Rob! You also gave me another idea of taking apart dollar store solar lawn light stakes and using the electronics to make some creepy garden gnomes.

  • TerryC / last month / 2

    This is inspired! One suggestion: consider Glad Press'n Seal Wrap (or maybe there’s some equivalent) for waterproofing circuitry prior to encapsulation. It’s a plastic film with a light adhesive on one side.

  • Purists will point out that there are differences between cement, concrete, grout, and mortar, though they can be somewhat abstract (had to get that pun in somehow!), though all four are related. What you have is technically a grout. One word of caution: do NOT try using “hydraulic cement” – it is meant for things like repairing cracks in swimming pools, and it expands as it cures. (It could crush the electronics.)

    One idea that occurs to me is to cast “project boxes” – boxes with space for the electronics to be installed later. You can cast in threaded inserts for screws to mount the boards and/or assemble the boxes later.

    As for molds, yes, you can use “off the shelf” stuff, or explore “making your own”, hand crafted, 3-D printed, vacuum formed, CNC milled, etc. (A thought about cardboard molds: rather than using “plain old cardboard”, use some of that plastic coated stuff that frozen foods often come in – it should be more immune to the moisture from the material being cast.)

    One last thought: I’ve found concrete to be a fascinating material. The Army Corps of Engineers published a handbook on the stuff several decades ago, at it can make interesting reading. Sorry, I don’t have my copy handy, so can’t give you the exact title. (I got my copy well before Mary was born, but I’d be very surprised if you can’t find a copy today.)

    • I happened to think of something from when I was a young kid back in the early 60s: One of the activities at “summer bible school” one year was carving blocks of plaster with some sort of additive to make it “softer” and easier to carve (they’d been cast in the old cardboard half-gallon milk containers). I remember trying this later with ordinary plaster of paris, but because we didn’t have the additive, it was much more difficult to carve. I never did find out what the additive was (I think I was maybe 10 or 11 years old at the time).

      Anyway, plaster is a bit more amenable to post-casting modifications.

    • I really like the idea of using freezer food cardboard. The texture would produce a smooth result and it would be easier to cut to form various shapes.

  • Of interest perhaps: RePWR: Wireless Power Transfer within Reinforced Concrete https://doi.org/10.1145/2996884.2996885

  • in case anyone else was wondering…i think she said : “ resinobsession.com ”

  • My suggestion is walkway tiles that light up when you step on them! You’d probably need alot of concrete….

  • I used concrete countertop style concrete for a bathroom floor due to mechanical constraints. It had a ton of additives including chopped fiberglass fibers. I would use fiberglass over sisal, less affected by humidity and won’t rot out on you someday. (Of course this is probably a total nitpick, the electronics will probably die first, years down the road).

    There are some great colorants available via the concrete countertop industry, most notably Cheng and Fishstone.

    Other things to note is that Aluminum and concrete don’t get along (electrolytic corrosion I think), but 304 Stainless is ok. So I’d definitely seal the electronics before encasing them.

  • A couple of ways you could waterproof your connections is (1) seal them in hotglue or (2) epoxy resin. I have done both and can say my thumb drive that had it’s case broken years ago has survived my washing machine and dryer and still works. Hot glue I have used in my outdoor ham radio experiments with good success.

  • Rob / last month / 1

    As you experiment, you might try using grout (as in tile grout) in the place of concrete. It’s easy to buy in smaller quantities and it comes in a variety of colors. Also, note that concrete is prone to cracks and in most cases should include some sort of reinforcement. For tiny little projects it may not matter, but for anything large, you would want some steel in there somewhere. Lastly, I’ve seen some pretty cool results with casing optical fiber into concrete.

    If you are literally going to take your designs and “cast them in stone”, then I would suggest a modular approach, where the individual modules are “cast in stone” and can be replaced with new modules. If you cast everything in stone and a component dies, you are pretty well done.

    • Sembazuru / last month * / 2

      For reinforcing concrete in these small items, would something like sisal fibers like in plaster mold forming? (Flexible sisal might be easier to manage than metal mesh. And if RF devices are embedded any metal reinforcement might adversely affect the signal.)

      I wonder how well pieces of window screen (either old-school metal or modern fiber) would work for reinforcing. Easily cutable into shape that allows for keeping a gap between the reinforcing edge and the outside surface for aesthetic reasons.

      • diz / last month / 1

        I don’t know about sisal specifically, but fiber-reinforced concrete (FRC) is a thing, so I imagine the answer is “yes”. If you call up a concrete company and request a load of concrete, they will ask you if you want fiber. It’s not as good as steel, but it’s better than nothing at all.

        Source: I poured a concrete pad for a hot tub a few years ago. I used FRC and rebar.

        • Being a (now semi-retired) engineer, I tend to notice a lot of things. More than 20 years ago, I daily drove by a section of freeway that was under construction. I happened to notice that they didn’t seem to be embedding any reinforcing steel, so I asked a civil engineer friend who worked for the state Department of Transportation about it. He said that they were using a fiber reinforced concrete, and that it was stronger than the steel reinforced variety, plus because of the avoided labor in getting the steel in just the right place, it was less expensive, and it meant a few days less construction due to not having to place the reinforcing steel. Although I don’t drive that section daily any more, I do go over it a few times a year, and haven’t noticed any deterioration (caveat: freezing weather is rare in the Phoenix area).

    • I never thought of grout. I liked the cement because it had a pretty thin consistency for getting into tight spaces and smoothing was easy enough with gentle vibration. I do like the variety that exists in grout – I’ll have to try that out.

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