This is a table I constructed as a long answer to the question, "can you make concrete touch-sensitive?"

The short answer to that question is yes. I was inspired to make touch-sensitive concrete while visiting the Tadao-Ando-designed Chichu Museum in Naoshima, Japan. The building's concrete was so buttery and smooth - it's impossible not to run your fingers over it. I wondered if there might be interactions you could trigger with the touch of such a tactile but seemingly inert substance.

My solution ended up being to combine powdered conductive material (metals, basically) with the concrete mixture, and then running a wire throughout the concrete as it dried and attaching that wire to the end of a capacitive sensing circuit. And it totally works.

--DISCLAIMER--

If you attempt your own version of this Instructable, I wouldn't recommend using neon light or any other piece of electronics that involves a high-voltage transformer. They tend to mess with the capacitive sensing circuit.

Step 1: Materials

Below is a quick list of the materials I used for the concrete table, the neon, and the controlling electronics.

The most important materials are those which are used to create the conductive concrete. I tried a lot of different powdered metals to increase the conductivity of the concrete. Iron oxide and graphite were the most effective, but graphite is slightly more expensive and it leaves a pencil-y residue on your fingers after touching it, even when it's part of the mixed and hardened concrete.

Conductive Concrete

25lb Rapid Set Cement All Multi-Purpose Construction Material
Black Natural Iron Oxide
Bare Copper Wire
mixing tools (bucket, stirring stick, drill for stirring)

Concrete Mold

MDF 0.75" x 4' x 8' panel
wood glue & nails
Clear Silicone Sealant
Paste Wax
Sandpaper (or preferably a power sanding tool)

Neon/Electronics

Arduino Uno
Relay Module
Capacitive Sensing Chip (could be really any capacitive sensing breakout board - I used the Atmel QT2120 chip)
Filled Neon tubing with electrodes
3000V Neon Transformer

Step 2: Neon

I created my neon loops at The Crucible in Oakland, CA. Neon seems to be kind of a dying art, at least in the United States. That's just my personal impression, but it's based on the fact that it's incredibly hard to find a neon studio in the Bay Area that will let you use their equipment. The Crucible is an incredible facility, but they stopped doing 'free fills' for students who take their classes - you're left to find someone who can bombard/fill your neon, which can be a bit expensive.

For other neon projects, I've used Shawna Peterson of Peterson Neon in Oakland. She's awesome.

Step 3: The Concrete Mold

I made my concrete mold in a very specific way. There are lots of variations on this process, but I'll walk you through what I've discovered to be the most effective concrete shaping system.

Basically, you build a mold out of cut MDF. Personally, I cut the MDF on a Shopbot, although you could easily cut it on a table saw (or by hand).

Each layer of this type of MDF is 0.75" thick, so you'll need several layers to make a complete mold (I used 3)

I cut the layers into identical shapes and then stacked them to make sure they were close to aligned. For this table, I used three total layers of MDF on the outside and two in the middle. The images you see above show the middle two layers being slightly lower than the outer three layers. This allows one layer-worth of depth (0.75") of concrete to fill in above the middle part of the table and become the full shape of the table.

You'll need to affix the layers of MDF together (I used wood glue and good, old-fashioned nails), and then sand them to make sure that no layers or overhangs will be visble on the table edge. To the touch, they should feel like one continuous piece of wood. Power-sanding tool definitely recommended.

After the edges are even, you'll want to coat the entire inside of the mold (where the concrete will be poured) with silicone so that none of the curing concrete can seep into or bond with the edges of the MDF. Use gloves.

Once the silicone has dried, you'll want to coat it all with paste wax to ensure an easy mold removal.

Step 4: Wiring the Mold

Before pouring the concrete, you'll want to make sure a copper wire runs through the negative space of your mold. This wire will eventually be completely encased by concrete. It is the channel through which you monitor for somebody touching the concrete.

This is not an exact science. You won't be monitoring for the location of where somebody touches the concrete, just if they touch it at all. It's a binary thing. So your wire can be generally spread throughout the mold.

A good rule of thumb that I discovered through trial-and-error is that your wire should be at least eight inches from each potential surface a person could touch. Any more distance than that and you risk not being able to sense their contact.

Step 5: Mixing and Pouring

The ratio: 4 parts (by volume) Rapid Set to 1 part Iron Oxide. The iron oxide threw off the suggested water amount from the Rapid Set box, so just add water until you reach the right consistency.

My process:

1. Mix the Rapid Set and Iron Oxide together in a 4:1 ratio in a large container.

2. Put an amount of this mixture in a separate, 'wet' mixing bucket, and add roughly 1/4 that amount of water or until the consistency is that of thick but malleable mud.

3. Put this wet mixture into the mold carefully, keeping potential air bubbles in mind. Forcefully push the mixture into any areas that might generate air bubbles, and then pat those areas to ensure the concrete settles evenly.

4. Work fast - you have 20-30 minutes before the Rapid Set hardens.

5. Smooth the still-wet surface with clean concrete or grout knives.

6. Wait at least 24 hours for it to fully cure.

Step 6: Finishing

After 24 hours of drying, it should be safe to remove the concrete from the mold. Do this carefully. I messed it up with this table. In some pictures you'll notice a hairline crack running across the surface of the table. This came from being too forceful/greedy while removing the mold.

Tips for mold removal:

use lots of paste wax beforehand!
be patient!
don't use percussive force (a hammer).
avoid torsion or other rotational forces on the mold.
find a place where the hardened concrete has separated from the mold and work outward from there, gradually expanding the separation.
be patient!

Sand the edges of the concrete to remove the appearance of layering from the MDF.

I attached hairpin table legs to the bottom of my piece.