Inlay Technique

image of a octopus in concrete

RockMill is known for using concrete in creative, artistic ways. One of those ways in known as “inlaid concrete.”

It starts out not looking like much. Which may lead you to wonder: How could a pool of grey liquid be transformed into something with fine details and clean edges?

Concrete being poured into a form

Artistic concrete made to look like two trees

It seems magical. But unlike magicians, we’re happy to bring you backstage and show you what it takes to make the wow moments happen.

Positives and Negatives

Nearly every project begins with a pattern. It’s the blueprint we follow and it shows the dimensions and characteristics of the finished piece. When we’re happy with the pattern, we transfer it to a mold that will hold the concrete.

To make inlaid concrete, we adjust the pattern so that hollow areas (negatives) are left in the concrete after it is taken out of the mold. We do this by creating raised areas (positives) in the mold. The liquid concrete flows around those raised areas, so when it hardens and we pop it out of the mold, empty spots are left where the mold used to be.

A polyurethane mold showing positives and negatives in concrete

The raised lettering in this polyurethane mold are positives. They create the negatives in the hardened concrete.

Roughing It Up

One of the cool things about concrete is that it’s possible to add new material to concrete that has already hardened. This behavior is important for the inlaying process. But it isn’t as straightforward as dumping a bucket of wet material into the negative spaces and swiping it all flat with a trowel.

If you took regular concrete and poured some more concrete on top of it, the bond between them would be very weak, because there would be hardly anything for the new material to grip onto. The surface needs to be prepared so that old and new will become one.

We do this by spraying the negative spaces with a special type of acid. This roughens things up and creates lots of surface area for the new concrete to interface with. After the acid has done its work, we wash it away and let the concrete dry.

Concrete negative spaces being soaked in acid in preparation for inlaying

This countertop has cured for a couple of days, and already been ground flat after demolding. We applied acid to the negative spaces and then scrubbed them with a small brush, to make sure the acid reached all corners.

Back for Seconds

It is now time to mix a new batch of concrete, which we pour into the roughened negative spaces.

Wet concrete has just been poured into the negative spaces of this bar top

It might look messy, but that’s all part of the plan. We purposefully over-fill the negatives so that when the concrete cures and shrinks, just the right amount of material will remain. If it looked perfect when wet, it would be full of holes when dried. The new layer needs to cure for at least 24 hours.

Finishing Touches

When the second layer has cured we grind it flat. This exposes sand grains in the concrete and reveals the seamless edge between the two colors.

It also exposes small bubbles that were trapped in the wet concrete. These “pinholes” are natural to concrete, but we don’t want them in the finished piece, so we get rid of them by grouting the surface with a mixture of cement, pigment, and liquid adhesive.

Inlaid concrete needs to be grouted with colors that match it

We color the grout to match the underlying concrete. When the grout has set up it is time for more grinding. After a combination of machine grinding and using hand-pads to get everything properly smooth, we thoroughly wash the piece and let it dry.

The concrete has been ground flat and excess grout removed to get it ready for sealing

This countertop is inlaid, grouted, ground, dried, and ready for sealer. It takes a lot of attention to detail and hard work to make inlaid concrete. The great thing is, all that effort shows up in the finished product.

What pattern would you like to see inlaid into concrete? The possibilities are as great as the imagination.

Inlaid concrete octopus. No actual cephalopods were harmed in the making of this piece.


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