Grinding Flush Over Metal Embedments

Question:
I can’t keep the surface of my countertops flat after polishing over metal embedments. The embedments end up sticking above the concrete. Why does this happen and how can I prevent it?

Answer:
The problem occurs because concrete wears away faster than metal when polished with diamond abrasives. Because the embedded metal is “gummier,” grinding wears it away at a slower rate and leaves the metal slightly raised above the concrete. I call this “pillowing.”

Pillowing is least pronounced when very little honing is done to the concrete. Concrete with a lightly polished cement finish will show almost no pillowing because very little concrete is removed. More heavily honed concrete, where the cream is completely removed and some of the finer aggregates are exposed, will tend to show more pillowing. And concrete that has exposed aggregate, where aggressive grinding and extensive honing are performed, will tend to show the most pillowing.

Preventing pillowing is not difficult, but the challenge is to anticipate how much concrete you’ll need to remove when grinding, honing or polishing so that the metal and the concrete are flush. The more concrete that needs to be removed, the deeper the metal embedment needs to be cast into the concrete to compensate for the concrete thickness that gets ground away. With polished cement finishes, very little material is removed so little or no depth compensation is needed.

Typically metal embedments are glued to the forms with 100% silicone caulk. Applying a thick layer of caulk, without squeezing out the excess, holds the face of the embedment away from the form so the surface will be sunken relative to the cast concrete surface. By adjusting the thickness of the caulk, you can adjust how deep the metal is cast. Some trial and error is involved, although it is possible to measure the thickness of the caulk. First measure the thickness of the embedment in several areas. Then glue it down and measure the height of the back of the embedment relative to the casting surface. Subtract the embedment thickness and this results in the caulk thickness.

Also be careful about the grit level you use when grinding. Using a coarser diamond grit (such as 50 or 100) will wear the concrete away much faster and show more pillowing than using a medium grit (200 or 400). Fine grits (600 +) will provide the most balanced results, with the metal and the concrete wearing away at about the same rate.