I was hired by Hanley Wood as a senior editor, because I had been a concrete contractor for many years, had experienced many of the problems and challenges affecting concrete, and had learned hands-on the skills of placing and finishing concrete. Writing about concrete from a contractor's perspective, we believed, would benefit the industry.
Although I now spend more of my time writing about concrete than actually working with it, I recently built concrete countertops for my own home. Here's what I learned in the process.
When my wife and I decided to completely replace our kitchen, the first decision was easy—the counter-tops would be concrete, and I decided to build them myself (decisions about cabinets took much longer). A contractor produces the best product possible for the money. But as an individual building for oneself, there's an opportunity to experiment with ideas and obsess on the details without the fear of losing money.
Because my brother has a great precasting facility, I decided to precast my countertops in his shop. I also wanted to wet diamond polish the surfaces, which is difficult to do with cast-in-place because of the mess it creates in the kitchen. Also, I wanted the countertops completely ready for installation when the cabinets were installed (in the end, they were ready before the cabinets were delivered).
You never know exactly how colored concrete will look until you make samples. The color had to complement the black cherry cabinets, so I made a dozen samples using the mix design for the project. I knew that the sample selected would be close to the final result, but there would still be differences. I was careful to weigh out each ingredient, and I converted the color amount to a percentage of the weight of port-land cement in the mix. I also diamond polished the cured samples to see the influence that process would have on the color.
Dimensioning is very important for precast work, so I checked and double-checked. One counter section fit into a corner, so the angle of the corner was noted. We bought an undermount sink, and that opening had to be dimensioned properly with the cabinet below. In all, there were three countertop castings.
The counters were cast upside down, so the bottom form surface was constructed with two sheets of ¾ -inch-thick high-density fiberboard screwed together. We drew out the exact dimensions for each casting on this base and mounted I 1½-inch wide forms to the lines. There were two radius corners, and we used dense polyfoam to form the curves. When the forms were ready for concrete, they were carefully supported and leveled so that the concrete sections wouldn't have 3-dimensional twists. A light application of form release completed the forming step.
The mix design
The concrete mix design we used was unusual in that it included no large aggregates. My brother has done extensive experimentation with variegated silica sand mixes combined with self-consolidating concrete (SCC) technology. To the SCC, we added structural fibers that wouldn't be visible after the diamond polishing process. The mix also included 658 pounds of portland cement (7-bag), an accelerator, a polycarboxylate superplasticizer, and integral color, and the resulting mix had a 0.40 water/cement ratio-resulting in a self-leveling concrete. We placed it late one evening, and it had sufficient strength for polishing the next morning (approximately 4000 psi).
I'm glad I performed most of the polishing soon after placement because a week later the concrete was too hard to get good production. I used a hand-held planetary grinder starting with 50-grit diamond pads, doubling the grit number with each step until I reached the last 3000-grit polishing step. The process is very-wet, and wearing a rain suit is a must. The resulting finish was very flat and smooth, with a nice luster— just what we wanted. I used a ½-inch dry-grind diamond router bit to radius the top edges of the counter. It provided a much better look than could have been produced by placing a radius mold into the form.
The hard part about installing precast concrete countertops is lifting them into position. The panel for the peninsula weighed 450 pounds and required the help of several friends. But the L-shaped sink panel was the most difficult because the opening for the sink made it very delicate—any twists during placement would crack it in the narrow portions surrounding the sink. I placed many dabs of thin-set mortar on the cabinet tops to support the panels with minimal stress. I also placed silicone caulk on the tops of the cabinet to serve as a glue to keep the countertops from sideways movement.
Sealers are the most problematic aspect for both concrete and stone countertops. They must resist acids, oils, dyes from coffee and other foods, detergents, abrasion, and be tolerant of high temperatures. I used a penetrating sealer because I didn't want it to show on the surface, but its resistance to acid stains from fruit and certain other food colors isn't good enough, and the search goes on for a better system.
There are a surprising number of challenges associated with building concrete countertops. And it's strange that with all that can be achieved with concrete these days, concrete countertops are helping our industry to increase the use of concrete. They are considered to be very “hot.” For those of you with concrete skills, I recommend constructing your own.