Cast-In-Place Concrete Counters, continued
There are many ways to color and decorate concrete. Integral color is created by adding dry or liquid pigment to the mix.
In mixing integrally colored material, the pigment is added to the water so that the color is evenly dispersed when the dry materials are added.
Surface treatments, which color the concrete after it's poured, include acid stain, color hardener, colloidal dye, and universal tints. Some colors require you to substitute white cement for the gray cement that's in conventional concrete. Additional effects can be created by embedding objects in the surface or by exposing colorful aggregate that was added to the mix.
Many customers opt for integral color because it's more uniform and because dings are less likely to show when the color goes all the way through. However, counters contain very little material, so it can be tricky getting the exact color the customer wants. Minor variations in the ingredients may produce noticeably different results.
Surface treatments allow for patterns, mottled colors, and effects that aren't possible with integral color. The color usually goes in about 1/16 inch, plenty far for the kind of wear a counter is likely to get. Surface treatments cost more than integral color because they add labor, but it's not a significant amount compared with the total cost of the job.
Placing and Finishing
After the concrete is mixed, we haul it into the building and place it in the forms. The mix is stiff, so it's difficult to spread. We use a float to spread it and a cordless concrete vibrator to prevent voids and increase workability by bringing moisture to the surface.
A cordless vibrator consolidates the concrete so that there are no bug holes and rock pockets in the edge.
Next, we use a straightedge to screed the concrete flush to the form. To ensure that the top is really flat, we screed it or rod it two or three times from different directions. A wood float comes in handy for cutting down highs and filling in lows. Once the counter is flat, we leave it alone until it's hard enough to finish. The concrete is ready when you can't push your finger more than 1/8 inch into the surface.
The edge forms on this peninsula are a handy guide for the screed. It's harder to flatten counters that butt to walls because there's no edge form along the back. In such cases, the crew screeds to a reference line or uses a level.
Most customers prefer a hard, smooth, troweled finish. We start by running a 1/4-inch edger against the edge of the form. This smooths the top and puts a 1/4-inch radius on the upper edge of the counter. Next, we remove the edge forms, including the ones for the integral splash and undermount sink. We don't remove the temporary shelf that supports the nosing for at least a week, or until the overhang cures enough to support itself.
Once the forms are out of the way, we finish the face and edge of the slab with a steel trowel. The trowel produces a smooth, tight surface. To make it harder and denser, we can spray a little water on the counter and trowel it some more. This technique also burnishes the concrete, giving it the aged, variegated look that many customers are looking for.
This counter is being given a hard-troweled finish. Note the integral backsplash.
We use an edger to put a radius on all exposed edges, including the top of the splash and the upper and lower edges of the opening at undermount sinks. A rounded edge looks better than a square edge and is less likely to chip. Edgers are designed for use on broad, flat surfaces, so we have to modify the tool to get it to fit on a curved opening or narrow edge. We also sometimes smooth edges with a taut strip of plastic sheeting.
Standard concrete-edging tools can be used to round the corners of the counter.
Another way to finish curved edges is to rub them with a taut strip of plastic sheeting.
Curing. Concrete is strongest when it's properly cured. One way to cure a counter is to mist it with water every half hour or so. Another is to seal it right after it's finished, but that usually limits you to the use of water-based sealers. We like to tent the counter with plastic. That keeps the humidity in and allows the concrete to cure properly. The plastic should be suspended over the surface so that it doesn't mess up the color by coming in contact with the concrete.
Polishing. Although it's not required, we're frequently asked to polish counters. We give the concrete a few weeks to cure, then polish it with a diamond polishing pad on a water-fed disk sander. Polishing densifies the surface and produces an attractive look by exposing sand fines or colored aggregate that was added to the mix. The finish is smoother than a trowel finish. We typically start at 120 grit and work our way up to 400 grit. On some jobs, we've gone as high as 3,000 grit, which produces a surface you can see your reflection in. Grinding is time consuming and messy, so we use as little water as possible and mask the surrounding area.
Concrete counters should be sealed to decrease the likelihood of staining, but there's no such thing as a stain-proof concrete counter. The most effective stain-prevention is 100% epoxy sealer, but it looks kind of plastic. Most people prefer a matte finish because it preserves the "concrete look." We have successfully used a penetrating sealer followed by a low-gloss polyurethane. Sometimes we use three to five coats of acrylic sealer followed by a coat of acrylic wax or beeswax. Before wax is used, make sure the homeowners understand that they will have to renew the wax several times a year.
There are enough sealers on the market that you could devote a whole article to them. They can be silicone, polyurethane, epoxy, lacquer, acrylic, or water-based. Before using any of them, it's important to read and carefully follow the manufacturer's recommendations. The common mistake is to apply the sealer before the counter is at the proper stage of curing.
While concrete doesn't cost much, concrete counters are expensive because it takes a lot of labor to produce them. We operate in an area with high wages and a high cost of doing business. Around here, concrete counters cost about $100 per square foot, which is comparable to the cost of granite. The cost in your area will depend on labor rates and the degree of local competition.
Tom Ralstonis a third-generation concrete contractor and the owner of Tom Ralston Concrete in Santa Cruz, Calif., which does structural, decorative, and specialty concrete work.