Twenty-five years ago concrete countertops were the domain of artisan contractors and high-end clients, mostly in California, looking for a one-of-a-kind surface. Today, homeowners nationwide request concrete for the same reason.
Top: The best concrete has a low water/cement ratio for strength. Bottom: After removing the front form board, float the front edge to eliminate bug holes.
Credit: Photos: Tom Ralston
I've spent my career working with concrete and I agree that it's beautiful, unique, and a medium that enables builders to offer truly signature work. But working with concrete requires advanced trade skills. Though it may look temptingly easy to build forms and screed a countertop, take some laps around the learning curve before working with it or even before you sub it out.
Installation Method. The debate among concrete countertop contractors is about the best way to install them: cast-in-place or pre-cast. For my money, cast-in-place (CIP) is the way to go. Unlike pre-cast, with CIP all work is completed on site and tuned–on-the-spot–to the existing space. CIP also eliminates templating, moving countertop pieces (several times), and joining separate pieces.
The trade-off, however, is that getting the right finish takes practice and experience. To keep the work on track and help prevent cracking or callbacks, focus on four main areas : precise form assembly, protecting the work area (you're working directly over installed base cabinets), the right concrete mix and reinforcement, and finishing skills.
Protect the Space. Ruining walls and cabinets costs money and wastes time. Tape 4-mil or heavier plastic at least 12 inches up the wall surrounding the countertop location. Then, run blue painter's tape around the top of the base cabinets where they contact the countertop location, and mask the cabinet faces.
Forming & Reinforcement
Use a 7-inch angle grinder with a flat disc to put a hard finish on the setting concrete.
Credit: Photo: Kipper Crete
Bottom Form. Precise forming–and a few tricks–sets the stage for primo fabrication. Begin forming by placing form material on top of the base cabinets. Lots of guys use plywood covered with 4-mil plastic, but John Kipp, owner of Kipper Crete Countertops in Broadway, Va., uses 3/4-inch PVC sheets because they're impervious to moisture and act as a "slip sheet" so that concrete can shrink without resistance toward the center of its mass as it cures–reducing the possibility of shrinkage cracking.
Bond Break/Screed. Kipp also places a strip of flexible material, like a thin piece of foam, cardboard, or etha-foam, against backsplash walls. He keeps the top of it level with the top of the countertop thickness. It makes a good bond breaker so the concrete again can shrink toward the center; later it serves as a screed guide. It's covered by the backsplash so you don't have to remove it. If your project includes concrete backsplashes, form and cast them separately, then glue them to the wall after the countertop is finished.
Countertop Nosing. The front edge of a counter is usually 1-1/2 or 2 inches thick while the counter itself is 1 to 1-1/2 inches thick. The trickiest form work is the nosing. Because the nosing usually cantilevers 1-1/2 inches past cabinet face frames, some contractors support their forms with 2x4s propped on the floor. As an alternative, you can extend the bottom form beyond the cabinet face; you can screw the form work into it. It's much simpler and easy to strip. You also can insert a form liner strip for contoured nosings, which eliminates using edging tools to do the shaping before the concrete sets.
Sinks and Penetrations. The easiest way to form sink openings is with rigid polystyrene foam. Use the template that comes with your sink as a pattern. Carefully cut the foam to shape and then cover the edge with packaging tape to make it smooth. You also can use foam to block-out for faucet holes, soap dispensers, etc., but it's easier and safer to cut them afterward with a coring bit.
Reinforcement. After building the forms and cutting patterns for sinks, etc., add reinforcement. This helps prevent shrinkage cracking–especially on inside corners–and helps prevent cracking caused by movement in the wood frame of a house or cabinets due to settling or fluctuating humidity. There's a wide range of reinforcement products that work well, including various sizes of rebar, steel or synthetic mesh, diamond lath, and fibers, all of which add tensile and flexural strength to the concrete. I like structural fibers best because they provide 3-D reinforcement. No matter what concrete reinforcement you choose, a good rule of thumb is to make sure it is suspended in the center of the slab.
Materials & Mixing
Colored concrete isn't like a painted surface. There is a 3-D depth appearance, which is part of the reason why people choose concrete.
Credit: Photo: Joe Nasvik
Contractors used to order the same ready-mix for their countertops as they used for floors and driveways, but the trend now is toward prepackaged countertop mixes available at construction supply houses or national decorative suppliers. I prefer using either of two types of prepackaged mixes. The first is a "well-graded aggregate" blend. Well-graded blends have multiple aggregate sizes and sand sizes, which allow the mix to stay together nicely when you place it, minimizing void spaces, reducing shrinkage, and increasing strength. The second mix is a self-consolidating concrete (SCC). SCC has fine, variegated silica aggregates. SCC mixes are very flowable, producing almost no bug holes, and they're very strong. The fine aggregate sizes are ideal for diamond polishing when you don't want to expose any coarse aggregates.
Mix It Right. The worst thing that can happen to your countertop is cracking. Using the right countertop mix helps minimize this possibility. Most are designed with low water/cement (w/c) ratios, which produce a more impervious surface, less shrinkage, and higher strength. But follow directions carefully. Don't add too much water in an attempt to make the concrete flow better. Some manufacturers add super plasticizers (that don't change the w/c ratio) to make the concrete flow better, and sometimes polymers are added to increase strength and retain moisture during curing. Use a container with graduations for quarts and gallons so you can make precise mixes.
Contractors often use plywood forms for concrete countertops (Fig. 1), but pouring over PVC (Fig. 2) helps prevent shrinkage, and a plastic form liner at the nosing creates a cleaner edge. Extending the form past the cabinet face can eliminate the need for a vertical brace.
Credit: Illustrations: Chuck Lockhart
You don't need a truckload of tools to finish concrete counters, but you need the right ones and you need to know how–and when–to use them.
Use a palm sander on the edge form and as a vibrator to consolidate the concrete and shake out voids. The best time to vibrate is right after placing and before strike-off.
Use a strike board or darby–a straight 1x4 or 2x4 works–to strike-off the concrete after placing it. The object is to pull the concrete out so the surface is level and flat.
Right after strike-off, go over the concrete with a magnesium hand float. This further flattens the surface and depresses the aggregate a bit, making paste available for troweling. Add concrete to low areas and float it in. It's easy for an experienced mechanic to see low and high spots; however, the main purpose of hand-floating is to remove the textured surface left by the strike board.
Also, striking pulls off a little more concrete along the edges than in the center of the pour, so add a little concrete along the edges and float it in to prevent dishing, creating sharp, crisp edges and a flat surface.
Do the final finishing with steel hand trowels. Wait until you can press your finger, with moderate pressure, about 1/8 inch into the surface before beginning. The timing of when concrete is dry enough to final finish is a gray area–it's very dependent on air temperature, concrete temperature, sunlight hitting the concrete, the type of portland cement in the mix, and the admixtures in the concrete. If temps are low, you may have to wait all day.
Or you could add an accelerator and speed things up. If the concrete temp is 90 degrees, you won't have enough time to finish. So watch what's happening with the concrete. Pressing it with your finger to see how hard it's getting is a time-honored method.
If you don't use a form liner to shape the countertop nosing, use a steel hand edger to cut the radius on the nosing. Edgers come in all sizes; the most popular is a 1/2-inch radius. Getting straight, evenly tooled edges requires skilled work.
Curing & Sealing
Curing. The best way to cure concrete is to "wet-cure" it for one week by putting wet towels or batting covered with plastic over it. The concrete builds strength without shrinking
during this time, so it can resist shrinkage forces better when it dries out afterward. The problem with wet-curing, though, is the efflorescence marks that result (no problem if you're diamond-polishing).
If I'm not diamond-polishing, I use curing sealers, which cure faster than wet-curing; curing sealers form a physical barrier over the concrete and prevent efflorescence, but they're not as strong as wet-cures.
Surface Sealing. Unfortunately, there's no one sealer on the market that satisfies every condition in a kitchen. The ideal sealer should withstand high temperatures; be stain-, abrasion-, and acid-resistant; repel oils and fats; not be affected by UV radiation; and never need replacing. Too bad there isn't one on the market yet with all of these properties. Installers must choose between epoxies, penetrating silicone-based materials, urethanes, waxes, acrylics, and silicates, each of which has individual advantages and disadvantages. I've used penetrating sealers to protect the diamond polish on my tops; however, they're not as stain- and acid-resistant as I'd like and using an epoxy would have been better. In the next year I expect to see a new generation of water-based poly-urea sealers that will probably be the best for kitchen concrete.
–Joe Nasvik is senior editor for Concrete Construction, a sister publication of Tools of the Trade.
The easiest way to color concrete is to add color during mixing. Color is made using metallic oxides to prevent the possibility of color fading over time, and the particles are sized so that the cement paste will lock them in place. Colors come as powder or liquid; liquids are referred to as "liquid-dispensed." Be careful to measure the same amount for each bag you mix. Also, add the same amount of water to each bag of mix because changes in water also can cause color variation.