Concrete is an increasingly popular countertop material, and with good reason: It’s cost-effective and durable, and you can customize it to fit almost any application. Despite its rock-solid finished appearance, uncured concrete is as malleable as clay, so working with it is almost more art than craft. But don’t let that intimidate you — with a little instruction, any DIYer can achieve great results using concrete.
To learn more about making concrete countertops, I recently visited expert Fu-Tung Cheng’s design studio in Berkeley, California, for a one-day seminar on pouring concrete countertops. Fu-Tung’s system for creating countertops has taken much of the guesswork out of the process. Here we show you a basic overview of his innovative approach; you can learn more about the process in his books.
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Making a concrete countertop mold
The first step in creating a concrete countertop is building the mold. This is one of the most important parts of the process; mistakes at this stage can result in serious disappointment, so take your time.
Start by drawing your project, including any special features (such as?) that you want to incorporate. Once the design is complete, you can create a template of the counter using lauan plywood strips. The template provides a full-size map of the final countertop and customizes it to accommodate irregularities such as walls that aren’t perfectly square.
Cut the template strips 1/2 in. shorter than the final dimensions and set them onto the cabinetry, starting with the back and front and then adding the sides. Glue the strips together with hot-melt glue. Mark the centerline, sink and faucet locations.
The template will have to be placed upside-down on a melamine mold base, so you’ll need to transfer all notes and measurements to the bottom side of the template. When you flip the template, keep in mind that top is bottom, left is right, and so on.
We used Fu-Tung Cheng’s Pro-form mold-making system to create our mold. The system consists of Styrofoam strips that are adhered to the mold base with double-sided fiberglass tape and then fitted with a special liner that gives the sides of the countertop a smooth finish. The kit is available in 2- and 2-1/2-in. depths.
Seal all corners and edges with black silicone caulk, which creates a rounded edge. Be careful with the caulking, as imperfections will telegraph into the poured counter.
Knockouts are mold parts that create holes in the counter for plumbing. Use the templates that came with your sink and faucet to create the knockouts. Cut the sink knockout from 2-in. (or 2-1/2-in., depending on the depth of your countertop) dense-foam insulation, and then attach the Pro-form liner to the sides, keeping any seams toward the front of the mold. For the faucet plumbing, build mushroom-shape knockouts by cutting two 2-in. holes out of 3/4-in. plywood with a hole saw; then attach the rounded cutouts to a 1-1/4-in. length of 1-in. dowel. This creates a 3/4-in. countersunk hole in the underside of the countertop.
Concrete is strong in compression, but not in tension. Rebar is used to give concrete flexible strength and to keep it from cracking under loads. Support the rebar within the mold using wire tied to screws positioned just outside of the mold. You can also add wire mesh tied to the rebar for more reinforcement.
Finally, you can add inlays or aggregates to the mold. Use your imagination — fossils, shells, coins, metals, tile and glass are just a few choices for inlays.
When adding inlays to the mold, apply a thin layer of silicone to the visible surface of each inlay. This will secure the inlays to the mold and prevent them from becoming lost in the concrete mix during pouring and vibration. Keep the layer of silicone thin so that the inlay will not end up lower than the surrounding surface. If you are adding aggregates to the mold, lightly spray the bottom of the mold with an adhesive to help the aggregates remain at the surface. Add more aggregates than you think you will want to see, as some will become lost in the mix during vibration.
At last the mold is complete. Check your work for any flaws, and carefully vacuum the mold to clean out any debris.
Mixing concrete for your countertop
To determine the amount of concrete you will need, multiply the countertop’s length by width by depth to find the total cubic inches; then divide by 1,728 to find cubic feet. (A typical 80-pound bag of concrete yeilds 0.60 cu. ft.) Use this number to also figure the amount of admixtures required for the concrete.
The kind of concrete you use is very important. Do not use air-entrained concrete. Developed during the 1930s to enhance resistance to freezing and thawing, air-entrained concrete contains billions of microscopic air cells, which relieve pressure from water as it freezes. However, these air bubbles can create small voids in the countertop surface. It may be a challenge to find concrete that is not air-entrained; we had to special order a supply from a local hardware store.
Admixtures can help to eliminate air bubbles as well as add strength and color to the concrete mix. Plasticizers reduce the amount of water necessary for mixing concrete and improve its workability. Less water is better when mixing concrete — too much can weaken it. Other additives include nylon fibers, which improve strength and reduce hairline cracks, and pigments, which add color. Premeasured admixtures that include pigment, water reducers and strength-enhancing fibers make mixing concrete easier.
Once you’ve prepared the concrete and admixtures, gather your tools and get ready to pour. You’ll need a vibrator, which you can find at most rental centers. (Choose the smallest one available.) Be sure to wear gloves, an air mask and goggles when working with concrete. Recruit one or two helpers; once the concrete is mixed, you’ll have limited time to complete the pouring process.
A concrete mixer takes the drudgery out of the job and improves consistency in the concrete mix. Start by putting the concrete into the mixer; then add the dry admixtures. Cover the opening with a piece of plastic to keep the dust inside and run the mixer for five minutes to blend the pigment and additives with the concrete. Stop the mixer and break up any clumps; then remix for a few more minutes.
Pro-Mix by Cheng Concrete specifies the amount of water to use with the mix. However, the instructions that come with any additives you’re using preempt the concrete-package instructions. Slowly add the water to the mix while the mixer is running. Mix the concrete for five to seven minutes; then turn off the mixer and let the concrete rest for three minutes.
Use buckets or a wheelbarrow to move the concrete to your form. Pour it into the mold and work it into the corners with your hands, being careful not to dislodge rebar or inlays. After the mold is half full, use a vibrator just under the surface of the concrete to help remove air pockets. Continue pouring until the mold is full; then vibrate again. Carefully screed off the excess concrete.
Trowel the concrete, working it to a smooth finish. Finally, cut the wires supporting the rebar and cover the slab with plastic to cure. Ideal curing conditions are a temperature of 75 degrees with a humidity level higher than 25 percent. Lower temperatures require longer curing; higher temperatures can speed up curing. In very hot conditions, a moist carpet or towel can help with moisture loss.
After four to six days the slab should pull away from the mold slightly, indicating that you can safely remove it from the mold. The concrete is still “green” (somewhat malleable? Explain term.), so use caution. Place protective foam under the concrete slab as you turn it over to prevent any damage to the edges. Once you’ve turned the slab right-side up, carefully remove the knockouts. Let the slab rest for two to three days.
Polish and slurry your concrete countertop
If you are happy with your countertop as is, you can move on to sealing and waxing. However, if you prefer a different look, you can grind and polish the surface to expose aggregates just beneath the surface of the slab.
You can use wet/dry diamond hand pads for polishing, but the best way to polish the surface is to use a variable-speed wet polisher. Start with a 400- or 800-grit pad to give the surface a smooth finish without exposing a large amount of aggregate. The more polishing you do and lower the grit number you use, the more aggregate you will expose. Personal taste determines the amount of polishing needed.
Wet down the slab before you begin. Work toward the water source and in sections, taking care not to push too hard on the surface while keeping the polisher as flat as possible. Stop periodically and squeegee the excess water from the surface to check for uniformity. Use hand pads to polish difficult-to-reach areas.
Slurry is used to fill any holes or imperfections in the surface of the countertop. It’s a paste made of fine cement, pigment and water that is applied with a putty knife. Wet the surface before you begin, and keep a spray bottle handy as you work. Rub the paste into the counter; then scrape off the excess. When all of the holes are filled, wipe the surface with a clean sponge.
Let the surface cure for 36 hours; then lightly repolish it. If necessary, repeat the slurry-and-polishing process until you’re happy with the finish.
Sealing and waxing your countertop
Concrete is porous and needs to be sealed to resist staining. The two basic types of sealers are topical and penetrating. Topical sealers sit on the surface of the concrete, encasing it in a protective layer. They are excellent for preventing stains but can scratch easily. They also must be completely removed before you can add a new coat. Penetrating sealers do not protect from staining as well as topical sealers, but they allow you to apply additional coats at any time. I recommend applying water-base penetrating sealer followed by a food-safe wax.
Before you begin, make sure the surface is clean and dry. Feel the countertop to check for residual grit or contaminants, and wash it if necessary. To seal, start with a 1-to-1 mixture of water and sealer, spreading it thoroughly across the surface in a circular pattern for at least five minutes. Next, apply full-strength sealer. Continue saturating the surface, making sure the entire surface is covered. Let the sealer soak for a few minutes; then wipe clean. Wait 24 hours and repeat this process for the best protection.
After the sealer has dried for several hours, apply wax with a soft cloth. Let it dry for a few minutes; then buff with a lamb’s-wool pad. The counter should have a soft luster when you’re done.
Installation and maintenance of your concrete countertop
Before you install the countertop, make sure that your cabinet can handle the weight. You may need to reinforce it with strips of solid wood or metal, or you can place a substrate of 3/4-in. plywood on it.
Use caution during installation. Recruit help to move the countertop, and dry fit it before you apply silicone caulk to adhere the counter to the cabinet. If the sink is an under-mount style, you may want to install it before adhering the countertop in place. In that case, drill holes in the underside of the countertop and then install Tapcon screws. You can also use a plywood substrate, routered to accept the sink, as the support.
The surface of a concrete countertop should be cleaned only with mild soap and water. Wax the counter about once a month and reseal it about once a year, stripping any wax before you begin.
If the countertop eventually needs refinishing, wet-polish it using the finest grit needed to remove stains and scratches; then reseal and wax. If the concrete chips, salvage the pieces and glue them back in place using epoxy adhesive.