How To Cast a Concrete Vanity Top

For a kitchen or a bathroom, concrete countertops shine - and they're easy to do, too.

Techniques, material availability and information have improved greatly in recent years, and today pouring your own concrete countertop is easier than ever. By tapping into new DIY-friendly resources, you can virtually guarantee that your casting project will be a success—even if you are a first-timer.

The advantages of a cast concrete countertop are many. The material is durable, inexpensive, low-maintenance and when handled properly, beautiful. Design options are nearly unlimited. You get to choose the size, color, thickness and finish and add special touches such as inlays or integral drainboards.

Build the form with white melamine and coarse-thread screws driven through pilot holes. Plan so the screw heads will be accessible when it’s time to remove the form from the cured concrete. Buy your sink first, and use the cutout template provided by the sink manufacturer to make the knockout. Three layers of 5/8-in.-thick particleboard are used here, but you can create a knockout from 2-in.-thick foam board. Apply a strip of foam tape around the edge of wood-base knockouts to provide some cushion and to make the countertop easier to release from the form.

Apply a neat bead of silicone caulk to all seams to seal out water and create a slightly eased edge on the concrete. Black caulk is easier to see. It’s much easier to cast holes for faucets than to drill them later. This urethane knockout plug (from Cheng Concrete Exchange) flares to 2 in. to create access for the mounting mechanisms on the underside of the faucet. To limit the number of knockouts, shop for a faucet with a single handle mounted on the spout. Instead of a standard backsplash, add a second layer of particleboard with a cutout to form a cast backsplash ridge. Round over the edges of the ridge cutout with a router and ogee bit. Be sure to allow for the extra ¾-in. thickness when cutting the form walls.

With extensive input from author and designer Fu-Tung Cheng and Jeff Osteen, his associate at California-based Cheng Design, I designed and built the 1-7/8 x 22 x 48-in. bathroom vanity top featured here. To prepare for the project I studied Cheng’s book Concrete Countertops (Taunton Press, 2002) and viewed the new companion DVD by the same name. I also used products supplied through Cheng’s Internet site to build the form and to mix, finish and seal the concrete. You can find comparable products at local home centers and concrete-products suppliers.

Casting a concrete countertop is a messy process, so plan to do the project in a workshop or garage or even outdoors. Off-site casting (as opposed to pouring the countertop directly onto cabinets) affects the design process in two principal ways. First, you’ll pour the project upside-down in the form. Second, if you’re creating a large countertop, you’ll have to build it in several manageable-size pieces and join them together on site.

Level the form on a sturdy worktable; then secure it to the table by driving screws (not too long) up through the tabletop and into the form. Affix inlays or decorative aggregate to the bottom of the form with spray adhesive. These jadeite pebbles will accent the surface of the backsplash ridge.

Thoroughly mix the dry ingredients. This includes the concrete mix, fiber reinforcement, pigments and water-reducing admixture. (The kit used here included everything except the concrete in premeasured containers.)

Design the countertop
In the design stage, consider the following variables:
Size: Length and width are dictated by your cabinets or vanity base. Measured from front to back, kitchen countertops are normally 25 in., and vanity tops are usually 22 in. Typical thickness is between 1-1/2 and 2 in. If you’re making a knockout for a sink, make the countertop the same thickness as the knockout. The knockout I used for this project was created with three layers of 5/8-in. particleboard, so I made the countertop 1-7/8 in. thick to match.
Reinforcement: Use a combination of rebar (No. 3 or No. 4), wire mesh (re-wire or steel mesh with a minimum 2-in. grid) and fiber reinforcement (polyethylene) added to the concrete mix. Design the countertop so there’s at least 2 in. of concrete between edges and knockout areas.
Concrete mixture: You can mix your own concrete, but buying 5,000-psi bagged mixture (such as Quikrete 5000) is far simpler. If you mix your own, avoid large aggregate (1/2-in. and bigger), but do not use sand alone. Add water reducer to the mix along with fiber reinforcement and pigments.
Color: Some people like the neutral gray tones of concrete, but most homeowners opt for colored concrete. Home centers usually stock two or three pigments (including brick red and black). For more options, visit a local concrete-products supplier or shop on the Internet. Colorful aggregate can liven up the appearance of the countertop.
Finish: You don’t necessarily need to do anything to finish a concrete countertop. But raw concrete stains easily, so in kitchens and bathrooms it’s usually a good idea to seal it. By grinding and polishing the surface you can create sophisticated effects such as exposed aggregate and a very high gloss.
Inlays and special effects: One unique characteristic of concrete is that you can embed decorative items such as coins and stones to make design statements. Integral drainboards and cavities that hold cutting boards are practical countertop accessories. With concrete, virtually anything is possible.

Add water and thoroughly mix the concrete; then shovel it into the form. Avoid disturbing the rebar cage. Work the concrete into the form, using your hands to press it into corners.

Repeatedly rap the form with a rubber mallet to vibrate the concrete so it settles and to remove air bubbles that will cause voids. Vibrate repeatedly as you fill the form. Strike off the surface of the concrete with a screed. (A piece of angle iron is used here.) You’ll need to work around the wire ties as best you can.

Strike off the surface again, screeding over the form edges in a sawing motion. When the concrete is smooth and level, cover it with plastic sheeting and let it cure for four days.

Build the form
The best material for building a concrete countertop form is white ¾-in.-thick melamine-coated particleboard. It is relatively inexpensive, and the smooth surface eliminates the need for a release agent. You can cut sheet goods with a circular saw as long as you use a straightedge cutting guide, but a table saw is the best tool for the job.

When you assemble a form, predrill for all fasteners, and locate screws so the heads will be accessible after the concrete is poured. Position the form on a sturdy worktable, and level it with shims if necessary. Attach the completed form to the worktable with screws, taking care not to drive screws into the concrete area. Support rebar reinforcement with temporary blocks while you suspend it from the edges of the form with wire. The rebar should be at least 1 in. away from all surfaces to prevent ghosting. Rest steel mesh or rewire on the rebar cage and then remove the temporary support blocks.

Mix and pour the concrete
If you have the space and the budget, rent a portable concrete mixer. You cannot mix concrete for countertops in small batches, so look for the biggest mixer you can find – 9-cu.-ft. capacity is a good size. Because of space limitations, I mixed the concrete in an extra-large mixing tub. Blend the dry ingredients (concrete mix, pigment powder, fiber reinforcement and water reducer) thoroughly before adding water. Once you’ve added the water, you’ll have a half-hour to get the material settled into the form. That may sound like plenty of time, but when you’re mixing by hand with a masonry hoe, it goes by very quickly.

Four days later remove the screws and (without prying against the concrete) carefully strip off the form boards.

Grind the surface. I used an angle grinder and a sequence of 50-, 100-, 200- and 400-grit discs to wet-grind the countertop.

After you’ve mixed the concrete to the consistency of thick oatmeal and made sure all the ingredients are blended, begin shoveling it into the form. Stop filling occasionally and work the concrete into corners and hard-to-reach areas with your hands. (Wear rubber gloves.) Also vibrate the concrete by repeatedly rapping the form with a rubber mallet or by using a rented tool for vibrating concrete called a stinger.

When the form is full of concrete, strike off the excess, working around the wire ties. Vibrate one last time and then cut the wires that support the rebar cage. Press the cut ends of the wires into the concrete and then strike off the surface again until it is smooth and even with the tops of the form walls. Cover the concrete with sheet plastic and let it cure for four days.

Grind, polish and seal
Make sure to have a helper on hand when you strip the form from the cured concrete. Remove all of the screws holding the form together and pry off the sides. Do not pry against the fresh concrete. Also remove faucet knockouts. Set a few strips of foam board on your work surface for protection; then carefully lift the countertop on edge and lay it onto the foam with the form panel facing up. If the panel does not pry off easily, drive wood shims into the gap to force it off of the countertop. (Do this along the back edge.) If you used a foam or urethane sink knockout, it should pop right out. I cut apart the particleboard knockout I used with a jigsaw.

Grinding exposes the aggregate in the concrete. For a lot of exposed aggregate, start with a coarse grinding disc (50-grit). To limit the dust, use a grinder with automatic water feed. You can also use a regular variable-speed grinder to wet-grind; just sponge water onto the concrete surface. Use a GFCI-protected power cord if your electrical circuit isn’t protected. Clean the surface frequently with a squeegee to prevent scratching.

Clean up vertical surfaces with diamond pads. To achieve a high gloss, wet-polish the horizontal surface with an angle grinder and 800- and 1,500-grit discs.

OPTION: For a smoother surface, apply a coat of colored slurry to the surface. The slurry should be mixed to the consistency of toothpaste and pressed into any voids with a putty knife.

Use a succession of finer grits to grind the surface. I used 50-, 100-, 200- and 400-grit discs. Technically, grinding with discs that are finer than 400-grit is considered polishing. I wet-polished the top with 800-grit and then 1,500-grit. You have about two weeks from the time you pour the concrete before the countertop becomes difficult to grind and polish.

For maximum smoothness, fill any voids in the countertop with slurry. Usually applied after grinding to 400-grit, the slurry contains cement, pigment, fine sand and water. Spread it onto the surface and press it into the voids with a putty knife. Let the slurry coat dry for a couple of days and then grind it off with the finest- grit disc you used. Repeat if necessary. To preserve the natural concrete look of the slab, don’t fill the edges with slurry.

After polishing, seal the concrete. You can use either a topical or a penetrating sealer. Cheng Design recommends a penetrating sealer because it doesn’t have the characteristic plastic look of topical sealers such as urethanes. Penetrating sealers don’t protect against stains as well, but you can add a few coats of carnuba wax instead and refresh as needed.

To install an undermount sink such as the one shown here, drill guide holes and insert metal anchor sleeves. The sink should come with mounting hardware that can be secured in the sleeves. Although concrete countertops are very heavy (ours weighs 175 pounds), it’s still a good idea to secure it to the base with a few masonry screws.

We mounted the completed bathroom vanity top on a painted metal base.