Kitchen + Dining
These countertops are durable, heat resistant, and relatively inexpensive.
BLACK+DECKER B+D Contributor 122 Projects
Concrete countertops have many pluses and few minuses. They are durable, heat resistant, and relatively inexpensive. But most of all, they are highly attractive and a great fit with contemporary styles.
Once used exclusively for outdoor building projects, concrete has expanded its range to become a premier material for indoor construction as well. Still utilized mostly to cast countertops and vanity tops, concrete is continually finding new applications inside the home, including fireplace hearths, floors, and even furnishings. Along with remarkable strength and extreme durability, concrete has charm and appeal unlike any other building material.
A concrete countertop may be cast in place or formed offsite and installed like a natural stone countertop. For a number of reasons, casting offsite makes more sense for most of us. In addition to keeping the mess and dust out of your living spaces, working in a garage or even outdoors lets you cast the countertops with the finished surface face-down in the form. This way, if you do a careful job building the form, you can keep the grinding and polishing to a bare minimum. In some cases, you may even be able to simply remove the countertop from the form, flip it over, and install it essentially as is.
Thorough planning and careful form construction are the keys to a successful concrete countertop project. One of the first issues to tackle is weight: concrete weighs about 140 pounds per cubic foot (roughly 25 pounds per square foot for a 2" thick countertop). Most floors should be able to support a heavy countertop, but be sure to inspect floor joists and framing, especially in older homes, to determine if any reinforcement is needed. If you are unsure, consult a building professional, your local building inspector’s office or a structural engineer.
The weight of the concrete is also a factor when it comes to cabinetry. Typical base cabinets should be reinforced at the back and across the top with 1⁄2" or ¾" plywood. Reinforcing cabinetry may increase the overall dimensions, which can cause problems with modular units or in areas with limited space.
After you design your project and determine the actual dimensions, you’ll need to estimate the amount of concrete you’ll need. Concrete is measured by volume in cubic feet; multiply the length by the wide and then by the thickness of the finished countertop for volume in cubic inches, then divide the sum by 1728 for cubic feet. For example, a countertop that will be 48-in.-long × 24-in.-wide × 31⁄2-in.-thick will require 21⁄3-cu.-ft. of mixed concrete (48 × 24 × 3.5 / 1728 = 21⁄3). The best way to achieve consistent results when mixing concrete is to use premixed materials. One 60-lb. bag of premixed high-strength concrete equals 1⁄2-cu.-ft. of mixed concrete. A number of online concrete outlets also offer a virtual rainbow of dry-mix color pigments that are formulated with a water reducer admixture. Water reducers limit the amounts of water the concrete mix uses to help produce a stronger mix and a smoother finished produce.
As you mix the concrete materials, blend all dry ingredients thoroughly in a motorized mixer for five minutes prior to adding liquid ingredients. Do not mix the concrete until the form is completely built, with any sink or faucet knockouts and the reinforcement in place. For best results, mix the concrete in a single batch with a power mixer. Because the mixing container on power mixtures should never be more than half full (one-third full on some models), you’ll need a relatively large mixer for all but the smallest countertops. For the island countertop projects shown here, a tow-behind nine cubic foot mixer was rented (yet another good reason for casting your countertop offsite). Once your casting is done, let the concrete cure for at least a week before you strip off the forms.
The basic supplies needed to build your countertop form and cast the countertop include: (A) Melamine-coated particleboard for constructing the form; (B) poultry netting or welded wire for reinforcement; (C) concrete sealer (product shown is adequate, but for better protection look for a sealer that has both penetrating and film-forming properties); (D) high/ early bagged concrete mix rated for 5,000 p.s.i.; (E) coloring agent (liquid or powder); (F) grinding pads (shown are 5" diamond pads ranging from 50 grit to 1,500 grit for grinding and polishing); (G) paste wax; (H) buffing bonnet for polisher; (I) fiber reinforcement (nylon); (J) acrylic fortifier, latex bonding agent (or water reducing admixture if you can locate it); (K) black or colored silicone caulk; (L) faucet set if installing sink; (M) sink (self rimming shown); (N) No. 3 rebar (3⁄8”).