Do-It-Yourself Concrete Countertops

An overview of the construction process

Our appreciation to DECORATIVE-CONCRETE.NET for allowing us to reprint this article.  Unfortunately, their website is no longer active. This article is both instructional and painfully honest presentation by a dedicated do-it-yourselfer and his experiences in making his own concrete countertops.  Hope it is as inspiring for you as it was for me!   To read Article 2, click HERE.

Build a counter that will make a lasting impression!

Concrete is widely used in modern construction to resemble other building materials such as wood siding and roof shakes, clay pavers and many types of natural stone for retaining walls and facing projects. It is a strong, durable alternative to many building materials and has the added benefit of its versatile use in and outside.

Commonly used as a patio surface, we know that it can withstand the summer and winter elements and can look great! Designers are using it for a lot more than just a place to put Weber grills and outdoor furniture, it is also a great place to put the dinner plate as well!

Designers are using it to create distinctive tops for kitchen cabinets and islands, bathroom vanities and tables. When properly colored and finished, a concrete countertop looks like natural stone or maybe solid-surface material - not a sidewalk or garage floor.

Granted, there are easier ways to build and install a countertop. Poured concrete is heavy and messy and must be carefully engineered and surfaced to look its best and last a lifetime. But materials are inexpensive and the payoff is huge when done right. If you make a concrete counter, count on it to become a conversation piece for anyone who sees it. Although concrete counters have been common for centuries in other parts of the world, chances are none of your relatives or friends has anything quite like it.

One of concrete's greatest advantages is that you can form it with a void in the middle to accept an undermount sink so you can sweep crumbs and spills directly into the basin rather than over a rim. You can't do that with a plastic laminate countertop.

Scott Andersen of Aggregates Co. in Minneapolis, Minnesota, is a woodworker who has been making concrete tops professionally for the past five years. He helped us tackle this stunning 48 x 54-in. kitchen island project to demonstrate how it's done. With Scott's pointers and a little patience, you can build a concrete top of your own. All you need are a few bags of concrete and some special ingredients to enhance the strength, color and smoothness. Scott recommends you start small for your first project (as we did) and precast the top outdoors or in your garage where you have plenty of space and ventilation.

For a color like ordinary concrete or darker, start with bags of premixed concrete. For a light color, plan to mix your own concrete from white Portland cement, mason's sand and pea gravel. To be sure you will like the effect, test various colors and finishes by forming square-foot samples. When you're done, at the very least you will have a few interesting stepping stones for your garden.

Making the form...

Build your form from 3/4-in.-thick melamine-coated particleboard. The plastic facing will help the concrete dry slower so it reaches its maximum strength. It also will prevent the concrete from sticking to the form when you disassemble it. Join the sides to the bottom and to each other with drywall screws, or use a pneumatic stapler as we did with two fasteners at each corner and one every 6 in. along the sides. To avoid splitting the dense particleboard, predrill the stock before installing the screws.

You can't drill or cut the slab after it is built, so create voids for a sink, range top or deck-mounted faucet with exterior plywood blockouts. Don't use particleboard - the edges will absorb water and swell. Make blockouts the same height as the sides so you can screed the concrete level with the top of the form. Secure them to the bottom of the form with screws.

Pouring the concrete...

The key to building a strong countertop is to fortify the slab with diamond lath, reinforcing rod and polypropylene fibers. It also helps to use plenty of Portland cement and just enough water in the concrete mix.

Cut the diamond lath with aviation snips so it will stop 1 in. short of the edges of the form and the blockout when installed. You also should install No. 3 (3/8-in.) rebar or reinforcing track around the sink cutout. Do all of the measuring and cutting before you begin to mix the concrete so it will be ready when you need it. While you pour the concrete in layers, it's important to minimize the time between steps so one batch sticks to the next.

The durability of a concrete countertop depends on how the concrete mixture is prepared. Blend the dry ingredients thoroughly using a mason's hoe with holes. Then measure the liquid ingredients carefully, mix them together and add them in stages, a little at a time. Liquid components include latex additive, liquid pigment and water.

Scott typically adds 2 quarts of white Portland cement per 60-pound bag of concrete to strengthen the mix. All concrete cracks as it cures, but the polypropylene fibers, diamond lath and reinforcing rod keep the microscopic cracks tight once they form.

While you'll use poly-fiber-reinforced concrete for the core of the slab, fiber-free concrete is required for all of the exposed surfaces because the fibers would produce fuzz that you couldn't sand off. To accomplish this, pack a small amount of regular concrete about an inch thick around all of the edges. Then fill the middle with reinforced concrete, add diamond lath and reinforcing rods, and finish up with a full layer of regular concrete.

For the different batches of concrete to bond together and look like a seamless block, keep the consistency and color the same. That's why it is so important to measure ingredients, especially the liquids. The concrete should be just damp enough to hold together when you form it into a ball.

While soupy concrete would practically level itself in the form, the drier high-strength mixture used in countertop projects must be pressed hard to compact it.

Tamp the concrete with a magnesium float to pack it tightly into the form, then work the surface with the float until smooth. This step is where the most sweat equity is required. Add small amounts of concrete as needed to fill low spots. If larger pieces of aggregate work their way to the surface, remove them and fill craters with fresh concrete. Continue adding concrete until the surface is smooth, well-packed and slightly crowned.

With a helper, strike (or screed) the surface using a piece of 2-in. square metal channel that is at least 18 in. longer than the form itself.

Saw the bar back and forth as you move along the top of the form with the leading edge slightly raised so it doesn't catch on the aggregate. Repeat the process until the surface is smooth and even with the top of the form.

Let the concrete set for about two hours, then hard trowel the surface with a steel trowel. Take care not to overwork the concrete; this can draw aggregate too close to the surface, causing pop-outs and weakening the countertop. If water starts to puddle on the surface, let the slab rest for 30 minutes and try again. Scott says it's better to trowel the surface briefly three times than to overtrowel it once.

If you are working in hot or dry conditions, cover the concrete with plastic or wet burlap as it cures to retard the curing and increase hardness. Let the concrete cure for at least 48 hours before removing the forms. The slower the water in the mix evaporates, the stronger the concrete will be.

Finishing the concrete...

Release the concrete forms by carefully separating the joints with a small, flat prybar. Unscrew the blockout, but don't remove it unless you plan to install an undermount sink and need to finish the inside edges. Otherwise, wait until you've actually installed the countertop.

Ease sharp edges and corners of the slab by sanding with a random-orbit sander and 100-grit sandpaper. Wear a respirator to avoid inhaling the silica particles.

Etch the surface with a 1-ounce-per-gallon solution of muriatic acid and water. For tops made with pigmented concrete, such as this charcoal-gray unit, the acid wash exposes more of the color and texture variations. Be sure to follow the safety precautions on the acid label regarding eye and skin protection and ventilation. When you are done, rinse the countertop slab thoroughly with water to remove the acid residue, then let it air dry.

Prepare a mixture of Portland cement, latex additive and liquid pigment to the consistency of peanut butter and apply it to the exposed surfaces using a rubber-faced grout float. Then plow (or skiff) the surfaces with the leading edge of the float raised so the material remains in all of the small voids but is removed from the flats. After this filler dries and cures for an hour, sand it with an orbital sander and 180-grit discs until you are satisfied with the smoothness. Plan on going through a lot of discs!

Installation on the cabinets.. get lots of help!

When it comes time to move and install the top, get plenty of help. The slab will be heavy (about 20 pounds per square foot) and awkward to maneuver through the house, and you must support it evenly to prevent stress fractures. The more hands, the better. Concrete normally takes three to four weeks to reach full strength. It's best to let it cure fully before attempting to move it, and leave the blockout in place when you do to support the narrow edge.

Apply a thin bead of construction adhesive or silicone caulk to the top edges of the base cabinet (if you use too much it will squeeze out and make a mess) and set the top. With the counter in place, cut the blockout into pieces with a jigsaw and remove it.

Seal the exposed surfaces of the slab with a concrete sealing product according to the manufacturer's directions. This will prevent the surface from absorbing food stains and odors and make it easier to clean. After the coating dries, buff out the surface using a Scotchbrite pad. Then apply a coat of acrylic clear finish such as Minwax Polycrylic. To achieve a high-gloss surface, use a car buffer or an electric drill fitted with a buffing pad.

Click HERE to go to Article 2 on Concrete Countertops!