Granted, there are easier ways to build and install a countertop. Poured concrete is heavy and messy and must be carefully surfaced to look its best and last a lifetime. But the 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.
All you need are a few bags of concrete and some special ingredients to enhance the strength, colour and smoothness. Start with a small garden project before you tackle the countertop. Try making a couple of stepping stones.
Making the form
Use 19mm melamine. The plastic facing helps the concrete dry slower so it reaches its maximum strength. It also prevents the concrete from sticking to the form. Join the sides to the bottom and to each other with screws. To avoid splitting the dense particleboard, predrill the stock before installing the screws.
Pouring the concrete
The key to building a strong countertop is to fortify the slab with galvanised wire grid. It also helps to use plenty of Portland cement and just enough water in the concrete mix.
Cut the wire grid with wire cutters to stop 25mm short of the edges of the form when installed. 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.
For more information on mixing concrete, visit the Concrete and Cement Institute. All concrete cracks as it cures, but the wire grid keep the cracks tight once they form.
While soupy concrete would practically level itself in the form, the drier high-strength mixture used in counter top projects must be pressed hard to compact it.
Tamp the concrete with a wood 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, screed the surface using a 50mm square metal channel that is at least 500mm 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. It is 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 solution of muriatic acid and water. For tops made with pigmented concrete, the acid wash exposes more of the coluor 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 to the consistency of peanut butter and apply it to the exposed surfaces using a rubber-faced grout float. Then plow 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
When it comes time to move and install the top, get plenty of help as the slab is heavy and you must support it evenly to prevent stress fractures. 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 odours 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. To achieve a high-gloss surface, use a car buffer or an electric drill fitted with a buffing pad.
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