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| by:
Robert Koci
photos: Dave Starrett |
COUNTER INTELLIGENCE |
| How
to build a concrete countertop from start to beautiful finish |
At
first blush, the idea of making a countertop from concrete has
all the appeal of an aluminum bird feeder, or a cast-iron guitar.
Isn’t it a bit like preparing vegetables on the garage floor?
Rolling cookies on the sidewalk? Not at all. First introduced
by designers who saw the aesthetic potential and versatility
of the material, concrete countertops are fast becoming the
answer to homeowners’ demands for something new in kitchen (or
bathroom) design that doesn’t cost $50 per square foot and require
experts to install. It is also an open door for new, creative
ideas from handy homeowners who want to try something a little
different.
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| It's
less expensive and more versatile than almost any
other countertop surface. Why didn't anyone think
of it sooner? |
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A properly done concrete countertop with a little creative input
is comparable to the most popular finishes available today.
It will be every bit as attractive, smooth and durable as ceramic,
natural stones like granite or slate, and solid-surface materials
like Corian. And it can be very economical—concrete costs a
fraction of those materials. Best of all, you don’t need professionals—concrete
is a forgiving material that is accessible to any homeowner.
Working it requires tools you probably have, and making a mistake
means, at worst, you own a couple of bucks’ worth of custom-made
patio stones.
That’s not to imply that concrete
doesn’t take any skill to install. You have to understand the
basics of how concrete reacts as it sets and cures (see Concrete
Foundations). Concrete is heavy and a little messy for someone
who is not used to working with it. Luckily we had Bill Loveys
from Ardex Engineered Cements on hand to help us through. If
you follow Bill’s steps, and try mixing and drying a few test
squares before diving in, you will discover an opportunity to
create a unique and personal countertop that none of the conventional
materials can beat.
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| Finishing in Style |
Concrete countertops can take a variety of finishes for truly
unique, individual looks. It can look like almost anything:
natural stone, solid surface, ceramics—even concrete. Test a
few finishes on concrete samples before committing yourself
on the actual countertop.
Some effects require a slightly
more difficult construction method, where the countertop isn’t
formed in place as we did. Instead, the concrete is poured into
a form that’s upside down (so the base of the form creates the
top surface of the counter). When it’s completely cured, the
whole form is removed, and the countertop is gingerly lifted
in place.
• For a solid colour, use liquid or powder pigment. Mix
with the concrete, or sprinkle it on the concrete while it’s
setting. The advantage of mixing pigments into the concrete
is that if the counter chips, the colour goes all the way through.
• You can mix batches of very dry (bread dough consistency)
concrete with different colours. Then place the mixes into the
form where you want them. To keep the colour from bleeding together,
you have to place it, tamp it down, and leave it. For really
distinct fields of colour, make the counter upside down, so
the surface of the counter remains undisturbed at the bottom
of the form.
• For a ceramic tile or terrazzo look, use a grinder
with a diamond wheel to cut shallow 1/8" to 1/4" lines after
the concrete has cured, then fill in with ceramic grout. Seal
the concrete before you grout so the grout doesn’t stain the
“tile” surfaces.
• It’s easy to embed ceramics, stained glass, terrazzo
strips, or almost anything you find in your workshop. Depending
on what you want to embed, make this counter upside down also.
To ensure the concrete adheres to your additions, use a polymer-modified
cement.
• Concrete is porous, so you need to seal it. The choice
between topical and penetrating sealers is aesthetic: topical
finishes give a glossy not-so-concrete look; penetrating sealer
keeps the concrete look but is less stain-resistant.
• The sealer can also be pigmented—another opportunity
to change the look.
• Sealant won’t stick until the concrete is completely
cured. Determine this by laying a piece of plastic on the surface
and weighing it down to ensure direct contact with the concrete.
If, after 24 hours, there is no dampness collecting under the
plastic, the concrete is ready for sealing.
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| Concrete Foundations |
Concrete is a material few woodworkers spend a lot of time with.
Knowing its basic properties before you get out the garden hose
and mixing bucket may help you avoid a few beginner’s mistakes:
• Concrete is made from three basic elements: cement,
water, and aggregate. The active ingredient is cement, made
from calcium carbonate (limestone) and aluminium silicates (clay).
The second ingredient is water. Mixed in, the water starts a
chemical reaction which hardens the cement. Aggregate (sand
or stone) is a passive filler ingredient that dramatically affects
concrete’s strength and durability.
• Concrete for building construction contains both sand
and crushed stone to act as aggregates. In fact, some would
say our countertop isn’t concrete at all unless there’s stone
in it. We asked concrete countertop builders, and they agreed—call
it concrete. The bigger faux pas is to call it cement (which
is just one ingredient of concrete).
• Cement shrinks as it cures. Aggregate strengthens the
mix by reducing overall concrete shrinkage, thereby reducing
voids and cracking. A common proportion of aggregate to cement
is three to one. This proportion maximizes the bonding power
of cement and the non-shrink properties of the aggregate.
• The water necessary to pour the concrete is always
more than is necessary to harden it—as you know if you’ve ever
left a bag of cement on a damp floor—and too much water weakens
the final product. The professional’s most important measure
of concrete strength is its “slump,” which is simply a measure
of its water content. We found a good ratio was 4 litres of
water per 30 kg bag of sand mix.
• The longer the curing time, the harder the concrete.
Drying too quickly weakens concrete. That’s why keeping it damp
with wet burlap, or occasionally soaking it with a sponge will
give you a better, harder finish. Most concrete takes up to
a month to harden completely.
• Concrete is very strong in compression, and weak in
tension. A countertop doesn’t need a lot of tensile strength
because it is so well supported by the cabinet base.
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| GUIDE
PART 1 | GUIDE PART
1 |
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