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    Crafting Concrete Countertops

    continued

    Second, vibrating the concrete helps remove any air that might be trapped in the mix; trapped air will form bubbles that create small holes and pits in the surface of the concrete as it sets up.

    The safest method for vibrating a small piece like a countertop is to hold a palm sander, without sandpaper, tightly against the side of the form for a few moments, working around the perimeter. (A reciprocating saw, with the blade removed, will also work.)

    You'll get even more effective vibration with a sander if you wrap it in a couple of extra-strong plastic bags and place it on the wet concrete, taking care not to disturb the rebar and insets. This approach puts your sander in serious jeopardy, however. Assume that if the bag tears, your sander will be ruined.

    A low-tech method is to rap gently and rapidly on the sides of the form with a hammer. But the method that's by far the most fun is the lowest-tech method of all: Simply stick your gloved hands into the concrete and work it by hand into and around all the knockouts, inlays, and rebar, now and then wriggling your fingers quickly. If you can entice a friend or two to help, so much the better. Sometimes, we'll use all three methods in the same pour, with good results.

    Vibrating Tools

    There are a number of professional concrete vibrators available, and we'll often use them on larger pieces. Stick vibrators "stingers" come in a variety of sizes and configurations (most rental outlets that carry concrete supplies have stingers). Some stick vibrators are round, some are square. We like the square ones, and use the smallest available (3/4 inch, or 19 millimeters across).

    Although stick vibrators are very effective, they have some serious drawbacks. For one thing, they're meant to be plunged into wet concrete; on a piece as shallow as a countertop, sticking the vibrator into the concrete raises the risk that you'll disturb the rebar and insets.

    We suggest that if you use a stinger, run it along the surface of the wet concrete, taking care not to let it dip into the concrete so far that it disturbs the rebar, knockouts, or inlays. This technique is very effective. It's also very messy. (To cut down on the amount of concrete that will inevitably splatter on nearby walls, the floor, and you and your colleagues, have a large piece of cardboard handy and someone to hold it over the vibrator as you move it along the concrete.)

    Vulnerabilities of the Medium

    While concrete is incredibly durable, it's tough only in certain ways; it will last for centuries, but it can also stain and scratch. Unsealed concrete is especially susceptible to the many acidic liquids likely to be in use around a kitchen sink, stove, or island. Wine, lemon juice, and balsamic vinegar are particularly hard on naked concrete; if not wiped up fairly quickly, these liquids can discolor the surface or even eat into the top layer of concrete, leaving a slight roughness.

    A concrete countertop, even a heavily used sink or stove run, can be kept in nearly pristine condition with the help of a professionally applied epoxy sealer or through routine maintenance with penetrating sealers and wax. But all this can take more effort than some people are willing to spend, and even then the best sealers aren't perfect.

    Penetrating sealers help resist stains, but they don't prevent them. And while epoxy sealers are fairly stain-proof, there are problems. They scratch, you can't set hot pots on them, and they put a distance between the surface and you that reads "plastic coating."

    But then no countertop material is perfect: Granite and marble also stain, stainless steel scratches easily, and Formica and Corian can be scratched or burned. Glass is fairly impermeable it doesn't stain and it's hard to scratch but it chips easily and it breaks. And while it's difficult, if not impossible, to repair some of these other materials, concrete is relatively easy to fix.

    Some people are bothered by concrete's vulnerabilities, or they don't like the work it can take to keep concrete looking as untouched as they might prefer. Anyone who is comfortable only with surfaces that never change glass tabletops, for instance or who obsesses about every little stain or scratch on the furniture probably isn't going to be happy with a concrete countertop.

    But to some people, the various stains, scratches, and crazing that accumulate with the passage of time on a concrete countertop aren't blemishes at all but a patina to be valued. It conveys warmth and a sense of history, like the blemishes on an antique wooden table or chair of the dents and scuffs on an old wooden floor.

    Fu-Tung Cheng heads the Berkeley, California firm of Cheng Design. His hands-on approach to residential architecture and kitchen and product design has won him several awards and recognition for innovation and creativity.

    This article is excerpted from Concrete Countertops: Design, Form, and Finishes for the New Kitchen and Bath, copyright 2002, available from Taunton Press and at Amazon.com.

    Discuss this article in the Architecture Forum...

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    ArchWeek Image

    Concrete countertops can work in any context. Here, its earthy quality complements a traditional design.
    Photo: Matt Millman

    ArchWeek Image

    Concrete can begin to stiffen quickly, so while one person places the concrete, the other can work it into tight places, making sure there are no voids that will weaken the piece.
    Photo: Matt Millman

    ArchWeek Image

    Decorative inlays in concrete are familiar in the form of colored terrazzo, usually made with white cement and colored aggregates. Once the concrete has cured, it's ground to a high sheen, exposing the color in a field of white.
    Photo: Matt Millman

    ArchWeek Image

    These counters were poured into crude molds deliberately to make them look rustic.
    Photo: Matt Millman

    ArchWeek Image

    It's most effective to vibrate concrete in layers, rather than all at once. After the first layer has been vibrated, add more and vibrate it again.
    Photo: Matt Millman

    ArchWeek Image

    These concrete countertops were cast and lightly ground.
    Photo: Matt Millman

    ArchWeek Image

    Concrete Countertops, from The Taunton Press.
    Image: The Taunton Press

     

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