How to Make a Concrete Countertop
by Fu-Tung Cheng, Cheng Concrete Exchange
So you've heard the buzz about the latest do-it-yourself trend—concrete countertops—and you're thinking of giving it a try. Granted, it's not an easy endeavor, but it can be truly rewarding if you are looking for a custom piece that's equally beautiful, functional, and will wow every single visitor to your home!
The concrete countertops phenomenon — pioneered by designer Fu-Tung Cheng, author of bestseller “Concrete Countertops” — has revealed the exciting nature of concrete as a sculptural, earthy and warm building material. In the hands of a do-it-yourselfer, concrete can provide a relatively inexpensive countertop alternative that doesn't require a high level of expertise to install, yet unleashes the builder's creativity in ways no other surface material can. And nowadays, with the ever-increasing array of instructional books, videos and training available, even the least experienced do-it-yourselfer can gain the necessary confidence to engage in a concrete countertop project.
This article aims to provide you with a basic overview of the concrete countertop building process, as implemented by designer Fu-Tung Cheng at his Cheng Design fabrication shop in Berkeley, CA. The concrete pouring technique described in the article is precast (as opposed to cast-in-place), which means that the countertop is poured at an offsite location (e.g. outdoors, a garage, a fabricator's shop etc.) and then installed in the kitchen or bathroom. This method provides the builder with the highest degree of versatility, by allowing him or her to integrate a variety of decorative and functional elements into the countertop.
There are a few things to keep in mind when preparing the pouring site for a countertop project. Assume that you will make a mess and ensure that you have ample space. Your countertop will weigh approximately 135 pounds per cubic foot, so the work surface must be level and able to bear the weight of concrete. The ideal ambient temperature should be between 50 and 90 degrees and the humidity greater than 25%, to allow your countertop to properly cure (a process during which the “green concrete” hardens by forming internal bonds of great strength).
At the Cheng Design fabrication shop, we use an admixture to help the concrete cure at an accelerated rate. If you were to follow a similar process, the total timeframe needed to complete the countertop would be approximately as follows:
Building the Template
To begin your project, measure the space where you wish to install the countertop and then create a full-scale, two-dimensional template of the countertop you plan to build. You can make the template by hotgluing 3” strips of luan (doorskin) together.
The template will reflect the exact shape and dimensions of the countertop, as well as the size and location of any sinks, faucets or possible problem areas. This is especially helpful if you are templating an existing cabinet or countertop, since the new countertop needs to fit perfectly into the given space.
Making the Mold
A precast countertop mold needs to be cast upside-down. After the countertop piece is cured and de-molded, you will flip it over, so the base of the mold becomes the top of the countertop!
Building the mold is the most time-consuming and challenging part of making the countertop, as it requires great attention to details. It determines the ultimate look of the countertop, including its size, shape (rectangular vs. curved), depth, edges (e.g. bullnose edges, drop-down edges) and the placement of elements such as sinks, drainboards, cutting boards, trivets, soap trays etc. If your countertop is very large (and inevitably heavy!), is it a good idea to divide the mold into two or more sections, which will be easier to lift and install.
You can make the mold out of melamine. You will use the previously made template for tracing and cutting the base of the mold and marking any openings for the sink and faucets. After cutting the walls of the mold (which reflect the depth of the countertop), screw the base and walls tightly together. To create the necessary openings for the sink and faucets, you will need pre-shaped knockouts (which you can buy or make yourself), which will be glued or screwed to the base of the mold. Finally, to make the mold watertight, seal all seams with silicone caulk.
Adding Decorative Inserts (Aggregates)
You'll now have the option to show off your artistry by embedding decorative objects into the countertop, like stones, coins, fossils, shells; basically any object with a flat surface can be inlaid into the countertop, adding unique character to the final piece. After you've selected the decorative inserts, it's best to glue them to the base of the mold to ensure they will stay in place during vibration and eventually show up in the final countertop piece.
In order to prevent the concrete from cracking, the countertop needs to be reinforced by using rebar and wire mesh. Bend the rebar into a frame lining the inside of the mold and then fill in the frame with a grid of wire mesh, creating something that looks like a cage. Lower the cage into the mold and hang it with the help of screws attached to the outer sides of the mold.
Mixing & Pouring the Concrete
After you have built and reinforcing the mold, the next step consists in preparing the concrete mix.
Different concrete manufacturers will have different recipes for making the mix. A typical recipe would include the following ingredients: bagged concrete (a combination of cement, sand and aggregates), water reducer for workability, polypropylene fibers for strength, varying amounts of pigment for color, and a minimum amount of water for hydrating the cement.
To simplify the mix design process for the do-it-yourselfer, several manufacturers (such as www.concreteexchange.com) are offering ready-made kits or product combinations which take the guesswork out of preparing the mix. In most cases, all you need to add to these products are water and bagged concrete from your local home improvement store; however make sure to always follow the manufacturer's instructions for best results!
The most important thing to keep in mind when mixing concrete is that the least amount of water used in relation to the cement produces the least amount of shrinkage and yields the strongest, most durable concrete. The ideal concrete mix should have the consistency of runny oatmeal.
Concrete can be mixed in just about anything: buckets, a wheelbarrow, or a washtub. However, there is really no substitute for a good mixer; it is the best way to thoroughly combine all of your ingredients. To mix the concrete, place all the ingredients into the mixer, with the exception of water, which needs to be added gradually. After the ingredients have been thoroughly mixed (which should take a few minutes), take out the concrete mix and carefully pour it into the mold. In order to reduce air pockets in the countertop and minimize cracking, the poured concrete needs to be vibrated in the mold, preferably with a professional-grade concrete vibrator. You can rent a mixer and vibrator from most tool rental establishments or source them online from specialized vendors.
After pouring and vibrating, cover your concrete piece with a plastic sheet and keep it in a warm, humid environment (but not directly in the sun!) so that it cures. With the concrete mix ingredients used at Cheng Design, the piece will be cured in 4 full days.
After 4 days, you can release the countertop from the mold by removing the screws which hold the mold together. The countertop should now cure for another 2-3 days before grinding. Each day of curing, the concrete gets harder and stronger. Any less than 2 full days of curing and the countertop may be too soft to grind, while curing for too long may make the piece very difficult to grind. Keep the piece in the same warm, humid environment as you did for the first curing process.
When you take the piece out of the mold and flip it over, its surface may appear dull and uneven; however grinding and polishing will bring out the best in it. It will help diminish blemishes, achieve a smooth, uniform look and expose some of the aggregates in the concrete.
The best way to grind the countertop surface is with a variable speed grinder and a set of diamond grinding pads, used progressively from the coarsest to the finest. At the end of the process, the concrete will achieve a polished and color-rich look. Again, you can rent all the grinding/polishing tools from a tool rental establishment or buy them from a specialized vendor.
Applying the Slurry
This optional step needs to be implemented only if your piece has a porous surface after grinding or if you have holes or chip-outs. A slurry coat consists of cement, pigment, water and water reducer, and is used for filling in the possible voids in the countertop. To achieve a uniform look, use a slurry matching the color of the countertop; for a more playful or intricate look, choose a slurry of a different color.
Sealing & Waxing
In the final step you need to seal and wax your countertop, which will help make it more stain-resistant. As a porous material, however, concrete can not be made completely impervious to staining. While stains and blemishes can usually be removed with light polishing and resealing, they are often embraced for the organic patina they add to concrete countertops.
Waxing brings out the natural luster of the concrete as well as the beauty of the decorative inlays. It also helps build stain resistance in the concrete countertop. After the countertop has been installed, you should ideally wax it once a month and reseal it whenever necessary.
If you are pouring concrete for the first time, chances are you won't end up with a professional-grade countertop. It's a good idea to make some small concrete samples beforehand, to test the final color and surface finish achieved through grinding. Keep in mind that the beauty of concrete is not its perfection, but its natural, organic character brought to life by your creativity and imagination. When crafted with patience, attention to detail, and possibly some practice, your concrete countertop can turn into a unique and timeless investment into your home. It can become the centerpiece of your kitchen or bath, inviting your guests to pause, admire it, and run their fingers along its gracefully smooth surface …
For more information on making DIY concrete countertops, please visit www.ConcreteExchange.com