Mold-Making
Q1: What should I know about concrete countertop mold-making?
Mold-making is an art — and science — unto itself. Molds can be simple or complex, designed for thick or thin, horizontal or vertical applications, and made from a variety of materials. Molds for precast projects are made in the reverse and negative of the shape you wish to create. The Concrete Countertops book and Concrete Countertops Made Simple Book and DVD Guide show you how to make a level, water-tight mold for a surface that is meant to be finished with a polisher and includes detailed mold-making tips, such as creating sink knockouts and incorporating divisions, or breaks, into longer pieces.
If you are interested in learning in a hands-on format, CHENG Concrete Training Academy offers a range of programs with varying degrees of mold complexity, from the very simple (1-day Concrete Counertop Essentials Workshop) to the highly detailed and complex (5-day Advanced Countertop Design Training) including training on fireplaces, islands and walls.
Q2: How thin or thick can I cast my concrete countertop?
For mostly aesthetic reasons, we (led by Fu-Tung) prefer thicker (from 2" to 3" thick depending on the project) rather than thinner 1-1/2" tops, as we like the sense of mass that a thicker top provides. Thicker tops also allow space in the mold for conventional ladder wire and rebar reinforcing methods.
The challenge of a thin concrete countertop is ensuring the reinforcement material lies flat within such a narrow space such that it isn't so close to the countertop surface that it creates a "ghosting" effect (a slight discoloration in the surface of the concrete along the lines of the wire or rebar). When casting a thin countertop, or other thin pieces like back splashes, floor tile, wall panels or toe kicks, we use carbon fiber mesh metal lath (stucco reinforcing) for structural support. For thin pieces, make them in a moderate dimension (not too long and narrow as they can crack and break very easily), and use additional care when handling and installing them.
Q3: Do your concrete countertops have integral sinks?
Yes, we have on occasion created integral sinks, as have many other fabricators (see our Concrete Countertops book for examples). One reservation to this technique is that constant water flow from the faucet to the concrete surface combined with the aggressive cleaning we typically give sinks, will eventually damage the top "cream" layer of the concrete, leaving a permanent blemish that will only get deeper over time. Another, is that in a kitchen application, the wear and tear from everyday use (washing heavy pots and pans, pouring wine or acidic juices in the sink, etc.) can significantly impact the finish and overall appearance of the sink area which is why we favor stainless-steel sinks in kitchen applications.
That said, nothing beats a cast, integral sink for looks—it's the long-term wear and tear of the sink that is affected. For those of you who decide to create an integral sink, a good option is to create one that incorporates a glazed tile or stainless wear surface at the bottom of the sink.
Q4: What are decorative inserts, or decorative aggregates?
Standard bagged concrete (like Sakrete 5000-Plus) contains natural stone aggregate from local quarries. When you grind or polish into the surface of your countertop, you are removing the "cream" layer to reveal these stones, which will vary throughout the country since bagged concrete is locally produced. Some people trying to achieve the "terrazzo" look are happy with revealing the aggregates that come with the bagged concrete. Others trying to achieve a truly unique or custom look will supplement natural aggregate with decorative aggregate (see Decorative Inserts in our online store) and place them into their mold. This addition is an aesthetic decision and has no impact on the structural performance or durability of your countertop. For more information on polishing, refer to "Polish and Slurry," pages 74-80 in Concrete Countertops Made Simple Book and DVD Guide or pages 136-142, in Concrete Countertops.
Q5: What decorative inserts do you recommend for each Pro-Formula color?
View Fu-Tung's Recommended Color and Aggregate Combinations for suggestions.
However, there is no right answer when it comes to choosing decorative aggregates (see Decorative Inserts) for your concrete countertop. Because this process is completely optional and based on personal aesthetics and preference, feel free to choose the insert or combination of inserts that you desire. Whether you are striving for a subtle monochromatic look, or a standout terrazzo-like finish, you are only limited by your imagination, so have fun!
Q6: How do I achieve a "terrazzo-look" with my concrete countertop?
Terrazzo is an age-old technique of casting bits of selected stone, glass or shell into another substance and then polishing the hardened surface away to expose the added material. In order to achieve the same look without having to add hundreds of pounds of selected decorative aggregates and glass to your concrete mix, you simply "seed" the countertop mold by scattering a few hand-fulls of selected decorative aggregates or glass into the mold before pouring concrete over them. You must spray the bottom of the mold with a light mist of spray adhesive before scattering the aggregates into the mold to insure the aggregate stay positioned where you've placed them (though a few loose ones may migrate away from the surface). We recommend 3M™ Super77 Spray Adhesive for lightly misting the bottom of the mold.
Our online store offers decorative aggregates and inserts—bits of semi-precious stone,tumbled glass or polished ammonite fossils—which can be cast into the countertop and then exposed by polishing into the surface once it's fully cured. You can polish into your countertop a little or a lot, depending on how much of the aggregate you'd like to expose and the aesthetic you're trying to achieve. For more information, refer to "Polish and Slurry," pages 74-80 in Concrete Countertops Made Simple Book and DVD Guide or pages 136-142, in Concrete Countertops.
Q7: How do I divide a large piece into sections? Also, how do you get a bullnose edge without pouring right-side-up?
In Chapter 2, "Building the Mold," of Concrete Countertops, Fu-Tung explains the simple technique of using an aluminum divide. A more sophisticated approach is to shape the metal divide into a "Z" shape that acts as a "key" between the two sections, creating a more intentionally designed type of joinery. Another option is to affix a half-dowel on the metal divide to act as the key.
There are many ways to make bullnose edges, none of which are very easy. We have made them by building up a plaster edge with a shaped edging tool (made from sheet metal), then smoothing and coating the plaster with shellac. We have also heat-formed plastic Mylar, laminate and foam edges as well. This, and other advanced mold-making techniques are covered in our Advanced Concrete Countertop Design Training.
