The homeowner fascinated with decorative concrete for floors and countertops now is asking for concrete sinks. Concrete contractors are challenged with understanding and meeting their expectations. But it isn't easy. The finishing and aesthetics of concrete sinks require precision and knowledge of concrete mixes, mold making, and integrating hardware.
The sink can be precast or cast-in-place, and designed as monolithic with the countertop, or as a ramp sink where the basin features a downward slope, a trough sink for multiple faucets, or a stand-alone vessel sink.
What Do Clients Expect?
Contractors often compare the concrete sink to a copper sink that develops a patina or "living finish" featuring scratches and color change. "A concrete sink is not like a stainless steel sink and it will not wear like one," says Wanda Ellerbeck, Grotto Designs, owner, Canmore, Alberta, Canada. Explaining that scratches and stains occur with concrete sinks, and that some maintenance may be required, will help eliminate frustrations the customer may have with the end product.
Not only does the customer need to understand concrete, the contractor needs to understand the customer. "Find out what inspires the customer's desire for a concrete sink," suggests Chris Becker, president, Becker Architectural Concrete, South St. Paul, Minn. "Explain the labor and the cost differences between stone, fabricated material, and the hand-constructed concrete sink. Have pieces in the casting shop to show the progression from the mold to the finished piece; sinks in the showroom should not be better than normal production."
Some contractors use Google's Sketch-Up software program to give customers a 3-D education about the sink-making process and the product. "The program is intuitive and simple and models the sink from all perspectives," says Sean Dunston, partner, Concrete Jungle, Colorado Springs, Colo.
When the customer understands the product and the process, then it is safe to proceed to the next step in making the sink.
Molds are necessary for monolithic cast-in-place sinks, or vessel sinks, whether used in a precast or cast-in-place environment. Molds are prepared for upside-down casts or for two-sided mold casts. The upside-down placement can be used with stiff mixes that also build up sides, fluid mixes, and the "hand-pressed" method. The two-sided mold is used for a vessel sink or for a monolithic piece that lacks joints or seams.
As a precaster, Ellerbeck makes her own sink molds. To do this, "use backward and inside out thinking," she says. "Imagine the shape of the negative space that produces the positive form and create a mold that removes easily; consider the desired finished texture. Plan the seams where they will be least noticeable when the mold comes apart for removal." You may imagine more exotic designs, but in the beginning keep shapes simple.
Becker decided the cost and time for mold making was not profitable and collaborates with a designer to make molds. Mold making is taught by many manufacturers of concrete countertop mixes and mold materials manufacturers.
Mix Design, Placement, and Cure
The prepared mixes for sinks and counter-tops vary. Some finishing characteristics are similar to basic concrete and some have a sticky finish; some mixes produce a high-gloss sheen while others a more cementitious surface. Mixes often include admixtures such as superplasticizers and water reducers for easy placement and high-performance concrete. "Countertop mixes need to have a low water-cement ratio, be impermeable and dense, and have low shrinkage and stain resistant characteristics," says Doug Bannister, owner, The Stamp Store, Oklahoma City. "Usually, the more fibers and superplasticizer, the harder the mix finishes when you are doing cast-in-place work."
The finishing characteristics are noticeable especially for cast-in-place and hand-troweled work. Techniques such as the hand-pressed method require a stiff clay-like mix to pack the molds. Some contractors make their own mix or use mixes by several manufacturers to offer different looks. "Begin with the manufacturer's recipe. Experiment with other mix designs on the side," advises Becker.
The concrete sink made in an upside-down mold has its top surface on the bottom. The thickness of the sink depends upon the contractor's mix design and the cast or pressed technology required. Thinner sinks range from ½ to ¾ inch with standard thickness at 2 to 3 inches. Ellerbeck uses Le Farge's Ductal mix and casts sinks at ¾ inch thickness.
"Casting the two-sided form requires a mix fluid enough to flow between the walls," says Jeffrey Girard, P.E., president, Concrete Countertop Institute, Raleigh, N.C. "Mixes can be made to act more fluid by vibration, but not everyone has vibrating tables or tools powerful enough to achieve the fluidity to fill forms completely. Although the granular material usually is smaller, the key to two-sided casting is the base concrete-whether it is stiff or can be made fluid. Some mixes can do both."