Some contractors are building precast countertops and sinks featuring surfaces as shiny as glass when released from their forms or molds. In the past, this look could only be achieved with diamond polishing. To get these results, you will need three things: a defoaming admixture, self-consolidating concrete (SCC), and very smooth form surfaces.
Defoamers have been used by admixture manufacturers for years to manage air entrainment produced as a byproduct when superplasticizers, especially those in the polycarboxylate family, are added to concrete. But recently a few contractors and manufacturers of prepackaged concrete mixes for countertop construction have started using them to reduce entrapped air in concrete for precast countertops and molded products such as precast sinks.
Defoaming admixtures change the chemistry of water. Kevin MacDonald, the vice president of engineering services for Cemstone, Mendota Heights, Minn., says they cause air bubbles in concrete (entrapped and entrained) to coalesce, forming bigger bubbles that move to the top of the concrete and escape. He adds that trials at Cemstone have reduced the air content of concrete to as little as .25%. These admixtures are very potent, so a little goes a long way, he continues.
The easiest way for a contractor to purchase a defoaming admixture is through Fritz-Pak, Dallas. Dionne Hutchings-Ojeda, Fritz-Pak's national sales manager, says only a little bit of the product Air Minus is needed, adding “as little as 1/8 to 1/4 of a teaspoon will treat an 80-pound countertop mix.” But beware—only the most experienced contractors should use it to manage air entrainment levels for concrete placed in freeze/thaw regions of the country.
Flowable SCC mixes make it possible for concrete to fill forms and remain plastic enough for entrapped air bubbles to rise to the surface. Polycarboxylate superplasticizers are the most effective way to increase both placing slump and spread of concrete, but this level of flowability requires mixes that won't segregate.
Depending on the form surface, these mixes can be vibrated or agitated with hand floats to help air bubbles exit the top of the fresh concrete.
Most countertop contractors use melamine-surfaced plywood as a mold surface for countertop forms. It's not an entirely smooth surface, and finished work features a light matte finish. To achieve a very smooth aesthetic, you need form surfaces, such as the proprietary polyester impermeable forming material that PreiTech, Evans, Ga., manufactures. Mike Eastergard, the company's owner, says their sheet goods can be placed on top of flat forms for countertop surfaces, and molds are available for sinks and other concrete uses. The smooth, glasslike surfaces impart the same finish to concrete. If you are making a countertop with these forms, Eastergard cautions against using a vibrator, because vibrating can cause air pockets to form underneath the sheets. However, he advises using an SCC mix. These sheets also make demolding very easy.
Mixes for concrete countertops are becoming increasingly sophisticated. The days of ordering a cubic yard of ready-mix concrete to cast a countertop may be over. More contractors buy prepackaged products developed for countertop work now. Superplasticizers can be added to a few of these products to produce highly polished surfaces as they come out of their forms.
Doug Bannister, owner of The Stamp Store, Oklahoma City, says his company's enCOUNTER product works well for this type of application. However, he advises contractors to reduce the amount of water added to one bag of mix from 80 to 62 ounces, and to add 1 ounce of the company's polycarboxylate superplasticizer product, which also includes the right amount of defoaming admixture. The result is an SCC that easily flows into molds or formed countertop areas. He suggests that you gently bump forms or molds to agitate the concrete to allow air bubbles to escape. Bannister says the use of this superplasticizer raises the cylinder breaks from 8870 to 13,400 psi.
Joe Garceau, co-owner of Butterfield Color, Aurora, Ill., says his company recently developed a countertop mix product that includes a liquid polymer to increase the flexural strength of the product. Its product also includes defoaming and polycarboxylate admixtures, as well as glass fiber reinforcement. He says the mix flows very well and advises contractors to paddle it gently with a mag float to help the concrete move into corners and release air bubbles. Butterfield Color offers the choice of either regular or lightweight aggregate, which reduces the weight by half for higher strength.
Building concrete countertops and sinks with the polished surfaces described doesn't mean that they shouldn't be sealed to protect them from staining or acid etching from foods. It is still necessary that they be properly sealed.