Cement, Aggregate and Reinforcement in Concrete Countertops
By Amy Johnson

When the use of a structural material — concrete — is scaled down from monumental slab to kitchen counter, what changes? Some things are the same, some things are smaller, and some things are just plain different. After all, very few customers want a miniature driveway poured on top of their kitchen cabinets. Differences in cement loading, aggregates and reinforcing mechanisms all set countertop mixes and installations apart from those used for traditional flatwork.

Cement and aggregates
“Cement is the core of mix design for any application, balancing workability, strength and cost,” says Jeffrey Girard, P.E., president of The Concrete Countertop Institute in Raleigh, N.C. “Cement is the glue, the strength, but also the Achilles heel of concrete. Too much cement relative to aggregate tends to create excess paste. It is creamier, more workable, it behaves more like clay, but cement paste is what shrinks, causing cracking and curling. So cement loading is the delicate balancing act that all engineers who design concrete mixes have to perform.”

The amount of cement required is affected by the surface area of the aggregates in a mix. Aggregates in countertops are by necessity smaller, with the largest typically 3/8 inch to 5/8 inch. Mike Heidebrink, president of Cheng Concrete, explains: “Each individual aggregate has a specific surface area. The smaller the aggregate, the greater the total combined surface area. The amount of cement you add to the aggregate is really (based on) a calculation of surface area. The function of cement is to coat the surface of all the aggregates.”