Methods for Adding Decorative Aggregate

Decorative aggregate placed in the bottom of a form to create a swirl pattern.

Question:
When I’m making precast countertops with exposed decorative aggregate, should I add the aggregate to the mix or put it in the bottom of the forms?

Answer:
Decorative aggregates, whether colored glass or crushed stone, can create interest and add pizzazz to a countertop. There are two basic ways of getting the aggregate into the countertop. One method involves mixing all the decorative aggregate in with the concrete, while the other involves first “seeding” or placing the aggregate into the empty forms before placing the concrete. You can use one method alone or combine them. But each has its pros and cons.

Mixing the aggregate into the concrete before casting is an easy way to achieve consistency, control and uniformity. The greatest advantage of this method is consistency in surface appearance, since the decorative aggregate is mixed uniformly throughout the concrete. Also, because the total amount of ingredients is known, duplication of the look is easy. With this method, you can vary the degree of aggregate exposure by changing the amount of decorative aggregate used and the size of the aggregate particles. Larger aggregates have fewer particles per pound, so the overall coverage may appear sparser than with smaller aggregates.

The biggest disadvantage of this method is the amount of aggregate required, which can significantly increase material costs. Only a small fraction of the decorative aggregate is actually exposed, and the rest remains unseen. In addition, mix design adjustments may be necessary, depending on the size and amount of decorative aggregate used. For example, large amounts of aggregate might call for substituting some of the ordinary aggregate with some or all of the decorative aggregate as well as a change in cement content.

The alternative method—preplacing the decorative aggregate into the forms—uses much less material, so it offers a distinct advantage when the decorative aggregate is expensive or only a small amount is available. This method also permits you to arrange the aggregate in specific patterns or locations as desired. The main disadvantage includes the tendency for the scattered aggregates to shift, move or get lost during casting. Sometimes you can glue the aggregate to the forms to prevent shifting, but this works best with larger pieces that have a flat gluing surface. Also, edges and vertical surfaces are difficult to seed so that they match the top surface. Replicating the look of scattered aggregates can be tricky too, since the effects of concrete placement and consistency can have a strong influence on the final appearance.

Either of these methods requires some degree of grinding to expose the embedded decorative aggregate. Larger, more rounded aggregates require the most grinding to expose a significant portion of their cross section. Small, angular particles take the least amount of grinding for exposure.