Specifiers of exposed aggregate finishes are increasingly interested in low profile etches that resemble limestone finishes, according to Chris Forgey, commercial development manager for Grace Construction Products, Cambridge, Mass. And by experimenting with integral colored concrete and white portland cement mixes, the look achieved is more controlled than what we normally expect with exposed aggregate concrete. There are a number of products currently being promoted to help achieve different etch depths. Surface retarders can be painted, rolled, or sprayed onto slab surfaces or into wall forms (where resistance to abrasion is important during concrete placement). And some products are specially designed to resist high temperatures.

Providing different levels of exposure

Lightly exposing only the sand aggregate is becoming popular. Workers used a buffing machine with a scrub brush head and a wet vacuum to pick up the slurry.
Lightly exposing only the sand aggregate is becoming popular. Workers used a buffing machine with a scrub brush head and a wet vacuum to pick up the slurry.

Manufacturers of retarders for exposing aggregate in concrete are making it easier to produce different levels of exposure on concrete surfaces. They advertise products to achieve light, medium, or deep exposures, with several different exposure levels available in each category. Products are usually colored, each color representing a different depth of exposure. They range from exposing the top of sand particles to revealing 1½-inch top-sized aggregate. The decision about which strength of retarding product to use depends on the size of the aggregate and the mix design. Sandra Sprouts, a principal scientist for Degussa Construction Systems R&D, Beachwood, Ohio, says that in order for the cement to properly bond the aggregate, the depth of exposure should not be more than 40% of the coarse aggregate's diameter.

In-form and top-surface retarders

In-form retarders are designed for use in horizontal or vertical form-work. They are solvent-based and look like paint, coming in different colors to designate the depth of aggregate exposure. When applied to form-work they provide an abrasion-resistant surface so that concrete, while being placed, doesn't remove retarder from the forms. Sprouts adds that for precast applications where castings are put through steam cure rooms, heat-resistant products that can withstand temperatures as high as 160° F are available. To facilitate the transfer of the retarder film to the concrete, a release agent is typically applied to the forms first, followed by two applications of the retarding agent

These two photos show work that is lightly exposed, with three different levels of exposure. The photo on the left shows the deepest and the photo on the right shows two different levels. The right side of the picture shows the least exposure.
These two photos show work that is lightly exposed, with three different levels of exposure. The photo on the left shows the deepest and the photo on the right shows two different levels. The right side of the picture shows the least exposure.

Top-surface retarders are water-based products that are spray-applied using low pressure equipment such as a garden sprayer. They also come in different colors. The depth of the aggregate exposure and the color help identify where and how much material has been applied. Regardless of the type of retarder used, the goal is to apply the retarder uniformly across the surface to ensure even exposure of the aggregate.

Top surface retarders can be removed the same day or the next day depending on the product selected and the timing. For next day removal, the slab is generally covered with plastic sheeting after the retarder application, however there are products available that do not have to be covered except to protect the surface from adverse conditions or contamination.

Mix design considerations

There are two methods for exposed aggregate work: seeding special aggregate onto the surface of freshly placed concrete and exposing the aggregate present in the mix. In the latter case, the amount of stone in a mix design is usually increased to achieve a more uniform distribution of aggregate on the surface. The top size and gradation of the aggregate is also important to the look. Choosing the right surface retarder depends on the size of the aggregate you want to expose and the depth of the exposure.