Often times I am asked “what is exposed aggregate?” Exposed aggregate is a type of concrete finish. To understand how the finish is produced we should first dissect the composition of concrete. Ready mix concrete is generally a mix of sand, cement, water, and aggregate or stone. The ratio of theses elements is adjusted to meet the specific needs of a project. For exposed aggregate the aggregate amount is increased and the type of aggregate is change to meet the desired finish. For example, if you want a pea gravel exposed finish, all of the aggregate added to the ready mix concrete is pea gravel. Once you have the proper exposed aggregate mix design the concrete is poured. After the concrete is poured it is treated much like regular concrete, it is struck to grade(meaning leveled off), then bull floated(smoothed with a large magnesium float), then edged and hand troweled(this makes sure the surface is flat and edges are sealed). After the concrete is finished it is allowed to partially set, then a surface retarder is sprayed on. The surface retarder allows the concrete under the surface to continue to harden while the surface does not. This is very important because we only want the surface to be able to wash away. After the concrete hardens to a specific point, the surface is ready to wash. We then, using a water hose and brooms, will begin to wash away the top layer of concrete. By washing the surface away we have exposed the internal aggregates of the concrete mix. This is where the term exposed aggregate comes from.
We have poured thousands of exposed aggregate patios, driveways, and walks. It is a great alternative to traditional concrete for a variety of reasons. Lets discuss why. A regular broom finished concrete driveway looks great the day you pour it. It is a large white area that looks like a great big canvas. Unfortunately this canvas usually doesn’t get painted with a beautiful picture. Instead it gets covered with leaf stains, oil drips, surface pits, and hairline cracks. First and foremost exposed aggregate is a very aesthetically pleasing finish. With an exposed aggregate driveway these common occurrences are much less noticeable. Since there are so many stones visible in the surface if one was to pit away you would never notice it. Because the surface is more stone than concrete oil drips and leaf stains are much more difficult to see. If the concrete does crack it is very hard to see the crack because of the busyness of the exposed aggregate finish. I have seen exposed aggregate driveways and patios we poured 20 years ago that look as good today as the day we poured them.