Fundamental differences exist between flatwork made with pervious concrete and standard concrete. The surface texture differs noticeably, and when it comes to holding water, they're at opposite ends of the spectrum. But neither of those differences means that pervious can't be decorative.
Understanding what goes into pervious concrete and how it differs from traditional concrete can help when determining how to incorporate decorative treatments. It is made with gap-graded aggregate, which means all the pieces are similar in size and fall within a relatively narrow range. This ensures that open spaces remain in the concrete, rather than being filled in with smaller coarse aggregate particles and fine aggregate, and allows water to pass through freely. Because of this, pervious concrete is being promoted to end-users as a stormwater solution that also happens to provide a paved surface.
Using gap-graded aggregate gives pervious concrete an open surface that looks like gravel and varies depending on the coarseness and type of aggregate used. For example, using a 3/8-inch top-size aggregate allows the surface to remain open to water infiltration but dense enough to not cause problems for pedestrians, such as catching high-heeled shoes. Using larger aggregate allows more water to pass through more quickly, and also produces a more open surface. The type of aggregate also affects both the surface appearance and its placement; pervious made with crushed stone or other angular aggregate requires more compactive effort than that made with rounded aggregate.
Mix design is a very important aspect of a successful pervious installation. Adding too much cement to a pervious mix actually may weaken the final product. Because the aggregate gradation and sizing reduces the overall surface area available for the cement paste to cover, cement requirements are surprisingly low. A mix with 3/8-inch aggregate may need something in the range of 400 to 500 pounds of cement per cubic yard of concrete, because the paste is most effective when it is no more than 0.015 inches thick. (For more information on pervious concrete, see “Learning To Do Pervious” in the October issue.
The best pervious concrete is placed relatively dry and stiff but with a paste that is wet and sticky. The water-to-cement ratio is usually between 0.26 and 0.35, and with a low cement content, that means it is also rather dry.
Adding a pound of 3/4-inch polypropylene fibers per cubic yard of concrete helps bond the coarse aggregate and wick water after the concrete hardens. However, getting the fibers to mix well in the stiff concrete can be a challenge and balls up to the size of tennis balls occasionally form in the mix. These do not cause structural concerns, but especially in decorative applications should be pushed to the bottom of the placement.
Placing pervious concrete is a straightforward and relatively rapid process. The basic steps include forming the edges, spreading the concrete, striking it off high and compacting it to grade, rolling in joints if desired, and wet curing. The steps go in rapid succession, so it is very important to have all tools and supplies on the site and in place where they will be needed prior to the arrival of the concrete. The goal is to place, compact, and cover the concrete within 20 minutes, which doesn't allow time to hunt for missing materials. (For a step-by-step explanation, see “A Lesson in Pervious."
Many of the decorative surface treatments normally applied to standard concrete, such as stenciling in the traditional sense, are not nearly as well suited to pervious concrete. However, because of the recent developments in mix design and placement methods, this material is a good medium for coloring and stamping.
The surest way to color pervious concrete successfully is by using liquid integral color. But because changing the mix proportions has a ripple effect on how easy it is to place and the strength of the finished product, even this requires great care. In all cases, it's important to work closely with an experienced ready-mix producer, especially to ensure consistency among loads.
Jim Miller of C2 Products, Cicero, Ind., has been placing pervious concrete for years and recommends beginning with a color that is one and a half to two shades darker than what you want the finished product to be, because it will lighten fairly quickly. Also, make test panels to be sure the result is what you are looking for, and to find out how adding the color affects the mix. You may end up having to reduce the amount of water you're using, moving toward the lower end of the w/c scale, due to the effect of ingredients in the integral color.