Check out the exclusive step-by-step detail of the project in A Lesson in Pervious.
As an environmentally friendly product that provides more than a paved surface to drive or park on, pervious concrete offers a new frontier for concrete contractors. Now contractors can offer a “stormwater solution” that allows water to pass through unimpeded, reduces runoff and stormwater treatment, and acts as a biological filter.
But pervious concrete, also known as “no-fines” concrete, differs from standard concrete in significant ways. The material is characterized by an open structure that includes 15% to 25% voids. When hardened, it requires both strength and openness to perform properly, so close attention must be given to mix design and placement techniques.
Even more than with standard concrete projects, success with pervious concrete requires contractors to collaborate with their concrete producer. They also need to learn some placing and finishing techniques.
Material considerations
Keeping a good balance among the ingredients is important for any pervious concrete mixture. The water to cementitious materials ratio is especially important and can be more critical to the success of pervious than it is for standard concrete. Because of the sensitivity of the mix, and the fact that it's a new material for many crews, it's best to work closely with the concrete producer and rely on the firm's expertise to provide a good, consistent mix.
Today the only useful standardized test for pervious concrete is the ASTM C 138, Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete. Standard concrete is generally in the range of 145 pounds per cubic foot, but pervious mixes range from 128 to 138 pounds per cubic foot.
Pervious concrete has virtually no slump, so the slump test is not applicable. Cores can be taken for checking thickness, unit weight, and permeability, and standard cylinders can be made with pervious concrete, but breaking them using standard test methods does not yield meaningful results. The ASTM committee on pervious concrete is developing new test methods, but none have been finalized yet. For the time being, the most reliable acceptance criteria are based on experience.
It's good practice to batch and ship the concrete a bit on the dry side. It's easy to add a little water, but a load of pervious that arrives too wet may have to be rejected. At the jobsite, check the material by sight—it should have a wet metallic sheen—and by feel to make sure it has the proper consistency.
The National Ready Mixed Concrete Association (NRMCA) says to check for proper moisture by forming a handful of the mix into a ball that should hold its shape. Experience has shown in some cases, though, that a somewhat wetter mix—one that leaves a paste coating on your hand when you grab a fistful and release it—will provide better placement results.
Pervious concrete mixes typically include a gap-graded coarse aggregate, with a top size from ¼ to ½ inch. Larger aggregate provides a higher degree of permeability but also yields a more open surface. Using an aggregate size in the 3/8-inch range provides a surface texture that is appropriate for many applications.
Crushed stone and gravel both can be used for pervious concrete, but produce different surface textures. The choice of aggregate also determines the cement content and how other ingredients in the mix work together, such as water requirements and the effectiveness of compaction. Generally, pervious concrete made with angular aggregate requires more compactive effort than that made with rounded aggregate, all that made with rounded aggregate, all other things being equal.
