Pervious concrete is a mix with 15% to 30% voids. The concrete industry has accepted that these values strike a balance between the hydrological and engineering properties of the concrete. When voids are less than 15%, the slab will not drain rapidly, but is durable; when greater than 25%, the slab will drain rapidly, but will lose some of the critical properties essential for long-term durability.
But the lack of standard testing procedures has placed a great deal of responsibility on the producer and contractor to ensure that they achieve durable pervious concrete pavements that have the required engineering and hydrological properties. At a recent pervious concrete project in Omaha, Neb., the Southeast Precinct Police Department, the producer, and the contractor used the current American Concrete Institute (ACI) recommendations and ASTM standards and achieved a successful placement.
The test used on this project for acceptance of the pervious concrete at the jobsite was ASTM C 1688, “Standard Test Method for Density and Void Content of Pervious Concrete.” Other tests, as we will describe, were used to verify the pavement’s engineering properties. The basis for the testing program was John Kevern’s paper, “Predicting Performance of Pervious Concrete Using Fresh Unit Weight,” originally presented at the 2012 National Ready-Mixed Concrete Association Concrete Technology Forum.
Pervious concrete is a concrete mix with little to no fines, relatively single-sized aggregate, and zero slump. The perception of pervious has been skeptical as to its freeze/thaw durability, clogging, and surface raveling. For a number of years, national focus has been directed at improving durability. Research has shown that with proper proportioning, many pervious mixtures demonstrate more than acceptable durability with the proper void content for stormwater infiltration.
The challenge is producing and placing pervious concrete with consistent in-place properties such as the void content suggested by successful research conducted in various laboratories. What makes this project important is the amount of testing conducted in the field to correlate the fresh properties (density and void content) of the pervious concrete with the critical hardened properties.
Density (unit weight)
Why should we be concerned with density? Currently, there is only one test to accept pervious concrete, ASTM C 1688-08. This test method provides the standard procedures to obtain “fresh” density specifically for pervious concrete. ACI 522.1, “Specification for Pervious Concrete Pavements,” indicates that the unit weight of pervious concrete must be within +/- 5 pounds/cubic foot (pcf) of the specified “fresh” unit weight for acceptance of the mix at the jobsite. Anything outside of this range may be considered unacceptable. ACI 522.1 also indicates the hardened density be within +/- 5% of the accepted hardened density. Still think density is no big deal?