Pervious concrete is a mix with 15 percent to 30 percent voids. Our industry has accepted that these values strike a balance between the hydrological and engineering properties of the concrete. When voids are less than 15 percent, the slab will not drain rapidly, but is durable; when greater than 25 percent, 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 we get 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 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 NRMCA 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 storm water 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 the Southeast Precinct Police Department 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 we can use 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 percent of the accepted hardened density. Still think density is no big deal?
Southeast Precinct mock-up
TAB Construction, Omaha, Neb., the contractor on this project, formed and placed 225 square feet for the mock-up test panel. The testing agency conducted ASTM C 1688 tests and found that the fresh pervious concrete had a unit weight of 129 pcf and a 17.5 percent void content.
After the concrete set, the lab cored the mock-up and found that the hardened concrete had an average unit weight of 115 pcf and an average void content of 26 percent (more than 8 percent higher than the fresh unit weight from the ASTM C 1688 tests). ASTM C 1688 is intended to be an acceptance test and not a predictor of in-place density. Comparisons, though, are often made.
The mock-up panel was also tested for in-place infiltration using ASTM C 1701, “Standard Test Method for Infiltration Rate of In-Place Pervious Concrete.” The average infiltration rate for 3 locations was measured at 752 inches of rain/hour, more than an acceptable rate. This mock-up was accepted by the engineer and the project team and the acceptable fresh unit weight for the project became 129 pcf with a tolerance of +/- 5 pcf, meaning that the range of acceptable unit weight was 124 to 134 pcf.
Although 129 pcf was deemed acceptable, visual inspection of the surface and paste indicated the mix could have used more moisture. After analyzing the cores, the owner, contractor, and producer all agreed that the infiltration rate could be a little lower than 752 inches/hour, in order to provide a more durable pavement. This meant increasing the density and having fewer voids than 26 percent. The ultimate goal was to get as close as possible to 20 percent. The team essentially used the mock-up to compare the fresh density to the in-place hardened density and considered 129 pcf almost a minimum to obtain during the placement, since it yielded 26 percent voids.
Placing the pervious
For the Southeast Precinct project, the contractor placed 17,000 square feet of pervious concrete. The pervious concrete was transported by ready-mix trucks, deposited using a conveyor, and struck off with a 6-inch diameter hydraulic roller screed.
As discussed at the pre-construction meeting, the testing agency was responsible for conducting ASTM C 1688 fresh density tests on every truckload as it arrived at the jobsite. In all, 33 of these tests were performed. Every load placed was marked from beginning to end and documented on the pavement accordingly. This allowed us to establish a correlation between the fresh density and the in-place void content (or density). Every load of pervious concrete met the tolerances, falling within the limits of 124 to 134 pcf, averaging 129.7 pcf with a standard deviation of 2.03 pcf.
The pervious concrete cured for 7 days under plastic and then the testing agency removed 21 cores, and performed 21 tests for infiltration. The cores were tested for hardened unit weight and void content using the newly adopted test, ASTM C 1754, “Standard Test Method for Density and Void Content of Hardened Pervious Concrete.” The average hardened void content was 21.7 percent and the average hardened unit weight was 118.4 pcf. The average infiltration rate was measured at 448 inches/hour.
So now the big question: Were the specifications met? The real answer is, no. Even though the accepted fresh density met specifications, hardened density fell outside the specification requirements of +/- 5%. Of the 21 cores, 9 hardened unit weights were outside of the mock-up’s accepted hardened tolerances. But the project was still accepted by the owner because the pavement sections that were denser than allowed still showed good draining characteristics.
Analyzing the data
After studying the 21 cores from this project, it is clear that the average in-place void content will be greater than what would be predicted by the ASTM C 1688 test for fresh density. The Southeast Precinct pavement had an average void content that was 4.8 percent higher than predicted. This correlates well with data collected from three other projects in Nebraska with different mix designs (Metro Community College 2.9 percent higher, UNL Parking Garage 5.9 percent higher, and Sarpy County Sheriff 10 percent higher). This tells us that for a given mix, it is extremely important to understand the relationship between the fresh density/void content and the hardened density/void content. Only by understanding this relationship can we truly predict in-place void content.
Some additional observations were:
- Even though the average increase from fresh void content to hardened void content for this project was 4.8 percent, 3 cores were actually denser than the fresh concrete tests.
- Unit weights from 130 to 131 pcf produced a uniform dense surface.
- The infiltration rate increases exponentially with increasing void content.
- More field studies need to be conducted to determine the acceptable hardened density and the range of acceptance.
If you can control fresh density, you can control in-place void content. There is a correlation between the two. This is particularly important in regions where specifiers, producers, and contractors are new to pervious concrete. These new markets need data for mixes being used locally. Simply specifying, producing, and placing pervious concrete isn’t good enough. We need to use all the standardized testing procedures currently established to create data that can be shared and analyzed to construct predictable pervious pavements, rather than the unpredictable results that have been experienced in the past.
Jereme Montgomery is the executive director of the Nebraska Concrete and Aggregate Association. Dr. John Kevern is an assistant professor of civil engineering at the University of Missouri-Kansas City.