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% higher than predicted. This correlates well with data collected from three other projects in Nebraska with different mix designs (Metro Community College 2.9% higher, UNL Parking Garage 5.9% higher, and Sarpy County Sheriff 10% 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:
- Even though the average increase from fresh void content to hardened void content for this project was 4.8%, three 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. PW
Jereme Montgomery is the executive director of the Nebraska Concrete and Aggregate Association (www.nebrconcagg.com). Dr. John Kevern is an assistant professor of civil engineering at the University of Missouri-Kansas City (www.sce.umkc.edu). This article first appeared in the November 2012 issue of Concrete Construction, Public Works’ sister publication.