Launch Slideshow

The finished parking surface at the Southeast Precinct of the Omaha police department.

Density is our Destiny

Density is our Destiny

  • The finished parking surface at the Southeast Precinct of the Omaha police department.

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    The finished parking surface at the Southeast Precinct of the Omaha police department.

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    Nebraska Concrete and Aggregate Association

    The finished parking surface at the Southeast Precinct of the Omaha police department.

  • Testing the fresh unit weight of the pervious concrete using test method C 1688.

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    Testing the fresh unit weight of the pervious concrete using test method C 1688.

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    Nebraska Concrete and Aggregate Association

    Testing the fresh unit weight of the pervious concrete using test method C 1688.

  • The pervious concrete was placed using conveyors and roller screeds.

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    The pervious concrete was placed using conveyors and roller screeds.

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    Nebraska Concrete and Aggregate Association

    The pervious concrete was placed using conveyors and roller screeds.

  • Twenty one cores were removed from the finished pavement to allow correlation of fresh and hardened unit weight and void content.

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    Twenty one cores were removed from the finished pavement to allow correlation of fresh and hardened unit weight and void content.

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    Nebraska Concrete and Aggregate Association

    Twenty one cores were removed from the finished pavement to allow correlation of fresh and hardened unit weight and void content.

  • There is a straight-line correlation between density and air void content.

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    There is a straight-line correlation between density and air void content.

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    There is a straight-line correlation between density and air void content.

 

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.

Conclusion

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.