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.

 

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% 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% (more than 8% 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 three 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, to provide a more durable pavement. This meant increasing the density and having fewer voids than 26%. The ultimate goal was to get as close as possible to 20%. 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% 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 seven 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% 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, nine 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.