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Proving Pervious Concrete's Durability

Proving Pervious Concrete's Durability

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    Workers place pervious concrete at a construction site. Recently, researchers have tried to develop a new tool to assess the material's durability.

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    This is a sample of good pervious concrete

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    These are samples of poor pervious concrete, with a bad one on the left, and the worst on the right.

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    Researchers used a 3-inch cylindrical tool to manually compact the pervious concrete in the test.

Based on variations in mix proportions, this round of testing was able to demonstrate a wide range of surface durability conditions. Researchers speculate the modified abrasion test may be able to detect a wide range of resistance to raveling. After examining the abraded samples, researchers confirmed that the aggregate was not being abraded, not affecting durability results.

Based on these rounds of testing, the team proposed the following guidelines for the modified test method:

  • Samples would be 4-inch-high, 4-inch-diameter cylinders.
  • Specimens would be cast to the mix design's void content.
  • Cylinders would cure for seven days in a covered cylinder mold.
  • Immediately after stripping, the cylinder would be weighed and measured.
  • The cylinder would be subject to 50 revolutions in the ASTM C 131 chamber without any balls.
  • The final weight would be calculated.
  • The mass loss would be calculated as a percent of the original specimen mass.

  • Expanded testing

    To test the effectiveness of the proposed procedure, a number of mixes were tested with a range of variables. To reduce the variability of the test, an attempt was made to cast the specimens at the designed void content with minimal vertical porosity distribution. Unfortunately, ASTM does not yet have a standardized process for casting pervious concrete specimens.

    To achieve this, the mass necessary to achieve that void content was weighed and placed into the cylinder mold. The cylinder was dropped 1 inch 10 times and then compacted manually, using a gyratory motion with a 3-inch- diameter cylindrical tool, until the cylinder was 4 inches tall. A plastic bag was immediately placed over the cylinder, which was allowed to cure uninterrupted for seven days. (There is currently no ASTM method for casting cylinders. When that method exists, it may be used.)

    Testing was done with a number of variables, including aggregate type, void content, fly ash addition, and water-cement ratio. Each series of tests demonstrated the anticipated range of results.

    In most series, as void content increased, raveling resistance decreased. In the series demonstrating the range of water-cement ratios from 0.25 to 0.34, the surface durability did not change. Reviewing the results from these series, it appears the test method works effectively for demonstrating a range of raveling resistance.

    This is not a shortcoming of the test. But the method, as performed on a cast cylinder, may vary from field installations, as contractors' methods play an important role in surface durability.

    Conclusion

    The test method effectively demonstrates a range of raveling resistance for pervious concrete. The specimens are easy to fabricate and easy to handle because of their small size. The short curing time mimics field-curing conditions and produces timely results. Based on the simple procedures for casting a specimen, the test may be performed on laboratory-mixed or field-batched concrete.

    But there are some shortcomings to this test method. In this study, only aggregate passing the 3/8-inch sieve was tested. We will still need to evaluate a wider range of aggregate sizes.

    The specimen fabrication techniques are not standard. The test does not provide a good range of results for durable mixtures. Specimens that were cast too wet, and thus sealed on top or bottom, should not be tested.

    The repeatability was tested blindly in this evaluation, but not widely enough to be conclusive. A full study of repeatability should be considered. Most importantly, no cores were tested in this project. To establish good correlations to field performance, cores must be evaluated.

    Matthew Offenberg is the Southeast U.S. technical service manager with Grace Construction Products and is an internationally recognized expert in the field of pervious concrete. He chairs ACI committee 522 on pervious concrete, is the founding chairman of the NRMCA committee on pervious certification, and is secretary of ASTM committee C 09.49, which is developing test methods for pervious concrete. A registered professional engineer, his received his undergraduate and graduate degrees from Purdue University. E-mail matthew.offenberg@grace.com. Visit www.grace.com.