Launch Slideshow

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Density is our Destiny

Density is our Destiny

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    Consolidated Concrete of Omaha, Neb., manufactured pervious concrete for the Metropolitan Community College in Omaha.

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    Target Void vs Actual Void Content

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    Understanding the density curve is critical to producing and placing pervious concrete. It is apparent that the unit weight and void content have an extremely linear relationship.

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    Estimated In Situ Voids vs Core Voids

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    ASTM C 1688 Voids vs Core Voids

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    A worker tests the fresh unit weight of pervious concrete using the ASTM C 1688 test method.

Traditionally, pervious concrete has been promoted to be a mix with 15 percent to 25 percent void structure. Our industry has accepted these values due to the balance of hydrological and engineering properties.

If there is less than 15 percent voids, the slab will not drain rapidly, but it is durable. With more than 25 percent voids, the slab drains rapidly but will lose critical properties essential for durability. A lack of standard testing procedures has placed much responsibility on the producer and the concrete contractor to ensure durable pervious concrete pavements with required engineering and hydrological properties.

A placement at the Metropolitan Community College in Omaha, Neb., utilized recommendations and ASTM standards to ensure success. ASTM C 1688, Standard Test Method for Density and Void Content of Pervious Concrete, was used with recommendations and practices to verify engineering properties.

The basis for the testing was derived from Dr. John Kevern's paper, “Predicting Performance of Pervious Concrete Using Fresh Unit Weight,” presented at the NRMCA Concrete Technology Forum: Focus on Sustainability last year.

Typical characteristics of pervious concrete are a mixture with little to no fines, relatively single-sized aggregate, with no slump. Public perceptions are skeptical regarding freeze-thaw durability, clogging, and raveling. For several years, the national focus has been directed at research and development to improve durability.

Research has shown that with proper proportioning of the mix design, many pervious mixtures demonstrate more than acceptable durable qualities and still have the proper void content for stormwater infiltration. The challenge is producing and placing pervious concrete that results in in-place properties such as void contents suggested by successful research projects.

Only recently, more standards have become available for pervious concrete producers and contractors. Metropolitan Community College is the first pervious concrete placement in Nebraska since the release of ASTM C 1688. This standard was used to predict in-place void content.

Unit weight

Pervious concrete mixtures are designed to possess desired strength and permeability at a particular void content, usually around 20 percent. However, a particular pervious concrete mixture can have a wide variety of unit weights, depending on workability and method of compaction, which also control the void content.

One of the most critical components regarding pervious concrete is understanding your density curve for the selected mixture. It is apparent that the unit weight and void content have an extremely linear relationship. This density curve not only helps a producer proportion a mix to produce proper targeted void content, but also helps the contractor control proper in-place void content.

Through trial batching, the producer, Consolidated Concrete LLC in Omaha, acquired six data points obtained from calculations according to ASTM C 1688. Those data points were then plotted on a graph to determine the linear relationship or density curve. From this data and discussions with the engineer, a fresh unit weight 135 pounds per cubic foot (pcf) and 12 percent void structure was selected as the desired value when tested by ASTM C 1688.

The value of 135 pcf is produced using ASTM C 1688. This standard is not to predict void content in the field, only per the standard. We assume that filling a ¼ cf bucket in two lifts, 20 blows with a standard proctor hammer on each lift, will not produce the same void content in place. We assumed that this value would yield in-place void structure of 15 percent to 25 percent. The owner, design team, general contractor, producer, and concrete subcontractor all agreed to try to achieve lower in-place void content of 15 percent to 20 percent to achieve durability.

Targeted unit weight

Project specifications referenced target unit weight +/- 5 pcf. Since the target unit weight is 135 pcf, the target range was agreed at 130 pcf-140 pcf for acceptance per ASTM C 1688. According to the curve, this would yield a target range of 9 percent to 16 percent. In-place target unit weight according to the density curve is 125 pcf, with target range of 118 pcf-131 pcf. According to the curve, this would yield an in-place target range of 15 percent to 20 percent.

Quality assurance

The owner made it very clear that “if the pervious slab fails, we need to know why.” Conversely, “if the pervious slab is a success, we need to know why.” The team needed to include as many standard testing procedures as possible as quality control measures to ensure a successful placement. Thus, the following quality assurance program was developed.

  • Concrete producer to test moisture in aggregate before batching operations.
  • Producer to conduct ASTM C 1688 at the plant.
  • Conduct inverted slump test onsite to verify workability.
  • Third party testing agency to conduct ASTM C 1688 onsite.
  • Estimate in-place unit weight and plot on curve to determine estimated in-place void content
  • Core in-place pervious to verify desired properties.
  • Test resultsTarget void content versus core void content

    Since the target range of 15 percent to 20 percent void content was established, a target void content of 17.5 percent was used. The cores varied in void content from 13.4 percent to 21.6 percent, averaging 16.7 percent—less than 1 percent from target.

    Estimated in-situ void content versus in-place void content

    The results of estimated in-situ were much closer than anticipated, averaging less than 3 percent deviation from predicted values. The estimated values were all higher than actual voids.

    ASTM C 1688 void content and results from cores

    It was assumed that we would gain an increase of 3 percent in void content to reach at least 15 percent (minimum per specs). Only two of the five cores increased more than 3 percent. This could have been due to the workability of the mixture, or possible compaction from foot traffic at the core location. More data is needed to create any correlating relationships in the field.

    ASTM C 1701 Standard Test Method for Infiltration of in-place Pervious Concrete

    Six ASTM C 1701 tests were conducted where the concrete was placed after corresponding ASTM C 1688 testing. A high value of 2016 in./hr. with a low of 62 in./hr. were documented with an average of just more than 700 in./hr.

    Conclusions
  • Understanding unit weight to predict in-field void content can be done utilizing a density curve and ASTM C 1688. The team chose a target range of in-place void content to be 15 percent to 20 percent. Of the five cores taken from Metropolitan Community College, the average was 16.7 percent voids, with a low test data of 13.4 percent and a high test data at 21.6 percent voids. The average unit weight of all batches was 133.12 (133.5 pcf target), with a standard deviation of 3.25 pcf (+/-2.5 percent from target).
  • Inverted slump cone test did not show workable results with a unit weight of much less than 130 pcf per ATM C 1688.
  • Target unit weight range of +/- 5 pcf seemed to be too broad. Upper limits would make the mix much like conventional concrete, and lower limits produced estimated void contents around 30 percent. With proper quality control measures in place, a target unit weight range of +/- 3 pcf may be more acceptable.
  • Estimated in-situ unit weights and void contents had good results closely matching the core samples. This test is less accurate with non-workable mixtures, and more accurate with workable mixtures.
  • A targeted unit weight of 135 pcf produced a very workable mixture, one that prior to developing a curve would have been considered too wet by any visual inspections. Cores demonstrated no paste draining off aggregate to plug voids toward the bottom of the slab.
  • Pervious concrete pavements can be produced and placed in a controlled fashion utilizing quality control measures and ASTM C 1688. Using a density curve to predict in-place void content is a valid approach to ensure owners and designers a quality product is being delivered. Producers, as well as contractors, need to develop a density curve, understand their curve, and most importantly, trust the curve.

    Jereme Montgomery is the executive director of the Nebraska Concrete and Aggregate Association. E-mailjereme@nebrconcagg.com. Dr. John Kevern is an assistant professor of civil engineering at the University of Missouri-Kansas City. E-mailkevernj@umkc.edu.


    Learn More at World of Concrete

    You can learn more about pervious concrete at World of Concrete, Feb. 5-8, in Las Vegas. Pervious Concrete Live is the industry's only live-action demonstration devoted solely to this increasingly important and popular building material. This event takes place throughout the show in the Gold Lot across the street from the North Hall of the Las Vegas Convention Center.

    In addition, an Introduction to Pervious Concrete Pavement Construction seminar takes place 1-5 p.m. Monday, Feb. 4. A pervious concrete written exam takes place 1-3 p.m. Tuesday, Feb. 5. Also, Pervious Concrete Field Project for Installer Certification takes place 10:30 a.m.-4:30 p.m., Tuesday through Thursday, and 10:30 a.m. to 12:30 p.m. Friday.

    Visit www.worldofconcrete.com for more information on this and other events.