Feb 09, 2007

Pervious Concrete Reaches Minnesota

Pervious concrete is described as a zero-slump, open-graded material consisting of hydraulic cement, coarse aggregate, little or no fine aggregate, admixtures and water. Pervious concrete systems are typically used to control storm water runoff for parking lots, low volume roads and walking trails, but have been used in applications such as swimming pool decks and greenhouse floors. These systems have been adopted as a Best Management Practice (BMP) by the Environmental Protection Agency (EPA) for mitigating storm water runoff and also have the potential for acquiring credits within the LEED Green Building Rating System.

Program Description
The available literature concerning pervious concrete indicates that information on the use of pervious concrete in severe environments, those exposed to severe cold temperatures and deicing materials, is limited. This article describes a project conducted by the Aggregate and Ready Mix Association of Minnesota (ARM) at MnROAD, the world’s largest pavement research facility under the Minnesota DOT, evaluating the use of pervious concrete in Minnesota.

The goal of this research was to provide the industry with a controlled study of pervious concrete in a severe environment, to evaluate three mixtures containing common Minnesota aggregate types, and to exhibit a placement for area contractors.

The project consisted of the construction of a sidewalk with 4 inches of pervious concrete laid on 6 inches of washed 1.5-in. concrete stone underlain with filter fabric. The base material was then “day-lighted” at the lowest elevation using a drain pipe, as the sidewalk was placed on existing clay subgrade. The three mixtures evaluated by this study included a 3/8-in. crushed granite mix, a 3/8-in. gravel mix with approximately 5% sand (by weight of total aggregates), and a 3/8-in. crushed limestone mix with 5% sand and the use of fiber reinforcement. It should be noted that the mixes were supplied by different local producers and that the mixtures’ proportions aside from the aggregate type were also variable.

Before placement, the base material was thoroughly wetted to provide additional moisture for curing. This is important as the water content of pervious concrete is significantly lower than conventional concrete, and there is effectively much more exposed surface area. The concrete was placed using a rear discharge truck, spread with come-alongs and consolidated using a roller-compacting screed. Jointing was then installed at 4-ft intervals using a joint roller, which forms the joints in a plastic state leaving a rounded edge. The edges of the sidewalk were also rounded using a hand tool. Rounded edges have been found to exhibit higher raveling resistance than sawn or cornered edges.

Immediately after finishing, the concrete was covered with plastic sheeting and cross-rolled with a steel roller to provide a smooth surface finish. The sheeting was then secured to the formwork, and the slab remained covered for seven days.

Material Properties
For informational purposes, the mixes were evaluated for several properties, both fresh and hardened, using the following methods. As a note, there are currently no standard testing methods for pervious concrete through ASTM, and it is recommended that conventional concrete methods not be used to evaluate pervious concrete properties.

The unit weight of the concrete was measured by several methods. The fresh unit weight was determined by ASTM C 172 and C 29 “Jigging Method.” Hardened concrete unit weight was measured through coring and though nuclear density testing.

Three consolidation methods were used to prepare 4 by 8 in. compressive strength and 6 by 6 by 20 in. flexural strength specimens, which were evaluated after 28 days of curing:

  1. Two specimens - filled and unconsolidated.
  2. Two specimens - filled in two lifts and tapped 15 times per layer with a rubber mallet.
  3. Two specimens - prepared in accordance with ASTM C 31.

De-molded unit weights were measured for all specimens. In addition, cores were taken from the sidewalk and from beam specimen remains and evaluated for unit weight, void content and permeability. Using a plot of the compressive/flexural strength vs. unit weight, the ARM then used the core unit weight to estimate the in-place strength of the pervious concrete.

Results
The sidewalk was placed in the fall of 2006 and is currently undergoing its first winter exposure. It should be noted that the sidewalk is to be maintained using normal snow removal and salting as the adjoining sidewalk areas. Currently, there are no signs of deterioration or clogging, and the system is believed to be performing well.

The mixes yielded the in-situ compressive/flexural strengths of approximately 2200/360 psi for mix #1, 1300/160 psi for mix #2 and 2750/360 psi for mix #3. The permeability results were 990 in./hour for mix #1, 2630 in./hour for mix #2 and 510 in./hour for mix #3. It should be noted that pervious concrete properties are highly dependent upon the state of compaction, and these values represent those that correlate to core or in-place density.

Will pervious concrete work in Minnesota? Truly, it is hard to conclude at this point based upon the limited research and field use in the area beyond two winter seasons. However, the results from this study and from other cold environments seem to support that there is potential.

Future investigation with this study will include the placement of a 500-ft cell of pervious concrete incorporated in the low volume road itself at the MnROAD facility. For more information on MnROAD, please visit http://mnroad.dot.state.mn.us/research/mnresearch.asp.

Pervious Concrete Resources
“ACI 522R-06 Pervious Concrete” (Guide)
“Pervious Concrete Pavements,” by Paul Tennis, Michael Leming and David Akers
ACI – www.concrete.org
NRMCA – www.nrmca.org
PCA – www.cement.org
The Center for Transportation Research and Education with Iowa State University has also published a research report with information on freeze-thaw resistance of pervious concrete mixtures, available at: http://www.ctre.iastate.edu/reports/mix_design_pervious.pdf

References
“ACI 522R-06 Pervious Concrete” (Guide), American Concrete Institute, May 2006.
Tennis, Paul, Michael Leming and David Akers. “Pervious Concrete Pavements,” PCA and NRMCA, 2004.

About the author

<i>Joe Clendenen is technical service engineer for Holcim (US), Inc., as well as technical service committee co-chair for the Aggregate and Ready Mix Association of Minnesota and associate member of ACI 522 Pervious Concrete.</i>

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