When Greenberg Gibbons was designing the future community center-style retail outlet in Owings Mills, Md., they focused on innovative ways to maximize upcycling of the site’s former occupant, the Sweetheart Cup Company. The goal was to reduce, reuse, recycle, and conserve natural resources, landfill space, and as much energy as possible.
If it were possible to do all the above and simultaneously reduce runoff, clean stormwater, and promote groundwater infiltration to replenish aquifers, it would be a win for Baltimore County and the entire state of Maryland.
The former site featured less than 10 percent pervious surfaces with much of the area being under a roof. After considering the site’s geography, topography, possible building orientation, parking space requirements, and stormwater best practices, porous asphalt with a stone reservoir was chosen to help restore the natural features of the land that would promote the infiltration of rainfall into the soil where it would be naturally filtered and eventually find its way to streams, underground aquifers, and eventually the Chesapeake Bay. The process is recognized as a best practice by the U.S. Environmental Protection Agency and the State of Maryland.
Creating the reservoir
The concrete materials — including the foundations, slab on grade, elevated slabs, walls, etc. —and the concrete masonry units (CMU) were crushed onsite and stockpiled for future use in the porous asphalt’s reservoir. The material was crushed into a 5-inch maximum size, per the geotechnical engineer’s direction.
“We crushed and processed around 100,000 cubic yards of material that was used in the reservoir under the parking lots,” said E.J. Youngling Jr., vice president of Chesapeake Contracting Group, Inc., the general contractor for the project. Chesapeake Contracting’s role was to take the design from Greenburg Gibbons and turn it into reality.
Ahmet H. Aydilek, Ph.D., a professor in the University of Maryland’s Department of Civil and Environmental Engineering, confirmed, “The recycled concrete aggregate (RCA) has good bearing strength and drainage properties, and meets all requirements for long-term performance of dense-graded aggregate base or subbase. Permanent deformation is less for 100 percent RCA, compared to natural aggregates.”
Managing stormwater and pollution
Allen Davis, Ph.D., the Charles A. Irish Sr. Chair in Civil Engineering at the University of Maryland, explained, “The use of porous asphalt provides beneficial management of stormwater generated at the site. This directly translates into benefits for the Chesapeake Bay. The bay has a Total Maximum Daily Load restriction, its “pollution diet,” specifically for sediment, nitrogen, and phosphorus. The porous asphalt with a storage reservoir directly impacts all three of these pollutants.
“Pollutant loads are determined by the product of the volume of runoff and the concentration of each pollutant,” Davis said. “The porous asphalt and the deep reservoir beneath the porous asphalt offer benefits for both water quality and runoff reduction. The porous asphalt itself can filter sediment. A significant fraction of the phosphorus concentration is affiliated with the sediment, so it is removed. A maintenance plan that includes regular vacuum sweeping of the porous asphalt will remove this accumulated sediment and keep the porous asphalt in good working order.”
Impervious surfaces, including some sidewalks and the drive aisles between the porous parking bays, were routed to the porous pavement reservoir. The total hardscape area is approximately 13 acres. The volume of storage (voids in the stone reservoir) necessary to ensure water quality — the ability of the reservoir to capture and treat runoff from 90 percent of the average annual rainfall — is 48,584 cubic feet.
John Ranocchia Sr., vice president of the Civil/Survey Department of Century Engineering Inc., noted that this volume of storage for water quantity would meet most of the requirements for the 100-year management. Century Engineering developed the site plan for the project, which included the geotechnical analysis for the reservoir.
On a perfectly level site, the reservoir would look like a rectangular prism underneath the porous asphalt pavement. This site presented a challenge to designers with grade and slope changes in the parking area. In order to have the rain soak into the ground beneath the pavement, earthen berms with geotextile coverings were created. This isolated the lateral flow of water underneath the porous pavement. The cross section of the stone reservoir varied from 6 inches to 28 inches, capped with a locking course to stabilize the larger aggregates.
“The subsurface storage will allow for capture of major rainfall events, with storage lasting days until the next rainfall,” Davis said. “During this storage, water can infiltrate into the native soils, recharging shallow groundwater, which can benefit, as opposed to harming, local streams. Stored subsurface water can also possibly create conditions conducive to biological denitrification, leading to loss of nitrogen from the runoff. A deep, high-volume storage reservoir below the porous asphalt will help to maximize volume and nitrogen reduction. Together, the porous asphalt represents a two-pronged approach to reducing the stormwater footprint for this site.”
Paving with porous asphalt
C.J. Miller LLC was hired as the site work subcontractor responsible for constructing the berms, placing the geotextile, placing the stone reservoir and choker layer, and paving both the dense-graded asphalt and the porous asphalt.
“We have had a lot of success in placing porous asphalt at this facility and others around Maryland. It is a good way to give back to the environment without impacting the usable footprint of a job site,” said C.J. Miller, president of C.J. Miller LLC.
“Extra care has to be taken during the mix production and especially at the time of placement,” said Vice President Billy Miller. High-quality aggregates, polymer modified asphalt binder, and stabilizers go into making a successful porous asphalt mixture.
The subcontractor placed more than 13,000 tons of dense-graded base mix, 3,000 tons of dense-graded intermediate mix, and 7,000 tons of dense-graded surface mix in the impervious asphalt pavement sections. The porous asphalt sections featured more than 5,500 tons of porous asphalt. The dense-graded sections were placed first, followed by the porous sections. Constant vigilance produced a unique stormwater management system that doubles as a parking lot for the thousands of patrons shopping in Owings Mills.
Shortly after the grand opening of Foundry Row, a heavy rain event demonstrated how the pavement was performing. With several inches of rain falling that day, the parking aisles were clear of standing water, allowing patrons to load their vehicles while keeping their shoes from being soaked.
“We are pleased with the performance of the porous asphalt at our facility,” Wegmans Manager Jared McLaughlin said. “When it rains, you can certainly tell the difference in pavements. The porous asphalt section is free from standing water.”
Marshall Klinefelter is president of the Maryland Asphalt Association (http://mdasphalt.org). This article is reprinted with permission from the March-April 2017 issue of Asphalt Pavement magazine, published by the National Asphalt Pavement Association (www.asphaltpavement.org).