Porous concrete becoming more popular among developers and public agencies

| November 25, 2014 | 0 Comments

Workers vacuuming alleys and parking lots may be an odd sight now, but they’ll become less so in the coming years as so-called “green” and “blue” streets become more common. The colorful names signify a growing trend in road and parking lot design: the use of porous asphalt and pervious concrete.

permeable concrete sidewalk

Pervious sidewalk concrete at the Southlawn housing development in Milwaukee, WI. From Aaron Volkening.

The typical street or parking lot today is neither porous nor pervious. When rain or melting snow runs off rooftops and pavement, their hard surfaces act as conveyor belts that dump dirt, leaves, oil, and other pollutants into storm drains, rivers, and other water bodies.

In the Chesapeake Bay region, stormwater runoff is the fastest-growing type of pollution, says the U.S. Environmental Protection Agency (EPA). It accounts for one sixth of all nitrogen and phosphorus, as well as one fourth of sediment, polluting the Chesapeake.

Such numbers have caused the EPA to lean on Maryland and other states surrounding the Chesapeake watershed to more aggressively reduce stormwater runoff. The EPA’s advocacy is paying off. Various projects in Maryland—park-and-ride lots in Baltimore and Allegany Counties, for instance — are incorporating pervious concrete into their design.

The trend isn’t limited to the mid-Atlantic. Last week officials in Boston attended a ribbon-cutting ceremony for the city’s second “green alley,” with cities nationwide — Los Angeles, Philadelphia, Overland Park, KS, among others — also installing pervious streets. Colleges and universities have been part of the wave, with many schools installing permeable parking lots and walkways. The practice aligns with their green building initiatives and helps schools better manage runoff when constructing new buildings on limited campus footprints or accommodating more vehicles due to increased enrollment or growing staff.

The swing towards greener streets signals a marked shift. Fifteen years ago, the EPA cautioned against porous pavement, pointing to its high failure rate. Improper installation and maintenance were the main culprits, officials said, but research has since shown how the right flow design and correct installation can lead to long-term success.

Cost continues to be a barrier to wider adoption of porous asphalt and pervious concrete. Installation of standard versions averages $2 to $5 per square foot, but pervious mixes can run two to four times as much. Paving blocks can be even more spendy, costing $15 to $20 per square foot if they’re laid by hand, say experts.

Permeable pavement costs more because a porous base is needed to capture runoff. Usually that means laying down a 6- to 8-inch layer of gravel and tilling or replacing compacted soils that drain poorly. It also involves more non-standard maintenance, such as vacuuming to clear out clogged pores. New developments can install porous concrete or asphalt at more competitive costs because the ground has to be prepared for paving anyway.

Other projects simply have to eat the higher costs because the benefits outweigh the fiscal disadvantages. In inner city neighborhoods, where open ground is scarce or nonexistent, permeable pavement is one of few options for reducing runoff. Porous concrete or asphalt also helps local governments, businesses, and schools avoid building new retention basis, freeing up more space for other uses.

For Ghassan Korban, commissioner of public works for the City of Milwaukee, the tradeoffs are worth it. “This is the price of doing business and being progressive for the future,” he said in June. Milwaukee received a $1 million grant from the EPA earlier in 2014 to replace some of its roadways with permeable pavement.

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Category: Parking Tech

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