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Permeable Pavement Could Be the Next Sustainable Requirement for Buildings

As his quest to make New York City more sustainable advances, Mayor Eric Adams just dropped a bombshell announcement. Along with NYC’s Department of City Planning Director and City Planning Commission Chair Dan Garodnick, Adams kicked off the next stage of his “City of Yes” initiative—a series of citywide policy changes aimed at modernizing NYC’s infrastructure to dramatically reduce carbon emissions and promote sustainable and equitable development practices. Of the array of ambitious goals “City of Yes” aims to tackle, many of which Adams believes can be achieved through sweeping changes to the city’s zoning laws, the announcement specifically mentions expanding the use of permeable pavement in city streets and public spaces to manage stormwater runoff and promote environmental sustainability. 

If you are unsure of how much Adams’ push for permeable pavement could reshape the city, remember that NYC has carried the nickname “Concrete Jungle” since the ‘60s. Almost all of the city’s sidewalks are made of concrete, and while concrete has been historically favored in the construction industry because it’s a durable and readily-available material, it’s not the most ecologically supportive. Concrete does not allow water to pass through it, which gets problematic when a city is almost completely covered in it. Whenever rain falls, water builds up on impervious surfaces (like concrete) and picks up pollutants such as oils, metals, and chemicals along the way. This polluted runoff then flows into the city’s stormwater system, which can cause flooding, erosion, and poor water quality in rivers, lakes, and other bodies of water. 

Concrete also contributes to the urban heat island effect, which occurs when the built environment traps heat, causing higher temperatures in urban areas compared to surrounding rural areas. The dark color and thermal mass of concrete absorb and retain heat, which exacerbates the effects of heatwaves and leads to increased energy use to keep buildings cool. To mitigate those effects, Adams’ initiative is opening the proverbial floodgates for permeable pavement to totally transform the city that never sleeps.

Permeable pavements, as the name suggests, are made up of porous surfaces which reduce the amount of stormwater runoff that is generated by traditional impervious surfaces, like concrete or asphalt. Instead of water running off these surfaces and into stormwater drains or nearby water bodies, permeable pavements allow the water to be absorbed into the ground, which helps to reduce flooding, erosion, and pollution. As the water seeps through the surface, some of it sticks to the pavement and gradually dries out. This is where thermodynamics steps in: because water needs heat in order to evaporate, it effectively cools the pavement’s surface and surrounding air when the sun is beaming, which means that it drastically reduces the sweltering heat island effect. 

Adams’ inclusion of permeable pavement in his master plan is a huge deal for the city, but it also stands to set a precedent for other cities to follow. As of now, only a handful of municipalities across America are exploring the use of permeable pavement. But as NYC takes a more aggressive stance to implement permeable pavement into its infrastructure, property owners in areas where stormwater and the heat island effect are of great concern are undoubtedly watching. If permeable pavement is mandated, property owners will have to learn about this relatively new technology, including what options they have. Because proper planning and design are essential to choosing the most appropriate type of permeable pavement for each property, it’s fair to say that the permeable pavement knowledge gap is a big problem. Here are some of the products currently available on the market.

Pervious concrete pavement

Pervious concrete pavement is a type of permeable pavement that allows water to pass through it and infiltrate into the ground below. It is made with a mix of coarse aggregate, cement, and water, with little or no fine aggregate, creating a highly porous material. The high porosity of pervious concrete allows it to act like a sponge, capturing stormwater runoff. This can help to reduce the chance of floods, improve water quality, and recharge groundwater resources.

Pervious concrete pavement is typically installed over a base layer of crushed stone and aggregate. The thickness of the pervious concrete layer can vary depending on the specific application, but is typically between 4 and 8 inches.

Pervious concrete pavement has several advantages over traditional impervious surfaces such as asphalt or concrete. It can help to reduce the amount of stormwater runoff that needs to be managed, while also providing a durable and long-lasting surface that can withstand heavy traffic loads. The lighter color of the pavement reflects more heat than traditional dark pavement, further helping to mitigate the urban heat island effect.

Pervious concrete pavement does have some limitations. It requires a well-designed sub-base and careful construction to ensure that it remains permeable and does not clog over time. It may also have a higher upfront cost compared to traditional pavement, although this can be offset by reduced stormwater management costs over time.

Porous Asphalt

Porous asphalt is another type of permeable pavement that is commonly used in parking lots, driveways, and other high-traffic areas. It is made by adding a special type of aggregate to the asphalt mix, which creates voids in the material, just like pervious concrete. The key difference between porous asphalt and pervious concrete is the type of binder used. Porous asphalt uses asphalt cement as the binder, which can make it more flexible and resilient than pervious concrete. The choice between the two will depend on the specific site conditions and design requirements. 

Because porous asphalt has a void space of about 16 percent (as opposed to ordinary asphalt’s 2 to 3 percent), porous asphalt requires a little more structure to remain stable. The design for applying porous asphalt consists of a minimum of four layers: an asphalt layer of 2 to 4 inches, a filter layer of 1 to 2 inches made of half-inch crushed aggregate, a 12-inch minimum reservoir layer made of 1- to 3-inch aggregate, and a layer of geotextile material such as landscaping fabric. 

Porous asphalt can even help clean the runoff as it passes through. When applying pavement, the stone reservoir layer is stabilized by the crushed aggregate filter layer and helps to remove pollutants. The reservoir bed, an extremely porous layer of open-graded, clean-washed aggregate with at least 40 percent of void space, is where treated runoff is kept. The migration of particles into the stone reservoir, which might clog the system, is stopped by geotextile material positioned between the reservoir bed and uncompacted subsoil. The cleaned water then percolates through the undisturbed soil base.

One of the benefits of porous asphalt is its ability to handle heavy traffic loads. Like pervious concrete, porous asphalt’s thickness depends on the traffic loads that engineers will reasonably expect for the area. However, porous asphalt is not nearly as thick—expect a range of 2 to 4 inches. Even though porous asphalt is thinner, it is a durable material that can withstand the weight of vehicles and other heavy equipment, making it suitable for use in parking lots, driveways, and other areas that experience high levels of use. It can also last up to 20 years with proper maintenance, making it a cost-effective solution in the long run.

One potential drawback of porous asphalt is that it may require more maintenance than traditional asphalt surfaces. Because it is porous, it may be more susceptible to damage from freeze-thaw cycles, and may require more frequent sealcoating to maintain its permeability. It may also require more frequent cleaning to prevent clogging of the voids and maintain its ability to allow water to pass through. Plus, porous pavement materials should be laid at least 100 feet away from sources of drinking water since they are ineffective at removing dissolved nutrients from water (this is also true of pervious concrete). In areas where oil, grease, or other pollutants of groundwater are anticipated, pre-treatment of runoff is advised.

Permeable Interlocking Concrete Pavement 

Permeable interlocking concrete pavement (PICP) is a type of permeable pavement that combines the benefits of permeability with the durability and versatility of interlocking concrete pavers. They are made up of individual concrete pavers that are designed to interlock with each other, creating a stable and durable surface.

PICP is installed over a sub-base of crushed stone and aggregate, which allows water to infiltrate into the ground below. The pavers are laid on top of this sub-base in a tightly interlocking pattern, with small gaps between each paver. These gaps allow water to pass through and infiltrate into the sub-base below.

There’s a size variation depending on what the surface will be used for. Precast modular components of various sizes and forms make up PICP pavers. For automobile zones, they are normally 80 millimeters (3 1/8 inches) thick, whereas pedestrian areas typically have 60 millimeters (2 3/8 inches) of thickness. 

PICP has several advantages over pervious concrete and porous asphalt. First, PICP has a lower risk of clogging. This is due to its interlocking design, which provides channels for water to flow through even if the surface becomes partially clogged. PICP is also easy to maintain, requiring minimal maintenance as it can be easily cleaned with a vacuum or power washer. PICP’s joints between blocks provide an additional channel for water to flow through, improving drainage performance and reducing the risk of standing water. Additionally, PICP has a lower life-cycle cost compared to traditional impervious surfaces, as it requires less maintenance and does not need to be replaced as frequently.

The use of permeable pavement can have a number of benefits for urban environments, and as it becomes more popular with policymakers, permeable pavement will seem more attractive to property owners as well, as it can provide a more aesthetically pleasing and environmentally-friendly alternative to traditional paving materials.

NYC might be the first city to require permeable pavement, but it certainly won’t be the last. Even without regulations requiring the installation of permeable pavement, building owners should consider permeable pavement as a way to increase the environmental sustainability of the property and prevent flooding. 

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