March 17, 2015 12:12

Permeable Pavers

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“Pretty soon, people won’t be able to enjoy seafood in Maryland anymore if we don’t clean up the waters in Pennsylvania,” said Marc Barnes, director of public relations for Pine Hall Brick Company, Winston-Salem, North Carolina.

It might sound like a minuscule problem, but the issue Barnes is describing is a very real concern in the stormwater industry. What exactly is posing a threat to our favorite seafood restaurants? The answer: polluted stormwater runoff.

Keeping up with stormwater runoff can be a full-time job—and for many of us it is! Stormwater management is especially challenging for contractors who work in long-established cities. Many of these older communities do not have the proper drainage systems in place to handle any flex in overflow volume. This is where the problem begins. The stormwater and sanitary sewage pipes in many towns are combined. When heavy rain flows over a city’s pavements, the additional volume can be overwhelming, leading to flooding, erosion, and the overflow of municipal sewage systems. If the volume is too great, the water cannot naturally migrate back into the ground, and instead ends up carrying pollutants into aquatic habitats.

As we continue to develop our cities, the natural cycle of water is affected by the large amount of conventionally-paved impervious surfaces, such as highways, sidewalks, and parking lots. These impervious pavements cause water to run off into the drainage systems too quickly. The flooding can carry a lot of debris and pollutants into natural bodies of water.

The dense material of conventional paving does not allow water to naturally saturate. Instead, the rainfall flows rapidly, running off the pavement. According to the American Society of Landscape Architects (ASLA), more than ten trillion gallons of untreated stormwater runoff finds its way into lakes, rivers and streams each year. The Environmental Protection Agency (EPA) considers runoff in developing areas to be one of the leading causes of water pollution within the United States.

The agency strongly encourages contractors to install green infrastructures and low-impact developments by awarding credits based on various point-rating systems. “Green infrastructures” refers to techniques that help conserve ecosystems while still providing benefits to the human community. These methods help reduce the industry’s potential negative impact on the environment. The goal is to reduce the amount of generated runoff, improve water quality, and utilize limited space more effectively.

The Leadership in Energy and Environmental Design (LEED) is a rating system developed by the United States Green Building Council as a way to evaluate the environmental impact of a building or community. By meeting specific sustainability goals, contractors can earn either a Platinum, Silver, or Gold rating. The system provides a framework for evaluating sustainable design, construction and maintenance of a jobsite through five different credit categories, including sustainable site development, water savings, energy efficiency, materials selection, and indoor environmental quality. The United States General Services Administration now requires all new federal building projects to achieve at least a Gold rating. This is up from the previously required Silver certification.

One of the most common green infrastructures being adopted into cities’ stormwater management plans is pervious paving as a best-practice installation. Not only can this type of pavement help the environment, but it can also help a contractor’s bottom line.

Pervious pavement has pores, or openings, that allow water to percolate directly into the subsoil, usually through an underlying stone retention layer immediately beneath the pavement (depending on the freeze/thaw cycle of the region) instead of simply running off. According to a study performed at the North Carolina State University Permeable Pavement Research Lab, “pervious pavements significantly and substantially reduced runoff volumes compared to conventional paving.”

“One of the main reasons why we install pervious paving systems is to meet the requirements for LEED,” said Brian Killingsworth, vice president of pavement structures, National Ready Mixed Concrete Association. But how should a contractor go about meeting these requirements? What steps should he take and why?

First, according to Killingsworth, contractors need to be educated on the common terms used when referring to pervious paving options. The term “pervious paving” is generally considered an umbrella term for all types of paving that strive to accomplish effective stormwater filtration.

Within the pervious umbrella category, there are also permeable and porous pavements (the three Ps). Although the literal definitions of all three are different, the stormwater industry uses them interchangeably, because their ultimate goals are similar. This can cause confusion when speaking with clients. In order to avoid any miscommunication, it is important to remain consistent when using the three Ps to describe the various management options. An easy way to illustrate how pervious paving works is by having clients imagine the process as if looking at a reservoir through a glass aquarium.

“First, you put in a layer of small rocks, then layers of progressively bigger rocks, and finally your pavers on top,” describes Barnes. “The water will trickle down through the surface, into the aquarium, filtering more and more through each layer.”

Permeable paving is very similar to traditional pavement, except without any fine particles. The lack of fine particles creates small open areas where water can drain through. Porous materials also have holes in their surface. However, they are not necessarily connected, but water can enter through the entire surface area.

The most common types of pervious paving used to obtain LEED credits are permeable clay pavers, permeable (or porous) asphalt, and permeable interlocking concrete systems. But regardless of which type of pavement is used, the benefits to the environment are still the same.

Permeable clay pavers

This option is popular among universities and cities that want to maintain a traditional aesthetic. Permeable clay pavers can closely mimic the appearance and color of their conventional brick siblings.

“You put a permeable clay installation right next to a conventional one and no one would know the difference,” said Barnes. Well, maybe not to the average person, but to the trained contractor’s eye there is a major difference between the two.

The permeable clay paver’s design allows runoff to flow across the pavers before seeping down through the aggregate-filled voids in between—first to a layer of small rocks, then to the big rocks.

The rock layers hold up the pavers, as well as provide a place to store the water. The water will seep into the earth in a day or so, if 18 inches of open-graded aggregate without fine particles is used.

In addition to working as a filtration device, these pavers aid in helping to prevent viruses, such as West Nile. When installed in stormwater basins, they can reduce the number of mosquitoes by eliminating access to stagnant water.

When rain hits conventional bricks, it quickly runs off downstream, causing destruction on its way. However, when rain falls on permeable pavers, it disappears through the permeable aggregate that fills in the interlocking joints.

Some contractors may try to install permeable pavers over gravel and sand. This is not recommended for any type of pervious paver. While this is called for when installing conventional pavers, a permeable design allows the ground below to breathe. Using gravel or sand merely clogs the system, making it ineffective.

Permeable interlocking concrete

The popularity of permeable paving systems continues to expand, as people search for ways to reduce their carbon footprints. Within the permeable-concrete world, flexible, plantable systems are growing more popular. These consist of grids of “concrete muffins” that conform to minor ground movements. They also promote drainage, making them a good choice for roads, equipment yards and parking lots.

Vegetated permeable-pavement systems are being used more in urbanized coastal areas and those adjacent to lakes and rivers.

Concrete pavements have a significantly lower life-cycle cost than alternatives such as asphalt. Although the initial cost of pervious concrete installation may be slightly higher, its increased durability saves money in the long run.

“Because of the two different binders found within concrete and asphalt, you will get two different performances between the two materials in each situation,” Killingsworth said. “It is important to understand how to manage your water.”

Unlike conventional concrete blocks, permeable concrete pavement can pass three to five gallons of water per minute. This allows the roots of nearby trees to go deeper to seek water, instead of growing upward and breaking up pavement, as is often seen on city sidewalks.

Installing and maintaining

“The best thing to do when handling a paving contract is to bring in an engineer,” advises Julie Messmer with Wolf Paving, Oconomowoc, Wisconsin. “Make sure the engineer has experience doing soil testing, because the condition of the soil needs to be determined first.”

Different types of soil have different infiltration rates. You need to determine whether you’re working with clay, sand or loam. Pervious paving installations are only feasible in areas where the soil allows for a reasonable rate of infiltration.

Soil can be tested in the field or at a lab.

continued on page 30

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In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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