Pervious concrete is one of the hottest topics in the world of land development today. It is not a new technology, but it’s a technology that is being embraced in a world of sustainable development and expensive land.

If you’ve never seen it before, it looks like pavement made out of a big, gray Rice Krispies treat. In technical terms, it is a concrete manufactured without fine aggregate. This creates a void structure in the concrete that allows stormwater to pass through at incredibly high rates — on the order of 500 in. per hour.

As a developer, you’re probably thinking and wondering how many wonderful ways you can use this on your new site. The purpose of this article is to demonstrate some projects that have been permitted and built around the United States and to share some ideas on how you may utilize pervious concrete in your next project.

Permitting
Every developer knows how difficult permitting is. With expensive real estate, tight environmental regulations, and land costs always an issue, pervious concrete is a tool that can make permitting easier.

There are five basic uses that pervious concrete has been permitted for in the United States. These are tree protection, wetlands protection, zoning credits, stormwater runoff reduction, and stormwater treatment. It should be noted that not all of these permitted uses are necessarily available in every city and county in the United States. Each local government will have their own permitting rules and may accept some or all of these uses in their land development code.

Tree Protection
As pervious concrete allows air and water to pass through the pavement surface, it makes sense that it would allow air and water to reach the roots of a tree. Thus, the first basic permitting use of pervious concrete is for tree protection. What that means to a developer is that he may pave under the drip line of a tree. For a developer who wants to build a sustainable site this allows healthy tree cover, to reduce the urban heat island effect. There may also be a financial benefit to the developer who simply doesn’t want to pay for removal and replacement of many new trees on his site.

As an example, a big box retail store was recently built in Hillsborough County, Florida. On the site there were six old oak trees that the developer was not allowed to remove without paying a heavy price. In Hillsborough County, if a developer removes such a tree it must be replaced with new trees at a replacement factor of up to two-to-one. For example, if a developer removed a 60 in. diameter oak tree, he may have to replace it with 120 inches of new trees. This replacement may be twelve 10 in. diameter oak trees, or twenty-four 5 in. diameter oak trees. No matter how you do the math, it’s a lot of trees. The engineer was able to design the parking lots to use pervious concrete within the drip line of each tree and incorporate the trees into the overall landscaping plan for the site.

An example of a land development code with this permitted use can be found at Hillsborough County, Florida in section 4.01.06.10.

Wetlands Protection
Preserving wetlands is crucial to preserving the quality of the environment. To preserve wetlands, many cities and counties have established a setback from the border of the wetlands within which land development activity may not occur.

Installing pervious concrete typically has little impact on the environment; it is often used for nature trails for this very reason. Additionally, pervious concrete may be permitted for use within the wetlands setback due to its low impact on the wetland ecosystem.

As an example, also in Hillsborough County, Florida, the land development code section 4.01.07.4 allows pervious concrete paving within the wetland setback. For a developer with a small site adjacent to a wetland, this can mean the difference between using a large percentage of that site or not.

This was exactly the case for a small drive-through coffee shop. The site was less than one acre and adjacent to a wetland. By being able to pave within the wetland setback, the site was able to create a safe turning radius for the drive-through lanes.

Zoning Credit
To encourage groundwater recharge, and the health of the natural environment, the zoning regulations in many cities and counties now require the site to meet an impervious surface ratio. That means on a developed piece of real estate, the site developer may only cover a percentage of that site with building, pavements, parking lots, and other impervious surfaces. The rest of the site must remain open to allow infiltration of stormwater to recharge the aquifer. Many studies have been done to show that the more pervious area there is on a site, the healthier that will be for the ecosystems and the groundwater supplies.

It is now common for cities and counties to count pervious concrete as pervious area in the impervious to surface ratio calculation for developed sites. Some areas count pervious concrete as 100% pervious, while others may count it as only 75% pervious. The credit given to pervious concrete counting toward pervious area is based on the local soils and their ability to recharge the aquifer. Research suggests, however, that pervious pavements, in general, many recharge more groundwater than a site in its predevelopment condition — this is due to the lack of evapo-transportation losses in the native vegetation and the rapid infiltration rates of pervious concrete pavements.

The city of Titusville, Florida, for example, counts pervious concrete as 75% pervious area when figuring a site’s impervious surface ratio. For a developer, this means that they can develop more  real estate while still recharging groundwater supplies.

Two pharmacies were built across the street from each other in Titusville. Both national chains used pervious concrete for the parking lots to allow these developments to fit on relatively small sites.

Stormwater Runoff
When a site is paved with conventional pavement, during a thunderstorm, the rain will run off the pavement into the drains and swales. From there, the water finds its way into our streams, rivers, and lakes. This doesn’t sound like much of a problem until you consider the tremendous volume of water this can be for a heavy storm, and all of the pollutants that water is carrying. This tremendous volume of polluted stormwater can damage the wildlife in our water bodies and cause massive erosion. It makes sense then that if the pavement is built with a pervious material the water won’t run off but instead drain straight through the pavement to infiltrate groundwater. With pervious concrete, downstream pollution and erosion can be reduced.

A limiting factor to this runoff reduction is the soil on the site. If the site has sandy soils, as is seen in Florida, the runoff reduction can be significant even in a heavy storm. The pavement will transport the water to the site soils, which will rapidly drain the stormwater. Looking at sites with less favorable soils, such as clays or silts, the runoff reduction would be less in heavy storm events if a rock base were not placed beneath the pavement. The rock base can be made to any thickness necessary to detain the designed storm event and reduce or entirely prevent runoff from the paved area.

As an example, the St. John’s River Water Management district in Florida is the permitting authority for stormwater for sites in northeast Florida including Orlando and Jacksonville. Their rules allow the engineer to use a runoff coefficient or curve number for a pervious concrete pavement, to be that of the site soils in their post-construction, compacted state. For the developer, when considering building a site with conventional pavements (and thus high impervious coverage) or pervious concrete pavements, the pervious concrete could reduce the pond size by a third.

The most desirable credit a developer may receive with pervious concrete, though, is the reduction in stormwater treatment volume necessary on site by the use of pervious concrete pavements. To achieve this, the site designer would design the parking lot as a conventional detention system, or as we call it in Florida, a retention system. A few keys to the system include a parking lot that is graded basically flat, curbing that extends from six inches above the top of the pavement to six inches below the bottom of the pavement, and a rock base to handle the design treatment volume. The pavement in combination with the rock base and elevated curb can be designed to handle any storm event.

Depending on the environmental conditions of the site, the permitting authority may require regular maintenance of the pervious concrete pavement to prevent clogging with silts or clays. The neat thing about this system is that the pervious concrete can store the first inch of stormwater within its 6-inch thickness. Additional stormwater treatment volume can be either in a rock base below the pavement, or held above the pavement in the parking lot.

It is suggested to prevent the pavement from being overlaid with an impervious material for the developer to put signage on the site indicating the pavement has a pervious concrete pavement. Further, the developer should have an annual maintenance contract with a local engineering firm to confirm that the pavement is performing as designed.

The Future of Pervious Concrete
Pervious concrete has been in use in the United States for over twenty years. It has only recently become more popular as construction techniques have improved, design methods have been enhanced, and real estate prices have increased to a point to justify its cost.

Currently, pervious concrete is most commonly used for commercial parking lots and some light residential streets. The future of pervious concrete is wider acceptance at the regulatory level, and use in heavier pavement applications such as arterials and highways.

The concrete industry is committed to providing contractors qualified to place pervious concrete. Unfortunately, pervious concrete is not an easy material to handle. Thus, it takes a specialty contractor, with the right equipment, knowledge, and craftsmen to place a pervious concrete pavement.

The National Ready Mixed Concrete Association (NRMCA) will begin certifying pervious concrete craftsmen later this year.  

“The pervious contractor certification is a unique but important program,” explains NRMCA’s Colin Lobo. “It ensures that the contractor placing pervious concrete is knowledgeable of the purpose of the final product and the details associated with its placement and subsequent maintenance needs, many of which are significantly different from conventional pavement placements.”

With growing interest from the development community, regulatory agencies, and departments of transportation, there is much research happening in the world of pervious concrete to better understand the long-term performance, stormwater treatment and mass balance, freeze-thaw behavior, highway noise reduction, and strength and durability characteristics of the material. SLDT