A Permeable Package

Sept. 23, 2014

The use of permeable and porous paving has dramatically increased in the last decade. Municipalities around the country now accept, and even specify, these products in both new and retrofit projects. The capability to provide runoff management with a pavement or pavers can save space and sometimes installation time, and it allows projects to meet stormwater requirements in an effective manner.

Many products are available for whatever projects you are working on. Are you working up a proposal for a new parking lot or retrofitting alleys and walkways? Do you need to meet runoff regulations and have no space for retention ponds or bioswales? Permeable, pervious, or porous materials could fit the bill.

Know Your P’s
Mark Walker of Kuert Concrete, in South Bend, IN, has a mission to educate industry professionals on the difference between the three P’s–pervious, permeable, and porous. He finds that many use the terms interchangeably, even though they refer to different products.

While the goal of limiting stormwater runoff is the same for all three P’s, the methods are different. Permeable pavers are made of concrete or clay and depend on aggregate placed between the blocks to provide the infiltration. Porous pavers use grids to hold aggregate or vegetation in place, which allows the water to soak into the subbase. Pervious pavers have voids interlaced through the bonded stone material so that water percolates through the paver, through base courses of rock, and into the soil.

Credit: Pavestone
Eco-Prioria pavers installed at Peterson Air Force Base

Iowa Lofts
Ingersoll Square, apartment lofts in Des Moines, IA, had an issue with stormwater runoff exacerbated by its downtown location and nearby impervious surfaces. A new parking lot was specified to include permeable surfaces, and the Polk County Soil and Water District assisted with outside funding to ensure an improvement in water quality.

The product chosen was the PaveDrain system. The system consists of Permeable Articulating Concrete Block/Mats, which are installed using construction equipment. Each block interlocks with six additional units, providing strength and stability in lateral and vertical directions. The contractor for installation was Corell Contractor Inc. of West Des Moines, IA, and the project manager for the contractor was Aaron Carlson, P.E.

A geotextile was installed on top of the subgrade and the base was built up with 18-inches of rock to provide drainage. The 1/4-inch joint between the PaveDrain blocks allows water to infiltrate into the sub-base while still providing a smooth surface. Sediment and organic material is washed down with the stormwater and it breaks down in the rock layer. The mats are placed over the rock and forced into place with a bump bar. The block has 7% open space in the surface and 20% in the storage area, allowing good infiltration, at an average rate of 4,306 inches per hour according to an independent test.

Carlson says the only significant installation challenge was restricted space on the jobsite. Because delivery trucks could not get close enough for direct placement, the mats had to be unloaded and handled multiple times. Fortunately, the modularity of the system, detailed numbering, and efficient scheduling meant that the installation was accomplished fairly easily.

Maintenance for the parking lot includes snowplowing with a rubber-tipped blade. Steve Krehbiel, with Quick Supply Company, is the local distribution representative for PaveDrain. He says, “Bring on the freezing and thawing; the system is made to flex or articulate.” While conventional asphalt or concrete tends to crack, the PaveDrain system will not.

Routine maintenance of the surface should be accomplished using the PaveDrain VAC head for sanitation trucks. The head squirts water into the joints and sucks the debris out. For extreme maintenance, the mattresses can be lifted out so that the underlying rock can be cleaned or replaced. Then the mats are reinstalled. Usually removal of only a few mats is necessary, but this option can improve the lifetime of the entire system.

Carlson says the mats were as easy to install as the company reps had claimed, and their help on the project was invaluable.

Credit: Pine Hall Brick
StormPave pavers were installed at several sites on the Wake Forest University campus.

Colorado Project
The Odell Brewing Company in Fort Collins, CO, planned an expansion of its existing facility that would include a large building and parking lot. One of the challenges for the project was that the building and parking lot would occupy a spot that was previously open detention pond space. Permeable interlocking concrete pavers (PICP) were the best solution for providing detention and water-quality treatment in a limited space. The project also incorporated dry wells, rain gardens, and bioretention or infiltration ponds.

The soil contained upper clays with lower sands and gravels. Pit run material was worked into the subgrade to stabilize it. After the site was graded, concrete curbs were added as well as interior parking islands with dry wells. A geotextile fabric was installed over the subgrade and ASTM #2 rock was spread over it to a minimum depth of 12 inches. The sub-base was compacted with a vibratory drum roller. The base course of 4 inches of ASTM #57 was placed next and compacted. The bedding course is 1/4- to 3/8-inch washed, fractured, open-graded stone smoothed into place. The rock courses are single-sized aggregate.

Pavers were placed by hand in a herringbone pattern to provide stability and longevity. Void filler is the same rock as the bedding course. The pavers used were Aqua-Bric supplied by Western Operations of Borgert Products Inc. based in Denver, CO. The multicolored Autumn Blend provides a touch of color among the plants included in the landscape.

Vice president of Northern Engineering Nick Haws, P.E., LEED-AP, who was involved in this project, says installation costs have run about $10 to $12 per square foot, which is three or four times the cost of asphalt or concrete in his region. However, he points out that this is the installation cost only and does not include costs for maintenance, which are typically much higher for traditional pavements.

The project held up well to a snowstorm, showing that snowmelt passed through the aggregate in the paver joints. A severe test of heavy rain and small hail occurred in September 2012. During the rainstorm, the parking lot had some ponding, but it had completely infiltrated in 14 hours and the parking lot was dry.

Permeable Street for Air Force Base
A street on Peterson Air Force Base near Colorado Springs, CO, suffered from a storm sewer system at capacity so that stormwater partly drained into a nearby creek. Older warehouse-type buildings in the area had downspouts that drained into the street during the infrequent but often heavy thunderstorms. Because of space constraints, a detention pond was not a viable option, and a complete replacement of the storm sewer system was too expensive. A less costly but effective solution was needed–PICP.

Fred Brooks, design engineer for the project, chose Eco-Priora pavers, manufactured by PaveStone Company of Colorado Springs, licensed by Uni-Group USA. LEED certification was crucial for base approval the PICP earned points toward certification.

Asphalt was torn up in a strip between parking spaces and the existing roadbed. The subgrade is clay, and a woven geotextile fabric was placed over this material. Sub-base material of ASTM #2 stone was installed 11 inches thick and overlaid with a 4-inch thick layer of ASTM #57 stone. Next was a 2-inch bed of #89 stone topped by the Eco-Priora pavers in a herringbone pattern. The pavers are 3-1/8-inch thick, and have an interlocking joint and a micro-chamfered top. The interlock provides strength to prevent lateral and vertical displacement even under load, and the chamfered top allows for smooth passage of wheelchairs. The pavers can handle flow rates up to 100 inches per hour. The Eco-Priora pavers were provided by distributor Mike Midyett, general manager of Pavestone of the Rocky Mountain Region.

Engineers are using the Peterson Air Force Base project as a test case. One section included a drainpipe at subgrade and another section does not have the pipe. Monitoring the two sections will show whether the pipe is necessary.

Brooks notes that the product has performed well under snowmelt conditions, and he is already planning more projects using Eco-Priora pavers.

Credit: City of Chicago
Pavers installed in the center of an alley

University Heritage
For the past two summers, Wake Forest University of Winston-Salem, NC, has worked on an extensive project, installing Pine Hall Brick’s StormPave permeable pavers on several sites around campus. Part of the reason for this project is to meet the city’s stormwater mitigation requirements. Another reason for choosing StormPave was to match the university’s red-brick architectural heritage.

According to Ryan F. Swanson, university architect, the installation was centered on Farrell Hall and Wait Chapel parts of campus. The StormPave pavers were also placed in a 24-foot plaza on the east side of Farrell Hall, which is the center for the university’s school of business. The plaza encircles most of the building and stretches out to a nearby road. Sidewalks were built to reach North Dining Hall and the newly built Dogwood and Magnolia residence halls. A walking path also allows students to pass from a parking area to Wait Chapel.

Other stormwater measures in addition to the permeable pavers were used in this project. Three bioswales were built to slow water flow from the campus into Lake Katherine and Silas Creek. The bioswales were constructed over a surface of crushed rock. Two large concrete cisterns, capable of holding 110,000 gallons, were built to collect rainwater and then dissipate it at a slower pace. Smaller cisterns were also constructed at different points on the campus.

StormPave pavers are clay blocks designed to be placed 3/8 inch apart to fulfill the requirements of the Americans with Disabilities Act, allowing for easy wheelchair passage. A variety of colors is available, so matching with the architecture of Wake Forest University was easily accomplished. The pavers are installed over a sub-base course of 1 1/2- to 3-inch cleaned stone; a 4-inch-deep base course of 3/4-inch stone (#57 aggregate); and a 2-inch bedding course of 1/4- to 3/8-inch stone (#89 aggregate). The #89 aggregate is also placed in the voids between the pavers. Water infiltrates through the aggregate in the joints and is captured in the lower levels of stone.

Reneé Lawson of Pine Hall Brick says that the cost of materials for permeable pavers is about 10 to 20% higher than for typical flexible base clay pavers. Installation costs vary, depending on the experience of the installers.

Most industry specialists agree that the increased materials costs are balanced by meeting stormwater requirements and by lower maintenance costs.

Cleaning porous concrete

The work was scheduled to be done while students were on summer break, leading to less interruption of campus life. One challenge on the project, besides meeting the more stringent stormwater requirements, was the limited space. By using a combination of permeable pavers, cisterns, and bioswales, the university is able to provide a great aesthetic and efficient stormwater management system.

To determine when maintenance of s permeable paver system is needed, Lawson suggests watching out for unwanted sediment that winds up in nearby beds of mulch or soil. If these appear, maintenance on the permeable pavers is required. The pavers should also be inspected after heavy storms to see if sediment has been tracked over the surface, and surface cleaning should be done before the sediment migrates into the joints.

Swanson wrote a post for the university’s website explaining to students the mechanism of the pavers and why they were necessary to meet stormwater requirements. The message will carry over when the students have left college and perhaps lead them to recognize the work of stormwater specialists in the future.

A Pioneer of Permeable Paving Projects
Bill Schneider, owner of Advancement Pavement Technology and Aqua Paving Construction, has pioneered the use of permeable paving techniques. More than 12 years ago, he installed some of the first permeable pavers used in the United States. In March 2014, he was awarded the first Lifetime Achievement Award by the Interlocking Concrete Pavement Institute for his work in the field.

One project that Schneider and his companies completed in 2008 was at Elmhurst College in Elmhurst, IL. A new dormitory was being built on the campus, but limited lot lines provided a challenge when designing the parking area. Asphalt would provide too much runoff into the neighboring areas, which included a cemetery and older homes across the street. Nearby homeowners had experienced water in their basements because of runoff from the site even before the parking lot was built. Space was not available for a retention pond.

Jay Womack, director of sustainable design with Wight & Company, had worked with PICP on other projects and recommended it for this one as well. Advanced Pavement Technology was the choice for products, including the Bio Aquifer Storm System, patented by Schneider.

Credit: Sustainable Paving Systems
The site for the waterfront rinse station, above, and the aggregate-filled surface

The site was excavated down 6 to 8 feet and a sub-base was filled with 3-inch crushed rock with no fines. Then 4 inches of base rock was installed, using 3/4-inch-diameter rock. The sub-base and base were compacted. Permeable pavers were installed in a herringbone pattern, using a mechanical installer developed by Schneider. The use of the installer can cut the installation costs by half. A single pipe runs through the site to handle emergency overflow.

Bruce Mather, executive director for facilities management, says the college needed to maximize parking, so placing stormwater detention below the lot was the best option and the pavers were a good way to accomplish that.

Maintenance challenges also arose. Mather says, “We struggled at first finding a method to remove cigarette butts from between the pavers without vacuuming up all of the granite chips [between the pavers]. We eventually found that using a leaf blower would remove the butts and leave the granite in place.” Crews also found that a beet-juice-based deicer works well and doesn’t stain the pavers. If snow removal is needed, crews plow the lot with a rubber-tipped blade.

The lot is now almost seven years old and has held up extremely well, even under heavy traffic at the construction stage. The college is planning to replace all asphalt parking lots with PICP.

Chicago Alleys
Chicago instituted its Green Alley program as part of the Streetscapes and Sustainable Design effort, with the goal of improving commercial areas, neighborhoods, and public places while also strengthening infrastructure. City engineer Ibrahim Hadzic says the Green Alley program involves reconstruction of alleys and resolution of drainage or flooding issues.

The first project was undertaken in 2001. Plastic pavers filled with gravel were used originally, but they failed. From 2001 to 2007, different designs and materials were tested. Since 2007, the city has incorporated bottomless catch basins, pervious pavers, porous concrete, and asphalt and has redone 25 to 30 alleys each year.

Porous concrete had been used on most of the projects; it did allow infiltration of stormwater, but cleaning was a challenge. The porous concrete often became clogged, and then the drainage issues were back. Engineers have found that they save time and work by installing a 1- to 2-foot-wide strip of Xeripave pervious pavers down the middle of the porous concrete or asphalt alleys. The pavers have a high infiltration rate, measured at 1 gallon per second per square foot, helping resolve drainage issues. The city requires detention beneath the pavers, so trenches were dug and filled with stone.

Hadzic says that the cost of the pavers and installation is about 20% less than that of some other products. The materials costs are higher, but that it is mitigated by other expenses that are lower. The fact that they city had to replace only a small strip of the alleys instead of repaving the whole surface led to substantial savings. The installation of pavers is less labor intensive than some other methods and can be completed in a shorter time so that the alleys can be reopened to traffic. Overall costs will be lower because additional stormwater retention or treatment ponds or added pipes are not needed.

According to Hadzic, the pervious pavers are extremely environmentally friendly. If necessary, they can be removed and reused. Some other products must be excavated and recycled by crushing, then new paving must be installed.

The pavers are especially useful where sewer connections are not required or when the existing water table is close to the surface. Their characteristics mitigate flooding problems that can occur. The combination of catch basins, porous pavement, and permeable pavers provides higher infiltration into the soil and reduces the amount of construction excavation.

Maintenance for the alleyways will include surface vacuum cleaning. Hadzic said the city has instigated a program using a combination of a vacuum and pressurized water. The process has worked well on the pervious pavers and the porous concrete.

In Chicago, snow and ice can be difficult; however, plowing with rubber tipped blades works well with the pavers. Mark Walker, director of business development with Kuert Concrete in South Bend, IN, a distributor for pavers used in the Chicago Green Alley program, says the pavers have a void content of 35 to 39%. Because water expands 9% when it freezes, there is plenty of space left in the paver, so the interior is unscathed by ice. He notes that the only time he finds ice in the pavers is when the base was poorly constructed. The aggregate base must accept water as fast as it enters from the pavers. Then there is no standing water and the pavers are fully drained into the underlying rock.

Finished Grasscrete rinse station

Boy Scout Camp
Camp Bud Schiele in western North Carolina provides camping experiences for up to 3,000 Boy Scouts each summer. The Piedmont Council Boy Scouts staff wanted a design to improve the waterfront rinse station. The area would have to withstand constant water use during the seven to eight weeks of camp and have the ability to be drained for the winter.

The product chosen was Grasscrete, manufactured by Sustainable Paving Systems based in Granite Bay, CA. Grasscrete is a different type of product than the previous examples in this article. It is a pervious concrete. The product is made by pouring concrete over “formers,” molds that leave voids in the concrete. Once the concrete has set, the molds are removed and the voids are filled with a variety of materials, including grass, other vegetation, or small rock. The infiltration rate for Grasscrete is similar to that of a typical lawn. The final installation is strong enough to support vehicles up to 65,000 pounds.

The contractor for the Camp Bud Schiele project was Carolina Bomanite Corporation of Charlotte, NC. In an area of 250 square feet, a base course of rock was installed and water lines were placed for the showers and rinse stations. The formers were put into place and the concrete was poured. Once the concrete was set, the molds were removed. A small aggregate was placed into the voids, which allowed complete drainage and easy-to-walk-on surface, even with bare feet.

Maintenance Critical to Success
As with most efforts in the field of stormwater management, maintenance of permeable paving systems is critical to the success of the project. Once design and installation challenges have been met, a schedule of maintenance needs to be conceived and kept.

A project undertaken by Gerry Kesselring of Contract Sweepers and Equipment of Columbus, OH, was challenging. The design of a parking lot had not taken into account runoff from the adjacent farmland. The parking lot was constructed with asphalt drives and pervious concrete parking stalls. Significant rainfall caused standing water on the site several inches deep.

When Kesselring was called in, he found that the pervious concrete was plugged and especially clogged along the edge by the farmland. The solution was what Kesselring calls the rejuvenation process. Using a regenerative air sweeper manufactured by Tymco, the operator sweeps the entire surface of the parking lot. The sweeper not only picks up surface dirt but also forces a jet of air into voids to dislodge debris, which is then vacuumed into the hopper. The areas that were most heavily clogged were cleaned repeatedly with the sweeper forcing air down into the spaces in the pervious concrete and removing the dirt. The crew performed pre- and post-cleaning tests to ensure success. After extensive cleaning, the parking lot was able to accept 600 inches of rain an hour with no ponding, including in what had been the most clogged section.

To keep the pervious concrete of this lot working as it should, a monthly sweeping program was started. Using a regenerative air sweeper, the operator must ensure that 100% of the pervious surface is swept. As Kesselring notes, “Even if the lot appears clean, it is necessary for the blast of air from the regenerative air sweeping process to get down into the pavement.”

Weather-related maintenance also had to be addressed. Grit and sand should not be used on the lot. Liquids for preventing snow and ice buildup are a better choice. This is not always possible when rain turns into snow, but when that event occurs, extra sweeping should take place after the weather has warmed.

Bright Future
The future is extremely bright for all types of permeable pavers and paving. The ability to combine pavement with stormwater retention and treatment lends an effective solution to situations with limited space or retrofitting of older properties. The overall cost savings of not having to construct retention ponds or other stormwater measures make these systems attractive to planners and designers.

With the varieties of permeable materials available, the right combination can be found for any project. A wide choice of colors can be used to match different styles of architecture and landscapes.

About the Author

Roberta Baxter

Author Roberta Baxter specializes in science and technology topics.

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