Part 2. Putting pavers to work
Part 1: Choosing products and installation methods
Whether it’s interlocking concrete pavers, drivable grass, or recycled tires, permeable pavement is becoming a best management practice of choice for many seeking to address water-quality concerns. Permeable pavement can also fulfill some Leadership in Energy and Environmental Design (LEED) and other sustainability requirements.
Another benefit: Not only is permeable pavement doing its job with regard to water quality and quantity, but it also is adding aesthetic value to property.
Adding Parking, Not Sewer Connections
The Heroes Gym was recently constructed at the site of an old, run-down YWCA building in South Dallas, TX, in an area targeted for revitalization. Mark Cuban, owner of the National Basketball Association’s Dallas Mavericks and the Heroes Foundation, funded the renovation of building the into a new basketball center for youth.
But the site was not up to code, and to develop the gym, 30 more parking spaces were needed to meet zoning requirements. The only area where parking could be added was in a wooded field without storm sewer connections.
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Photo: O’Donald Engineering
The site of Heroes Gym was originally not up to code, and to develop the gym, 30 more parking spaces were needed to meet zoning requirements. |
Adding the connections would have been cost-prohibitive, says David Greer, an engineer in training for O’Donald Engineering. Additionally, the parking area was uphill from a residential property.
To address both parking and storm sewer concerns, O’Donald Engineering chose Invisible Structures’ Gravelpave2, based on a seminar the company had presented. Gravelpave2 is
manufactured of porous geotextile fabric that is molded to a 1-inch-high integrated ring and grid system. It is placed on top a porous base course, anchored with galvanized anchors, and filled with decorative gravel.
Invisible Structures also makes Grasspave2, a porous system consisting of a sandy gravel base course, Hydrogrow polymer-fertilizer mixture, a ring and grid structure, sharp concrete sand, and grass seed or sod.
“Grasspave2 will disperse the point load of a tire or a fire truck outrigger, take the point load and spread it out into the base course, so it will do a similar effect to what any surface paving will do,” says Dustin Glist of Invisible Structures. “Gravelpave2 will hold aggregate inside the void space in the cylinders and disperse the load to an underlying porous base course.”
Both products remain porous throughout their lifetime, Glist adds.
“The Gravelpave2 is filled with open-graded aggregate uniform in size, such as a small quarter-inch gravel, that remains in the product itself, and when you drive over it, it inhibits rutting and washboarding that you normally see with gravel surface,” he says.
O’Donald Engineering prepared a report for South Dallas, which approved the use of Gravelpave2. “We got our permit, and that saved us from having to put in a storm sewer system and having to go through a lengthy process to get that unit approved by the city,” says Greer. “It saved quite a bit of money.”
Greer points out that because the project was for a nonprofit organization, the budget was tight. Just as the project was underway, the economy started to falter. The budget was reduced to two-thirds of its original amount.
“It would have probably killed the project to have to pay for $200,000 worth of storm sewer that they weren’t expecting,” says Greer. “It could have been a potential deal breaker if we weren’t able to do this.”
Greer says it took about a week to install approximately 7,000 square feet of Gravelpave2.
Glist notes that unlike porous concrete or porous asphalt, there is no special training for installation of Gravelpave2 or Grasspave2. “It would be similar to what a landscape or paving contractor would normally do,” he says.
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Photo: Rehbein Environmental Solutions
Rehbein chose the EPIC system in conjunction with Netlon’s ATS for 0.5 acres of green space that stores and treats water from a 3.75-acre watershed. |
Greer says the most important aspect of the project was using the right type of gravel—a small-diameter stone.
“We used a compacted subgrade and put down what Gravelpave2 specified,” says Greer. “We used an eight-inch sandy gravel base course, which is a typical cross-section for Gravelpave2.”
The soil at the site was a hard-packed clay not conducive for drainage, says Greer. “It helped prove our point in that the existing ground was impermeable to begin with, and when we add this Gravelpave2, it was actually going to improve the drainage conditions instead of making it worse,” he says.
Glist says the product is generally up to 60% of the total installation cost for a given project, with the remaining cost being for the base course, the fill material, and the labor. With Grasspave2, the seeding also accounts for a portion of the cost.
“The porous base course is not going to significantly impact cost, because you would need a base course in any kind of paving of material, whether it be asphalt, concrete, brick pavers, or our products,” says Glist. “The difference is we are very similar to most state Department of Transportation road base specifications, but there are cases where they have too many fines, and in that case we can mix it with a little bit of sand to retain that porosity and still get the strength of a load-bearing base course.”
Greer says he expects minimal maintenance on the parking area. “The owners are aware they need to keep it clean and make sure there’s not a lot of creeping with the gravel,” he says. “We did spec out a concrete border around the parking lot just to help keep all the gravel in place and keep the soil from eroding out.”
Glist notes that while there is a “slight risk” of the system becoming less porous if external sediment from a swale or streambank lining is directed to it, “in general terms, Grasspave2 and Gravelpave2 will not clog in normal conditions.
“Airborne particulates and any tire movement particulates coming into contact will not affect the porosity in any practical sense,” he says.
Because the gym parking lot involved compact car spaces, making it challenging to apply paint striping down on the gravel, O’Donald Engineering put in wheel stops with the compact spaces marked on them so drivers would know the parking space width.
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Photo: Rehbein Environmental Solutions
Installation of the EPIC system and liner |
A challenge inherent to the site was that there were many old-growth oak trees present.
“One of the selling points of the Gravelpave2 was allowing a permeable surface at the drip line of the trees and the canopies. The city arborist informed us it would keep those trees viable and in good health,” says Greer.
After his firm’s first experience with Gravelpave2, Greer anticipates specifying it for future projects.
Glist says Gravelpave2 can handle unlimited amounts of traffic. “The gravel is not affected by the amount of traffic. It needs to be a low speed of about 20 miles per hour or less, so it works well for parking lots, trails, access roads, and golf cart paths, for example, and it also has 5,700 psi compressive strength.
“Grasspave2 is for intermediate to light traffic use,” he continues, “so it’s not for grocery store parking, but it is for church parking and overflow parking, an access road, or a fire lane. Grasspave2 will hold up, but to make it look aesthetically pleasing, grass can only take so much traffic, so we recommend less than five trips per day for Grasspave2. We like to see a load speed under 20 miles per hour.”
Both products also have been independently tested for wheelchair access and can be used for ADA-specific requirements up to a certain slope, Glist says.
He also says that Grasspave2 and Gravelpave2 have held up in cold climates where plowing is often necessary. The recycled high-density polyethylene will flex during freeze-thaw cycles and can be plowed. “We have installations in Alaska and many in Canada,” he notes.
“There is potential for the [plow] blade to catch our product,” says Glist. “If it is caught, it can be repaired, but it can be prevented by skids or raising the plow blade and taking a little extra care. We have installations in every perceivable weather condition and our products have held up.”
Recycled Tires in Florida
Permeable pavement made from recycled tires is getting some attention in North Port, FL, where stormwater manager Elizabeth Wong is assessing its effectiveness through the use of test strips of Flexi-Pave, manufactured by K.B. Industries.
“We take primarily the passenger tire, which is the worst to get rid of because of the amount of steel and core that exists within that tire,” explains K.B. Industries’ CEO, Kevin Bagnall.
Flexi-Pave performs to LEED standards, says Bagnall. “We also are very cognizant of the fact that we have to have a material that made complete sense in the construction industry,” he adds.
North Port had planned to install Flexi-Pave in four parking spaces in a new development. Wong says although Flexi-Pave is installed by manufacturer representatives, “they will certify us if we as a city want to install it ourselves to save money. You have to take a class and make sure you know how to do it correctly.”
Flexi-Pave has no mechanical joints and requires minimum maintenance and a minimum amount of equipment to mix onsite, says Bagnall. The base course needed for Flexi-Pave depends on engineering requirements. “A base course for a trail is not as robust as a base course for a parking lot,” he says.
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Photo: Mark Hadley of W & H Pacific
Stormwater quality is an issue of concern in Oregon to prevent pollutants from running into the streams, and local agencies have strict requirements on water-quality treatment. |
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Photo: Mark Hadley of W & H Pacific
Hadley used asphalt pavement in the center aisle of the parking lot and used the SF-Rima permeable pavement in the parking stalls. |
Bagnall says an example of the type of aggregate used is No. 57, an inch and a half, clean, which would permit the water to infiltrate as fast as it passes through the Flexi-Pave itself. Crushed concrete is another aggregate option. On trails, weed fabric can be laid down with Flexi-Pave placed over it, creating a trail system with minimum impact to the trees, he says. Aggregate options affect installation costs.
Although she has not had enough time to judge the performance of the Flexi-Pave, Wong says it’s showing promise to date. “If you have pollutants such as roadway oil and grease, it will go through this permeable pavement and get seeped into the ground,” she notes. “The ground has a wonderful natural filter for pollutants. The grease is degraded with the naturally occurring bacterial action. The suspended solids hopefully will not get trapped in the pores of the pavement.
“Flexi-Pave manufacturers say if there are fine sediment particles going through it, it will not stay in the pores, but will eventually work itself down into the soil underneath.”
Because Flexi-Pave is made of recycled tires, it’s got a “rubbery feel,” says Wong. “It flexes a little bit so the particles can work themselves out of it downward into underground soil. Porous concrete is still rigid. So if it gets trapped in the pores of the concrete, you’re going to have to physically remove it through vacuuming it. That part doesn’t attract me too much because of maintenance.”
Wong doesn’t believe the Flexi-Pave will require a great deal of maintenance.
“You do have to take care of it. You can’t expect a truck to pour a lot of dirt on it and still expect it to be porous. But with the normal suspended particles in stormwater, I think it can probably handle it.”
Wong sees Flexi-Pave as being most useful for sidewalks or parking spaces, but like most permeable pavement, it’s not suitable for high-velocity traffic. She adds that she doesn’t know how it will handle wear and tear over time.
She sees benefits in using it as sidewalk cover. “I’ve heard when you walk and jog on this, it is really good for your knees,” says Wong. “It’s got a tad bit of give, so it’s not as hard as concrete.”
With respect to its load-bearing capacity, Bagnall says, “While any paver is affected by truck weight, Flexi-Pave has been specified for some fire stations and can handle garbage trucks and even a few loaders.”
As for its effectiveness in cold climates, Bagnall says Flexi-Pave can be hit with a plow and absorb the shock.
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Photo: SF Concrete Technology
Installation of the permeable concrete paving stone system |
Making the Most of Stadium Space
Rehbein Environmental Solutions Inc. (RESI) was called upon to provide a solution to the challenge of finding a sustainable way to manage stormwater within usable green space last spring at the TCF Bank Stadium on the University of Minnesota’s East-Bank campus.
There were several inherent challenges. For one, it was a small project site within a highly developed urban area. Little space was available to meet stormwater demands, and the green space that was available needed to be maximized. The solution had to be both environmentally and economically viable.
Rehbein chose the Environmental Passive Integrated Chamber (EPIC) system in conjunction with Netlon’s Advanced Turf System (ATS) for 0.5 acres of green space that stores and treats water from a 3.75-acre watershed.
“The EPIC system is a non-clogging drainage and subsurface irrigation technology that utilizes passive, natural processes of water movement to manage and direct water resources,” says Mark Apfelbacher of Rehbein. “EPIC systems reduce water pollution, decrease irrigation water use by 50% to 85%, and manage massive stormwater volumes using living systems of green space to treat and reuse water.”
Netlon ATS is constructed of non-UV-treated polyethylene, which will break down quickly if exposed to sunlight, says Apfelbacher.
The EPIC system, constructed of recycled polypropylene, is composed of three elements: a liner pan, the injection-molded EPIC chamber, and the sand fill that covers and surrounds the first two components. Water flows throughout the chamber through holes, and the liner pan catches and retains the water.
The system uses capillary attraction to provide subsurface irrigation and drainage, consuming less water than traditional surface or drip irrigation systems. Stormwater surge or runoff is retained in the system for reuse or slowly released in a controlled manner. Additionally, greywater may be used for irrigation, which reduces or eliminates the need for a freshwater irrigation source.
The Netlon ATS consists of 2-inch by 4-inch polypropylene mesh pieces blended into the top layer of soil to bio-mimic vegetative roots. The pieces are either machine-blended or manually mixed into the top soil layer.
The system offers the turf surface “memory” properties that allow it to spring back into shape after heavy or repetitive use. The system is resistant to ripping and tearing and can support heavy vehicles without developing ruts.
The EPIC system and Netlon ATS must be installed either by a RESI-approved installer or through guidance by a RESI employee, says Apfelbacher.
“We typically use local contractors and train them on the job,” he says. “Once one installation is completed and checked, a contractor is typically RESI-approved.”
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Photo: SF Concrete Technology
A central bioswale acts as a backup system to the pavers during large storms. |
Typical maintenance needed with turf grass and landscaping can be accomplished without changes in practices, Apfelbacher says. “It will need deep aeration every few months due to vigorous vegetative growth,” he adds. “The EPIC system maintains moisture in the soil. It only needs water when the plants evapotranspirate the water in the soil.”
Moisture sensors are typically used to dose water through the system and allow for the moisture to wick up via capillary rise to the desired moisture percentage, says Apfelbacher, adding that this can be accomplished with simple monitoring systems.
As for plowing Netlon in cold climates, the plow will need to be on skids so as to prevent scalping of the turf crown, says Apfelbacher, adding that NetPave 50 is a more ideal solution for sites requiring frequent plowing.
The EPIC system and Netlon ATS were used within the landscape plaza outside the stadium, eliminating the need for a traditional stormwater pond and creating usable green space capable of supporting heavy vehicles. The area treats more than 3.5 inches of rainfall in 24 hours.
Properly installed, the load-bearing capacity is greater than H-20 loading, says Apfelbacher. “Eight inches of Netlon ATS has a load bearing capacity of more than 80,000 pounds,” he says. “Netlon ATS has a one-to-one ratio to load bearing capacity of road base gravel. If you need six inches of road base for typical vehicle transportation, you need six inches of Netlon.”
The EPIC system provides not only subsurface irrigation but also stormwater quality and quantity management, removing 80% total suspended solids, 85% total phosphorous, and more than 60% of nitrates.
Apfelbacher explains that when receiving rainwater runoff, the system filters water through aggregate materials, naturally cools it through a geothermal process, and detains it in the void spaces of the aggregate profile.
“Living systems of vegetation and microorganisms clean and reuse water though physical uptake and biological breakdown of pollutants,” he says. “Much of our current work in temperate climates revolve around stormwater management.”
Apfelbacher says system “attains the ultimate goal of recycling in that the products are totally reusable without any intermediary processes. By design, the components are not glued, are non-pressurized, are protected from UV degradation, and are completely reusable.” For instance, RESI has relocated plastic and natural components that have been established more than 10 years to be reused in another location.
“The plastic components can be extracted and reused, or shredded and ground for traditional plastic recycling methods,” says Apfelbacher. “The environmental equation of total energy and human power used in any recycling process is rarely analyzed as a whole.
“In efficient recycling systems, the energy total positively impacts environmental goals; in other recycling systems, there is more energy lost than are positive environmental gains.”
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Photo: SF Concrete Technology
Joints are filled with gravel. The base layer is about four inches thick. |
He adds that the natural components are the living system that provide the water treatment, retention, and increased irrigation efficiencies.
“The primary components by weight are sand and gravel; these products are always locally obtained,” he says. “If the EPIC system is recycled, the sand and gravel materials can be reused as fill or grading materials. Surface vegetation and roots are compostable in traditional methods.”
He says there are no concerns about pollutants reaching the groundwater if large amounts of runoff are being infiltrated. “We use living systems and basic sand filter technology,” he says. “This is not new science; nature has been doing this for eons.”
Rehbein Environmental Solutions also uses other technologies, including NetPave 50 Porous Pavers from Conwed, manufactured from recycled plastic, and green roof systems.
Protecting an Oregon Stream
The vast majority of Oregon rainstorms are low in volume and long in duration.
When Mark Hadley of W & H Pacific, a landscape architecture and engineering firm, was asked to design the 32-acre Tualatin Hills Park and Recreation District-Portland Community College Rock Creek Recreational Facility in Portland Community College with sustainable elements, he had to take into consideration a creek that ran along the western edge of the property. Stormwater runoff needed to be treated or eliminated before it entered that creek.
Stormwater quality is an issue of concern in Oregon to prevent pollutants from running into the streams, Hadley notes, and local agencies have strict requirements on water-quality treatment.
For the 275 parking-bay areas, Hadley chose to use the SF-Rima system, inspired by observing the system widely in use in Europe. “As a landscape architect and designer, I took a special interest in looking at how European communities have incorporated sustainable stormwater management using permeable pavers,” he notes.
“I spent a couple of weeks in the Lake Lucerne area. They have big water-quality issues, and at a lot of the marinas and apartment complexes around the lake; permeable pavements were used everywhere. The most predominant paver for new construction and contemporary applications was the SF-Rima paver.”
The system is from SF Concrete Technology, which has a patented permeable concrete paving stone system and licenses US companies to make the products, including SF-Rima permeable pavements, VS5-Eco, VS5-Drain, SF Eco-Duct, and SF Matoro-Drain.
Helga Piro, owner of SF Concrete Technology, says the products should be installed by certified installers; such training is provided by the Interlocking Concrete Pavement Institute (ICPI).
SF-Rima is the most popular of the company’s products, says Piro. It can be installed in two different ways. For water drainage, it is ideally installed mechanically in close assembly with joints filled with gravel and spacer touching stone with a stone requirement of 22.8 stones per meter. Applications include low-trafficked roads, driveways, industrial and commercial parking lots, sidewalks and patios.
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| Parking for Florida’s Ferndale Preserve consists of 26,000 square feet of a plantable concrete system. |
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| Drivable Grass conforms to irregular ground surface contours and allows roots to penetrate to the soil below. |
For turf growth, the product is installed with larger joints—spacer on spacer—and filled with a soil-gravel mix.
The base course consists of existing soil, a subgrade, and a sub-base of 6 to 8 inches with No. 2 aggregate. The base layer is about 4 inches thick with No. 57 aggregate. The bedding consists of No. 8 aggregate and is normally 1 to 1.5 inches thick.
While SF Concrete Technology can take “quite a bit of a traffic load,” Piro says her company does not recommend it as pavement for ongoing heavy traffic.
As for maintenance, Piro says that while the base layer might need to be replaced eventually, if the pavement is kept clean and free of dirt and debris through vacuuming, it should sustain itself. The pavement can be plowed in cold climates, she adds.
The system permits water to pass down to the voids in a sub base reservoir layer of crushed, open-graded stones. Water is held until it seeps into the soil below.
To accommodate load capacity issues that would occur because buses would frequently be parked at the facility, Hadley used asphalt pavement in the center aisle of the parking lot and used the SF-Rima permeable pavement in the parking stalls.
To accommodate runoff from larger storms, a bioswale in the center of the parking lot acts as a backup system to the pavers, drawing excess water and allowing filtration. Catch basins also have been incorporated. The project was completed over a year ago.
Hadley says one of the positive attributes of the SF-Rima system is that it has a high percentage of openings between the pavers to allow the water to flow through. He used 16 inches of an open-graded drain rock as a reservoir that will hold the infiltrated water. The next 2 inches contain a finishing stone to get a leveling course with no fines, allowing the water to flow through. Pavers were laid atop that. Crushed gravel was placed between the stones.
The SF-Rima was installed by machine in a way that was “fairly efficient and cost-effective,” says Hadley.
“One of the downsides of permeable pavement is that over time, silt and small particles will get in and clog up the drainage,” says Hadley. “My observations of this product was that it probably had the least chance of plugging up over time, because it had one of the highest percentages of openings compared to other products.”
Hadley also viewed the pavers as an environmental educational tool for the public. He incorporated some onsite information that explains how water can flow through the joints between the pavers.
“It does have a nice look,” he adds. “It clearly identifies the parking area from the drive aisle. You can get them with color additives, but we used a standard gray concrete color. It’s a nice contrast to the asphalt and has a very neat and orderly appearance.”
Additionally, the system is expected to meet the sustainability requirements.
“The parking lot is permeable on a one-to-one ratio,” says Hadley. “Every square foot of permeable pavement reduces pollutants by the same square footage. We’ve greatly reduced the runoff from the site by using these. We’ve used the permeable pavement from a water-quality standpoint as well as a water-quantity standpoint.
“By reducing the quantity of runoff as well as by infiltrating that water, we’ve also minimized the water-quality detriment going into the stream.”
Hadley anticipates he will continue to use permeable pavements in future projects, especially as it relates to such factors as LEED certifications.
“Permeable pavements are here to stay and will only increase in our development projects,” he says.
Durable Parking at Ferndale Preserve
Ferndale Preserve, a 192-acre park in Ferndale, FL, on the site of a former orange grove was recently restored as a long-leaf pine and wiregrass ecosystem, featuring hiking, equestrian, and multipurpose trails that offer vistas of Lake Apopka.
One of the goals of Lake County officials was to establish grass in the parking area, which was surfaced with 26,000 square feet of Soil Retentions’ Drivable Grass, a plantable concrete system.
The concrete-based flexible and permeable pavement product allows stormwater to filter through into the subsurface. That’s important in Florida, where stormwater can be retained during the state’s dry season and accommodate the state’s high water table needs.
Drivable Grass is designed to flex and conform to irregular ground surface contours along predefined linear grooves. Root penetration takes place through pre-manufactured holes and cracks down into the subgrade soil. The mat’s porous nature allows moisture to filter into the underlying soil, increasing onsite stormwater storage and minimizing offsite water flow.
Tom Eicher, Lake County’s manager of parks and trails, notes the Drivable Grass installation on the lot went smoothly. The parking lot was graded and the base material was laid down.
The Drivable Grass mats were placed on top and measures 2 feet by 4 feet, weighing about 95 pounds apiece. Soil Retention has since switched to a 2-foot-square mat, primarily because of the weight, Eicher notes.
Eicher, who describes the product as having the appearance of an upside-down cupcake tin, says the water seeps through the holes of what would be considered the raised portion of the tin. Grass grows in-between the “muffins.”
Drivable Grass allows contaminants, such as vehicle drippings, to filter through the aggregates installed below the product. Lake County had previously tested Drivable Grass with fire trucks as the new park would be hosting horse trailers with lots of weight.
The park features a few small retention ponds, but Eicher says they exist primarily to help collect some of the runoff from the concrete driveway. Drivable Grass helps create a pervious area for stormwater.
“This was the only option that really made sense,” says Eicher. “The area getting into the park is relatively soft sand, and if we did anything other than asphalt, people would be getting stuck and there would be problems.”
Eicher notes the project stayed within budget and the product will be considered for future use. Drivable Grass requires no other maintenance than to mow the grass, he adds.
Additional Options
Other options in permeable pavement systems abound, such as Cellular Concrete.
“The success of permeable pavers in controlling, capturing, containing, treating, and infiltrating runoff from stormwater and snowmelt is dependent on many factors,” says Phill Domask of Cellular Concrete. “Key to success is the design and successful installation of the sub-base and subgrade/recharge bed.”
He points out that innovation in foam liquid concentrate technology provides an engineered geotechnical fill solution for improving stormwater runoff detention in situations where permeable pavements are used. Pervious forms of cellular lightweight concrete (PCLWC) can be used for the sub-base.
“PCLWC technology improves runoff detention, providing more detention capacity than typical granular pervious fill solutions,” says Domask.
He says that special sub-grade design provisions are required in the design of parking lot and other permeable paver applications for areas with native soils containing significant amounts of clay, silts of high compressibility, muck, and expansive soils.
“Highly organic materials must be excavated and replaced, either with soils containing high amounts of coarser fill material or with PCLWC,” he says. “A permeable, open-cell, lightweight concrete fill, PCLWC stabilizes soil without disturbing or redirecting natural water flow.”
Domask points out that typical granular pervious fill materials have a detention capacity of about 2.4 gallons per cubic foot, whereas PCLWC has a detention capacity of 4.8 gallons per cubic foot (641.3 liters per cubic meter).
“The higher detention capacity of PCLWC is especially useful in permeable pavement project situations faced with high site water tables,” says Domask.
“As communities struggle to strike a balance between economic growth and environmental issues, innovative construction materials can provide effective solutions for improving stormwater runoff detention and support more sustainable development.”
Part 1: Choosing products and installation methods