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The Double Life of Porous Pavers - Curb Appeal with Function

By Dustin Glist, Invisible Structures, Inc.








The days of slapping concrete and asphalt down to make a paved trail walkway, or driveway may be numbered. Commercial properties, public spaces, and residential developments have upped the standard on what they require from a paved surface. Land restrictions, stormwater management, and other environmental regulations have turned the growing housing market and other land developers to seek solutions that can address these issues.






Porous pavement and permeable pavement surfaces temporarily store rainwater runoff before infiltrating into the subsoil. This porous surface replaces traditional pavement, allowing rainwater that would normally runoff into rivers and streams, to infiltrate directly into the soil and receive water quality treatment. Hydrocarbons from cars, pet waste, nitrogen, and other pollutants are filtered and removed by bioremediation, before polluting the ground water.


Stormwater Solutions

Discerning designers, homeowners, and municipalities have turned to Landscape Contractors for information on how to provide attractive surfacing for walkways, pedestrian areas and driveways while addressing stormwater issues. Many stormwater problems can be mitigated with porous and permeable pavers.

Within the last year, Mark Tiernan, with Fred Adams Paving in Morrisville, N.C., has seen an increase in developers turning to porous pavers. "A big house on a small lot has to account for stormwater runoff," says Tiernan. "The impermeable roof line produces a lot of water runoff." Fred Adams Paving offers several solutions to address the needs of stormwater and provide paving - Grasspave2 and Gravelpave2, manufactured by Invisible Structures, Inc., Golden, Colo. and permeable interlocking concrete pavers like Eco-Stone and Ecoloc by Uni-Group USA.

Porous pavement and permeable pavement surfaces temporarily store rainwater runoff before infiltrating into the subsoil. This porous surface replaces traditional pavement, allowing rainwater that would normally runoff into rivers and streams, to infiltrate directly into the soil and receive water quality treatment. Hydrocarbons from cars, pet waste, nitrogen, and other pollutants are filtered and removed by bioremediation, before polluting the ground water. Asphalt, concrete or other impermeable surfaces would otherwise carry these pollutants to storm sewers, streams, and reservoirs.






Workers easily spread the gravel fill inside a gravel porous paver installation at a horse and pedestrian walkway. According to park officials, the installation has reduced monthly maintenance on the walkway by 80 percent.


Permeable interlocking concrete pavers (PICP) are similar to traditional pavers contractors have installed for decades. Interlocking shapes, notched corners, or enlarged joints between PICPs allow for water infiltration. These void spaces can provide enough permeability to significantly reduce stormwater runoff. Manufacturers of these pavers provide a wide variety of colors.

Grass and gravel flexible porous pavements are made up of plastic cylinders connected on a grid system to provide a flexible and structural, load-bearing surfaces. A grass porous paver, as the name suggests, has a full grass-covered surface and looks no different than a normal turf area or lawn. A gravel porous paver has a filter fabric backing attached to the grid and can be filled with a variety of aggregates to integrate with other landscaping. These pavers are extremely strong and have a large amount of void space for almost no stormwater runoff. Unsupported grass areas will compact and rut under the pressure of an automobile. Unsupported gravel drives and roads do not remain porous when driven upon and eventually cause runoff similar to asphalt or concrete. Flexible porous pavers and permeable interlocking concrete pavers transfer the automobile load to the base course underneath.

Landscape Contractors have recognized the many benefits of installing porous paving. "One of the main reasons is curb appeal," Tiernan adds. "The homeowner or the developers want the house to look good." A grass driveway, a rustic gravel driveway, or an attractive pattern of paver can add the finishing touch to a custom home looking to stand out. Flexible grass pavers can add a park-like or pastoral feel to many areas normally needing hard paving.






Workers fill and compact the path with a 1/4'' aggregate. The porous paver grid is provided in large rolls to make for a relatively quick installation. A gravel porous paver has a filter fabric backing attached to the grid and can be filled with a variety of aggregates to integrate with other landscaping.


Flexible gravel porous pavers can have any number of colorful aggregates to add a rustic sense and attractive appeal to a path or driveway. PICPs come in a variety of colors and patterns to enhance and complement a site.

"Builders are also looking for long-term benefits - mainly long-term durability." Tiernan says. Tiernan is referring to the cracks associated with aging concrete and asphalt, making them less attractive and shortening the life span. Asphalt will need to be re-surfaced in eight to fifteen years and concrete has a life span of 20 - 25 years. Both asphalt and concrete can expect to have cracking occurring within the first year. Flexible grass pavers and flexible gravel porous pavers have life spans of 60 years and 25 year, respectively. Permeable concrete pavers have a lifespan of 20-25 years.






Before and after: Grass and gravel flexible porous pavements are made up of plastic cylinders connected on a grid system to provide a flexible and structural, load-bearing surface. A grass porous paver has a full grass-covered surface and looks no different than a normal turf area or lawn. A grass pave system requires mowing, fertilization and water, but does not need to be aerated as the rings and sand combination will prevent compaction and allow for air infiltration.









Environmentally Friendly

Porous pavements have other environmentally enhancing qualities that contractors like to purport to their clientele. Porous pavers cool the air (urban heat island mitigation), reduce soil erosion, promote tree growth, and capture and retain airborne dust. Porous pavements are also noted for having sound reducing qualities over impermeable surfaces. Flexible porous pavers have the additional benefit of being made from 100 percent recycled plastic.

With increased infiltration of stormwater into porous pavers, runoff is reduced and the velocity of the run-off is also reduced.

"Grasspave2 provides an air-conditioning effect through transpiration," notes Kate Wright, Engineer with Invisible Structures, Inc. Golden, Colo. "It is the same reason your lawn is cooler than dirt or concrete. Grass paved surfaces can be several degrees cooler that surrounding areas." Light colored aggregate in a flexible gravel porous paver and light colored interlocking pavers can also reduce air temperatures compared to impermeable surfaces. "Light energy is not absorbed and transformed into heat as easily in Gravelpave2 with a light gravel fill," Wright continued.

With increased infiltration of stormwater into porous pavers, runoff is reduced and the velocity of the run-off is also reduced. This has a tremendous effect on offsite flooding, erosion, and eco-systems. Stream and creek beds are spared erosion and higher temperature waters caused by increases in velocity and pollutants normally associated with impermeable run-off. Higher water temperatures are detrimental to fish, aquatic vegetation, and other animals. Erosion due to increased velocity is harmful to mature trees.






Here, porous pavers are in the process of being installed at Reliant Stadium in Houston. Porous pavers have environmentally enhancing qualities that contractors like to purport to their clientele. Porous pavers cool the air (urban heat island mitigation), reduce soil erosion, promote tree growth, and capture and retain airborne dust. Porous pavements are also noted for having sound reducing qualities over impermeable surfaces.


Porous paving promotes tree growth and can help retain mature trees on-site. Trees that would normally have to be removed to pave an area with asphalt or concrete, can often times remain intact when porous paving is used. Porous paving will not suffocate the root system and provides enough water and air infiltration to sustain a healthy-mature tree. Landscape Contractors can plan for more trees and retain more trees when using porous paving.

Permeable interlocking concrete paver installation is nearly identical to normal concrete paver installation.

Airborne dust particles can be problematic in suburban or sub-rural areas especially close to gravel and dirt roads. Contractors wanting to help alleviate airborne pollutants can recommend porous pavers. Unlike hard surface pavers, which can compound the dust problems, porous pavers can retain the particles and absorb many into their cross sections. Washed or "clean" aggregate fill between 3/16'' and 3/8'' will not create dust normally associated with a gravel road.






Dust particles can be problematic in suburban or sub-rural areas. Contractors wanting to help alleviate airborne pollutants can recommend porous pavers. Porous pavers can retain the dust particles and absorb many into their cross sections. Washed or "clean" aggregate fill between 3/16'' and 3/8'' will not create dust normally associated with a gravel road.









Landscape Contractors have an opportunity to reduce traffic noise for sub-urban and urban areas. Since the late seventies, porous pavers have been used to reduce traffic noise in Europe. The idea is catching on in the U.S. and many state DOT's are currently installing porous paving to reduce noise. Noise reducing pavements have a better reduction effect on noise from light vehicles than on noise from heavy vehicles. Decibel reduction levels vary depending on the porous paver used.

Flexible porous pavers can be used in driveways with slopes up to 5-7 percent. Wright notes that this is comparable to asphalt and concrete and studies have shown porous paving is less susceptible to surface freezing and actually thaws ice and snow faster than hard surfacing. Wright also adds, "Our products are flexible so you won't see heaving and shifting problems associated with even brick pavers in freeze thaw cycles."

For these porous system minimal maintenance is required. A grass-pave system requires mowing, fertilization, and water - just like any other lawn. A flexible grass porous system does not need to be aerated, as the rings and sand combination will prevent compaction and allow for air infiltration. Commercial aerators may also damage the plastic structure. A reinforced gravel system's maintenance may include occasionally brooming the aggregate back into low spots. An additional quarter to half inch layer of top-dress gravel may be necessary every five or 10 years. Interlocking concrete pavers require routine inspection, in some cases vacuuming, and occasionally adjusting a shifted paver block or area.

Residential and small commercial drives, walkways, and many other paved surfaces are perfect candidates for these products. "We find that when the public is informed on the benefits of porous paving, they are very receptive to the idea of using them in their own homes," states Wright.






Installing Porous Pavers

Installation procedures for flexible porous pavers are straightforward. Examine the site and check the slope. Porous paving systems are designed to absorb most of the rain that falls on them; however, torrential rainfall amounts still should be directed away from structures by a minimum of one percent slope.

  • Determine the depth of the base course if it has not already been specified.
  • Excavate down to the depth of the base course plus one inch - the height of the flexible porous paving plastic structures. The existing soils are then compacted to 95 percent modified proctor density.
  • Irrigation lines, for grass paver driveways or to traverse to a different section, can be laid. The use of swing joints is recommended as they can withstand vehicle tire pressure while static heads can be damaged and inevitably will need replacing.
  • Place and compact a sand and gravel base course, similar to a road base. Porous pavers require that the base be pervious. Pervious base courses usually have less fines, approximately 4-5 percent #200 sieve, by volume. Check the manufacturer's literature for sieve sizes and requirements. Compact the base material in minimum six-inch lifts. Hosing down the area with water can aid this process. For a grass porous paver, sprinkle on the Hydrogrow mixture - a polymer/fertilizer compound to help aid growth - with a fertilizer spreader or by hand. Hydrogrow should be applied at a rate of one pound per approximately 100 sq ft.
  • Connect each roll to its adjacent roll via the male and female connectors. The plastic pavers can be cut easily, with garden shears or tin snips, to go around trees, curbs, to conform to the shape of the driveway, and to allow for irrigation heads.
  • The next step is anchoring down the flexible gravel porous paver system with the provided eight-inch nails and washers (a grass paver does not need to be anchored, as the grass roots will permanently stabilize the matrix).
  • Next, for a grass paver, fill the rings level to the top with clean sand. Sand provides structural integrity and the drainage necessary for a long lasting grass porous paver. Do not fill the rings with topsoil.
  • For a gravel paver, the next step is to fill the rings with your top-dress gravel. Fill a gravel porous paver 1/4 to 1/2 inch above the top of the rings. Gravel fill should be clean, sharp, hard, and about 3/16 inch to 3/8 inch.
  • For a gravel paver, installation is now complete and your client's driveway is ready immediately.
  • For flexible grass porous paver, the next step is to lay thin cut sod or to hydroseed directly on the sand. Rainfall amounts, sun exposure, and aesthetics are all variables to be considered when choosing a grass type. If sod is applied, use a sod roller to ensure good contact between the roots and the sand/top of the rings. If hydroseeding is applied, extra watering may be necessary the first growing season. In both cases, wait three mowing cycles to drive or park on the grass pave system.







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September 17, 2019, 10:58 pm PDT

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