Think about what happens when rainwater falls on an impervious surface in a big outdoor parking lot studded by the occasional tree: The water dampens the surface, which instantly becomes saturated. Only a minute percentage of water that penetrates the trees’ canopies to reach their curb-bound planters becomes available to the trees’ roots. The rest almost immediately starts flowing to drain grates or perimeter drainage details and is lost to a stormwater-collection system.
The trees are helped only marginally by the life-giving rain, and the water that flows to the drainage system – basically to waste – is contaminated by oil not to mention the countless particulate byproducts of the use of automobiles that come to take advantage of available parking spaces.
This is not a pretty picture, which is why landscape designers and contractors have long sought solutions in the form of pervious surfaces and more recently are taking a long look at a relatively new product known as pervious concrete.
BIG, HARD SURFACES
The advantages of porous pavements have been known for a good while, basically because surface-water runoff from non-porous surfaces has long been a problem in urban environments, where paving typically occupies twice the surface area of the buildings it surrounds. A penetrable-surface option allows for water uptake by plants and trees and also reduces the need for capturing and diverting rainwater runoff.
Porous or pervious concrete is one such material. Pervious concrete is a paving material with built-in voids that allow water and air to penetrate past the surface and all the way through to the substrate. It is mixed at the same concrete plants and made with the same ingredients as conventional dense concrete – that is, Portland cement, water and aggregate.
The difference is that the aggregate in pervious concrete is single-sized, with no sand or fine materials added to fill the voids between larger particles. The remaining single-sized aggregate particles leave voids that give the material its porosity and allow water to reach all the way through the material to reach a base course (or “base reservoir”) of additional single-size aggregate.
|When installed properly, pervious concrete allows water to pass right through the surface to the substrate (left), so using the material means you need to consider what to do with excess water that may collect below grade (middle left and middle right). Installed incorrectly, the material will not allow adequate water passage, the pores will clog and cracking will result, as seen at right.
By design, pervious pavements solve water-related urban environmental problems at the source.
To quantify points made earlier, common dense pavements generate two-thirds of the excess runoff in urban watersheds and essentially all of the hydrocarbon pollutants found in stormwater-control systems. These same dense pavements also cramp tree roots in compacted soil, robbing them of air and water, limiting their growth and significantly affecting their life spans.
Conversely, porous pavements empower a landscape’s natural water-retaining function by bringing water back into contact with the underlying soil – or, at the very least, assist in that process by filtering and storing water within the pavement’s structure. As such, they facilitate groundwater replenishment, sustain urban stream flow in dry summers, biodegrade the oils from cars and trucks and allow tree roots to breathe.
Better still from a cost/benefit viewpoint, the combining of stormwater management with pavement function in a single structure enables porous pavements to be used at reduced cost compared with dense pavements that require design and installation of separate stormwater-control facilities.
Even where the soil beneath pervious concrete is only slightly or slowly permeable and therefore prohibits significant soil infiltration and water will be discharged through perforated pipes set at the bottom of the pavement, a porous pavement can support runoff comparable to that of off-pavement reservoirs and ponds. Indeed, the outflow is later, slower and smaller in volume than the rainfall entering from above.
AN INSIDE LOOK
The idea of a pervious-concrete pavement first emerged in Florida in the 1970s as part of a general quest to find a durable yet porous surface that would take advantage of the state’s typically sandy soils and reduce requirements for off-pavement stormwater-control facilities.
The products that resulted have been used beyond Florida since the 1990s, first in other areas with sandy soils and since 1996 on clay soils in inland regions. Use in the west began in 2001, while applications in Texas began making news in more recent times.
|In this application over clay, a geotextile is positioned below an aggregate base to keep the plastic clay soil from flowing into and clogging the aggregate’s voids (left). The concrete material contains only enough water to make a paste that sticks to the aggregate, so it must be pulled down the chute and raked into place (middle left). Once applied, the material os rolled (not screeded) to compact the surface without smearing the paste across the surface pores (middle right). Next, the surface is covered for several days to prevent premature evaporation of the small amount of water in the mix (right).
What transpires within the permeable pavement is remarkable. In the pavement’s voids, for example, naturally occurring microorganisms that digest oil leaked from automobiles will take hold and thrive. The constituents are digested and released as carbon dioxide, water and very little else. In other words, essentially none of the oil makes it to the bottom of the pavement: It ceases to exist as a pollutant.
Other harmful substances including such metals as cadmium and lead (released by automobile corrosion and wear) are captured in porous pavements’ voids along with minute sediment particles to which the ions are characteristically attached. Capturing these metals prevents them from washing downstream and accumulating in the environment.
Porous pavements are seen as a possible solution to a number of urban problems, the result being that lots of questions are being asked and numerous studies are being conducted to put them to the test.
A recently completed, eight-year-long program researched these materials and offered answers to the questions that had inhibited their acceptance in the design and installation marketplaces. It included interviews with 170 experts in the field, reviews of 800 technical articles and reports and firsthand examination of 280 installations of all types of porous pavements throughout North America.
Pervious concrete was just one of the materials studied. For more information, check CRCPress.com, Amazon.com or BarnesandNoble.com.
Below the surface, pervious concrete will give urban trees the rooting space they need to grow to full size and will enable them to provide the shade, cooling and air quality for which trees are planted and prized. As a rule of thumb, a tree’s rooting zone must be at least as large in area as the tree’s canopy at maturity. Beneath a properly installed porous surface, there is no practical limitation on where roots can travel to seek water, air and nutrients.
Beneath the pavement in a properly installed system, the rooting zone will be made with an aggregate base made of large, single-sized aggregate that bears the pavement’s load. Into the aggregate’s void spaces is mixed 15 to 20 percent (by volume) of nutrient- and water-holding soil, while the remaining (unfilled) void spaces maintain aeration and drainage. In effect, this mixture establishes the base as a “structural soil.”
The composition of this soil base takes a number of forms depending upon need. Some installers are using natural, locally available stones to facilitate water passage, for example, while others use expanded-clay aggregate intended for additional water-holding capacity. The combination of porous pavement and structural soil is in fact a revolutionary way to integrate healthy ecologies into thriving cities: a living tree canopy above, the city’s traffic on the ground and living tree roots below.
In appearance, a correctly installed pervious-concrete surface is vividly recognizable, with uniformly open surface pores, obvious permeability and surface particles firmly attached. By contrast, a poorly installed surface will have a closed, smeared-over aspect and low permeability and within a year will begin to unravel, with loose particles rolling around on the surface.
Construction with pervious concrete is not more difficult than construction with conventional dense concrete – but there are differences in the process that must be recognized and variations in specifications and procedures that must be followed. Publications provided by the Florida Concrete & Products Association (www.fcpa.org) are helpful here, and the American Concrete Institute is about to release its own guide (www.aci-int.net).
It is important to find a fully qualified installer. These contractors generally have credentials or certifications from the Portland Cement Pervious Association (www.petrusutr.com) and/or the National Ready Mixed Concrete Association (www.nrmca.org). For complete assurance, it can’t hurt to have them install a test panel before proceeding with the remainder of the project.
|When properly installed, pervious concrete is kind to trees, allows for excellent control of runoff and can even serve as the end rather than the beginning of a drainage system in a parking lot that is designed without curbs, drainage inlets, swales or culverts.
The reason for the caution is obvious: As a “zero-slump” mixture (meaning its water content is very low), the exact water content of the pervious-concrete mix is critical. Too much water makes the paste drain to the bottom of the slab, while too little prevents the paste from curing completely. With the current state of the technology, evaluating the exact water content requirement depends quite completely on the expertise of the installer at time of placement.
It should also be noted that pervious concrete is just one of several porous paving materials available for selective use on specific sites. Porous asphalt, open-jointed blocks, porous aggregate and porous turf are just a few examples, each of which presents distinctive advantages and disadvantages for specific applications.
The key is detailed site analysis, which should be conducted to determine which material is best suited to the job. At a minimum, that analysis should consider traffic types and levels, different needs for hydrology, appearance, tree rooting and cost. The necessity of selecting among alternative materials should, one thinks, force designers to pay closer attention to pavements as multi-function design elements.
For its part, when compared with other porous paving materials, properly installed pervious concrete can bear heavy traffic loads and can readily be stained to match or blend with colors of nearby natural or architectural features.
Even with optimal installation and despite its great durability, care needs to be taken to protect pervious-concrete surfaces from sedimentary clogging. They should be situated, for example, both to facilitate runoff of excess water and to prevent sediment from washing onto the surface.
Conditions below a pervious-concrete surface are important to successful applications.
Sub-grade compaction during construction, for example, greatly reduces infiltration rates once water moves through the porous surface material – which has its conceptual downside – but it may be necessary for stability where the subgrade is plastic clay or fill or the pavement will be too thin to compensate structurally for soft, wet soil.
By contrast, compaction might be completely unnecessary and the material’s native infiltration rate preserved where the subgrade is native cut (and therefore has in situ compaction and stability) and adequate pavement thickness can compensate for any sub-grade softness.
Temperature can also influence the effectiveness of pervious concrete, which is why much current research is being conducted with these materials in progressively cooler climates, including locations in Maryland and Washington State. Preliminary results indicate that the material’s cold-climate durability can be enhanced by air entrainment and polymer-fiber reinforcing.
To that end, surface drainage should move water away from pavement edges in every possible direction. On downhill sides, large (and numerous) curb cuts should be added as needed, while on uphill sides, swales should be added as required to divert potential incursions of sediment-laden runoff from upslope surfaces.
If pervious concrete does become clogged by sediment, vacuuming is the only effective means of removal. Low-pressure washing may be used to help mobilize sediment before vacuuming, but high-pressure washing only drives sediment deeper into the pores. As an option, sweeping can be helpful, but only when followed immediately by vacuuming.
To be sure, porous pavements in general and pervious-concrete surfaces in particular now constitute only a minute fraction of the paving done each year in the United States. They are gaining ground, however, because they represent the solution to a significant urban problem. Moreover, they offer a cost-effective option when the fact is considered that they reduce the need for off-pavement stormwater-treatment facilities.
Successful installation of these surfaces requires knowledge and care. Each must be selected, designed, installed and maintained correctly, and every site has differences from others that must be taken into account. That said, however, this technology has great promise in the movement to replenish renewable resources, restore regenerative processes and produce cleaner, healthier, safer, more-sustainable cities in which more of us can work, play and live.
Bruce K. Ferguson is a professor of landscape architecture and director of the school of environmental design at the University of Georgia. An expert in the field of porous pavements, his design practice includes consulting on specialized stormwater and green-building issues in urban development and redevelopment projects. He has conducted extensive research on pavement materials and uses the results of that research as a basis for lecturing and consulting on design projects nationwide. Among his books are Stormwater Infiltration, Introduction to Stormwater and, most recently, Porous Pavements. He holds a bachelor’s degree in architecture from Dartmouth College as well as a master’s degree in landscape architecture from the University of Pennsylvania. Ferguson is a fellow of the American Society of Landscape Architects, a past president of the Council of Educators in Landscape Architecture and a recipient of the council’s Outstanding Educator Award in recognition of career contributions to landscape architectural education in North America.