Ever mindful of the continuing pressure to address air and water quality issues, a number of agencies, metropolitan planners and consultants are turning to pervious concrete pavements.
Pervious concrete has been used since the 1980’s for a diverse range of applications including driveways, sidewalks and parking areas. More recently, because federal mandates have increased the demand for storm water management, a growing number of regional and local authorities have been looking at pervious concrete as a way of meeting environmental requirements.
What is pervious?
Pervious concrete is a mixture of cement, coarse-graded aggregate and water. Little or no sand is used in the mix. Fibers and admixtures also may be added to impart added strength or other properties, but the common denominator is the control of water and cementitious content as well as the absence of fines.
The result is a paste—a very low-slump mixture—that forms a thick coating, which stays in place around aggregates without flowing off during mixing and placing. Using just enough paste to coat the particles maintains a system of interconnected voids, typically about 15 to 35% voids.
Because of its high void percentage, pervious concrete also is relatively lightweight, typically 100 to 120 lb/cu ft. Another characteristic related to the void percentage is limited compressive strength, although it is not uncommon to see compressive strengths of 500 psi to 4,000 psi, which are acceptable for a wide range of applications, including pavements.
After placement, pervious concrete looks somewhat like a sponge—and for good reason. The cured pervious concrete typically allows about 3 to 8 gal of water per minute to pass through each square foot, in a free-flow condition. It is this feature that put pervious concrete pavement in the sights of many municipalities and townships.
Pervious concrete pavements can capture rainwater and then allow it to seep into the ground, thereby recharging groundwater, reducing storm water runoff and meeting U.S. Environmental Protection Agency (EPA) storm water regulations. The use of pervious concrete is among the EPA’s recommended best management practices. Increasingly, pervious concrete is being cited by other agencies and geotechnical engineers across the country as a viable means of managing storm water runoff on a regional and local basis.
There is also a simple, but often overlooked, benefit of concrete that is attributable to its naturally white to light gray color. Concrete pavements—including pervious concrete—are inherently light reflective, which means they absorb less heat from solar radiation than darker asphalt pavements.
With porous concrete pavements, the relatively open pore structure of pervious concrete stores even less heat than asphalt or other conventional paving systems. This helps lower the urban heat island effect, an increase of air temperatures in urban areas in contrast to cooler surrounding rural areas.
These features are important because as urban temperatures rise, cooling demands and electrical consumption increase. Demonstrating just how dramatic the difference can be, a recent experiment conducted in Rio Verde, Ariz., involved thermographic imaging of an asphalt roadway and an adjacent concrete parking lot (plain jointed concrete and not pervious concrete) at the same time of day. There was a 30°F differential in temperatures between the asphalt and the concrete surfaces.
Pervious concrete pavements can further reduce the urban heat island effect because they are said to protect trees and greenery. Trees and plants have a hard time growing in some areas covered by impervious surfaces, because air and water have difficulty getting to the roots of vegetation. But not so with pervious concrete pavements, which can help promote the growth of trees and plants by allowing air and water to reach root structures more freely than impervious structures. As a result, planners can plant vegetation strategically, allowing for more shaded areas, which in turn can reduce the temperatures.
To appreciate the role and significance that pervious concrete can play in a storm water management strategy, consider that the EPA storm water regulations set limits on the levels of pollution in U.S. streams and lakes. To meet these regulations, local officials have considered two basic approaches: reducing the overall runoff from an area and reducing the level of pollution contained in runoff.
Efforts to reduce runoff include zoning ordinances and regulations that reduce the amount of impervious surfaces in new developments, including parking and roof areas; increased green space requirements; and the use of “storm water utility districts” that levy an impact fee on owners based on the amount of impervious area.
Efforts to reduce the level of pollution from storm water include requirements for developers to provide systems that collect the “first flush” of rainfall, usually about 1 in., and then treat the pollution prior to release. According to the EPA, first flush is a condition in which a disproportionately high pollution load is carried in the first portion of the discharge or overflow.
By using pervious concrete pavement to collect the first flush of rainfall and allowing it to percolate into the ground, soil chemistry and biology can then treat the polluted water naturally. Thus, storm water retention areas may be reduced or eliminated altogether, allowing increased land use. By collecting rainfall and allowing it to infiltrate, groundwater and aquifer recharge is increased, peak water flow through drainage channels are reduced and flooding is minimized.
Pervious concrete pavements reduce the need for large detention ponds, because the paved area acts as a detention area. This translates to savings in labor, construction and maintenance of detention ponds, equipment and other related system components.
Expensive irrigation systems also can be downsized or eliminated. In reducing runoff from paved areas, pervious concrete reduces the need for separate storm water retention ponds and allows the use of smaller-capacity storm sewers. This allows land use for purposes other than sewers and ponds.
Concrete pavements have a significantly lower life-cycle cost than alternatives such as asphalt. Although the initial cost of pervious concrete or asphalt pavements may be slightly higher than conventional materials, concrete almost always provides cost benefits over its intended design life because of its superior durability and strength. Quite simply, it requires fewer repairs than asphalt and needs virtually no maintenance. In fact, over time, asphalt will cost about three times more than concrete pavement on average.
Perhaps less well known than the life-cycle cost benefits is the fact that first costs of concrete are typically in line with asphalt, given the recent spikes in oil pricing. As the price of oil has risen, so too has the price of asphalt. The price of asphalt (PG-58) has increased from $190/T to $270/T in the past year, according to the Engineering News Record.
Other costs lurk beneath the surface as well. Many government agencies are now implementing storm water impact fees for all impervious areas. As regulations further limit storm water runoff, it is becoming more expensive for property owners to develop real estate due to the size and expense of the necessary drainage systems. Pervious concrete can reduce these fees for the property owner by helping to minimize demands on sewer systems.
Pervious concrete also is economical in that it minimizes the need for runoff retainers, reducing property costs. There is very little overproduction since it is made directly onsite and as needed, and it can be recycled once it has reached the end of its life cycle. Thus pervious concrete is widely recognized as the lowest life-cycle cost option available for paving.
This pavement technology creates more efficient land use by eliminating the need for retention and detention ponds and other storm water management devices. In doing so, pervious concrete has the ability to lower overall project costs on a first-cost basis.
Safer by design
Of course, concrete also offers some inherent safety benefits. For example, a recent study by the University of Illinois showed stopping distances of about 162 ft or approximately 10.8 car lengths on dry concrete, while dry asphalt required about 190 ft or approximately 12.7 car lengths. In sharp contrast, wet asphalt required 440 ft of stopping distance, or about 29.3 car lengths, compared with wet concrete at 316 ft, or about 21 car lengths.
In addition to providing better friction and skid resistance, concrete pavements offer other safety, cost and environmental benefits, including:
- A non-rutting surface that does not trap water;
- Excellent surface drainage;
- Better light reflectance;
- Fewer repairs and fewer work zones over life of pavement;
- Comparable initial cost to asphalt; and
- Lower ownership cost than asphalt.
The effects are also dramatic with pervious concrete. By reducing surface oils, the pavements further enhance the inherently safe driving surface. Additionally, recent research has shown that bacteria living in these cellular voids act to break down many of the pollutants, making the pervious pavement even more effective for water filtration.
As a result of these and other benefits, the interest in pervious concrete pavements for low-volume municipal and county roadways is increasing. Pervious concrete surfaces also can improve safety during rainstorms by eliminating ponding and glare at night, spraying and the risk of hydroplaning.
Although pervious concrete has long been used for driveways, sidewalks and parking areas, there is a growing interest in using it for low- to medium-volume roadways. As environmental performance specifications and wastewater regulations become more stringent, a growing number of consultants, planners and other officials are looking to pervious concrete pavement as a vital tool in meeting the challenges that lie ahead.