The environmental and financial benefits of pervious concrete for paving commercial parking lots have long been recognized in the warmer climates of the United States. But as pervious concrete makes its way into the colder and longer winters of Midwestern states such as Michigan, commercial developers and municipalities are beginning to recognize the cold weather benefits of this proven paving material. Anecdotal evidence has shown that both snow and ice leave the surface of pervious concrete more rapidly when melting than traditional concrete and asphalt pavements because the resulting water can drain down rapidly. Additionally, since there is no ponding of moisture, there is not the same propensity for re-freezing as with conventional pavements. These features alone make pervious parking lots safer in winter for drivers and pedestrians.
Pervious concrete is a structural concrete pavement with a large volume (15 percent to 35 percent) of interconnected voids. Like conventional concrete, it's made from a mixture of cement, coarse aggregates and water. However, it contains little or no sand, which results in a porous, open-cell structure that water passes through readily. The open-void structure in the porous pavement allows cooler earth temperatures from below to cool the pavement. These factors allow porous pavement systems to approach natural ground cover in heat-absorbing and storage capacity. Unlike traditional concrete or asphalt, pervious pavements provide improved filtration and an enormous amount of surface area to catch oils and chemical pollutants. Moreover, certain types of porous pavement can pass 3 gallons to 5 gallons of water per minute, which is far greater than most conceivable rain events and highly effective in controlling storm water drainage.
With over 20 years of use in warmer climates, pervious concrete has demonstrated that porous concrete parking lots can double as water-retention structures, reducing or eliminating the need for traditional and costly storm water management systems such as retention ponds and sewer tie-ins. Pervious concrete is especially environmentally friendly because it allows rainwater to seep into the ground through the pavement and consequently:
- Vegetation is naturally watered, reducing the need for costly irrigation,
- Ground water is recharged,
- Water resources are preserved,
- Storm water runoff is reduced, and
- Storm water runoff quality is improved, while hydrocarbon pollution from asphalt pavements and sealers is eliminated.
The initial costs for pervious concrete pavements are slightly higher than those for conventional concrete or asphalt paving because pervious concrete is thicker than regular concrete. But the life cycle costs for pervious concrete are substantially lower. Here are some of the savings provided by pervious concrete:
- Pervious concrete provides lower installation costs. According to the Center for Watershed Protection, installing traditional curbs, gutters, storm drain inlets, piping, and retention basins can cost two to three times more than low-impact strategies for handling water runoff, such as pervious concrete. Projects that use pervious concrete typically don't need storm sewer ties-ins, which eliminates the cost of installing underground piping and storm drains. Grading requirements for the pavement are also reduced because there is no need to slope the parking area to storm drains.
- Pervious concrete permits the use of existing sewer systems. Pervious concrete may also reduce the need for municipalities to increase the size of existing storm sewer systems to accommodate new residential and commercial developments.
- Pervious concrete provides increased land utilization. Since a pervious concrete pavement doubles as a storm water management system, there is no need to purchase additional land for installing large retention ponds and other water-retention and filtering systems. That means developers and property owners can use land more efficiently and maximize the return on their investment.
- Pervious concrete provides lower life cycle costs. Pervious concrete is a sustainable paving material, with a life expectancy equal to that of regular concrete. Most parking areas, when properly constructed, will last 20 to 40 years.
As previously mentioned, a major benefit of using pervious concrete in cold climates is that it provides little opportunity for the formation of surface ice. First, rains will not pond on pervious concrete; therefore, the formation of ice is almost completely eliminated. If all is working as it should, the rain will immediately infiltrate the porous pavement. Although snow could pile up, when it melts the snow becomes a liquid that readily infiltrates the pervious concrete. As a result, pervious parking lots will be safer to drive and walk on with little if any standing ice or slush.
Recent studies at the National Concrete Pavement Technology Center (CP Tech Center) at Iowa State University in Ames, Iowa — where tests under freeze-thaw conditions have been applied to pervious concrete mixes — have shown that this pavement system can withstand the powerful expansion forces of freezing water in colder climates. Research at Iowa State has identified pervious concrete mixes that will meet freeze-thaw durability requirements. The traditional ASTM test for freeze-thaw is 300 cycles of freezing and thawing with less than 85-percent mass loss. One of the mixes developed at Iowa Sate will provide that with 95-percent retention of mass.
There are also steps that can be taken when placing pervious concrete in colder climates with longer and greater number of freeze-thaw cycles. The following precautions are recommended to enhance the freeze-thaw resistance of pervious concrete:
- Use an 8-inch- to 24-inch-thick layer of clean aggregate base below the pervious concrete.
- Attempt to protect the paste by incorporating air-entraining admixture in the pervious mixture.
- Place a perforated polyvinyl chloride (PVC) pipe in the aggregate base to capture all of the water and let it drain.
Not every situation warrants all three safeguards. The safeguards are organized in the order of preference. For example, a pervious concrete sidewalk at Pennsylvania State University in State College, Pa., has shown good performance over five winters while it has only an 8-inch-thick layer of aggregate base underneath the pervious concrete.
When you consider all of the environmental, financial and safety benefits of pervious concrete in both hot and cold weather, it truly is a paving material for all seasons and for all of the right reasons.