Permeable pavement (also known as pervious or porous concrete) is a specific type of pavement with a high porosity that allows rainwater to pass through it into the ground below.
Through this movement, pervious concrete mimics the natural process that occurs on the ground’s surface, consequently reducing runoff and returning water to underground aquifers. It also traps suspended solids and pollutants, keeping them from polluting the water stream. Pervious concrete has many applications, most commonly:
- low-volume pavements
- residential roads and driveways
- parking lots
- low-water bridges
- well linings
- walls (including load-bearing walls)
- swimming pool decks
Pervious concrete was first seen in the 1800s in Europe and was used for various structural purposes, including load-bearing walls, infill panels, and pavement surfacing. It became popular again overseas after World War II due to the scarcity of cement.
Although not a new innovation, pervious concrete has only been implemented in the United States in the past fifty years. The concept was proposed in the 1960s in hopes of reducing floods, raising water tables, and replenishing aquifers, while a decade later the Environmental Protection Agency (EPA) began testing it to determine its cost and efficiency. These tests were done at various sites in Delaware, Pennsylvania, and Texas, with Texas being the most significant site since it was the only one to have installed monitoring instruments.
The first official design guide for pervious pavement was co-written in 1977 by Edmund Thelen and L. Fielding Howe in Philadelphia, PA. Titled “Porous Pavement,” this document provided the groundwork for permeable pavement education and is still referred to today for guidelines and information.
Permeable concrete is now used in multiple cities throughout the U.S. and its number of applications has grown drastically over the past ten years, from driveways and sidewalks to commercial and multi-acre spaces.
The Ins and Outs of Permeable Pavement
Permeable concrete consists of cement, a coarse aggregate, and water, with little to no fine aggregates (sand or clay). That is why permeable concrete has a very rough and uneven appearance.
The American Society for Testing and Materials (ASTM) has a set of standards for both pervious and non-pervious concrete. ASTM calls for the following percentage of air content (or voids) within pervious concrete:
20% ± 5%: Low porosity, high strength
30% ± 5%: High porosity, low strength
When compared to the required void percentage of non-porous concrete, which ranges from 3%-7.5%, the difference in overall structure can be easily seen. The high void percentage required for pervious concrete lets stormwater run easily through the material and seep into the ground below, with typical flow rates ranging from two to 18 gallons per minute! More standards and tests can be purchased or viewed on ASTM’s website.
Water and aggregate are added in specific amounts to attain pervious concrete with a high air content and just enough cementitious paste to coat particles and interconnect voids. The low cement and high air content results in reduced strength, hindering pervious concrete from being used on highways, certain streets, or heavy loading areas. The mixture of all added materials, however, can be altered to allow sufficient strength for certain needs.
Types of Pervious Concrete
There are multiple types of permeable concrete, all of which are used for different purposes:
- Porous Asphalt is used on highways to remove excess water.
- Single-Sized Aggregate contains no binder and is commonly known as loose gravel. It can commonly be seen in very low-speed applications such as driveways or pathways.
- Plastic Grids allow for a 100% porous system and are growing in popularity due to LEED project requirements. These grids help reinforce gravel driveways, parking lots, and fire lanes. Plastic grids can also be planted with grass.
- Porous Turf can be used for areas with occasional parking, such as stadiums or churches.
- Permeable Interlocking Concrete Pavers are individual units that can be laid out in an interlocking grid pattern, with in-between spaces commonly filled with grass or small stones. This type of paving is popular in public areas due to its architectural appeal.
- Permeable Clay Brick Pavers are similar to interlocking pavers, but are composed of fired clay.
- Resin-Bound Paving is a mixture of a clear resin and aggregate, used for areas with pedestrian and vehicular traffic, including walkways, driveways, and parking lots.
- Bound Recycled Glass Porous Pavement is a mixture of post-consumer glass with resins and binding agents. Made by FilterPave Products, this colorful pavement prevents glass waste from ending up in a landfill. Recycled glass pavement is appropriate for both pedestrian and vehicular traffic.
Benefits of Pervious Concrete
- Eliminates runoff
- Recharges groundwater
- Traps suspended solids and pollutants
- Reduces surface temperatures and, therefore, the heat island effect
- Eliminates the need for retention basins and water collection areas
- Eliminates costs for retention basins, curbs, gutters, and other water collection installations
- In winter conditions, typically requires much less salt or other de-icing products than traditional pavement types
- Lower installation costs (no underground piping, storm drains, or sloping/grading needed)
- Low life-cycle costs with an equal life expectancy to that of regular concrete: 20 to 40 years when correctly installed
Factors to Keep in Mind
- Runoff Volumes: A pervious pavement project should be properly designed to accommodate the amount of stormwater runoff that is expected in the area. If not adequately designed, the water table below the pavement can rise, preventing stormwater from being absorbed into the ground.
- Because pervious concrete has such a high void content and its overall strength is generally lower than that of regular concrete, it is not recommended for highways, high-volume streets, potential spill sites (in case of clogging), and heavy loading areas.
- While it is estimated that porous concrete can be two to three times more expensive than regular asphalt or concrete, cost savings are simultaneously achieved as stormwater installations are not necessary.
- Certain types of pervious pavements require frequent maintenance due to the possibility that solids and particles may get trapped and clog pavement pores. If the proper “vacuuming” or flushing is not carried out, pervious concrete will assume the traits of impervious concrete.
- Siting should be a major factor when considering permeable pavements. If the ground surface exceeds a 20% slope, stormwater will run downhill over the permeable pavement as opposed to being absorbed by it as intended.
Permeable paving can be one part of building green parking lots, which can also include rain gardens, art, trees, solar covers, and other creative elements. Paving and landscaping choices have a large effect on the environmental impact of parking lots. See here for examples of how Pittsburgh parking lots have been made greener! For more information on green parking, visit Green Parking Council or see this Green Parking Lot Resource Guide from the EPA.
Local Examples of Permeable Paving
- Pervious sidewalks in Four Mile Run
- Residential application on Mt. Washington
- Grass grid parking at Phipps Conservatory and Powdermill Nature Reserve