Pervious concrete is a unique and innovative means to manage stormwater. When pervious concrete is used in building site design, it can aid in the process of qualifying for LEED 2009 Green Building Rating System credits.
Pervious concrete has been used successfully in many construction applications, including parking lots, streets, plazas, nature trails and walkways. While pervious concrete has a surprising number of applications, its primary use is in pavement. Pervious concrete is a performance-engineered concrete made with controlled amounts of aggregates, water and cementitious materials to create a mass of aggregate particles covered with a thin coating of paste. A pervious concrete mixture contains little or no sand, creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15 percent and 25 percent voids are achieved in the hardened concrete and flow rates for water through pervious concrete are typically around 480 inches/hour (0.34 cm/s), although they can be much higher. Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures, but sufficient strength for many applications is readily achieved.
Pervious concrete pavements are usually supported on a base layer of uniformly sized stone to form a basin where rainwater can be stored before percolating into the soil below. The system essentially forms a dry detention pond that can reduce the need for expensive stormwater drainage and wet pond detention/retention systems, allowing for more effective land use. In effect, the pervious concrete pavement system functions: 1. as a paved surface for driving, parking or walking and 2. as a retention basin for storing rainwater during a storm event.
Leadership in Energy and Environmental Design (LEED®) is a rating system developed by the U.S. Green Building Council (USGBC) to evaluate the environmental performance of buildings. LEED provides a framework for evaluating building performance and meeting sustainability goals through five credit categories: sustainable site development, water savings, energy efficiency, materials selection and indoor environmental quality. LEED points are not gained directly by the use of a product but rather by meeting a specific sustainability goal of the rating program. Pervious concrete can contribute to credits earned under many LEED categories.
Pervious concrete is frequently used to obtain the specific points listed below. It should be noted, however, that pervious concrete can also contribute to Innovation in Design category points, and while it is not explicitly approved for SS Credit 7.1-Heat Island Effect — Non-Roof, pervious concrete can reduce heat reflected back into the environment and the design may be submitted for interpretation.
SS Credit 6.1 — Stormwater Design — Quantity Control
SS Credit 6.2 — Stormwater Design — Quality Control
The intent of these credits is to limit disruption and pollution of natural water flows by managing stormwater runoff, increasing on-site infiltration and eliminating contaminants. Pervious concrete can contribute to this credit by allowing stormwater to soak through and infiltrate to the ground below. Pervious concrete can also reduce the pollutant loads by filtering contaminants as the water is transferred through the pavement. On building sites where the existing imperviousness is greater than 50 percent, SS Credit 6.1 requires reducing the rate and quantity of stormwater runoff by 25 percent from the two-year, 24-hour design storm. On building sites where the existing imperviousness is less than 50 percent, the requirement specifies that the post-development peak discharge rate and quantity from the site shall not exceed the pre-development peak rate and quantity. Generally, by incorporating a pervious concrete pavement system on-site, the project can meet these criteria and obtain the LEED points for these credits.
WE Credit 1 — Water Efficient Landscaping
The intent of this credit is to limit or eliminate the use of potable water for landscape irrigation. To earn this credit, potable water for irrigation must be reduced by 50 percent when compared to a mid-summer baseline case. The gravel sub-base under pervious concrete can be used to store stormwater for irrigation, helping to satisfy this credit. If no irrigation is required for a project, two points may be earned.
MR Credit 4 — Recycled Content
The intent of this credit is to increase the demand for building products that have incorporated recycled content material, reducing the impacts resulting from the extraction of new material. Almost all ready mixed concrete contains recycled materials in the form of supplementary cementitious materials (SCM) such as fly ash, slag or silica fume. The use of SCMs or recycled aggregate in pervious concrete or base material contributes to recycled content needed for this credit. SCMs are considered pre-consumer recycled material and recycled aggregate from a demolished project are considered post-consumer recycled material.
MR Credit 5 — Regional Materials
The intent of this credit is to increase demand for building products that are extracted and manufactured locally, thereby reducing the environmental impacts resulting from their transportation in addition to supporting the local economy. To meet the intent of this requirement, 10 percent (based on cost) of the total materials must be harvested, extracted or recovered within 500 miles of the project site. An additional point is awarded for 20 percent regional materials. The majority of materials in pervious concrete, and most ready mixed concrete, are considered regional materials. Projects with large amounts of concrete can meet the required 10 percent or 20 percent regional materials to meet this credit.
Pervious concrete is a perfect material for environmentally conscious business owners to use in parking lots, drives, plazas and walkways. With both environmental and safety benefits, pervious concrete can provide a low-maintenance and durable paving material that will assist in gaining the highly regarded LEED certification.
Erin Ashley is senior director of sustainable construction at the National Ready Mixed Concrete Association (www.nrmca.org). A version of this article appeared in NRMCA’s Infocus magazine.