The Green Side of Precast Concrete

By Andrew Zumwalt-Hathaway

Clients are demanding greener, more sustainable buildings. Precast concrete manufacturers, contractors and specifiers are increasingly being asked to provide a product that can demonstrate environmentally responsible attributes.

One measure of environmentally sensitive buildings is the LEED (Leadership in Energy and Environmental Design) Green Building Rating System, which “scores” a building’s sustainability according to a host of features, including materials. Understanding the LEED rating system, how the sustainable attributes of precast concrete contribute to LEED certification and the documentation procedures that demonstrate compliance with LEED will enable precast concrete manufacturers and contractors and specifiers to respond to the market and develop more sustainable products and procedures.

The LEED Green Building Rating System
Established in 1998, LEED is a voluntary, consensus-based national standard for developing high-performance, sustainable buildings. Developed by members of the U.S. Green Building Council (USGBC) representing many segments of the building industry and environmental science, LEED has several goals:

• Define what qualifies as a green building by establishing a common standard of measurement;
• Promote integrated, whole-building design practices;
• Recognize environmental leadership in the building industry;
• Stimulate green competition;
• Raise consumer awareness of green building benefits; and
• Transform the building market.

While the current number of registered LEED projects only accounts for about 5 percent of the U.S. building market, this number has grown from 12 certified pilot LEED projects in 1999 to 1,262 registered LEED projects as of April 2004.

LEED certification can be achieved at the Certified, Silver, Gold or Platinum levels based on the number of LEED credits awarded after a project meets seven LEED prerequisites. The LEED rating system divides these credits and prerequisites into five categories: Sustainable Sites; Water Efficiency; Energy & Atmosphere; Indoor Environmental Quality; and Innovation and Design Process. Prerequisites and credits are based mostly on established governmental or trade group standards. The number of points a project earns by satisfying the environmental performance criteria of the various LEED credits determines the level at which a project can be certified.

While the LEED rating system was initially designed for commercial high-rise office buildings, LEED 2.1 for New Construction includes requirements suitable for multi-family residential high-rise projects. The USGBC is currently developing guidelines for residential and laboratory projects as well. The USGBC is also currently making the final revision to the LEED for Existing Buildings Rating System.

Environmental attributes of precast concrete
Precast concrete has environmental attributes that can make it preferable to cast-in-place concrete. Plant casting is more resource-efficient in the use of formwork and reduces waste generated at the construction site. Also, standardized precast elements can be installed more quickly and efficiently, resulting in faster construction times and less on-site equipment noise and emissions. Gaining additional recognition is precast/prestressed concrete, which uses less material to achieve similar load-bearing capacity.

These types of environmental attributes are important in terms of environmental excellence and contribute to the material’s embodied energy. However, these attributes relate only indirectly to existing LEED points. For instance, reducing waste generated on site makes it easier for a project to capture points awarded in the Construction Waste Management credit for diverting 50 percent to 75 percent of construction waste from area landfills. Less formwork on site also means that less wood has to be purchased for a project, which makes it easier to earn the Certified Wood credit, as certified wood can be an expensive option for formwork.

Precast concrete products do relate directly to four LEED credits and a total of eight points: Site Credit 6, Stormwater Management; Site Credit 7, Heat Island Reduction; Materials & Resources Credit 4, Recycled Content; and Materials and Resources Credit 5, Regional Materials. In addition, there may be opportunities for precast concrete to contribute to capturing LEED Innovation Points.

Stormwater Management
LEED Site Credit 6, Stormwater Management, awards one point for measures taken to manage or reduce stormwater run-off. Precast permeable paving systems reduce run-off by allowing infiltration of rain into the subsurface.

LEED has a simplified calculation to demonstrate compliance based solely on the run-off coefficient of site surfaces. Typically, according to the LEED Reference Guide, precast pervious paving systems have a run-off coefficient of 0.60. The run-off coefficient relates the amount of run-off to the amount of precipitation received. Therefore, a run-off coefficient of 0.6 indicates that 60 percent of the rain falling on pervious paving is run-off and 40 percent is able to percolate into the subsurface. Precast pervious paving systems would likely need to be combined with additional measures like green roof assemblies or rain water harvesting to fully comply with this LEED credit.

Pervious paving can also help a project earn a second LEED point within the Stormwater Management credit for stormwater treatment. For this point, LEED requires EPA Best Management Practices that effectively remove at least 80 percent of the total suspended solids (TSS) and 40 percent of total phosphorous (TP) from stormwater volumes leaving the site. As with the credit above, additional measures like infiltration basins may be required for a project to fully comply with this credit, but precast pervious paving systems do contribute toward compliance.

Heat Island Reduction
The LEED Site Credit 7, Heat Island Reduction, awards two points for measures that reflect heat from buildings and site paving. The “Heat Island Effect” is the result of constructed surfaces absorbing solar energy and retaining heat, which raises urban temperatures and increases building cooling loads.

The first point is achieved by specific measures adopted to reflect solar heat gain from a project’s non-roof surface area. One of these measures requires that 30 percent of a site’s non-roof impervious surfaces either be shaded, have an albedo of at least 0.3, or be an open-grid permeable paving system. (Albedo refers to solar reflectance, and an albedo of 0.3 means that 30 percent of all the energy striking a reflecting surface is reflected back into the atmosphere and 70 percent of the energy is absorbed by the surface.) Light-colored pavers can usually meet the 0.3 albedo threshold.

Projects can also earn this point by constructing underground or aboveground parking structures, which is a practice that lends itself to the extensive use of precast concrete materials.
The second Heat Island Reduction credit requires that 50 percent of a building’s roof area be covered by a vegetative roof assembly, or that 75 percent of a building’s roof area be covered by ENERGY STAR-compliant roofing materials with an emissivity of at least 0.9. There are new roof pavers currently on the market that meet these criteria.

Recycled Content
The Recycled Content credit in the Materials and Resources Section awards one point if, in aggregate, 5 percent of a project’s total material value (by cost) is comprised of post-consumer and/or post-industrial recycled content. A second point is awarded for reaching a 10 percent threshold. Since these percentages are based on cost, buildings that employ precast concrete structural systems can more easily earn the credit if precast concrete manufacturers are able to include high percentages of recycled content into their products.

The EPA’s new initiative, “Coal Combustion Products Partnership,” promotes the use of industrial byproducts in concrete to make it a more sustainable material. Industrial byproducts well suited for replacing portions of portland cement include fly ash and ground-granulated blast furnace slag. Using fly ash in concrete generally decreases permeability, improves sulfate resistance and allows for lower water content, thus improving overall performance. Ground granulated blast furnace slag consists of silicates, aluminous silicates of calcium and blast furnace slag from molten iron production. It is often used in concrete requiring maximum durability, higher strength, fire-resistance, better insulation and lighter weight.

One of the drawbacks to using fly ash or slag in concrete is increased curing times, but this is not as great a concern in precast manufacturing as it is in cast-in-place applications where construction schedules can be delayed. These types of industrial byproducts are considered post-industrial recycled materials, and while they do improve concrete performance, they typically account for only 2.5 percent to 5 percent of the overall concrete material.

Steel used for concrete reinforcement is typically composed of 95 percent post-consumer recycled content, but perhaps the biggest area where precast concrete materials can contribute to this credit is in the use of recycled aggregate, as it typically accounts for 70 percent to 80 percent of the concrete volume. While this practice is not common in the mainstream concrete market, precast manufacturers have more opportunities to test and experiment with design mixes that include recycled aggregate.

The most common materials recycled as concrete aggregate are glass cullet and crushed recycled concrete itself. Most glass, including types that cannot be reused in recycled glass bottle manufacturing (such as clear window glass) can be used as concrete aggregate, but only in non-structural applications such as precast pavers.

Regional Materials
The Regional Materials credit in the LEED Materials and Resources section awards one point for projects that, in aggregate, utilize materials manufactured within a 500-mile radius of the project site. Manufacturers who provide materials to projects within 500 miles can contribute toward a project’s achievement of this credit.

According to LEED, “manufacturing location” refers to the point of final assembly of a product. No matter where the various materials that make up precast concrete originate, the precast plant is the location that needs to be within 500 miles of a project.

A second point is available in the Regional Materials credit if, of the 20 percent of regionally manufactured materials, 50 percent of the raw materials that make up a product originate from within a 500-mile radius of the project. For example, if the aggregate was quarried within 500 miles of a given project site, that material would help contribute to earning this point.

Innovation and design process
LEED also awards up to four Innovation credits for projects that can demonstrate exemplary performance meeting stated LEED requirements. For instance, if a project exceeds the 10 percent recycled content threshold by another 5 percent, it can pick up an extra LEED point.

Innovation credits can also be earned by employing sustainable design strategies not included in LEED. For instance, close collaboration with structural engineers on a given project to develop innovative systems that are more resource efficient or less energy intensive may earn a project an additional point.

Documentation procedures
Precast concrete manufacturers and contractors will be called upon by the client to provide documentation of a given product’s environmental attributes. Design teams experienced with LEED will include these documentation requirements in the submittal requirements of the bid documents. However, LEED documentation requirements are commonly left out of a project’s submittal requirements, which results in lost time tracking down information on recycled content or albedo after the fact.

The site credits addressed above will require the manufacturer to have tested for the albedo or actual TSS and TP removal efficiency.

Precast concrete manufacturers will also be asked to certify the recycled content of their products. In many cases this will require the manufacturer to break out the material value of the cement, sand, aggregate and rebar, and provide the respective percentages of recycled content.

The Regional Materials credit will require that precast companies certify the location of manufacture as being within a 500-mile radius of the project site, which is obviously easy to determine. However, manufacturers will also need to track the origins of raw materials and break out the material values as with the recycled content credit.

This is easy to determine for materials like raw aggregate where the quarry can be easily identified, but the origin of recycled-content steel rebar or recycled aggregate may be more difficult to determine. Those in charge of procurement for precast manufacturers will need to investigate the origins of these materials to provide the appropriate documentation.

* Faddis Concrete Products is a certified plant under NPCA’s Quality Assurance/Plant Certification Program.