Precast Concrete and LEED
Precast concrete's local proximity, energy efficiency, recyclability, and minimal waste are keys to meeting environmental standards that are gaining client interest.
Precast Concrete Sandwich Panels
Precast concrete insulated architectural panels can be attached to steel, precast concrete, or cast-in-place concrete frames. The panels can be finished on both sides, allowing the interior face to serve as the interior wall without additional finish or furring out. Panels can even be radiused to create a frame that surrounds a glass curtain wall.
Precast sandwich panels can help achieve LEED certification in a variety of ways. Panels can be recycled, manufacturing locally and have a energy efficient precast envelope. These attributes help reduce the expended energy needed to manufacture, transport, and erect structures - all important LEED requirements.
According to the LEED Energy & Atmosphere (EA) Prerequisite 2 (Minimum Energy Performance), buildings must meet ASHRAE 90.1-1999 energy-efficiency and performance requirements or the standards of a more restrictive local code, if it exists. "The requirements for the ASHRAE standard are cost-effective and not particularly stringent for concrete," says David Shepherd of the Portland Cement Association (PCA). Requirements can be calculated from tables in Appendix B of the ASHRAE standard.
Precast concrete's key benefit comes from its thermal mass, which helps the material store heat and moderate daily temperature swings. When precast concrete is used in insulated sandwich wall panels, in which a layer of insulation is sandwiched between two wythes of a concrete panel, the material can produce high R-values and lower HVAC needs. In addition, large precast concrete panels have a minimal number of joints, reducing uncontrolled air infiltration. These attributes can help a project earn as many as ten LEED credits in the Optimize Energy Performance category of the standards.
Thermal Mass Not Appreciated
According to the ECCO, recent studies, including one by the U.S. Department of Energy (DOE), have demonstrated the true benefit of thermal mass. The DOE report indicated that mass in exterior walls reduces annual energy costs in a building. The U.S. Department of Housing & Urban Development (HUD) and the National Institute of Standards and Technology (NIST) have also conducted studies.
According to the ECCO study, "modeling and testing have proven that the combination of insulation with thermal mass forms a superior wall system exhibiting the benefits of both. The most benefit comes from placing the insulation inside the thermal mass, as in insulated sandwich wall panels. Adding insulation to the interior wall, another commonly used approach, isolates the wall from direct contact with the interior, reducing the benefits of the wall's thermal mass." Regardless of placement in the wall, "thermal mass reduces loads and shifts peak loads in most climates," says Martha G. VanGeem, principal engineer and group manager for the Building Science & Sustainability Group at Construction Technology Laboratories Inc. in Skokie, Illinois.
"The guiding principle for all thermal-mass standards has been performance, whether of the individual components or the overall building envelope," says the ECCO report. "These standards have successfully translated the behavior of thermal mass into understandable and easy-to-use terms. The result is that thermal mass has become a feasible element of building design." With precast's ability to help meet LEED standards, the benefits of thermal mass will become more apparent to designers in the future.
Building Reuse (Materials Credit No. 1)
"Specific durability can be hard to quantify, because it depends on so many variables," says PCA's Shepherd. Those factors can include weather, maintenance and finishes. "Compared to concrete, many other building materials simply don't last as long without significantly more maintenance. High-performance concrete is really the way to go if you want to achieve durability." Precast concrete's durability can also eliminate the need for interior partitions and exterior cladding, and the panels may only need to be recaulked approximately once every 20 years. This reduction in the use of chemical-based materials increases a building's environmental friendliness.
Because total precast concrete systems offer long interior spans via double tees and hollow core floors and roofs, buildings are easier to remodel or reconfigure as tenant needs change. This ensures the structure can remain in place longer with only minor adjustments which do not impact the LEED rating.
Construction Waste Management (Materials Credit No. 2)
Concrete's inorganic composition makes it an ideal material to be recycled, and it is frequently crushed and reused as aggregate for road bases or construction fill, says Shepherd. As with the Building Reuse credit, these are future options available only after the building has been completed and is subsequently being considered for other uses.
Source: Environmental Council of Concrete Organizations (ECCO), 2004
ECCO indexed the environmental impact of various materials compared to concrete. Iron-ore mining, for example, has 2.25 times the impact as concrete-aggregate extraction.
Precast concrete offers other waste-saving benefits, says VanGeem. "Less material is required to produce precast components because precise mix designs and tighter tolerances can be achieved," she says. Less concrete is wasted because quantities of constituent materials are tightly controlled in the precast plant.
Waste materials are also more likely to be recycled because concrete production takes place in a single location under controlled conditions. For example, greywater - a type of wastewater - is often recycled for use in future batches. Between 5 and 20 percent of aggregate in a mix can consist of recycled concrete, and sand used in finishing can also be reused. Steel forms and other materials used in casting also are reused many times.
"There also is less dust and waste at a construction site, because only the needed precast components are delivered," she notes. "And there is no debris from formwork and associated fasteners. Fewer trucks and less time are needed because concrete is made offsite. That's particularly beneficial in urban areas where minimizing traffic disruptions is critical." Less noise is produced at the site, which reduces noise pollution.
"A properly designed precast concrete system will result in smaller structural members, longer spans, and less material used on site," VanGeem says. This translates directly into economic savings. It also results in environmental savings. Using less material means using fewer natural resources and less manufacturing and transportation energy.
Recycled Content (Materials Credit No. 4)
The primary reason that supplements contribute to concrete's environmental friendliness comes from the cement itself, which comprises 7 to 15 percent of the total volume of a precast component. Cement is a manufactured product and uses energy during production, though the industry has reduced energy used per ton of cement by 35 percent since 1972. Water comprises 14 to 21 percent of precast concrete's volume, while aggregate accounts for the rest.
The most common supplementary cementitious materials are fly ash, silica fume, and slag cement. Fly ash is the residue remaining from fossil-fuel power plants, silica fume is a byproduct of the electric-arc furnace used in the production of silicon or ferrosilicon alloys, and slag cement is created from iron blast-furnace slag. Each of these waste products would end up in a landfill if they were not reused. In many cases, the use of fly ash and other supplementary materials can produce a more durable product than a total cement mix can provide.
Too Much Fly Ash?
"The best approach is to talk with the producer in advance and avoid prescribing an amount of fly ash beforehand," Shepherd says. "Tell the producer to use the maximum amount that's effective and give him a performance standard for the application, not just a percentage. The goal for long-term performance of a building will optimize the use of fly ash, not maximize it."
Local/Regional Materials (Materials Credit No. 5)
Precast concrete can meet both of these requirements in most cases. Typically, precast plants are located within 200 miles of a project, and the raw materials used to produce the precast concrete components, such as cement, aggregate, prestressing strand and rebar, are usually obtained from sources within 200 miles of the precast plant. The local proximity advantage leads many designers to replace granite, stone, and other imported and transported products with precast concrete panels.
Innovation in Design (Innovation and Design Process)
Craig A. Shutt Managing Editor Ascent Magazine, Precast/Prestressed Concrete Institute, Chicago
LEED POINTS AVAILABLE FOR PRECAST CONCRETE PROJECTS
Source: Environmental Council of Concrete Organizations (ECCO) plus 2/5/03 Credit lnterpretation Request to LEED