Precast Concrete Life-Cycle Assessment and High-Performance Design
Study Results
The study yielded three important results that have far-reaching implications for design professionals and the industry as a whole:
Building Environmental Performance Is Dominated by the Operating Energy Stage
Operating the building over its lifetime—that is, the use phase—was shown to have the greatest overall impact on the environmental performance of a building. In fact, more than 90 percent of the impacts relating to global warming, acidification, respiratory effect, eutrophication, photochemical smog, and total primary energy occur in the use phase. While the numbers vary somewhat among the four cities studied, in all cases at least 97 percent of the total primary energy (TPE) and 96 percent of the global warming potential attributed to the building over its lifetime occur in the use phase. This is a primary reason why environmental impacts should be evaluated over the full life cycle of a building.
Environmental Performance of Precast Concrete Buildings Is Competitive
The peer-reviewed study also confirmed a basic conclusion of most balanced LCA studies of commercial buildings, namely that there is presently not a significant difference in life-cycle impacts between steel, cast-in-place concrete, and precast concrete building systems. Although concrete is sometimes perceived to have a higher environmental impact due to energy use and carbon dioxide emissions associated with manufacturing portland cement, the fact is, as shown by this research, precast concrete does not impose additional environmental burden than other materials over the full building life cycle.
What is Precast Concrete?
Precast concrete consists of concrete (a mixture of cement, water, aggregates, and often admixtures) that is cast into specific shapes at a location other than its final in-service position. The concrete is placed into a form, or mold usually made of wood or steel, and cured before being stripped from the form, typically the following day. These components are then transported to the construction site for erection into place.
Photo by The Exoro Group and GSBS
The Salt Lake City Public Safety Building used high-performance, insulated, precast panels embedded with terra cotta tiles to meet its aesthetic and energy-efficiency goals. The panels also provide the structure with the required blast resistance.
Precast concrete is reinforced with either conventional reinforcing bars, prestressing strands with high-tensile strength, or a combination of both. Prestressing is a method of reinforcement where the steel strands are tensioned, with the tensile force released into the concrete after the concrete reaches the minimum required strength. Often the precast concrete industry uses pre-tensioning, where the steel strands are tensioned in the form before the concrete is cast. Once the concrete is cured to a specific strength, the strands are cut (detensioned). As the strands, having bonded to the concrete, attempt to regain their original untensioned length, they bond to the concrete and apply a compressive force.
Photo © Paul Turang
The design-build Century project includes an exterior comprised of 170,000 square feet of architectural precast concrete with multiple finishes, including Italian travertine veneer cladding at the base.
Precast/prestressed concrete can contribute to high-performance design in many ways. It is a versatile and durable material, produced in a factory by highly trained personnel with virtually no waste, under stringent quality control measures. Precast components can be transported for just-in-time delivery, with little or no required lay-down space. Precast concrete can be quickly erected on the jobsite, with minimal disruption to the site, and precast's thermal mass can save energy and increase user comfort. In addition to providing versatility in aesthetics, structural design, and use, precast concrete inherently provides a high level of resiliency which protects against multiple hazards such as fire, severe storms, explosions, and even earthquakes. The material contains no volatile organic compounds (VOC), nor does it provide a food source for mold which helps maintain a healthy indoor environment. It is also an effective sound insulator and can help maintain more uniform indoor temperatures.
Photo by Jacia Phillips Photography
Precast concrete has tremendous aesthetic versatility. Here, a five-color, random blend terra cotta tile pattern was embedded into insulated precast concrete panels, providing a cost-effective and efficient method for construction.
Precast Components—A variety of components can be fabricated from precast concrete, meeting a range of project needs. Customized pieces, sizes, and shapes can be created in most cases to meet specific needs. The most common components that precast concrete manufacturers produce and that designers incorporate into their projects include floors and roofs; walls; building structural components (beams, columns, etc.); piles; transportation components; and modular components. Architects are well advised to consult with a producer early in the design phase to determine what components will work most efficiently and review specific sizes, joint locations, and other details that can create cost-efficient options.
The following 10 environmental impacts were evaluated for each of the 15 building cases, in each of the four cities. To provide a simplified characterization, the following discussion focuses on the coefficient of variation (COV) of the results for the 15 building cases for each environmental impact. For detailed life-cycle impact assessment results, architects can refer to the full LCA report.1
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