• Global warming potential, total primary energy, acidification, respiratory effects, eutrophication, photochemical smog, and solid waste impacts. Within a given city, the COV of results for the 15 building cases relating to global warming potential, total primary energy, acidification, respiratory effects, eutrophication, photochemical smog, and solid waste was 2 percent or less. This means that for a given climate, there is little difference among the building cases for these seven environmental impact categories.

• Water use impact. Within a given city, the COV of results for the 15 building cases relating to water use was 14 percent, meaning that for a given climate, there is a notable difference among the building cases for water use impact. Water use in the operating energy stage is the same for all cases. However, buildings with steel structural systems—regardless of envelope system—have greater water use impact in the construction stage because they were assumed to employ masonry elevator and stairwell walls, which embody significant water use. Precast concrete's water use impact was generally less than steel construction and roughly equal to cast-in-place construction.

• Abiotic resource depletion. Abiotic resources are those resources that come from non-living, non-organic material such as land, fresh water, air, and metals. Within a given city, the COV of results for the 15 building cases relating to abiotic resource depletion was 36 percent, demonstrating that for a given climate, there is a great difference among the building cases for abiotic resource depletion. The majority of the abiotic resource depletion impact occurs in the manufacturing and construction stages. Regardless of city, the buildings with largest abiotic resource depletion impact were those with steel structural systems. Specifically, the manufacturing stage abiotic resource depletion impact of just the floors and roofs for buildings with steel structural systems was greater than that for the entire building for either cast-in-place or precast concrete structural systems.

• Ozone depletion. The COV of results for the 15 building cases relating to ozone depletion varied from 0 to 15 percent, depending on the city. This means that the difference among the building cases for ozone depletion depended on climate. Most ozone depletion impact results from the use of extruded polystyrene (XPS) insulation. Use of XPS in roofs, which is relatively consistent for all building cases in this study, is the primary driving factor. All the buildings in the study have XPS insulation in the roof, and during the maintenance stage, when the roof is replaced every 20 years, the insulation is also replaced. Over the life of the building, more XPS insulation is used in the roofs of all buildings than is used in the walls of the buildings with cast-in-place or precast concrete envelopes. However, XPS is also sometimes used in concrete walls (both cast-in-place and precast). This means that in climates where wall insulation is used (for this study, all cities except Miami), the concrete-wall building cases had greater ozone depletion impacts. For Miami, there was essentially no difference among the building cases for ozone depletion.

Continuously Improving—The Environmental Performance of Precast Concrete

Although manufacturing stage impacts are a small percentage of the overall environmental impact of buildings, one goal of the LCA study was to identify manufacturing stage impacts for the constituent precast concrete elements (hollow-core slabs, wall panels, columns, beams, double tees). This is important to the precast concrete industry, so that it can continuously improve any environmental hot spots in its manufacturing processes. Unpacking this data revealed some minor differences—about 6 percent—among the various element types. Overall, the LCA revealed a few key environmental hotspots in the manufacturing stage impacts for all precast concrete element types including use of portland cement, which accounts for the majority of the environmental impact in most categories, and plant energy used in the fabrication process. This accounts for 23–25 percent of the total primary energy and 13–14 percent of the global warming potential for the manufacturing stage.