Thursday, December 29, 2011

Excerpts on Steel and Concrete LCA


The following excerpts provide comparison between steel and concrete industry-sponsored LCA studies.

From Modern Steel Construction (Weisenberger 2010):

“In all but one category, the structural steel building outperformed the concrete-framed building; the comparisons, expressed as percentages, are as follows:

• Global warming potential: 9% less equivalent CO2 per sq. ft
• Acidification potential : 8% less mol H+ equivalents per sq. ft
• Eutrophication potential: 9% less kg N equivalent per sq. ft
• Smog potential: 14% less kg NO equivalent per sq. ft
• Non-renewable energy primary demand: 1% more MJ per sq. ft

The only category in which steel lagged concrete was energy demand, and only by one percentage point.”

The author goes on to acknowledge:

“While the study did conclude that the steel-framed building had a lower environmental impact in four of the five categories, it should be noted that the differences in these categories did not reach a confidence threshold of a 15% difference to conclusively recommend one material over the other.”

Similarly, from a recent PCA-sponsored study (Masanet 2012) the author concludes:

“Total life-cycle GHG emissions in the baseline scenario are estimated at 14,350 Mg CO2e for the steel framed building and 14,080 Mg CO2e for the concrete framed building.”

The author later states:

 “.. for some climate zones the superior option switched between scenarios, which suggests that the technology options might play a key role in determining which option has lower life-cycle GHG emissions in practice. For such instances, improved understanding and modeling of regional and technological variations in materials pathways is of particular importance for credible materials comparisons.”

Friday, December 23, 2011

Comparing LCA Results with Biogenic Carbon

One study from New Zealand (Fernandez 2008) demonstrates the varying carbon emission results by comparing LCAs of the same building designed using four different primary building materials and two different LCI data sources.  Embodied carbon emissions factors in Figure 2 sourced from the a New Zealand dataset (Alcorn 2003) included biogenic carbon (negative indicating carbon sequestration) while the factors in Figure 3 from the GaBi inventory did not. 

Figure 1, Embodied CO2 for Four Buildings, Alcorn Database
Because the treatment of biogenic carbon in an LCA can result in significantly different conclusions, the World Resource Institute/World Business Council for Sustainable Development Greenhouse Gas Protocol (World Resources Institute 2011) recommends that biogenic carbon absorption and emissions be reported as a separate inventory item.  LCAs that report biogenic carbon in this manner are most transparent.  The LCI data for U.S. wood processes and products reported in the National Renewable Energy Laboratory’s LCI database and in CORRIM Reports does not include biogenic carbon, however, the carbon sequestered in a piece of lumber can be estimated based on wood chemistry.   

Figure 2, Embodied CO2 for Four Building Types, GaBi Database
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