How Sustainable Is Seattle's Concrete?
By Erica Fischer and Adam Slivers

Concrete mixes that meet requirements of Seattle Building Code Section 1905.1.11 and Directors Rule 11-2014 are waived of plant inspections by the building official on an annual basis. This collection of mix designs streamlines approval of concrete mixes on a project-by-project basis. The SEAW Sustainability Committee obtained the mix designs for 390 continuously approved concrete mixes, conducted a life cycle assessment of the mixes (evaluating the mixes for the six impact categories listed in LEED v4 “Building product disclosure and optimization - environmental product declarations” Option 2), and compared these values to the industry averages listed in the National Ready Mix Concrete Association (NRMCA) Industry Benchmark Report for the Pacific Northwest region. The committee conducted their own environmental assessment on the mixes because only a handful of mixes in Seattle have Environmental Product Declarations (EPDs).

For the purpose of this article, we will focus on the results from the calculated Global Warming Potential (GWP) of these mixes. The goal of the study was to understand the environmental footprint of very common concrete mixes used in Seattle and how they compare with the NRMCA regional benchmarks. Seattle’s collection of continuously approved concrete mixes provides local engineers with data to compare local concrete to environmental benchmarks created for LEED v4.  LEED v4 has been available for over two years, but most building design professionals will transition to LEED v4 when LEED 2009 becomes obsolete on October 31, 2016. Rather than focus on individual attributes such as recycled content and local resources, LEED v4 promotes the holistic approach of life cycle assessment to evaluate the energy used and emissions created through manufacturing, using, and disposing of building materials. Manufacturers can provide information about their products’ environmental performance through EPDs in six impact categories (such as global warming potential). Concrete is one material where structural engineers can make an impact on reducing carbon emissions and embodied energy of buildings.

Figure 1 shows the GWP of all of the continuously approved concrete mixes in Seattle. As expected, higher strength mix designs have higher GWP, largely due to the increased quantity of cement. We found that on average 96% of the GWP of concrete is from cement. The plot also shows that there is a large range of GWP for concrete of the same 28-day compressive strength. Reducing cement content of a concrete mix reduces the GWP of the concrete mix. 

Figure 1: GWP of all continuously approved concrete mixes in the database

Figure 2 shows the data for the continuously approved concrete mixes without any cement substitution (black dots) and with 50% cement substitution with slag (orange dots). This plot shows that a 50% cement substitution results in about a 34% reduction in GWP of concrete. Figure 2 also shows that the continuously approved concrete mixes were below the regional baselines developed by NRMCA. 

Figure 2: GWP of continuously approved concrete mixes with 0% (black) and 50% (orange) cement substitution with NRMCA baselines (dashed lines) 

LEED v4 provides a project team with one Materials and Resources credit when an EPD is provided. For an additional credit, the majority of building materials must be at least 10% below the industry average, in three out of the six impact categories. We found that in general the commonly-used continuously-approved mixes are an improvement on the NRMCA averages for the Northwest region. This means design professionals in Seattle can pursue two additional LEED v4 credits:

  • Credit 1: Utilize the reductions in environmental impact due to concrete mix selections to qualify for Whole Building Life Cycle Assessment improvements (v4 MRc: Building life-cycle impact reduction Option 4)
  • Credit 2: Obtain EPDs from the concrete supplier showing that project mixes demonstrate environmental impacts below industry averages (v4 MRc: Building product disclosure and optimization - environmental product declarations, Option 2).   

In summary, Seattle’s continuously-approved concrete mixes are quite “green.”  The SEAW Sustainability Committee found that the mixes generally exceed average environmental impacts calculated by NRMCA for the Pacific Northwest.  Smaller than average proportioning of cement is the most critical component to reduce the impact of concrete mixes on the environment. Design teams may take advantage of this attribute by specifying those mixes with lower impacts to gain LEED v4 points for whole building life cycle assessment. If local ready mix providers were to provide more third party verified EPDs, designers could leverage greener concrete for additional LEED credits in the Seattle region.

Erica Fischer (efischer@degenkolb.com) is a Design Engineer at Degenkolb Engineers and Adam Slivers (aslivers@lundopsahl.com) is an Associate at Lund Opsahl.  Other members of the SEAW Sustainability committee that contributed to the concrete mix database and analysis include Sean Augustino, Blake Doepker, Shana Kelley, Gino Mazzotti, Luke Ruggeri, Kyle Steuck, Rachel Vranizan, and Morgan Weise.