Carbon Group Post 3: Example Demonstrating How SCMs Can Reduce Embodied Impacts of a Concrete Building

This is the third in a planned series of blog posts on topics that are discussed in depth in the SEI Sustainability Committee’s forthcoming technical report, Building Structure and Global Climate, due out later this year.

In this example, a cradle-to-gate LCA was conducted to determine the embodied impacts of concrete on a building to compare the Global Warming Potential (GWP) of a reference building using typical concrete mixes with moderate amounts of Supplementary Cementitious Materials (SCMs) such as fly ash and slag and a proposed building using concrete mixes that have relatively high volumes of fly ash and slag. The building is an 18 story residential tower located in the northeast United States. Compressive strengths for each structural element are identified in Figure 1.

Figure 1. Specified compressive strength of concrete for an 18 story residential tower.

The first step in the analysis is to identify typical concrete mixes for the reference building. In 2014 (updated in 2016), National Ready Mixed Concrete Association (NRMCA) published benchmark mix designs and their environmental impacts for eight different regions in the United States (www.nrmca.org/sustainability). This example uses the benchmark mix designs for the Northeast region. 

The next step is to estimate mix designs that have significantly lower GWP than the benchmark mixes that still meet the performance criteria (strength, durability, etc.). This example uses high volume SCM mixes from the NRMCA Industry-Wide Environmental Product Declaration (EPD). A summary of the concretes selected for the reference and proposed building are provided in Tables 1 and 2.

Table 1. Mix designs selected for the reference building (from NRMCA benchmark report)

Concrete Element	Specified Compressive Strength (psi)	Portland Cement (lbs/yd3)	Slag (lbs/yd3)	Fly Ash (lbs/yd3)	SCM content
Mat Foundation	6000	782	119	82	20%
Basement Walls	5000	741	112	78	20%
Floors B2-1	5000	741	112	78	20%
Floors 2-18	5000	741	112	78	20%
Shear Walls	6000	782	119	82	20%
Columns	8000	967	147	102	20%

Table 2. Mix designs selected for the proposed building

Concrete Element	Specified Compressive Strength (psi)	Portland Cement (lbs/yd3)	Slag (lbs/yd3)	Fly Ash (lbs/yd3)	SCM Content
Mat Foundation	6000 psi	256	342	256	70%
Basement Walls	5000 psi	242	323	242	70%
Floors B2-1	5000 psi	512	0	341	40%
Floors 2-18	5000 psi	581	0	249	30%
Shear Walls	6000 psi	427	256	171	50%
Columns	8000 psi	503	302	201	50%

Using the Athena Impact Estimator for Buildings (Athena IE) software (www.athenasmi.org), the reference building and proposed building were defined using the proposed mixes in Table 1 and 2 respectively. Athena IE has the NRMCA benchmark mixes and the NRMCA Industry-Wide EPD mixes pre-loaded into the software. The software also permits the user to define new mixes based on the existing mixes in the library or completely new mixes if that information is available from a concrete producer.

Once all the concrete information is defined for each project, the user can then run a report that will provide the estimated GWP, along with other impacts, for each building. The reference building will represent the largest impacts and the proposed designs will represent lower impacts. The results for this example showed that the reference building has a GWP for concrete of 6.14 million kg CO₂ while the proposed building has a GWP for concrete of 3.92 million kg CO₂ meaning that the high volumes of fly ash and slag mixes resulted in 36% reduction in GWP as shown in Figure 2.

Figure 2. Summary of GWP for reference building and proposed building.

Keep in mind, this is example was simplified for illustration purposes. It only considered the effects of concrete during the material extraction and manufacturing stage on the environmental impacts of the building. The Athena IE software does contain environmental impact information for most materials and products used in buildings and allows input of operational energy data for conducting a more comprehensive whole building LCA.

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Q1:Which is better, steel or concrete?

Which is better, steel or concrete?

Author: Frances Yang; Contributors: Steve Buonopane, Kate Simonen, Adam Slivers

Although many trade-sponsored studies promote one type of structural material as more sustainable than the other, the results across all concrete vs. steel studies we have seen reinforce the conclusion that the likely variation within each material is greater than the difference between materials (Weisenberger 2010, Masanet 2012, Arup 2010).

Figure 1-1: Results from literature review of 13 embodied carbon studies

This conclusion is influenced by two primary factors. First, all structures are truly composite systems of steel and concrete, with significant amounts of both concrete (foundations, fill in deck) and steel (rebar).   Second, differences in the goal, scope and functional units of various LCAs leads to greater differences in results than the difference between the concrete and steel figures of each study (Hsu 2010).  Variability across material types within a single LCA study are often less than 5%, yet variability of the same material types across multiple LCA studies are on the order of 25% to 35% (Ramesh 2010).

Figures 1-2, 1-3: Courtesy of S. Hsu at MIT

Another very important consideration in LCA is the full life-cycle.  Most studies only include cradle-to-gate or cradle-to-site impacts, meaning from extraction of the raw materials to leaving the product factory gate or completion of construction on site.  Truly holistic evaluation should include the full life-cycle.  A recent MIT study compared steel, concrete and wood frame commercial and residential structures, and quantified the carbon emission impact of building systems over their complete life cycle. This study found that the greatest differences in impact were in the operation phase of the life-cycle (Ochsendorf 2011). See Q2 and Q3 for ideas on how structural design and specifications can affect operational impacts.

So a more effective question the SE should ask is, “What can a structural engineer do, to lower the environmental footprint of the building within the structural system and material(s) selected?”  LCA can help you identify the material or system which has the most potential for effective sustainability improvements within your specific project constraints. Most often, the selection of the main structural system is based on traditional performance criteria—strength, fire resistance, durability, constructability, low vibration, noise control, aesthetics and cost—just to name a few. Sustainability criteria, including environmental impacts, are additional performance criteria that should be considered in the design process on all projects.

Material efficiency, designing for adaptability and deconstruction to enable reuse, using salvaged materials, integration of structural system with building systems to optimize energy use such as design to utilize thermal mass or to optimize daylighting potential, reducing cement where appropriate in concrete mixtures, and specifying more environmentally responsible sourcing -- structural engineers can implement all of these measures to some degree all buildings.  (See Q2 for more detail.)  As every building project has unique criteria and available resources, the best approach for SEs is to maximize these opportunities, and use them to inform materials selection, rather than the other way around.

At the same time, variability in accounting is a current problem in arriving at truly comparable results, and structural engineers need to do the following things to address this problem:

1. Be clear on the boundaries and accounting method used in LCA studies to insure final comparisons consider the full life-cycle impacts.  This first requires an understanding of the methodological differences that can impact LCA results.  See Q5 for the various accounting methods seen in life-cycle inventories and tools.
2. Focus on reducing impacts that are within their control through design and specification.  See Q2, plus Q3 for examples specific to operational energy performance, and Q9 for resilience.
3. Engage in creation of product category rules (PCRs) and environmental product declarations (EPDs).  See Q6 for more on PCRs, EPDs, and whole building LCA standardization
4. Recognize the broad range of environmental impacts related to material extraction, manufacturing, use and end of life.  See Q7 and Q8 for more information on LCA metrics.

References

Arup (2010) “Embodied Carbon Study: Study of Commercial Office, Hospital and School buildings,” The Concrete Centre, United Kingdom. 

Hsu, S. (2010) “Life Cycle Assessment of Materials and Construction in Commercial Structures: Variability and Limitations,” Massachusetts Institute of Technology, Cambridge, Massachusetts. 

Masanet, E., Stadel, A., and Gursel, P. (2012) Life-Cycle Evaluation of Concrete Building Construction as a Strategy for Sustainable Cities, SN3119, Portland Cement Association, Skokie, Illinois. 

Ochsendorf, J., et al. (2011) “Methods, Impacts, and Opportunities in the Concrete Building Life Cycle,” Massachusetts Institute of Technology, Cambridge, Massachusetts. 

Ramesh, T., Prakash, R., Shukla, K.K. (2010) “Life cycle energy analysis of buildings: An overview,” Energy and Buildings, 42 1592-1600. 

Seppo, J.; Horvath, A., and Guggemos, A. (2006) “Life-Cycle Assessment of Office Buildings in Europe and the United States,” Journal of Infrastructure Systems, March 2006 Issue. 

Weisenberger, G. (2010) “And the Winner is... A framing system’s environmental impact depends on more than just the choice of materials,” Modern Steel Construction, Aug 2010 Issue.

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