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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