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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Q2: Most Effective Strategies

What are some of the most effective design strategies that I can put into practice as a structural engineer?

Author: Frances Yang; Contributors: Steve Buonopane, Lionel Lemay, Kate Simonen, Dirk Kestner

Design for material efficiency

• Design that maximizes efficient use of materials by not oversizing simply for ease and speed of construction can reduce an estimated 5% to 7.4% of embodied carbon, excluding operational energy. (Anderson 2009)
• On the other hand, designs with unconventionally long spans tend to incur a 20% increase in embodied carbon to the structure, or 10% to the whole building.(Arup 2010)

Design for adaptability and deconstruction to enable future change in function

• This is highly dependent on the assumptions of future use, but a simple estimate performed by Anderson and Silman indicates that a 43% embodied carbon savings can be achieved if the design doubles the structure’s lifespan. (Anderson 2009)
• The Deconstructable and Reusable Composite Slab invented by a team of structural engineers enables reusability of nearly all components.  A precast system set on reusable steel beams loses the efficiency of composite action, and therefore demands approx. 30% more material.  This new system retains composite action while allowing for disassembly and reuse.   This concept won an award in the 2007 Lifecycle Building Challenge. (Webster 2007)

Figure 1: Deconstructable and Reusable Composite Slab. Image © Simpson Gumpertz & Heger Inc.

Use salvaged materials

• A design that uses 50% salvaged steel sections showed a 34% decrease in embodied carbon. (Anderson 2009)
• NREL’s building in Colorado used 124 salvaged pipe columns for 88% of the columns on the project.  A cradle-to-gate LCA on the columns found that when compared to equivalent manufactured pipe columns, the use of salvaged columns reduced CO2 emissions by 65%. Total energy was reduced by over 76 %. (Guggemos 2010)

Minimize cement in the concrete

• Use of slag, a waste material, to replace, 50% of conventional cement in the concrete alone resulted in a 38% carbon savings for a concrete framed structure, and a 22% savings for the same structure with a steel frame.  (Anderson 2009)
• Another study for the Concrete Centre confirms the previous findings by Anderson and states that specification of cement content in concrete mixes can affect the embodied carbon of the structure by +/-33%. (Arup 2010)  Read more on sources of variability in Q5.
• The lowest reduction cited was in a study by MIT that found that increasing supplementary cementitious material (SCM) substitution in a concrete building from 10% to 25% decreased pre-use GWP by only 4.3%. The same study also found that Increasing SCM substitution from 10% to 50% in ICF walls can reduce the pre-use GWP by 12% to 14%. (Ocshendorf 2010)
• Furthermore, Anderson found that a design using both concrete containing 50% slag and 30% recycled aggregate, in combination with 50% salvaged steel, can reduce embodied carbon by an estimated 41% to 45% of embodied carbon. (Anderson 2009)

It should be noted that these were based on a direct replacement of cement with SCM’s, such as slag or fly ash, meaning the reduction in cement was equally the percent recycled content.  Structural engineers should be careful when reviewing concrete mix submittals that the supplier has effectively reduced the quantity of cement, not merely added SCMs to increase recycled content.

Integrate the structural system for optimal operational energy performance

See Q3 for these.

Design and promote resilient structures

Traditional LCA methodologies do not consider the risk of building life and life of building components to catastrophic hazards. Some methods of integrating LCA and damage assessment are under development. A broader need is for resilience-based design to make its way into standard practice. See Q9 for more on this topic.

Specify more environmentally responsible sourcing

The issues around biogeniccarbon are complex.  Life cycle assessment does not currently capture all the metrics relevant to healthy forests yet importance of responsibly managed forests is undeniable.  See Q5 for more on biogenic carbon and support behind responsible forest management.

In summary, there are numerous design and specification strategies structural engineers can implement to improve the environmental performance of buildings both upstream and downstream of construction.  See our Bibliography, and particularly TheSustainable Design Guide for Structural Engineers, for more detail on how to implement all of these.

References

Anderson, J., Silman, R. (2009) “A Life Cycle Inventory of Structural Engineering Design Strategies for Greenhouse Gas Reduction,” Structural Engineering International, March 2009 Issue.

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

Guggemos, A.A., Plaut, M.; Bergstrom, E.; Gotthelf, H.; and Haney, J. (2010) “Greening the Structural Steel Process: A Case Study of the National Renewable Energy Laboratory,” Proceedings of the 2010 Structures Congress, ASCE, Reston, Virginia.

Kestner, D., Goupil, J., and Lorenz, E. (2010) Sustainability Guidelines for the Structural Engineer, Sponsored by Sustainability Committee of the Structural Engineering Institute of ASCE, Reston, VA: ASCE, 978-0-7844-1119-3.

Marceau, M., and VanGeem, M. (2008) Comparison of the Life Cycle Assessments of an Insulating Concrete Form House and a Wood Frame House, SN3041, Portland Cement Association, Skokie, Illinois.

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. (2010) “Life Cycle Assessment (LCA) of Buildings Interim Report,” Concrete Sustainability Hub, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Webster, M., Kestner, D., Parker, J., Waltham, M.. (2007)  “Deconstructalbe and Reusable Composite Slab,” Winners in the Building Category: Component – Professional Unbuilt, Lifecycle Building Challenge <http://www.lifecyclebuilding.org/2007.php>

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Coal Waste Processors Sue EPA

Coal waste products, like flyash, have long been used as complimentary cementitious materials to improve strength and durability of concrete while reducing cement content and therefore the embodied carbon of concrete. Until recently, even the EPA has been supportive of the commercial use of such materials. Now the EPA is taking a second look at the heavy metal content in these byproducts. A final ruling on whether to classify these materials as hazardous waste remains in limbo, but the uncertainty has angered the largest coal waste producers who have filed suit against the EPA demanding a deadline for the ruling.

Jim Vallette of the Healthy Building Network has published an interesting article outlining the positions of each side on this case. Online at https://app.e2ma.net/app/view:CampaignPublic/id:1405374.11099227719/rid:e87b8c542d06d8efb7270d2f375cc924

This issue was addressed in a previous blog article on this website: House Bill Takes on EPA Ruling Process

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10 Steps to Greener Concrete

According to the World Business Council for Sustainable Development (WBCSD), “concrete is the most widely used material on earth, apart from water, with nearly three tons used annually for each man, woman, and child.” Most structural engineers are familiar with efficient design practices for working with concrete, but there are sustainable considerations that should also be taken into account. There are at least three areas in which the characteristics of concrete can be used sustainably:

1.     Exposed concrete can be used as both an interior or exterior finish, thereby reducing the additional material cost of cladding, painting, installing drop ceilings and sheeting, etc.

2.     Concrete structures intrinsically have more thermal mass – a property that enables heat energy to be absorbed, stored, and later released, giving greater comfort in the both the heat of summer and the cold of winter.

3.     Carefully considered mix designs can reduce the embodied carbon intrinsic to concrete, improve long-term durability, and still provide sufficient workability.

Unfortunately, the production of portland cement releases a high volume of carbon. Approximately 40% of embodied carbon is associated with powering the extremely hot furnaces needed for the transformation. Cement manufacturers are experimenting with ways to become more efficient. (4) Structural engineers can specify that cement be sourced from plants scored by the Energy Star Industrial Focus Program with an Energy Performance Indicator (EPI) above 75.

The remaining 60% of embodied carbon in cement is a result of calcination, the intrinsic chemical reaction whereby limestone is transformed into clinker, on the way to becoming cement. Therefore, the primary means of greening concrete is to reduce the amount of portland cement in the mix.

Fortunately structural engineers can have great control over the mix designs selected. Some relatively simple steps can be taken to ensure that more sustainable concrete mixes are used on your job site. These can be simplified into four rules of thumb: (5) reduce water content, (6) use complimentary cementitious materials (CCMs), (7) use the maximum aggregate size, and (8) specify proper strengths.

Reduce water content. Keep the water/binder ratio low, and although less cement is used, the same strength can be achieved. A low w/b is also good for durability. Based on the relationship of specific gravity between concrete and water, a w/b ratio greater than 0.32 is most likely to result in free water that is not bound to the binder paste. This results in unwanted voids and drying shrinkage as the free water evaporates (instead of being consumed in the chemical reaction).

High slump is often desirable for workability. (9) Fly ash and superplasticizers help improve workability without increasing water. The spherical shape of fly ash acts as a physical lubricant and thus aids in cement hydration. Water-reducers likewise increase slump, however, too much of these admixtures can cause segregation and excessive bleed water. A good rule of thumb is to limit the water-reducers or superplasticizers to 2 percent of the mass of the binders.

Fly Ash (Meryman 2007)

Use complimentary cementitious materials (CCMs). Fly ash, slag, natural pozzolans, and ultrafines can be used in lieu of portland cement. Many such materials have less embodied carbon or are recycled industrial byproducts. The basic chemical reaction between portland cement and water produces calcium hydroxide (CH).  Many CCMs then react with the CH to produce calcium silicate hydrate (C-S-H), which provides a much stronger bond, particularly around the aggregates.

With regard to durability, C-S-H is known to be a much denser product and therefore less permeable. Non-cement binders also tend to reduce the heat of hydration. Although specific alkali-silica reactions are known, CCMs generally enhance both strength and durability. (10) For a more in-depth analysis of durable mixes, designers can utilize the Life-365 freesoftware from the National Ready Mix Concrete Association (NRMCA).

Use the maximum aggregate size. This reduces the surface area that the binder paste needs to cover thereby keeping the past volume lower. Normally available aggregates are stronger than the surrounding hardened paste.

Specify proper strengths. Choose target strengths at ages that realistically reflect the needs of the project. Recall the above described chemical reaction involving CSMs: C-S-H takes longer to develop and the conventional 28-day period may not be sufficient. If possible, specify 56 or even 90 day strength. This gives mix designers at the batch plant more freedom to utilize some CCMs.

The above recommendations were sourced from SustainabilityGuidelines for the Structural Engineer, Chapter 3.2 – Concrete. Current and former SEI Sustinability Committee Members influential in authoring the referenced chapter include: Helena Meryman, Sarah Vaughan, Alan Kren, and Iyad Alsamsam. This summary is by Ken Maschke, P.E., S.E., LEED A.P., associate with Thornton Tomasetti in Chicago, IL.

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Gray to Green: How to Make Cleaner Concrete

Sustainable concrete has captured the imagination of always provocative Popular Mechanics. A recent web article explores radical new ways to green concrete. Most structural engineers are well versed in supplemental cementitous materials like fly ash and blast furnace slag, but have you considered rice husks, sewage sludge, and geopolymers? The article also suggests using prcelain from recycled toilets for aggregate and hempcrete blocks as an alternate to CMU. Finally, a PM article wouldn't be complete without some exploration of space. Strategies are discussed for converting moonrocks to traditional or sulfur-based concretes.

http://www.popularmechanics.com/home/how-to-plans/masonry/gray-to-green-how-to-make-cleaner-concrete#slide-1

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House Bill takes on the EPA ruling process. What's best for Fly Ash?

Most of you will recall the EPA is considering whether CCRs that are not designated for beneficial reuse (such as fly ash in concrete) should be regulated as a Hazardous Waste or Household Garbage.  There are nuances to the ruling but that is the crux and either way beneficial reuse would still be allowed.  One of the raging debates is whether a hazardous designation will hamper or encourage higher recycling and reuse rates.   Now the House of Representatives has introduced a bill that would undermine the EPA ruling and comment process and their authority to regulate or not regulate based on considered analysis of science, economics, public comments and existing law.
The following link to a short article provides an interesting and concise update (and I’ve pasted some excerpts as a teaser):
http://www.wtrf.com/story.cfm?func=viewstory&storyid=98061
“ “During my 10 years working for the hazardous waste disposal industry, I noticed that hazardous waste disposal companies lost market share over time to recyclers and beneficial users,” according to Scott Slesinger, now legislative director for the Natural Resources Defense Council. “Market economics made this obvious — the higher cost of disposal led to finding cheaper alternatives.”
H.R. 1391, the “Recycling Coal Combustion Residuals Accessibility Act of 2011,” would prevent the U.S. Environmental Protection Agency from regulating coal combustion residuals, abbreviated as CCRs and often called “coal ash,” under the hazardous waste subtitle of the Resource Conservation and Recovery Act.
That would leave only one of two approaches the agency proposed in June 2010: regulating CCRs in the same manner as household garbage.….
….In an April 14 House of Representatives Energy and Commerce Committee hearing prompted by McKinley’s bill, Mathy Stanislaus, Assistant Administrator for the EPA’s Office of Solid Waste and Emergency Response, expressed the agency’s concern about the bill.
“We want to make an informed ruling,” Stanislaus said. “This would remove that one option and not allow us to make our decision based on all the data and the 450,000 comments.” ”

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