Tuesday, March 20, 2018

Carbon Group Post 7: Fiber Reinforced Polymers (FRP)

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Fiber reinforced polymers (FRPs) are often used for retrofit and specialized applications even though they are not common for primary structural systems. For those unfamiliar with FRPs, carbon fiber and glass fiber sporting equipment such as surfboards, bicycles, and golf clubs may provide the best example of the material. FRPs are applied by impregnating a fiber such as carbon or glass with a polymer such as epoxy. Once the polymer hardens, the fibers provide the bulk of the structural strength and stiffness while the polymer maintains the shape and protects the fibers. FRPs are convenient in that their properties are customizable by varying the size and orientation of the fibers.

Application of near surface mounted FRP strips for supplemental strengthening (courtesy of  Simpson Gumpertz & Heger Inc.)

This blog post is concerned with the climate impact of FRPs. How high are their greenhouse gas (GHG) emissions? What place do FRPs have in minimizing climate impact of structures?
The greenhouse gas (GHG) impact of FRPs has been summarized in a chapter of a recent ASCE publication, Structural Materials and GlobalClimate: A Primer on Carbon Emissions for Structural Engineers.  This post presents the main conclusions of the FRP portion of that publication.
By weight, the GHG emissions from FRPs are relatively high when compared to materials such as steel or concrete. However, FRPs can be competitive with other materials on a GHG-basis due to their high strength-to-weight ratios and their potential for use in retrofit applications that greatly extend the life of structures.
The high strength-to-weight ratios of FRPs makes them competitive with other materials despite their high emissions per unit weight. Sufficient strength can be achieved with a relatively low amount of material. For example, the strength of a carbon-epoxy FRP is around 220 ksi, while most steel is well below 100 ksi. A comparison of GHG emissions based on the amount of material required to resist the same force at ultimate stress suggests that FRPs emit between 18% and 29% of the GHGs emitted by steel. Details of this comparison can be reviewed in Structural Materials and GlobalClimate: A Primer on Carbon Emissions for Structural Engineers.
A second reason that FRPs are competitive with other materials is their utility in retrofit applications that can greatly increase the service life of a structure. If the judicious use of FRPs can avoid the need to demolish and reconstruct, then all of the GHG emissions associated with those operations can be saved. The net impact, even if GHG emissions from FRPs are relatively high, can be much lower in the retrofit case.
Each situation is different and it is impossible to make general conclusions about the overall GHG emissions from any given project and no material is hands down better than any other. Rather, the GHG impacts of each material are one of the many factors for their selection in a structural system. There are many other aspects of the behavior of FRPs that influence its use as a structural material (e.g, cost, environmental resistance, and brittle behavior of FRPs requiring higher safety factors) but there is no doubt that FRPs have a place in environmentally-responsible construction.



FRP column test specimen with longitudinal strips and wraps in the transverse direction


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Saturday, February 24, 2018

Carbon Group Post 6: Concrete Masonry Units (CMU)

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The Carbon Working Group continues our series of blog posts on topics from SEI Sustainability Committee’s newly-released technical report, Building Structure and Global Climate.

As we write this post about the carbon footprint of masonry materials, we see more Environmental Product Declarations use in construction.  Environmental Product Declarations, or EPDs, are documents that quantify a product's embodied carbon footprint and other environmental impacts, and are used to achieve LEEDv4 Materials and Resource credits.  We referenced EPDs throughout Building Structure and Global Climate. Last fall, California's governor signed into law the Buy Clean California Act, which will require some building materials manufacturers for publicly-funded infrastructure projects to report carbon footprints through EPDs.  House Bill 2412 in Washington State, aims to enact something similar.

Concrete masonry is one of the four major structural material systems covered in the committee's technical report, along with wood, concrete and steel.  Concrete masonry unit walls are combined with wood, steel, or concrete floor systems to create many low-rise building types, such as warehouses, shopping centers, offices, and single- and multi-family residences.  While four times as much ready-mix concrete is used in the U.S., concrete masonry units are produced by the billions each year by over 1,000 plants in North America.

Concrete masonry units (CMU) are simply a form of precast concrete with very little water to create zero-slump blocks.  Like ready-mix concrete, the manufacturing of portland cement accounts for more than 90% of the carbon dioxide emitted to produce CMU.  Therefore, as with ready-mix concrete, higher-strength CMU block results in higher global warming potential.

In addition to portland cement, fine aggregate, and water, CMU can be made with a variety of ingredients, including granulated coal ash, expanded blast furnace slag, pumice, shale, slate, clay, and crushed glass. In terms of global warming potential, ingredient variations that use heat to expand aggregates for lightweight block show an increase in global warming potential.  Substituting recycled materials or industrial byproducts for virgin aggregate have a relatively insignificant effect on global warming potential (however beneficial in reducing depletion of finite resources).

Concrete masonry doesn't get built with just CMU, it requires mortar, grout and steel reinforcing.  Like the concrete, mortar and grout is typically made of cement, aggregate, water, and an additional ingredient, hydrated lime.  It turns out that the biggest contribution that engineers can make in reducing the global warming potential of concrete masonry lies in the grout.  This is because grout needs lots water to make it flow into CMU cells, and so as not to dilute its strength, grout needs lots of cement. By adding grout to every cell of CMU, for the same volume of wall assembly, the embodied carbon dioxide can be triple that of an ungrouted CMU wall since the cells are about half the volume.

As environmental product declarations become more common, manufacturers of masonry products will have incentives to reduce global warming potential and other environmental impacts.  We expect to see more manufacturers replacing portland cement with supplementary cementitious materials, such as fly ash and slag cement.  These industrial byproducts have lesser global warming potential because the energy used to create them are attributed to coal-fired power production and iron smelting, respectively.

Structural engineers have at their discretion many aspects of building design that can make a significant difference in global warming potential.  These include right-sizing CMU compressive strength, using the ASTM C476 strength method for proportioning grout, minimizing the extent of grouted cells, considering when lightweight CMU is necessary, and weighing the environmental impacts and thermal envelope performance of CMU walls compared to other wall assemblies.


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Thursday, December 21, 2017

Carbon Group Post 5: Wood

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Perhaps you’ve noticed that big wood-framed buildings are in the news lately:



Figure 1: Composite Timber Slab at UMass Design Building

Wood is hot for good reason. Buildings account for nearly 40% of U.S. climate-altering carbon dioxide emissions, and about 20% of those emissions are related to building construction and maintenance. Materials matter, and wood structure usually has the smallest carbon footprint of any of the primary structural materials.

The just-published ASCE report, Structural Materials and Global Climate, explains why wood structure has such a low carbon footprint. When trees are converted to structural framing, including sawn lumber and engineered products such as laminated veneer lumber, glu-lam beams, and CLT, the carbon dioxide that the trees metabolized into wood fiber is sequestered. As long as that wood is protected from decay or combustion, that sequestered carbon will not contribute to climate change. In contrast, the production of other structural materials such as steel and concrete emits significant carbon dioxide emissions.

Keep in mind that the carbon balance of living forests is complex and not yet fully understood. Harvesting wood has carbon dioxide emissions impacts that are not generally considered in life-cycle assessment such as soil disturbance and burning of tree residue. Some studies show that poorly managed forests actually emit more carbon dioxide when harvested than if they had been left alone. Therefore it is good practice to specify lumber harvested from sustainably managed forests.

Here is a simple example of how you can compare the climate impact of different structural options. On a weight basis, the “embodied carbon” in cold-formed steel framing is 2.28 lbs of CO2 per lb of steel, whereas the figure for sawn lumber is only 0.15 lbs/lb. In and of itself, this information is not all that meaningful, since steel is stronger than wood; we must look at “functional equivalency.”

So, let’s say you have a 12-foot span and want to use joists at 16” o.c. The 2012 International Residential Code specifies that 2x8 SPF #2 at 16” o.c. can span 12’-3” (for the 10 psf DL, 40 psf LL case). The IRC table for cold-formed steel joists calls for 800S162-33 under the same conditions. Converting to psf of floor area, the wood framing is 1.7 psf and the steel framing is 1.1 psf. In terms of carbon dioxide emissions, then, the wood option is 0.25 psf and the steel option is 2.4 psf, nearly ten times higher (Figure 2). In practice we should look at the entire building to account for all aspects of the construction, including those that may vary between wood- and steel-framed buildings; nevertheless, this simple comparison starts a compelling argument for the climate benefits of wood construction.

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Thursday, August 31, 2017

Carbon Group Post 4: Do you know how to achieve a carbon-efficient steel-framed building? Your fabricator does.

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A common misconception within the Architecture/Engineering/Construction community is that structural steel is a carbon-intense, “dirty” product which sabotages the natural environment by utilizing large amounts of mined content.  In reality, the steel structure being fabricated for your current project has a decent chance of containing metal from a car similar to the one you were driving when you were 16.  
Figure 1 - The structural steel life cycle Credit: American Institute of Steel Construction

Domestic structural steel and rebar are produced with an average of 90% recycled content.  The process in which they are produced transitioned in the late 1970s from the Basic Oxygen Furnace (BOF) process to the Electric Arc Furnace Process (EAF).  This shift resulted in a 24-fold increase in productivity - from 12 hours per ton of steel to one-half of an hour per ton.  Steel products are also transported efficiently in the US, with the majority of steel being transported via barge or rail rather than truck.

Some lighter gage material, like decking and light gage studs, as well as Hollow Structural Sections (HSS) are made via both EAF and BOF.  The BOF process utilizes approximately 30% recycled content.   It is important to be able to differentiate between production processes for all types of steel used on your project when accounting for the carbon impact.  (In this post, as well as the carbon working group white paper, the term “carbon” is used to mean “carbon-dioxide equivalent.”) Versions 2 and 3 of the LEED rating system assume a 25% recycled content for all steel products as a default unless documentation via mill-specific data is provided.  In LEED v4, mill-specific recycled content data can be used to achieve the Leadership Extraction Practices option of the Building Product Disclosure and Optimization credit. 

The cradle-to-gate Environmental Product Declarations (EPDs) released by the American Institute of Steel Construction (AISC) in 2016 for fabricated hot-rolled structural shapes, fabricated HSS, and fabricated plate material provide a full accounting of material sourcing, production, transportation to fabrication shop, and labor and processing in the shop. However, only fabricators who were members of the Institute when it was produced (or have since joined and submitted environmental data) are able to legally use these EPDs to account for carbon content on their structures.

The EPD for Fabricated Hot-Rolled Structural Sections gives results for the impact category of Global Warming Potential (GWP) as 1.16 tons CO2e per ton of steel produced, with approximately 85% of the total impact coming from the furnace production process.  With the material production impact being the dominant contributor to the total impacts, it is important to be sensitive to overall material use when lessening carbon impacts is the overall goal.  
Figure 2 - Comparison of structural steel frame in the Empire State building as constructed in 1931 vs. now.  Carbon emissions numbers include material production only.  Credit: American Institute of Steel Construction

Material, however, is only one side of the coin regarding overall sustainability.  An argument can be made that designing an erection-friendly structure can also lessen carbon impacts by reducing schedule and saving weeks of labor in the field, although this has yet to be quantified by an official EPD or Life Cycle Analysis (LCA).   

In order to find the “sweet spot” where material efficiency, up-front cost, life-cycle cost, and resiliency come together, one needs to discuss specific project goals with the structural steel fabricator as early as possible – preferably as a preconstruction partner.  The fabricator will be able to educate the design team on local material supply, desired connection schemes, how material can most efficiently run through their shop, as well as be efficiently sequenced in the field. They can aid in implementing green goals such as overall material reduction (both with a material efficient structure and by exposing structure to reduce finishes), material reuse, and integrated process credits. 

The Steel chapter of the upcoming White Paper “Structures and Carbon” describes and compares production processes used domestically.  Specific strategies to produce a carbon-efficient structure are presented.  We look forward to your reviews and comments!
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Thursday, July 20, 2017

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

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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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