Can anyone point me to some good resources/articles/commentary on embodied carbon of CMU walls?
My hunch (unproven, untestested) is this: when used as a true structural wall (bearing wall, shear wall, or retaining wall) and designed efficiently, CMU is likely to have a lower embodied carbon intensity than a functionally equivalent cast in place concrete wall. But when used simply as a non-load bearing partition or even as an exterior backup wall to a facade taking lateral loads, CMU is likely to have a higher embodied carbon intensity than a functionally equivalent metal stud wall.
I would welcome any comments supporting/refuting this claim, or any insights about CMU in general.
I haven’t really tested this extensively but from my perspective, you should take a look into the material intensities as well, i.e. how much you’d be using in each case.
Now, I’ve run just a very quick comparison using One Click LCA.
Option #1: CMU, lightweight at 3750 PSI
Option #2: Readymix, lightweight. 4% fly ash at 4000 PSI
Option #3: Precast Concrete Wall element, 4400 PSI, 0% recycled binders
and the results are heavily in favor of CMU (see img below).
EPDs for another producer in California:
[https://www.angelusblock.com/sustainable_design/sustainability-downloads.cfm]
This one is kind of neat because they have developed a CMU carbon calculator spreadsheet tool (reportedly reviewed by Athena) which is pretty transparent and allows a very high degree of specificity with regards to block size/shape, mix design, rebar adn grout pattern, etc. I believe the data are specific to this producer, though.
I have not carried out the detailed comparisons that I asked about in the OP, but if I do I will post here.
Here is a blog post that the SEI Sustainability Committee posted on the topic. There are additional resources in Structures and Global Climate ASCE publication. Not quite answering your exact question, but at least a resource on the topic.
The Athena EcoCalculators could be useful for this.They (used to) have predefined wall assemblies and you could compare on an area basis just by selecting what seems equivalent or what you consider, without having to create the whole assemblies yourself. I say used to, because I believe Athen discontinued the support for this tool, but here is a sample of the tool for you. They had separate spreadsheet for commercial and residential, and for various regions in North America. Here is a summary of the Athena assemblies by structural layer.
CMU are sometimes filled with concrete and rebar, so that should be part of the calculation if it isn’t already. You’d probably find it in Athena but not in the EPD for CMU.
You are right that the EPDs for CMU don’t include the rebar, and it was not clear to me whether grout was considered. The spreadsheet tool from Angelus Block I noted above does incorporate EPD data for rebar and grout in addition to the CMU block itself. The assumptions tab notes that the rebar EPD data is sourced from Canadian data in Athena, which is not consistent with the region the block is produced in, but I still appreciate how comprehensive the tool is.
I need to install Athena IE check out the options within that. In Tally I believe there are explicit options for grout & rebar in CMU walls, but the individual contributions of these components are not broken out in the reported quantities.
I am a CMU producer located in the northeast, and have spent a lot of time of this very topic. We have EPDs for almost 400 of our different CMU mix designs.
In general, CMU uses less cement compared with cast in place concrete. This is due to it being made from dry cast concrete, and some of it’s strength is gained by the compaction and vibration of the block machine. As such, the strength of CMU is not dependent on cement content alone. In addition to this, there are several factors that affect the GWP of CMU, and the biggest is cement content.
I’d like to preface this by saying I am not speaking for the whole industry here, only for our CMU. There are regional similarities, but there are also regional differences. Our structural CMU is usually around 2700 psi, which exceeds the ASTM C-90 structural requirement of 2000 psi. We do make high strength units for special cases, which could be up to 3750. The global warming potential of our high strength units (3750) is almost 10% higher than our regular structural block of (2700) psi. Another factor for us, and this is a regional difference as well, is light weight aggregate. We use a manufactured light weight aggregate, made from expanded shale. The GWP of our lightweight CMU is substantailly higher compared to our normal weight CMU. BUT, I know of a CMU producer on the west coast that uses a natural light weight aggregate generated by volcanic activity, and their GWP is similar for both light weight and normal weight. Another fun fact, we found that a CMU using recycled concrete aggregate brought our GWP up.
Now, on to the topic of carbon sequestration. First, a quick/simplified overview of why this happens…when cement is produced, limestone is heated and chemically broken apart releasing CO2. When water and aggregate is added to the resulting cement powder to make concrete, CO2 is converted back to limestone within the matrix of the concrete. This is part of the reason concrete gets hard, and is also one of the reasons concrete shrinks. Sequestration has been historically called carbonation. Due to the unique structure of dry cast concrete, CMU has more potential to sequester CO2 during its manufacturing and use phase than poured in place concrete. The CMU industry is in the process of updating our industry PCR with UL Environment, and sequestration language will be added to our PCR. There are several sequestration technologies currently on the market that can increase the amount of CO2 that CMU sequesters.
Another thing to think about….depending on the climate zone, thermal mass benefits can also effect the embodied carbon of the assembly materials. Due to thermal mass, CMU walls use less insulation to meet energy code requirements. Take a look at this comparison for climate zone 5. The same amount of insulation in a metal stud wall vs a CMU wall has very different results for energy code compliance.
I was the author of the SEI blog linked above, which was a summary of the CMU chapter of Building Structure and Global Climate. I’m happy to see that Angelus has added a simple calculator and especially pleased that it validates my earlier estimate that grout can triple the embodied content of a CMU wall.
Grout has a high embodied carbon content - higher than most concretes - because there is excess cement to balance the excess water needed to make the grout flow. This counteracts the efficiency of CMU manufacturing.
The embodied carbon of a CMU wall is the same as the same volume of concrete wall made of 3000 psi concrete. That’s what the Angelus calculator reports for a CMU a wall including grout, and comparing that to the median GWP from Climate Earth’s Concrete Selector tool. (https://selector.climateearth.com/).
The engineering wrinkle is you don’t need to fully grout a wall. At a minimum, only the cells than contain rebar need grout, which may be one quarter of the cells. For partitions and very light loads, they can remain ungrouted, Many engineers (either through laziness or inexperience) may specify more grout than is required. I was one of those over-specifiers in the past. So you could cut that impact by 1/3 for hollow partitions.
As for carbonation, in my research for the book showed that the benefits are small. CarbonCure technology, that uses carbonation during manufacturing has a very small improvement. For carbonation that occurs over the life of operation, in my opinion it is too little too late. The damage is done when the CO2 is put in the atmosphere during manufacturing (sunk cost), and we have no time to wait for the sequestration to occur to combat global warming. You need more surface area exposure for carbonation to make a dent.
The only take-away I learned about carbonation is demolished concrete can be ground up and used as fill. If left exposed to the air, the additional surface area can rapidly expedite the carbonation (which ends up decaying the concrete anyway).
I’m happy to discuss further.
Heidi, thanks for the discussion about different strength, normalweight vs lightweight, insulation properties, and carbonation/sequestration. This is all very helpful.
In your example, I assume the metal frame will easily be made to pass with some kind of insulation between the studs. But depending on the insulation used, this could have a large or small impact on the total GWP of the assembly: https://zeroenergyproject.org/2020/06/12/does-your-insulation-have-low-embodied-carbon/. It has become apparent to me recently that as a structural engineer trying to reduce GWP, I need to learn a lot more about insulation, too, beyond identifying which kinds of insulation are “bad” or “good.” The exterior wall framing system used, the required additional insulation, and the appropriate types of insulation are inextricably linked. The same applies for roof assemblies.
Adam, I really appreciate this post. For any situation where CMU is called for, it’s helpful to know how much of an outsize impact the grout has. I would say that relatively few engineers consider themselves CMU design “experts,” yet most need to specify CMU from time to time. As engineers with limited time to complete a project, we make choices about what to optimize, and I agree with you that CMU walls are often fully grouted as either a time-saving measure or a feel-good measure - I’ve done this myself, too. In light of this info, though, I would spend more effort to minimize grouting on any future projects with significant CMU, whether structural or non-structural (as even the non-structural CMU is typically covered by our spec and typical details).
