Dear Colleagues,
I proposed CarbonCure for a project and the structural engineer in the meeting was opposed to it. He later sent me detailed reasoning by email. I have not used this technology and would like to hear other more experienced opinions on the technology. The project we are working on is well integrated with climate, 99% sDA, shooting for zero-energy and zero-carbon for operation. We are also using mass timber but need some concrete. My reasoning is that even if it is a 5% reduction in embodied carbon it is a good thing since every bit counts. I also understand that reducing cement and efficiency are important. Below is the original email (engineer's wording) and there are two images that pasted in the email that are also attached. I would appreciate any comments.
Thank you,
Pablo
the email:
Yes I raised the concerns – the “greenwashing” term was actually used by the Architect we were researching together with at the time. I’ll discuss in more detail what I found from a previous review of the available materials from their website, and discussions with Central concrete at the time who have implemented this in all their plants in Northern California. The gist of CarbonCure is that they are a method to promote more carbonation of concrete, which is beneficial to the strength final concrete product and does not adversely impact durability. It is this improvement that allows a reduction of cement usage.
Structural discussion:
CarbonCure’s website has a number of different documents with trial batch reports (from 2014):
This appears to be the mix design unchanged and just adding CO2 at different mixing times. The end result was about 15% increase in design concrete strength.
Here’s a case study they provide: http://go.carboncure.com/rs/328-NGP-286/images/CarbonCure%20Ready%20Mix%20Technology%20Case%20Studies.pdf
The purpose of this appears to be investigating reduced cementitious content (both cement and slag/flyash), and then adding CO2 to that reduce binder mix. This also shows the strength gain from the process. It’s a little strange that they are not adjusting just one variable (Portland cement only) to show the impact – maybe this was a shoot for the moon test.
Another paper that is not highlighted on their website, but referred to in their other papers: https://info.carboncure.com/hubfs/Downloads/White%20Paper%20Downloads%20-%20New%20Website/Properties%20and%20Durability%20of%20Concrete%20Produced%20Using%20CO2%20as%20an%20Accelerating%20Admixture.pdf
This one is interesting. Utilizing the same concrete mix, they varied admixtures and CO2 dosage (presumably absorption rate stayed the same) – the results are striking, as at the higher dosage rates, concrete strength actually dropped. The conclusion is a good read – I think it warrants further exploration on their part, but I don’t know if they did. Perhaps they have enough data to consider this an outlier and a throwaway.
To be honest, concrete mix design is chemical wizardry these days (we’ll come back to this) and the RMC producers can come up with any combination to make it work. So in summary, if a GC came to me and said they want to use this mix with CarbonCure in it, I feel comfortable that they can make the strength and I will not reject it (as long as they have done their test mixes and provide data).
Technology Discussion
This is (one of the) the patents filed for CarbonCure:
https://patents.google.com/patent/US9388072/en
“It will be appreciated that the level of carbonation also depends on the efficiency of carbonation, and that inevitably some of the carbon dioxide delivered to the mixing cement mix will be lost to the atmosphere; thus, the actual amount of carbon dioxide delivered can be adjusted based on the expected efficiency of carbonation. Thus for all the desired levels of carbonation as listed, an appropriate factor may be added to determine the amount of carbon dioxide that must be delivered as a dose to the cement mix; e.g., if the expected efficiency is 50% and the desired carbonation level is 1% bwc, then a dose of 2% bwc would be delivered to the mix. Appropriate doses may be calculated for desired carbonations at an efficiency of 5, 10, 20, 30, 40, 50, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, or 99%”
I believe this what they refer to as “precise calculated dose” – there is some level of estimation of the expected efficiency. There’s no way to know what the efficiency is before hand for a mix, only after mixing and testing. Perhaps with time they get enough data to narrow the range – so right now we have to assume that each ready-mix concretr (RMC) producer has done sufficient number of tests(?) to sort of get this dosing amount right.
Speaking of testing, the application contains this blurb: “11. The method of claim 10 wherein the testing comprises determining, for each carbon dioxide dose tested, a carbonation level of the test cement mix, a compressive strength of the test cement mix after hardening, a flowability of the test cement mix, or a calorimetry curve for the test cement mix, or a combination thereof.”
This is also referred to in their sample spec language (also attached). There’s nothing in the patent about how to perform / control this test, and is also not described in the specs. In speaking with Central, they noted they have no way to test / verify how much is absorbed. Perhaps someday the academic exercise will move into the real world. Presumably environmental carbonation won’t muddy the picture if you test early enough. The verification statement is also very vague – the concrete producer provides verification of the in-situ mineralization of CO2, but what’s the target? If efficiency is low for some reason, there could be very low absorption and if the test shows its low, then what? It’s not something you can fix after the fact.
With regards to rest of the specifications – it lacks anything concrete, pardon the pun. They leave the percentage of carbon and cement removal up to everyone else using their product, the reasoning could be that each mix is unique and needs to be tested individually with the RMC provider with their specific setup, for the individual job – no requirement for say 5% reduction in cement minimum, on average. There doesn’t seem to be a way for the design team to determine a good replacement rate / quantity since none of us are concrete mix design professionals –honestly this is going to be driven by what the GC / concrete subtrade tells the RMC producer with respect to the concrete performance.
Sustainability Discussion
Final reference: http://go.carboncure.com/rs/328-NGP-286/images/Calculating%20Sustainability%20Impacts%20of%20CarbonCure%20Ready%20Mix.pdf
This is where they provide a sample calc of what the potential CO2 reduction is, and I understand they want to be optimistic about their projections.
If you go by their numbers, you can see that the biggest contribution is avoidance of CO2 from cement production, by a long mile. And assumptions are everything in this calc, and more importantly the baseline used to make this comparison. Per this paper, the baseline is a mix with 570lb of cement per cubic yard, basically pure Portland cement mix. We regularly see high replacement numbers for cement already – even in the second link from the top, the cement content for their two test mixes were 240lbs/yd3, which halves the benefit.
So here we come back to concrete design being wizardry – in providing you with GWP reductions, they have to compare against another mix, and this is where things can be fuzzy / unverifiable. They have a library of thousands or more of design mixes – how do we know they are doing an apples-to-apples comparison, and not simply feeding us with made up numbers with a fictitious mix that was never going to be used? There is no way for us as structural engineers to tell, because mix designs are not industry standard – they hold all the cards in this area.
CO2 absorption sounds magical and sexy, but clearly the contribution is minor and the greatest goal is less cement usage – so why not simply take all the slag and flyash mixes we have been using, and reduce cement by 3-4%, and deal with the slight strength loss? Those mixes regularly go over design by a significant margin anyway. There’s no way to sell this of course – reduced cement content doesn’t come with an EPD.
Economics:
The reps for Central whom I had discussions with noted that they would include CarbonCure (and reduce cement content) at no additional cost to the buyer – hopefully that’s the same in Florida. They also said there was little to no benefit to their bottom line when I asked. It clearly costs some amount of money to retrofit plants, maintain equipment and transport & store the liquid CO2. Best case scenario is that Central is a true believer and is willing to take a small hit to be more sustainable. CarbonCure is probably making money from licensing the technology. Worst case, I would hate to be part of a system that allows some to take advantage of carbon reduction / offset credit sales without meaningfully making a difference in our fight for long-term sustainability.
This has been a bit of a long post, and ultimately I’m wary because of the lack of verifiability of the magnitude of benefits. Given that we have all been naïve in the past (thinking of the abandoned mega solar farms in CA and the resource cost to build them), I am trying to be more thoughtful and critical to the next hottest thing, even if it is from our more environmentally friendly neighbours from Canada. We know that “reduce” is one of the few reliable approaches to sustainability, hence I think as structural engineers our best focus should be a commitment to efficient designs and reducing materials used – but again that’s not sexy and doesn’t come with an EPD.
I was initially excited when I was introduced to this over a year ago, now I have more questions – it’s nice to see comparisons with mixes using other cementitious replacements. Keep in mind I’m merely a structural engineer, and not a trained sustainability professional so I welcome any further thoughts and discussions.
Regards,
Thank you,
