What are the implications of specifying low carbon concrete?

I’m trying to understand what are the implications of specifying lower carbon concrete and would appreciate any insights to this. Ideally, I want to be in a position to spec a certain % of cement replacements in a mix and give a rough idea of what the cost implications are and what the impact may be on the curing times. This is vital information to get the contractor on board but I’ve not found they as willing to do the research.

Has anyone put together a table similar to the one below? This CO2 saving is a rough calc based on the CRMCA EPD. And the cost implications I’ve made up. It would also be good to add other proprietary mixes or alternative options that could be considered (maybe adding limitations and/or locations available columns?)

It really depends on the project requirements and weather. I would start with a baseline of using PLC/GUL (up to 10% carbon reduction and no cost) and then work with your supplier to find the optimal mix design for your needs.

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The NY/NJ Port Authority has specified at least 30% cement replacement since the late 1990s. If you email me at susan.dorward@raritanval.edu I can send you their spec document and contact information.

It’s not about SCM replacement, it’s about cement content. A mix with 500 pcy of cement + 50 pcy SCM vs a mix with 500 pcy cement + 100 pcy SCM will have very similar impacts. The second one will likely be stronger, but SCM is still an indirect metric. If you’re going to use prescriptive specifications, I would recommend maximum cement content and refer to the NRMCA regional benchmark as a baseline.

Like @ajauer said, PLC/GUL is also a great way for easy reductions. You can also be more cognizant of specifying curing time (i.e. maybe you can go with 56-day strength for foundations). Next step up is going with performance-based requirements by specifying maximum GWP as demonstrated by a product-specific EPD. https://www.portlandoregon.gov/brfs/article/731696


There’s tonnes of good content on this in the UK market. Here’s a good starting point for you, where you’ll find a section on this topic, and references to more good resources:

@martintorres, I would agree that cement content is important, but it’s a bit of both matters, isn’t it? You want to limit cement content and include cement replacements. In cases where we’ve specified cement replacements, contractors worried about strength gain (and overestimating the impact of the cement replacement on strength gain delays) will just massively increase the total cement content, counterproductively. So you want to spec cement replacement and limit total cement content.

Yep, well put! I just meant cement replacement is effective as a means of reducing total cement content needed to achieve the same strength/cure time. The total cement content is still the best proxy for GWP outside of an EPD, SCM replacement is one way of reducing it.

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Does anyone know of good data on whether using conventional SCM (fly ash and slag) actually reduce carbon once transportation distances are accounted for? I’m based in Vancouver, Canada, and the nearest blast furnace is 1000s of miles away, the nearest coal fired power plant is 100s of miles away. I’ve found lots of good UK data, but everything is essentially down the road there. I ask in part because on one of my current projects the contractor is using imported slag from Australia - in this case to get the right mix properties rather than any consideration of carbon. But that got me thinking about transport distances generally. Thanks!

Hi Ian, If you’re looking at the data from a concrete EPD, the transport impacts are included. From what I understand the SCM’s in the Pacific North West can be barged in from Asia. Even so, the impacts are less than cement. I have heard of studies on this nature from the mid-west but can’t remember the source. @jgregory, do you have references on this topic?

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Thanks Kate. Unfortunately the EPDs I’ve found so far are averaged for all ready-mix concrete in Canada. That doesn’t give me much confidence that the numbers are representative. Hopefully transport emissions for SCM are just rounding errors on the totals, but I’d like to see the data to demonstrate it.

Hi Ian,

We supply most of the slag to the Lower Mainland and are working on an EPD now that the updated PCR has been published. At this point in time we’re undecided if it will be a verified Type III, or a Type II as the intent is to use the data internally.

If you need something verified, your only current option is to use the SCA EPD available here: Slag Cement EPD This is being updated and should be available this year. You can see that the GWP is significantly lower than cement. As Kate has pointed out, for North America, slag is available in the Great Lakes area from domestic origins. The slags available on the West coast are from Asia. We import granules in approximately 50kt shipments and that means that the GWP of ground slag is lower than the last published SCA EPD based on the work we have done so far and that does include that Trans-Pacific freight.

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Thanks Matt. That’s really helpful. I was concerned that the GWP figures in the EPD from the Canadian Ready-Mix Concrete Association might be skewed if transportation distances aren’t accounted for, but from your input it seems that’s not a real concern.

There are many concrete producers who import SCMs from other countries. Ocean transport has a low-carbon impact, so I wouldn’t be concerned that this would offset the environmental benefits of using SCMs. I was also going to suggest the slag cement EPD that Matt mentioned as a useful reference point.

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@martintorres Martin has a really good point. There needs to be a focus on cement content reduction vs. SCM%. @Ross If you are interested, I can share a copy of the ZGF Concrete LCA Tool for rapid LCA of a concrete mix design. The data is mostly from the U.S. but we are working on a version adapted to Canada.

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@Ross One challenge with I’ve seen in the Canadian market is that suppliers do not share the ingredients of a mix design as the data is proprietary in Canada. This makes understanding the true carbon impact of a mix design difficult as for instance a 500 lbs cement + 100 lbs fly ash mix and a 550 lbs cement + 110 lbs fly ash mix are both 20% SCM mix designs but with different carbon footprints.

Great comments from everyone thanks!

What I think will get general contractors/clients on board with low carbon mixes is a table similar to what I posted originally with indicative costs and structural consequences. It would be great to have a concrete supplier do this for a range mixes (if it hasn’t already been done). Has anyone done anything like this or know a good (and willing) contact who would be interested in this kind of study?

@baha.sadreddin - I’ve actually been sent the ZGF concrete LCA tool (working on a project with you). It’s certainly interesting but for the moment I was more looking at the implications in terms of cost and curing times rather than the carbon side of things as they are the main hurdles for contractors adopting alt mixes.