When should you engage your contractor and concrete supplier about low-carbon mix design?

Hi CLF Community!

Reaching out on behalf of SEAOSC for a project where the Sustainable Design and Concrete Committees are teaming up to collect and share best practices to help engineers initiate the conversation around low-carbon concrete.

  • Have you had a conversation with a client, contractor, or concrete supplier about using low-carbon concrete on a project?
  • When is the right time to start talking about low-carbon concrete? How can you initiate? Letter? Specifications?
  • As a contractor / supplier, when do you need to be making decisions about mix design? And impacts like data availability or testing protocol for 56-day specified strength? Others?
  • What issues have come up that have inhibited adoption of lower CO2e mixes? Curing time? SCM availability? Others?

If anyone has any examples (letters, specification language, etc.) or lessons they are willing to share, it would be much appreciated! We’re hoping to get feedback from across AEC!


A couple resources that come to mind that could be helpful for reference:

Hi Luke-

I work for LafargeHolcim US and you can see some of our plants on the map of “industry leading low-carbon producers”! We launched our ECOPact range of low-carbon concrete in the summer 2020. We don’t operate on the West coast but if you want some feedback coming from the Washington, DC & Boston areas I would be happy to chat. Feel free to contact me: cecile.roman@lafargeholcim.com.

As a low-carbon concrete producer, we try to engage early on in the project development with engineers & architects. We hold webinars/talks about GWP reduction targets, EPDs, low-carbon specification guidelines, embodied carbon and technical properties of concrete with higher SCMs replacement rates. I would love to share some lessons learned from these conversations, and also to hear your feedback as a structural engineer.

Thanks for starting this topic!

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Hi Cecile and Luke,

Would love to be part of this conversation and to hear the lessons learned from past conversations or projects. If you do initiate a conversation or set up a meeting, would you mind looping me in as well?



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When doing your bidding with the foundation contractor, require a 25% fly-ash or slag in your concrete’s mix design. You may need to bid out to 3-4 contractors until one of them is willing to change from their normal 100% Portland cement mix design.

Also, reach out on your own to 3-4 ready mix suppliers in your area and have a list ready of who has fly ash or slag in stock. So when you speak with your foundation contractor, then they can’t tell you that it’s not possible.


Hi Luke,

I’m a residential builder/general contractor in Philadelphia and on my current project the biggest problems I had were finding a contractor willing to use a different mix and sourcing material. One contractor used fly ash about 10 years ago who had problems with the concrete curing, which nearly led to a lawsuit: He swore off using these mixes.

Sourcing was the other hurdle, I called three suppliers who had no experience with mixing slag or fly ash into their mixes. Next I spoke with my architect who said his only experience was that another contractor he worked with figured out that you can drive to New Jersey to pick up a truck of fly ash and drop it off at the concrete supplier. I looked around a bit to find that fly ash supplier without any luck.

At some point there just isn’t enough time to source all the materials. So @ScottMorris is right, if you can get me a list of suppliers, I would be happy to find a contractor to use the material.


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I would like to point out that an important carbon-reduction method is almost being ignored by the industry. That’s the reuse of return concrete. about 6% of the total produced concrete is coming back to the plant unused. In many cases ‘just’ overordered. This concrete is dumped in 95% of the cases. In few instances blocks are made. ASTM regulation makes it possible to reuse this concrete in a new load. respecting basic guidelines. Think of the enormous and instant reduction using this concrete in buildings.

Not an answer to you question…sorry for that, but I have a problem with the lack of attention this topic gets as it its really low-hanging-fruit that’s being overlooked.

Grt Rob


We’re planning on including GWP targets for our mixes as part of our specifications on jobs that we think the suppliers will be able to quantify them (through EPDs or otherwise). But setting these targets is tricky and feedback so far from larger suppliers is that the benchmark data from CRMCA is actually quite conservative. We’re reaching out to various suppliers to get some guidance on this -@Cecile_Roman is this something you can help with. Even if East coast it would still be helpful.

@ScottMorris - Minimum % fly ash doesn’t necessarily mean a lower GWP rate - you could have a mix with high % of fly ash but an overall increase in cement compared to a mix with no fly ash. It’s the cement to water ratio which is more relevant (and harder to specify).

@rpiosik - Interesting point. I’m wondering how easy it is to incorporate the dregs of a previous mix into a new one and know what the results are? Do they have to be like for like? And presumably it’s not done currently as there’s very little cost incentive to do so?


Hi @Ross,

Where in Canada are you working. I can connect you with the right Canadian Lafarge people. Feel free to drop me an email at matt.dalkie@lafarge.com.


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Right after the structural designers have determined the concrete performance requirements.
Then the materials engineers as well as site teams shall come onboard to set suitable parameters from the concrete technology and constructability perspective and then try and optimise in terms of minimising cementitious content.
With all this to hand, concrete suppliers should be engaged to produce the final mix design.


@matt.dalkie - based in Vancouver. Will fire you an email thanks!

@jharmony Two very valid points that are often raised during conversations:
1/ concrete contractors sometimes have their bad experience using slag or FA on a project, dating back from 10 or even 20 years ago. There are several answers we can give:

  • with new research and mix improvement we can now say with confidence that using low carbon concrete won’t make any difference in the placement and workability of mixes. Suppliers can design mixes with the exact same properties as standard concrete, even with high-early or self placing concrete.
  • slag mixes with lesser replacement rates have been used for many years in the Mid Atlantic region for example without performance loss. Higher cement replacement rates are also more and more used in other countries around the globe, especially in Europe. We really need to collectively share the success stories of low-carbon concrete projects to change the general perception and have it become mainstream.
  • a good way to start could be to use low-carbon concrete for applications where it can improve concrete properties (such as mass concrete) or be perceived as low risk (pavement, foundations etc)

2/ sourcing issue: if you’re interested send me an email cecile.roman@lafargeholcim.com
I can connect you with our cement folks in Philadelphia. They will help you find suppliers for slag and low-carbon concrete.

@rpiosik Very interesting, I wasn’t aware of that possibility either. I agree with @Ross the big question is how to use rejected mixes into a new one without loss of quality. There would also be a timing issue if the RMX truck goes to the project location and then back to the plant.

@Ross I agree the best way to specify low-carbon concrete is not to require a FA or slag percentage. Instead, it is best to give a GWP target (based on NRMCA regional averages as you mentioned, or coming from an LCA calculation). We generally advise engineers to specify for each class of mix:

  • strength
  • exposure class
  • max aggregate size
  • max GWP or GWP reduction compared to a baseline
    And nothing more! Concrete suppliers are able to provide mixes that answer to those specs, and optimize the cement and SCM content in order to reach the performance without overdesigning (that could lead to wasting materials and increasing GWP).
    About NRMCA averages: with ECOPact we can typically reach between 30% to 45% reduction in GWP compared to the US Northeast regional averages. And you will hear more from my colleagues in Canada (thanks @matt.dalkie !)
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@Ross Hi Excuse for the late response. Please allow me to address your questions in the reply to @Cecile_Roman .

Hi, @Ross @Cecile_Roman
I will try to make this a short reply for now. When there is interest we could always discuss further.
I don’t blame anybody of not being fully aware of the fact of ‘return concrete’. I have been working over 30 years in the industry and always kind of heard about it but did not gave it any attention. Few things have changed over the course of the last decade. We are more focused on the Embodied Carbon that comes with concrete and we have different technology available. At least these 2 made ‘me’ realize that there is a real problem that is overlooked but can be addressed better than it could have been in the past. Happy that also the ASTM has since several years a standard (ASTM C1798) that permits reuse of fresh concrete. Long story short. Yes its is under this standard possible to reuse fresh concrete, but (addressing Ross’s question), its not easy to do a manual assessment on the return concrete (slump, mix design, temp, volume, age etc.) according ASTM. Further we have to deal with this negative connotation of return concrete…like its not good anymore (open-box feeling), but the fact is that this is good and fresh concrete…never been used. Bad thing is that the amount of return concrete is only getting bigger. This die the fact that contractors almost per definition over-order. The penalty for return concrete is not high enough as the costs they would have for a small shortage of concrete would be much higher on labor and job-time loss. Anyhow, what is possible to-date to address the problem : Technology (sensor) that can be mounted in the truck is able to measure extreme accurate the mentioned concrete properties of the return concrete : slump, temp, water added and volume. With a real-time interface to the batch system also the age of the return concrete and the mix design is known. With all of this the return mix can be ‘matched’ with new loads and can be top-loaded within all specs and regulations. When the concrete would run out of set-time its possible to add a Cement Stabilizer to the return mix to stop hydration for any wanted time (also within ASTM regulations). I see this answer is longer than anticipated ;-), but want to say that of course not all concrete can be reused but with the new technology, that takes care of the measurement and ‘administration’ at least 50% of the return concrete could be reused. This would be a 100% win on the Embodied Carbon and not to speak about the huge win in value to the bottom line.


Capitol Aggregates (CA) and I believe Alamo Cement (both in San Antonio Texas) Have been producing ASTM C595 Type 1L and a number of other blended and ternary blend cements for some time now. All with OPC replacements of between 15 and up to 70%. They have also sponsored the Development of the EPS for the materials through the NCRMA. Speaking with the Technical director he said they have not been as aggressive on the development of other EPDs and renewing the one they did produce because industry (mostly structurals and spec writers) haven’t been demanding it. The cost of developing an EPD is about $30k according to him and I heard that from other sources as well.
He also pointed out the OPC carbon issue is well known it the industry. Also the cost of fuels and supply of feed stock is putting pressures on producers to find less costly ways to make structural concrete. He said in time of shortages of clinker you may be getting one of the blended cement because they are generally less costly to produce. The lower carbon is a side effect for them.
Lesson: specify anything but ASTM C150 in order to create the demand. Produces will respond. The professionals are going to have to that the lead at least on this point.

Great discussion here.

On projects where my firm (Atelier Ten) is consulting on embodied carbon reduction, we like to start the discussion usually in the DD phase. Concrete bid packages can go out in early CDs, so we like to have enough time before that for us to perform a whole-building LCA and identify the contribution of concrete to the whole building, as well as potential for reduction.

We have an LCA Kickoff with the design team and GC if they’re onboarded, talk about goals related to concrete optimization. This makes it way to structural specifications or key notes in structural drawings, with one of three approaches:

  1. Cement Replacement (specify min SCM % for different applications) - commonly understood, but not pegged to GWP reduction directly. Also moderately prescriptive.

  2. Cement Reduction (specify max cement lbs/cy for different applications) - not common, but easy to understand for subs, and directly pegged to GWP reduction. Highly prescriptive.

  3. GWP cap (specify max GWP in kgCO2e/cy for different applications) - not common, moderately difficult to understand for subs, obviously pegged to GWP, and least prescriptive.

Ideally, the GC gets feedback on these targets from their potential bidders, and includes an explicit mention of these in bidding instructions. Once the ready mix supplier is onboarded, you can have a dedicated meeting to talk about these sustainability goals that mix designs will need to meet in addition to other performance criteria, and get their feedback.


Thank you for that breakdown Prateek and @Ross and @Cecile_Roman.

For the GWP cap or embodied carbon cap method, wouldn’t the ready-mix supplier need to have an EPD? Outside of big cities, it isn’t common for ready-mix suppliers to have a plant-specific EPD, so the next best thing is to spec option #2 and #1?

@ScottMorris Suppliers would need to have either EPDs or find most similar mix in proprietary software (Tally, Oneclick etc.). Our thinking is that, soon enough, all suppliers will need to be producing this information.

In the past year I have found that the awareness and response to GWP and EPDs has gone from “I don’t have a clue what you’re talking about” to “we’re looking into it…”. So we’re getting there…!

Agree! Getting there!

The earlier the better, is pretty much the answer to everything regarding decarbonization.

I think it is important for everyone to remember, especially if you have total carbon emission targets for your design, that CarbonCure sells their CO2 sequestration as an offset, so technically the building does not get carbon credit for implementing that technology.

We have focusing on portland cement replacement as that seems to get the biggest reduction in kgco2e per cubic yard. We work with whomever the structural consultant is to try and get whatever carbon performance data is possible for the design into the spec in DD as well.