@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 firstname.lastname@example.org
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:
- 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 !)