Polished low carbon concrete slabs?

Materials
1

Hi folks,

Any advice on challenges and limits to cement reduction/SCM replacement on concrete polished slabs? I’m getting a lot of questions from residential high performance contractors interested in low carbon concrete options. I saw an old thread on this but it didn’t provide much advice beyond rock popping issues. So, I thought I’d try again. Any other insights, case studies, or project pictures to share?

Thanks,
Jordan

2

Hi Jordan - we’ve gotten feedback in the past that once you get above 30+% SCM replacement (so typically with slag) that the slab finishing and polishing quality can deteriorate - slag makes your concrete “sticky”. We’ve tried looking at 50% slag mixes for slabs but were advised against using these high SCM mixes for polished slabs by our concrete supplier and polishing sub. That being said, if I was an Owner who cared about this kind of stuff I would think about paying the extra $$ to mock up a polished high SCM slab and learn from that mock up. My guess is that our suppliers/polishers had one or two bad experiences with polishing high SCM slabs where they had tried it for the first time and understandably don’t want to take the risk on it again unless they’re allowed/paid to run some additional trials to figure out a QC process when polishing high SCM slabs.

3

Thanks, Mark. That 30% slag value matches some of the initial feedback from concrete installers. If we have a local successful project with higher replacement rates, I’ll aim to put a case study together.

4

Please do! I think a lot of this has to do with the perceived risk of the unknown so if there are success stories that will help quell those concerns.

5

Jordan/Mark,

Ready mix supplier here. Affirming your discussion, that you can certainly go higher than 30% SCM for concrete polished slabs.

The 2 biggest factors are (1) time to initial set and (2) time to final set.

SCM content is one of the factors, but not the only factor. For instance source of SCM is a big deal, fly ash on the East Coast is different than the West Coast. West Coast slag can replace cement at ~1:1 weight ratio. More East Coast slag is needed to replace an equivalent weight of cement. Temperature is also a big deal.

All that being said a lot of communication is critical if you are trying to reduce the carbon impact of your concrete. Certainly a lot of finishers feel comfortable <30% SCM, because they’ve always done it that way and it would be on them if the work wasn’t performed properly. However, that will artificially limit your ability to make your projects more environmentally friendly.

Aaron Fisher
afisher@emcoblock.com
Ernest Maier and Bay Ready Mix

6

Hi All, interesting question and discussion. What about other (growing in popularity) SCM’s, such as ground glass? Oh and, since fly ash is fairly toxic I am not sure that polishing it and leaving it exposed is a good idea?

7

Slag and fly ash do not enhance the end product of polished concrete. Suggest discussion with local suppliers and contractors; develop trial mix designs varying SCM percentages and request Environmental Product Declarations to confirm carbon contribution of each. Check with cement suppliers for availability and cost of reduced or zero carbon product offerings. Consider using carbon offsets.