In looking a bit more at the discount rate approach, I’ve begun to understand it more fully. For others following along, I found this paper helpful for using the methodology alongside a simple LCA for context. I think this discounting approach is valid, and underscores the importance of avoiding emissions today - which is also a key takeaway from our work. That the concretes with the lowest lifecycle emissions, also had the lowest cradle-to-gate emissions and lowest carbon uptake through carbonation. So maximizing carbon uptake potential is NOT the appropriate decision. Again, I’ll point to Table 5 in Breton et al. 2018 that points at all the different dynamic approaches that have been taken to account for the issue of time (your approach Will, is described as the Discounted Global Warming Potential - note though that this approach does not use a time horizon of 100 years for GWP, but rather an infinite one).
I agree that designing to prevent carbonation induced concrete corrosion is the primary thing to do. Build something once that is designed to last as long as possible while avoiding emissions today. Yet, carbonation is going to occur when we hydrate cement to form CH and C-S-H, so we can account for it. And there is (and will be) so much mortar and concrete out there, carbonating, that we are nearly at the Gt per annum uptake of carbon just through the carbonation of cementitious materials. It’s also useful to see that concrete is only 68% of cement usage.
LCA is a useful methodology for comparing design decisions, rather than arriving at exact emissions due to the high uncertainty surrounding LCIs and the GWP values they present. We are also typically using GWP100 which is a midpoint indicator, rather than an end-point one (such as Global Temperature change Potential, GTP), so accounting for potential climate feedbacks are outside the scope of nearly all building-scale LCAs. Using an end-point indicator such as GTP has much higher uncertainty (+/- 90% or so), while GWPs have uncertainties of +/- 26% or so at 100 years (I believe these numbers originated from the IPPC AR5 report, Ch. 8).
Re: Pat’s comment, the methodology presented in the EN 16757 PCR is a great starting point for understanding how to include carbonation in an LCA (for those not familiar with cement chemistry). It is a simpler model that doesn’t account for some new understandings in cement hydration/carbonation mechanisms and how to deal with SCMs - which the model presented in Souto-Martinez et al. 2017 (and others) includes.