One big problem with cities is that they are mostly made of concrete – the vast majority of which is reinforced concrete. We have initiated a project that looks at the inevitable decay of the steel reinforcement of concrete. This challenge is uniquely situated between technical and historical domains of analysis and has direct impact on both mitigation and adaptation to climate change. The decay takes place over a historical timespan of a century and has significant technical infrastructural material science implication. Reinforced concrete only came into widespread use around 100 years ago and is now ubiquitous. For of half its century of deployment the mechanisms and rates of oxidation and failure were on empirically, not analytically, understood.  Only since 1968 has the physical-chemical process of carbonation been understood as the rate limiting aspect. Carbonation kinetics driven by CO2 diffusion that creates a front of pH lowering reactions that travels through the concrete and eventually arrives at the steel reinforcement where the lowered pH allows oxidation to occur and the expanded volume of rust to cause cracking and delamination of the reinforcement from the concrete it was supporting. Our research found a significant gap in how this problem is considered by historians and engineers. For historians the focus is on philosophical questions of material – not on its technical components, and for the engineers the timespan of 100 years meets the “design life,” and is therefore only considered as a future sufficiency rather than limitation. We research both the direct decay of concrete, and also more broadly how concrete operates on the urban environment structurally, aesthetically, and thermally.