Abstract The fib-Model Code for Service Life Design, which is referenced within Model Code 2010, considers different deterioration mechanisms of concrete structures. In particular, it proposes a physical model for the assessment of the carbonation depth in time, which, for existing structures, requires data, such as the type of cement and the water-to-cement ratio of the concrete, that are often unavailable. In the paper the theoretical results obtained with the fib-model are discussed and compared with experimental data obtained during an extensive campaign carried out on cast-in-place uncracked concretes of in-field exposed existing r.c. structures from a highway infrastructure. This comparison has highlighted the key role played not only by the environment, but also by the quality of the concrete through the inverse effective carbonation resistance of concrete, RNAC,0-1, on the evolution of the carbonation depth in time. The direct measurements of the carbonation depth on existing r.c. structures allows the inverse effective carbonation resistance of concrete to be determined and correlated to the concrete mean compressive strength at 28days obtained from compressive tests on cores taken from the investigated structures.