Low-carbon concretes in infrastructures : Applicability, potential and carbon footprint focusing on the Parisian Metro Line L18

Abstract: Low-carbon concretes are more and more commonly used to limit the environmental impact of concrete structures. Among concrete structures, infrastructures are characterized by their high load-bearing function (foundations, bridges, aqueducts, dams, etc.). They are distinguished by their large size, their interaction with aggressive environments and by their longer life span than ordinary buildings. It is difficult to find alternatives to concrete for these structures. Low-carbon concretes therefore offer a very interesting environmental opportunity. In order to support the prescription of these innovative materials, it is necessary for a project supervisor to have a deep understanding of them. Bibliographical research of scientific, standards and industrial sources is carried out to understand the state of the art of the material. Then, a hand-calculation of carbon footprint at the scale of an infrastructure project -a section of the L18 of the Grand Paris Express- is made following the methodology of Life Cycle Assessment and the data available to a structural project supervisor. The analysis has essentially analytical and comparative purposes. It shows that the environmental impact of concrete is between 42% and 52% at the scale of the structural envelope of the case study. However, as detailed below, these results are very sensitive to the environmental data chosen for the transports, and especially the reinforcement. By using low-carbon concretes, standardized or not, a carbon optimization is proposed. This could reduce the environmental impact of the project by about 13% by using alkali-activated concretes.However, this study also highlights the importance of not restricting the low-carbon transition to binders alone, as limits to their optimization will be reached. The end product of construction is the structure, not the concrete. Considerations must include low-carbon concrete along with transportation issues, construction methods, alternative structures, other materials including steel and more.