Quantifying Environmental Impacts Across Multiple Future Lifecycles of Steel and AAC Structures: Results, Uncertainties, and Methodological Challenges

Although circular strategies in construction can substantially reduce environmental impacts (EI), their effectiveness depends on aligning technological capacities with socio-economic conditions, requiring a future-oriented assessment. This study quantifies the EI of steel-frame and autoclaved aerated concrete (AAC) structures under various circular end-of-life scenarios, including multiple future lifecycles in Poland. A prospective consequential life cycle assessment (LCA) was conducted, integrating Shared Socioeconomic Pathways (SSP) for sensitivity analysis. The inventory was based on literature, spatial analysis, and expert input. Two methodological challenges were identified: lack of material quality differentiation in LCA databases, limiting reuse modelling, and difficulties in defining robust functional units across multifunctional technologies. Results indicate substantial mitigation potential from AAC recycling, particularly in single-family housing. EI of concrete recycling depends on distances and substitution of energy-intensive materials. High-quality recovery is critical for steel (>70% recycling or reuse). SSP modelling revealed the ‘crediting paradox’, highlighting the need for clear methodological boundaries.