Closing the loop of construction materials : a guidance for integrating quality in life cycle assessment

The construction sector is among the most resource- and waste-intensive industries worldwide, responsible for over half of global raw material consumption and approximately one-third of total waste generation. Although reported recovery rates for construction and demolition waste (CDW) are relatively high, such as the 89 percent reported for Europe, these figures often do not reflect the actual quality of the recycled materials. A significant share of recovered CDW is used in low-grade applications, raising concerns about its contribution to high-value material cycles. Within the framework of a sustainable circular economy (CE), one key objective is to retain materials in loop for as long as possible while preserving their quality. As such, distinguishing between genuine recycling and downcycling becomes essential. For accounting environmental impacts of CE, Life Cycle Assessment (LCA) is a widely applied methodology. However, when applied to strategies aimed at closing material loops in the construction sector, LCA faces specific methodological challenges. While LCA follows a standardized framework, its practical implementation involves multiple methodological choices that require guidance for accurate application within the CE context. Specifically focusing on closing the loop of construction materials, differentiation between recycling and downcycling is a relevant topic of discussion both from the definition and quantification perspective. At this point, material quality becomes decisive, and within LCA, how to integrate material quality and the effect of modelling choices becomes important. This thesis aims to advance the application of LCA within the CE context by developing methodological guidance specifically focused on closing material loops in the construction sector. The guidance is derived from both systematic literature reviews and case studies. The first review provides a detailed methodological evaluation of LCA studies addressing CDW recycling, revealing inconsistencies in modelling practices and a lack of transparency, particularly in substitution accounting. The second review adopts a broader lens, examining how the concept of quality is defined and applied within CE literature. It identifies considerable variation in usage and a general absence of clear, operationalized definitions. To complement these reviews, three case studies were conducted. Two studies address CDW management, with a particular emphasis on differentiating recycling from downcycling. The first examines the recycling process itself, using a parameter-based LCA approach to identify the key factors influencing the comparative performance of conventional and advanced recycling technologies. The second expands the system boundary to include demolition practices, comparing partial and highly selective demolition scenarios and their influence on the composition and quality of resulting material flows. The third case study explores the end-of-life (EoL) challenges associated with an emerging material, carbon fiber reinforced concrete (CFRC), which, despite its environmental benefits during production, lacks established recycling pathways. Here, a technical analysis was used to estimate separation yields for LCA modelling due to the absence of inventory data, a typical constraint in early-stage technologies. The case studies collectively highlight several cross-cutting and scope-specific considerations essential for LCA in CE contexts. Applying LCA within a CE context requires attention to several critical elements, particularly when accounting for material quality. One important example is the need for a functional unit that reflects both quantity and quality aspects of the system under study. This, along with other modelling challenges, is addressed in the guidance provided. In particular, the guidance includes tailored considerations for quality-sensitive modelling of recycled aggregates, identifying factors such as technical performance, regulatory limits, and market dynamics that influence substitution accounting. The relevance and influence of these factors, however, vary with the assessment scope. Similarly, the case studies revealed some case-specific considerations. For instance, in system boundaries limited to CDW recycling processes, the choice of substitution coefficient and transport distances substantially affect results. By contrast, in building-level assessments, their influence is rather low, while modelling of selective demolition becomes more critical due to its effect on the quantity and quality of post-demolition material flows. To support robust interpretation, the case studies demonstrate the necessity of applying sensitivity and uncertainty analyses, particularly where input data are limited or assumptions influence outcomes. By synthesizing findings from literature and applied case studies, the thesis provides structured and practical guidance for enhancing the application of LCA in CE context with a specific focus on the construction sector.