Limit state design for biodeterioration - a new paradigm for management of fungal risks in biobased building materials




Lepage, Robert

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Biodeterioration is the leading cause of failure in buildings. Organic materials, key components of our built infrastructure, are particularly vulnerable to biotic attack (i.e. fungal growth) and can suffer from pre-mature failure. These failures are responsible for billions of dollars of direct damage to wooden structures in Canadian buildings. The impacts of failure can range from mild surface disfigurement, allergic reactions, mycoses, to direct life-safety concerns from compromised building structures. These impacts all have different failure modes and it is therefore prudent to consider how these failures manifest. The limit state design framework is an approach used by engineers to describe the risks of failure. It defines the probabilistic failure envelope of an inherent resistance being exceeded by a given load. The competing loads and resistances, in this case, consist of the fungal growth potential versus the intrinsic resistance of the substrate. Another key feature of limits state design is that it describes differing thresholds of failure depending on the potential impacts. This framework is desirable in application for biodeterioration in buildings. However, prior attempts to adopt these concepts into biodeterioration models have met with limited success. This dissertation is the first to effectively apply a limit state design framework to biodeterioration by considering two key states: serviceability limit state (i.e. surface fungal growth), and ultimate limit state (i.e. incipient decay). First, a database of fungal deterioration was created using Penicillium chrysogenum and Gloeophyllum trabeum fungi inoculated on jack pine (Pinus banksiana) prisms. These prisms were careful controlled for both moisture content and temperature, while minimizing ambient contamination. Photo documentation using a 20x USB microscope permitted evaluation of the surface disfigurement of the ascomycete fungus (serviceability state), and non-destructive flexural testing permitted the identification of incipient decay with the wood rotting basidiomycete (ultimate limit state). A serviceability limit state model was created using a population growth equation to describe the probability of detecting fungal growth as a function of substrate type (heartwood or sapwood), moisture content, temperature, and time. The model was contrasted with empirical tests on a mouldy roof in Vancouver, BC, and shows promising results that surpass the limitations of competing mould models. The method to develop the ultimate limit state model has been delineated in this dissertation, but further work is required. Future scopes of work are provided to address the limits and areas of uncertainty revealed by this research, but the results can help reshape the narrative of biodeterioration risk assessments for the built environment.



Biodeterioration, Wood, Fungi, Limit State Design