International joint field trial on the moisture performance of wood - Set up and first results

Wood durability to decay stems from its inherent resistance to fungi resistance to fungi. In this regard, wood's ability to withstand moisture uptake plays an important role as the lower threshold for wood decay is a moisture content of 25%. However, the aspect of moisture resistance is not yet sufficiently emphasized as a durability factor, both in the assessment of durability as a material property and within the context of durability-focused design principles. This international joint field trial was designed to provide a comprehensive comparison of different laboratory-based test variables and their potential for predicting the outdoor moisture performance of wooden elements over time at climatically different locations. Nineteen research groups representing 15 different countries from six continents teamed up to conduct field exposure tests and record wood moisture content over time. These results were then compared with more controlled moisture uptake and release tests in laboratory. Several of the latter tests showed a strong ability to predict moisture performance of exposed wood specimens under varying climatic conditions. The results should support the development of methods for incorporating ‘moisture performance’ of wood into existing durability classifications. Simultaneous exposure of matched test specimens under several climatically different conditions facilitated the development of a multi-year time series in just a few months. The approach might be considered for future projects to reduce the time frame for developing relevant data.

This study aimed to compare decay and durability classifications between standard specimens and mini-blocks across different incubation times for various wood species and fungi. The experiment utilized two specimen formats: standard EN 113-2 (2021) specimens (15 x 25 x 50 mm) and mini-blocks (5 x 10 x 30 mm) exposed to two fungi (Coniophora puteana and Trametes versicolor) for varying incubation periods (6 to 16 weeks). A total of ten wood species were tested and the durability of wood species was classified per EN 350 (2016) standard based on median mass loss percentages.

This study clearly demonstrated that mini-block trials yield dissimilar outcomes compared to the European standard test (i.e. EN 113-2, 2021) at 6, 8, or 10 weeks of incubation. This discrepancy extends to both durability classifications based on median percentage mass loss and those based on x-values. When incubated with T. versicolor, the data provided a more consistent comparison between the two methods than incubation with C. puteana. Hence, while the mini-block tests can be considered useful as a rapid initial screening tool, e.g. for evaluating new wood protection systems, caution is warranted for its use in durability classification due to the potential for generating misleading results. Therefore, the main conclusion drawn from this research is that the mini-block test is not advisable as a substitute for conventional durability classification assessments.