Relevance of natural pre-weathering for laboratory decay tests with native, modified, and preservative-treated wood
A Pilgård, C Brischke, L Meyer
The on-going development of new wood protection systems is hampered by the long term field tests currently in use. New accelerated test methods and novel methods for faster and more accurate evaluation of wood protection methods are requested. For both field decay tests and accelerated laboratory decay tests, limitations are imposed. This study is part of the research program ‘WoodBuild’, which aims at developing a comprehensive test methodology for determination of wood durability. Field tests under various exposure conditions are regarded as well as laboratory tests. An overall objective is to find methods having the potential to reflect real life situations for all use classes. Therefore this particular study aimed at investigating the differences in mass loss with a mini-block test (Bravery 1979) following ageing with either leaching (EN 84) or natural weathering. The aim of this paper is not focussing on the durability (and definitely not on the efficacy) of any treatment but to highlight the differences in durability between naturally weathered samples and laboratory leached samples. The present study indicates that natural weathering and laboratory ageing procedures influence various wood species and various wood treatments differently with regard to durability. For preservative treated wood, none out of six treatments had a significantly higher mass loss for P. placenta when the samples were pre-weathered compared to pre-leached. For T. versicolor, 3 out of 6 treatments showed higher mass loss after natural pre-weathering. For the modified woods, 2 out of 7 treatments had a significantly higher mass loss for P. placenta when the samples were pre-weathered compared to pre-leached. When exposed to T. versicolor, 4 out of 7 treatments had a higher mass loss for P. placenta when the samples were pre-weathered. The mass loss for T. versicolor for the softwoods and hardwoods did not seem at all to be affected by the pre-treatments. Although, when exposed to P. placenta, 3 out of 9 softwoods and 1 out of 5 hardwoods had a significantly higher mass loss when the samples were pre-weathered compared to pre-leached. The present study indicates that the impact of natural weathering on mass loss seems not only to be treatment dependent but also wood species and test fungus dependent. The effects of natural weathering might be explained by the losses of active substances from treated wood but also the biological influence of microorganisms is a contributing factor. Taken this into account, it seems very important to consider the effect of any pre-treatment on both, the test materials and the untreated and per definition non-durable reference species.