Durability tests of wood materials related to constructions in Björnlandet National Park

The aim of the present study, primarily initiated by the Swedish Environmental Agency, is to study the durability of different wood materials related to and used in constructions in Björnlandet National Park in northern Sweden (63°97´N, 18°05´E), completed during 2014 and 2015. Of particular interest is to determine the durability properties in a scientific way of local materials used, such as mountain-grown Norway spruce and locally grown Scots pine and larch. The effect on the performance of charring and superficial wood-tar treatment is also studied.

The study consists of a laboratory test according to ENV 807, a field trial according to EN 252 and a service trial on-site in the national park, including constructions such as load-bearing piles, ramps, foot-bridges and platforms. Results from the tests according to ENV 807 and EN 252 as well as the initial inspection of the constructions and components of different materials are presented in this report.

During the initial inspection of the constructions on site, then in use for 1-2 years, no decay was found besides a small initial attack on one single pile of larch.

The accelerated tests in laboratory and field have provided interesting information on the durability properties of the different materials and the ranking between them from a durability point of view which can be concluded as follows:

• The local materials show moderate durability with marginal differences between the materials, with the exception of the spruce materials which show lower durability, comparable to the durability of pine sapwood.

• Heartwood of locally grown pine and larch show basically equal durability properties.

• Slow-grown wood will not enhance the durability properties.

• Superficial treatment with wood-tar contributes so far to a somewhat increased durability. However, the primary positive effect is most certainly owing to the smell and appearance of wood treated with wood-tar, which in certain environments contributes to a positive perception. Charring does not add any value from a durability point of view.

It is important that the results of the laboratory and field tests are interpreted with some care and common sense. The relatively extensive decay achieved with all materials but the preservative-treated wood does not necessarily mean that the constructions will be subject to early decay. With respect to the geographical location of Björnlandet National Park in the inland of northern Sweden, where the decay hazard in general is moderate, the local materials can thus be expected to have a reasonably long service.

However, results from the tests indicate that efforts to select local materials/use "traditional" treatments most certainly will not result in longer service life than can be achieved with corresponding materials commercially available.

Finally, it is pointed out that:

• Avoiding moisture traps by correct design of components and constructions is vital for the service life when "traditional" local materials and treatments are used.

• Preservative treatment will in general give critical wooden constructions a longer service life. The use of untreated wood involves more uncertainty with expected service life. Regular inspections and maintenance are therefore particularly important for constructions critical to personal safety (e.g. foot bridges, ramps) or infrastructure (e.g. bridges).

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Urgent future objectives to complete are the inspections of the field trial until remaining stakes have been exposed for five years, and to follow up the service trial in Björnlandet National Park with inspections in 4-5 years, i.e. in 2022 or 2023 in order to confirm or reassess the preliminary conclusions.