Abstract
Abstract The moisture-dependency of the fracture energy for unmodified and acetylated Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth) has been investigated. Specimens were conditioned at relative humidity levels of 20, 75, and 97%, as well as dry and water-saturated. At moisture contents below 15%, the fracture energy increased with increasing moisture content for both unmodified and acetylated wood, while it decreased for untreated wood at higher moisture contents. A significant difference in moisture-dependency was found, indicating higher fracture energy for unmodified wood compared to acetylated wood at similar moisture contents. Additionally, to assess the impact of the increased brittleness for structural applications, the fracture energy was compared at equal relative humidity levels. The largest difference was seen at 75% relative humidity with approximately 50% lower fracture energy for acetylated wood. No significant differences were found for water-saturated samples. The moisture-dependency of the fracture energy, combined with the reduced hygroscopicity of acetylated wood, is suggested to be one, but not the only, contributing factor to the lower fracture energy of acetylated wood compared to unmodified wood at equal humidity levels. These observations have importance for structural design since design codes often assess material parameters based on ambient humidity.
Highlights
In order to reach milestone targets in mitigating the climate change, many operators within the building sector are exploring possibilities to increase the use of timber in loadbearing structures
At moisture contents below 15%, the fracture energy increased with increasing moisture content for both unmodified and acetylated wood, while it decreased for untreated wood at higher moisture contents
The findings demonstrated larger fracture energy for unmodified wood compared to acetylated wood for the moisture content levels investigated (0–15%)
Summary
In order to reach milestone targets in mitigating the climate change, many operators within the building sector are exploring possibilities to increase the use of timber in loadbearing structures. An increased use of wood in outdoor load-bearing structures would open possibilities for new architectural expressions. This should, in turn, increase the awareness about those possibilities and about the environmental and climatic benefits associated with the use of wood in constructions. Wood used in outdoor conditions must be protected from moisture to avoid excessive swelling and shrinking, as well as fungal degradation. To overcome these drawbacks, many different wood modification methods have been studied. The resulting change of the chemical constitution of the cell wall affects most physical attributes of the material: acetylated wood exhibits a decreased equilibrium moisture content, a lower maximum cell wall moisture content (Rowell 2006), and due to bulking of the cell wall, it exhibits less fibres per cross section area compared to its unmodified state (Rowell 1996)
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