Fracture behaviour of acetylated wood : Material characterisation and dowel-type connections

Sammanfattning: In order to increase the competitiveness of timber as a building material in outdoor applications, durability and dimensional stability must be ensured. Acetylation enables an environmentally friendly way to increase both durability and dimensional stability of wood, without introducing harmful substances to the environment. The focus of the present research is to examine the possibility of adding a structural value by acetylation of wood species native to the Nordic region, rarely used for loadbearing structures outdoors due to poor durability and dimensional stability. Yet, before a large scale use is possible, the mechanical properties of acetylated wood must be examined. The research presented in this dissertation focuses on the brittleness of acetylated wood, both at a clear wood level as well as in structural applications.Fracture characteristics are hereby defined by properties that influence the brittleness, considered by the stiffness, tensile strength and fracture energy. For Scots pine conditioned at a relative humidity of 60% and a temperature of 20◦C, no significant impact of the acetylation process was found for the stiffness along the grain, nor for the tensile strength perpendicular to the grain. However, for Scots pine and birch examined at various relative humidity levels, the fracture energy was found to be significantly reduced for acetylated wood at relative humidity levels up to 97%. The largest difference between unmodified and acetylated wood of the same species and at equal climate conditions was approximately 50%. The studies demonstrated a clear moisturedependencyof the fracture energy for both unmodified and acetylated wood, but it was suggested that the fracture energy is lower for acetylated wood compared to unmodified wood at similar moisture contents. The lower fracture energy of acetylated wood when compared to unmodified wood at equal relative humidity levels can thus partly, but not solely, be explained by the reduced hygroscopicity of acetylated wood.To evaluate the implications of the increased brittleness in terms of structural applications, single doweltype connections made from acetylated and unmodified Scots pine were studied. Results were compared to Eurocode 5 estimations to evaluate the validity of current design provisions for acetylated wood. It was found that, for all the tested enddistances for loading parallel to the grain, joints made from acetylated wood failed in a brittle manner. Nevertheless, connections made from acetylated wood demonstrated a significantly higher embedment strength and loadbearing capacity parallel to the grain, and Eurocode 5 provided conservative estimations. For loading perpendicular to the grain, a reduced splitting capacity was found for acetylated wood compared to unmodified wood, and here the loadbearing capacity wasoverestimated by Eurocode 5.The most important conclusion from the research presented herein is the increased brittleness of acetylated wood compared to unmodified wood. Special attention is, hence, required in structural design using acetylated Scots pine and birch. In case of doweltype connections, or other loading situations where stress concentrations occur, measures should be taken to avoid premature brittle failure modes. This risk may for instance be limited by increasingm spacings between fasteners as well as endand edgedistances, and/or reinforcement of joints. Further studies are needed to increase the knowledge of how the acetylation process will impact loadbearing structures. Although the research presented herein reveals one disadvantage with acetylated wood, it can still outperform unmodified wood in moist conditions thanks to e.g. increased dimensional stability and durability.