Investigations to improve the durability and dimensional stability of wood using heat treatment have been carried out since the early 20th century. Despite quite a long history of research work, industrial scale manufacturing of heat treated wood is still in its beginnings in Finland. To better control the properties of heat treated wood, two classes – Thermo-S and Thermo-D – were introduced in 2003. Thermo-S (190 ºC, classified as moderately durable according to EN 113) is recommended for indoor objects, e.g., fixtures and furniture, whereas Thermo-D (212 ºC, classified as durable according to EN 113) is suitable for outdoor purposes, e.g., claddings, environmental structures and garden furniture. The aim of the study was to find out how uniform industrially produced heat treated wood is considering wood strength. Scots pine planks after Thermo-S and Thermo-D treatments were provided by a significant Finnish manufacturer. The planks were first machined to sample pieces for two loading tests: static bending (ISO 3133) and compression (ISO 3132). Before testing the strength with Lloyd Instruments EZ50 device, wood density and average ring width were determined. About 100 samples per treatment were measured. Untreated pine wood was used as a reference material. The data was analysed with statistical methods. Bending strength was about 15% higher in Thermo-S than in Thermo-D treatment, whereas the average compression strength was slightly higher for Thermo-D samples. Strength properties increased linearly with increasing wood density. Variation in the strength properties was large due to large variation in growth rate and wood density.
Keywords: Scots pine, heat treatment, strength properties
Authors
Rikala J.
Department of Forest Sciences, University of Helsinki
Havimo M.
Department of Forest Sciences, University of Helsinki
Sipi M.
Department of Forest Sciences, University of Helsinki
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