Moisture-related properties of modified timber - an experimental study

Doktorsavhandling, 2006

Wood has long traditions as a structural and building material. In those cases in which its natural features are not sufficient, chemical modification can eliminate some shortcomings. The overall aim of this investigation was to strengthen the role of timber as a building material by assessing the suitability of modified timber in construction applications by studying key properties such as density, modulus of elasticity (MOE), swelling/shrinkage behaviour and creep. This work comprises two parts: a pre-study and a main study. A large variety of material properties and their interactions were studied mainly on the small specimens, while the creep behaviour was studied on the full-scale specimens. In the pre-study, eight modification methods were screened: acetylation, maleoylation, succinylation, furfurylation, modification with N-methylol acryl amide, modification with reactive linseed oil derivatives, heat treatment in vegetable oil, and modification with methylated melamine formaldehyde resin. The wood materials were pine heart and sapwood, beech, birch and spruce. The specimen size was 10 x 10 x 200 mm. A total of 491 specimens were studied. A close interconnection between equilibrium moisture content, anti-swelling-efficiency and stiffness stabilisation was observed. The MOE appeared to be statistically unaffected, whereas the impact bending strength appeared to be reduced. The four modification methods used in the main study were acetylation (AC), heat treatment in vegetable oil (HT), furfurylation (FA) and modification with methylated melamine formaldehyde resin (MMF). The wood material was sapwood of Scots pine. A total of 2449 small specimens (10 x 10 x 200 mm) and 132 full-scale specimens (45 x 70 x 1100 mm) were tested. Significant changes in material properties took place. AC increased the density and reduced the equilibrium moisture content (EMC), MOE, shrinkage and creep. FA increased the density and reduced EMC and creep. MMF increased density and EMC but reduced creep, HT reduced EMC and creep. All four modification methods reduced the relative creep significantly. The creep factor was always lower for modified timber than for the controls. Untreated and modified timber have different material properties and deformation behaviour. Density is suitable for the estimation of creep deflection for untreated timber but not for modified timber, whereas the MOE is suitable for the estimation of creep deflection for both, untreated and modified timber. The use of modified timber as a building material is possible in non-load-bearing components such as wall cladding in wet rooms and garden furniture and in constructions for components, such as floor joists when serviceability limit state governs the design. However, the strength properties such as shear and tensile and design properties for mechanical connection must be determined in order to use modified timber in load-bearing structures.