Wooden cladding boards in cyclic moisture conditions : studies of cupping, moisture distribution and swelling stress
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Doctoral thesis (article-based)
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Helsinki University of Technology Laboratory of Structural Engineering and Building Physics publications, 128
AbstractOne reason for the decreased durability of modern claddings is considered to be the use of deformation-sensitive cladding boards. At the end of the nineteenth century cladding boards were about 40 mm thick, whereas present-day instructions recommend about 20 mm thick boards for claddings. The first aim of the study was to compare the cupping sensitivity of heat-treated and non-heat-treated cladding boards during cyclic moisture conditions. The second aim was to investigate the progress of the moisture profile in wooden cladding board as a result of short-term single-sided water soaking. The third aim was to investigate the development of swelling stresses in the tangential direction in spruce (Picea abies) as a function of water soaking time in order to provide basic data. The cupping studies were conducted with full-scale test walls in laboratory conditions with specimens made of heat-treated and non-heat-treated timber. The moisture distribution study was based on a theoretical approach, after which the modeling results were compared to those of previously published studies. Experimental swelling stress results were compared to the theoretical calculations obtained from different models. On the basis of the cupping results, the thickness and heat-treatment of the board affected both the rate and the extent of curving during cyclic moisture conditions. The species of wood was also found to be an important consideration for selecting stable boards for the cladding. On the basis of the moisture distribution results, the transverse moisture profile could be predicted using a transport model in which air pressure and other resistance factors were compensated using an apparent surface emission coefficient. The modeling results were almost identical to the experimental observations. On the basis of the swelling stress study it appears that the magnitude of the swelling stresses in spruce samples in the tangential direction was approximately 1.2 MPa.
cladding, cupping, moisture distribution, swelling stress
- Virta, J., Koponen, S. and Absetz, I., 2005. Cupping of wooden cladding boards in cyclic conditions – A study of boards made of Norway spruce (Picea abies) and Scots pine sapwood (Pinus sylvestris). Wood Science and Technology 39 (6); 431-438.
- Virta, J., 2005. Cupping of wooden cladding boards in cyclic conditions – A study of heat-treated and non-heat-treated boards. Building and Environment 40 (10); 1395-1399.
- Virta, J. and Koponen, S., 2004. Free cupping of cladding boards caused by capillary penetration: An experimental study of untreated and heat-treated boards. Proceedings of the 8th World Conference on Timber Engineering (WCTE 2004). Vol. 3; 591-594.
- Virta, J., Koponen, S. and Absetz, I., 2005. Modelling moisture distribution in wooden cladding board as a result of short-term single-sided water soaking. Building and Environment, in press.
- Virta, J., Koponen, S. and Absetz, I., 2006. Measurement of swelling stresses in spruce (Picea abies) samples. Building and Environment 41 (8); 1014-1018.