Lumber Properties

Treated Wood

When wood is to be used outdoors and has to face the elements, it should have proper treatment for protection.  Wood is in a constant battle from decay and insect attack.  The type of treatment used in the construction of the boat was Micronized Copper Azole.  In this type of treatment, microscopic sized particles of copper are suspended in water rather than being dissolved in a chemical reaction.  This method is approved by the International Code Council for building code requirements(1).  This treatment was chosen because it was environmentally friendly and goes along with the recycled products.  It has received Environmentally Preferable Product certifications by the Scientific Certifications Systems(2)

Over time even treated wood will begin to feel the effect of weathering on the wood.  Weathering occurs when the lumber starts to break down as a result of weather conditions.  The four types of organics materials on wood are cellulose, hemicellulose, lignin, and extractives and they are all effected by weathering in a different way(3).  Although the beginning changes like color loss do not seem major, they can have drastic effect on the surface chemistry of the wood. If such changes are only surface related and there is no crack formation on the surface, the tensile strength of material decreases and elongation stays almost constant. If there is crack formation, the cracks act as notches that reduce both tensile strength and elongation very rapidly(4). The biggest weathering effect the raft would have to endure would be water and moisture content.  Wetting and drying lumber roughens the surface and raises the grain.  This may result in many small cracks which will lead into larger and deeper cracks.  This will cause the lumber to loose its strength and eventually break while in use.  In a wet environment, it is very important to have treated wood.

Figure 1: The top two figures show healthy pieces of wood unexposed to the elements.  The first is a tree with thin cell walls while the second has thick.  The next set of pictures show the effects of weathering after 30 days.  The bottom two show show the side the tubes that allow water to run through the tree.  The bottom left is unexposed while the bottom right has been exposed for 30 days(3).

[1] UserSpecification For Treated Wood, U1-12, 2012

[2] J. Hansen. (2010). International Center for Technology Assessment [Online] Available: http://nanotech.lawbc.com/uploads/file/00068945.PDF 

[3] Buschow, K.H. Jürgen; Cahn, Robert W.; Flemings, Merton C.; Ilschner, Bernhard; Kramer, Edward J.; Mahajan, Subhash (2001). Encyclopedia of Materials - Science and Technology, Volumes 1-11. (pp: 9715-9731).  Elsevier.

[4] Wypych, George (2003). Handbook of Material Weathering (3rd Edition).. ChemTec Publishing.