WOOD - Materials for Civil and Construction Engineers - page 400

dependent. It is a linear polymer (aliphatic carbon compound) having a high
molecular weight. The main building block of cellulose is sugar-glucose.
As the tree grows, linear cellulose molecules arrange themselves into highly
ordered strands, called fibrils . These ordered strands form the large struc-
tural elements that compose the cell walls of wood fibers.
Lignin accounts for 23% to 33% of softwood and 16% - 25% of hard-
wood by weight. Lignin is mostly an intercellular material. Chemically,
lignin is an intractable, insoluble, material that is loosely bonded to the cel-
lulose. Lignin is basically the glue that holds the tubular cells together. The
longitudinal shear strength of wood is limited by the strength of the lignin
bounds.
Hemicelluloses are polymeric units made from sugar molecules. Hemi-
cellulose is different from cellulose in that it has several sugars tied up in
its cellular structure. Hardwood contains 20% to 30% hemicellulose and
softwood averages 15% to 20%. The main sugar units in hardwood and
softwood are xylose and monnose, respectively.
The extractives compose 5% to 30% of the wood substance. Included in
this group are tannins and other polyphenolics, coloring matters, essential
oils, fats, resins, waxes, gums, starches, and simple metabolic intermediates.
These materials can be removed with simple inert neutral solvents, such as
water, alcohol, acetone, and benzene. The amount contained in an individ-
ual tree depends on the species, growth conditions, and time of year the tree
is harvested.
The ash-forming materials account for 0.1% - 3.0% of the wood mater-
ial and include calcium, potassium, phosphate, and silica.

10.3 Moisture Content

The moisture content of a wood specimen is the weight of water in the spec-
imen expressed as a percentage of the oven-dry weight of the wood. An oven-
dried wood sample is a sample that has been dried in an oven at 100°C to
105°C (212°F to 220°F) until the wood attains a constant weight. Physical
properties such as weight, shrinkage, and strength depend on the moisture
content of wood.
Moisture exists in wood as either bound or free water . Bound water is
held within the cell wall by adsorption forces, whereas free water exists as
either condensed water or water vapor in the cell cavities. In green wood,
the cell walls are saturated. However, the cell cavities may or may not con-
tain free water. The level of saturation at which the cell walls are completely
saturated, but no free water exists in the cell cavities, is called the fiber sat-
uration point (FSP). FSP varies from species to species, but is typically in
the range of 21% to 32%. The FSP is of great practical significance, because
the addition or removal of moisture below the FSP has a large effect on prac-
tically all physical and mechanical properties of wood, whereas above the
FSP, the properties are independent of moisture content.