Quantcast Equilibrium Moisture Content

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2.4.3.1 Green Wood.  Wood cut from a green log often contains as much or more
than its oven-dry weight in the form of sap or water. The cell walls in green wood are fully
saturated and swollen, and the cell cavities are partially to completely filled with water. Water in
wood cell cavities is called free water.
2.4.3.2 Fiber Saturation Point. After wood has dried to about 30 percent mois-
ture content, it is at the fiber saturation point. In this state, the cell cavities are emptied of free
water, but the cell walls are still saturated and, thus, still in their weakest condition. At moisture
contents above the fiber saturation point, wood can be attacked by decay fungi.
2.4.3.3 Bound Water. The water remaining in the cell walls after wood has dried
to the fiber saturation point is called bound water. The bound water is held by physical forces of at-
traction within the cell walls. Just as a sponge shrinks and hardens as it dries, wood also shrinks as
it dries below the fiber saturation point. Wood will not decay when only the bound water remains.
2.4.3.4 Equilibrium Moisture Content. Since wood is a hygroscopic material,
the amount of water which the wood will lose depends on the relative humidity. That is, it responds
to changes in atmospheric humidity and loses bound water as the relative humidity drops, regaining
bound water as the relative humidity increases. For a given relative humidity level, a balance is
eventually reached at which the wood is no longer gaining or losing moisture. When this balance of
moisture exchange is established, the amount of bound water eventually contained in a piece of
wood is called the equilibrium moisture content of the wood.
2.4.3.5 Shrinking and Swelling.  Wood shrinks and swells due to the loss or gain
of bound water from the cell walls. The amount of movement depends on the amount of water
gained or lost, the orientation of the wood cells and species (Table 2-2).
Wood should be dried to its anticipated equilibrium moisture content after installation, to minimize
problems (shrinking, swelling, checking and warping) due to changing moisture contents. Table 2-4
shows the recommended moisture content values for various wood items at the time of their installa-
tion.
Tangential shrinkage is generally 1.5 to 2 tunes that of the radial shrinkage. Since radial and tangen-
tial shrinkage is not equal, and because most boards contain some combination of the two gram pat-
terns, warping will result if green lumber is allowed to dry without restraint. Warping includes cup,
bow, twist, as well as diamonding, and crook (Figure 2-10). Round products such as poles, piles
and posts develop surface checks and deep cracks for the same reason. Checks or cracks that
penetrate the preservative shell on treated products provide entryways for decay fungi. Consequent-
ly, decay frequently is associated with cracks that go deep into the wood products. Therefore, when
poles are inspected for internal decay, at least some borings should be taken in close proximity to
cracks.
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