BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

10 U.S. DEPT. OF AGRICULTURE HANDBOOK NO. 207
Most information on kiln-seasoning of Caribbean
timbers has been developed by the Forest
Products Research Laboratory in England. Frequeut
reference is made in this text to the kiln
schedules recommended for different, timbers in
their “Kiln Operator’s Handbook” (222). One of
their more recent publications, No. 42, provides
new schedules for faster drying and more severe
kiln conditions. Any reader interested in kilnseasoning
of tropical American timbers should acquire
these publications for details on kiln
schedules and recommended operating techniques.
The U.S. Forest Products Laboratory, Madison,
Wis., also lists kiln schedules in its report D-1791
that may be used for tropical hardwoods by experienced
kiln operators.
SHRINKAGE AND MOVEMENT
Freshly sawed lumber contains from 30 to more
than 300 percent water, based on the ovendry
weight of the wood. The percentage of moisture
varies according to species. In the green condition,
dense, close-grained wood contains less
moisture than open-grained, less dense wood. As
an example, very heavy woods often contain 30 to
40 percent moisture in the green condition, compared
to as much as 300 percent or more in some
very light woods such as balsa.
Water is present in wood in two forms; as free
water in the cell cavities and intercellular spaces
of the wood and as absorbed water held in the
walls of the wood elements. When the free water
is removed but all the absorbed water remains in
the cell walls, the wood is said to hare reached the
fiber saturation point. This condition occurs at
about 30 percent moisture content but may vary
considerably between species. Shrinkage does not
occur in wood until this point has been reached.
However, as a piece of wood dries, the outer part
is reduced to a moisture content below the fiber
saturation point sooner than the interior. Consequently,
the whole piece may show some shrinkage
near the surface before the average moisture
content reaches the fiber saturation point.
Wood shrinks as it loses moisture below the fiber
saturation point. Wood that, has reached the airdry
condition at. 15 percent moisture content has
undergone about half the shrinkage possible. For
each 1 percent loss of moisture-below the fiber
saturation point (30 percent moisture content),
wood shrinks about, one-thirtieth of the total possible
shrinkage. Shrinkage values are normally
calculated and reported as the total shrinkage
from green to ovendry wood, and expressed as a
percent, of the original green volume.
For Caribbean timbers, shrinkage values are
presented on this basis in table 4, and in the sections
discussing shrinkage of the timbers in the
text. Shrinkage from green to 15 percent moisture
content (the air-dry moisture content often used
for timbers in this area) amounts to half the total
shrinkage. Shrinkage to 10 percent and 20 percent
moisture contents amounts to 2 / 3 and 1 / 3 of the
total shrinkage respectively.
Wood generally shrinks about twice as much in
the direction of growth rings (tangential) as
across the growth rings (radial) and very little
along the grain (longitudinal). Consequently,
wood that contains cross grain (interlocked) or
irregular grain will shrink more longitudinally
than straight-grained wood because of the tangential
and radial surfaces. The total shrinkage in
all directions quoted as one sum is termed volumetric
shrinkage.
The relationship of radial shrinkage to tangential
shrinkage is often an indication of the
inherent tendency of a wood to cup and otherwise
distort during seasoning. Generally, woods
that have a low ratio of tangential to radial
shrinkage season with less cupping and other defects
than woods with a higher ratio. Likewise,
because of the greater shrinkage in the direction
of the growth rings, “flat-sawed” or “plainsawed”
boards are more a t to cup and surface check
than “quartersawed” or “edge-grained” boards.
Flat-sawed boards are also subject to greater
shrinkage across the surface and less in thickness
than quartersawed boards.
I n genera!, heavier pieces of wood shrink
more than lighter pieces of the same species.
There are, however, many exceptions where heavy
species shrink less than those that weigh less and
where lightweight woods have large shrinkage
values. Recent studies at. Yale University (250)
show that tropical woods, with few exceptions,
undergo lower shrinkage than United States
woods of similar density. They found no correlation
between volumetric shrinkage and specific
gravity, indicating something other than
weight was the dominant factor controlling
shrinkage.
Shrinkage during drying is to some extent a
criterion of subsequent shrinkage and swelling
that will occur in the wood as a result of changes
in atmospheric conditions. Wood is exposed to
continuous daily and seasonal changes in relative
humidity. Its tendency to absorb or give off
moisture to come into balance with the surrounding
air is accompanied by swelling or shrinking
of the wood. This movement may be in the same
proportions as the initial shrinkage in seasoning.
In the absence of specific movement values, it IS
reasonable to assume a dimensional change of onethirtieth
of the shrinkage from green to ovendry
for each l-percent change in moisture content.
However, recent studies at the Forest Products
Research Laboratory in England show that all
woods do not undergo dimensional changes in
response to changed atmospheric conditions in
proportion to their initial shrinkage values (108).
They found that some woods shrink appreciably
in drying, yet may undergo comparatively small
dimensional changes in use.