BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

8 U.S. DEPT. OF AGRICULTURE HANDBOOK NO. 207
Table 2 also carries both the nominal specific
gravity based on green volume and ovendry
weight of wood and the true specific gravity
based on the air-dry volume and weight of the timber.
Several different specific gravity values are
often reported in the literature for each timber,
and sometimes cover a considerable range. This
is to be expected as many factors are included that
influence the density of wood, Density often
varies considerably between sites and country of
orgin and, as many of the timbers have extensive
distribution, a covsiderable range in density is expected.
The specific gravities and weights in table
2 are, therefore, the averages of the individual
published results from the areas included,
Most specific gravities and weights per cubic
foot quoted in the tables and text for air-dry wood
are based on a moisture content of 12 percent.
The change between 12 and 15 percent moisture
content is so minor that no reference is made to the
few data based on the higher moisture content.
Some difficulty is encountered in listing specific
gravities and weight per cubic foot for the wood
of those timbers with two or more species. Where
the difference between species is great, the gravity
and weight are cited for each species. In other
cases, these are combined and represent the timber
group. Where this information is available for
only one or two species of a trade group, it is stated
for the individual species rather than the entire
group.
SEASONING
Wood can be air-seasoned in the open or kilnseasoned
in dry kilns using artificial heat and
humidity. Few dry kilns are available in the
Caribbean area so most lumber is air-seasoned.
Because of their diffuse porous nature, tropical
hardwoods generally season with less defect than
most ring porous woods in the temperate zones.
The tropical climate of the Caribbean area presents
both favorable and unfavorable conditions
for air-seasoning. One combination bringing
about favorable conditions is high temperature
and relative humidity throughout the year that
allow continuous and uniform drying. This reduces
the possibility of serious surface checking,
end splitting, cup, bow, and other seasoning
defects.
On the other hand, lumber piles should be
roofed or under some cover for successful airseasoning;
otherwise, the frequent tropical rains
may prevent any appreciable amount of drying
during the rainy season or, for that matter, any
time of the year. When properly protected from
ram, lumber will season through out the year in
most areas of the American tropics. Under cover,
lumber 1¼ inches thick will usually air-season in
2 to 6 months. Lumber will normally air-season
to a moisture content between 15 and 20 percent,
depending on the species, location, and time of
year.
Table 3 groups the different timbers according
to their ease of air-seasoning. The timbers,
divided into three groups, are rated on the time
required for each to season and the amount and
severity of defects that normally occur. Of necessity,
only a small number of groups are used.
Thus, where two timbers are quite similar, one
may fall in the bottom of one c ass and the other
in the top of the next lower class. This is unavoidable.
To a marked extent, the grouping
presents the average air-seasoning; it does not
cover the possibilities of improving the seasoning
qualities of the different timbers by bettering
seasoning practices.
Many timbers are degraded during seasoning
because of an excessive rate of drying. This can
be largely overcome by reducing air circulation;
that is, by closer piling, by use of thinner stickers,
and by placing shields or covers on one or more
sides of the lumber stacks. Other factors such as
sticker spacing, width of lumber stacks, and exposure
to sun or rain are important in the drying
rate and amount of seasoning defect.
End splitting and end checking occur during
the seasoning of some timbers. But these faults
may be prevented by coating the ends of boards
with a moisture-resistant paint, pigments, waxes,
or other material to prevent the over-rapid loss
of moisture at these places. A commercial mixture
of asphalt and plastic roof coating has been
used by the Tropical Forest Research Center with
excellent success; when end-coated with this material,
60 species of tropical hardwoods were
seasoned with no appreciable end defect. The
nailing of end cleats or narrow wooden strips to
the ends of boards or planks is of only limited
value in air-seasoning; during kiln-seasoning this
practice can even cause end splits to develop or
extend.
The humidity and temperature conditions in
the Caribbean area are very favorable for the
development of sap-stain fungi in logs and lumber.
The best ways to prevent this type of damage are
early conversion of logs after felling or storage of
logs under water. The application of antiseptic
sprays to ends and places from which the bark
is removed will also protect most species of logs
for 1 or 2 months if wood-infesting insects are
not prevalent. Unfortunately, insects are a problem
in tropical areas. To prevent their entry
into logs and the transmission of fungi to the
wood, applying an insecticide to the log is also
necessary. In some species, adding an end coating
is also required to prevent seasoning checks
through which insects or fungi can enter (242).
Molds and stains are confined largely to the sapwood;
their colors vary. Molds are not responsible
for much staining. The discoloration caused
by them is mostly superficial and largely due to
the cottony or powdery surface growths easily
removed by brushing or light surfacing.