BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

COMMERCIAL TIMBERS OF THE CARIBBEAN 15
conditions favorable for decay. Decay is an everpresent
hazard in the tropics where the conditions
of moisture and temperature are ordinarily ideal
for the development of fungi. Unless preservative
treatment is applied, only timbers with considerable
decay resistance should be used for posts,
poles, railway ties, foundations, and other uses.
where the timber may become damp or wet. Decay
resistance is not important where wood is used
for furniture, cabinetwork, millwork, and other
interior or protected uses. Then too, wood will
not ordinarily decay in exterior uses where it is
subject to frequent wetting, unless the construction
is such that the wood is prevented from drying out
after each wetting.
Most decay occurs in wood with a moisture content
above the fiber saturation point–about 30
percent moisture content. On the other hand,
wood that is continuously water soaked or continuously
dry will not decay. Thus, the underwater
portion of piling and bridgework or air-dry
timber at 20 percent or less moisture content is
safe from damage. Wood kept at the air-dry
moisture content of 15 to 20 percent in the Caribbean
area is, consequently, in no danger of decay.
However, decay is almost certain to occur if the
construction allows any part of a board or timber
to remain wet for long periods of time.
The description in the text of the decay resistance
of each timber is based on the durability of
untreated heartwood, which is more durable than
sapwood. It is doubtful if the sapwood of any
species is durable without preservative treatment.
Information on durability was available from published
results of graveyard tests, pure culture
laboratory tests, and from published observations
on the durability of the different woods while in
use.
Information derived from graveyard tests, in
which untreated heartwood posts of tropical
species (either round or square) are set in the
ground and inspected periodically for decay, is
available from Surinam, British Guiana, Trinidad,
St. Lucia, Panama, Puerto Rico, England, and
the United States. The results sometimes vary
between countries but are generally in good
agreement.
In pure culture tests small heartwood specimens
are exposed to fungal attack under closely controlled
laboratory conditions, and the relative
durability of different woods is determined on the
basis of the loss in weight through decay. The
results of these tests at Yale University and the
U.S. Forest Products Laboratory are in very good
agreement. However, the results of graveyard
tests and pure culture tests are not in agreement
for some timbers. When this occurs, the laboratory
tests generally show the higher decay resistance.
It appears that the laboratory tests
conducted under closely controlled conditions may
be the most reliable. In the laboratory, it is not
difficult to distinguish heartwood from sapwood;
but in graveyard tests, it is sometimes very difficult
in timbers without, well-defined heartwood to
be certain whether the posts used contain heartwood
or sapwood. This may be the reason for
some of the discrepancies between different graveyard
tests.
Table 6 lists the timbers by durability classes, according
to the published results of field and laboratory
tests. Four classifications are used: Very
resistant, resistant, moderately resistant, and nonresistant.
When authors report differences in
tests and results, the most consistent or reliable
rating is used. Timbers reported to be quite variable
in durability and those rated on the basis of
general reputation alone are indicated by
footnotes.
RESISTANCE TO ATTACK BY
TERMITES AND OTHER INSECTS
The sections on durability in the timber descriptions
contain a brief summary of the published
information on damage by wood-attacking insects.
Unfortunately, very little information is available
concerning insect attack on the Caribbean timbers
except for the comprehensive laboratory tests conducted
by G. N. Wolcott (262, 263) on the resistance
of woods to attack by the West Indian
dry-wood termite (Cryptotermes brevis Walker)
and a less comprehensive study of damage by subterranean
termites of Trinidad timbers (39).
Most information on other insect damage to logs
and sawed products is based on casual observations
and experience which, although generally
reliable, is often influenced by local conditions
peculiar to a small area.
The most common wood-attacking insects in the
Caribbean region are the ambrosia beetles (Scolytidae
and Platypodidae), both dry-wood and
subterranean termites (Order Isoptera), and
powder-post beetles (Bostrychidae and Lyctidae).
Ambrosia beetles, more often called pinhole borers,
are a common menace in green logs and lumber
and sometimes attack living trees. Both unseasoned
sapwood or heartwood may be attacked.
Attack by these insects is evidenced by numerous
holes 1 / 50 to 1 / 8 inch in diameter, according to the
species of pinhole borer responsible. Entry of
pinhole borers is often associated with staining
of the wood around the holes. Damage by this
insect is liable to be severe if logs are not converted
soon after felling or if they are not protected
by insecticide sprays.
Attack ceases when the timber is seasoned.
Damage can be prevented in freshly sawed lumber
or living beetles destroyed by dipping the product
in a water solution containing 0.2 percent of
gamma benzene hexachloride. It is also a common
practice to include in the same solution an
ingredient to prevent sap stain and keep the
lumber bright.