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the pile, is attached to the strips. The strips permit air to circulate beneath the cap and, thus, keep
the wood dry.
Because pile caps on working piers are often damaged or pulled off by hawsers, it is helpful to treat
the top with a water-soluble preservative before cap installation. These treatments remain inactive as
long as the cap remains effective, but they are activated whenever water penetrates. Borer Damage.  When an engineering determination is made to repair
rather than replace borer damaged piles, several options for repairing and preventing additional
damage are available. NAVFAC MO-104 provides a number of repair techniques for fender and
bearing piles. Repair may involve partial replacement of damaged wood with treated wood and rein-
forcing weakened areas. Prevention of damage is accomplished by encasing the pile in an imperme-
able barrier. Barriers.  Impermeable barriers are used to protect the wood below the
water line. These barriers prohibit the entry of new borers into the wood and kill the established
borers by limiting the available oxygen supply. The barriers must cover a zone from below the mud
line to above the high tide line and completely encase the piling. They may provide protection from
marine borers for 25 years or more and are advisable when inspection reveals a 15 to 20 percent loss
of cross sectional area from marine borer attack. Numerous materials have been used, but 30 to 60
mil polyvinylchloride or polyethylene wraps and two inch thick concrete are probably the most com-
mon. Polyvinylchloride was used initially, but more recently two-layer polyethylene sheets have
been used. A 20 mil sheet is heat shrunk before the piling is driven. To protect against abrasion, a
150-mil polyethylene sheet is attached at the intertidal zone or to fender piles with aluminum alloy
nails. However, corrosion will occur if these nails are used on wood treated with copper-chromium-
arsenic (CCA) or ammonical copper-arsenic (ACA) preservative. Protected piles should be in-
spected periodically for damage to the barrier. Procedures for installing plastic barriers are given in
NAVFAC TSM-B10, Installation of Flexible Plastic Barriers on Marine Borer Damaged Wood
7.3 TIMBERS AND OTHER HEAVY WOOD MEMBERS. Timbers are those members five or
more nominal inches in their least dimension. They are also called beams, stringers, posts or gir-
ders. Inspection procedures and control techniques for timbers and other heavy wood members are
similar to those for poles and piling as already discussed. Additional information unique to the
product is provided below.
7.3.1 Problems To Be Found. Nature of Timbers.  Timbers are different than poles or piles in that they
are sawed products. With the exception of Southern pine, they usually do not contain a continuous
sapwood band which is easily treated. Therefore, incising of most large timbers will improve the
preservative treatment. In addition, many timbers will contain a pith center which results


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