WOOD - Materials for Civil and Construction Engineers - page 420

Termites are perhaps the most destructive organism that attacks wood.
The annual damage attributed to termites exceeds losses due to fires. Ter-
mites enter structures through wood that is close to the ground and is poor-
ly ventilated or wet. Prevention is achieved by painting and otherwise
prohibiting insect entry into areas of unprotected wood through the use of
screening, sill plates, and sealing compounds.

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10.11.4 Marine Organisms

Damage by marine boring organisms in the United States and surrounding oceans
is principally caused by shipworms, pholads, Limnoria , and Sphaeroma . These
organisms are almost totally confined to salt or brackish waters.

10.12 Wood Preservation

Paints, petroleum-based solutions, and waterborne oxides (salts) are the
principle types of wood preservatives. The degree of preservation achieved
depends on the type of preservative, the degree of penetration, and the
amount of the chemical retained within the wood. Paints are applied on the
surface, while the other preservatives are applied under pressure to increase
penetration into the wood.

■ 10.12.1 Petroleum-Based Solutions

Coal-tar creosote, petroleum creosote, creosote solutions, and pentachlorophe-
nol solutions are the oil-based preservatives. These preservatives are very
effective, but are environmentally sensitive. They are commonly used where
a high degree of environmental exposure exists and human contact is not a
concern. Applications include utility poles, railroad ties, and retaining walls.

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10.12.2 Waterborne Preservatives

The typical solutes used in waterborne preservative mixtures are ammonia-
cal copper arsenate, chromated copper arsenate (CCA) and ammoniacal cop-
per zinc arsenate. The advantages of the waterborne preservative over the
oil-based are cleanliness and its ability to be painted. The disadvantage of
these treatments is their removal by leaching when exposed to moist condi-
tions over long periods of time. These preservatives are also environmental-
ly sensitive and must be applied under carefully controlled conditions. The
level of potential danger to humans from contacting wood pressure treated
with CCA is controversial. Trade groups are supportive of the product, but
several other agencies see potential health effects. By 2003, the wood preser-
vation industry agreed to stop using arsenic-based preservatives for prod-
ucts intended for residential use or direct human contact. CCA can still be