BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

Although in-place preservative treatments have been effective in extending the service life of
certain timber elements and in some locations throughout the United States (e.g. utility poles and
railroad ties), there are a number of general concerns with the potential use of in-place
preservative treatments on this bridge including the following:
• Use of toxic chemicals that are harmful to humans and marine life that can run off of the
surfaces (surface treatment), leech out of splits and checks in the wood (internal
treatment) and/or can be spilled during the work with corresponding potential that the
work will not be permitted,
• Limited duration of effectiveness (10 years for internal treatment and 5 years for surface
treatment) due to leeching of chemicals from the timber with corresponding need for
periodic inspection and reapplication,
• Limited effectiveness of treatments due to the poor diffusion and absorption especially
when certain conditions needed for proper diffusion and absorption are not present (e.g.
specific moisture content), which potentially results in chemicals not diffusing or
absorbing to the areas of fungal decay,
• Weakening of smaller structural members due to the need to drill a significant number of
holes in the members to install the chemicals (internal treatment),
• Limited initial effectiveness due to slow diffusion and absorption rates, which permits
decay to take place before the treatment has become fully effective,
• Limited performance data on certain newer treatments.
In addition, as the timber stringers are of limited thickness (only 6” thick), the drilled holes for
internal in-place treatment will likely reduce the load carrying capacity of the members and may
require load restrictions. In order to provide access to the top of the timber stringers to perform
the work, the deck must be removed. As the preservatives have a limited service life and need to
be periodically reapplied, it would be necessary to remove the deck in the future to provide
access to the tops of the stringers for reapplication. As the new timber wearing surface and
structural deck are anticipated to have a longer service life than the estimated 10-year service life
of the preservative treatment, it would be necessary to temporarily remove and reinstall the deck,
when this would otherwise not be necessary, which significantly increases the cost of
retreatment. It may be possible to drill the holes from the bottom of the beam. However, this
introduces a potential environmental concern if one or more dowels that plug the holes were to
come loose due to dimensional changes in the timber from changes in moisture content,
permitting the chemicals to drop into the water. Water-diffusible fungicides (e.g., boron and
sodium fluorides) reduce some of the risks associated with the environment and human contact
with the toxic chemicals. However, these chemicals are a recent development with ongoing
research. Limited testing results have shown that the effectiveness of these chemicals can vary
significantly. With the significant above concerns and risks, in-place internal treatment would
not be a prudent alternative for the timber stringers.
(NOTE: If the decision is made to pursue in-place preservative treatment of other larger
members that do not have the same access concerns and/or concerns with human contact (e.g. the
timber cap beams), it is recommended that a solid fumigant such as methylisothiocyanate
(MITC) (available in capsule form) be used as this chemical fumigant would likely provide the
best alternative when balancing safety and effectiveness. Minor decay during the slow diffusion
Repair/Rehab. Feasibility Study March 10, 2011
Bridge No. C-07-001 (437) 24 Final Report