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Issues Impacting Bridge Painting: an Overview
Executive Summary
The bridges of the United States are under
constant attack from the environment. Whether it is the sudden
yet violent effects of floods or earthquakes or the long
term effects associated with sunlight, rain, deicing salts
and freeze/thaw cycles. To often, in the design of a bridge,
little thought is given to the mitigation of these effects
and seldom are provisions made for ease of future rehabilitation.
The quality of the environment has always been
of concern to the general populace as well as to the government.
Concerns over the potential pollution of the environment,
by lead and other heavy metals, during the removal of older
paint systems from bridges has resulted in regulations severely
limiting the options available for paint removal. Prior to
1985 open air blasting with grit was the standard method
of paint removal. While inexpensive it resulted in paint
debris scattered over a wide area. Today, open air blasting
is illegal and has been replaced with removal in containment.
Containment means that the bridge or a portion of the bridge
is enclosed thereby containing the paint debris. Since this
concept requires engineering analysis and construction and
potential movement of the enclosure the costs for paint removal
has dramatically increased oftentimes without meeting the
regulatory goals.
With the advent of containment the emphasis
has shifted to the worker. The use of containment has led
to increased concentrations of particulates within the structure.
With the increase in particulate concentrations has come
regulations as to the attire, respiratory protection, and
decontamination of workers within the containment. Increasing
regulation has led to decreased productivity which again
has resulted in increased cost to the bridge owners and the
inability to adequately meet the needs of the infrastructure
with the available funding.
Additional regulations have been enacted limiting
Volatile Organic Compounds (VOCs) for architectural and maintenance
coatings. These regulations have resulted in the application,
onto bridges, of modified paint systems with limited available
information associated with their durability. The use of
these paints to replace or repair existing systems is a gamble
that the states must take in order to meet regulatory compliance.
The purpose of this study was to review aspects
of the rehabilitation process in light of the above mentioned
limitations, assess to the degree possible the state of the
art, arrive at conclusions and make recommendations where
applicable. To this end five technical and one economic tasks
were performed. Below are selected conclusions and recommendations
resulting from the various tasks.
- TASK A - ECONOMIC ANALYSIS
A life cycle cost (LCC) model has been developed for
the maintenance painting of steel bridges. The model
computes the Equivalent Uniform Annual Cost (EUAC) for
a given coating system with a particular initial cost
and known life expectancy under a given set of financial
conditions (interest and inflation rates). The cost data
is presented on a dollars per square foot basis. This
allows the user to calculate an average yearly cost for
controlling corrosion of a steel bridge using a particular
coating system based upon the estimated usefull life
of the structure. The model shows that the coating life
expectancy plays a major role in the determination of
life cycle cost of maintaining the paint system and the
structure. This is particularly true for coating life
expectancies in the lower range. An increase in life
expectancy from 6 to 10 years can mean a life cycle cost
savings of nearly 60% for ____ economic conditions 7%
interest and a 4% inflation rate. The corresponding percentage
decrease in life cycle cost with increasing life expectancy
diminishes as the coating life expectancy extends outward
towards 40 years but still remains significant to 40
years.
What is quite clear from the economic analysis that
a new way of costing the removal of old paint and the
application of new paint is necessary. For this or any
other model to be an effective tool, costing needs to
be made in an itemized manner and more information about
the durability of VOC compliant paint systems is necessary.
What is also clear is that the cost of material i.e.
paint is the least costly element of paint removal and
recoating. It is penny wise and pound foolish not to
buy the highest quality paint for recoating.
- TASK B - WORKER PROTECTION/PAINT REMOVAL
As discussed above the regulations concerning paint
removal and worker safety are complex and under constant
revision. The preferred method of removing lead containing
paints is containment and total removal. Once the paint
is removed it no longer presents a danger. Unfortunately,
it is expensive and this study has found that the potential
exists for particulates with diameters less than Ü are
generated by steel grit blasting and laser ablation.
For these particles many HEPA filters have limited capture
potential. Additionally, similar results would be expected
for any process that pyrolyses the paint. It is recommended
that the presence of these particles be determined in
an unambiguous manner and technology developed to insure
their capture.
- TASK C - WASTE TREATMENT AND DISPOSAL
The purpose of this task was to evaluate the Toxicity
Characteristic Leaching Test (TCLP) in the presence of
iron to determine whether adequate stabilization occurs
in order that the debris can be disposed of in commercial
waste sites. This study and others concludes that the
TCLP test is indicative of the stabilization of the waste
at the time of the test and is not a reliable indicator
of future resistance to leaching. Insufficient information
is available at this time as to the actual "typical" environment
in a commercial dump site to define an improvement or
replacement for the TCLP test.
- TASK D - ALTERNATE COATINGS
Many of the alternative coatings, with the exception
of metallizing, have sufficient data to make recommendations.
In the case of mettalizing the technology can be recommended
for use in fabrication shops and as a replacement in
the field were appropriate. As part of this task the
difficulties faced by fabrication shops and the concept
of "overcoating" are discussed. Additionally, problems
were identified and future research to affect solutions
to those problems was outlined.
- TASK E - ACCELERATED TESTING
One of the difficulties of responding to changing regulations
is the lack of time to evaluate new paint systems before
they must be used. This task identified modifications to
existing tests that, if implemented, might provide better
correlation between accelerated tests performed in the
laboratory and durability in the field. Additionally,
laboratory tests were evaluated to determine their applicability
in the field.
- TASK F - PRODUCTIVITY IMPROVEMENT
In this task an evaluation was made to determine where
sensors could best influence productivity and result
in improvements to coating performance. Since it is commonly
acknowledged that over 70% of coating failures are associated
with poor surface preparation a series of sensor technologies
were evaluated. They included enhanced monochrome CCD
techniques to determine the degree of surface rusting,
Color CCD techniques to evaluate blast cleaned surfaces,
and thermal wave imaging for the quantitative evaluation
of painted surfaces prior to paint removal.
While this study has reviewed and evaluated existing and
future rehabilitation practices it raises more issues than
it answers since the issues are far more complex than they
appear.
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