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Issues Impacting Bridge Painting: an Overview
Chapter 5. Task D - Advanced Coatings
Table of Contents
INTRODUCTION
The rehabilitation of steel structures, in general, and
bridges, in particular, is a significant drain on resources,
including manpower and budget. As restrictions on both the
permissible methods of paint removal and the VOC content
of paint systems increase, the options available to bridge
owners diminish.
For years, standard maintenance painting practice for steel
bridges has entailed the use of open-abrasive blasting for
the complete removal of existing paint prior to repainting.
Open-abrasive blasting not only removes the existing paint,
but also cleans the steel surface of corrosion products and
millscale while providing a roughened surface profile desirable
for the adhesion of new paint. In the past, lead-containing
paint provided a reliable economical solution to the long-term
corrosion protection of these structures without requiring
extensive surface preparation such as the removal of millscale.
Recent concerns over the effects of lead as a toxic substance
in the environment and the release of VOCs during the application
of paints has caused a radical change of direction to be
pursued in the painting of steel highway bridges. The primary
objective of task D was to investigate advanced coatings
that have the potential to meet the changing needs associated
with the rehabilitation of steel bridges.
BACKGROUND
Organic coatings are applied onto steel surfaces
to provide a barrier to minimize the interaction of the steel
and corrosive
elements of the environment. In order to be successful, these
coatings must delay and limit ingress of the aggressive corrosive
elements: water, oxygen, and ionic species such as Cl- and
SO4=. These aggressive corrosive elements
can reach the substrate/coating interface either by intrusion
through defects or by diffusion through the protective coating.
While the incidence of defects can be reduced by proper coating
application based on good quality control practices, the
diffusion of oxygen and salts in solution will always take
place. However, the rate of diffusion varies widely, based
on the coating system chosen. Considerable research has been
performed on the mechanism of coating degradation. However,
due to the complexity of metal/coating and coating/environment
systems, theories do not exist that quantitatively or even
qualitatively describe the coating degradation processes
and relate them in a manner that allows the prediction of
the performance of coating systems during service. FIGURE
52 describes one model, developed by NIST, indicating
the major factors that they perceive as influencing the durability
of a coating system.
Aging is one of the factors that affect coating
durability and is a direct measure of the success or failure
of the
system to provide its stated goal of protecting the substrate.
It is primarily aging that determines when, and to what
degree, a coating system is removed and replaced.
As mentioned in task A, advanced coatings can
benefit the maintenance of steel bridges within the following
options:
(1) Total paint removal and recoating, and (2) Partial
paint removal and overcoating.
In addition, advanced coatings
can be of benefit during the rehabilitation of steel bridges
that require replacement
of structural components.
Total paint removal and replacement may be
the best economical option if more than 20 percent of its
protective coating
system has failed. Within the context of total paint removal,
several options exist which are based on the integrity
of the structural elements. If component replacement is not
necessary and/or can be deferred, a VOC-compliant paint
system
with an expected lifetime comparable to the next planned
rehabilitation of the structure can be chosen.
If component replacement is necessary, a coating
system applied in the fabrication shop should be chosen such
that
it can both perform and be repaired in the field. However,
fabrication shops have to face increasing challenges in
this area, again due to the implementation of ever more stringent
VOC requirements as discussed below.
DILEMMAS FACING FABRICATION SHOPS
Increased Shop Painting
For many years, highway agencies specified paint systems
for new bridges that were reasonably long-lasting and relatively
simple for steel fabrication shops to apply. Initially, shop
painting involved the application of a primer coat that was
supplemented by additional coat(s) of paint applied after
the steel was shipped to a job site and erected. Multi-coat
paint systems offering enhanced performance have gradually
been adopted by many highway agencies, resulting in a desire
to limit or eliminate field painting. This is partially due
to the increased control of the painting and quality control
practices in a shop environment. At least eight highway agencies
have adopted the practice of complete shop painting, limiting
field painting to touch up work.
Containment For Maintenance
Painting
Environmental regulations currently prohibit environmental
releases of particulates generated by abrasive blasting during
maintenance painting of bridges in urban areas. Worker exposure
to these particulates necessitates the use of protective
respirators, even when abrasive blasting operations do not
involve lead-based paints. In the future, it is likely that
all bridge maintenance painting incorporating abrasive blasting
will require total containment. Highway agencies anticipating
that eventuality are currently seeking extremely durable
shop coatings to forestall the need for maintenance painting.
Fabrication Shop Problems
In the past, fabrication shop personnel considered shop
painting as a nuisance, however, it was necessary to sell
fabricated steel. In many cases, the quality of painting
operations in those shops took a back seat to other activities
such as welding. The forgiving nature of the early oil-alkyd
paints promoted that antipathy. The advent of advanced structural
steel paints, such as the inorganic zinc/vinyl systems demonstrated
the need for improved quality control. For example, poor
control of wet thickness for inorganic zinc paints resulted
in "mud cracking." As more highway agencies have required
the shops to perform more painting work using high-performance
paint systems, painting has had greater impact on shop profitability
and the quality of shop painting has became more critical.
Regulation of Fabrication Shops
Fabrication shops are point sources of waste
generation. As a result, they are easy targets for regulation
by State
departments of environmental protection (DEP's). Those agencies
promulgated regulations restricting the VOC content in paints.
In addition, if materials are used that exceed these limits,
the fabrication shops are faced with a limit on their total
yearly VOC emissions. Highway agencies specifying paints
with high-VOC content have been forced to employ lower VOC
coatings or the fabrication shops nearing their VOC point-source
limits are unable to supply the components.
Current limitations on VOC content of paints
and the total allowed yearly VOC emissions by fabrication
shops will most
likely be tightened in the future. The VOC regulations
for paints, as the result of the ongoing regulation-negotiation
activities related to architectural industrial maintenance
will most likely reduce the VOC limits on bridge maintenance
paints to 336 g/L (2.8 lbs/gal). Most states require for
new construction that point sources use paints with a VOC
limit of 420 g/L (3.5 lbs/gal). In States restricting the
total yearly VOC emissions from fabrication shops, those
restrictions probably will be more limiting than that for
the VOC content.
A solution to the problems of both highway
agencies and fabrication shops would be for State highway
agencies to
specify water-based paint systems such as the acrylic systems
now offered by a number of paint companies. However, while
these systems may meet present and projected VOC restrictions,
they are not as durable, particularly in high-salt environments,
as conventional solvent-based systems. In addition, waterborne
paints are less application-tolerant than their solvent-based
counterparts. They require close control of application
conditions (e.g., temperature and humidity) and surface cleanliness
(e.g., sensitivity to surface oils and other contamination).
Fabrication shops able to apply high-performance solvent-based
paints have experienced problems with waterborne systems.
Due to shop layouts and the proximity to other operations,
some of those problems are difficult to resolve and severely
limit painting operations when waterborne systems are employed.
Several highway agencies have experienced reduced performance
using those systems on new bridges.
Many polymer manufacturers and paint companies
have invested heavily in water-based paint technology. With
continued research,
some of the problems currently experienced with waterborne
systems will be remedied over the next 5 to 10 years. However,
it is problematical that coating durability of waterborne
paints will improve sufficiently to match that of some
of the solvent-based systems presently in use, such as inorganic
zinc.
The 100-hundred percent solids paint technology
employs two-component systems that react chemically and polymerize
to form a solid film. The coatings are automatically mixed
during the feeding process or internally mixed in the spray-gun
head during application. Common two-component systems include
epoxies, polyureas, and polyurethanes. The 100-hundred
percent
solids painting generates no VOC's. Typically, onethe 100-percent
solids paints are applied in high builds 0.5 to 0.75 mm
(20 to 30 mils). The coating components react and dry quickly
(typically within 15 to 30 min), facilitating shop handling.
Special application and handling equipment, coating materials,
and operator training are required. Coating costs will
initially
be somewhat higher than for conventional solvent-based
paint systems. These costs are anticipated to be offset by
somewhat
longer service lives. The 100-hundred percent solids paints
are presently being employed on chemical plants, boat hulls,
and railway cars.
The Union Carbide Corporation has developed
a proprietary paint application method, the Unicarb System,
that uses carbon
dioxide to replace up to 80 percent of the solvents in
conventional solvent-based paint systems. Carbon dioxide
gas is introduced
in a spray gun during application. It improves paint atomization
during spraying and acts as a carrier for the paint. The
method has the advantage of being able to employ current
solvent-based paint technology, while achieving a significantly
reduced VOC release level. The reduced level of VOC's causes
the paint to dry fairly rapidly (depending on the paint
system), facilitating handling. Special application equipment,
coating
materials (i.e., compressed gas-carbon dioxide), and operator
training are required. Coating costs will initially be
slightly higher than for conventional solvent-based systems.
The same
coatings applied using the Unicarb System in place of conventional
solvents have had relatively lower permeability. Therefore,
longer service lives may be obtained. The Unicarb System
has been employed for commercial products; it has been
demonstrated successfully in several areas, including automotive
and aircraft
applications. The system as presently employed appears
to be more economically attractive for higher production
rate,
repetitive operations.
OVERCOATING
Spot repair and overcoating of compromised
coatings involves: (1) cleaning the surface of the bridge
with pressurized water
to remove salts, soils, and other contaminants, (2) mechanically
removing loose corrosion products and paint, (3) spot priming
areas where paint and corrosion products have been removed,
and (4) applying coats of paint over the entire surface.
The conditions required for successful overcoat application
are: (1) adequate adhesion and mechanical properties of the
old paint, (2) compatibility of the overcoat system to existing
coating system, and (3) proper surface preparation. In the
past, many highway agencies maintained bridge paint systems
by overcoating. Typically, existing lead-based alkyd systems
were overcoated with similar or identical paints that provided
years of acceptable service. Maintained alkyd paint systems
have lasted well over 15 years. Often, oil-alkyd painted
bridges were overcoated until they had paint builds as high
as 0.75 to 1.0 mm (30-40 mils).
Overcoating has some potential advantages compared
to full removal with containment. It minimizes the disturbance
of
the existing paint, which in turn, limits the generation
of (possibly hazardous) wastes and minimizes precautions
necessary for preventing waste discharge, worker exposure
to lead, and efforts required to dispose of generated wastes.
Repair and overcoating operations do not require expensive
containment enclosures. Costs for repair and overcoating
are low (typically one-fifth to one-third that for full
removal with containment) and overcoating. Overcoating may
extend
the service life of the in-place coating system by 15 years
or more. Low initial painting costs coupled with potentially
significant extension of service life are very attractive
to State highway agencies strapped with limited painting
budgets and large backlogs of bridges needing to be repainted.
However, there exists significant danger of short-term
failure of the complete system if the original coating system
does
not have sufficient adhesion/mechanical properties or if
a non-compatible repair system is chosen.
The uncertainty with this maintenance option
is that long-term experience with many of the newer paint
systems as repair
material is lacking.
Factors Affecting Performance
and Durability of Overcoating Systems
In the following section, we will discuss the factors that
affect the success or failure of an overcoating job. The
relative importance of the discussed factors is not clearly
understood. Furthermore, in many cases, the current test
methods available do not clearly characterize various factors.
Adhesion/cohesion
The use of adhesion tests to rate the ability of an existing
paint system to be overcoated and to estimate the durability/compatibility
of the repair merit review. Current tests are time-consuming
to perform, difficult to repeat, and the meaning of the results
are subject to individual interpretation. For example, large
differences have been observed between the test results when
different methods are used. Existing paints may be brittle
and lack cohesion and intercoat adhesive strength (i.e.,
typically occurring between an alkyd primer and existing
intermediate or topcoats of alkyd paints containing aluminum
pigments). Knifing adhesion tests such as ASTM D3359-90 (Measuring
Adhesion by Tape Test) typically fracture old alkyd paints
causing intercoat (cohesive) failure. The pull-off test,
ASTM D-4541 (Standard test Method for Pull-Off Strength of
Coatings Using Portable Adhesion Testers), measures adhesion
between existing coats or between the primer and the substrate,
or the cohesive strength of a specific coat depending upon
which component fails first. Brittle alkyds may provide only
a 1B or 0B rating using the knifing adhesion test. If used
on existing paint that contains several overcoated layers,
the knifing adhesion test may provide similar values. However,
a pull-off adhesion test on the same system may provide readings
in excess of 2067 kPa (300 lb/in2).
Surface Contamination
The presence of soluble salts on existing paint surfaces
and in corrosion products pose a distinct threat to repair
coating's durability. Soluble salts must be removed in order
to provide extended paint durability even though their effects
are difficult to assess. They are difficult to detect in
the field and their concentration may vary depending on level
of deicing salt application, bridge design, and structural
location. Currently, the primary tests for chloride surface
contamination are wet chemical tests that are slow to perform
and are questionable as to accuracy and precision.
THERMAL SPRAY TECHNOLOGY
Introduction
As can be seen, the dilemmas facing facility owners and
fabrication shops are complex. One potential answer may be
thermal spray (TS) technology. TS is a coating process that
has been proven in FHWA, National Association of Corrosion
Engineers (NACE), U.S. Navy (USN), and other sponsored programs
to provide superior long term corrosion protection in a variety
of environments. As part of task D, Thermal Spray was evaluated
by both ongoing field tests and also as part of task E, Accelerated
Testing.
Thermal Spray Coatings
The term "thermal spray" is commonly used
to describe a family of coating technologies associated with
the application
of thick coatings onto components in order to reduce or eliminate
the debilitating effects of wear and corrosion. In general,
material in either powder, wire, or rod form is introduced
into a region of high enthalpy (FIGURE 53). Within this region, the material is brought
to either a plastic or molten state. As part of this process,
an expanding gas is used to accelerate the droplets onto
the component surface, forming a coating. As the droplets
impact onto the surface, they form lenticular splats (FIGURE
54) and form a coating layer. The coating is built up
to the desired thickness by multiple passes over the component
surface. As with any coating, the surface must be properly
prepared in order to insure coating adherence. In the case
of thermal spray coatings applied in open atmosphere, the
proper surface preparation is the roughening of the surface
either by grit blasting or chemical etching in order to generate
the asperities necessary for the mechanical bonding of the
coating to the component surface. In addition, thermal spraying
is a line-of-sight process with optimum coatings being obtained
when the angle of impingement of the molten droplets is perpendicular
to the surface being coated. Depending on the process being
used, excellent coatings can be obtained when the droplets
impinge at angles between 90 and 45o relative
to the surface being coated. Below 45o, evaluation
of the coating microstructure determines the applicability
of a given coating and process to the problem being addressed.
Flame spraying, the oldest form of thermal
spraying, has been used since the late 1800's. In its simplest
form, it
consists of a nozzle assembly wherein a fuel (acetylene,
hydrogen, propane, etc.) is mixed either with oxygen or
air and undergoes combustion external to the nozzle. The
flame
front is stabilized by matching the flame propagation speed
to the average unburned gas velocity. Heat transferred
to the nozzle also aids in anchoring the flame front. For
materials
in powder form (FIGURE 55a),
the powder is injected into the flame in a manner designed
to optimize the heating of the powder. For materials in
wire or rod form (FIGURE 55b),
the flame is concentric to the wire or rod in order to
maximize
the uniform heating of the wire rod. A coaxial sheath of
compressed gasses around the flame acts to atomize the
molten particles and accelerate them toward the substrate.
The main
advantages of these processes are low capital investment
costs and ease of operation. Because of the relatively
small size of the equipment and the ease of operation,
the process
is field-portable, and there is little restriction to the
size and complexity of components that can be coated. However,
particle velocities are relatively low and, therefore,
the coating porosity can be as high as 20 percent.
The electric arc process is illustrated in FIGURE 55c and involves the continuous feeding of
two wires into a device such that the wires converge at
a point in space. The wires are held at different electrical
potentials, such that an electric arc is generated between
them. These wires are, in essence, consumable electrodes
and are continuously melted. A jet of gas, usually compressed
air, is used to atomize the molten material and accelerate
the resultant droplets onto the component surface. It is
not necessary for both wires to have the same chemical
composition
resulting in alloyed coating, however, adjustments to the
feeding mechanism are necessary to compensate for these
differences. Very high deposition rates are achievable by
this process,
but the atomization process generates more fumes than other
thermal spray processes. In addition, this process frequently
results in porosity levels of 25 percent by volume. Recent
advances in the design of electric arc equipment have incorporated
the use of Coanda accelerators. The Coanda accelerators
use the geometry of the flow chamber to enhance the syphoning
action of the atomizing gas, thereby increasing the volumetric
flow through the same cross-sectional flow area. This results
in higher particle velocities and coatings of higher density.
Thermal Spray Materials
As with any coating process, the proper choice of coating
materials may determine whether or not the desired goals
are achieved. Historically, thermal spray coating systems
have used pure zinc, pure aluminum, and zinc/aluminum alloys
for applications involving corrosion protection of steel
structures. Being metallic, the coatings offer additional
protection in high-wear areas.
Zinc
Zinc provides long-term
corrosion protection to steel through galvanic action at
the zinc/steel surface as well as by its ability to protect
itself with its own corrosion byproducts. Zinc has a lower
oxidation potential than iron and will therefore preferentially
corrode, preventing the steel from rusting. In addition,
this properly provides cathodic protection to any small
discontinuities or damage done to the zinc coating that may
expose the steel
components. Being a reactive metal, zinc readily forms
a protective corrosion-product film. When exposed to air,
a
very thin layer of zinc oxide forms. When exposed to moisture
in the atmosphere, the zinc oxide reacts with the moisture
to form zinc hydroxide. During the drying process, the
zinc hydroxide reacts with carbon dioxide to form an insoluble
zinc carbonate layer on the surface, providing excellent
protection to the underlying zinc.
Aluminum
Aluminum provides
a barrier to the corrosion of steel by the formation of
an inert aluminum oxide layer on the surface of the coating.
When damaged, this coating is self-healing. Like zinc,
aluminum
has a lower oxidation potential than steel with respect
to iron and therefore provides galvanic protection to the
steel
substrate. Unlike zinc, pure aluminum has not been extensively
thermally sprayed onto steel bridges and structures although
extensive use by the U.S. Navy indicates aluminum is more
corrosion-resistant than zinc in marine environments. FIGURE 56 describes the estimated service life for
aluminum thermal spray coatings.
Zinc/Aluminum
With the
proven protection associated with zinc and the implied
improved performance of aluminum in salt environments, alloys
of zinc
and aluminum have been developed. Initial work in Japan
indicates that such alloys, particularly 85 percent zinc/15
percent
aluminum alloy, have advantages over all-zinc coatings.
This alloy has successfully been applied to steel bridges
within
the United States. FIGURE 57 describes
the estimated service life for Zn and 85/15 Zn/Al thermal
spray coatings.
Recently, the application of polymers such
as ethylene acrylic acid EAA) by thermal spray technology[A
has been
suggested for corrosion prevention on bridges. Unfortunately,
unlike the metallic coatings described above, success has
been limited for two reasons. First, control of the substrate
temperature is critical for adhesion and to date no definitive
method for insuring the proper temperature has been proposed.
Second, like most paint systems, these coatings are solely
barrier coatings without any additional protective mechanisms
such as those exhibited by the metallic coatings.
Sealers
Although zinc,
aluminum, and their alloys provide galvanic protection,
additional protection can be obtained by sealing the porosity
with acrylic
urethanes, polyester urethanes, vinyls, phenolics, epoxy
sealers, or thermally sprayed polymers.
INFRASTRUCTURAL APPLICATIONS OF
THERMAL SPRAY TECHNOLOGY
There is a history of corrosion protection
by aluminum and zinc thermal spray coatings for structural
steel work:
buildings, bridges, towers, radio and TV antenna masts, steel
gantry structures, high-power search radar aerials, overhead
walkways, railroad overhead line support columns, electrification
masts, tower cranes, traffic-island posts, and street and
bridge railings.
The interior of steel-hopper rail cars for
hauling coal have been sprayed with aluminum for sulfuric
acid corrosion
protection and with aluminum composite for both corrosion
and abrasion protection. Steel car exteriors have been
sprayed with zinc for atmospheric- corrosion protection.
Zinc thermal spray coatings used to protect
potable water pipelines and storage tanks as specified in
ANSI/AWWA D-102-78,
American Water Works Association Standard for Painting
Water-Storage Tanks. (5). Aluminum
and zinc thermal spray coatings are used on sluice gates
in irrigation
systems and canal lock gates in shipping canals. These
coated components have required virtually no maintenance
for decades.
In marine applications, ship structural areas
and components are preserved with aluminum and zinc thermal
spray coatings.
The U.S. Navy routinely uses aluminum thermal spray coatings
in new ship construction and in the overhaul, repair, and
maintenance of ship structures and for a wide range of
shipboard components, especially those in topside and wet
spaces. The
British, Australian, and New Zealand Navies use a duplex
zinc (base) and aluminum (top) thermal spray coatings system.
Zinc thermal spray coatings complement hot-dip
galvanizing when fabrications are excessively large or otherwise
cannot
be hot-dip galvanized. Zinc thermal spray coatings are
used for repairing galvanized coating damaged during the
fabrication
process (e.g., welding, cutting and joining areas) and
for maintenance recoating. Here, zinc spray is particularly
advantageous
because it ensures the uniformity and reproducibility of
the galvanized coating thickness.
THERMAL SPRAYING OF STEEL BRIDGES:
A SHORT HISTORY
Thermal spraying of bridges is not a new idea.
Since the 1930s, zinc spraying has been extensively utilized
in Europe,
and in many countries zinc spraying is specified as the only
corrosion protection system for new bridge construction.
To date, several hundred bridges have been thermally sprayed
to provide long-term corrosion protection. TABLE
8 lists several significant bridges, the year of metallization,
and the last date when the coatings were inspected and found
to be intact. In the United States, the oldest known bridge
to be thermally sprayed is the Kaw River Bridge in Kansas
City, Missouri. In 1936, the bridge was sprayed with between
0.25 to 0.30 mm (10 to 12 mils of pure zinc. The bridge with
the most notoriety is the Ridge Avenue bridge in Philadelphia,
the underside of which was sprayed with 00.25 mm (10 mils)
of pure zinc in 1937. The Ridge Avenue Bridge is a railroad
overpass and therefore has been subjected to severe corrosion
environments due both to its exposure to the effluence of
generations of steam and diesel locomotives as well as to
road salts. Fifty-five years later, the zinc coating is still
effectively protecting the bridge. The Forth Road Bridge
in the United Kingdom is noteworthy in that it is the largest
structure in the world to have its entire outer surface thermally
sprayed. In 1961, over 18,140 Mg (20,000 tons) of structural
steel was metallized with a 0.125-mm (5-mil) coating of zinc,
and upon assembly, supplemented with three coats of paint.
The Pierre-LaPorte Bridge spanning the St. Lawrence River
near Quebec City, Canada, is the largest onsite metallized
structure in the world. The original construction of the
bridge was completed in 1970. The corrosion protection system
consisted of painting with a lead silico-chromate, oil, and
alkyd system. Maintenance and repainting of the bridge began
in 1975. In 1977, the Province of Quebec decided to zinc
spray the entire understructure of the bridge [160,000 m2 (1.8
million ft2)]. An economic study concluded that
while the paint system would have provided an 8-year lifespan
requiring frequent touchups, the alternate zinc spray system
would give a probable life of 20-25 years, requiring no major
touchups. As recently as 1991, the coating was providing
superior corrosion resistance on the bridge.
Since 1985, Ohio has coated four bridges with
the thermal spray process. The first bridge selected consisted
of five
lines of 21 WF 62s, 32 m (102 ft) long, over a stream.
The surface area of steel to be coated was 309 m2 (3323
ft). The bridge was to have a new concrete bridge deck
constructed on the existing beams. The metallizing of the
beams was to
be done after the deck was replaced. The beams were required
to be sandblasted to a white metal surface (SP 5) and the
85 percent zinc/15 percent aluminum metallizing was to
be applied [0.15-2.0 mm (6-8 mils) thick] followed by 0.0375
mm (1.5 mils) of epoxy, and 0.05 mm (2 mils) of a urethane
topcoat. The metallizing had to be applied within 4 h of
sandblasting to avoid flash rusting of the freshly blasted
surfaces. The bid price on this first bridge for surface
preparation, metallizing, epoxy intermediate coat, and
urethane
topcoat was $30,000 or $95.35/m2 ($9.03/ft2).
This cost did not include containment of blasting debris
since this work was done before containment was a requirement
in Ohio. The actual time required for metallizing the beams
was 8 working days. To date, this bridge remains in excellent
condition and exhibits no rusting of the steel beams.
A second bridge was selected to be metallized
in 1985. This structure was considerably more complicated
to metallize
because: (1) it is a bridge over an Interstate highway
and (2) the beams are composed of riveted plates and angles.
The bridge consisted of four lines of riveted beams each
being 111 m (364 ft) long. Again, this was to have a concrete
deck replacement with the metallizing to be done after
the
deck had been replaced. Containment of the blasting debris
was not a requirement on this project. The bid price for
cleaning and painting this bridge was $217,000 or $87.44/m2 ($8.28/ft2).
In an effort to better evaluate the effectiveness
of the metallizing, it was decided not to topcoat the beams
with
the epoxy and urethane, but rather only seal the metallized
surface with a clear sealer. This bridge, being situated
over an Interstate highway, is subject to much salt spray
and to date, the beams remain in excellent condition.
At the time of metallizing these two bridges,
standard paint systems were costing $36.96/m2 ($3.50/ft2)
and it seemed unlikely at that time that metallizing would
ever become cost-competitive on a first-cost basis. However,
recent cost increases in standard systems are narrowing
the margin between metallizing and conventional painting.
A third bridge, which carries a local road
over a four-lane divided highway, was selected for metallizing
in 1988. This
bridge was also to receive a reinforced concrete deck replacement
with the metallizing of the steel beams to be completed
after the deck was replaced. The bid price for metallizing
the
four lines of 30 WF steel beams, 117.2 m (384.5 ft) long,
was $237,000 or $91.66/m2 ($8.68/ft2).
This price included surface preparation, but not full containment
(which was still not required at that time).
TEST PANEL PREPARATION
As part of this task, 210 test panels were
prepared for field and accelerated testing. Initially, it
was proposed
that in addition to thermally sprayed test panels, paint
systems under testing at the Mathis River Bridge in New Jersey
would be included in testing. This was proposed to provide
a baseline comparison between the paint coating systems and
the thermally sprayed coating systems. Unfortunately, many
of the paint systems tested at Mathis River are no longer
VOC-compliant and, therefore, it was decided that paint systems
meeting existing and proposed VOC limits would be used in
this study. TABLE 9 details the
thermally sprayed samples prepared and TABLES
10a and 10b detail the
paint systems samples prepared.
All coatings were applied
onto standard 102-mm by 152-mm by 4.76-mm (4-in by 6-in
by 3/16-in) A-36 steel panels purchased
from KTA Tator. All samples were degreased in a vapor degreaser/ultrasonic
bath containing 1,1,1 trichloromethane. They were then
grit-blasted using 36 grit aluminum oxide at 511 kPa (80
lb/in2)
to a SP-5 white metal finish with at least a 0.051-mm (0.002-in)
profile as shown in FIGURE 58.
Using ANSI/AWS C2.18-93, Guide
for the Protection of Steel with Thermally Sprayed Coatings
of Aluminum, Zinc, and
their Alloys and Composites, the zinc (FIGURE
59), aluminum (FIGURE 60), and the 85/15 zinc/aluminum (FIGURE 61) samples were prepared. The EAA coating (FIGURE
62) was applied using manufacturer supplied parameters.
In addition, the paint systems were applied following manufacturers'
guidelines.
A typical set of samples for field testing
was composed of one each of paint systems 1 through 5 and
one each of
the thermally sprayed zinc, aluminum, and 85/15 zinc/aluminum.
In addition, one each of the zinc, aluminum, and 85/15
zinc/aluminum test samples had one-half of each panel sealed
with the cycloaliphatic/aliphatic
amine epoxy. Finally, one each of the 85/15 zinc/aluminum
test samples had one-half of each panel thermally sprayed
with the EAA copolymer. One set of test panels has been
deployed to the I-55 bridge with three remaining sets to
be deployed
in early 1994. Follow-on evaluation of these panels will
occur as part of an ongoing 4-year program on overcoating.
The remaining samples were supplied for evaluation under
task E.
CONCLUSIONS AND RECOMMENDATIONS
The most critical factor in the implementation
of advanced coatings is the relative lack of history as it
pertains to
the durability of these coatings with the exception being
thermally sprayed coatings. One difficulty with the implementation
of thermally sprayed coatings is the relative age of the
infrastructure and limited access to portions of the bridges
where advanced corrosion has occurred.
Since many bridges have suffered loss of cross
section, the application of thermally sprayed coatings retard
corrosion,
but cannot replace lost steel. In addition, since the regions
of most serious corrosion are in the area of expansion
joints and bearing areas, thermal spray technology's need
for line
of sight can limit the quality of coating applied. This
should not preclude use of this technology when the above-mentioned
limitations do not contraindicate its use.
Two immediate areas where thermal spray technology
can be implemented are in structural replacement and fabrication.
In these applications, if the steel at an expansion joint
was sprayed with 85/15 Zn/Al it would provide long-term
corrosion
protection and minimize loss of cross section.
The use of paint for overcoating requires an
understanding of the durability of these paint systems and
a fuller understanding
of the equivalent uniform annual cost as demonstrated in
task A. A partial list of companies and State DOT's addressing
the technical and economic aspects of overcoating include:
Ocean City Research, Inc., BIRL, the industrial research
laboratory at Northwestern University, IDOT, KTC, NCDOT,
LADOT, VIDOT, WI-DOT, etc. |
