Scheduled for completion late in 2002, a project to widen the San Mateo Bridge over California’s San Francisco Bay could guide future transformations of 20th century bridge spans to handle the traffic volumes of the 21st century. And it could demonstrate how advanced elastomeric coating technologies can be employed to waterproof precast concrete and protect it against corrosion.
The longest bridge in the world when it was completed in
1929, the original San Mateo-Hayward Bridge was a two-lane, low-level structure
of just 35 ft in elevation. It connected Highway Rte. 92 between San Mateo and
Alameda counties. A 1967 renovation of the bridge by the California Department
of Transportation (Caltrans) added another lane in either direction.
In the ’80s and ’90s, annual traffic volume on
the San Mateo Bridge grew 70% to an average of 72,000 vehicles, according to
Caltrans. By 2010, an estimated 95,000 autos will cross the bridge in both
directions.
To handle this volume, Caltrans began a construction project
in 2000 to build a new parallel trestle bridge on the north side of the
existing bridge that will serve the west-bound traffic with three travel lanes
and shoulders on both sides. The east approach also will be widened to three
travel lanes in each direction, thus creating six continuous lanes in both
directions.
To protect the precast concrete components in this expansion
project, Caltrans issued a call for bids to coat every concrete girder, deck
plate, end cap and piling with a minimum 63-mil layer of coatings. Polyurea was
specified as the coating of choice. More than 3 million sq ft of concrete would
be coated. Caltrans required that just one contractor supply the multiple
coating materials and one contractor apply the coatings.
For protection purposes
Polyurea coatings are ideal for protecting concrete and
steel against the effects of moisture, abrasion and corrosion. In general, they
are a fast-curing, elastomeric polymer coating that is waterproof, resistant to
many chemicals and resistant to abrasion.
In addition to these overall benefits, polyurea coatings
provide the following features:
• Spray
applied for high productivity with set times of just seconds to approximately
30 minutes;
• Low
or no volatile organic compounds (VOCs) emitted by the coating;
• Seamless
and resilient;
• Variable
mil thickness in one application;
• Excellent
adhesion;
• Resistant
to solvents and many other chemicals;
• Tolerant
of varying weather conditions during application (polyurea formulations can
cure at temperatures ranging from -40?F to 350?F and in high
humidity);
• Color-stable
formulations resist ultraviolet (UV) degradation; and
• Broad
range of elongation (from 5% to 400% and more).
Polyurea coatings are derived from the reaction of an
isocyanate component and a resin blend component. They incorporate raw
materials similar to those used to manufacture polyurethane coatings that
protect furnishings and automotive finishes, though polyurea coatings are
formulated for industrial-grade conditions and performance.
Covering 35,000 sq ft a week
Caltrans hired Balfour Beatty Construction Inc., San Mateo,
Calif., to erect the new bridge additions. Pomeroy Corp., Petaluma, Calif., won
the contract to produce the pre-cast concrete structures and provide a
workspace for applying the coatings. To procure and apply the coatings, Pomeroy
selected Harding Lawson Associates, now operating under the name Mactec
Constructors, Petaluma, Calif., managed by Frank Limas and Dave Harris.
Mactec turned to the Polymers Division of Elastomers
Specialties Inc., Broken Arrow, Okla., for the elastomeric coating products.
Elastomer Specialties supplies an epoxy mortar/grout, an epoxy primer, an
aromatic polyurea coating and an aliphatic polyurea coating.
The ability to spray-apply polyurea coatings contributed to
production efficiencies in managing the monumental logistics of coating some
3.2 million sq ft of precast concrete throughout the construction project.
Each week, the contractors must supply approximately 35,000
sq ft of coated-concrete parts to the San Mateo bridge construction site. This
calls for the efficient, timely execution of a four-step coating process on
90-ft-long girders, 120-ft-long pilings and 41-sq-ft deck plates.
The weather tolerance of the polyurea coatings was particularly
important in this year-round project. The San Francisco Bay area’s widely
varying temperature and humidity conditions would have greatly interfered in
the effective application of many other coatings. However, the contractors
developed formulas of the polyurea coatings specifically for the variety of
climate conditions throughout the year.
More than minimum
Physical properties of the elastomeric polyurea coating on
the San Mateo Bridge meet and, in many cases, exceed Caltrans’
specifications. For example, adhesion of the coating is 800 lb per sq in.
(psi), well above the minimum specification of 250 psi. Tensile strength is
3,300 psi and elongation is 400%, also above the minimum specifications.
According to Addis Ambaye, transportation engineer for Caltrans
on the San Mateo Bridge project, the goal of utilizing a polyurea coating is to
yield a 20-year service life for the new concrete additions to the bridge. He
said no short-term maintenance should be required.
Bayer Corp., which supplies Elastomer Specialties with the
isocyanates used to formulate its elastomeric polyurea coatings, tested samples
of the coating for weatherability. The elastomeric polyurea coating exhibited
no cracking or bubbling and only a slight loss of gloss.
Elastomer Specialties also brought its expertise in polyurea
coating chemistry to bear on a challenge posed by this application. Aromatic
polyureas provide a flexible coating, but they are not light-stable. Aliphatic
polyureas are light-stable, but they are brittle.
Elastomer Specialties reformulated its aliphatic polyurea,
which forms the top coat on the San Mateo Bridge to produce a color-stable top
coat with a coefficient of linear thermal expansion (the rate at which a
material expands when heated and contracts when cooled) similar to the aromatic
polyurea coating it is applied over. As a result, the two polyurea coatings on
the San Mateo Bridge will expand and contract along with each other and with
the concrete substrate of the bridge. Polyurethane technology provided by Bayer
Corp. aided Elastomer Specialties in achieving this important improvement.
Work the size of a football field
According to Limas, superintendent at the Petaluma
production site for Mactec Constructors, a football-field sized staging area
was created at the Petaluma, Calif., site of The Pomeroy Corp.
The logistics of supplying a steady flow of concrete parts
to the construction site call for coatings that cure quickly. Following are the
general steps to coating each concrete component for the San Mateo Bridge.
Workers complete these steps in a 24-hour day.
• Sandblast
the precast concrete parts to provide a smooth surface for the coatings. The
sandblasting reveals bug holes, holidays and pin holes in the concrete, which
must be sealed;
• Hand-apply
a coating of PoxyPrime BF, a 100% solids, no-VOC, silica-free epoxy
mortar/grout that is specifically formulated to fill bug holes, holidays, pin
holes and cracks in concrete;
• Once
the epoxy mortar/grout is applied, spray-apply the PoxyPrime epoxy primer. This
epoxy provides an excellent adhesion surface on dry or moist concrete surfaces
for a top coating. It can be applied over a wide temperature range from
20?F to 170?F. It cures rapidly;
• Spray
a 50-mil intermediate coating of ElastoGard ARC aromatic polyurea spray over
the primer. This coating provides excellent chemical resistance, thermal
stability and UV resistance for the concrete parts. It cures to a tack-free
surface in approximately 20 seconds;
• Apply
5-10 mil of PolySpar HP aliphatic polyaspartic coating, which is based on new
polyurea technology. It is a low film thickness, high opacity topcoat for
polyurea coatings that provides color stability and excellent UV and weathering
characteristics. The fast-curing, moisture-resistant coating produces a highly
abrasive-resistant, high-gloss, smooth finish. It is tack-free in 15 minutes;
and
• Load
coated concrete parts onto a barge for shipment to the San Mateo Bridge
construction site via the Petaluma River.
