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 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. To handle the large volume of vehicles, Caltrans began a construction project to build a new parallel trestle bridge on the north side of the existing bridge that will serve the westbound traffic with three travel lanes and shoulders on both sides. The east approach was also 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. To understand why they went with Polyurea, lets go over some main points so you understand just what this material is and why it works so well for this type of project.
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).
WHAT IS POLYUREA?
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
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 had to supply approximately 35,000 sq ft of coated-concrete parts to the San Mateo bridge construction area. This called for the efficient, timely accomplishment of a four-step coating process on 90-ft-long girders, 120-ft-long pilings, and 41-sq-ft deck plates. The weather threshold 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 considerably interfered in the effective application of many other coatings. However, the contractors developed formulas of the polyurea coatings specifically for various climate conditions throughout the year.
MORE THAN MINIMUM
Physical properties of the polyurea coating on the San Mateo Bridge meet and, in many cases, exceed Caltrans' specifications. For example, the coating adhesion is 800 lb per sq in. (psi), well beyond the minimum specification of 250 psi. Tensile strength is 3,300 psi, and elongation is 400%, also over the minimum specifications.
The goal in using a polyurea coating was to yield a 20-year service life for the new concrete additions to the bridge. No short-term maintenance should be required when using a polyurea.
An additional coat of Polyurea also forms the topcoat on the San Mateo Bridge to produce a color-stable topcoat with a linear thermal expansion (the rate at which a material expands when heated and contracts when cooled) similar to the aromatic polyurea coating is applied over. As a result, the two polyurea coatings on the San Mateo Bridge expands and contracts along with each other and with the bridge's concrete substrate.
WORK THE SIZE OF A FOOTBALL FIELD
A football-field-sized staging area was created at the Petaluma, Calif., site of the project. The logistics of supplying a steady flow of concrete parts to the construction site call for coatings that cure swiftly. Following are the general steps to coating each concrete component for the San Mateo Bridge. Operators complete these steps in a 24-hour day.
• Sandblast
the precast concrete parts to give a smooth surface for the coatings. The sandblasting reveals bug holes, holidays, and pinholes in the concrete, which must be sealed;
• Hand-apply
a coating of PoxyPrime BF, 100% solids, no-VOC, silica-free epoxy mortar/grout that is specifically formulated to fill bug holes, holidays, pinholes and cracks in concrete;
• Once the epoxy mortar/grout is applied, spray-apply the PoxyPrime epoxy primer. This epoxy gives an excellent adhesion surface on dry or moist concrete surfaces for a top coating. It can be applied over a wide temperature range from 20F to 170F. It cures rapidly;
• Spray a 50-mil intermediate coating of ElastoGard ARC aromatic polyurea spray over
the primer. This coating gives 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 give color stability and excellent UV and weathering
characteristics. The fast-curing, moisture-resistant coating provides 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.
If you would like to learn more about Polyurea or would like to learn how to become an applicator, we suggest getting in contact with ArmorThane. With over 30 years in the business, they are the go to company when it comes to Polyurea.