Cal-Poly coatings

April 2, 2018
Polyurea elastomeric protects precast concrete on the San Mateo Bridge in San Francisco

About the author: Limas is Mactec Constructors’ site superintendent for the San Mateo Bridge widening project; Harris is the coatings superintendent for Mactec Constructors; and Ishmael is director of the Polymers Division of Elastomer Specialties Inc.

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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.

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