The Port Lavaca Causeway, stretching between Port Lavaca and Point Comfort, Texas, is the longest bridge in the massive Lone Star State. The 2.25-mile (3.62 km) span allows passage over Lavaca Bay, while also allowing ships to access nearby Texas ports. The causeway was built in 1961, but by 2016 it required some major rehabilitation work, prompting the Texas Department of Transportation (TxDOT) to budget more than $16 million for the project.

The causeway consists of 194 simple prestressed beam bridge spans and one three-span continuous steel girder unit. The substructure is reinforced concrete supported on pile-cap footings bearing on driven precast/prestressed concrete piling. The rehabilitation, which has continued into 2019, covers five items:

1. Address reinforcing steel corrosion/resultant concrete distress (cracking/spalling) by completing concrete structure repair and wrapping columns with carbon fiber reinforced polymer.

2. Address cracking along main reinforcing steel in concrete pile caps by supplementing footing with steel-reinforced concrete encasements.

3. Add zinc spray cathodic protection (CP) on bent caps with elevation nearest water to curtail future corrosion-related distress.

4. Clean and metallize all existing steel rocker bearings to protect.

5. Clean and apply thermal spray aluminum (TSA) coating to three-span continuous steel unit.

“There was a lot of metallizing on that job,” said Nik Parianos, superintendent of painting and metallizing for M&J. “It’s a good product.”

A report¹  written for the Federal Highway Administration provides an explanation of the process:

Metallizing is a type of thermal spray coating (TSC) that produces a durable metal coating. The TSC metal typically applied to steel structures such as highway bridges is either pure zinc or a zinc/aluminum alloy. A feed wire of the TSC metal is heated to a molten state by flame or electric arc and propelled by air spray onto a surface. The metal solidifies upon contact with the substrate to form a durable metallic coating. Prior to TSC application on bridges, the surfaces are cleaned thoroughly using abrasive blasting.

The Hard Part First

When Parianos and his crew of eight to 12 people arrived on the jobsite in March 2017, they tackled the most difficult part first: CP on the 16,604 square feet (1,542.6 m²) of concrete pier caps. However, close to the abutments, each end of the bridge had about five caps that were buried in the dirt. “I had to dig that out to about 2 ft [0.6 m] below the caps so I was able to pressure wash it,” recalled Parianos. “Just a regular shovel and a lot of guys. Working with the tides, it took me a couple days just to dig. That was one of my biggest problems.”

The crew then began surface preparation. The first step was pressure washing using Honda 5,000 psi (34.5 MPa) machines. Then came the sand blasting: Size 20/40 silica sand from Sand Express was put into a 6-ton Marco pot with a dryer system and an accompanying Sullair 1,600-cfm (2,718.4 m³/min.) compressor.

While Texas requires only a 150 psi (1.0 MPa) result from a dolly pull adhesion test on concrete, pressure washing prior to blasting leads to superior adhesion. “I didn’t want to pressure wash,” said Parianos. “But it gave a better quality. We got a dolly pull of 450 psi [3.1 MPa], which is three times higher than what the state asked for on those concrete pier caps.”

CP on concrete required installing a steel tie-bar to the concrete reinforcing steel (rebar) and connecting it to the thermal spray zinc (TSZ) applied on the concrete surface with a bolt and washer. The M&J crew then drilled through the concrete caps and into the rebar to connect the rebar to the TSZ. As Parianos explained, “I fill that [drilled hole] in with epoxy. Then I blast it, I put my metal plate — it’s like a washer plate — on it, and I bolt it down. When I metallize the cap, that’s what creates your positive anode. It goes around the whole cap to keep it from rusting — keeps the rebar from rusting and keeps the concrete from deteriorating. After you metallize it, you have to put a water-based zinc spray on it, which dries within minutes, and that seals everything up.”

For the metallizing of the concrete caps, M&J used a Thermion Precision Arc 4.8 with a 99 percent zinc wire feed. A generator from United Rentals supplied the necessary 200 to 250 kW for three Thermion machines. Although the American Welding Society (AWS) joint standard requires an average thickness of 10 mils (254.0 microns) of metallizing material, most contractors and their clients will accept a range of 8 to 12 mils (203.2–304.8 microns). Finally, Sherwin-Williams’ Zinc Clad XI was sprayed on with a Graco Xtreme at an average thickness of 1.5 to 2.0 mils (38.1–50.8 microns) to seal the thermal spray coatings on the reinforced concrete pier caps.

Rockers and Shoes

According to Parianos, the next part of the job called for M&J to “clean and metallize all existing steel rocker bearings,” but again, it wasn’t quite so simple. “There were about 3,000 sets of shoes and rockers combined,” he said. “They’re not just rocker bearings. They’re also just regular shoes that don’t move.”

Proper surface preparation was essential for this work, and Parianos has high standards. Though most specs call for a minimum surface profile of 2.5 mils (63.5 microns), “I won’t let my guys metallize any less than 3 [mils, or 76.2 microns],” said Parianos.

The joint standard NACE No. 12/AWS C 2.23M/Society for Protective Coatings (SSPC) Coating Systems (CS) 23.00:Specification for the Application of Thermal Spray Coatings (Metalizing) of Aluminum, Zinc, and Their Alloys and Composites for the Corrosion Protection of Steel contains requirements for the application of zinc and  aluminum alloys to steel substrates using thermal spray. To start, silica sand was again used to abrasive blast the rocker bearings and the shoes to an anchor profile of at least 3 mils. Parianos recalled, “We only needed 500 [psi, or 3.4 MPa, from the dolly pull] on the rocker bearings, and we were getting well over 1,000 [psi, or 6.9 MPa].”

Per Parianos, some challenges included putting a containment on each pier, working from pick boards on barges below the bridge, and then moving that equipment as they went. The M&J crew ran cables and connected safety crosslines to personal safety belts for their fall protection. But as soon as the waves would get high, the crew would have to shut down operations for the day. “Water and power don’t mix,” commented Parianos.

It was also during this time that Hurricane Harvey made landfall in Texas, but as Parianos recalled, “the only thing I had to drop were my 30 pick boards.”

Spanning the Gap

The final part of the project, said Parianos, was to “clean and apply TSC aluminum to the three-span continuous steel unit.” This was on the highest part of the bridge, which he estimated reaches 70 or 80 feet (21.3 or 24.4 m) in the air. M&J’s crew members installed Safespan to have a hard deck under the working area, as well as impermeable tarps along the vertical sides.

Other personal protective equipment (PPE) included Nova last hoods by RPB Safety during the blasting and metallizing, respirators from 3M, fall protection lanyards, gloves, safety glasses, and work boots. TSC operations create metal dust, which can be harmful if inhaled. As Parianos pointed out, “With the metallizing, you can’t wear a regular half-face respirator or a full-face. Everybody needs to be in supplied air. The metallizers are in it, my vacuumers, my blasters.”

A #25 steel grit blasting abrasive helped achieve the proper profile on the steel, meeting NACE No. 1/SSPC Surface Preparation (SP) 5: White Metal Blast Cleaning. Because the steel had existing lead paint underneath an original aluminum coating, M&J had to contain it using a reclaiming unit from Atlantic Design. “It’s got the vacuum on it, separates all my waste, then it takes my grit back into the pot for my guys to continue blasting,” Parianos said.

Pure aluminum (99 percent) feed wire was used on the steel spans with the Thermion machines. Lastly, Sherwin-Williams’ Macropoxy 920 Pre-Prime was applied using Graco Xtreme sprayers to an average wet film thickness of 1.5 to 2.0 mils (38.1 to 50.8 microns). This penetrating sealer reaches all the way to the substrate, filling the porosity of the TSC.

Long-Lasting for the Long Haul

When it comes to preserving bridge structures, metallizing the concrete and steel surfaces offers long-term protection against corrosion. Upfront costs may be higher than with other coating methods, but the cost across the entire life cycle should be far less. After this project, TxDOT should have a long-lasting coating system on the Port Lavaca Causeway and far less maintenance over the coming years.

Reference

1 “Cost Effective Alternative Methods for Steel Bridge Paint System Maintenance, Report IX: Field Metallizing Highway Bridges,” Corrpro Companies Inc., 2001.

Editor’s note: Reprinted with permission from Infrastructure Insights (August 2019).