EVERETT — Concrete is falling off in chunks, rebar is rusting and thousands of people drive over the westbound U.S. 2 trestle every day.

While the girders on the trestle’s underbelly have been slowly deteriorating for more than 20 years, state transportation officials say there’s no doubt the bridge is safe for drivers. Repairs are being made this summer and fall.

About 37,000 vehicles per day use the trestle in one direction or the other.

“There’s no problem with people driving on the structure,” said Chad Brown, project engineer for the trestle repair. “The structure is safe.”

Workers are restoring the exposed rebar, replacing the concrete on the girders and sealing them to keep them waterproof. The repairs are expected to preserve the girders on the now 43-year-old span until 2026 or longer. The $8 million project began earlier this month and is expected to be finished in the early fall.

The roadway on the trestle is built atop girders made of concrete and rebar, running lengthwise with the roadway, sitting atop crossbeams that in turn sit on pillars. The eastbound half of the trestle was built with timber in the 1930s and rebuilt in sections in the 1990s, according to the state. The westbound side was built in 1968.

By1987, crumbling began to show on the concrete beams on the underbelly of the westbound structure, and the bridge was declared “structurally deficient.”

The term is a federal designation meaning that a bridge has a part or parts that will eventually need to be repaired, not that it is unsafe, according to the state. Washington currently has 143 bridges in this category, including seven in Snohomish County.

The first rehabilitation on the westbound trestle was not done until 2007, when girders on the east end of the westbound structure were repaired.

“Projects are defined and then prioritized based on future available funds,” said Harvey Coffman, a bridge preservation engineer for the state, in an email.

“We begin our planning at a 20-year forecast. That would put these bridges right in that time frame. During that 20-year span we inspected the bridge at regular intervals to ensure that it was capable of operating safely.”

There’s no average amount of time for a bridge to be considered structurally deficient before it is repaired, Coffman said. It depends on funding and the severity of the problems with the bridge, he said.

Under the state’s routine inspection program, bridges are examined every two years. Some with more structural issues are put on a yearly schedule, and the trestle has been inspected every year since 2003, Coffman said.

The section currently being repaired was done four years after the other section because of funding issues and environmental regulations involved in working over Deadwater Slough, officials said.

The entire trestle received a seismic retrofit job in 1994, Coffman said.

The current repairs are being done in the same manner as in 2007. The bridge at the far west end of the trestle, over the Snohomish River, is newer and won’t be repaired in this cycle.

Crews build scaffolding to get close to the ceiling of the structure, about 20 feet above the ground. Then they tap the walls of the girders with a hammer, listening for hollow spots.

Any weak concrete is chipped out. In places where rebar is exposed, the metal is measured to determine if it still has most of its circumference. If so, the rust is buffed off and the rebar is coated with sealant.

So far, no cases have been found where the rebar is rusted beyond repair, engineers said.

Engineers are confident the moisture has not further penetrated the beams. So far, the farthest into any beam that weak concrete has been found is a couple of inches, Brown said.

A wooden mold is applied to the beam and concrete is then injected through holes to fill up the areas previously chipped or fallen out. After the concrete dries, sheets of a carbon-fiber mesh material are applied to the concrete to add tensile strength, Brown said. It helps keep the girders from being pulled lengthwise, he said.

Then a seal coating is sprayed on the girders to provide waterproofing and the girders are painted.

The technique is supposed to keep the girders intact for 15 years, but it could be much longer, Brown said.

“If they’re maintained, they can last forever,” he said.

Traffic on the westbound trestle will be shut down for up to 65 nights through early October for the concrete and carbon-fiber mesh work. These phases have to be done when there’s no traffic on the road so vibrations won’t prevent the concrete from setting properly or keep the mesh from bonding to the concrete, engineers said.

The trestle was closed for three nights on June 9, 14 and 16 while two sections were repaired. It’s possible the work will take less than the 65 nights, Brown said. The next closure is planned for July 12.

If the girders are not repaired, beams and the rebar would continue to deteriorate, and longer closures would be needed, engineers said.

In a worst-case scenario, entire girders would have to be replaced, Brown said — in which case the state would have to consider replacing the entire structure.

The trestle is nowhere near this point, officials said.

The trestle’s structural integrity “is well above the more serious and critical levels,” Coffman said.

Nancy Singer, a spokeswoman for the Federal Highway Administration, said federal engineers say the work being done on the trestle appears to be appropriate.

“State departments of transportation are the owners and operators of infrastructure and are in a position to select the best approach to addressing the needs of a bridge,” she said in an email.

“The U.S. Route 2 project that calls for replacing old cracking concrete, removing corrosion from the steel frame, and reinforcing the girders on the underside of the viaduct seems to be based on sound and established approaches to bridge repair and rehabilitation.”

Bill Sheets: 425-339-3439; sheets@heraldnet.com.