More damage found to concrete wall in Bay Bridge tunnel

Elementary school teacher Liz Gustin was recently hit by a piece of concrete from the Yerba Buena tunnel which fell near the passenger side of her car while trying to avoid it in San Francisco, California. She shows a popped back tire and damages to the passenger side on Friday, February 12, 2016. less Elementary school teacher Liz Gustin was recently hit by a piece of concrete from the Yerba Buena tunnel which fell near the passenger side of her car while trying to avoid it in San Francisco, California. She ... more Photo: Liz Hafalia, The Chronicle Photo: Liz Hafalia, The Chronicle Image 1 of / 7 Caption Close More damage found to concrete wall in Bay Bridge tunnel 1 / 7 Back to Gallery

Corrosion in the Bay Bridge’s Yerba Buena Island tunnel that caused a concrete chunk to break loose last month, narrowly missing a motorist, is more widespread than Caltrans first believed, The Chronicle has learned.

Caltrans has found 12 spots in the tunnel where concrete was in danger of breaking away from the wall since a Jan. 30 incident in which a tire-size piece fell into traffic on the lower deck, causing more than $3,000 in damage to a car that ran over it, officials said.

A Caltrans official initially described that incident as isolated, but subsequent tests found more locations with corrosion-caused damage, all in the eastbound lower tunnel. They ranged in size from roughly 3 by 3 inches to 2 by 3 feet, and were found on both sides of the tunnel, said Ken Brown, a Caltrans engineer who oversees Bay Bridge maintenance.

It’s possible the problem is still more extensive. Brown said Caltrans found the latest problem spots by using a basic test — workers tapped the concrete with a hammer to see if it sounded hollow. But that method can’t detect less-severe cracks and corrosion on the large portions of the 1,800-foot-long tunnel that are covered with paint, and Caltrans will have to use more sophisticated inspections there.

Nothing urgent

Despite the potential for additional damage, Caltrans officials downplayed the danger to motorists. Dan McElhinney, deputy district chief for the agency, said the immediate issue of falling concrete has been resolved by removing loose material from the tunnel walls. He called the corrosion “minor, but important.”

“We have checked all the walls — there is no risk of spalling in the near term,” McElhinney said, using the term for concrete falling away from a surface due to corrosion. “We don’t see anything urgent at this point.”

Brown added, “We don’t think we have a big issue right now. But until we finish our investigation, we don’t know anything for certain.”

Leaking rainwater

The corrosion culprit, Brown said, is rainwater, which is apparently leaking through some of the more than 250 drain openings on the upper deck to the tunnel below.

The upper and lower tunnels are separated by a deck that was installed in 1964, after rail service was eliminated on the lower deck. Reinforced-concrete sections that make up the deck sit atop 12-inch-wide concrete ledges on either side of the tunnel, cushioned at contact points by half-inch-thick Masonite pads.

Over the years, rainwater leaking through the drain openings has apparently soaked some of those 512 pads, causing them to expand downward and create cracks in the lower tunnel’s concrete walls, Brown said.

Water can then flow into those cracks and corrode steel rebar, which expands and pushes the concrete away from the wall — a phenomenon known as “pop-out.” That’s what happened in the Jan. 30 incident, which Caltrans officials said was unprecedented in the lower tunnel.

On the Masonite pad where the concrete failed last month, Brown said, “it looked like it had swelled, and that it had frayed.”

Caltrans said at the time that the tunnel had been visually checked for corrosion damage in July and that crews had found no problems. But the hammer test, Brown said, apparently had not been done since 2004.

Brown said that besides replacing compromised concrete in the lower tunnel, Caltrans may have to remove hundreds of Masonite pads and replace them with rubberized pads. That should make it less likely that new cracks will form to allow rainwater to infiltrate the lower-tunnel walls, Brown said.

“We would clean it off, prep and put the concrete back in,” he said.

Brown said the work would probably require at least nighttime lane closures, but that the extent of the job is still unknown.

“We’ll look at the big picture and try to figure out what to do then,” Brown said.

The Masonite may contain asbestos, experts say, which would require that the work area be sealed off, complicating the job for crews and motorists.

Mystery system

Adding to Caltrans’ challenge is that the 1960s drawings for the tunnel deck do not spell out exactly how the drainage system works, Brown said. Without that information, Caltrans faces a challenge in figuring out how to keep water from flowing into the Masonite pads that aren’t replaced.

“We are looking at both keeping the water out and dealing with any issues we have with the bearing pads,” Brown said. “The long-term goal is to prevent any kind of continued deterioration so we don’t have any major issues.

In the meantime, Caltrans has to test the rest of the lower tunnel for damage. Because much of the walls are covered with paint and road dust, workers will have to use X-rays or other advanced equipment to look for hidden cracks, Brown said.

“We would have to bring in specialists,” he said.

Federal Highway Administration officials will help guide the process, Brown said. “We brought (the agency) in to advise us, to get a feel from what they are seeing in other tunnels,” he said.

Brown said that although the wall is not considered at danger of catastrophic failure, the incident last month is a reminder that even isolated corrosion can be a threat to motorists.

“I would say any time a piece of concrete falls from a structure, it’s a concern,” Brown said. “We’ve seen some minor cracking in the past, but nothing to indicate something would be popping off.”

Aging infrastructure

Brown said the cost of repairs will come from a fund of about $70 million a year for rehabilitation projects on the Bay Area’s seven state-owned toll bridges. “We don’t have exact costs,” he said.

McElhinney said such problems are bound to become apparent as the state’s roads and bridges age.

“We have to do some more work, both in this tunnel and elsewhere, to improve our aging infrastructure,” he said.

Jaxon Van Derbeken is a San Francisco Chronicle staff writer. Email: jvanderbeken@sfchronicle.com Twitter: @jvanderbeken