Age, freeze-and-thaw cycles and salt contributed to the brew of deterioration that caused 1,200 pounds of concrete to fall in chunks from the underside of a freeway overpass in St. Paul on Saturday, the state's top bridge engineer said Sunday.

After an inspection in August, officials knew the bridge deck was breaking down, but nothing suggested that pieces would break off within the year, said Dan Dorgan, state bridge engineer with the Minnesota Department of Transportation.

Two vehicles were hit -- one on the windshield, one on the hood -- and debris from the chunks, which fell from the underside of the Maryland Avenue bridge, choked off traffic on Interstate Hwy. 35E for more than eight hours as crews inspected the overpass and knocked off other loose concrete as a precaution. Traffic backups stretched for miles in both directions on the north-south thoroughfare, which shuttles nearly 140,000 vehicles per day through downtown St. Paul.

No injuries were reported, according to the State Patrol.

"Certainly, it's an issue that concerns us, but the bridge is structurally safe," Dorgan said. "For 50-year-old concrete, that's the type of deterioration you often see."

The bridge, constructed of steel beams and concrete, was built in 1958, remodeled in 1973 and got minor repairs in 1992.

MnDOT is replacing many bridges from that era, Dorgan said, noting the Crosstown Commons and Unweave the Weave constructions projects in progress this summer.

Overpasses built in the 1960s or later used prestressed concrete beams, which resist corrosion better, Dorgan said.

Routinely inspected

The 6-foot-by-9-foot-by-1-inch patch that peeled off could be described as a veneer for the underside of the steel beams that span the length of the bridge and support it, Dorgan said. Those beams are structurally unaffected by the loss of concrete, he said.

Inspectors routinely look for cracks and signs of water coming through the concrete; damp spots are telltale signs the concrete isn't as solid as it should be, he said.

Delamination -- the weakening of the concrete bonds -- and spalling -- the falling-off of concrete -- are early indicators of deterioration for bridges of this age, he said.

"Typically, when delamination is seen, where it appears that it could break loose, then we'd have maintenance come out and knock it off so we don't have these incidents," he said.

Of Saturday's incident, he said: "This isn't acceptable. Our goal is to catch these before they let loose, but it's not always completely apparent."

After the chunks fell into northbound lanes of I-35E at about 4:45 p.m., the State Patrol closed the six-lane freeway in both directions. Northbound lanes reopened after midnight and the southbound lanes about 2:30 a.m. Sunday, said MnDOT spokesman Kent Barnard.

Saturday's incident happened within a week of the one-year anniversary of the I-35W bridge collapse over the Mississippi River, which killed 13 people and injured more than 100. That disaster triggered widespread inspection of the state's 13,000 bridges.

Saturday's incident probably won't prompt broad inspections of freeway overpasses, Dorgan said.

According to MnDOT's Statewide Bridge Inspections database, the Maryland Avenue overpass, bridge No. 6513, was last inspected on Aug. 15, 2007, and gets examined every year.

It was given a sufficiency rating of 77 out of a 100 -- considered "structurally deficient." Generally, bridge inspectors say a bridge with a rating below 80 means it needs rehabilitation, while ratings of 50 or less may indicate the bridge needs to be replaced.

The report indicated that the concrete deck underside had some delamination in the past, but had been repaired.

While the sloughed-off concrete might not affect the bridge's strength, it poses a serious risk to people underneath, Barnard said.

The Maryland Avenue bridge is set to be replaced in 2014 along with the Cayuga Bridge, a 1,285-foot span just north of downtown, as part of a $200 million reconstruction of I-35E.

Inspectors will probably head out this week for another look at the Maryland bridge, Barnard said. "We're definitely on top of this one," he said.

Chris Havens • 651-298-1542