OCEANSIDE -- Emergency repairs have begun on an Interstate 5 overpass that has dropped chunks of concrete onto Oceanside Boulevard at least twice in the past few months, a state Transportation Department official said Tuesday.

Deck repair work, which should begin in two to three weeks, is expected to take place at night, likely beginning at 9 p.m., according to Edward Cartagena, a spokesman for Caltrans. Workers will close no more than three lanes at a time, he said, and will use quick-setting concrete to allow all lanes to reopen each morning while work is under way, sparing northbound commuters the daily traffic snarl that would result if lanes remained closed during daylight hours.

In preparation for the night work, Cartagena said, workers have begun installing scaffolding and concrete forms underneath a narrow portion of the 10-lane overpass.

The crews are preparing to replace a 2-foot-wide, 150-foot-long section of bridge deck that has begun to crumble under the strain of an estimated 189,000 daily vehicle trips. The cost of the repairs is estimated at $500,000, Cartagena said.

The work was deemed an emergency after a 7-foot-long, 1-foot-wide chunk of concrete fell from the overpass to the road below on Feb. 17.

Several weeks later, on April 27, three lanes of northbound I-5 in the same area were closed during the morning rush hour while Caltrans crews worked to patch a 6-by-6-inch pothole on the overpass.

Cartagena said Tuesday it took time for the work to begin because the agency needed to select a contractor, design the repairs and perform preparatory work.

"Everything that has been done has been fast-tracked, but that doesn't mean that it's instantaneous," Cartagena said.

Problems at the site first emerged in May 2008, when a 10-by-20-inch chunk of concrete from the overpass fell onto the roadway. No one was injured in that incident, or in the subsequent ones, but early this year, local leaders began to call for action from Caltrans.

A Caltrans engineer who inspected the overpass after the 2008 incident found that the falling concrete is the result of a previous widening effort.

Caltrans widened the northbound section of the overpass from two to four lanes in 1963. Rather than tear down the old two-lane bridge and build a four-lane replacement, workers simply built a new two-lane bridge right next to the old one, removing the old structure's guard rail. Workers then covered the gap between old and new with a concrete covering.

Inspectors found that the overhang between the bridge's old and new sections was not reinforced to modern standards and is weaker than the rest of the deck. They insist that the bridge is structurally sound, but say that the weak area, which is in a truck lane, needs to be reinforced.

Cartagena said temporary scaffolding and netting is in place to prevent any additional concrete from falling onto Oceanside Boulevard.

Call staff writer Paul Sisson at 760-901-4087.