Spokesman Travis Phelps said the framework is in place and workers are tightening bolts and welding sections on the temporary structure that's 24 feet wide and 160 feet long.
He said they'll be installing panels Monday that will be paved with asphalt to form the new bridge deck.
Some other work remains, such as divers' inspecting piers, so the department isn't saying yet what day the Mount Vernon bridge will reopen.
Traffic has been detoured since May 23 when a truck with and oversize load struck the 58-year-old span, causing it to crumble. The bridge carried 71,000 vehicles a day.
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