Photo: Leah Millis / The Chronicle 2014

Caltrans officials have one word to describe the planned rebuild of a deck of Highway 101 at Alemany Circle, north of the interchange with I-280: “Carmageddon.”

The $25 million project, scheduled to take place in July, affects a spaghetti tangle of freeway often called the “Alemany Maze,” with spiraling ramps to connect 101 with 280. Like a tentacled octopus, it stretches north to the Bay Bridge and south to Daly City and San Francisco International Airport, with prongs swooshing in every direction.

Built in 1950, it’s starting to decay, with rebar rusting and concrete crumbling beneath the roadbed. Caltrans workers say they can’t keep doing one patch job after another.

So, crews will replace an 800-foot section of the deck and strengthen the bridge columns underneath. The work is simple: break up the concrete on top of the deck, replace the steel below, pour more concrete and let it set. But it requires closing a congested strip of freeway and rerouting traffic for three weeks, reducing lanes in the northbound direction from three to two.

On any given day, the quarter-mile span between Cortland Avenue on the north and Silver Avenue at the south carries 244,000 vehicles. Transportation planners are now asking all those drivers to change their routine for a month, either by working remotely or taking public transit. They estimate the project will cause backups for 6 miles, in addition to all the typical traffic snarls in the area.

“If you think you can get through here, get it out of your mind,” said Caltrans spokesman Jose Moreno. “You’ll be trying to thread the needle ... so find another way.”

The original “Carmageddon” occurred in Los Angeles in 2011 during a highway construction project that, thanks to public cooperation, turned out to be less of a traffic nightmare than everyone had feared.

The state agency is negotiating with local public transit systems to beef up service, including extra BART cars and ferry boats from San Francisco to San Mateo County. Officials may also provide shuttles for residents who live in the area.

Still, board directors for the San Francisco Municipal Transportation Agency were concerned the project might have warping after-effects on city streets, or on the busy Alemany Farmers’ Market nestled beneath the freeway.

The directors fired questions at Caltrans Bay Area Director Tony Tavares when he presented the project at a board meeting on Tuesday.

“I know it’s July, but it’s still a time when you have a lot of ... tourists and people coming into the city,” said Director Gwyneth Borden, who lives in the Outer Mission neighborhood that surrounds the maze. She noted that the freeway links to the airport, which is flooded with tourists in summer.

“I would agree with you that whole entire area is completely congested with no work going on, and it’s going to get even worse during this period of time,” Tavares said. He said the agency may pay for parking control officers, shuttles and the proposed additional ferry service to get people out of their cars.

Caltrans has offered an incentive for contractors to reduce the project timeline: $1 million per day, up to a limit of $8 million — and a penalty for delays. The agency has also considered other construction options, like closing the entire highway for five or six days, and getting all the work done at once.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan