A two-lane closure on Interstate 5 threatens to bring a stretch of freeway to a near standstill this weekend for motorists going north through downtown Seattle.

Two lanes will be closed alongside Lake Union, from 9 p.m. Friday to 4:30 a.m. Monday.

And incidentally, the Highway 520 bridge will be closed in both directions from 11 p.m. Friday to 9 a.m. Sunday.

The northbound I-5 shutdown will close two of the four freeway lanes all weekend, from Olive Way to Lakeview Boulevard East, while workers fix expansion joints and concrete throughout the corridor.

Unlimited Digital Access. $1 for 4 weeks.

Beyond Lakeview, two lanes will be closed all the way across the Ship Canal Bridge from 5:30 a.m. to 5:30 p.m. both Saturday and Sunday. The state decided to extend the construction area north, taking advantage of a chance to repair the giant bridge deck.

(And in the wee hours, 11:59 p.m. Friday to 4:30 a.m. Saturday, an additional lane will close while crews set a concrete barrier, to protect their colleagues working on the deck joints — then, a similar closure is needed to remove barriers 48 hours later.)

The Mercer Street on- and offramps, Olive Way onramp and Lakeview Boulevard offramp will remain open.

Even on a normal weekend, northbound traffic tends to be stop-and-go middays approaching the Convention Center downtown, where state officials and lawmakers failed to locate $23 million this year to widen the Seneca Street bottleneck.

The state won’t predict how bad delays this weekend will be.

The I-5 work zone serves a quarter-million vehicles daily. Weekend events in Seattle include a Sounders FC match
at 7 p.m. Friday, a Seahawks game at
5:30 p.m. Sunday, the Boats Afloat Show
at South Lake Union and the St. Demetrios Greek Festival in the Montlake neighborhood.

The Highway 520 closure extends in both directions, from Interstate 405 to Montlake Boulevard. On Monday morning, drivers will find that lanes have been shifted on the Eastside, while 40 truckloads of concrete will be poured to build foundations for a new “West Connection Bridge,” which will link the new 2016 floating bridge to the old fixed highway, at the west side of Lake Washington.

Julie Meredith, program director for the 520 project, said crews there will finish in plenty of time for Seahawks fans to cross the lake.

These are the last major construction closures of the summer, the state Department of Transportation says.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom