The harbour city's iconic Bastion Street Bridge is set to undergo a facelift 40 years in the making.

The bridge connects Nanaimo's Old City Quarter neighbourhood to downtown.

Untouched for decades, the crossing will get a long-awaited retrofit, which includes seismic modernization work.

"The last major upgrade of the bridge took place over 40 years ago, in 1978," said Nanaimo's manager of engineering projects, Phil Stewart. "This major upgrade will extend the life of the bridge by 20 to 30 years."

The project is expected to take several months to complete. The massive concrete deck of the span will be completely replaced, meaning motorists will be unable to use the crossing.

Construction begins on March 25, but the bridge will not be closed to motor vehicle traffic until significant deck work begins in May.

Nanaimo city staff say a pedestrian walkway will be maintained throughout the project.

The city's engineering department says drivers will need to use alternate routes into the downtown core for nearly six weeks between May 21 and June 28.

Rehabilitation work will also include decorative signs on the deck level and a general clean-up below the bridge.

The bill for the improvements is expected to cost approximately $2.3 million dollars.