City of Los Angeles

This contemporary "extradosed" bridge design is the recommended replacement for Downtown's historic 6th Street Viaduct, which suffers from a condition described as "concrete cancer."

DOWNTOWN LOS ANGELES — After a one-year delay, the clock will again start ticking for a historic Downtown landmark on Wednesday as environmental review resumes on a project that will most likely result in the demolition and replacement of the 1933 Sixth Street Viaduct.

The span is the largest of the dozen historic bridges that cross the L.A. River. It stretches 3,500 feet, from Matteo Street in the Arts District to Boyle Avenue in Boyle Heights.

It also has concrete "cancer," specifically a condition known as Alkali-Silica Reaction (ASR) that causes the concrete to crumble from the inside, steadily reducing the chances that the structure would survive a major earthquake.

Reaching community consensus on a cure has proven to be a long, slow road. The project's first community meetings were held four and a half years ago.

At sometimes heated meetings, community members from Downtown and Boyle Heights argued for a solution that would save the historic structure.

Project engineers have been clear throughout that preservation and retrofit options would be inadequate given the way that ASR has attacked so much of the structure's concrete.

Sixth Street is the only one of the river spans to suffer from ASR. It was built using an innovative on-site concrete plant, and the ingredients in the concrete aggregate just happened to contain the mix necessary for the condition to take hold.

Rebuilding a replica of the historic design is also problematic. Advances in engineering have made the structure's tightly-spaced legs unnecessary, and modern railroad codes would make it impossible to replicate the columns that currently sit between the tracks along the river.

In the environmental documents that go to the city's Board of Public Works on Wednesday, the project team recommends a contemporary "extradosed" bridge design, which features a cable-supported deck suspended from low towers. A recommended realignment would soften the existing structure's odd bend, widening the roadway and sidewalks and adding bicycle lanes.

The $401 million budget includes $365.5 million in federal funds, $29.7 million in state Proposition B funds, $5.6 million from the city and $0.2 million from other state sources. The City of Los Angeles owns the bulk of the structure, but Caltrans owns the section over the 101 freeway.

The project is tentatively scheduled to go to City Council for environmental certification on November 16. It got close to this point two years ago, getting as far as a hearing in front of the Cultural Heritage Commission in July of 2009. The process was put on hold last summer so that Caltrans could conduct a review of the environmental documents that was required under changed federal rules.