Eric Richardson

The 1933 6th Street Viaduct, viewed from the 4th street bridge in a 2007 photo.

DOWNTOWN LOS ANGELES — The 6th Street Viaduct has cancer, but the path to get it into treatment has not exactly been a smooth one.

The project’s Environmental Impact Report was supposed to be released this summer, but on Tuesday city staff emailed members of the Community Advisory Committee to let them know that further review by Caltrans would result in a delay of approximately six months.

That would place the release of the document four years from the project’s public kickoff. Built in 1933, the structure is considered to be the gem of the collection of historic spans that cross the Los Angeles River as it cuts through Downtown. It stretches 3,500 feet, from Matteo Street to Boyle Avenue in Boyle Heights.

Despite its status as a Historic Cultural Monument, the city’s project staff feels that the viaduct must be torn down due to a condition known as Alkali-Silica Reaction, or ASR. A combination of high alkali content and certain reactive elements in the aggregate cause the concrete to crumble from the inside, steadily reducing the chances that the structure would survive a major earthquake.

The viaduct is the only one of the spans to suffer from ASR. It was built using an on-site concrete plant specially constructed for the job, and the local materials led to the toxic mixture.

If the viaduct must be torn down, opinion has been split on how to replace it. Some want a re-creation of the historic structure, while project staff have pushed for a more modern design.