The6thStreet Viaduct Bridgein Los Angeles—the site of numerous Hollywood movie scenes—has reached the end of its career. Los Angeles city engineers are internally reviewing the Request for Proposals document for a final design to replace the structure, which consists of two prestressed concrete viaducts and a double steel arch center span, with a cable-stayed structure.
The Los Angeles City Council voted late last year to replace the3,500-ft-long, 46-ft-wide viaductin Boyle Heights, just east of downtown. The viaduct extends east-west across the Los Angeles River, Union Pacific Railroad and Metrolink tracks, US 101, and several local streets. Listed on the California Register of Historic Resources, the bridge’s concrete components have the equivalent of cancer.
洛杉矶市公共工程系发言人托尼亚·杜雷尔（Tonya Durrell）说：“这座桥病很恶心。”“它在混凝土中具有碱 - 硅反应（ASR）。它就像癌症一样，它正在持续且不可逆转。它只是必须被拆除，并建造了另一座桥。”
ASR是混凝土结构中众所周知的潜在弱点。化学过程基本上是一种在产生凝胶的聚集体中二氧化硅的攻击和分解。然后，凝胶取水并膨胀。凝胶就像一块海绵并膨胀，可以看到从染色的混凝土区域渗出。然后，混凝土开始从膨胀的凝胶的压力中裂开。在整个桥梁中可以看到凝胶和裂纹的可见迹象。
The 6thStreet Viaduct Bridge is the only one of the many bridges over the L.A. River to have the fatal condition. Ironically, it was built with innovative concrete techniques for its time. A concrete plant was constructed on-site to produce the material for the bridge. “We now know how to proportion the concrete mixture so you don’t get ASR,” says Rachel Detwiler, an engineer and editor-in-chief of the Precast Prestressed Concrete Institute (PCI) Journal, “And you can choose different materials as long as you have a favorable chemistry. But at the time that the bridge was built, they did not know about Alkali-Silica Reaction.”
But being aware of the problem has not eliminated it. ASR has derailed even more contemporary projects. For example, in 1999, a 440-lb slab of concrete tunnel smashed onto the roof of a speeding Shinkansen bullet train in Japan due to weakening from ASR.
2004年,地震脆弱studies determined that given the current rate of deterioration, the bridge has a 70% possibility of collapse over the next 50 years in a significant seismic event. This is in contrast to the accepted collapse probability of 5% or less over 50 years.
The city is carefully monitoring the bridge, which is still open to traffic, says John Koo, city bridge improvement project manager. He adds that the RFP for final design will be released "in the near future." The $401-million, mostly federally funded replacement bridge will start to take shape in three years after the design gets finalized with the ideas and approval of its aesthetically sensitive L.A. community. Once construction begins, the new bridge will be completed in about four years and will include modern amenities like wider sidewalks with room for pedestrians and cyclists.
这座桥建于1931年,在电影like "Grease" and "Terminator 2" is no stranger to bad news. Since just after it opened it has been plagued with material failures. Two pylons were removed in the 1940s because of the compromised condition of the concrete. In the 1980s, in an effort to shore up the bridge, its deck was stripped and its concrete sealed with a waterproof coating. And more recently, epoxy injections patches were administered to prevent moisture incursions, has left stains, according to 2007 official documents of the 6th Street Viaduct Seismic Improvement Project.
