Investigators still do not know exactly why the I-35W bridge collapsed into the Mississippi River in Minneapolis on August 1, killing 13 people and injuring about 100. But a succession of less spectacular failures over the years has raised concern among the country’s bridge engineers. New materials promise to make routine repairs less costly and intrusive, an important consideration in an era in which money for infrastructure is tight. “The days of letting a bridge deteriorate and then simply replacing it are going away,” says Mark Hirota, a consultant with Parsons Brincker­hoff and a former bridge engineer for the state of Oregon.