In a press conference Thursday afternoon, Lowe's Motor Speedway personnel and engineering consultants described the cause of last Saturday night's pedestrian bridge collapse as being due to corrosion of the reinforcing cables in the structure. The cause of the corrosion was described as chloride attack of the steel cables.
Steel is extremely susceptible to attack by chloride ions.
Dr. Manning stated that the only way this would have indicated a problem would have been if there had been rust stains visible on outside of the bridge. There were no such stains visible. As a part of the manufacturing process for this type of tee, a plug area is created in the center and filled with a grout-like material. Our investigations have revealed these two double tees failed in the center section where there was substantial corrosion of the steel cables. The preliminary results from investigators of the grout material at the center section shows unexpectedly high concentrations of calcium chloride in the grout material.