The Bridge
The James B Edwards Bridge over the Wando River near Charleston SC is a pair of pre-cast post-tensioned concrete box girder bridges. A failure of post-tensioning cable, which are inside the box girder of the bridge, caused a closure of the westbound spans mid-day on May 14, 2018. Weekly inspections of the 92 cables inside the bridge had been occurring since a repair project in 2016. (Wikipedia)
The bridge was designed by the same company (Figg Bridge Group) that designed the Florida International University pedestrian bridge that collapsed.
The Closure
SCDOT will complete a temporary repair by June 11, 2018. Temporary repair means adding one or two additional cables. After the temporary repair is completed on June 11 , they will begin the permanent removal and replacement of the broken cable. The permanent replacement work will happen with the bridge open with traffic.
There are 8 post-tensioning cables in each segment of the bridge. I believe the cables are the length of the segment. Nineteen 7? strand cables are bundled into the 4" cable that is incased in concrete grout inside a PVC pipe to form one of the 8 cables in each segment.
History
SCDOT requested funding in 2002 to see if a new testing method would help inspect the post-tensioning cables because "the strands are encased in concrete and grout-filled tubes making non-destructive inspection of the strands extremely difficult." The SCDOT Request for funding was a request to participate in a research program.
The Florida Department of Transportation has had a lot of experience with corrosion in this type of bridge. Florida published a report New Directions for Florida Post-Tensioned Bridges. In that report they state: " As a result of recent findings of corrosion of prestressing steel in post-tensioned bridges, the Florida Department of Transportation will be changing policies and procedures to ensure the long-term durability of post-tensioning tendons."
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