Vacuum Grouting successfully introduced as a new standard
The Mid Bay Bridge is a precast segmental bridge with a total length of 5,872 m and is composed of 140 spans of 41.5 m each plus a main span of 68.6 m. The bridge is about nine years old. (See Fig. 1)
During a routine inspection of the bridge in August of 2000, two 19-0.6" external tendons were found with severe signs of distress. The corrosion had completely deteriorated the tendon section, causing the tendon failures.
The Florida Department of Transportation called for an emergency plan to repair the structure. The general contractor awarded the contract was Granite Construction Co., who in turn, hired DSI for materials, equipment and technical advice for the detensioning and replacement of 11 external tendons. This work was completed from October to December 2000.
The inspection performed by the Florida Department of Transportation also found voids behind the bearing plates in a significative percentage of the external tendons of the bridge. Initially, the contractor plan was to regrout the voids by using a tremie tube method. DSI, however, proposed the use of vacuum grouting for this repair. Mock-ups were built to demonstrate the adequacy of this method (see Fig. 2).
The mock-ups consisted of tendons partially grouted in order to artificially reproduce the actual field conditions. A darker grout was used for the first grouting phase, while a lighter colour was used for the vacuum grouting phase in order to visually inspect the performance of the grouting after the dissection of the mockup. Figs. 3 and 4 show sections of the dissected mock-up at the anchorage and at an intermediate location, respectively. Fig 5 shows the poor results obtained when the tremie tube method of grouting was used in a mock-up built for testing purposes.
The results of the mock-up testing proved the higher quality of the vacuum grouting and this was the method selected for the grouting of the voids. More than 700 voids were vacuum grouted between January to May of 2001. Fig. 6 shows the DSI vacuum grouting equipment used inside the bridge box for the grouting operation.
The problems experienced by the Mid Bay Bridge originated a nationwide bridge inspection activity that confirmed that the grouting practice in the USA until then (conventional grouting materials, conventional grout pump/mixers and non-qualified labor) had contributed to the problems of voids in post-tensioning tendons. The vacuum grouting method has now been adopted by many states as the solution for these problems.
DSI is now considered the actual leader in the vacuum grouting technique in the USA. Since the completion of the Mid Bay Bridge, we have vacuum grouted voids in post-tensioning tendons at the BQE Bridge (precast segmental bridge) in New York, at the Sydney Lanier Bridge (cable stayed bridge) in Georgia, at the Charles River Crossing Bridge in Massachusetts, at the C & D Canal Bridge in Delaware (cable stayed bridge) and are currently working at the Cochran Bridge in Alabama (cable stayed bridge).