Magnetic Flux Leakage Device for Evaluation of Prestressed Concrete Box Bridges

About the report:

Corrosion of prestressed concrete reinforcement in bridge components is a major concern for state transportation agencies.  The severity of the problem is amplified by the fact that the corrosion-induced damage in concrete reinforcement often does not show up on the surface until significant propagation has occurred.  Moreover, prestressed reinforcement can undergo brittle fracture at lower cross-sectional losses when compared to non-prestressed reinforcement.  As evidence, some prestressed bridges in Virginia built since the 1960s have begun showing premature corrosion-induced deterioration.  This created a need for a nondestructive evaluation technique that could reasonably determine areas of damage at early stages of corrosion.

Since a ready-to-implement commercial tool is not available, a prototype for a new magnetic flux leakage (MFL) device for the evaluation of hidden prestressing strand under the surface of prestressed concrete box beams was designed, developed, and tested in the laboratory.  This device works based on the MFL principles of nondestructive evaluation.  The novel features of this device are portability, lower cost, and real-time results, and it can be used by a bridge inspector with minimal training.  The device uses advanced sensors and microcontroller technologies to deliver a battery-powered MFL inspection system device capable of detecting corrosion damage in prestressing strand beneath 2 in of concrete cover. 

The device was verified to work well in the laboratory and was minimally deployed in the field to check for any issues.  An extension of the study could improve the field-implementable device to include better display of magnetic flux data and semi-automation to identify corroded areas of box beams.  However, further improvement of the device in terms of analyzing and displaying results and fabricating a lighter, portable, self-containing instrument was beyond the scope of the Virginia Transportation Research Council in terms of resources. Therefore, the Virginia Transportation Research Council should propose a research needs statement and seek proposals for a follow-up study using implementation or other sources of funding to improve the device in terms of analyzing and displaying results and fabricating a lighter, more portable, self-containing instrument for field deployment for inspecting bridges in service.

Condition evaluation of the prestressing strands in the box beams prior to severe corrosion-induced damage will lead to informed decision-making regarding whether to repair, replace a single beam, or replace the entire superstructure.  This device can provide an indication of section losses in prestressed strands in areas with shallow concrete cover, where no off-the-shelf inspection equipment can provide this critical information on the condition of prestressed box beams.  This makes the outcome of this study highly significant for the infrastructure of the Virginia Department of Transportation.  Implementation of this technology will lead to improved safety of the traveling public, better performing box beam bridges, and cost savings to the Virginia Department of Transportation from making appropriate maintenance decisions.

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