The University of Toledo University Transportation Center : UTUTC-IU-15: Magnetic Sensor for Nondestructive Evaluation of Deteriorated Prestressing Strand - Phase II

The objective of this project is to develop a prototype magnetic sensor that can reliably estimate the remaining cross sectional area of exposed or hidden corroded prestressing strands in a laboratory setting. This proposal is a request for phase II funding. Phase I completion has been extended until December 31, 2009.

Dominant mechanisms for sudden failure of bridges are corrosion and fatigue. These are slow developing phenomena and adequate sensors to detect them do not currently exist. Despite national studies, no effective nondestructive sensor technology has been identified for prestressing strand corrosion. An effective sensor for corrosion will be able to get a snapshot of the corrosion in time by sensing the corrosion byproducts or a direct change in member properties due to corrosion. This is a radical departure from the usual process of measuring a quantity, such as strain, which is a secondary effect of corrosion. The need for the proposed sensor is particularly acute in Ohio where there are many prestressed box girder bridges.

The phase I work is ongoing, but we have two clear findings. 1) Magnetic detection can reliably distinguish cross sectional areas and 2) our present magnet is not large enough.  The first finding shows that development of a practical sensor can be undertaken with a reasonable prospect of success. The second finding shows whether we detect the remanent magnetism or the magnet field, we will need to come much closer to magnetic saturation than is possible with the present small electromagnet. In addition, our work has highlighted the need for a stronger theoretical basis and our initial interaction with a large industrial electromagnet firm has shown the importance of developing a commercial partnership. Overall, the phase I work has given us confidence that the core idea of magnetic detection of prestressing strand area is sound and set the agenda for the proposed phase II work.

In phase II, we need to secure access to a larger better designed electromagnet, to broaden and deepen our theoretical base, to identify difficulties that could be fatal to the development of the sensor and carry out proof of concept resolutions, and to engage a commercial partner. A larger electromagnet that can reach a higher level of magnetic saturation and is designed for use with concrete will allow us to get an adequate signature of the remaining strand cross section. Achieving an effective magnetization through concrete requires an understanding to the magnetic properties of concrete and of the magnetic fields for objects not in contact with the magnet. Based on our work, the fundamental difficulties that could be fatal to the sensor development are the need to magnetize our specimen from one side and the need to “see” through concrete. Access to a simple, larger electromagnet can provide proof of concept resolution for these issues. A commercial partner will have larger electromagnets, expertise to help determine what characteristics of concrete are critical, and experience in the engineering and fabrication of practical electromagnets.