The University of Toledo University Transportation Center : UTUTC-IU-12: Magnetic Sensor for Nondestructive Evaluation of Deteriorated Prestressing Strand

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. The proposed sensor is based on the Hall effect.

Unexpected failure of prestressed concrete box beam bridges due to prestressing strand corrosion has occurred. To determine the remaining flexural capacity of a prestressed bridge, the engineer must know the useable cross sectional area of the strand. The corrosion of the exposed strands is manifest, but their useable area must be estimated. The state of the strands where there is no spalling is unknown. It is desirable to be able to estimate the area of these strands without removing their cover. Despite national studies, no effective nondestructive sensor technology has been identified.

Bridges with prestressing are widely used. In the northern states, 25% of total area of bridges is prestressed. The Ohio Department of Transportation (ODOT) has estimated that prestressed concrete box beam bridges represent approximately 10% of the bridge square footage in the Ohio bridge inventory.

Visual inspection is the method currently used to detect corrosion in prestressing strands in box beam bridges. Visual inspection is not adequate even when it is known a priori that the structure has extensive deteriorated strand. An accurate and convenient nondestructive inspection technique for detecting in-situ corrosion of prestressing strand, particularly in box beam bridges, would improve safety for the traveling public and assist in better maintenance planning.

The state-of-the-art in magnetic sensor capabilities has advanced to the point where design of a practical sensor can be undertaken with a reasonable prospect of success. The magnetic properties of rust are vastly different than the magnetic properties of uncorroded steel. Therefore, with a sensitive enough sensor and appropriate imaging techniques, the volume of uncorroded steel can be interfered from the magnetic properties of the specimen.

This research is an initial step on the road to the development of a practical magnetic sensor for in situ estimation of the cross sectional area of corroded prestressing strands. This sensor should be able to estimate the area of exposed or hidden corroded strands. Such a sensor would be immediately useful to inspect, rate and manage the inventory of precast box beam bridges and other prestressed bridge structures.