Development of a Sensor for Nondestructive Inspection of Deteriorated Prestressing Strands in Prestressed Concrete Bridges

Prestressed concrete bridges have failed due to unobserved corrosion in prestressing strands. This paper describes proof of concept testing of a magnetic sensor based on magnetic flux detection to estimate the remaining cross-sectional area in corroded prestressing strands inside prestressed concrete beams. Prestressing strand deteriorates over time due to various causes. The primary corrosion mode is rusting of the strand. The magnetic properties of rust are vastly different from those of steel. Thus, a magnetic sensor is theoretically possible. An electromagnet and Hall sensor array were developed to detect the cross sectional area of a steel specimen when it is magnetized from one side and at a distance from the magnet face. In this series of experiments, the electromagnet is used to induce a magnetic field in the solid steel bars of various diameters and at distances of up to two inches from the magnet face. At near magnetic saturation, the induced magnetic field was reliably proportional to the cross-section of the steel bar. The completed experiments with steel bars are discussed. The ongoing laboratory experiments with embedded strand and planned field tests are described. The initial results confirm that if a combination of magnetic field and remnant flux techniques are used with an appropriate electromagnet, in-situ detection of remaining cross sectional area may be possible.

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