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.
