To detect reinforcing steel corrosion, Gordon H. Reigstad of White Bear Lake, Minn., and Hanley S. Reigstad of Sunburg, Minn., developed an invention for the economical nondestructive testing of prestressing members. Their "Method and Apparatus for Detecting Tendon Failures Within Prestressed Concrete" received U.S. Patent 6,119,526, and is now assigned to Tech Research in St. Paul, Minn.

The detection apparatus includes a cart equipped with a metal detector and a distance meter that is designed to be drawn across a concrete slab along a length of tendon. The metal detector includes a control unit that is electronically coupled to a sensor or probe.

As the cart is moved over the concrete slab, the metal detector takes readings representative of the steel tendon, and the distance meter measures the distance traversed by the cart. The detection apparatus also includes a controller that interfaces with the metal detector and the distance meter. The controller is designed to sample the readings generated by the metal detector at predetermined distance intervals along the tendon to determine the present state of corrosion in prestressing steel within a concrete slab.

The controller can send digital signals to a processing unit such as a computer. The computer processes the digital signals, calculates corresponding distance data, and generates a graph in which the data generated by the detector is plotted vs. the corresponding distance data to produce a profile for the length of tendon.