Because incompletely grouted posttensioned ducts result in voids, the steel strands are vulnerable to premature corrosion. This paper describes a nondestructive evaluation (NDE) procedure that allows bridge owners to ensure that posttensioned ducts are properly grouted (i.e., have no voids). The NDE procedure uses time-domain reflectometry (TDR), a technique developed by electrical engineers for locating discontinuities in transmission lines. TDR involves sending a signal created by a step-pulse generator through a transmission line, determining whether the signal is reflected back, and, if it is reflected back, using the elapsed time to determine the location of the discontinuity. Prior research funded by the Delaware Department of Transportation and the National Science Foundation has shown that TDR can be used to detect corrosion on strands and can be implemented in the field. To detect and evaluate voids, the transmission line is placed either in or adjacent to the region where a void is suspected. The presence of a void affects the electric field surrounding the transmission line and causes a distinct reflection. Data are presented to show the measurement of both the relative size and the position of voids. The effects of environmental conditions, such as moisture content, temperature, and material contained in the void (e.g., corrosion products) also are reported.
Abstract