Detecting Faults in Posttensioning Ducts by Electrical Time-Domain Reflectometry

A series of laboratory-based experiments is described in which an electrical time domain reflectometry (ETDR) system is used to locate, identify, and size a number of different artificial voids and wet sections in ducts filled with sand and containing a variety of steel cables. The echo waveforms were digitized using an ETDR instrumentation system and the signals passed to a computer for storage and processing. Data analysis using simple regression techniques, in conjunction with simple transmission line theory, suggests that a system based on the same principles could be used to identify and size similar voids in posttensioning ducts used to carry prestressing cables in load-bearing concrete structures such as bridges, retaining walls, and load-bearing platforms. Although considerably more research into the usefulness of this technique is required, initial results suggest that ETDR sensors, incorporated into the ductwork at the construction phase, may lead to the development of self-monitoring or so-called smart structures.