Pioneering work by Prof. Charles Dowding at Northwestern on TDR cable shearing and detection of water have unique application to monitoring two significant bridge performance parameters, foundation scour and cable moisture and degradation. Both of these applications take advantage of the unique geometry of pulsing a long cable and analyzing the reflected signature, which is easily digitized and transmitted telemetrically. TDR's EM reflections are produced by changes in the capacitance of a coaxial cable. These reflections can be produced by severing, distortion of the cross section, or changes in the conductance of the medium between or surrounding two conductors, be they coaxial or parallel. Since the electronics are above ground or at the end of the cables they can be replaced or repaired easily. The sensor is the cable that either exists or can be tailored to meet specific sensing needs. For instance, the entire cable can be probed from one end and/or large soil and rock volumes can be monitored with a single cable.
One of the objectives of the proposed research is the development of a key element in the TDR system, the pulsing card, to allow operation in a low power consumption environment and dispersed deployment geometry. Development of this key element will advance measurement of bridge performance parameters as well as several others because all of these approaches employ the same pulsing instrument.
While field placement of TDR instruments is envisioned to be either centralized or dispersed, equipment exists now only for placement at relatively centralized and protected sites. These sites must provide a protected environment for the TDR pulser, a relatively bulky and separate instrument, a multiplexer, a telemetric device (radio or modem based) and a digital processor or computer. These multi component systems are adequate for proof of concept but are not sufficiently robust or power independent to allow dispersed deployment in remote and hostile environments typically associated with infrastructure monitoring.
The TDR research embodies three basic activities: 1) development of a cable installation procedure for scour detection, 2) initial testing of signal transmission through twisted, multi strand cables, and 3) fabrication of a miniaturized, low power consumption pulsing card for remote operation.
The U.S. Bureau of Mines (USBM) and the U.S. Army Corps of Engineers (USACE) will cooperate in the funding of the development of TDR technology in general and a miniaturized TDR pulsing card for remote operation. At present USBM has engaged Northwestern in a 5 year contract to develop water pressure and rock mass deformation technology and multiplexing and data analysis software. This contract will form the backbone for this initiative because of the obvious stability.
The broad applicability has sparked the interest of such a broad range of agencies. Its development would be well received by the infrastructure monitoring community. The U.S. Army Corps of Engineers (USACE) has funded this component through the Construction Productivity Advancement Program (CPAR).
Transfer to the construction industry of the TDR technology will be accomplished through three principal mechanisms: 1) a technology transfer symposium, 2) three field demonstration projects sponsored by owner operators, and 3) the marketing of instrumentation manufacturers.
The field demonstration projects with owner operators should supply information needed to allow these agencies to more aggressively design for the use of the innovative TDR technology. The marketing of TDR technology which will naturally follow should need no additional financial support from the ITI.
SCOUR DETECTION
For additional information on scour detection, please refer to the paper which Prof. Dowding has prepared on this subject.
BRIDGE PIER CRACK DETECTION
Also during the first year, an investigation will be undertaken to determine the usefulness of TDR techniques for 1) flaws in long steel cables under tension and 2) changes in moisture in cable sheaths. These tests will be conducted in the laboratory with available equipment and are of a preliminary and exploratory nature.
TDR PULSING CARD DEVELOPMENT
Please consult the project description for the TDR pulser for monitoring infrastructure facilities.
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