Problems associated with the corrosion and failure of prestressing wire and strand inexisting concrete structures have historically been difficult to evaluate and manage. Unbonded posttensionedtendons, in particular, have experienced numerous corrosion incidents primarily due topreviously unrecognized inadequacies in corrosion protection systems, improper installation, orenvironmental exposure before, during and after construction. Conventional destructive methods ofevaluating can be costly and often inconclusive due to the selective nature of sampling. These methodsinclude such procedures as making small excavations into the concrete to provide inspection windows tothe prestressing steel, or de-tensioning and removal of select prestressing tendons.A non-destructive system of continuous acoustic monitoring of prestressed concrete structures has beendeveloped and successfully used. The system provides data on newly-occurring wire or strand failure andcan be used to determine the rate of failure over a finite period of time. The system can identifyprestressing steel failure at localized areas of active corrosion in a structure so that further investigationand remedial measures can be cost-effectively planned. The system is based on a similar principle to thatused in seismic monitoring. An array of passive acoustic sensors are distributed on a structure andconnected to a data acquisition system. The system detects and evaluates all acoustic activity occurringon the structure. Using hardware and software filtering techniques, acoustic data from events of interest,including the acoustic energy released when a tensioned wire fails, are acquired and transmitted to acentral data processing facility over the Internet. Proprietary software is used to analyse the data and todetermine the time and location of any wire breaks that may have occurred during the monitoring period.This information is made available to engineers and owners through a password-protected website.Statistical modelling techniques permit forecasting of future failure trends, thus providing the owner andengineer with valuable information to assist in structural and economic decision making.The system has been successfully deployed on post-tensioned buildings and parking structures in Canadaand the United States since 1994. Over 500,000 square metres of post-tensioned flat slab is currentlybeing monitored with the technology, and a number of repair projects have been undertaken using datagenerated by the system.
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