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Currently 90% of bridges built in California are post-tensioned box-girder. In such structures the steel tendons are
the main load-carrying components. The loss of prestress, as well as the presence of defects or the tendon breakage, can
be catastrophic for the entire structure. Unfortunately, today there is no well-established method for the monitoring of
prestressing (PS) tendons that can provide simultaneous information related to the presence of defects and the level of
prestress in a continuous, real time manner. If such a monitoring system were available, considerable savings would be
achieved in bridge maintenance since repairs would be implemented in a timely manner without traffic disruptions. This
paper presents a health monitoring system for PS tendons in post-tensioned structures of interest to Caltrans. Such a
system uses ultrasonic guided waves and embedded sensors to provide simultaneously and in real time, (a)
measurements of the level of applied prestress, and (b) defect detection at early grow stages. The proposed PS
measurement technique exploits the sensitivity of ultrasonic waves to the inter-wire contact developing in a multi-wire
strand as a function of prestress level. In particular the nonlinear ultrasonic behavior of the tendon under changing levels
of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation
at (ω). Moreover this paper also present real-time damage detection and location in post-tensioned bridge joints using
Acoustic Emission techniques. Experimental tests on large-scale single-tendon PT joint specimens, subjected to multiple
load cycles, will be presented to validate the monitoring of PS loads (through nonlinear ultrasonic probing) and the
monitoring of damage progression and location (through acoustic emission techniques). Issues and potential for the use
of such techniques to monitor post-tensioned bridges in the field will be discussed.
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