This paper reports on the status of ongoing collaborative studies between UCSD, University of Bologna and University
of Pittsburgh aimed at developing a monitoring system for prestressing strands in post-tensioned structures based on
guided ultrasonic waves (GUWs) and built-in sensors.
A Semi-Analytical Finite Element (SAFE) method was first used to compute dispersion curves of a pretwisted
waveguide representing a seven-wire strand. The strand embedded in grout and surrounded by a concrete media was
subsequently modeled as an axisymmetric waveguide. The SAFE method allows to account for the material damping and
can be used to discriminate low loss guided modes.
Experimental tests targeted at the defect detection and prestress level monitoring were performed. Notch like defects,
machined in a seven wire strand, were successfully detected using a reflection-based Damage Index (D.I.) vector. The
D.I. vector was extracted from GUWs measurements which were processed using Discrete Wavelet Transform (DWT).
A four dimensional Outlier analysis was performed to discriminate indications of flaws.
In a parallel study, transmission measurements were collected to identify wave features sensitive to prestress level in
strands embedded in post-tensioned concrete blocks. The most sensitive features are being investigated further to assess
their reliability in a monitoring system whit sensors embedded in a real post-tensioned concrete structure.
This paper describes a method based on novelty detection and Discrete Wavelet Transform for damage detection. A statistical discordancy test is used to determine the presence of outliers in a set of features extracted from the Discrete Wavelet Transform of ultrasonic guided-wave signals; the outliers indicate a faulty condition of the structure under investigation. The proposed approach, which is general, is applied to the quantitative detection of notch-like defects in steel strands used as cable-stays or prestressing tendons. The effect of the number of features and the effect of the ultrasonic noise level on the sensitivity of the method to the presence and the size of the defects are discussed.