SAFE modeling of waves for the structural health monitoring of prestressing tendons

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.