Reliability of UT for the grouting inspection in PT bridges: A case study

The inspection of the condition of prestressing steel in post-tensioned structures is a challenging issue because of the potential presence of corrosion in the tendons not warned by any other visible degradation condition. The economic unfeasibility and technical difficulty of performing a large number of semi-destructive tests (endoscopies) require the identification and practical application feasibility of non-destructive testing techniques to optimize the inspection procedure and enhance its accuracy. Many real-world examples have evidenced that ineffective grouting, or even the lack of it, triggers the corrosion mechanism that, if not detected early, can lead to the brittle failure of the bridge. Therefore, failure of posttensioned structures is normally associated with ineffective grouting. For this reason, it becomes of interest to localize and quantify grouting defects and voids in the tendons. Some non-destructive techniques are currently available; one of these being the Ultrasonic Tomography. This paper presents the results of the combined application of Ultrasonic Tomography and endoscopies in the detection of un-grouted regions in a real bridge. The reliability of the Ultrasonic Tomography results is then obtained through the Probability of Detection (PoD) curves which are checked against the results given by the endoscopic testing campaign.