An integral methodology for the special inspection of concrete bridges with bonded post-tensioned cables

The inspection and the consequent assessment of prestressed concrete bridges with post-tensioned cables is a complex issue. The system was conceived in an era in which there were no doubts about the durability of the concrete. Moreover, avoiding the cracking of the concrete meant the total protection of the high-strength steel cables against corrosion. There was also the additional protection grouting layer. Thus, the first designers did not worry about providing inspection methodologies. When the first sudden collapses occurred, engineers questioned the possible damage mechanisms. Nowadays, a consolidated inspection method is still missing. The research focuses on two major subjects, i.e. how to perform the inspection and how many tests to carry out for a reliable result. Some Non-Destructive Tests (NDTs) were selected and tested both in the laboratory and in situ. The required characteristics must be the easiness of application, the low effort in terms of time and money, and their reliability. The lab campaigns were executed mainly to calibrate the procedures and collect data. Ground Penetrating Radar (GPR) was selected for individuating tendons, while Ultrasonic Tomography (UT) for locating void regions. Even if the absence of grout is not sufficient for the corrosion trigger, the hypothesis of correspondence of void and corrosion is conservative. Seven specimens were realised to state the accuracy of the two techniques. The GPR was reasonably accurate in locating the tendons. It was possible to obtain a Normal distribution with a good fit to the data, with parameters mean of 0.702 cm and a standard deviation of 1.52 cm, describing the error. The UTs’ reliability was assessed by the Probability of Detection model. It resulted that, stated 95% confidence and 90% probability of detection, the system can detect voids longer than 30 cm. The remaining prestress estimation allows to state the losses' entity and indirectly identify damage. The X-ray diffraction technique was selected for the stress estimation on the tendons. During the first laboratory campaign, only wires were tested. The laboratory results showed that the methodology is accurate if the residual stresses are known, and the load level does not influence the results. Given that outcome, another campaign was performed, and it showed that the variations in residual stresses within similar sample groups are not negligible (for wires residual stresses from -30 to -190 MPa, and strands from 120 to 240 MPa). If the residual stresses are considered, a constant relative error in the order of 20% is obtained. The saw-cut method was chosen for estimating the stresses on the concrete. The samples tested in the laboratory were very simple, aiming to assess the accuracy of the procedure. The results were not satisfactory; out of twelve tests, only in one case did the test correctly estimate the actual stress. The in-situ testing campaign was executed on seven similar structures to assess the applicability of the NDTs in real conditions. The research deals also with the issue of the number of samples at different levels of evaluation, based on the procedure proposed by the Federal Highway Administration (FHWA), adapted to the Italian Standards, and to different levels of confidence for more accurate evaluations. Finally, an overall inspection protocol is thus proposed, trying to summarize the main outcomes of the research, by the definition of a multi-phase methodology for the special inspection in the assessment of PT bridges. The proposal follows the same philosophy as the Italian Guidelines, providing more effort (and tests) for the riskiest structures while trying to optimise resources. The last part of the thesis concerns the accurate assessment, i.e. how to obtain and use inspection results for bridges requiring accurate evaluation because the special inspection did not provide a conclusive classification of the actual safety of the bridge.