DEVELOPMENT OF NEW WIRELESS SENSORS AND METHODS FOR CIVIL ENGINEERING APPLICATIONS

In the proposed research, effective and innovative monitoring solutions inspired by the principles of low cost, miniaturization, system energy autonomy, and measurement reliability, for remote and widespread monitoring of historical and civil structures in general, have been developed and tested. The advantages of new wireless measuring methods over traditional wired technologies and devices have been investigated. New methods and devices have been proposed for different monitoring purposes. These methods and devices have been developed and tested through experimental campaigns, in laboratory and in situ, and have been validated with wired traditional techniques for comparison. The new measuring principle of a new RFID-enabled wireless strain gauge has been tested in static and dynamic conditions, respectively by measuring the elastic modulus of three different materials and by measuring induced environmental vibrations on a steel cantilever. The accuracy of the proposed new technique has been proved to be high, reaching a maximum interrogating distance of 20 meters in laboratory and outdoor, very suitable for structural monitoring. The use of commercial UHF-RFID tags for civil engineering purposes has been investigated, specifically for the monitoring of out-of-plane displacements, representing an application novelty since commercial tags are usually used in logistics and other purposes. The feasibility of the application of this technique was assessed by laboratory and in situ experimental campaigns. The response of the Tags in laboratory environment demonstrated to be very satisfactory, proving that the application of these wireless RFID tags is feasible, potentially very reliable. In situ experiments showed a weaker response of the Tags due to environmental interference caused by the high presence of metal which affected negatively the transmission of the electromagnetic signal, and consequently the indirect measurements of displacements. Despite some limits, the application is promising and opens up new scenarios for the design of new wireless tags suitable to meet the required needs. A new wireless RFID sensor for crack width monitoring has been developed and tested in laboratory through a three-point bending tests on different materials. Compared to traditional crack-width measurement procedures, the proposed new technique results to be potentially more suitable, with an interrogation distance up to 1.5 m in this raw state, which could be extended more to allow the positioning of these devices in points difficult to access by traditional wired sensors. The devices are currently in a raw form and require technological development in order to be applied on large scale. Some critical aspects related to the strong influence of metal can be overcome in future steps of design of the sensors and further investigation is surely required. In this thesis a large-scale approach to wireless structural monitoring has been also investigated. A sensor network with commercial wireless tri-axial MEMS accelerometers was deployed on the deck of a butterfly-arch stress-ribbon pedestrian bridge located in Fuzhou, Fujian, China, to record the structural response under ambient vibration and perform dynamic identification, finite element modelling and parametric updating. A satisfactory agreement between the model prediction and the experimental data was achieved. The most accurate model has been chosen to be used as baseline for long-term monitoring of the bridge. In the SHM field, this last study demonstrates the importance of the modelling strategy in simulating the dynamic behaviour of complex structures, as the one considered in this research, in comparison to traditional ones which can be modelled following simplified modelling procedures.