Repository of DUTH

Structural health monitoring in existing and retrofitted RC elements/members using smart piezoelectric materials (Doctoral thesis)

Παπαδόπουλος, Νικόλαος/ Papadopoulos, Nikolaos

The effect of time on Reinforced Concrete (RC) structures, seismic excitations, accidental actions, corrosion of steel reinforcement, and explosive failure due to fire are the most important causes of structural failures, which require high repair and maintenance costs. It is now common place in the global scientific community that the agents mentioned above, which occur mainly in existing structures, pose significant risks of sudden collapse, with incalculable consequences and potential casualties. In addition, the absence of targeted seismic considerations in the pre-1985 building provisions is an essential and critical factor in increasing the structural vulnerability of structures and infrastructure. Moreover, the country’s structural wealth aging should be further considered. Considering and combining the above references, it is clear that the systematic supervision and monitoring of the integrity of structures and infrastructures is vital for their proper and timely maintenance, ensuring the intended level of performance and total control of their quality. Structural Health Monitoring (SHM) is an emerging and continuously developing area involving several techniques and represents a reliable solution for assessing structural integrity. In particular, some of these techniques can be applied and even operate in real time. Moreover, the primary objective of performing structural integrity monitoring of structures and infrastructure is to enhance their safety and reliability. In addition, the application of SHM increases their lifetime through targeted repair and reinforcement work. However, despite the remarkable, rapid development that has taken place in the applied technologies and proposed methodologies, SHM’s research faces limitations. The complexity of the structures, the noise introduced in the acquired data, the accuracy of measurements, the reliability of the operation and robustness of the sensors, and the high cost of implementing the SHM methods are some of the most important limitations. The Electro-mechanical Impedance (EMI) technique can be a reliable solution to overcome certain limitations, such as local inspection and the complex geometry of structures. In this Ph.D. thesis, the EMI technique is implemented by exploiting the piezoelectric phenomenon through the installation of piezoelectric sensors in the host structures and the implementation of a Wireless System for SHM applications called “WiAMS” (Wireless Admittance Monitoring System), which was designed by Professor Konstantinos Providakis at the Technical University of Crete. xxxiii WiAMS is a low-cost remote control device that was developed and implemented in a series of experimental tests, which were conducted at the Laboratory of Reinforced Concrete and Seismic Design of Structures of the Democritus University of Thrace, within the framework of the experimental program of this Ph.D. thesis. The experimental project’s primary objective is to evaluate the effectiveness of the proposed ΕΜΙ technique by employing piezoelectric sensors and the WiAMS to monitor RC elements’ structural integrity. Therefore, the efficiency and sensitivity of the method in detecting changes in mechanical impedance and its influencing parameters in RC elements are investigated. In particular, the investigation concerns the study of the influence of the main basic failure mechanisms occurring in existing and retrofitted concrete elements on the EMI measurements, which are implemented using WiAMS and piezoelectric patches mounted on the RC elements under consideration (host structures). Therefore, the performance of the proposed SHM technique on RC beams under dominant shear and under dominant bending was studied. In addition, the performance of the proposed technique in SHM of single-story RC discrete frames without and with cross metal connector reinforcement was investigated. At the same time, the SHM of the metallic link of the cross-bracing was also examined to evaluate the efficiency of the proposed technique. In addition, the technique was evaluated in applications of retrofitted RC beams: a) with a U-shaped jacket of reinforced cement mortar and b) with a Carbon-Fibre Reinforced Polymer (CFRP) rope. In addition, a pilot application of the proposed SHM technique was carried out on a single-span RC road bridge by installing piezoelectric sensors and an autonomous Wireless System of Structural Health Monitoring. The latter creates the conditions for a broader method application, even in inaccessible RC elements or infrastructures. From the results of the experimental program and their analysis, using common statistical indices and emerging machine learning methods, many valuable and promising conclusions for the optimization and redevelopment of the proposed methodology and the reliable application of the technique in large-scale RC elements and real structures are derived. Moreover, within the framework of this Ph.D. thesis, several prototype applications of the WiAMS device were carried out, which create the conditions for further investigation for the transformation of the method from a strictly local, with limited potential for wide application, to a global one, by creating a network of several local SHM foci.