Microwave propagative imaging for nondestructive evaluation of concrete structures

Resumen

In the course of the 20th century, reinforced concrete has posicionat itself as one of the major building materials. Steel fibre reinforced concrete (SFRC) and conventional steel bar reinforced concrete (SBRC) are two coexistent reinforcing techniques with particular associated threats, such as insufficient fibre mixture or corrosion, respectively. Advanced nondestructive evaluation techniques are required to guarantee the integrity of such structures. In the last decades, microwaves are being considered as a potential tool for the assessment of structures where visual inspection is not possible, owing to their capability to sense and penetrate light-opaque materials with a fair trade-off between penetration and resolution. Based on the good precedents of microwaves in the imaging field, this thesis is devoted to give an insight on microwave nondestructive evaluation techniques for structural monitoring of concrete structures under different operational conditions SFRC, SBRC and concrete itself). Accurate assessment of concrete structures using microwaves depends on reliable measurements of EM waves propagating through them. For this reason, the EM modelling of homogeneous concrete structures is first conducted in the thesis, comprising a comprehensive understanding of the interaction of EM waves with the structures and the consequent analysis of the wave propagation parameters (speed, attenuation, permittivity). Propagative analysis with regular distributed sensors is studied for the modelling and assessment of first, the fibre density in SFRC structures, and second, the electromagnetic signature of corrosion damage in SBRC structures. Both frequency and time domain analysis are performed, and the suitability of using transmission and reflection evaluation geometries is discussed in each case. The harsh and variable nature of the scenarios constitutes an additional challenge: difficulty to manipulate or access to certain parts of the structures, severe environmental conditions, rough surfaces, etc. In this context, new configurations/systems which might be better suited for these environments, based on the use of collaborative embedded antennas, are explored. On the one hand, collaborative embedded antennas are proposed for the quantification of variations on physical parameters of the environment, through remote measurements of their input impedance. On the other hand, the use of hybrid arrangements combining embedded and external antennas is proposed to simplify microwave imaging setups in terms of number and complexity of the antennas, while still providing reasonable reconstructed images of the dielectric properties of the structures. For the practical implementation of the embedded antennas, enhanced RFID sensors using MST are proposed.