Non-destructive approach for defect detection in concrete structures by em waves with FDTD techniques

Abstract

In this study; concrete samples are divided into three groups and both simulation and non-destructive measurements are made using the electromagnetic characteristics of concrete and construction materials that can be used in concrete structures. Concrete structures may contain defects due to environmental factors such as humidity and temperature, exposure to sudden energy changes such as earthquakes or wear over time. These defects need to be checked for the service life of the concrete structure. The most basic defects in concrete structures are surface cracks that can be found on the concrete surface and can go down to a certain depth, or internal cracks that cannot be observed from the outside. Cracks do not have a definite shape due to the natural formation process. In our study, rectangular step function and dynamic geometry are used to express surface cracks. By adding the narrowing rectangles one after another towards the inside of the concrete, a deeper crack is expressed, while a relatively shallow crack is expressed by the succession of circles, with the origin of one circle being the tangent point of another circle. The area between the two curves used in the integral definition is used to express the inner crack. The crack in the interior of the concrete is the area between the two curves, namely the Riemann integral region, and this region is assumed to be filled with air. As it is known from the Riemann integral definition, the finer the partition of the area between the two curves is taken, the closer the area is to reality, so the crack modeling closest to reality is made by taking the fine partition of the air-filled area in the concrete. A concrete structure does not only consist of concrete, but also rebar, mortar, pavement and masonry can be found together with concrete. In addition, in environments with very high humidity, composite materials can be used together with concrete in the construction of piles. Considering these situations, modeling of concrete filled composite piles is made together with layered rectangular concrete structures containing mortar, masonry and soil. After determining the geometry of concrete samples, crack shape, content of layered samples and shape of concrete filled composite materials, samples are simulated with the help of FDTD, which is widely used in the simulation of electromagnetic wave propagation. Gaussian waves are used as input signals in simulations because frequency modulation is easier compared to other waveforms. In the numerical simulation, an absorber boundary condition is used to terminate the calculation region and to prevent the waves coming to the boundary from re-entering the calculation area. The results are compared using both transverse electric field and transverse magnetic field waves in the simulations. Two rectangular concrete specimens with surface cracks at different depths, one cylindrical concrete specimen with internal crack, and one cylindrical concrete specimen with rebar and avoids are prepared and dried in air. The prepared samples are measured in an anechoic chamber in the frequency range of 0.4-4.0 GHz using Vivaldi antenna array and vector network analyzer and scattering parameters are obtained. The reflection and transmission coefficients obtained from the scattering parameters and the reflection and transmission coefficients obtained from FDTD calculation are compared and the results are shown in figures.