Static Detection of Thin Layers into Concrete with Ground Penetrating Radar

Abstract

Ground Penetrating Radar (GPR) is a nondestructive technique particularly well adapted to the inspection of concrete structures and can help to determine the structure inner geometry or to detect damaged areas. When the GPR is used on structures containing thin layers, for example the sealing layer of a concrete bridge deck or the void into a masonry wall, it is important for the radar user to know the minimum thickness required to detect and estimate the thickness of those layers.The theory of thin layer detection is based on a sine wave but, in reality, most commercial GPR systems emit a large frequency band wavelet, which undergoes attenuation into the layer. To analyze the influence of those realistic conditions on the reflection coefficient of a thin layer, we combined experimental measurements and numerical FDTD simulations. The experimental results matched the numerical predictions well, presenting a fast attenuation compared to the theoretical predictions. Nevertheless, for thicknesses inferior to λ/11, the reflection coefficient could still be considered as linearly dependent of the thickness to wavelength ratio.

Pathologies of concrete in Saint-Vincent Neo-Byzantine Church and Pauchot reinforced artificial stone

“…This structure has been confirmed by GPR technique [18]: the principle of radar is to issue a high frequency electromagnetic wave in the wall. The wave will propagate into the medium and be reflected at the interface between different environments (eg at the transition between the concrete and vacuum or between concrete and reinforcement).…”

“…These last one may be related to the presence (confirmed by radar [18,19]) of metal pipes discharging rainwater from the central dome. The position of the vertical cracks visible on the front of the pillars coincides with the hyperboles (↑) identified by the radar (Figure 22).…”

Detection of near-field, low permittivity layers with ground penetrating radar: Analytical estimation of the reflection coefficient

“…However, the wave front of the multiple reflections and of the lateral wave can be observed in the layer. For the layers with intermediate thicknesses (0.3 λ -0.5 λ), the observed behavior is intermediate [16]. It is thus expected that the reflection coefficient will have to be described by different equations, depending on the layer thickness.…”

“…3, the reflection coefficients calculated by the spherical equations (5)- (11) and by Fresnel equation (1) are compared to reflection coefficients obtained through 3D FDTD modelling (with the program GprMax [15]), in the case of a concrete (ε' r1 =7.7) -air interface situated at a distance of 10 cm from the investigation line. The numerical reflection coefficient is obtained by dividing the measured amplitude by the amplitude reflected by a perfect reflector [16]. The tests are performed for an incident frequency of 2.3 GHz.…”

“…For each path, the different reflections are calculated using Fresnel equations (1)- (2) [16]. We will refer to this method as the first terms estimation:…”