Ground Penetrating Radar: an Application to Estimate Volumetric Water Content and Reinforced Bar Diameter in Concrete Structures

Non-destructive evaluation of reinforced concrete structures is an increasingly important field in the construction and civil engineering community. A large number of pathologies affecting both the concrete and the reinforced bar inside the concrete are related to the presence of water and its spatial development. In this context, the quantification of both water content and bar diameter is an important part of the diagnosis of concrete. The aim of this paper is to present the results of two important studies undertaken in order to estimate the volumetric water content and bar diameter in concrete structures. The dielectric properties of concrete were studied as a function of volumetric water content. Experimental tests were carried out using an HP impedance analyzer. More than 800 samples of concrete were analyzed and a new relationship between the relative dielectric constant and volumetric water content was obtained. Further experimental tests were carried out by using GPR travel time tomography to study the effectiveness of the new relationship between the relative dielectric constant and volumetric water content. The study was subsequently focused on rebar sizes estimation inside concrete structures. Models were reproduced in the laboratory using a range of rebar sizes from 8 to 22 mm. These models, which represent rebars in a concrete medium, were used to detect rebar's position and study the rebar sizes as a function of the ratio of the cross polarized to normal polarized electromagnetic wave amplitudes. GPR measurements, using the 1 GHz center frequency antenna, were performed on the model reproduced in the laboratory. The results of the GPR measurements were compared with the results of a simulation forward model, proposing an empirical relationship between rebar sizes and the ratio of the cross polarized to normal polarized electromagnetic wave amplitudes. Furthermore, several experimental tests were carried out by using a GPR to study the effectiveness of the proposed relationship.