GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 645 and money, non destructive tests (NDT) can be used successfully to monitor the structures during their life without interfere with their work. The use of the geophysical techniques in the framework of the NDT opens new possibilities for analysing also restoration operations both for reinforced concrete and for masonry structures. Strong potentialities are offered also by the possibilities of the geophysical methods to give the necessary indications for the characterization of not well-known engineering elements that are realized in the recent past in cases where the projects are unknown or totally lost. This is particularly true for the foundation structures (or sometimes unfortunately absent) belonging to building with severe settlement phenomena where it is fundamental their identification and characterization before to intervene with the necessary rehabilitation works (Capozzoli et al. 2019; Rizzo et al. 2019). The paper presents some laboratory tests completed at the Hydrogeosite Laboratory where GPR and ERT are applied for analysing reinforced concrete structures to identify advantages and limits of these techniques for engineering issues. The work analyses the potentialities of a non-conventional approach based on the integrated use of GPR, cross-borehole tomographies (CHERTS) and 3D ERT acquisitions with the simultaneous use of surface and borehole electrodes. The laboratory tests are performed on masonry and foundation structures realized ad hoc with the reinforced concrete technology. ERT and GPR. ERT is a geophysical active method belonging to the resistivity techniques and is based on the introduction of direct current with a couple of electrodes inserted in the ground and the subsequent recording of the potential difference generated in the soil thanks to the passage of the electrical current. The theoretic basis of this well-known technique lies with the Ohm’s law and the target of this method is the localization of the electrical resistivity variations of the subsurface materials induced by the presence of anomalous bodies into the soil. Generally, a switched square wave is the current waveform used (Binley and Kemna, 2005) and an advanced “multimeter” called georesistivimeter is used to implement particular sequence for generating the elecrical. The data acquired are expressed in form of apparent resistivity (ρ) and then they are interpreted in terms of real resistivity and depth by means of inversion software. The aim of the inversion procedure is to compute the ‘best’ set of resistivity values, which satisfies both the measured dataset and some a priori constraints, in order to stabilize the inversion and constrain the final image (deGroot-Hedlin and Constable, 1990). The approach with cross-borehole resistivity imaging provides a great advantage compared to other conventional surface electrical resistivity tomography, due to the high resolution at high depth (obviously depending on the depth of the well instrumented for the acquisition). This method has been shown to provide good information on the distribution of electrical properties of the subsoil at high depths and, in some cases, a detailed assessment of dynamic processes in the subsurface environmen. However, there are two critical issues: the first one is due to borehole electrodes characteristics that usually cause a high data noise levels; the second one is the reciprocal distance of the boreholes that reduce the sensitivity of the analysis. The other geophysical method adopted only for surface acquisitions is the GPR that is an active technique based on the introduction of electromagnetic (EM) waves that, in presence of variation of EM impedance of the investigated medium, is able to identify scattering phenomena occurring in the subsoil. In detail variations of physical properties of the investigated medium as dielectric permittivity (ε, F/m), electrical conductivity (σ, S/m) and magnetic permeability (μ, H/m) are recorded as reflections characterized by a particular amplitude or phase. Variations of physical properties influence strongly the velocity of propagation of e-m waves and energy attenuation. Analyses of variations of velocity are used to create permittivity map according to the fundamental relationship for non-dispersive medium v=c/√ε, where v in the propagating velocity of the material, c in velocity of emwave into the void (2.998 × 10 8  m/s) and ε represents the relative permittivity of the material [3]. Study Cases. Two reinforced structures are studied in laboratory conditions to test the effectivity of the two geophysical techniques. The first one (S-1) is an inverted-T foundation