GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.2 ___ GNGTS 2023 For each survey with the georadar 30 radargrams of 500 mm length are acquired; the trace increment set is 1.250 mm (8 trace/cm), and the sampling is 512. Each radargram contains a total of 400 A-scan traces. After each georadar survey, Self-Potential (SP) acquisition was carried out on the sample. The SP signals is based on the electrical and electrolytic continuity between rebar in concrete, reference electrode on the sample surface and a multivoltmeter. The SP was acquired by a fixed cable to the rebar and a non-polarizable calomel electrode as reference. The measurements were carried out by covering the entire surface of the sample according to a predetermined mesh of 54 points. The non-polarizable electrode was connected to the multivoltmeter, while the positive pole was connected to the armature bar. At each movement of the electrode, the measurement of the spontaneous potential in (mV) was carried out. The procedure adopted for the acquisition of spontaneous potential is that indicated in current legislation UNI 10174:2020. The experiments were performed at Applied Geophysical laboratory of University of Ferrara, where reinforced concrete samples of about 50 cm x 30 cm were cast, with a central ribbed steel rebar of 10 mm diameter and 40 cm long. The steel rebar was partially painted with a waterproof resin in order to leave only the central part uncovered for a length of 8 cm, in this way the localized corrosion interested only the exposed portion of the rebar. On the edges of the concrete sample and on the lower surface of the sample, the same waterproof epoxy resin was applied leaving an exposed area of 8 cm2 in correspondence of the uncovered reinforcing bar. In this way chlorides diffusion occurred only from the lower exposed surface of the sample, and we avoid edge phenomena which disturbed the previous investigations (Fornasari et al., 2022). Tests were carried out simultaneously on two identically constructed reinforced concrete samples, exposed to distilled water (sample “A”) and the second, exposed to a solution contaminated with chlorides (sample “B”). Both samples were partially immersed for only 1 cm form the lower surface. Sample "A" was taken as a reference as it represents an initial passive condition for the rebar. Tests were carried out on sample "B" under various chlorides concentrations. An initial NaCl concentration of 0.1 % was adopted for 7 days, then the concentration was increased to 1% and finally to 3.5% for further 7 days. The 2GHz C-Thrue GPR antenna by IDS and Self-Potential with non-polarized calomel referenced electrode acquired by multivoltmeter, were used for rebar corrosion monitoring. Simultaneously, investigations were carried out on the sample to determine the Ecorr and the Rp. The collected data were used for an integration observation to define the evolution of the phenomenon of corrosion on the reinforcement steel rebar. During natural corrosion exposure the corrosion potential (Ecorr) values were measured by an external saturated calomel electrode (SCE) and polarization resistance (Rp) values were determined potentiostatically by applying an anodic polarization of +10 mV vs. Ecorr for 300 s and by dividing this anodic polarization by the stable anodic current finally measured. The accelerated corrosion was designed in order to reach different theoretical levels of mass weight loss in the steel rebar, which were of 2%, 5%, 10% and 20%.