Many water pipes used in distribution networks (cooling system, fire network, drainage…) are constituted of a core welded sheet coated with reinforced concrete. Steel constitutive of these pipes, in particular when in contact with sea water, are subject to corrosion. This corrosion involves reduction of the metallic core sheet thickness, which could affect the water-tightness and the mechanical resistance of the pipe. The knowledge of the residual thickness of this steel sheet is a good indicator of the damage state, remaining life time of this sheet and general water-tightness. Therefore, a common feasibility study between the CEA and EDF was carried out in order to demonstrate the feasibility of a steel sheet thickness measurement using electromagnetic methods. The CIVA modelling software dedicated to Non Destructive Testing was used during this study to understand physics phenomena and to optimise the probe design. This study shows that the use of a low frequency electromagnetic field and a micro-Fluxgate (high sensitivity magnetometer developed by the CEA) allows to measure the thickness of the metallic core sheet covered by a layer of concrete (without reinforcement) with a thickness up to 30 mm. The study also showed that a discrimination between a low thickness sheet and a nominal sheet thickness of 2 mm is possible through a layer of concrete with a thickness up to 40 mm in spite of the steel reinforcement bars.
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