CONTACTLESS MONITORING OF THE FRONT OF THE PROPAGATION OF CHLORIDES OR CO2 IN THE COATING OF WORKS DEVELOPMENT OF A SYSTEM OF ORPHAN SUSTAINABILITY SENSORS

Abstract

Chlorides and CO2are the two main ions which, havinggone through a concrete coating, result in the depassiva-tion and the corrosion of the reinforcement.Instrumentation by monitoring of potential requires con-tacts or wired systems to record these fronts, sources ofunwanted interfaces, and often leading to a late finding ofthe depassivation of the steels. In addition, the nature ofthe cement does not always correlate reliably potentialfalls and initiation of corrosion of steels.The present system is designed to anticipate the corrosionof steels. It consists to position, during the pouring of thefresh concrete, small isolated steel sheets, of about 100 μmthick and some centimeters diameter, between the surfaceof the concrete and the first front of steels. The sheets areoriented perpendicularly to the diffusion front. When incontact with chlorides or CO2, sheets are oxidized, in thesame manner as a steel reinforcement.The principle is to detect, in the hardened concrete, theoxidation state of the steel sheets. The system uses the ther-mal properties of iron to check its oxidation state. Whenthe metallic iron is subjected to a variable magnetic induc-tion, it creates surfaces currents (Lenz’s law) which lead toa quick and important warm up of the iron sheet (Ohm’slaw). The sheets are therefore subject to heating by induc-tion from the surface. Thermal response of sheets is thenevaluated with an infrared reader since the surface of theconcrete, and tells us about the state of the sheets, oxidizedor not. Indeed, unlike a healthy sheet, oxidized sheet willgive its thermal response with a low amplitude and a rapiddecay. Sheets, placed in rafters to regular spaces betweenthe surface and the reinforcement, and previously locatedin the structure, it is thus possible to determine the positionof the front of diffusion at any given time, and to anticipatebefore any repair than ions chlorides or CO2lead to thedepassivation of the steels.