The presence of aggressive agents (such as chloride ions brought by seawater) in reinforced concrete structures is responsible for the corrosion of the steel rebars. A Structural Health Monitoring technology is developed as a new passive preventive method that would allow for the detection of and for the ability to follow the presence of chloride ions in the cover concrete of reinforced concrete. This technology, referenced as Functional Magnetic Material (FMM), consists on the measurement with an external interrogator of a Magnetic Observable (MO), partially shielded by a patch and corrodible by chloride ions. This paper presents the results of a parametric experimental study, allowing the validation of the concept of this technology, by highlighting the variation of the MO while considering the geometry and the corrosion level of the patch (based on its Relative Mass Loss—RML), as well as the distance between the samples and the interrogator. The results show that the MO of the FMM significantly varies with the increase in the RML of the patch. A 10%-RML for the patch is sufficient for detecting a variation of the MO of the FMM, and the relative variations of the MO are strongly dependent on the distance between the FMM and the magnetometer, as well as the patch’s thickness.

Rebar corrosion represents a major threat to the durability of reinforced concrete structures, primarily in marine environments. Various Non-Destructive Evaluations (NDE) have been developed to detect rebar corrosion; although most of these have delivered successful results, a lack of reliable techniques for proper corrosion prognosis still remains. Under the French Research Agency (ANR) project’s “LabCom OHMIGOD” framework, we introduce here a novel embedded tool to evaluate the level of cover concrete contamination from aggressive agents responsible for causing corrosion. This tool is divided into two parts: a reactive part exposed to corrosion, and a permanent part protected against corrosion. Using magnetic materials in both parts entails “Functional Magnetic Materials” (FMM) and generates a Magnetic Observable (MO). Through the evolution of corrosion on the reactive part, its magnetic properties become affected, which in turn modifies the MO. By means of regular monitoring of MO variations, it is possible to evaluate the aggressive agent ingress. Consequently, by using a variety of FMM tools placed at different concrete depths, it is possible to indirectly evaluate the rebar corrosion risk. This paper presents a numerical model of the tool employing Ansys software. The underlying objective is to investigate tool accuracy through its key parameters, namely, geometry, relative distance to the receiver, coupling effect, and border effect from the rebar. Simulation results demonstrate that by choosing an efficient geometry for the reactive part (25 mm × 25 mm × 1 mm) and position for the tool (between 1 and 3 mm), both a sufficient MO variation range and a negligible coupling effect can be obtained when the FMM is more than 5 cm from any ferromagnetic material.

Dans les environnements marins, la durabilité des structures en béton armé est générale- ment dégradée par la corrosion due à la pénétra- tion d’agents agressifs. Cette thèse vise à proposer une nouvelle méthode de SHM magnétique, utili- sant un dispositif intégré dans le béton de couver- ture, comme solution technologique pour surveiller la pénétration des agents agressifs. Ce nouveau dispositif est appelé Functional Magnetic Materials (FMM) car il est composé de matériaux magné- tiques et ferromagnétiques. Il propose un dispositif autonome et peu coûteux qui génère des obser- vables magnétiques (OM) passives mesurables à distance avec des interrogateurs externes. En rai- son de l’évolution de la corrosion de la partie ré- active du FMM par le front d’ions chlorure, l’OM générée est modifiée en conséquence. Ceci per- met de corréler l’état de corrosion du FMM en fonc- tion des variations de l’OM et d’estimer également la probabilité de contamination du béton. L’objec- tif principal de cette thèse est de développer une méthode de traitement du signal, basée sur l’es- timation de la variation du moment dipolaire ma- gnétique en fonction de la variation de l’OM, à tra- vers un modèle analytique de Biot Savart et avec une méthode d’inversion. Pour atteindre cet objec- tif, par modélisation numérique du FMM, la plage de variation de l’OM et la tendance ont été éva- luées comme modèle de référence. Ensuite, via la solution analytique, la quantité de moments dipo- laires magnétiques a été estimée en fonction des variations de l’OM. Enfin, la méthode proposée a été étudiée expérimentalement, en utilisant une va- riété de dalles en béton et différents états de cor- rosion pour la partie réactive. Les résultats obte- nus montrent que la méthode proposée apparait comme une nouvelle solution de pronostic ND ap- pliquée à la corrosion.

Limestone is selected as test material to study the influence of corrosion on dielectric relaxations because of its high porosity (40-50%) that quickens the observation of the corrosion diffusion. The corrosion process is launched by an iron plate on the surface of the limestone imbibed in the brine. The complex dielectric properties from 50 MHz to 1.5 GHz have been measured since most ground penetrating radar (GPR) applications cover this frequency band. To analyse the dielectric relaxation associated with the corrosion products (mainly Fe rust, mixture of hematite and magnetite), the dielectric modulus has been used as a tool to suppress the influence of an apparent direct current conductivity ( $\sigma_{dc}$ ) brought by the imbibition in the brine. By finding out the feature relaxation peak of the Fe rust at a certain frequency, the content of Fe rust in the limestone can be estimated by the application of the complex refractive index model (CRIM). We hope that this study on the dielectric relaxation of Fe rust in limestone under saline environment can serve as a model which can eventually adapt for concrete structures in our future works.