Modeling of ultrasonic wave propagation in concrete to improve the diagnosis of civil engineering structures

Monitoring of civil engineering structures is a major challenge for building engineers, maintainers and owners. Ultrasonic non-destructive testing (NDT) is used to characterize concrete, without degrading it, because of its relationship to its mechanical properties and composition. Such ultrasonic testing is therefore suitable for the control and in-situ monitoring of structures. However, since the wavelengths used are of the same order of magnitude as the sizes of the constitutive elements of the concrete, the measured signals result from successive diffusions and thus from numerous waves resulting from multiple scattering. These are therefore complex to analyze. In order to optimize ultrasonic techniques, it is thus necessary to better understand the wave-material interactions in this type of medium and to better model the associated phenomena. Analytical homogenization models are often used to simulate the coherent part of waves propagating in a medium. But these models are often limited to the presence of heterogeneities at low concentration. In order to go beyond these limits, in this thesis a two-dimensional numerical model describing the propagation of ultrasonic waves in a heterogeneous medium, adapted to concrete, is built in the SPECFEM2D software package, which based on the spectral-element numerical method. This model is compared to analytical models, and validated experimentally using a synthetic medium with high heterogeneity by comparing the two effective parameters of coherent waves: phase velocity and attenuation. This numerical model also makes it possible to take into account the viscoelasticity of the mortar by means of a quality factor. This quality factor is determined from measurements made for a series of mortars that we study. The complete set of numerical tools developed in this work can be used for several purposes: firstly, to carry out studies to evaluate the influence of certain parameters on wave propagation, such as the shape, orientation and distribution of aggregates, and secondly, the simulation of the measurement configurations implemented for a structure in order to optimize them in terms of the parameters involved, in particular the frequency of the propagating waves. This better control of the measures will ultimately lead to better diagnosis. In future work, the development of this numerical modeling tool will have to be continued to better take into account the interfacial transition zone (ITZ) that exists between the aggregates and the mortar (even if in this thesis a first study of this zone is proposed), as well as damage and cracking in concrete. (author)