Ultrasonic evaluation of a steel-concrete structure using embedded Fiber Bragg Gratings (FBG) and piezoelectric sensors
Résumé
Concrete is one of the most widely used materials for civil engineering structures. The casting process is particularly well suited to the deployment of buried sensors able to sample a ultrasonic wave field in three dimensions, a valuable piece of information to evaluate the internal properties of a structure, its health state and its evolution over time. Fiber Bragg Gratings (FBGs) sensors can be used as embedded acoustic receivers to acquire such point-wise ultrasonic data. The technical advantages of this sensing technology are numerous: the reduced size of the optical fiber offers a very small footprint on the studied structure, it can be used over a very broad frequency range with very small phase or amplitude distortions, FBGs are highly resistant to harsh environments such as high temperatures, salinity or radioactive environments, thus increasing the longevity of the embedded sensors. The interrogation of FBG requires dedicated state-of-the-art opto-electronics that must be adapted to the imaging problem (bandwidth, size of the heterogeneities/aggregates, nature of the source). In this study, we present a Steel-Concrete wall model equipped with several embedded optical fibers as well as several embedded and surface piezoelectric (PZT) sensors. We compare the performances of the different embedded sensors for several sources of ultrasonic waves applied on the surface of the wall for frequencies ranging from a few tens to a few hundreds of kHz. The analysis of the signals reveal that a significant part of the energy is transmitted to the concrete through the surface liner. Using a collection of random hammer impacts applied on the surface, we evaluate our ability to reconstruct empirical Green’s functions between pairs of sensors buried into the structure using passive imaging methods. These measurements will provide useful information on the internal structure of a steel-concrete model and its temporal evolution.
