In Situ Distributed Strain Monitoring of a Small-Scale Reinforced Concrete Structure Using Combined FBG/OFDR Fiber Optics Sensing for Assessment of Its Seismic Degradation Mechanisms

Abstract

An experimental program has been defined by EDF (French electric utility) and CEA with the aim to improve the knowledge of the wall-slab junctions’ behavior, by means of reduced-scale mock-ups made of reinforced concrete (RC), representative of Nuclear Power Plant (NPP) building structural elements. As part of the Tamaris platform (CEA), the Azalee shaking table provides 3D excitation. A wall-slab ¼- scaled mock-up was designed, constructed, equipped with both Optical Fiber Sensors (OFS) and conventional instrumentation (accelerometers, displacement & strain gauges, Digital Image Correlation (DIC)) and vibration-tested. OFS provide in-situ local or distributed strain measurements, complementary to other methods. Gathering in situ distributed strain data is thus of considerable interest since it helps reducing experimental variance and eventually improving modeling accuracy. Fiber Bragg Grating (FBG) and Optical Frequency-Domain Reflectometry (OFDR) sensing techniques are well suited to Structural Health Monitoring (SHM) of RC structures during seismic evaluations. As a proof-of-concept, sensing rebars were embedded within the wall-slab mockup. Artificial ground motion test sequences were applied producing cyclic bending. Wavelength/frequency data were recorded by FBG/OFDR units during several runs with accelerations growing up from 0.18 g up to 2 g. Dynamic strain, natural frequencies, and both local (Bragg) and distributed (OFDR) permanent strains were then compared to both modeling and complementary data. OFS sensing reveals itself as a helpful investigation method for Structural Health Monitoring (SHM) of RC structures, in order to assess for seismic degradation mechanisms. To the authors’ knowledge, this experiment is a world first involving both in situ FBG and OFDR measurements simultaneously in this context.