Quantitative strain measurement and crack detection in RC structures using a truly distributed fiber optic sensing system

► We monitor a concrete beam during a mechanical test with a fiber optic system. ► The sensing system has a high strain sensitivity and a centimeter spatial resolution. ► Fiber optic measurements were consistent with conventional sensors and modelling. ► Externally bonded, embedded near and far from the rebars fibers showed same results. ► The sensing system was able to detect cracks earlier than visual inspection. In this study, a truly distributed sensing system was used to monitor the mechanical behavior of a representative-scale reinforced concrete structural element tested in four-point bending. The innovative measuring chain consisted of an OBR interrogation unit (Optical Backscatter Reflectometer, based on relative Rayleigh measurements) paired with fiber optic sensors which were bonded to the surface of the test specimen or embedded within the concrete volume, near and away from the reinforcing steel bars. Such a configuration provides both high strain sensitivity and spatial resolution in the centimeter range. Strain measurements performed on the loaded beam (i) were found to be consistent with both experimental data obtained from conventional vibrating wire gauges and theoretically predicted strain profiles, and (ii) showed no influence of the sensor location on quantitative results (externally bonded sensors, cables embedded near/far from the rebars). Besides, the distributed sensing system under consideration offered the capability to detect and localize cracks, at a much earlier stage than visual inspection.