NON LINEAR STRAIN MEASURES ON CONCRETE STRUCTURES BY MEANS OF EMBEDDED OPTICAL FIBER SENSORS

Optical fiber sensors have become a widely used tool to perform life-long monitoring of large structures. Different set-ups of sensors can be used to obtain information at different scopes. In this paper, the application of a portable Brillouin optical time domain analysis sensor applied to reinforced concrete beams is presented. The sensor is made up of a single-mode optical fiber applied to the structure that enables the measurement of tensile and compressive axial strain along the beams. The fiber is embedded into a concrete casting without particular installation care and fixed along the reinforcement bars so that a complete correlation can be obtained regarding axial strain and displacements at different heights of the beam in linear and non-linear bending response. The use of four sensing fibers positioned along the beam's length at different vertical positions makes it possible to read the strain variation within the cross-section. In accordance with Bernoulli's hypothesis, this allows curvature diagrams to be obtained across the structure. The measured curvature is used to set up the moment–curvature relationship, which highlights the possibility of the sensor monitoring structures continuously in space and time. From the experiment the theoretical and experimental results are compared and it will be seen how they can actually be reliable in order to perform a monitoring on civil engineering structures. This allows to validate the response of a distributed fiber optic sensor and to promote its use for structural health monitoring (SHM).