Quasi-distributed optical fibre extensometers for continuous embedding into concrete: design and realization

This paper reports on the design and realization of a new optical fibre sensor for continuous measurement of concrete strain over very long distances. We propose a composite-made wave-like sensor body that enables continuous bonding between optical fibre and concrete. Finite element analysis shows that the stiffness of the optical fibre can be adapted to that of the concrete, reducing the strain concentrations and the need for a theoretical calibration factor. Moreover, unlike the body of traditional I-shaped sensors, a wave-like sensor body enables a symmetrical response in tensile and compressive loadings whatever the contact conditions may be. We have realized optical fibre extensometers based on this technique and on a low-coherence interferometry method for the measurements. Despite its wave-shape, the proposed sensor body does not introduce any losses or strain that would result from fibre microbending. The process of realization is adapted to distributed sensors and sensors with a very long gauge length. Furthermore, several sensors were multiplexed, demonstrating the ability to perform quasi-distributed measurements. Once embedded into concrete cylinders, the optical fibre sensors were tested under compressive and tensile loading, and compared with traditional sensors. The allocation of one of the multiplexed sensors as a dummy gauge provides self-compensation for thermal effects and ambient fluctuations. As a result, and without any calibration factor, optical sensor measurements perfectly agreed with external linear variable differential transducer measurements.