New optical fibre coating meant for embedding distributed sensors into concrete

This paper reports on the design and realization of a new optical fibre sensor body meant for embedding distributed optical fibre sensors into concrete. We have carried out a finite element analysis which shows that the proposed wave-like sensor enables continuous bonding between optical fibre and concrete, symmetrical response in traction and compression whatever the contact conditions may be. The stiffness of the optical fibre can so be adapted to the concrete, which reduces strain concentrations and need for a theoretical calibration factor. Experimental validations are carried out, showing that despite its wave-shape, the proposed sensor body does not introduce any losses or strain that would result from fibre macrobendings. The process of realization is very cheap, thus dedicated to distributed sensors. We have realized optical fibre extensometers based on this technique and on low-coherence interferometry method for the measurements, including an original approach of thermal effect compensation. Preliminary results are very encouraging.