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Structural health monitoring is an emerging area of strategic engineering with a great potential for extending the service life of civil infrastructures and reducing their maintenance costs. In spite of the deflection measurement being one of the main parameters for assessing the real state of a bridge, the measurement techniques and the existing deflection transducers often do not respond to the necessities of structural monitoring. Therefore, a novel displacement transducer to measure bridge deflections based on noncontact measurement technique supported by the liquid leveling method was developed. The measurement of bridge deflections is performed using a hydrostatic level position along the structure as an absolute reference and without any external physical reference to the ground. Fiber-optic sensors based on fiber Bragg grating technology are the sensing units of this original bridge deflection transducer. The transducer performance obtained in a set of experimental tests has shown an accurate linear response and a very good behavior under the environmental conditions simulated in a climatic chamber. A field application at the Lezíria Bridge across the Tagus River, Portugal, evinces the ability of the present transducer to be applied in both temporary and permanent structural health monitoring systems with automatic and remote data acquisition.

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Long-gauge deformation sensors have opened new possibilities for the health monitoring of civil engineering structures. They are particularly suitable for applications in structures built of inhomogeneous materials, such as concrete, and with complex strain fields, such as bridges, buildings, dams, whenever the global structural behaviour assessment is of the interest. Different technologies and measurement principles have been developed for measuring average strains over measurement bases that can reach tens of meters with resolutions in the micrometer range. In this work, the performances of seven commercially available alternative solutions, based on fibre Bragg-grating, Fabry-Perot interferometry, stimulated Brillouin scattering, low-coherence interferometry and traditional vibrating-wire technology, were tested and directly compared both in laboratory and in field conditions. The results are presented and discussed, aiming at the assessment of the main characteristics of each technology, and taking into account the principal requirements of in-field civil engineering applications. The efficiency of a monitoring method based on long-gauge sensors is illustrated through an application at the Ricciolo Viaduct in Switzerland.