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In many concrete bridges, deformations due to static and dynamic loading are the most relevant parameters to be monitored in both the short and long term. Strain monitoring give only local information about the material behavior and too many of such sensors are therefore necessary to gain a complete understanding of the bridge evolution. Recent advances in measurement technology have demonstrated that optical fiber sensors are suitable for monitoring full-scale structures. A network of such sensors installed inside a bridge enables the measurement of parameters such as internal deformation and temperature. In the past four years, our laboratory has installed hundreds of fiber optic deformation sensors of varying sizes in concrete and composite steel-concrete bridges. These sensors give useful information during the construction phases and about the long-term geometrical deformations of a bridge under static load. Recently it has been found that these sensors can also be used to measure the quasi-static part of the dynamic deformation of a bridge under traffic load. The measurement technique relies on low-coherence interferometry and it guarantees high resolution, high precision and long-term stability. Nevertheless each measurement takes a few seconds, thus it is inadequate for monitoring the dynamic behavior of bridges, where measurement frequencies of up to 1 kFIz are necessary. This paper also presents a new technique to demodulate at a high frequency the signal from the same sensors used for long-term monitoring. With its large dynamic range of several mm it allows for the monitoring of bridges under dynamic loads whilst maintaining the configuration of the original fiber optic sensor that has been proven to be reliable.The security of bridges requires periodic monitoring, maintenance and restoration. Excessive and non-stabilized deformations are often observed and although they rarely affect the global structural security, they can lead to serviceability deficiencies. Furthermore, accurate knowledge of the behavior of bridges is becoming more important as new structures become lighter and as and increasing number of existing bridges are required to remain in service beyond their theoretical service lives. Monitoring, both in the long and short term, helps to increase the knowledge of the real behavior of the bridge and in the planning of maintenance intervention.In the long term, the static monitoring needs of an accurate and very stable system able to relate deformation measurements often spaced over long periods of time. On the other side, dynamic analysis of bridges, or short term monitoring, requires of a system capable of measuring deformations occurring over relatively short periods of time. Currently available monitoring transducers, such as inductive and mechanical extensometers, GPS, microbending sensors or accelerometers are only suitable for performing measurements in a short range of frequencies. Moreover, some of these techniques are still in the development stage and only used in labora...

Abstract.A novel demodulation technique for performing dynamic deformation measurements using a path-unbalanced Michelson interferometer is reported. The method is based on the rf amplitude modulation of a low-coherence source, and demodulation is achieved by tracking in the frequency domain the position of the minimum of the detected intensity. This technique is particularly suitable for deformation measurements in civil engineering structures where deformations of the order of few millimeters over the sensor length are expected. The method features a measurement range of at least 10 mm, sensitivity better than 10 m, and dynamic deformation measurements with a bandwidth up to 100 Hz.