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Long term in-situ test of a low-cost fiber-based crack monitoring system

In this paper we present a low cost crack monitoring system and the results obtained during a more than one-year in-situ test in a prestigious monument in Venice. The system uses displacement sensors based on the variation of the transmitted power through plastic optical fiber links with the displacement, a solution that has been devised to enable continuous monitoring with performance similar to those offered by glass fiber sensors, but with a remarkably lower cost. Several transducers with different embodiments have been fabricated and tested; a modular unit able to control up to seven optical transducers plus the reference, and humidity and temperature sensors has been developed too. This unit can be interfaced wireless with other neighboring units and send periodical reports to a remote control center via the GPRS network. Long term lab tests have evidenced the necessity of a reference transducer to compensate for the effect of temperature in order to achieve a very good sensitivity. In-situ tests have confirmed the lab experiments and have allowed the determination of the crack width evolution due to day-night, summer-winter thermal excursion.