Management and safety of existing concrete structures via optical fiber distributed sensing

The use of Optical Backscatter Reflectometer (OBR) as a distributed fiber optic system to measure strain and detect cracking in concrete structures is investigated. A laboratory experience in a concrete slab is first presented and after this pilot test, a full-scale bridge load test was used to investigate the effectiveness of the OBR technology monitoring system when applied to real structures. The bridge monitoring consisted of the installation of 100m of optic fiber in the soffit of the prefabricated bridge beams. Strain readings obtained from the optic fibers were compared to results from previous identical load tests applied to the bridge, confirming the high accuracy inherent in the optic fiber technology. Subsequently, the cooling tower of a power plant was monitored using the OBR sensor system. A total of 300 meters of cooling tower are monitored and 30,000 points on the concrete surface (inner and outer of shell) are interrogated. The obtained results show the feasibility of this technique despite the roughness of the concrete surface and the heterogeneity due to the presence of aggregates of different sizes. All applications verify that the OBR sensor is not only able to detect appearing cracks that are hardly visible, but it is also able to perform robustly and with high accuracy up to a load level producing a crack width in the range of 1 mm. Moreover, the OBR frequency signal is acquired properly and provides correct strain values without breaks, even at a high load level. Regarding economic impact, deployment of the OBR monitoring system presents a financial advantage over equivalent monitoring techniques (number of points interrogated)