ESR11: Development of optical fibre distributed sensing for 
SHM of bridges and large scale structures


The technique of fiber optic sensors has been used in Structural Health Monitoring (SHM) for 30 years. Over these years, the technique has been developed until obtaining measures with accuracy similar to the standard strain gages and extensometers. The use of Optical Backscatter Reflectometry (OBR) is a promising measurement technology for Structural Health Monitoring (SHM) as it offers the possibility of continuous monitoring of strain and temperature along the whole fibre in comparison with local sensor placement (as the Bragg gratings).

OBR differs from other frequency-domain techniques in that it is sensitive enough to measure levels of Rayleigh backscatter in standard single-mode fiber. Several applications to materials used in the aeronautical construction have demonstrated the feasibility of such technique. These materials (composites, steel, aluminium), apart from having a smooth surface where the bonding of the sensor is easily carried out, they also have a continuous strain field when subject to external loading.

The feasibility of using OBR in SHM of civil engineering constructions made of concrete is even of greater interest, mainly because in this type of structures it is impossible to know where the cracks may appear and therefore severe cracking (dangerous for the structure operation) can appear without warning if local measuring sensors are not placed in the particular location where the crack appears.

UPC will work with Crack Ingenieria to overcome some limitations in the application of such type of sensor to concrete structures such as (1) the roughness of the concrete surface and the heterogeneity due to the presence of aggregates of several sizes, and (2) the fact that reinforced concrete cracks at very low level of load, appearing as a discontinuity in the surface and the strain field that may provoke a break or debonding of the optical fiber. This technology has the ability to test a structure over extended gauge lengths.

The objective of the project is to study the spatial resolution and strain accuracy obtained with optical distributed fibre when applied to concrete elements, and to see the potentiality of detecting crack or abnormal deflections without failure or debonding. Therefore, the most suitable bonding adhesives as well as the technique of attachment of the fibre to the concrete will be investigated. The long-term reliability of the sensor measurements will be investigated too. Finally, criteria for the design and deployment of a permanent monitoring system based on these sensors will be presented.
Criteria for the design and deployment of a permanent monitoring system based on the use of Distributed Optical Fibers and Optical Backscatter Reflectometer for SHM.
This project involves a secondment of some months to COTCA, S.A. (former Crack Ingenieria) supervised by Dr. Sergi Villalba. The ESR will learn from COTCA’s practical experience on SHM and instrumentation, and in particular, on the use of the Optical Distributed Sensor Interrogator technology based on swept-wavelength coherent interferometry to measure temperature and strain using optical fiber.
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