In this paper, a method to obtain crack initiation, location and width in concrete structures subjected to bending and instrumented with an OBR system (Optical Backscattered Reflectometer) is proposed. Continuous strain data with high spatial resolution and accuracy are the main advantages of the OBR system. These characteristics make this Structural Health Monitoring (SHM) technique a useful tool in early damage detection in important structural problems. In the specific case of reinforced concrete structures, which exhibit cracks even in-service loading, the possibility to obtain strain data with high spatial resolution is a main issue. In this way, this information is of paramount importance concerning the durability and long performance and management of concrete structures.The proposed method is based on the results of a test up to failure carried out on a reinforced concrete slab. Using test data and different crack modelling criteria in concrete structures, simple non-linear finite element models were elaborated to validate its use in the localization and appraisal of the crack width in the testing slab.

The versatility and easy installation of Distributed Optical Fiber Sensors (DOFS) compared with traditional monitoring systems is an important characteristic to consider when facing the Structural Health Monitoring (SHM) of real world structures. The DOFS used in this study provide continuous (in space) strain data along the optical fiber with high spatial resolution. The main issues and results of two different existing structures monitored with DOFS, are described in this paper. The main SHM results of the rehabilitation of an historical building used as hospital and the enlargement of a prestressed concrete bridge are presented. The results are obtained using a novel DOFS based on an Optical Backscattered Reflectometry (OBR) technique. The application of the optical fiber monitoring system to two different materials (masonry and concrete) provides also important insights on the great possibilities of this technique when monitoring existing structures. In fact, the influence in strain transfer between the DOFS and the bonding surface is one of the principal effects that should be considered in the application of the OBR technique to real structures. Also, and because structural surfaces generally present considerable roughness, the procedure to attach the optical fiber to the two monitored structures is described.

Abstract. This paper provides an overview of the use of different Distributed Optical Fiber Sensor systems (DOFSs) to perform Structural Health Monitoring (SHM) in the specific case of civil engineering structures. Nowadays, there are several methods available for extracting distributed measurements from optical fiber, and their use have to be according with the aims of the SHM performance. The continuous-in-space data is the common advantage of the different DOFSs over other conventional health monitoring systems and, depending on the particular characteristics of each DOFS, a global and/or local health structural evaluation is possible with different accuracy. Firstly, the fundamentals of different DOFSs and their principal advantages and disadvantages are presented. Then, laboratory and field tests using different DOFSs systems to measure strain in structural elements and civil structures are presented and discussed. Finally, based on the current applications, conclusions and future trends of DOFSs in SHM in civil structures are proposed.

This paper outlines the second part of an experimental study to show the capabilities of distributed optical fiber sensors (DOFS) in their application to the structural health monitoring (SHM) of the shear performance of concrete structures. SHM seeks to obtain the shear crack characteristics of concrete elements: detection, localization and quantification of shear damage. The two first were discussed in the first part of the experimental study. In the present paper, the quantification is dealt with by proposing a method to obtain the mean shear crack width in concrete beams. The method is based on the experimental data obtained by a DOFS bonded to the concrete surface. First, the basis of the methodology are presented and, later on, experimentally checked by testing of three partially pre-stressed concrete (PPC) beams subjected to a shear test with increasing level of load. The DOFS were deployed in the web of the beams to conform a 2D grid mesh to measure the strain profile along two orthogonal directions. The experimental data was obtained using an OBR (Optical Backscattered Reflectometer) system with high spatial resolution and sensitivity that allow a complete mapping of the cracking pattern and to obtain the required data for the calculation of the crack width. The results show the feasibility of the proposed method in calculating the shear crack width when compared to the results from traditional instrumentation.

For the first time, a gold coated single mode optical fiber has been used to detect a liquid sodium leakage on a pipe of secondary circuit pipe mock-up of nuclear fast reactor (Gen IV) by means of Optical Frequency Domain Reflectometrybased on Rayleigh backscattering. During 150 min of the experiment we were able to detect and monitor the evolution of a liquid sodium leakage on the surface of the pipe.

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An experimental methodology to obtain the shear cracking pattern in concrete elements is presented. The method is based on the use of Distributed Optical Fiber Sensors (DOFS) connected to an Optical Backscattered Reflectometer (OBR). Using this OBR system and a 2D grid conformed by one or two DOFS, the crack patterns of three partially pre-stressed concrete (PPC) beams subjected experimentally to shear failure, were obtained for increasing level of load. The 2D distributed fiber optic sensoring mesh was formed by attaching the fiber to the shear span of each beam using an epoxy adhesive. The importance of a correct DOFS attaching procedure to the concrete surface to obtain accurate results is described, and the principal advantages of DOFS to complement the use of discrete sensors in concrete experimental shear tests are shown. The proposed technique is a powerful tool to be implemented in the structural health monitoring in shear of concrete structures, where the variable inclined cracks are difficult to monitor by other experimental techniques using discrete sensors.

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