Structural health monitoring of reinforced concrete beams by means of embedded fiber Bragg grating sensors

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ABSTRACT This study aims to investigate the mechanical behavior of FRCM composite-strengthened concrete beams using embedded FBG sensors. FBG sensors were installed both on th e tensioned surface of the concrete beam and on the PBO mesh woven, that had been applied using cementitious mortar without any epoxy resin. Conventional strain gauges were used to compare results measured from the FBG sensors. Under three-point bending, a marked difference between strains measured in the concrete and those gotten on the reinforcement net was observed. A theoretical model is presented to explain the observed discrepancy. Keywords: Fiber Bragg grating sensor; embedded sensor; concrete structures rehabilitation and retrofitting; strain measurement; flexural strength; fiber reinforced cementitiuos matrix (FRCM). 1. INTRODUCTION The conventional concrete rehabilitating technique by means of beton-plaque has been proven to be an effective strengthening solution in civil engineering applications but it has many disadvantages such as high weight and corrosion due to exposure to harsh environment that may eventually in crease the overall maintenance co st. This has encouraged the research of new materials and technologies that allow for a reduction of the installation costs. The use of fiber reinforced plastics (FRP) for concrete beams strengthening and columns confinement is an example of the results of the research efforts that have been devoted to this important task in the last few years and represents a noteworthy improvement with respect to traditional methods. In this context, the use of embedded optical fiber sensors (FOS) as a real-time health monitoring measurement system is very attractive, since it might provide a powerful method for substantially improving the durability and the safety of the retrofitted structure [1-3]. The use of FOSs in concrete was first suggested by Mendez et al in 1989 [4], while the earliest use of fiber Bragg grating (FBG) sensors as a structural health monitoring device in a real bridge structure was demonstrated some years later [5]. Since then, several research groups have reported on a variety of applications of FOSs embedded in concrete structures to detect strain, vibration, corrosion or cracks development in both laboratory demonstrations [6-10] and field projects [11-13]. FBG sensors have also been successfully embedded in glass fiber/epoxy laminated composites to monitor thermal and residual strain of the host material [14-15]. Recently, Lau et al [16] addressed the mechanical behavior of composite-strengthened concrete beams under three-point bending by embedding FBGs at the interface between th e concrete surface and the bonded composite patch. They found that the strain on the concrete surface wa s always higher than that meas ured in the reinforcement. Conventional FRP systems make use of epoxy resins to glue the woven to the structure, so that they form a single unit conveying stress from the structure to the fibers. But epoxy resins have well known limitations: i) they cannot be applied on dump substrates; ii) they cannot be applied at temperatures of less than 10°C or more than 30°C; iii) finally, they are not fire or heat resistant, because once they have hardened, they return to a viscous/rubbery state, making structural reinforcement ineffective. To overcome these drawbacks, an innovative fiber reinforced cementitiuos matrix (FRCM) system that do not rely on epoxy resins was launched on the Italian market since 2001. This study aims to investigate the mechanical behavior of FRCM-strengthened concrete beams using embedded FBG sensors. FBG sensors were installed both on the tensioned concrete surface and on the PBO net reinforcement.