PSI - Issue 11

Esequiel Mesquita et al. / Procedia Structural Integrity 11 (2018) 138–144 Author name / Structural Integrity Procedia 00 (2018) 000–000

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The optical signal reflected by the FBG is sensitive to physical parameters, as deformation and temperature changes. The dependence of the λ B with these two parameters is given by Equation (2): ∆∆λ BT = 2 �Λ eff + eff Λ � ∆ + 2 �Λ eff + eff Λ � ∆ ( 2) where the first and second term of the equation represent the strain and temperature induced wavelength shift, respectively. �⋀ ∂ ∂ n l + n ∂ ∂ ⋀ l ��⋀ ∂ ∂ n l + n ∂ ∂ ⋀ l � Some advantages of the Bragg grating technology include: high sensitivity, high capability of multiplexing, no magnetic interference and low signal losses. These characteristics have been contributed to the dissemination of this optical technology, especially in the sensors industry (Antunes et. al., 2009). 2.2. Sensor description The FBG used in this work was inscribed in single mode Boron doped photosensitive fiber (Fibercore PS1250/1500) using a KrF excimer UV laser and the phase mask technique, with a pulse energy of 5 mJ and a pulse repetition of 500 Hz. After experimental characterization, the FBG sensitivity to mechanical deformations yielded to a value of 1.35 pm/ µε. The prototype of the FBG optical sensor for bond-slip monitoring of old RC elements is composed by two metallic components, specifically brass, with 32.00 mm of total height, 5.00 mm of thickness and 41.00 mm of total length. The superior component presents an “¬” form with a hook placed at the top and center of the total length, while the inferior component has an “ ˾ ” form. The optical fiber with the FBG was fixed between the vertical parts of these two components. The inferior and superior surfaces was not in direct contact and the minimum spacing between them is of 1.00 mm in the extremities. A polyurethane spray (PU) layer was employed to fulfill the space between the two metallic surfaces, and allowed them to move independently. In fact, the inferior metallic component of the sensor was designed to be move along with the reinforcement rebar, while the superior component was planned to be hooked on the concrete. Thus, once that the superior component of the FBG sensor was fixed in the concrete and the inferior component was immobilized on the reinforcement rebar, any movement of the reinforcing bar will deform (by compression or traction) the optical fiber with the FBG fixed between the two components. Consequently, a shift on the reflected Bragg wavelength will be obtained, proportional to the relative movement between the rebar and the concrete. 3. Specimen detailing and experimental setup The implemented prototype of the FBG sensor for RC bond-slip monitoring was tested in a RC sample prepared according to Annex D of the EN 10080 (European Standard, 2005). A concrete sample with dimensions of 0.20 m x 0.20 m x 0.30 m with a centralized rebar embedded with diameter of 16.00 mm was used to perform the prototype tests, according to proceedings described in EN 12390-3 (CEN 2003). The compressive strength of the concrete used was 25 MPa. The FBG sensor was positioned centralized on the rebar surface, at 75.00 mm of the right extremity of the RC specimen. The Figure 2a) shows the bond-slip sensor positioning inside the RC specimen, and the sensor arrangement during the specimen preparation step. Following the specimen preparation, the sensor was connected to the optical data acquisition system (Figure 2c) and the RC specimen casted (see Figure 2b). During the first 24 hours, the sensor signal was monitored in order to identify potential damages in the sensor or signal losses provoked during the casting phase. After that, the sensor was disconnected and the RC specimen submitted to concrete cure process by 28 days, in a humidity chamber (relative humidity of 95% and 20 ºC of temperature), till the pull-out testing. The general view of the experiment can be seen in Figure 2a).