PSI - Issue 28

N.A. Kosheleva et al. / Procedia Structural Integrity 28 (2020) 1883–1891 Author name / Structural Integrity Procedia 00 (2019) 000–000

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4. Results Embedded fiber-optic sensors didn’t have any additional protective coatings, except for the regular polyimide coating. In addition to the samples described earlier, small-sized samples with embedded optical fibers were made to analyze the internal structure of the sample in the vicinity of the embedded optical fiber (see Fig. 4). Figure 5 shows a cross-section of one of these cement samples with embedded optical fiber.

Fig. 4. Small-sized samples.

On Fig. 6a and 6b the strain values for cylindrical and prismatic samples, obtained using the embedded FBG sensors are presented. The data is shown for two of the studied samples (hereinafter referred to as sample 1 and sample 2) for the first three days after casting. The initial setting time of the cement, which corresponds to 5.3 hours from the casting was considered as the starting point on the graphs (Yun, Jang and Wang, 2012). This time was taken as the reference point for calculating the strain from the recorded Bragg wavelength shifts of used FBG sensors, since it is assumed that at this time interval the sufficient adhesion of the cement mixture (Kara and Korjakins, 2013) and the optical fiber is achieved for qualitative measurement of the strain behavior of the investigated material. The data obtained from the FOSs show a significant increase in strain of the samples in the period from 6 to 12 hours after casting. This growth can be explained to ongoing internal chemical reactions (hydration) which is accompanied by the release of heat.

Fig. 5. Internal structure of the sample in the vicinity of the embedded optical fiber.