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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active operation and unfavorable environmental influences. Traditionally, monitoring includes measuring different parameters such as strain, displacement, rotation as well as acceleration and etc. at various points along the structure. Electrical gauges are traditionally used for such measurements. The development of photonic technologies has led to the widespread application of fiber-optic sensors (FOSs) for monitoring the mechanical state of such structures as bridges (Tennyson et al. , 2001; Lin et al. , 2005), dams (Taheri, 2019) and power plants (Majumder et al. , 2008). FOSs have a number of advantages over electrical gauges, such as: high sensitivity, high accuracy, corrosion resistance, immunity to electromagnetic interference, and the ability to perform distributed measurements using single optical fiber. One of the major advantages of FOSs lies in the possibility of embedding them into the material. In most studies, various types of protective coatings (packaging layers) are used in the Bragg grating area when optical fiber is embedding into a material (Leng et al. , 2006; Majumder et al. , 2008). Metallic, silicone, composite and other protective coatings have great practical importance (R. Scott et al. , 2019), because optical fiber is at risk of breaking when embedding into material, even with a standard polymer coating such as polyimide or acrylate. However, the use of FOSs with additional protective coatings leads to a number of problems: incomplete transfer of strains from the host material (concrete, cement) to the optical fiber and mismatch between the mechanical behavior of the material in the FBG area and in the host material in which the optical fiber is embedded. To restore the real picture of the mechanical state, it is necessary to use the recalculation of the strain values obtained with the FOSs (Huang et al. , 2019). At the same time, the correction coefficients must be found in laboratory conditions and require knowledge of the strain distribution of the host material at the points of the embedded sensors location. Concrete is a composite material in which the binder is cement mixture. The change in the properties of the cement mixture largely determines the mechanical properties of concrete. Analysis of the literature shows that the use of FOSs in concrete structures is associated with the assessment of their mechanical state. At the same time, the use of FOSs embedded into material makes it possible to obtain new experimental information about the mechanical behavior of the material. The paper investigates the possibility of using point FOSs to assess the strain state of samples made from a cement mixture. While various aggregates in the form of stones, gravel and sand have stable properties, cement mixture plays a significant role in the strength and properties of the concrete structure (Kara and Korjakins, 2013). It is the cement that affects the change in the mechanical state of the samples over time, in particular, during its formation. The optical fiber used for embedding has no additional protective coatings except for polyimide. Despite the fact that this variant of embedding the optical fiber into cement mixture or concrete has some limitations for application in structural construction, the use of optical fiber without additional protective packaging in laboratory conditions allows to obtain a more reliable picture of the mechanical state of samples made from a cement mixture. 2. Experimental setup and samples An experimental stand is shown in Fig. 1 that was designed and created in order to perform the task associated with the use of embedded FOSs to assess strains in cement samples during formation. This stand allows to measure the shrinkage and the internal temperature of the test samples made of cement mixture, as well as the temperature and humidity of the environment. The stand allows to provide the long-term experiments, including the stages of hardening and drying of the sample and was designed in such a way that samples with different geometric shapes could be placed there. At the preparatory stage of the experiment (see Fig. 1), two types of empty molds were placed inside a steel frame (1) of the size 80  100  110 cm. The optical fibers with FBG sensors were placed inside the molds (2) and their position was centered as shown in Fig. 1. Six FBG sensors with 5 mm sensor length and 60 mm distance between sensors were written in each optical fiber line. After placing the FOSs, the cement mixture molds were filled a layer by-layer (with compaction and vibration) to the level of the upper free boundary of the mold, so that the FOS lines became immersed into cement mixture. The vertical position of the FOS lines was provided by a given pretension with the help of attached weight. To measure the shrinkage of the sample, the laser position sensors (3) were fixed and pointed to the upper free surface of the cement mixture. Also, the temperature change was measured at all stages of hardening and shrinkage by a temperature sensor (4) immersed into the sample. Ambient temperature and humidity in the vicinity of the samples were measured by a sensor (5) placed near the upper free surface of the molds.