Fundamental Studies on the Use of Distributed Fibre Optical Sensing on Concrete and Reinforcing Bars

Abstract

Distributed fibre optical sensing (DFOS) allows for quasi-continuous strain measurement in a broad range of gauge lengths and measurement frequencies. In particular, Rayleigh backscatter-based coherent optical frequency domain reflectometry has recently registered a significant application increase in structural concrete research and monitoring thanks to its numerous merits, such as high resolution and low invasiveness. However, it is not a plug-and-play technique. The quality of the acquired data depends highly on the choice of the fibre optical sensor and the methods of instrumentation and post-processing. Furthermore, its unprecedented resolution and sensitivity allow capturing local effects not well documented so far. This paper analyses the suitability of DFOS based on Rayleigh backscatter for reliably measuring strains and discusses the origin and structural relevance of local variations in the results. A series of experimental investigations are presented, comprising tensile tests on bare reinforcing bars and concrete compression tests. A critical analysis of the results leads to a best practice for applying DFOS to reinforcing bars and concrete, which establishes a basis for reliable, accurate measurements in structural concrete applications with bonded reinforcement.

Application of Distributed Fibre Optical Sensing in Reinforced Concrete Elements Subjected to Monotonic and Cyclic Loading

“…The difference between the DFOS strain profiles and the one obtained from the simulation is provided in Figure 2c. The results confirm the observations reported in the companion paper [28]: at the start and end of the length of interest, the polyimide coated fibres measured a sharp strain increase (i.e., short activation length), while the strains measured by the acrylate-coated fibres showed a gradual increase to the same level as the other two fibres (i.e., long activation length). The slip between fibre and coating causes this variance in the activation length and is also the reason why the acrylate coated fibre is unable to reproduce sharp changes in the strain profile (refer to Figure 2f): unlike both polyimide-coated fibre types (PG and PS), the acrylate-coated fibre could not reproduce the plateau in the strain profile and measured lower maximum strains.…”

“…This example illustrates the ability of DFOS to measure strain gradients and discontinuities, which is essential in order to obtain meaningful measurements in structural concrete elements. The polyimide-coated fibres are recommended as they demonstrate a higher accuracy, as reported in [23,28]. The results highlight the necessity of temperature compensation in long-term tests.…”

“…Recently, Monsberger and Lienhart (2021) employed DFOS to extract the curvature and bending moment in structures by double integration of the obtained strains [27]. [1][2][3][4][5][6]; and (b) fibre optical sensors installed on a reinforcing bar using either fibre Bragg gratings (FBG) [10] or Distributed Fibre Optical Sensing (DFOS) [14,18,24,28].…”

“…The limitation to small deformations is provided by the measurement ranges of the spectrometers currently in use (typically up to 12,000 µm/m). Galkovski et al (2021) reported the possibility to extend the measurement range of the spectrometer by setting intermediate reference states and superposing the data afterwards [28]. Their method enables measurement of strains up to 25,000 µm/m.…”

Scientific Applications of Distributed Acoustic Sensing: State-of-the-Art Review and Perspective

“…Distributed fiber sensors are used to measure the distribution of temperature [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ], stress/deformations [ 22 , 23 , 24 ], or vibro-acoustic properties [ 25 , 26 , 27 , 28 , 29 , 30 ] of various objects. The last application allows more information about the investigated object to be obtained in contrast to the static measurement of temperature or stress.…”