Long-Term Performance of Distributed Optical Fiber Sensors Embedded in Reinforced Concrete Beams under Sustained Deflection and Cyclic Loading

Abstract

This paper explores the performance of distributed optical fiber sensors based on Rayleigh backscattering for the monitoring of strains in reinforced concrete elements subjected to different types of long-term external loading. In particular, the reliability and accuracy of robust fiber optic cables with an inner steel tube and an external protective polymeric cladding were investigated through a series of laboratory experiments involving large-scale reinforced concrete beams subjected to either sustained deflection or cyclic loading for 96 days. The unmatched spatial resolution of the strain measurements provided by the sensors allows for a level of detail that leads to new insights in the understanding of the structural behavior of reinforced concrete specimens. Moreover, the accuracy and stability of the sensors enabled the monitoring of subtle strain variations, both in the short-term due to changes of the external load and in the long-term due to time-dependent effects such as creep. Moreover, a comparison with Digital Image Correlation measurements revealed that the strain measurements and the calculation of deflection and crack widths derived thereof remain accurate over time. Therefore, the study concluded that this type of fiber optic has great potential to be used in real long-term monitoring applications in reinforced concrete structures.

Keywords: Rayleigh backscattering; cyclic loading; distributed optical fiber sensing; performance indicators; reinforced concrete.

Figure 1

Geometry of the beam specimens, reinforcement layout and DOFS configuration (all measurements in mm).

Figure 2

Loading setup for the long-term loading experiments (all measurements in mm).

Figure 3

Definition of the loading procedure for the cyclically loaded beams.

Figure 4

(a) Time evolution of the load for the beams subjected to sustained deflection where four selected times of interest (t₁, t₃, t₃, t₄) are indicated by markers. (b) Strain profile at one of the tensile reinforcement bars of beam 5 at the beginning of the test. Triangular markers indicate the positions of identified cracks and dashed lines highlight the position of two sections of interest, a crack in the shear span (Section 1) and a crack in the constant moment region (Section 2).

Figure 5

Incremental variation of measured and calculated magnitudes in the beam over time for Section 1 and Section 2: (a,b) strain; (c,d) curvature; (e,f) crack width. (g) Variation of ambient temperature and the corresponding temperature-induced strain.

Figure 6

(a) Distribution of curvature increment along the beam for the instants t₂, t₃ and t₄ with respect to the initial instant t₁; (b) evolution of the maximum deflection in beam 5 calculated from the DOFS strain measurements together with the corresponding evolution of applied load.

Figure 7

Load history of the beams subjected to cyclic loading. A zoomed-in region is presented to show the variability of the load cycles both in load magnitude and pulse duration and how the corresponding measurements of the DOFS and DIC correlate with the applied load. A further zoomed-in region is presented for the load, DOFS deflection and DIC displacement to illustrate the noise of the load-control system and the averaged values used for the analysis of results.

Figure 8

Strain profiles at the tensile reinforcement of beam 4 at different times for a loading cycle of ca. 28.5 kN.

Figure 9

Evolution of strains over time represented as the ratio between the DOFS strain measurement and the section bending moment and evolution of crack width for Section 1 (a,b) and Section 2 (c,d).

Figure 10

Evolution of the midspan deflection calculated from DOFS strain measurements for (a) beam 3 and (b) beam 4.

Figure 11

Ratio of the deflection increment calculated from the DOFS and measured by the DIC for (a) beam 3 and (b) beam 4.

Figure 12

Results of the DIC showing the field of principal strains to highlight the position of cracks on the surface of beam 4 along the region of constant bending moment (top). Relationship between increment of crack width and applied load for selected cracks (middle). Crack width error between the DIC and DOFS measurements (bottom).