The Smart Nervous System for Cracked Concrete Structures: Theory, Design, Research, and Field Proof of Monolithic DFOS-Based Sensors

Abstract

The article presents research on the performance of composite and monolithic sensors for distributed fibre optic sensing (DFOS). The introduction summarises the design of the sensors and the theoretical justification for such an approach. Lessons learned during monitoring cracked concrete are summarised to highlight what features of the DFOS tools are the most favourable from the crack analysis point of view. Later, the results from full-size laboratory concrete specimens working in a cracked state were presented and discussed in reference to conventional layered sensing cables. The research aimed to compare monolithic sensors and layered cables embedded in the same reinforced concrete elements, which is the main novelty. The performance of each DFOS nondestructive tool was investigated in the close vicinity of the cracks-both the new ones, opening within the tension zone, and the existing ones, closing within the compression zone. The qualitative (detection) and quantitative (widths estimation) crack analyses were performed and discussed. Finally, the examples of actual applications within concrete structures, including bridges, are presented with some examples of in situ results.

Keywords: civil engineering; composite sensors; cracks; detection; distributed fibre optic sensing DFOS; laboratory; monolithic sensors; reinforced concrete; strains; widths.

Figure 1

Comparison between capabilities of distributed fibre optic sensing DFOS and spot measurements in crack detection [8].

Figure 2

(a) John William Strutt (Lord Rayleigh); (b) Léon Brillouin; (c) Chandrasekhara Venkata Raman [6].

Figure 3

Example cross-sections of (a) single-mode optical fibre in its primary coating, (b) layered sensing cable with steel strengthening insert and (c) monolithic strain sensor with the corresponding views of their external surfaces (d–f).

Figure 4

The scheme for crack analysis based on the DFOS system: (a) qualitative analysis (detection, location); (b) quantitative analysis (width estimation) [6].

Figure 5

(a) Simplified scheme for a bond-slip crack model for reinforced concrete [51]; (b) simplified graphical interpretation of spatial resolution in distributed sensing.

Figure 6

(a) Footbridge reinforced with stiff monolithic sensors; (b) concrete slab with the flexible monolithic sensor not influencing the structural behaviour [8].

Figure 7

Possibilities of the sensors’ installation methods: (a) installation in near-to-surface grooves; (b) surface gluing (on steel bar); (c) embedding inside the concrete.

Figure 8

(a) Spatial visualisation of the reinforced concrete beam equipped with DFOS tools; (b) close-up to the selected tools; (c) the view of the beam during concreting.

Figure 9

(a) The view of the beams after concreting; (b) thermal-shrinkage cracks within the upper part; (c) mechanical cracks within the lower part during bending tests.

Figure 10

(a) Example beam during four-point bending test; (b) Rayleigh-based data logger with an optical switch connected to embedded DFOS tools; (c) reference readings.

Figure 11

Designed research schedule for S-type beams.

Figure 12

Beam S3/compression zone: strain profiles registered by (a) monolithic sensor M2 and (b) layered sensing cable C2 with steel strengthening insert.

Figure 13

Beam S3/tension zone: strain profiles registered by (a) monolithic sensor M2 and (b) layered sensing cable C2 with steel strengthening insert.

Figure 14

Beam S3/tension zone: crack-induced strain peaks over subsequent load steps registered by (a) monolithic sensor M2 and (b) layered sensing cable C2 with steel strengthening insert.

Figure 15

Beam S3/tension zone: strain profiles in length and load step domain registered by (a) monolithic sensor M2 and (b) layered sensing cable C2 with steel strengthening insert.

Figure 16

Strain profiles over length measured by monolithic sensor M2 and layered cable C2 (beam S3, load step no. 26, force = 50 kN).

Figure 17

Installation of the monolithic sensor by tying to the existing reinforcement.

Figure 18

(a) The view of the slab during concreting; (b) example strain profiles with crack analysis 20 h after concreting.

Figure 19

(a) Rędziński Bridge in Wrocław, Poland—general view (photo: W. Kluczewski); (b) the close-up of the lower crossbeam connecting two inclined legs of the pylon.

Figure 20

(a) Installation in near-to-surface grooves; (b) cross-section of the beam with locations of the sensors; (c) side view of the crossbeam with sensors marked over its length.

Figure 21

Example strain profiles over selected sections of the lower and upper monolithic sensor with crack width analysis.

Figure 22

(a) Design of the collector with monolithic sensors; (b) the view of the collector with marked locations of monolithic sensors; (c) near-to-surface groove filled with injection.

Figure 23

(a) Cross-section of the collector [58]; (b) location of the circumference monolithic sensor [58]; (c) the view of the crack documented before installation.

Figure 24

Strain profiles measured by the monolithic sensor (T) along the test section showing the primary discontinuities in the concrete collector closing during strengthening process. Example section from 132 to 146 m. Measurement P01—after installation of GRP panels, P02—during grout injection, P03—after completed grout injection, P04—after filling the collector with sewage [58].