Monitoring of Large Diameter Sewage Collector Strengthened with Glass-Fiber Reinforced Plastic (GRP) Panels by Means of Distributed Fiber Optic Sensors (DFOS)

Abstract

Diagnostics and assessment of the structural performance of collectors and tunnels require multi-criteria as well as comprehensive analyses for improving the safety based on acquired measurement data. This paper presents the basic goals for a structural health monitoring system designed based on distributed fiber optic sensors (DFOS). The issue of selecting appropriate sensors enabling correct strain transfer is discussed hereafter, indicating both limitations of layered cables and advantages of sensors with monolithic cross-section design in terms of reliable measurements. The sensor's design determines the operation of the entire monitoring system and the usefulness of the acquired data for the engineering interpretation. The measurements and results obtained due to monolithic DFOS sensors are described hereafter on the example of real engineering structure-the Burakowski concrete collector in Warsaw during its strengthening with glass-fiber reinforced plastic (GRP) panels.

Keywords: collector; concrete; cracks; displacements; distributed fiber optic sensing DFOS; glass fiber reinforced polymer GFRP; layered cables; monolithic sensors; strains; strengthening; tunnels.

Figure 1

The view of Burakowski concrete collector during its technical inspection.

Figure 2

GRP test panel inside a collector before installation.

Figure 3

Examples of sensing cable designs: (a) with plastic coating; (b) with metal coating and single or multiple plastic coatings; (c) with metal coating and additional metal reinforcing elements and with single or multiple plastic coatings.

Figure 4

The effect of slippage inside layered sensing cable when measuring crack opening in concrete member under axial tension. As the load increases, the crack peak is reduced, instead of higher. Blue—force of 6 kN, Red—force of 10 kN. When measured correctly, an increase in force should result in increase in the measured strain peak.

Figure 5

Cross-section of DFOS measurement sensor used for strain measurement. 1—monolithic core, 2—optic fiber [7].

Figure 6

Crack analysis in reinforced concrete beam using monolithic DFOS sensors. Sensor was embedded inside the beam at the height of main bottom reinforcement (courtesy by [8]). Graphs show strains development during increasing the load (local peaks correspond to cracks near to the bottom surface); (a) initial phase; (b) middle phase; (c) final phase.

Figure 7

Sensor locations along the collector.

Figure 8

Sensors used in the monitoring system. (a) Sensor A—EpsilonRebar, (b) Sensor B—EpsilonSensor [8].

Figure 9

(a) EpsilonRebars delivered to the site in coils; (b) arrangement of DFOS sensors (type A) in the cross section of the collector.

Figure 10

Spatial visualizations: (a) longitudinal sensors arrangement; (b) scheme of installation inside the groove filled with chemical anchor.

Figure 11

View of the sensor on the collector wall inside the groove during filling it with chemical anchor.

Figure 12

Examples of diagonal cracks of a collector concrete structure.

Figure 13

Scheme of DFOS sensors (type B) installation in the cracked section of collector.

Figure 14

Scheme of DFOS sensors arrangement in walls of the existing collector. I—section outside the loops, II—intersection of sensors A and B.

Figure 15

Strain profiles measured by 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.

Figure 16

Relative vertical displacements (change in shape) of the collector after installation of GRP panels and grout injection.