The Challenges and Advantages of Distributed Fiber Optic Strain Monitoring in and on the Cementitious Matrix of Concrete Beams

Abstract

Distributed fiber optic strain measurement techniques have become increasingly important in recent years, especially in the field of structural health monitoring of reinforced concrete structures. Numerous publications show the various monitoring possibilities from bridges to special heavy structures. The present study is intended to demonstrate the possibilities, but also the challenges, of distributed fiber optic strain measurement in reinforced concrete structures. For this purpose, concrete beams for 3-point bending tests were equipped with optical fibers on the reinforcement and concrete surface as well as in the concrete matrix in order to record the strains in the compression and tension zone. In parallel, an analytical approach based on the maximum strains in the uncracked and cracked states was performed using the Eurocode 2 interpolation coefficient. In principle, the structural design correlates with the measured values, but the strains are underestimated, especially in the cracked zone. During load increase, structural distortions in the compression zone affected the strain signal, making reliable evaluation in this zone difficult. The information content of distributed fiber optic strain measurement in reinforced concrete structures can offer tremendous opportunities. Future research should consider all aspects of the bond, sensor selection and positioning. In addition, there is a lack of information on the long-term stability of the joint and the fiber coating, as well as the effects of dynamic loading.

Keywords: cementitious matrix; concrete beams; distributed fiber optic strain monitoring; structural health monitoring.

Figure 1

Exemplary scale structure of fiber optic sensors; (a) optical fiber protected only by coating; (b) structure of a sensor cable.

Figure 2

Experimental setup and sensor arrangement.

Figure 3

Optimal (left) and defective (right) adhesive joint according to [36,57].

Figure 4

Fiber applied to the surface of the reinforcement.

Figure 5

Results of structural design and rebar strain measurements for all specimens and load steps.

Figure 6

Results of structural design and concrete surface strain measurements for all specimens and load steps.

Figure 7

Applied fiber on the concrete surface and defects due to loading application.

Figure 8

Structural design results and concrete matrix strain measurements for all specimens and load steps.