Environmental Durability of an Optical Fiber Cable Intended for Distributed Strain Measurements in Concrete Structures

Abstract

The present study investigates the environmental durability of a distributed optical fiber sensing (DOFS) cable on the market, commonly used for distributed strain measurements in reinforced concrete structures. An extensive experimental program was conducted on different types of specimens (including samples of bare DOFS cable and plain concrete specimens instrumented with this DOFS cable) that were exposed to accelerated and natural ageing (NA) conditions for different periods of up to 18 months. The instrumentation of both concrete specimens consisted of DOFS cables embedded at the center of the specimens and bonded at the concrete surface, as these two configurations are commonly deployed in the field. In these configurations, the alkalinity of the surrounding cement medium and the outdoor conditions are the main factors potentially affecting the characteristics of the DOFS component materials and the integrity of the various interfaces, and hence impacting the strain transfer process between the host structure and the core optical fiber (OF). Therefore, immersion in an alkaline solution at an elevated temperature or freeze/thaw (F/T) and immersion/drying (I/D) cycles were chosen as accelerated ageing conditions, depending on the considered configuration. Mechanical characterizations by tensile and pull-out tests were then carried out on the exposed specimens to assess the evolution of the mechanical properties of individual component materials as well as the evolution of bond properties at various interfaces (internal interfaces of the DOFS cable, and interface between the cable and the host structure) during ageing. Complementary physico-chemical characterizations were also performed to better understand the underlying degradation processes. The experimental results highlight that immersion in the alkaline solution induced a significant and rapid decrease in the bond properties at internal interfaces of the DOFS cable and at the cable/concrete interface (in the case of the embedded cable configuration), which was assigned to chemical degradation at the surface of the cable coating in contact with the solution (hydrolysis and thermal degradation of the EVA copolymer component). Meanwhile, F/T and I/D cycles showed more limited effects on the mechanical properties of the component materials and interfaces in the case of the bonded cable configuration. A comparison with the same specimens exposed to outdoor NA suggested that the chosen accelerated ageing conditions may not be totally representative of actual service conditions, but provided indications for improving the ageing protocols in future research. In the last part, an analysis of the distributed strain profiles collected during pull-out tests on instrumented concrete specimens clearly illustrated the consequences of ageing processes on the strain response of the DOFS cable.

Application of Distributed Optical Fiber Sensing Technology to the Detection and Monitoring of Internal Swelling Pathologies in Massive Concrete Blocks

“…It is worth noting that the durability of this DOFS cable was investigated in previous research under ageing conditions very similar to those considered in the present study (see [ 27 ]).…”

“…The adhesive was a bi-component epoxy system X120 commercialized by the HBM Company (Darmstadt, Germany). Its durability under the same ageing conditions as those used in this study was investigated in a previous paper [ 27 ].…”

Durability in Alkaline Environment of a Fiber Optic Sensor Bonded at the Surface of Reinforcing Bars for Distributed Strain Measurements in Concrete Structures

“…To verify if the DOFS instrumentation is resistant to alkaline environments at temperatures up to 60 °C (these ageing conditions are commonly used for durability studies on FRP rebars [6,8]), half-length of each instrumented sample was submitted to these harsh conditions. Before being partially immersed in the ageing solution, each specimen was first submitted to an initial tensile test using an Instron universal testing machine of capacity 250 kN.…”

“…If the durability of DOFS bonded at the surface of concrete or embedded within concrete has been investigated in a recent work [6], to the author's knowledge, the ageing behavior of DOFS bonded to the rebars of RC structures is still an open issue. In the present study, an experimental program was proposed to investigate such behavior.…”

Improving Prediction Accuracy and Extraction Precision of Frequency Shift from Low-SNR Brillouin Gain Spectra in Distributed Structural Health Monitoring

“…There are a number of applications where obtaining accurate data on the temperatures and strains of a material is essential. For example, in [ 10 ] the authors illustrate the age-induced degradation on the Rayleigh-based distributed sensor response. To implement such a task, the distributed systems must have a resolution of the order of tenths of a microstrain.…”