Optimization of the adhesion of fiber‐optic strain sensors embedded in cement matrices; a study into long‐term fiber strength

Abstract

Fiber-optic strain sensors to be used in materials research and for long-term monitoring the state of chemically stressed concrete, e. g. sewer pipes, cooling towers, have to be intimately embedded in cement matrices. They should be coated with very thin alkaline-resistant coatings. This is necessary to achieve a reliable strain transfer from the matrix to the fiber sensor. In order to clarify possible ways to replace thick coatings, which are unstable in cement environment, optical fibers have been modified and investigated with regard to their chemical compatibility, their strength behavior after alkaline attacks and their adhesion behavior to the matrix. The surface state of differently treated optical fibers intended for embedding has been characterized; additional strength estimation of alkaline attacked fibers was carried out. All estimations have been compared with those of bare alkaline attacked fibers. The real adhesion behavior was determined using the fiber push-in test technique (indentation). Recent results suggest that fibers coated with very thin organic layers have remarkable adhesion properties. However, the fiber strength after exposing to alkaline pore solution (pH = 13.9) is worse than that of bare fibers. Optimum fiberkement bond strength showed fibers, which were coated with a crosslinked polymer, produced in gas plasma.

Fiber-optic strain sensors are making the leap from lab to industrial use: reliability and validation as precondition for standards

“…When the load cycles were repeated at 80 °C, the gauge factor rose to 0.8. 16. Patch E only showed good performance for temperatures up to 60 °C (not as specified!).…”

“…The testing facility allows characterizing the interface bonding behavior of thin sample pieces with embedded optical fibers. Details are described in [16]. This method provides information about the coating's bond strength of embedded sensor fibers and allows evaluating appropriate strain transfer.…”

How do application-related issues influence the reliability of fiber optic strain measurements?

“…The testing facility allows characterizing the interface bonding behavior of embedded fiber optic deformation sensors. Details are described in [3]. This method is primarily used to determine the appropriate sensor coating for a specific measurement task.…”

Fiber optic sensors for deformation measurements: criteria and method to put them to the best possible use

“…The most common method to check the adhesion quality of an embedded sensing element is the single fiber push-in test. This method and alternative tests are described more in detail in [12]. A schematic diagram and a photograph and of the testing arrangement for the push-in test are represented in Figure 2, and 3, respectively.…”

“…Testing arrangement for push-in tests: a) schematic, b) indenter needle for fiber diameters of 125 µm[12] Push-in test machine at BAM Typical test result when evaluating the strain transfer characteristic (abscissa: displacement of the indenter, ordinate: force at the indenter)…”