Organic-Inorganic Hybrid Sol-Gel Materials for Optical Fiber Sensing Application

Abstract

The Integration of advanced sensing devices into concrete and reinforced concrete structures (RCS) is a rational approach for the assessment of repair options and scheduling of inspection and maintenance strategies. The imediate advantages are cost reduction and a more reliable prevention of unpredictable events. The use of optical fiber sensors (OFS) for such purposes has increased in the last few years due to their intrinsic advantages such as lightweight, corrosion resistance, immunity to electromagnetic interference and multiplexing capabilities. In most OFS, the chemical transducer consists of immobilized chemical reagents, placed in the sensing region of the optical sensor by direct deposition or by encapsulation in a polymeric matrix. The choice of the support matrix influences the performance of the sensor, which is governed by factors such as the permeability to the analyte, suitability for reagent immobilization and adequate optical response. In the last few years, OFS, cement-based strains and nanomaterial based sensors have been reported and implemented in diverse structures. It has been demonstrated that they could monitor several physical and chemical properties directly related to concrete and RCS durability. However, most of them show poor and inconsistent responses, signal drift due to dye leaching, reduced pot life and unsuitability to operate at pH > 12. New organic-inorganic hybrid (OIH) sol-gel materials were obtained using a functionalized siloxane 3-Glycidoxypropyltrimethoxysilane (GPTMS) by the reaction with different oligopolymers (referred to as Jeffamine), namely Jeffamine® RFD 270, Jeffamine® THF 170 and Jeffamine® T403. The OIHs were doped with different silver nanoparticles contents and also with different contents of pH indicators (i.e. phenolphthalein and orange II) . These materials were developed for application on OFS to monitor the pH concrete and chloride ions. The chemical stability of these materials was studied in simulative concrete solutions (pH > 12.5) and in mortars by electrochemical impedance spectroscopy (EIS). The materials synthesized were characterized by UV–Visible spectrophotometry, EIS, fluorescence spectroscopy and Fourier-transform infrared spectroscopy. The produced OIH materials show promising properties to be coupled to advanced OFS to monitor pH and chloride ions in concrete. The real-time monitoring of these parameters will contribute to understand/antecipate the progress of RCS deterioration mechanisms. Furthermore, integrated systems will contribute for the development/improvement of theoretical models to predict with high accuracy the service life of concrete and RCS under the action of combined effects.