Formation of ettringite and gypsum from sulfate attack together with carbonation and chloride ingress have been considered as the most serious deterioration mechanisms of concrete structures. Although electrical resistance sensors and fiber optic chemical sensors could be used to monitor the latter two mechanisms on site, currently there is no system for monitoring the deterioration mechanisms of sulfate attack. In this paper, a preliminary study was carried out to investigate the feasibility of monitoring sulfate attack with optical fiber excitation Raman spectroscopy through characterizing the ettringite and gypsum formed in deteriorated cementitious materials under an optical fiber excitation + objective collection configuration. Bench-mounted Raman spectroscopy analysis was also conducted to validate the spectrum obtained from the fiber-objective configuration. The results showed that the expected Raman bands of ettringite and gypsum in the sulfate-attacked cement paste can be clearly identified by the optical fiber excitation Raman spectrometer and are in good agreement with those identified from bench-mounted Raman spectrometer. Therefore, based on these preliminary results, it is considered that there is a good potential for developing an optical fiber-based Raman system to monitor the deterioration mechanisms of concrete subjected to sulfate attack in the future.
Formation of ettringite and gypsum from sulfate attack together with carbonation and chloride ingress have been considered as the most serious deterioration mechanisms of concrete structures. Although Electrical Resistance Sensors and Fibre Optic Chemical Sensors could be used to monitoring the latter two mechanisms in situ, currently there is no system for monitoring the deterioration mechanisms of sulfate attack and hence still needs to be developed. In this paper, a preliminary study was carried out to investigate the feasibility of monitoring the sulfate attack with optical fibre Raman spectroscopy through characterizing the ettringite and gypsum formed in deteriorated cementitious materials under an ‘optical fibre excitation + spectroscopy objective collection’ configuration. Bench-mounted Raman spectroscopy analysis was also used to validate the spectrum obtained from the fibre-objective configuration. The results showed that the expected Raman bands of ettringite and gypsum in the sulfate attacked cement paste have been clearly identified by the optical fibre Raman spectroscopy and are in good agreement with those identified from bench-mounted Raman spectroscopy. Therefore, based on these preliminary results, there is a good potential of developing an optical fibre Raman spectroscopy-based system for monitoring the deterioration mechanisms of concrete subjected to the sulfate attack in the future.
KEYWORDS: Photonic nanostructures, Near field scanning optical microscopy, Dielectrics, Spatial resolution, Silicon, Finite element methods, Near field optics, Raman spectroscopy, Numerical simulations, Laser energy
A new concept of “photonic nanojet” SNOM is proposed in this paper and the system is based on a dielectric
microsphere which is mounted on a cantilever. The dielectric microsphere works as a superlens to focus the laser energy
into a small volume with subwavelength spatial resolution. The numerical simulation by using Finite Element Method
(FEM) has been done to optimize the parameters of the photonic nanojet of dielectic microsphere for “photonic nanojet”
SNOM. The microspheres with different diameters have been investigated numerically and the results show that bigger
microspheres produce higher intensity “photonic nanojets”. The simulation result on the interaction between a silicon
cylinder and a photonic nanojet reveals that a “hot spot” is formed inside the silicon cylinder and is confined into a small
volume. Therefore a new area on high spatial resolution spectral analyzing for nanostructures is in prospect.
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