Abstract
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.
Highlights
Concrete, during its working life, is undergoing various interactions with the environment in the forms of water penetration, ion, and gas diffusion
Pure sulfate-bearing samples: Prior to the characterization of the sulfate products formed in the hardened cement sample, one pure gypsum sample and one pure ettringite sample were first analyzed under bench-mounted Raman spectrometer so that the spectra obtained can be used to verify the results from the optical fiber system
After these spectra were processed with background subtraction and the Lorentz peak-fit using OriginPro 8.6, the signal-to-noise ratio (SNR) of the main peak of silicon (i.e., 520 cm−1) were calculated according to the method specified in ASTM E579-04.29 The corresponding results are presented in Table 2 below
Summary
Concrete, during its working life, is undergoing various interactions with the environment in the forms of water penetration, ion, and gas diffusion. Several durability problems, such as carbonation,[1] chloride ingress,[2] and sulfate attack[3,4] could be induced These three deterioration mechanisms are the main culprits for the degradation of concrete durability. Some feasibility studies on using optical fiber Raman spectroscopy to monitor the concrete subjected to various deterioration mechanisms have been carried out by the authors. Gypsum and ettringite, the most commonly formed deterioration products in sulfate attack, were characterized using a Raman spectrometer with fiber assembly as excitation path (hereafter, optical fiber excitation Raman spectroscopy/spectrometer). Based on the results reported in this paper, the feasibility and potential of optical fiber Raman spectroscopy as a remote characterizing and monitoring technique for concrete durability are discussed
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