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.
Effective devices for the monitoring of chloride ions are required in industry this work reports on the development of a
reflective long period grating-based senor for the measurement of chloride ions in solution. The device is based around a
long period fibre grating (LPG)-based Michelson interferometer where the sensor was calibrated and evaluated in the
laboratory using sodium chloride solutions, over a wide range of concentrations, from 0.01 M to 4.00 M. The grating
response yields shifts in the spectral characteristic of the interferometer, due to the change of refractive index of the
solution surrounding it. The work was stimulated by the need for the more effective measurement of the ingress water-borne
ions in structural concrete. It was found that the sensitivity of the device could be enhanced over that obtained
from a bare LPG by coating the LPG-based interferometer with gold nanoparticles. Devices using this approach were
also evaluated and their performance cross-compared.
Optical fibre sensor technology has advanced rapidly in recent years, with a feature of current work a strong applications-focus which has become one of the major driving forces for innovation in the field. This paper focuses upon the nature of the background needs and the development of technologies well suited to tackling a range of problems in the assurance of the quality and stability of modern structures, and as a consequence the optimization and the minimization of the costs associated with maintenance of such structures. A number of topical sensor applications are considered and results of recent work presented.
Carbonation-induced corrosion of steel is one of the principal causes of deterioration of reinforced concrete structures. When concrete carbonates, its pH decreases from a value in excess of 12.6 to less than 9 and, hence, a measure of the pH is an indicator of the degree of carbonation. This paper describes the development, testing and evaluation of two types of fibre optic sensors for the pH monitoring. One of these used a sol-gel based probe tip, into which an indicator dye has been introduced and the second used a disc containing an indicator operating over a narrower range of pH with shorter lifetime. Both were connected to a portable spectrometer system, which is used to monitor the spectral changes in optical absorption of the probe tip. A white light source to interrogate the active elements is used as the systems operate in the visible part of the spectrum. The two types of sensors have been found to be sensitive to the changes in pH due to carbonation, but the response time depended on the thickness of the coating material in the case of the sol-gel sensor. The durability of the sensors is still under investigation. The disc type sensor has a life span of approximately 1 month and, hence, it is not suitable for embedding in concrete for long-term monitoring of pH changes. However, it can be used for assessing the pH in vivo. The harder sol-gel is more durable and, hence, has a slower, but acceptable response time.
Methods for estimating the lifetime of reinforced concrete structures are being investigated with a fiber optic chemical sensor system, using a sol-gel as the matrix for the active material, for embedment in concrete for evaluation and testing. Results are presented on recent work.
Features are derived from wavelet transforms of images containing a
mixture of textures. In each case, the texture mixture is segmented, based on a 10-dimensional feature vector associated with every pixel. We show that the quality of the resulting segmentations can be characterized using the Potts or Ising spatial homogeneity parameter. This measure is defined from the segmentation labels. In order to have a better measure which takes into account both the segmentation labels and the input data, we determine the likelihood of the observed data given the model, which in turn is directly related to the Bayes information criterion, BIC. Finally we discuss how BIC is used as an approximation in model assessment using a Bayes factor.
We address the problems of (1) segmenting coarse from fine granularity materials, and (2) discriminating between materials of different granularities. For the former we use wavelet features, and an enhanced version of the widely used EM algorithm. A weighted Gaussian mixture model is used, with a second order spatial neighborhood. For granularity discrimination we investigate the use of multiresolution entropy. We illustrate the good results obtained with a number of practical cases.