This is a fundamental study of a non-destructive and contactless equipment to evaluate the chloride ion on concrete, with near infrared spectroscopy. Non-destructive measurements on Friedel's salt, generated by ingress of chloride ion into concrete, have been studied; characteristic absorption unique to each substance was utilized to qualitative and quantitative analyses. However, there has been a difficulty in separating absorption peak of Friedel’ salt from that of other hydrates. This study investigates the method to extract the absorption peak of Friedel's salt by Bayes’ theorem for quantitative evaluation, and its validity was confirmed using some conventional methods.
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