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Carbonation does not cause deterioration of concrete but damage on durability of concrete because of corrosion of steel embedded in hydrating cement paste. In order to prevent reinforcing steel from corrosion, it is very important to maintain passivation by measurement of the depth of carbonation and repair. Laboratory techniques that can be used to determine the depth of carbonation are indicators, X-ray diffraction, infra-red spectroscopy and thermogravimetric analysis, etc. In those methods, phenolphthalein method is most useful in practical measurement because the method is easy and rapid to perform despite low reliability. Even though researches, which are carried out to improve the reliability of phenolphthalein method through standardization of usage, are being conducted, phenolphthalein method still has potential problems. 1) It is impossible to measure the degree of early carbonation and 2) the quantity of OH- with varying depth because the pink color indicates the presence of Ca(OH)2 in accordance with pH 8.9.In this study, many indicators are selected and applied to improve the reliability of phenolphthalein and solve the potential problems.