Effects of chloride ions and hydroxide ions on corrosion of steel bar in hardened cement composites

Currently, many of the infrastructure in Japan has passed more than 50 years of service life and decrepit has become a serious problem. Therefore, realization of effective and efficient maintenance of existing structures is strongly demanded. In general, the steel bar embedded in concrete is located in highly alkaline environment as over pH value 12 due to the cement hydration. And the rebar under highly alkaline environment is covered by a passivation film, so it is hard to corrode. Carbonation of concrete due to the reaction between carbon dioxide and cement hydrate as a result of the penetration of the carbon dioxide of the air into concrete, which cases the pH value in pore solution of concrete to fall. Carbonation destroys the passivation film on the steel bar in concrete, and corrosion progresses. In addition, it is well known that the passivation film is destroyed due to the existence of chloride ions, and the ratio of chloride ion concentration to hydroxide ion concentration ([Cl^-]/[OH^-]) is used as the index for onset of steel bar corrosion. However, there is no suitable method to estimate the pH value in concrete, although it is very important to measure the chloride ion concentration and pH value at the location of steel bar for understanding the remain service life of concrete structures. Considering these backgrounds, we investigated a method for estimating the pH internal concrete in the method of estimating pH value, and it carried out to measure the concentration of the hydroxide ions eluted from the powder collected from concrete by drilling. In this study, we also examined the influences of soluble chloride ions concentration and hydroxide ions concentration eluted from the powder on the corrosion of the steel bar. 2 types of specimens were prepared according to binder type. First one is OPC specimen which used only ordinary Portland cement as binder to make mortar. Second one is BB specimen. BB specimen is 45% replacement of ordinary Portland cement with grand granulated blast furnace slag. The BB specimen is expected to be lower internal pH because the amount of cement is small. For the test method, specimen was immersed in 10% NaCl solution for 3 days and drying at 20 degree Celsius and RH 60% for 4 days, of which total is 7 days as 1 cycle, were cyclically continued. The corrosion situation of steel bar was checked by the visual observation and the amount of soluble chloride ions concentration at the location of steel bar were measured at every cycle. As a result, for the OPC specimen, the steel bar in mortar began to corrode when the ratio of [Cl^-] to [OH^-] over 6.0 in the range of [OH^-] = 0.01 to 0.02 mol/L. And these ions concentration could be obtained by using the proposed drilled method.