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Corrosion state of reinforcing steel in concrete as affected by concrete curing conditions
Abstract
An experimental investigation has been carried out to evaluate the effect of curing condition on the corrosion activity of the reinforcing steel in concrete. It has been shown that as long as the concrete is kept immersed in water, the passive film will not form. This may be due to the lack of oxygen at the interface of the steel and concrete. The corrosion evaluation of concrete using the half-cell potential is misleading when testing concrete which is continuously immersed in water. High potential values will be obtained, although no corrosion, activity exits.
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Corrosion monitoring of a reinforced concrete specimen was carried out for a period of 648 days starting from the day of its casting. The monitoring period included 53 days curing of the specimen in potable water, followed by exposure in the dry conditions of the laboratory for the remaining period. The test results indicate that the reinforcing steel did not attain passivity as long as the concrete specimen was immersed in water for curing. After taking the specimen out of water, the reinforcing steel took more than a year to attain a passive state.
This paper presents the findings of a laboratory investigation in which the applicability of Tafel plot and linear polarization resistance techniques in the corrosion rate measurements of reinforcing steel in concrete have been evaluated. The use of these electrochemical techniques in concrete is in a developing stage. Corrosion rates of reinforcing steel in concrete specimens made from three different mix proportions and immersed in salt solution for two years have been measured and compared with that obtained from metal loss determination.
Chloride induced corrosion is a major cause of the deterioration of steel reinforced concrete structures in marine environments, and in Northern environments where deicing salts are used. The time-to-corrosion initiation and subsequent corrosion induced damage is related to the time that it takes chloride ions to reach a critical level at the steel. In this paper it is demonstrated that chloride ingress into concrete follows Fick's Diffusion equation for properly cured concretes. It is shown how the diffusion coefficients and chloride surface concentrations can be used to predict the chloride profile as a function of time. The effects of concrete mixture proportioning and concrete admixtures on the diffusivity of chloride and chloride corrosion threshold level are shown. The results of several experiments and models developed show that reducing the water-to-cement ratio and increasing concrete cover over the steel greatly reduce the chloride ingress as recommended by the American Concrete Institute codes. Furthermore, even more dramatic decreases in chloride penetration can be obtained by the use of microsilica in the concrete mixture. It is also shown that calcium nitrite when admixed into the concrete significantly increases the chloride levels at which severe corrosion will occur. The above results are used to estimate the time to corrosion and failure for different types of reinforced concrete structures. These models can be used by a design engineer to estimate the life of reinforced concrete exposed to chloride ingress.