Corrosion of steel in concrete is studied typically in uncracked concrete. In the field, however, concrete often has cracks that extend to the reinforcing steel. Electrochemical corrosion testing was performed on cracked concrete of two qualities. Results were compared to physical examination of the embedded reinforcement. Corrosion resistance improved significantly as the concrete properties and reinforcement cover approached that recommended in American Concrete Institute 318. Calcium nitrite additions to the concrete reduced corrosion significantly. Results indicated testing in cracked concrete should be performed in concrete representative of that specified in ACI 318.
EXPERIMENTALSpecimens were produced to compare reinforced concrete meeting minimum ACI specifications and recommendations (referred to as ACI-type beams) to concrete meeting the minimum recommendations of CEN/TC104 (CEN-type beams).
Concrete Beam PreparationBoth ACI-type and CEN-type beams used Black Steel 615, Grade 60, No. 4, reinforcing bars (rebars) of 13 mm (0.5 in.) diameter (Figure 1).Each beam contained three rebars, with CEN-type beams substituting the top rebar with a smoothsurface, mild-steel bar. All rebars were pickled in a 10% sulfuric acid (H 2 SO 4 ) solution, rinsed in water, and dried with methanol. The H 2 SO 4 bath gave all rebars the same initial condition with respect to existing corrosion product. Ends of the bars were covered with electroplater's tape, leaving a 508 mm (20 in.) length of bar uncovered. A total of 20 concrete beams (12 CENtype and 8 ACI-type ) were fabricated, with dimensions of 762 mm by152 mm by 152 mm (30 in. by 6 in. by 6 in.). For all CEN-type beams, concrete cover was 25.4 mm (1 in.) over the smooth top rebar, while ACItype beam concrete cover over the top bar was 38 mm (1.5 in.). Table 1 shows all fresh and hardened concrete properties for CEN-type and ACI-type beams.
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 recommemded 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.
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