Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements (2006) / Chapter Skim
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Pages 13-20
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From page 13... ...
This is the rate of diffusion of the chloride ions into concrete and is designated by the use of a coefficient termed the diffusion coefficient (D)
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From page 14... ...
As long as the steel remains in good alkaline concrete, the passive layer will prevent corrosion initiation on the surface of the steel. This manual is primarily concerned with bridge superstructure elements and exposure to chloride ions from deicing salts.
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From page 15... ...
Time to Damage Time to damage is the sum total of the time required for chloride ions to diffuse down to the steel depth and accumulate in concentrations in excess of the corrosion threshold (Ti) plus the time required for sufficient corrosion to have occurred to generate the required amount of rust (or expansion)
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From page 16... ...
. A set of equations has been derived for the transport of oxygen and chloride ions through the concrete cover, the mass sources and sinks of oxygen and corrosion products, the cathodic and anodic electric potential, and the flow of ionic current through the concrete electrolyte.
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From page 17... ...
obtains probability distribution information on diffusion coefficient, clear concrete cover, and surface chloride ion concentration from the existing structure. It subdivides the concrete element into finite elements and determines the time to corrosion initiation for each finite element using the error solution to Fick's second law of diffusion.
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From page 18... ...
are selected from the statistical distribution functions such that the probability of selection of any value from the function is the same and can be applied to Equation 6. It should be noted that this particular solution of the Fick's second law assumes that C0 is constant during the diffusion process, although for an actual structure C0 increases with time as chloride ions from the environment accumulate in the concrete.
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From page 19... ...
If sufficient chloride ions are present to initiate corrosion, then corrosion-induced damage in the near future is expected, and only very aggressive corrosion mitigation techniques, such as cathodic protection and electrochemical chloride extraction, can be used to control the corrosion process. However, if the chloride ion concentration distribution at the steel depth is low and corrosion is not expected to initiate in the near future, less expensive corrosion control systems -- such as 19
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From page 20... ...
The second method uses the field data collected for the service life model. The diffusion coefficients, the surface chloride concentrations, and the clear concrete cover can be used in conjunction with the diffusion model presented for black steel to determine the distribution of chloride ions at a given steel depth at a given age.
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