| Corrosion of reinforcing bars ("rebars") in concrete is the most destructive mechanism contributing to damage in concrete bridges in the U.S. Corrosion products that are formed are two to six times more voluminous than the corroding steel and the resulting expansion causes the concrete to crack and eventually spall thereby accelerating structural deterioration. The alkaline environment provided within concrete protects steel rebars from corrosion due to the passive layer that is formed on their surface. However, chloride ions from deicing salts or from a marine environment that diffuse to the surface of rebars destroy the passive layer and initiate corrosion. While considerable research has been devoted to predicting the time to corrosion initiation, studies on predicting the time to cracking of the concrete cover from the time of corrosion initiation have been far fewer. Much of the treatment has been empirical or based on a constant corrosion current.;This dissertation develops a holistic model of the electrochemistry of corrosion that accounts for the diffusion of oxygen, moisture and chloride through the concrete and rust layers, the densification of rust due to confinement, the diffusion of rust into the concrete pores, the development of internal pressure due to rust build-up, and cracking of the concrete cover. Experimentally measured anodic polarization curves for the corrosion of mild steel in alkaline media at varying chloride concentrations, the limiting cathodic react7on, and models of the diffusion of oxygen and chloride through the concrete and rust layers, were used to determine the corrosion current in both the anodic and cathodic controlled regions. The boundary conditions, concrete quality, and cover thickness were included in the diffusion calculations. The relationship between the corrosion current and the pressure build-up due to the corrosion products for different concrete cover thicknesses and concrete quality was established through experiments using an accelerated corrosion test with an impressed current. Experiments were conducted to determine the conditions under which Faraday's Law could be used to accurately estimate the steel mass loss due to corrosion in the accelerated corrosion test. Results from the accelerated corrosion experiments were used to calibrate a model relating the rust build-up to the internal pressure generated on the concrete cover, allowing for the densification of rust due to confinement and the diffusion of rust into the concrete pores. Results from finite element analysis with an inelastic smeared crack concrete model were used to calibrate a simple analytical model of the critical internal pressure required to cause cracking of the concrete cover. The various sub-models were then linked together to predict the time for cracking of the concrete cover from the time of corrosion initiation.;Results from the model compare reasonably well with data reported in the literature from laboratory and field observations, as well as other models, although the variation in the data is considerable. The developed model was used to study the time for cracking of the concrete cover for different boundary conditions, concrete quality, and cover thicknesses. The model indicates that the time for corrosion initiation and the time from corrosion initiation to cracking of the concrete cover are comparable for certain boundary conditions and concrete quality. |
