Incipient corrosion in concrete repair.

One of the main causes of deterioration of reinforced concrete structures is the corrosion of the reinforcement. This may be as a result of carbonation or chloride diffusion into the concrete. During the lifetime of any reinforced concrete structure it is likely to require maintenance and repair. Repair materials are used as a form of "corrosion prevention" on deteriorated areas of a structure. The properties of the repair material offer maximum protection to the steel at the repair. Combining two cementitious materials with different properties can create a difference in oxygen or chloride concentration between two materials, creating a galvanic cell. Matching the physical properties of the repair and substrate materials may mitigate the corrosion of the reinforcement. The theoretical and experimental work to support the idea of matching properties is currently limited.This project has examined the effect of combining repair and substrate materials with different physical properties on the corrosion of reinforcement embedded in these materials. The initial part of the experimental programme examined the physical properties of typical repair and substrate materials to quantify the range of the properties. This concentrated on mass-transport properties such as, the density, porosity, permeability, chloride diffusion and resistivity of each material. The electrochemical compatibility of the materials was measured using potentiodynamic polarisation measurements. This showed that steel embedded in materials with lower permeability coefficients had lower corrosion potentials. The permeability and chloride concentration in the materials were used to produce test specimens with range of differences in cell potentials between the steel embedded in the repair and substrate materials. The specimens of the different repair substrate combinations were exposed to a saline solution for twelve months. Measurements of the resistivity, half-cell potentials and corrosion currents of the specimens monitored with time assess the corrosion rate of the specimens. Two methods were used to measure corrosion currents, impedance spectroscopy and linear polarisation resistance. The half-cell potential measurements indicated that a low permeability material would be anodic when combined with a high permeability material, which would be cathodic. The corrosion current of the material identified as anodic by half-cell potentials was higher for larger mismatch in permeability. This indicated a higher corrosion rate in the anodic material for substrate repair combinations that result in large differences in halfcell potentials. This would suggest that a galvanic cell was formed due to oxygen concentration differences between the repair and substrate materials.The results from the experimental work were used to model the distribution of current between the anodic and cathodic sites. The model shows that it is the difference in potential that has the greatest influence on the current flowing in the cell and the resistivity of the material controls the distribution of the current within the cell. This indicted that corrosion would be concentrated at the interface between the repair and substrate material with the low permeability material being anodic. The study shows that the corrosion resulting from a disparity in properties between repair and substrate materials is likely to be small. However higher corrosion rates may occur at the interface between repair and substrate that may require additional corrosion protection systems to be used. Matching the permeabilities of materials would not be practical, as permeability has been found to change with time altering the match between repair and substrate.