Methods of remedial treatment for carbonation-induced corrosion of reinforced concrete
The importance of carbonation-induced corrosion has grown in recent years owing to the increasing age of reinforced concrete structures. Among the various remedial treatments for the alleviation of the deterioration process, applications of corrosion inhibitors and surface coatings are highlighted in this thesis owing to their ease of practical application to such structures as railway viaducts. The factors that may enhance the effectiveness of the above two methods in terms of retardation of embedded steel corrosion have investigated as follows: (1) Electrochemical injection of corrosion inhibitors into concrete and (2) Fatigue resistance of surface coatings. (1) After the concentration threshold of electrolytes of three organic base corrosion inhibitors, namely ethanolamine, guanidine, and arginine, required for steel passivation had been investigated by steel immersion tests, the inhibitors were injected into fully/partially carbonated cement-based materials from external electrolytes under the influence of an electrical field. The penetrations of the three inhibitors into the embedded steel cathode were satisfactory in terms of steel inhibition. The field-induced penetration was markedly affected by the pK. values of the inhibitors and the pore solution pH. When the electrochemical treatment was applied to partially carbonated cementitious materials, adequate accumulation of the inhibitors was also attained at the cathode for steel passivation; however, the migration of cationic inhibitors was found to be discouraged in the carbonated region by the lowered current densities effectively applied to this region owing to its large resistivity. Mathematical modelling was performed for simulation of the proposed electrochemical inhibitor injection. A model based on the Nernst-Planck equation, taking account of dissociation equilibria and solubility products of the relevant species, and activity coefficient of molecules, yielded a reasonable agreement with the experimental data. In the application of this model to the 2-D cases, representing the domain with a resistor network that could simulate the current distribution within the material resulted in good prediction of concentrations of the species observed in the experiments. The long-term effectiveness of the electrochemical inhibitor injection was monitored for a reasonably long period, whilst the treated concrete specimens were exposed to cyclic wet/dry conditions. As a result, the injected corrosion inhibitors were found to be effective in promoting steel passivation, and ethanolamine showed the best performance with the smallest steel corrosion rates observed during the experiment. (2) For the investigation of properties of surface coatings affecting their long-term fatigue resistance, fatigue tests with a total of 18 million cycles were carried out under varied temperatures for several coatings bridging a substrate crack whose properties( width and dynamic amplitude) were determined by on-site survey methods. It was found that the thickness and the composition are two important properties of surface coatings that significantly influenced their long-term durability when they were exposed to varied environmental temperatures.