|Ph.D Student||Kenny Amit|
|Subject||The Micro Structure of Concrete around Embedded Steel|
Influence on the Chloride Threshold for Chloride
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Amnon Katz|
|Full Thesis text|
Chloride induced corrosion is the main cause for corrosion initiation of reinforced concrete (RC), which has significant economic implications. Elevating the chloride concentration for steel depassivation (Clth) may significantly reduce RC life cycle cost .
A work to find any relationship between the concrete microstructure, at the interfacial transition zone (ITZ) of embedded steel rebar, and the Clth was performed .
Concrete mixes were cast with horizontal or vertical rebars. The specimens were used for corrosion test and to characterize the steel-concrete ITZ. Corrosion initiation was detected by potential shift, and was validated by visual inspection of rebars, and for several specimens by electrochemical impedance .
A model of an interface was analyzed as well to validate results . Microstructure of the ITZ was analyzed using backscatter secondary electron microscopy image analysis. The properties of the ITZ which were analyzed are: thickness, porosity, and steel-concrete distance . The ITZ thickness ranges from 19 μm to 529 μm, and steel-concrete distance ranges from 2 μm to 83 μm .
In addition to the image analysis, mechanical test of the steel-concrete bond was done .
In vertical rebar casts, the steel-concrete distance tends is be smaller as the mix becomes more fluid. In horizontal rebar casts, the steel-concrete distance is influenced by processes that occur during settlement of the fresh concrete and early hydration. The ITZ thickness around horizontal rebars and its variation depends on mix properties related to bleeding.
Chloride threshold values in this study ranged from 0.56 to 7.98 gr Cl/kg concrete. This range overlaps the published range. For both rebar orientations, the Clth correlated with the steel-concrete distance. The Clth of the horizontal rebars correlated with the ITZ thickness as well. The Clth of the vertical rebar was correlated with some mix properties which are associated with the steel-concrete distance. Smaller steel-concrete distances are associated with elevated Clth .
A mathematical model of concentration polarization of hydronium was modified and examined with data which represent steel-concrete interface. Considering the pH of concrete components, the pH-distance relationships found in the model fits the experimental results . Hereby affirm the polarization concentration as a mechanism by which ITZ influence corrosion initiation. In addition, these results imply preferential precipitation of minerals with different pH's at the ITZ. A similar phenomenon of different minerals close and away from a steel rebar was observed in the simulated ITZ model as well.