Significance of macrocell corrosion of reinforcing steel in partially carbonated concrete: numerical and experimental investigation

Concrete carbonation is usually assumed to induce microcell corrosion of reinforcing steel by uniform depassivation. However, the carbonation front evolves continuously as a slow, time-dependent process and concrete carbonation is often non-uniform in real concrete structures. Hence some areas of the steel reinforcement network in concrete are likely to be depassivated before others and, consequently, the theoretical conditions of macrocell corrosion appear to occur quite frequently in real concrete structures. This paper therefore focuses on steel corrosion phenomenology in partly carbonated concrete structures. The conclusions are supported by an experimental and numerical study based on accelerated macrocell corrosion tests, carried out on partly carbonated concrete specimens with embedded active and passive steel bars. The specimen design allowed for electrically connecting one active bar with one or several passive bars in order to form macrocell corrosion systems with various cathode-to-anode (C/A) surface ratios. Despite the high resistivity of the carbonated concrete layer, direct measurements of galvanic current revealed high macrocell corrosion rates, even in cases of low C/A ratio. The significance of galvanic coupling in carbonation-induced corrosion was thus confirmed experimentally. In addition to experiments, a synthetic, but comprehensive, theoretical description of macrocell corrosion is proposed and finite element simulations of experiments are reported. The agreement between numerical and experimental results is demonstrated regarding both potential field and C/A influence on macrocell current intensity. This consistency highlights the relevance of the modeling and simulation approach.