Corrosion rate evolution in concrete structures exposed to the atmosphere

The data on corrosion rate values measured on-site in real size concrete structures are scarce, while the data bank of values in laboratory specimens is relatively larger. The majority of the experiments in the laboratory have been performed in chambers of controlled relative humidity and temperature, however real outdoor climate usually is characterized by day–night and seasonal temperature cycles. These cycles, or natural weathering, influence the internal relative humidity of the concrete and the corrosion rate of the steel. In the present paper, results of corrosion rate of steel in chloride containing concretes exposed to natural weathering, are presented.

Four main weather events have been identified to influence the corrosion rate of reinforcements due to the changes of the hydrothermal situation of the concrete: (a) day–night cycles, (b) seasonal cycles, (c) extreme temperatures and (d) rain periods. In unsheltered conditions it is the rain (moisture content of the concrete) which controls the corrosion rate. In concretes sheltered from rain it is the temperature the controlling factor of the moisture content and then, of the corrosion rate. Moisture is well represented by the electrical resistivity. A pure Arrhenius trend of the corrosion rate could not be found because several counter balance effects develop when temperature changes. The resistivity is the parameter that more comprehensively represents the corrosion rate.