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The corrosion of steel elements in reinforced concrete can cause considerable damage to civil-engineering structures. Early detection of rust is therefore very important. The aim of this paper is to evaluate the possibility of using nonlinear ultrasonic spectroscopy with a single exciting harmonic frequency and the impact-echo method for monitoring the corrosion of concrete-covered steel. For the research we manufactured concrete beams, reinforced with a standard reinforcing-steel bar passing through their centre. After concrete curing and drying, the samples were exposed to a 20 % carbon dioxide atmosphere. After the concrete pH decreased below 9.6 throughout the sample volume, the samples were immersed into a 5 % water solution of NaCl and subsequently placed into a drying oven. Measurements were carried out before the carbonation of the concrete, after it, and then after every 20th cycle of the accelerated degradation by chlorides. Nonlinear-ultrasonic-spectroscopy (NUS) methods are based on the fact that crack-induced nonlinearity makes a sensitive material-impairment indicator. The impact-echo method uses short-time mechanical impulses applied to the surface of a test sample, producing elastic waves. These waves spread throughout the sample and reflect from the surface but also from the microcracks and unobservable defects inside the sample. To verify the correctness of the NUS and impact-echo-method results, additional measurements were carried out (a confocal scanning microscope). It was proved that both methods could be used for monitoring the corrosion of concrete-covered steel.

Possibilities of NUS and Impact-Echo methods for monitoring steel corrosion in concrete