Detection of initiation of corrosion induced damage in concrete structures using nonlinear ultrasonic techniques.

Structural failure caused by corrosion of the reinforcing steel in concrete structures is quite common. In most cases, corrosion cracks appear on the surface at a late stage, leaving inadequate time for taking any measures. This paper investigates the detection of corrosion damage in reinforced concrete elements by using nonlinear ultrasonic (NLU) techniques. Various linear ultrasonic and NLU techniques were adopted to identify the most sensitive technique and ultrasonic parameters for corrosion induced damage detection at its early stage. It is observed that the linear techniques are not very effective in detecting corrosion induced damage. The sideband peak count-index (or SPC-I), a relatively new and promising technique, has been found to be an excellent indicator for the detection of corrosion induced damage initiation. However, its efficacy for detecting corrosion induced damage has not yet been reported. The present study shows that the SPC-I-based NLU technique outperforms (with the highest sensitivity) all other NLU techniques for detecting the onset of corrosion in steel and micro-crack formation in the surrounding material. As the corrosion progresses and cracks appear on the surface of the concrete, the efficiency of the SPC-I slowly weakens and other technique(s) are found to be quite efficient at that stage.