Characterization of concentrated and distributed cracks in concrete using a harmonic wave modulation technique

In this work, the damage progression of concrete cracks presented in concentrated form caused by mechanical load and presented in distributed form caused by chemical attack is characterized by a harmonic wave modulation technique. The relative amplitude of sidebands in frequency domain due to the modulation effect of two propagating ultrasonic waves is considered as the damage index for concrete samples. The low frequency signal in this research is a harmonic wave produced by an electromagnetic exciter to avoid the uncertainty of man-made influence previously where the low frequency pulse signal was generated by the instrumented hammer. Both the first order sideband corresponding to classical nonlinearity effect and the second order sideband corresponding to hysteresis nonlinearity effect are simultaneously analyzed through the ultrasonic experiments for damaged samples induced by the sulfate attack and bending tests. The nonlinear parameters present a good distinction between damaged and intact samples for both concentrated cracks and distributed cracks and have an increase in orders of magnitude, showing the well-round feasibility of developed nonlinear wave modulation technique. Furthermore, the nonlinear parameters in the wave modulation technique present an excellent correlation with the nonlinear parameter obtained by the second harmonic generation method for bending cracks of the same reinforced concrete, indicating that the wave modulation method is capable of discriminating different state of damage presented in concentrated cracking form.