Acoustic Nondestructive Evaluation of Concrete

The resonance method, also known as the Impact-Echo method, is an acoustic nondestructive method based on the principle of introducing a mechanical shock to the test body and recording the response of the test body to this excitation pulse. It is a common nondestructive method used in defectoscopy in various technical fields. Its particular variant, IE, is adapted for testing building materials, elements, and structures. Mechanical waves can propagate in three forms:

longitudinal waves (P-wave),

item transverse (shear) waves (S-wave), and

surface waves Raigley waves (R-wave).

These waves move through the material at a speed depending on the acoustic impedance Z of the material being measured. If the wave hits the interface of materials with different acoustic impedance, the mechanical energy of the wave is reflected, refracted, or absorbed. For an example, air has an acoustic impedance of 1.275 kg $·$ m ${}^{-2}·$ s ${}^{-1}$ , whereas concrete has an acoustic impedance equal to $10.35\times {10}^6$ kg $·$ m ${}^{-2}·$ s ${}^{-1}$ . This phenomenon is described by Snell’s law [25 (link)]. Due to this phenomenon, mechanical waves are reflected at the interface between the cement composite and the air cavity or surrounding environment. At the same time, mechanical waves interfacing with cracks, steel, and other materials that may be embedded in the concrete mass are affected. As a result, the incidental mechanical wave on the piezoceramic sensor has complex characteristics. The procedure of testing by IE method is illustrated in Figure 3.
The standard process for evaluating this waveform consists of applying a fast Fourier transform (FFT) and converting the measured signal from the time domain to the frequency domain. Then the dominant frequencies that can be assigned to the expected shape frequency are evaluated. In addition to these parameters, however, many other parameters can be assessed on the frequency spectrum that are no longer covered by the standard NDT approach within the IE method. Therefore, the aim of this paper is to verify whether the use of even nonstandard parameters obtained from measured acoustic signals can improve the accuracy in the classification of thermally degraded plain concrete test bodies.