How Impact-Echo Works

Schematic Diagram of Impact-Echo Method

A short-duration mechanical impact, produced by tapping a small steel sphere against a concrete or masonry surface, produces low-frequency stress waves (up to about 80 kHz) that propagate into the structure and are reflected by flaws and/or external surfaces. The wavelengths of these stress waves are typically between 50mm and 2000mm -- longer than the scale of natural inhomogeneous regions in concrete (aggregate, air bubbles, micro-cracks, etc.). As a result they are only weakly attenuated, and propagate through concrete almost as though it were a homogeneous elastic medium. Multiple reflections of these waves within the structure excite local modes of vibration, and the resulting surface displacements are recorded by a transducer located adjacent to the impact. The piezoelectric crystal in the transducer produces a voltage proportional to displacement, and the resulting voltage-time signal (called a waveform) is digitized and transferred to a computer, where it is transformed mathematically into a spectrum of amplitude vs. frequency. Both the waveform and spectrum are plotted on the computer screen. The dominant frequencies, which appear as peaks in the spectrum, are associated with multiple reflections of stress waves within the structure, or with flexural vibrations in thin or delaminated layers.

The fundamental equation of impact-echo is d = C/(2f), where d is the depth from which the stress waves are reflected (the depth of a flaw or the thickness of a solid structure), C is the wave speed, and f is the dominant frequency of the signal. The frequency f is obtained from the results of a test. To determine thickness or depth of a flaw, the wave speed C must be known. It can be measured by observing the travel time of a stress wave between two transducers held a fixed distance apart on the concrete surface or by performing a test on a solid slab of known thickness and observing the dominant frequency. In the latter case the equation is rearranged to give C = 2df (where d is the known thickness).

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For an in-depth study of the technology, refer to the book written by the inventor of the method, Mary Sansalone.