Impact Echo


Impact-echo is a method for nondestructive evaluation of concrete and masonry structures, based on the use of impact-generated stress (sound) waves that propagate through a structure and are reflected by internal flaws and external surfaces.

  • It provides independent, nondestructive measurements of the thickness of concrete slabs with an accuracy of 3% or better.

  • It can determine the location and extent of flaws such as cracks, delaminations, voids, honeycombing, and debonding in plain, reinforced and post-tensioned concrete structures.

  • It can locate voids in the subgrade beneath slabs and pavements.

  • For masonry it can determine thickness and locate cracks, voids and other defects where the brick or block units are bonded together with mortar.

  • It is not adversely affected by the presence of steel reinforcing bars.

How It Works


A short duration impact, produced by tapping a small steel sphere against a concrete or masonry surface, is used to generate low-frequency stress waves (1 to 60 kHz) that propagate into the structure and are reflected by flaws and external surfaces. The use of long-wavelength, low-frequency stress waves distinguishes impact-echo from traditional ultrasonic methods.

These stress waves propagate through concrete as though it were a homogeneous elastic medium. Surface displacements caused by the arrival of reflected waves at the impact surface are recorded by a transducer, located adjacent to the impact site, producing an analog voltage signal proportional to displacement. The resulting voltage-time signal, called a waveform, is digitized and transferred to the computer memory, 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, and they provide information about the thickness of the structure, its integrity, and the location of flaws.


The Impact-Echo Test System


The principal components of a test system are a cylindrical hand-held transducer unit, a set of spherical impactors, a notebook computer, a high-speed analog/digital data acquisition system, and a software system that controls and monitors the tests and displays the results in numerical and graphical form. Power is provided by internal batteries in the notebook computer and data acquisition system.

Instrument Configurations Used in Testing :

Flaw Detection &
Thickness Measurement

The configuration is used for routine impact-echo testing. It weighs about 18 lbs (8.2 kg).

Wave Speed Measurement
The configuration is used for direct measurement of wave speed, by measuring the travel time of a P-wave between two transducers a fixed distance apart on the surface.With a 7.5 lbs notebook computer, the system weighs about 13 lbs (5.9kg).

ASTM Standard for measuring concrete thickness: ASTM Standard C1383-98, "Standard Test Method for measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method" appears in the 1998 American Society for Testing and Materials (ASTM) Book of Standards.