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General Impact Echo Principles
A tap on the surface of concrete produces stress waves. These waves are reflected by surfaces where there is sudden change in acoustical properties. For example, at the bottom surface of the concrete, or at cracks and internal defects in the concrete. On arrival at the surface of the concrete it meets another major acoustical change (air/concrete) and is again reflected back into the concrete, continuously repeating this cycle until all the energy is dissipated. As the wave continues to oscillate within the concrete it produces small movements at the concrete surface which are detected by the impact-echo equipment. These signals are transformed into characteristic frequency responses which can be interpreted by experienced impact-echo technologists. Each test takes a few moments to complete and most interpretations of the signal response can be determined immediately. A major advantage to this nondestructive test method is that only one side of the structural member need be accessible to complete a test.
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 New Transducer Technology
Although the principles of impact-echo technology have been known for over 40 years, it is only recently that new signal sensing and processing techniques have been developed which greatly enhance the quality of the impact-echo signal. Early impact-echo equipment typically used accelerometers to detect the impact-echo signals. The quality of signal detection using these types of transducers limited the use of this type of instrument.
The advanced instrumentation used by our company directly detects the surface movement caused by the impact energy. This technique substantially enhances the clarity and accuracy of the processed signal and enables trained operators to interpret secondary signal characteristics.
The test head requires the impact to be manually applied and has been designed to be used in more difficult circumstances. It is most useful when testing rough surfaces such as shotcrete, masonry, sanded overlays and deteriorated concrete. Additionally, this head is splash proof, allowing its use in damp conditions such as underground structures. These features, together with significantly increased signal clarity, allow our company to use this technology more effectively in many more difficult construction problems than has previously been possible.
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