Nondestructive Test Methods

Impact-Echo

Impact-Echo is a nondestructive test method specifically developed for concrete and masonry structures. The method has achieved unparalled success with flaw detection in concrete and masonry structures. WDP has been an industry leader in the application of impact-echo in the evaluation of concrete and masonry structures and in the development of new applications for the method.

The impact-echo method uses transient stress waves generated in the concrete or masonry by an elastic, low-energy impact on the surface of the member. As the stress waves propagate through the material being tested, they are reflected by air interfaces within the member and the external boundaries of the member. Examples of air interfaces are: delaminations, voids/honeycombing, and cracks.

Wave arrivals at the surface of the concrete or masonry are recorded by a displacement transducer placed near the impact point. The record of displacement versus time is transformed into the frequency domain for ease of signal analysis. The presence and nature of any internal flaws or external interfaces can be determined from analysis of the time-domain waveform and frequency spectrum.

Impact-echo is often confused with ultrasonic methods such as pulse-velocity and pulse-echo. Unlike ultrasonic methods, impact-echo utilizes lower frequency sound (2 to 20 kHz, typically). The use of lower frequencies allows impact-echo to overcome the high signal attenuation and noise frequently encountered with ultrasonic methods.

Typical applications of Impact-Echo Testing include:

  • Slab thickness measurements (ASTM C-964)
  • Detecting delaminations, cracks and voids
  • Evaluating unconsolidated concrete
  • Locating voids in grouted tendon ducts
  • Locating subgrade voids beneath foundation slabs
  • Evaluating mine shafts and tunnel liners
  • Finding voids in grouted masonry
  • Evaluating distributed damage in concrete (ASR, DEF)