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The impact-echo method is a non-destructive evaluation method in which internal pressure waves are excited through the thickness of a plate-like structure.  By reviewing the frequency content of the internal reflections of the wave, the wave velocity can be inferred, and internal flaws, voids, and delaminations can be detected by interruptions of the wave propagation.  This method has been in use for over a decade as a means of non-destructive evaluation of concrete slabs and masonry walls.  In the usual application of this method, a displacement transducer is put in positive contact with the surface of the structure under investigation, and the surface is struck with a hardened steel ball.  In the air-coupled impact echo method, displacement transducers are replaced by a microphone held 1-2 cm from the surface within an acoustically isolated enclosure In the present project, the air-coupled impact-echo method is evaluated for use in the condition assessment of historic properties, particularly concrete slabs and masonry walls.

The laboratory concrete slab specimens included intentionally cast-in defects, of different sizes and at different depths, and the ability of the system to detect these defects was examined.  The fundamental frequency was recorded over a grid of test points superimposed on the specimen, and colored contour plots of this frequency were developed.  The contour plots were very effective in indicating the locations of the defects, in both laboratory and field specimens.  The laboratory masonry specimens were built as 6 foot square two-wythe brick walls.  In addition to a control specimen, the walls included intentional defects such as unfilled collar joints, steel inclusions, and disrupted bond.  The walls were subjected to a testing program and the results of each test point were compared to a standard set of waveforms correlated to masonry defects, such as poor bonding, honeycombing, or very poor bonding.
It was concluded that the air-coupled impact-echo method is a promising alternative to displacement based impact echo analysis.  In laboratory and field trials, the system was very effective in locating areas where invisible defects were present.  The application of a microphone enclosure is significantly easier than a spring-loaded displacement transducer: it can be left on a horizontal surface, and needs minimum pressure to hold to a vertical surface.  The frequency plot of the acoustic signal is easy to read, containing very distinct peaks in the frequency response.  The system is applicable to concrete slabs between 2″ and 18″ in thickness, and to masonry walls up to 6 wythes.

This research was made possible through Grant MT-2210-10-NC-07 from the National Center for Preservation Technology and Training (NCPTT).

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