IMPACT ECHO (IE)
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The results from the IE tests using portable rolling IE Scanner system show good agreement with the
actual defect design. In the IE testing by the authors to date, the clearest indication of the presence of
grouting defects is the apparent increase in the thickness due to a reduction in the IE resonant frequency
as a result of the decrease in stiffness associated with a defect. No direct “reflection” from the ducts with
grouting defects was observed in these experiments. This is because of larger wavelength generated by
the impactor inside the Impact-Echo Scanner. In this study, with the help of 3D visualization, the IE
scanning was able to identify voids as small as 9% depth lost or 20% circumferential diameter lost of a
101.6 mm or 4 inch diameter steel duct. Areas of disagreement between planned voids and IE results
need to be investigated to determine the cause with X-ray radiography in the future. Studies from the
mockup slab shows that half size and full size voids can be detected with the IE tests in 4.72 and 3.94
inches in diameter. Only full size voids can be detected inside ducts with a diameter of 3.15 inches.
However, once the concrete cover is 5.5 inches and higher, the IE results become intermittent and
unreliable. In summary, it is easier to detect grout defect in ducts with bigger diameters. In addition, the
deeper the duct is inside the concrete, the harder it is to detect grout defects with the IE tests.
ACKNOWLEDGEMENTS
The financial support from the NCHRP-IDEA program of the Transportation Research Board of the
National Academy of Sciences which made this research project possible is greatly appreciated by the
authors. Our sincere gratitude goes toward Dr. Herbert Wiggenhauser of BAM Laboratory in Berlin,
Germany for sharing a mock-up slab for Impact-Echo testing with our research team. The authors would
also like to express their gratitude to Mr. Jim Fabinski of EnCon Bridge Company (Denver, Colorado) for
donating a full scale bridge girder for use of this research and the assistance of Restruction Corporation in
grouting the ducts.
REFERENCES
[1] Concrete Society Technical Report No. 47, “Durable Bonded Post-Tensioned Concrete Bridges”,
Concrete Society, 1996
[2] Woodward, R.J. and Williams, F.W., “Collapse of the Ynys-y-Gwas Bridge, West Glamorgan,”
Proceeding of The Institution of Civil Engineers, Part 1, Vol. 84, August 1988, pp. 635-669.
[3] Florida Department of Transportation (FDOT) Central Structures Office, “Test and Assessment of
NDT Methods for Post Tensioning Systems in Segmental Balanced Cantilever Concrete Bridges, Report,
February 15, 2003.
[4] J. S. West, C. J. Larosche, B. D. Koester, J. E. Breen, and M. E. Kreger, “State-of-the-Art Report
about Durability of Post-tensioned Bridge Substructures”, Research Report 1405-1, Research Project 0-
1405, Texas Department of Transportation, October 1999.
[5]
Sansalone, M. J. and Streett, W. B., Impact-Echo Nondestructive Evaluation of Concrete and
Masonry. ISBN: 0-9612610-6-4, Bullbrier Press, Ithaca, N. Y, 1997 339 pp.
[6] D. Sack and L.D. Olson, “Impact Echo Scanning of Concrete Slabs and Pipes”, International
Conference on Advances on Concrete Technology, Las Vegas, NV, June 1995
[7]
ASTM C1383 "Test Method for Measurement P-Wave Speed and the Thickness of Concrete
Plates Using the Impact-Echo Method".
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