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Impact-echo Testing of the Aeration
Basin Walls
Wastewater Treatment Plant
The aeration basin was a rectangular concrete tank with approximate
dimensions of 242'x 98'x 21' high. The tank was divided into several
chambers with the dividing walls with thickness between 14" and 16".
The walls were reinforced with either #5 or # 6 rebar placed at 6"
on center. The hairline cracks appeared on some of the walls after
a year. Reportedly, testing of the  chambers
for water tightness showed no measurable leakage rates. However, at
some places of the walls, seepage of moisture took place through the
hairline cracks. Attempts to seal the cracks were unsuccessful and
concerns were raised of possible significant internal voids in the
concrete. Concrete Science, Inc. was contracted to locate such suspect
voids in the walls.
Concrete Science, Inc. conducted impact-echo testing on the suspect
walls. Test results were very consistent and repeatable. Analysis
of the frequency spectra of all the testing showed only the frequencies
associated with the thickness of the walls. Frequency variation was
in generally accepted range. None of the test results (away from the
patched areas) showed frequencies associated with interior voids.
In the repaired areas of the cracks where the wall surfaces were wet,
impact-echo testing showed "normal" results indicating absence of
significant crack openings. The test results showed that some of the
repaired patches were delaminated near the surface.
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