IMPACT ECHO (IE)
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(The exact position of the defects is not revealed to the public herein in order for others to be able
to perform future blind tests.)
- Varying numbers and position of wire strand cables (individual diameters of 0.6 inch) in the
tendon ducts.
EXPERIMENTAL IE SCANNER RESULTS FROM THE MOCKUP GIRDER
Interpretation of Impact-Echo Data
Localization of grouting discontinuities through Impact-Echo in the mockup girder was based on an
analysis of variations in the Impact-Echo thickness echo depth/frequency. A direct echo from the void or
duct wall, measured as an Impact-Echo frequency corresponding to the depth of the discontinuity (given
by the formula in equation (1) above) has not yet been observed with the IE Scanner. However, as
previously found by others, the IE results indicate the presence of well-grouted, filled tendon ducts by a
nil to minor increase in apparent wall thickness over a grouted duct (typically on the order of 12.7 mm or
0.5 inches or less but larger in the research due to the 3 day age of the comparatively weak grout versus
the hardened, mature concrete). Grouting defects (Styrofoam voids) inside the ducts cause a more
significant increase of the apparent wall thickness in IE results as presented herein vs. well-grouted
ducts). This is in accordance with the interpretation of the Impact-Echo signal as a resonance effect,
rather than a reflection of a localized acoustical wave, i.e., the void may be simply thought of as a hole in
the web wall that due to decreased section stiffness causes a reduction in the resonant IE echo frequency
and a corresponding increase in thickness.
Discussion of Impact-Echo Results
The Impact-Echo tests were performed using a rolling IE Scanner at 1, 3 and 8 days of grout age after the
duct grouting process was completed by Restruction Corporation of Sedalia, Colorado. This paper
presents results from the South and North walls for testing conducted 3 days after the grouting of the
ducts was completed. Ultrasonic Pulse Velocity tests were performed on a sample of the grout placed on-
site in a 406.4 mm (16 in) long duct section. The grout velocity at 3 days old was 11,400 ft/sec,
indicative of solid grout but about 25 % slower than the velocity of the mature, hardened web wall
concrete. The results from the IE scanning of the South wall are presented in a thickness tomogram
fashion as shown in Figures 5– 7 from the South wall and in a normalized thickness tomogram as shown
in Figures 8 - 10 from the North wall. Figures 5 – 10 all show the experimental results compared with the
actual defect designs of the ducts. The drawing of the actual defect design is placed above the
experimental results of the duct for comparison purposes.
South Wall – Top Duct:
The IE Scanner results from the South Wall with the actual defect design of the
top duct placed above the IE results of the top duct are shown in Figure 5. The results are interpreted to
indicate that the grout defect started to appear at a length of 1.93 m (76 in) and becomes clearly evident at
length of 2.92 m (115 in) from the west end of the duct. The location that the defect starts to appear
corresponds to void with 11% depth lost or 20% circumferential perimeter lost. The location that the
grout defect become more evident corresponds to void with 59% depth lost or 57% circumferential
contact diameter lost. The dominant frequency of the fully grouted duct is approximately 6.4 kHz,
resulting in an apparent Impact-Echo thickness of 28.37 mm (11.17 in.). The dominant frequency shifted
downward to approximately 5.37 kHz for an empty duct, which corresponds to an apparent Impact-Echo
thickness of 340 mm (13.4 in). This is a relatively large thickness shift of over 30% compared to the
nominal thickness of the wall. The interpretation of the Impact-Echo Scanner results, however, shows a
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