IMPROVING THE ACCURACY OF IMPACT-ECHO IN TESTING
POST-TENSIONING DUCTS
C. Colla
via Piacenza 51, 29016 Cortemaggiore (Pc), Italy
ABSTRACT. The need for greater accuracy in positioning metal ducts in concrete and in detecting their
grouting faults motivated the application of the scanning impact-echo method on site for testing a posttensioned beam. 2-D data display allowed recognition of signal patterns improving interpretation
reliability with regard to lateral position and concrete cover of ducts, and their grouting condition.
Partially grouted ducts presenting small voiding were discriminated from fully grouted ducts and findings
compared with cores. New interpretation criteria were highlighted and alternative wave behavior
considered.
INTRODUCTION
Concern exists worldwide over the safety and durability of post-tensioned
structures as a result of voids around the tendons in the ducts. Faults in tendon duct
grouting permit the formation of voids in the duct, which can give rise to ingress of water
and chlorides, corrosion, and increased stress. Considerable difficulties have arisen in
terms of inspection techniques for these structures and also NDT has had so far a limited
success. Available non destructive investigation techniques for ducts include X-ray,
ultrasonic, electrical resistance, magnetic perturbation, georadar, etc. although their use
presents a number of limitations and requires engineering expertise and interpretation.
Today's civil engineering needs are for fast-to-apply and easy-to-interpret NDT methods
with the condition that their findings are capable of providing quantitative assessment
information for use in damage analysis and maintenance planning. Where possible,
available testing techniques have to be modified and improved in order to ease
applicability on site with minimum disruption activity, enhance data quality, provide
complete and reliable information with data visualization in real-time, independently from
user know-how. Furthermore, the results of the testing should be made available in a form
which can be easily understood by the engineer responsible for the structure, for example
in the form of 2- and 3-D data sets which can be related to structural cross-sections.
WORK OBJECTIVES
The work reported herein presents methodology and results from an advanced non
destructive testing technique, the scanning impact-echo method, applied on site with the
aim of evaluating post-tensioned concrete infrastructure. The data analysis from 2dimensional data visualization aims at providing accurate quantitative information with
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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regard to concrete thickness, lateral location of metal post-tensioning ducts, estimation of
their concrete cover and, finally, investigation of their grouting condition.
A critical interpretation of the impact-echo data seeks to improve the accuracy of
the technique on a number of tasks and introduces the recognition of signal patterns in data
imaging as well as the consideration of alternative wave propagation paths in concrete
around tensioning tendons. The highlighted signal patterns constitute still unreported data
interpretation criteria for discriminating between fully grouted and partially grouted ducts.
Result verification on large cores sampled from the tested areas have proven that
the criteria provided for data interpretation are highly reliable and thus that the technique
has the capability to evaluate existing post-tensioned concrete and the potential for quality
control of new construction.
PRINCIPLES OF THE IMPACT-ECHO TECHNIQUE
The impact-echo method relies on frequency analysis of wave echos propagating
inside the structural element under investigation. Through a mechanical shock generated at
the surface via a metal tip, the transient stress wave propagates from the impact point in the
concrete half-space as surface, compression and shear waves. The surface wave amplitude
will dominate over the other waves at the concrete surface but will decrease exponentially
with depth. Instead, shear and compression waves propagate into the structure along
spherical wavefronts. Particle displacement due to shear waves is maximum in a direction
approximately at 45° on the impact direction whilst compression wave displacement
presents maximum amplitude in the direction of impact and its energy dominates that of
the shear wave. The impact-echo method focuses on multiple P-wave reflections between
the impact surface and the opposite surface or intermediate interfaces with sufficient
impedance variation and dimension in comparison with the wavelength employed.
These transient wave resonances are registered via a sensor located on the surface,
close to the impact point, and their distribution of amplitudes and frequencies is analysed
in the recorded spectra. Relevant signal reflections in the structure are singled out by the
frequency position of peaks with dominant amplitudes. The depth (d) of interfaces may
then be estimated from the relation between P-wave speed (vp) and peak frequency (/) frequency being the inverse of wave propagation time - as shown in figure 1 left [1]:
= vp/2f.
FIGURE 1. Principle of 1-D impact-echo testing (left) and 2-D scanning impact-echo (right).
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(1)
An advancement in the possibilities offered by the traditional impact-echo testing
method, based on data collection at local points, is the scanning version of impact-echo [2].
Here, thanks to high density data collection at discrete points equally spaced along survey
lines, impact-echo waveforms and spectra may be visualized on site in real time also as 2dimensional data imaging (Fig. 1 right).
These plots, or impactechograms, may be directly related to structural crosssections of the object under investigation as one axis represents the testing position on the
concrete along the survey line, whilst the other axis indicates the recorded frequency or
corresponding values of calculated depth. Intensities of gray tones represent amplitudes of
wave reflection at interfaces. In the impactechograms, the relevant wave reflection
frequencies (those with maximum amplitude) are more easily identifiable and analyzed
than in peak comparison of single spectra from scattered measurement stations. In fact, it is
the immediate recognition of signal patterns in the 2-D data plots that allows a more rapid
and reliable analysis of data from scanning impact-echo.
WAVE PROPAGATION FOR THE AIMS OF TESTING POST-TENSIONED
CONCRETE
The wave resolution is a critical point particularly in the case of testing posttensioned concrete. The relative small dimension of the ducts and particularly of the
tendons inside in comparison to typical impact-echo wavelengths and the approximate
round shape of the tendons hardly represent an ideal reflection surface for the aims of
impact-echo testing. From the equation of wave propagation,
v =ft
(2)
where v is the wave speed and A is the wavelength, it is evident that the relationship
between wavelength and dimension and depth of the duct is fundamental in detecting the
metal tendons or any air pockets in the grout inside the ducts. For this reason, the use of
higher frequency waves, with their shorter wavelength, will be preferable in post-tensioned
concrete testing. These are characterized by higher maximum useful frequency, fmax,
although their maximum amplitudes is lower and attenuation due to absorption and
scattering is greater. So that if in most impact-echo applications, broadband frequency
stress waves in the range up to circa 30 kHz are commonly employed, for the testing of
ducts frequencies up to 50 kHz and above are more appropriate. The minimum useful
wavelength of a broadband frequency wave is obtained from the maximum frequency, see
equation (3). The maximum frequency of useful energy, fmax, may be approximated from
the impact contact time, tC9 as seen in equation (4).
A<min ~ V' Jmax'
fmax=1.25/tc.
\*)
(4)
It has been reported that, given a wave of sufficiently short wavelength, a flaw or
other inhomogeneity in concrete can be detected via impact-echo depending upon its
lateral dimensions in function of depth. If the lateral dimensions are at least one-fourth of
the depth, the presence of the flaw can be detected; its depth can be determined if the
dimension is greater than one-third of the depth [1].
Furthermore it is required that the depth of the flaw be greater or equal to half of
the wavelength for the waveform to capture the periodic response between impact surface
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and top of the flaw. Further, the flaw's lateral dimensions will determine which frequency
components will be reflected, i.e. those with wavelength smaller or equal to the flaw's
dimension. In addition, the applicability of the method to bonded post-tensioning tendons
depends on the geometry of the structure and the location and arrangement of the tendon
ducts [1]. The tendon ducts themselves, when made of thin steel are invisible to the
impact-echo waves whilst the grout inside the ducts present acoustic impedance similar to
concrete.
The expected impact-echo response to the main cases that may be present in posttensioned concrete is represented in figure 2, where the black arrows represent the
schematic propagation path of compression waves. According to [1], in the presence of a
void duct (case b), to a shifted thickness frequency of the plate,/r, (lower frequency than
for the solid plate response of case a and equation (1) is registered due to a longer wave
propagation path) is associated a duct frequency peak, f& corresponding to compression
wave reflection at the top side of the void in the duct, as follows:
a
(5)
(6)
where a is a shift factor depending on the size and position of the duct, T is the plate
thickness and dvoid is the duct concrete cover.
In presence of a partially void duct, with the void not directly at the top of the duct,
case c in figure 2, the impact-echo response of the plate will be somehow similar to the
previous one, but with the reflection at the duct occurring at a slightly lower frequency
than in case b. If dpan void is the depth to the air pocket in the duct and 7 is a correction
factor for the flaw depth, it will be:
fd = (0.96vp/2dpartvoid)-r
CO
Finally, in presence of a fully grouted duct, case d, part of the wave will travel
through the tendons and reach the bottom of the plate, while part of the energy will be
reflected at the grout/tendons interface. Here, however, because of the higher acoustic
impedance of steel in comparison to concrete and grout, the compression wave will not
undergo a phase change such as at a concrete/air interface (concrete has a higher
impedance than air). Therefore the reflected wave arriving back at the impact point will
not be sensed by the transducer until its second arrival, when the wave is incident upon the
impact surface as a tension wave, after a phase change occurred between the first and
second propagation into the solid. With these motivations, the following equation has been
proposed for the concrete cover of a filled duct [1], with the notation that however the
frequency response registered at the tendons will be higher than the value predicted by
equation (8), where dtendon is the depth to the shallower tendon wires:
fd = Vp/4dtendon
llr..
J;*.. , . , , . ,
(8)
^
FIGURE 2. Cases of impact-echo responses in post-tensioned concrete.
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DATA COLLECTION AND INTERPRETATION
During the dismantle of a 30-year old highway bridge, part of a post-tensioned
concrete I-beam has been made available for non destructive field testing and scanning
impact-echo measurements have been carried out [3]. The web thickness of the girder
(3.65 m high) was smaller than 30 cm and contained corrugated steel ducts (circa 67 mm in
diameter) spaced about 28 cm on centerline at the area of testing. Twelve 1-cm diameter
metal tendons were contained in each duct. Vertical measurement lines run on the web for
a length just over 0.9 m and oriented approximately transversally to the known direction of
the post-tensioning ducts.
The main component of the measurement equipment consisted of a single impactecho testing unit on which are assembled both the wave impact source - a small-size metal
blade driven by a solenoid- and the transducer for detection of the wave reflections. On
the concrete of the girder object of this investigation, the operating frequency of the
commercial testing equipment (Olson IE II) was contained on average in a range between
10 Hz and 23 kHz. The equipment settings during data collection included a rate of
150375, 2048 points and a frequency resolution smaller than 0.1 kHz. A PC with dedicated
software was used for data collection, visualization and storage.
As an example, the unfiltered impactechogram obtained from data collected along
one of the vertical survey lines is shown in Fig. 3 left, as frequency plot in a range from 3
kHz to 15 kHz (other relevant signals were not recognized outside this frequency range).
The darker tones of gray in the image represent higher signal amplitudes.
The horizontal black band at about 7 kHz frequency is the most evident feature in
the image. It represents the wave reflection at the backside of the web and allows
calculating the thickness of the concrete web. An estimation of wave propagation velocity
in concrete of 3800 m/s and the use of equation (1) led to evaluate the concrete web as 270
mm thick. This thickness frequency line has a regular horizontal ongoing which testifies of
the uniform web thickness and constant wave velocity in the concrete, thus providing
indication that the concrete is homogeneously good. Only between stations 10 and 35, a
small shift in the thickness frequency towards lower frequencies is registered. At the
station point at 22 cm, in the middle of this region, the web shifted thickness frequency
value recorded is 6.53 kHz, thus a frequency drop of circa 4% is registered. As a shifted
thickness frequency is associated with the presence of a unfilled or partially filled duct see fig. 2 cases b and c -, this position was associated with the presence in the plate of a
duct with grouting faults.
Other important features in the image are strong reflections appearing approximately
between 1 1 and 12 kHz frequency. Their signal intensity is weaker than the thickness fre-
;;;:::;-;:t;
:
ft
"if
1ft
FIGURE 3. Impact echogram (left), core with grouting flaw (middle) and core with filled duct (right).
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quency, but clearly recognizable in the impactechogram. These features are interpreted as
the presence of three ducts in the concrete [3].
In particular, between station 15 and 25, a high intensity reflection region appears
between 10.5 kHz and 12 kHz frequency, with maximum peak amplitude at 11 kHz.
Associated with this peak is what appears as a hyperbola extending downwards and with
its branches reaching the plate thickness frequency in the region where a shift in the
thickness frequency has been described. The peculiar hyperbolic shape of this reflection,
so far unrecorded in published literature of impact-echo data from post-tensioning ducts or
other structures, constitutes a well recognizable signal patter in the impactechogram. Due
to its appearance in combination with a small shift of the thickness frequency towards
lower values, this feature was interpreted as a supplementary data interpretation criterion
for a duct with grouting faults [3], either an unfilled or most likely a partially filled duct.
The concrete cover to the void or partially filled duct can be calculated from equation (6)
in the first case or following equation (7) in the second. The estimated duct depth would be
contained between 173 mm and circa 158 mm if a frequency shift of about 1 kHz were
supposed in the second case. The lateral position of the duct along the measurement line
was considered at the point of highest frequency in the hyperbola signature, thus with
center at station 22.
In addition to this duct, as mentioned above indications of two other ducts appear in
the impactechogram. Two short horizontal segments are discernable at 11.9 kHz between
stations 45 and 55, and between stations 72 and 82 respectively. At the position of these
ducts, with estimated center at station 50 and 78, were combined neither hyperbolic
signatures nor shift in the web thickness frequency, thus leading to interpret their grouting
condition as good. By using equation (8), the concrete cover of fully grouted ducts was
estimated smaller than 80 mm for both these ducts.
DESTRUCTIVE VERIFICATION OF RESULTS
To verify the impact-echo data interpretation from the 2-dimensional analysis
exposed above, the position of large cores (200 mm in diameter) was carefully chosen and
marked on the concrete surface. In particular, two cores (see figure 3 middle and right)
were bored along the measurement line corresponding to the data of figure 3-left The
inspection of cores sampled from the tested area has permitted to accurately measure both
the web thickness and the ducts concrete cover, besides ascertaining their lateral position.
Furthermore, the grouting condition of the ducts could be observed on the samples.
The concrete cores showed a thickness between 275 mm and 280 mm; this
permitted to adjust the estimated wave propagation speed to 3920 m/s and to re-calculate
the depth values provided above to more accurately represent real figures of duct concrete
cover. The lateral position of the three ducts as indicated by the analysis of the impact-echo
data was found to be correct, with a small error ranging between 5 mm and 10 mm. This
value was considered satisfactory.
Observation of the core from the area on the left hand side of figure 3-left showed
that the grouting condition of this duct was faulty, with the duct partially unfilled by the
presence of a small air void between the tendons on one side of the duct section. This void
reaches almost the top of the duct (figure 3-center). Thus this check confirmed the impactecho data interpretation of an unfilled or partially filled duct. It was also possible to verify
the condition of the middle duct, thanks to a second core. The duct appeared fully grouted
(figure 3-right), thus again satisfactorily confirming the data interpretation.
Instead, the verification of the concrete cover values for the same two ducts proved
the estimated vertical position as indicated by the impact-echo interpretation to be
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erroneous: excessive in one case and conservative in the other. For the partially grouted
duct a depth of 97 mm was measured on the core whilst the re-calculate impact-echo value
yields 163 mm by using equation (7) as above, thus an error of+68%. For the neighboring
duct, fully grouted, the real depth to the shallower metal tendons, which in this case
coincides with the duct depth, is 96 mm while the impact-echo interpretation indicates 82
mm (error:-15%).
Therefore, if the precision of the lateral location of ducts in concrete and especially
the interpretation of duct's grouting condition have shown to be reliable, the depth
indications for both ducts were considered unsatisfactory.
CONCRETE COVER ACCURACY FOR FULLY AND PARTIALLY GROUTED
DUCTS
The above observations show that the employed equations (7) and (8) failed in
providing satisfactory values of concrete cover for partially grouted and fully grouted ducts
respectively. In particular, the recorded signal frequency for these 2 categories of ducts
were very similar, and the criterion of a partially filled duct exhibiting a frequency almost
double than that of a grouted duct - implicit in the comparison of equations (7) and (8)-,
could not be verified in the data from this testing. Similar observations and error ranges to
those highlighted above were verified on all the data collected on this structure.
Therefore, alternatively to the solutions sketched in fig. 2, other options of
compression wave propagation paths in concrete in the presence of filled and partially
filled ducts were empirically considered in view of the recorded frequency values read in
the data.
One plausible explanation is to consider that the wave is not reflected at the
concrete/tendon interface but that it travels around the tendons, thus reaching the tendons
at the furtherer side from the impact point, and from there it travels back to the impact
surface after being diffracted by the metal tendons. This would imply that the frequency
values registered in the data correspond not to the depth of the shallower tendons but to the
deepest. Without the use of the modified equation (8), it would be possible to calculate the
concrete cover of filled ducts simply by using equation (1) and considering that the
obtained value is the sum of the concrete cover plus the diameter of the bundle of tendons:
Uduct + "^tendons ~ Vp '2jrear tendon
(9)
where dduct is the concrete cover of a filled duct, 0tendons is the tendon bundle diameter
(which can be readily estimated) and frear tendon is the frequency corresponding to the
deepest tendon.
The re-calculation allowed to obtain a greatly improved value of depth for the fully
grouted duct and the error was contained within 9 mm (+5% or less) by considering
0tendons = 60 mm.
The impact-echo response for the partially grouted duct is somehow more
complicated and a clear explanation for the hyperbola signature need to be developed. It is
possible that due to the presence of the air pocket in the grout that changes the stiffness of
the plate at this positions, the wave generates a transient resonance that can be detected
also before and after the impact and sensor are positioned at azimuth over the duct thus
allowing to record the highlighted hyperbola feature. It may also be possible that at reading
stations over the region of the hyperbola, shear waves contribute to detect the small flaw in
the duct, as the air pocket will constitute a concrete/air interface of high impedance
contrast. Note that shear wave amplitudes present a maximum in a direction approximately
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at 45° to the impact surface and shear wave wavelengths are almost half of longitudinal
wave's. In addition, the flaw in this case presents a boundary (its larger lateral dimension)
almost perpendicular to the impact surface. It would be:
Wpart void ~ Vs '^Jpart void
v •*•"/
where vs is the shear wave propagation velocity (vs = 0.62 vp) and fpan voi(j is the frequency
corresponding to the reflection at the impact-near side of the small flaw in the duct.
In this case, an approximate calculation considering the flaw depth at 105 mm,
shows that the depth error would be contained within 5 mm (or circa 5%).
CONCLUSION
The scanning impact-echo, a non destructive evaluation acoustic technique, was
applied on site for investigating a post-tensioned concrete beam and visualizing in real
time cross-sections of the concrete web containing metal tensioning ducts (67 mm in
diameter). Thanks to recognition of clear signal patterns in the impactechograms produced,
the method was successful in calculating the thickness of the concrete beam and in locating
precisely the lateral position of the ducts. It was also accurate in evaluating correctly the
grouting condition of the ducts by identifying voiding in one of the ducts and in indicating
filled ducts. Autopsy of the beam in the form of large cores confirmed that discrimination
between fully grouted and partially grouted ducts was reliable. The 2-dimensional data
visualization allowed to single out a hyperbolic feature which can be used as new
interpretation criterion for partially grouted ducts with a small flaws.
Furthermore, option hypotheses of shear wave reflection and longitudinal wave
diffraction in concrete around ducts were formulated as complementary to compression
wave reflection behavior in impact-echo testing, which led to improve figures of concrete
cover calculation for post-tensioning ducts. Further investigations of this aspect are needed
and comparison with analytical prediction is required.
ACKNOWLEDGEMENTS
The article includes reference to research work carried out by the author as former
BAM (Bundesanstalt fur Materialforschung und -prufung) employee under BASt research
project FE 86.017/2000/B4. Funding of the project by the Bundesanstalt fur StraBenwesen
(BASt) and Dr J. Krieger, who acted as project officer, are gratefully acknowledged, as
well as facilities provided by BAM.
REFERENCES
1. Sansalone, M. and Streett, W., Impact-Echo, nondestructive evaluation of concrete and
masonry, Bullbrier Press, Ithaca, N.Y., 1997, 340 pp.
2. Colla, C., Schneider, G. and Wiggenhauser H., "Automated impact-echo: method
improvements via 2- and 3-D imaging of concrete elements", in 8th Structural Faults
and Repair, edited by M.C. Forde, Engineering Technics Press, Edinburgh, 1999, CDRom.
3. Colla, C., "Impact-echo", in Technical material investigation at the demolition of the
Haiger viaduct, edited by M. Krause et al., BAM-BASt internal research project FE
86.017/2000/B4 report, Berlin-Bergisch Gladbach, v. July 2001, p. 29-57, (in German,
unpublished).
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