D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
879–884
EVALUATION OF THE DEGREE OF DEGRADATION USING THE
IMPACT-ECHO METHOD IN CIVIL ENGINEERING
OCENA STOPNJE DEGRADACIJE V GRADBENI[TVU Z
UPORABO METODE ODMEVA ZVO^NIH VALOV
Daniela [tefková, Kristýna Tim~aková, Libor Topoláø, Petr Cikrle
Brno University of Technology, Faculty of Civil Engineering, Veveøí331/95, 602 00 Brno, Czech Republic
stefkova.d@fce.vutbr.cz
Prejem rokopisa – received: 2015-06-30; sprejem za objavo – accepted for publication: 2015-12-15
doi:10.17222/mit.2015.150
Non-destructive methods such as Impact-echo method are based on the acoustic properties of the material that are dependent on
its condition. It allows the studied progress development of micro-defects in the structure of the material and is thus suitable for
monitoring the building structure’s condition. This acoustic method allows us to identify and locate defects and is thus suitable
for monitoring the building structure’s condition. The application of this method is widespread; it can be used in mechanical
engineering, power engineering and in many industries as well as in the construction industry. Impact-echo uses a short-time
mechanical impulse (a hammer blow) that is applied to a surface of the test sample and is detected by means of piezoelectric
sensors placed on the surface of the sample. The impulse is reflected by the surface but also by micro-cracks and defects of the
specimen that are under investigation. From thus obtained signal the frequency spectrum is determined and is found to be the
dominant resonance frequency using fast Fourier transformations. The dominant frequencies give an account of the condition of
the structure or determine the location of flaws, at which the waves are rebounded. The signal is digitized by means of a data
processing system to be transferred into a computer memory. A piezoelectric MIDI sensor takes the signal response and it is
brought to the input of an oscilloscope TiePie engineering Handyscope HS3 two-channel with 16-bit resolution. This paper
reports the results of measurements by the Impact-echo method on three applications in civil engineering. The results are
obtained in the laboratory during the hardening process in quasi-brittle materials such as alkali-activated slag mortars, the
degradation of concrete samples by corrosion caused by the action of chlorides and the degradation of composite materials
based on cement by high temperature.
Keywords: predominant frequency, high temperature, impact, mortar, specimens
Neporu{na metoda, kot je npr. odmev zvo~nih valov, temelji na akusti~nih lastnostih materiala, ki so odvisne od stanja
materiala. Metoda omogo~a {tudij napredovanja mikronapak v strukturi materiala in je zato primerna za pregled stanja gradbene
strukture. Ta akusti~na metoda omogo~a odkrivanje in dolo~anje polo`aja napake in je zato primerna za pregled stanja zgradbe.
Metoda ima {iroko uporabnost, saj se lahko uporablja v strojni{tvu, energetiki ter v mnogih drugih industrijah kot tudi v
gradbeni{tvu. Metoda odmeva zvo~nih valov uporablja kratke mehanske udarce (udarec s kladivom) po povr{ini preizku{anega
vzorca, odmeve pa se dolo~i s piezoelektri~nim senzorjem, ki je tudi name{~en na povr{ini. Impulz odseva povr{ino in tudi
mikrorazpoke in napake v vzorcu, ki se ga preiskuje. Iz tako dobljenega signala, se dolo~i spekter frekvenc in s pomo~jo hitre
Fourierjeve transformacije se poi{~e prevladujo~a resonan~na frekvenca. Prevladujo~e frekvence poka`ejo stanje zgradbe ali pa
dolo~ijo lokalne pomanjkljivosti, na katerih valovi posko~ijo. Signal se digitalizira s sistemom obdelave podatkov, da se ga
lahko prenese v spomin ra~unalnika. Piezoelektri~ni sensor MIDI prevzame odbit signal, ki se ga usmeri na vhod dvokanalnega
osciloskopa TiePie Engineering Handyscope HS3, z lo~ljivostjo 16 bitov. ^lanek predstavlja rezultate meritev z metodo odmeva
zvo~nih valov na treh primerih v gradbeni{tvu. Rezultati so dobljeni v laboratoriju med procesom utrjevanja v kvazi krhkem
materialu, kot je z alkalijami aktivirana `lindra v maltah, degradacijo betonskih vzorcev zaradi korozije, povzro~eno s kloridi in
visoko temperaturno degradacijo kompozitnega materiala na osnovi betona
Klju~ne besede: prevladujo~a frekvenca, visoka temperatura, udarec, malta, vzorci
1 INTRODUCTION
In civil engineering, efficient non-destructive quality
control plays an important role in the optimization of
resources for manufacturing, maintenance and safety.
The impact-echo method is a useful non-destructive
technique for flaw detection in concrete. It is based on
monitoring the surface motion, resulting from a short-
time mechanical impulse. This method overcomes many
of the barriers associated with flaw detection in concrete
that occur during ultrasonic methods. One of the key
features of this method is the transformation of the
recorded time-domain waveform of the surface motion
into the frequency domain. The impact gives rise to
modes of vibration and the frequency of these modes is
related to the geometry of the tested object and the
presence of flaws.1
A short-time mechanical impulse, generated by tapp-
ing a hammer against the surface of a concrete structure
(Figure 1), produces low-frequency stress waves that
propagate into the structure.2,3 Thus generated waves
propagate through the specimen structure and reflect
from the defects located in the volume of the specimen
or in the surface. Surface displacements caused by the
reflected waves are recorded by a transducer located
adjacent to the impact.4 The signal is digitized by an
analogue/digital data system and transmitted to a com-
puter memory. This signal describes the transient local
vibrations, which are caused by the mechanical wave
Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884 879
UDK 669.9:620.19:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)879(2016)
multiple reflections inside the structure. The dominant
frequencies of these vibrations give an account of the
condition of the structure that the waves pass through.5,6
The signal analysis from the impact-echo method is
most frequently performed by using frequency spectra
obtained from the fast Fourier transform. Fourier ana-
lysis converts time to frequency and vice versa. A fast
Fourier transform (FFT) is an algorithm to compute the
discrete Fourier transform (DFT) and it is inverse. The
results of fast Fourier transforms are widely used for
many applications in engineering, science, and mathe-
matics. Equation (1) is the expression of the Fourier
transform for a continuous function:7
x t
a
a
nt
T
b
nt
Tn n
( ) cos sin= + ⋅ ⎛⎝
⎜ ⎞
⎠
⎟ + ⋅ ⎛⎝
⎜ ⎞
⎠
⎟⎡
⎣⎢
⎤
⎦⎥
0
2
2 2π π
n =
∞
∑
1
(1)
a
T
x t
nt
T
tn
T
= ⋅ ⎛⎝
⎜ ⎞
⎠
⎟∫
2 2
0
( ) cos
π
d
b
T
x t
nt
T
tn
T
= ⋅ ⎛⎝
⎜ ⎞
⎠
⎟∫
2 2
0
( ) sin
π
d
2π
T
= 
where an and bn can be calculated from function x(t)
using the following relations.
2 EXPERIMENTAL PART
For the impact-echo method a short-time mechanical
impulse (a hammer blow) was applied to the surface of
the specimen during the test and was detected by means
of a piezoelectric sensor (Figure 2). The impulse reflects
from the surface but also from micro-cracks and defects
present in the specimen that are under investigation. The
frequency analysis can be carried out from the response
signal by means of a fast Fourier transform and thus
dominant resonant frequencies are found. An MIDI
piezoelectric sensor was used to pick up the response and
the respective impulses were directed into the input of an
oscilloscope TiePie engineering Handyscope HS3 two-
channel with a 16-bit resolution.
2.1 Material for the hardening process of alkali-acti-
vated slag mortars
The mixture consisted of 450 g of fine-grained
granulated blast furnace slag [tramberk 380 (specific
surface area 380 m2 kg–1), 180 g of sodium silicate (water
glass) with a modulus of 1.6, 1350 g of silica sand
(maximum grain size of 2.5 mm) and 95 mL of water.
The amount of admixtures was 0.1 % of mass fractions
with respect to the slag. CNTs were added in the form of
a well-dispersed aqueous dispersion containing 1 % of
mass fractions of multi-walled carbon nanotubes
D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
880 Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884
Figure 2: Images of measurement with the Impact-Echo method
Slika 2: Posnetka merjenja z metodo odmeva zvo~nih valov
Figure 1: Principle of the Impact echo method
Slika 1: Princip metode odmeva zvo~nih valov
(Graphistrengths CW 2-45). Since CNTs are commonly
not water-soluble, the dispersion also contained carboxy-
methyl cellulose (68 g/L) as a dispersing agent.8 The
slurry was poured into steel moulds 40 mm × 40 mm ×
160 mm to set. The samples were demoulded after 24 h
and one set was tested (marked 0d) and the second set
was immersed in water for another 28 d before testing
(marked 28 d).
2.2 Material for the degradation of concrete samples
by corrosion caused by the action of chlorides
The mixture for the production of concrete samples
was composed of cement CEM II/B – S 32.5 and 1350
kg of sand with a fraction of aggregate 0–4 mm and
225 L of water. The high water ratio resulted in easier
penetration of the degradation agents into the concrete
structure. These samples of dimensions 50 mm × 50 mm
× 330 mm were reinforced with one standard reinforcing
bar of diameter 10 mm and a length of 400 mm passing
through the centre of the beam.
The samples were demoulded after 24 h and placed
in the water for 27 d, then the dried samples with natural
humidity were exposed to accelerated degradation by
chlorides. The samples were immersed into a 5 % water
solution of NaCl for 16 h and then subsequently placed
into a drying oven with an air temperature of 40 °C for
8 h.
2.3 Specimens intended to be subjected in the heating
process
Mortars of dimensions 40 mm × 40 mm × 160 mm
were produced using a type CEM I 42.5 R Portland
cement (^eskomoravský Cement-Heidelberg Cement
Group) and a water-to-cement ratio (w/c = 0.46) and
quartz sand from Filtra~ní písky, s.r.o. for the preparation
of the test mortar mixture in a ratio of 1 to 3, in com-
pliance with the ^SN 721200 standard. The specimens
were left in the moulds for 24 h, then cured in water for
27 d and finally air-cured for 31 d at laboratory tem-
perature (25±2 °C) and a relative humidity of 53±5 %.
After initial curing, the specimens were dried at a
temperature of 60 °C for two d. Subsequently, the speci-
mens were subjected to gradual heating in a furnace at
200 °C, 400 °C, 600 °C, 800 °C, 1000 °C and 1200 °C.
The temperature increase rate was 5 °C/min. A dwell of
60 min at each temperature was provided, in order to
find out the effect of temperature on the specimens. After
heat treatment, the specimens were left to cool down
under laboratory conditions.
3 RESULTS AND DISCUSION
3.1 Material for the hardening process of alkali-
activated slag mortars
The experiment was employed to determine the
microstructural changes during the hardening process of
the alkali-activated slag composite with different admix-
tures. Changes in the density of the material due to the
process of hardening as well as the creation of micro-
cracks due to the time of curing are reflected in the shift
of dominant frequencies. Figure 3 shows the shift of the
resonance frequency during 14 d after demoulding
without curing. The frequency of the reference specimen
(AAS) increased by about 37 % during the first 48 h
from the start of the measurement and then decreased to
a steady value of around 18 % from the initial value. The
dominant frequency of the AAS+CNT (AAS+HPMC)
specimen started 21 % (15 %) above the initial dominant
frequency of the reference specimen (AAS). The domi-
nant frequency of the AAS+CNT (AAS+HPMC)
increased by about 35 % (30 %) from the initial value
during the first 24 h and then decreased to a steady value
of around 23 % (25 %) from the initial value. The pro-
cess of hardening and the formation of a hard and dense
structure caused an initial increase of the dominant
frequencies. At a later time, the frequencies are again
slightly shifted towards lower values. This phenomenon
is probably associated with the drying process, which is
followed by the shrinkage of the AAS matrix and the
formation of micro-cracks. Figure 4 shows the shift of
the resonance frequency during 14 d after 28 d of curing.
The dominant frequency decreased for all the measure-
D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884 881
Figure 4: Change of relative dominant frequencies over time – after
28 d of curing
Slika 4: Spreminjanje relativne prevladujo~e frekvence s ~asom – po
28 dneh utrjevanja
Figure 3: Change of relative dominant frequencies over time - without
curing
Slika 3: Spreminjanje relativne prevladujo~e frekvence s ~asom – brez
utrjevanja
ment times to a stable value. For the AAS specimen
there was a stable value of about 70 % of the initial
value. For AAS+CNT (AAS+HPMC) there was a de-
cline of about 25 % (20 %) from the initial value. This
decline is mainly associated with the drying process,
which is followed by shrinkage of the AAS matrix and
probably the formation of micro-cracks. Whereas that
dominant frequency obtained for specimens with admix-
tures was higher than for the reference sample, then both
admixtures have a positive effect on the formation of a
structure of alkali-activated slag composite. Both cellu-
lose derivatives that were added to mixture are able to
retain water. These admixtures prevent the material from
rapidly drying and the subsequent formation of the
micro-cracks caused by drying shrinkage, which gene-
rally occurs during hardening of the samples. Carbon
nanotubes employed in one set of samples can act as a
micro-reinforcement, it participates on the improvement
of the mechanical properties.
3.2 Material for degradation of concrete samples by
corrosion caused by the action of chlorides
Measuring the effect of corrosion on reinforced con-
crete samples was carried out on two sets of samples.
The first set of samples was a reference and the second
set of samples was subjected to corrosion by using
accelerated degradation by chlorides in an aqueous NaCl
solution. The measurements were always carried out
after 20 cycles of alternating immersion in an aqueous
NaCl solution and drying in an oven. In Figure 5 we can
see the relative change of the dominant frequencies,
when the piezoelectric sensor was placed at one end of
the concrete beam and hit by a steel hammer at the oppo-
site end of the concrete beam in the longitudinal direc-
tion. Figure 6 shows the relative change of dominant fre-
quencies for the same samples, but measurements were
carried out so that the sensor was placed at one end un-
covered reinforcement and the hit was made at the
opposite end of reinforcement in the longitudinal direc-
tion. The signal passing through the degraded sample
changes the frequency by reflections on the inhomoge-
neities, which is an indicator of changes in the structure.
The results are referenced to a relative value and are
displayed in the form of an arithmetic average (obtained
from three independent measurements for reference sam-
ples and from six measurements for degraded samples)
and standard deviations as error bars. In the first case, for
measurements of the samples on concrete, the dominant
frequencies of reference samples was a decrease of
2.4 %, which can be explained by a lack of water for the
hydration of the concrete, therefore this decrease is not
as significant for immersed samples, only about 1.0 %.
In the case of measurements on reinforcement the pheno-
menon associated with hydration is not apparent, it is
only within the measurement error. Between 20 and 40
cycles of degradation occurred in the samples to the
formation of observable cracks due to expansion of the
corrosion products. This phenomenon is reflected in a
significant decrease of the monitored frequencies. For
measurements on concrete the decrease was 2.5 % and
for a measurement on reinforcement it was 1.9 %. Other
frequency changes are no longer significant. The results
obtained correspond to the processes that occur during
the degradation of the reinforced concrete and reveal
damage reinforced concrete.
D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
882 Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884
Figure 7: Shift of dominant frequency induced by degradation at
elevated temperatures (Arrangement U0-S0)
Slika 7: Premik prevladujo~e frekvence povzro~en z razpadanjem pri
povi{anih temperaturah (Postavitev U0-S0)
Figure 5: Change of the relative dominant frequencies during degra-
dation cycles (impact and sensor on material)
Slika 5: Spreminjanje relativne prevladujo~e frekvence med cikli pro-
padanja (udarec in senzor na materialu)
Figure 6: Change of relative dominant frequencies during degradation
cycles (impact and sensor on reinforcement)
Slika 6: Spreminjanje relativne prevladujo~e frekvence med cikli pro-
padanja (udarec in senzor na betonu)
3.3 Specimens intended to be subjected in a heating
process
The mortar specimens of the compositions were
exposed to the temperatures of 200, 400, 600, 800, 1000
and 1200 °C. The test results of impact-echo are pre-
sented below. Figure 7 presents the change of dominant
frequency versus temperature at which the specimens of
mortar compositions were subjected (arrangement
U0-S0; longitudinal waves). For this measurement, the
sensor was placed at the specimen’s end at its centre line
direction, while the specimen was hit at the opposite end
at the centre line direction – arrangement U0-S0. Longi-
tudinal waves, which propagate within the sample at a
speed of about 5100 m/s, can affect the mortar element
oscillations. The exposure at elevated temperatures
causes a decrease of the dominant frequency, leading to
the conclusion that the material’s elastic modulus for
each composition also decreases (E = 4·(f·L)2). Predo-
minant frequencies are shifted towards to the lower fre-
quency range in the course of the degradation. The
change is more rapid in the temperature range
400–600 °C, where are intense impurities changed. It is
clear that the predominant frequencies shifted down to-
wards the lower frequency region. For the specimen that
underwent a thermal stress at a temperature of 1200 °C it
is clear that the predominant frequencies shifted upwards
towards the higher-frequency region. It is evident that
a structural change, accompanied by the creation of new
crystal phases, takes place in the specimen at tempera-
tures of about 1200 °C. Figure 8 shows the change of
dominant frequency versus temperature when the
arrangement was the U1-S1 one. In this case, transverse
waves (gradual waves) are predominantly spread
throughout the specimens. The difference between the
U0-S0 and U1-S1 arrangements is that in the latter the
measurement took place with the sensor being placed at
the mid-point and perpendicular to the specimen. The
specimen was hit at the mid-point opposite to the sensor.
The dependence of frequency on temperature was similar
to that observed when the U0-S0 arrangement was used,
however the frequency values were lower in the case of
the U1-S1 arrangement. This is similar to the case of the
U0-S0 arrangement (Figure 7). The comparison of Fig-
ures 7 and 8 indicates that the frequency change is
slower when the arrangement U1-S1 is applied. In gene-
ral, acoustic methods illustrated the physical changes in
the structure of all the tested materials. A reduction of
the predominant frequency values was observed. More-
over, it was also observed in every case of elevated tem-
perature. The heating up to 110 °C resulted in a loss of
capillary water and a reduction of the cohesive forces
(weakening of bonds) due to moisture evaporation. At
about 170 °C decomposition of gypsum occurred, result-
ing in expansive spalling. Between 250 and 300 °C the
hydrated cement phases were decomposed, while above
300 °C it resulted in Ca(OH)2 decomposition. Further
temperature increases up to 300 °C or 400 °C intensified
the cement paste’s thermal decomposition and degrada-
tion. All the mentioned changes resulted in the embrittle-
ment and hardening of the tested materials. Thus, the
observed reduction of frequency values was assumed to
be due to the formation of micro-cracks.
4 CONCLUSIONS
The paper deals with the results of measurements
using the Impact-echo method on three applications in
civil engineering. The aim of this paper was to study the
application of the impact-echo method for the detection
of flaws in composite materials during different stress
situations (setting and hardening in air, degradation by
corrosion caused by chlorides and exposing to elevated
temperature). It is known that the impact response signal
of a specimen is composed of frequencies corresponding
to the modes of vibration of the specimen. A shift of the
dominant frequency to a lower value is a key indication
of the presence of the flaw. From the results obtained in
the framework of our research group and the results
demonstrated in this paper it can be summarized that the
frequency inspection carried out by means of the
Impact-echo method makes a convenient tool to assess
the quality and lifetime of these composite materials
when exposed to stress situations.
Acknowledgement
This paper has been worked out under the project
GA^R No.16-02261S supported by Czech Science
Foundation and the project No. LO1408 "AdMaS UP –
Advanced Materials, Structures and Technologies",
supported by Ministry of Education, Youth and Sports
under the "National Sustainability Programme I" and
under the project No.J-16-3254 supported by Faculty of
Civil Engineering of BUT.
D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884 883
Figure 8: Shift of dominant frequency induced by degradation at
elevated temperatures (Arrangement U1-S1)
Slika 8: Premik prevladujo~e frekvence povzro~en z razpadanjem pri
povi{anih temperaturah (Postavitev U1-S1)
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D. [TEFKOVÁ et al.: EVALUATION OF THE DEGREE OF DEGRADATION USING THE IMPACT-ECHO METHOD ...
884 Materiali in tehnologije / Materials and technology 50 (2016) 6, 879–884