APPLICATION OF THE FREQUENCY INSPECTION METHOD TO LIGHTWEIGHT CONCRETE SPECIMEN TESTING

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29 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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12th International Scientific Conference, April 20
-
22, 2009 Brno, Czech Republic

*
Mgr. Iveta Plšková,Ph.D., BUT in Brno
-

Faculty of Civil Engineering,Department of Physics,
plskova.i@fce.vutbr.cz

**
Ing. Barbara Kucharczyková,Ph.D., BUT in Brno
-

Faculty of Civil Engineering,Department of Building Testing,
kucharczykova.b@fce.vutbr.cz

*
**
Ing. Michal Matysík, Ph
.D., BUT in Brno
-

Faculty of Civil Engineering,Department of Physics,
matysik.m@fce.vutbr.cz


APPLICATION OF THE F
REQUENCY INSPECTION
METHOD TO LIGHTWEIGH
T CONCRETE SPECIMEN
TESTING



Iveta Plšková
*
, Barbara Kucharczyková
**
, Michal Matysík
*
**




Abstract: Current research and development of non
-
destructive frequency
-
inspection
method show these met
hods to be very promising for material testing and defectoscopy in
the near future. This method is based on the physics of elastic stress wave propagation in
bodies. An exciting impulse, being realized, for example, by a mechanical impact on the
specimen
surface, gives rise to low
-
frequency stress waves to propagate within the
structure and reflect on cracks and the specimen surface. The specimen response to the
exciting impulse is picked up on the surface by means of a sensor and transmitted to a
computer

for frequency analysis. The predominant frequencies may be associated with
multiple reflections within the structure, carrying information on the structure integrity
and defect localization. The paper presents some results of our experimental study of th
e
application potential of the frequency
-
inspection method. Our experiments focused on the
testing of lightweight concrete specimens



1. Introduction


According to the relevant standards, there are three kinds of concrete: lightweight, plain and
heavyw
eight concrete. By definition, the volume mass of lightweight concrete is less than
2000

kg/m3. Depending on the intended application, the lightweight concrete group consists of
three subgroups: 1) thermal insulating type, 2) structural insulating type for

supporting and
insulating applications and, 3) structural type for load
-
bearing capacity.


2. Experiment


Lightweight concrete specimens have been studied in our experiments. Fresh concrete mix
consisted of the following: 0
-
4 mm natural gravel and sand, L
iapor CZ4
-
8/600 lightweight porous
aggregates, CEM I


42,5 R cement, fly
-
ash, plasticiser and water. Water and lightweight porous
aggregates were gauged by volume, all other components were gauged by mass.

Joists of dimensions 100 × 100 × 400 mm were made

of the lightweight concrete. To produce
the test specimens, moulds were filled progressively in two layers, each of them being vibrated for a
period of 30 s. After 24 hrs, the joists were removed from the moulds to be placed into a tank with
PE
-
film
-
cover
ed wooden slat grids and a constant water level at the bottom. The tank was kept in
the laboratory at 20 ± 1°C and relative humidity RH 50 ± 5%. The joist volume mass amounted to
1700 kg/m
3

after 28 days.

Hardened
-
concrete specimens were prepared for compr
ession tests. A notch 8
-
mm wide and
33
-
mm deep (one third of the joist height) was cut at the joist centre. Subsequently, first specimen
measurements were carried out prior to the compression tests. The joist was placed on 2 supports.
Force F was applied t
o the joist at its centre (see Fig. 1), which is the case of three
-
point bending of


a notched joist. The force F was increased gradually until a crack occurred (it arose at the notch
root, Fig. 2). The force was applied slowly in order to prevent the joist

breakage, with only the
formation of a crack being desirable. Subsequently, the second specimen measurement stage was
carried out .


Fig. 1 Three
-
point bending of a notched joist.




Fig. 2 Post
-
stress TLB1 specimen


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it is able to detect only such defects which are unstable during the
structure loading (not in absolute magnitude, but only as magnitude changes).

To test the condition of lightweight concrete specimens,
either of the following AE methods
may be used: passive AE method (so
-
called frequency
-
inspection method), and active AE method,
studying the acoustic emission from loaded structures.


If a passive AE method is applied, mechanical waves are generated by me
ans of a hammer
stroke on the material under test. The response is picked up to show whether or not the material is
homogeneous or whether the structure is disturbed in any way. The inhomogeneity region may be
localised in the material if several sensors a
re used simultaneously to pick up the signal. The
homogeneity degree is usually assessed using fast Fourier transform or time
-
frequency (wavelet)
analysis of the AE response.

A metal hammer of a mass of 169 g, which was hinged in a fixture ensuring a cons
tant release
level, h=2 cm, was used to hit the specimen. The specimen response to the exciting impulse was
detected by means of a piezoelectric sensor of Sedlák S7 type, whose operating frequencies range is
from 100 Hz to 50 kHz. The sensor was attached t
o the specimen surface. The response voltage was
fed into the input of a Yokogawa DL1540CL digital oscilloscope and further processed by means of
a special signal
-
analysis software package.

The first measurement stage involved intact specimens. The effect

of the material structure
inhomogeneities and the joist notch on the signal propagation was studied. In the second stage,


post
-
compression
-
test specimens were measured. A correlation between the response frequency
spectrum and the specimen structure deter
ioration was examined.


3. Measurement Results


The measurement results are presented in Figs. 3


6. An example of specimen No. 1 is shown.
Figures 3 and 4 give the time
-
domain responses for measurements was made before and after a
loading test. The frequ
ency spectra from both measurements are shown in Figs. 5 and 6.


-
0
.
2
-
0
.
1
0
0
.
1
0
.
2
0
0
.
0
2
0
.
0
4
0
.
0
6
0
.
0
8
0
.
1
0
b
e
f
o
r
e

t
e
s
t

l
o
a
d
t

[
s
]
U

[
V
]

-
0
.
2
-
0
.
1
0
0
.
1
0
.
2
0
0
.
0
2
0
.
0
4
0
.
0
6
0
.
0
8
0
.
1
0
a
f
t
e
r

t
e
s
t

l
o
a
d
t

[
s
]
U

[
V
]

Fig. 3 Time
-
domain response record for
specimen No. 1. Measured before a loading test.

Fig. 4 Time
-
domain response record for
specimen No. 1. Measured after a loading test.

0
3
0
0
6
0
0
9
0
0
1
2
0
0
0
2
0
0
0
4
0
0
0
6
0
0
0
8
0
0
0
1
0
0
0
0
b
e
f
o
r
e

t
e
s
t

l
o
a
d
f
[
H
z
]
S

[
r
e
l
a
t
i
v
e

u
n
i
t
s
]

0
5
1
0
1
5
2
0
2
5
0
2
0
0
0
4
0
0
0
6
0
0
0
8
0
0
0
1
0
0
0
0
a
f
t
e
r

t
e
s
t

l
o
a
d
f
[
H
z
]
S

[
r
e
l
a
t
i
v
e

u
n
i
t
s
]

Fig. 5
The power spectral density versus
frequency plot for specimen No. 1. Measured
before a loading test.

Fig. 6 The power spectral density versus
frequency plot for specimen No. 1. Measured
after a loading test.


Figure 3 shows a waveform recording of specime
n. The impulse duration of response was 43
ms. The attenuation ratio was found to equal


= 48 s
-
1
. Figure 5 shows the corresponding power
spectral density (in relative units) versus frequency plot for this specimen. A dominant frequency
f
0

= 39
27 Hz is observed. Figure 4 shows a response waveform detected from the specimen after a
loading test. The response duration was 24 ms, and the attenuation constant increased to


= 138 s
-
1
. The spectral density vs. frequency plot (Fig. 6
) shows that the number of significant
frequencies increased in the range from 2.5 kHz to 6 kHz. The dominant frequency appears to have
shifted to f
1

= 5602 Hz. Table 1 shows dominant frequency before and after test load of tested
specimens. Average values

of a dominant frequency and the respective variance coefficients are
shown in Fig. 7.



Tab.1. Dominant frequency before and after test load of tested specimens

Specimen

Dominant frequency [Hz]

before test load

after test load

No. 1

3900

5591

No. 2

387
2

5869

No. 3

3475

5987

No. 4

4029

5674

No. 5

3779

6129

No. 6

3927

5602



0
1000
2000
3000
4000
5000
6000
before test
load
after test load
Average values of a dominant
frequency [Hz]
0
1
2
3
4
5
COV [%]
Mean value
COV in %

Fig.7 Average values of a dominant frequency before and after test load for all six specimens, mean
values and variance coefficients.


4. Conclusion


The frequency
-
inspectio
n analysis was applied to specimens of lightweight concrete.
Remarkable resonance frequency shifts were found, and an increase in the number of resonance
frequencies occurred in the course of the degradation tests by compressive loading. Average
changes of

resonance frequency of testing specimens were 2000 Hz. From the results we can see
that the frequency
-
inspection method is a prominent non
-
destructive testing method.


Acknowledgments


This research has been supported by project of GACR No.103/09/P247 an
d No. 1M0579.


References

[1]

MATYSÍK M., KOŘENSKÁ

M., PLŠKOVÁ I., KUCH
ARCZYKOVÁ B., KEPRT
J
.:
Application of the non
-
linear ultrasonic spetroscopy method to lightweight concrete specimen
testing.
In Workshop NDT 2007
. Brno, Brno University of Technology. 2007. pp.

82

-

87.

[2]

PLŠKOVÁ, I
.; KUCHARCZYKOVÁ, B.; PAZDERA, L.; TOPOLÁŘ, L. Využití akustické
emise při testování trámců z lehkého betonu s různými typy vláken.
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WORKSHOP NDT 2008.

1. Brno, Brno University of Technology. 2008. p.

102

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978
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