R. [TOUDEK et al.: ACOUSTIC AND ELECTROMAGNETIC EMISSION OF LIGHTWEIGHT CONCRETE ...
547–552
ACOUSTIC AND ELECTROMAGNETIC EMISSION OF
LIGHTWEIGHT CONCRETE WITH POLYPROPYLENE FIBERS
AKUSTI^NA IN ELEKTROMAGNETNA EMISIJA LAHKEGA
BETONA S POLIPROPILENSKIMI VLAKNI
Richard [toudek1, Tomá{ Tr~ka2, Michal Matysík1, Tomá{ Vymazal1,
Iveta Pl{ková1
1Brno University of Technology, Faculty of Civil Engineering, Veveøí 331/95, 602 00 Brno, Czech Republic
2Brno University of Technology, Faculty of Electrical Engineering and Communication, Technická 3058/10, 616 00 Brno, Czech Republic
matysik.m@fce.vutbr.cz
Prejem rokopisa – received: 2015-06-29; sprejem za objavo – accepted for publication: 2015-07-08
doi:10.17222/mit.2015.138
This paper is focused on failure monitoring in lightweight concrete (special high-performance concrete that contains porous
aggregate with a low bulk density) with high-strength polypropylene fibers under mechanical loading. The aim was to determine
how the cracks’ generation intensity in the tested concrete samples depends on the fibers’ length and quantity. Our diagnostic
method is based on a measurement of the acoustic and electromagnetic emission signals, which occur when solid dielectric
materials are mechanically stressed. Several groups of lightweight concrete samples with various types and concentrations of
high-strength polypropylene fibers were prepared for our experiment. We made two-channel measurements of the concrete
samples from each group for defined loading conditions. The first channel was electromagnetic emission (EME) and the second
was acoustic emission (AE). The electromagnetic emission and acoustic emission methods are promising methods to study the
generation and behavior of cracks. The main advantage of EME and AE is their ability to be detected already in the stressed
stage, which prevents macroscopic deterioration in solids. From the obtained results it can be concluded that the generated
cracks’ intensity is significantly affected by the presence of polypropylene fibers and by their length and dosage.
Keywords: acoustic emission, electromagnetic emission, lightweight concrete, fibers
^lanek je usmerjen na pregled po{kodb pri mehanskem obremenjevanju lahkega betona (poseben visoko zmogljiv beton, ki
vsebuje porozne sestavine z majhno gostoto), z visokotrdnostnimi polipropilenskimi vlakni. Namen je bil ugotoviti, kako je
intenziteta nastanka razpok odvisna od dol`ine in koli~ine vlaken. Diagnosti~na metoda je temeljila na merjenju akusti~nih
signalov in signalov elektromagnetne emisije, ki se pojavijo kadar je trden dielektri~ni material mehansko obremenjen. Za
eksperiment je bilo pripravljenih ve~ vrst lahkih betonov z razli~no vrsto in koncentracijo visokotrdnostnih polipropilenskih
vlaken. Pri dolo~enih pogojih obremenitve smo izvr{ili dvokanalne meritve vzorcev betona iz vsake skupine. Prvi kanal je bila
elektromagnetna emisija (EME), drugi pa akusti~na emisija (AE). Metodi elektromagnetne emisije in akusti~ne emisije sta
obetajo~i metodi za {tudij nastanka in obna{anja razpok. Glavna prednost EME in AE je, da ju je mogo~e odkriti `e med
stanjem napetosti, kar prepre~i lokalne makroskopske po{kodbe v trdnem stanju. Iz dobljenih rezultatov je mogo~e zaklju~iti, da
je intenzivnost nastajanja razpoke mo~no odvisna od prisotnosti polipropilenskih vlaken, od njihove dol`ine in odmerka.
Klju~ne besede: akusti~na emisija, elektromagnetna emisija, lahki beton, vlakna
1 INTRODUCTION
Cracks very often generate in the structures of both
normal and lightweight concrete. They can interfere only
with the surface of the concrete body, or penetrate the
whole volume. In both cases these cracks have a highly
negative affect, not only on the static and deformation
characteristics of the concrete, and thus the static beha-
vior of the structure as a whole, but also lead to an
intensive reduction of the durability. The source of these
cracks can be the volume changes of the concrete (plastic
shrinkage and settlement, heat of hydration, autogenous
shrinkage, drying) or by external strain (cracks from
bending and shearing stress).1
When a solid is exposed to mechanical stress, emis-
sion of electrons, ions, ground-state and excited neutrals,
free radicals, electromagnetic emission in the frequency
range from tenths of Hz up to gamma radiation and
acoustic emission may take place under certain condi-
tions. This phenomenon is generally termed fracto-emis-
sion. Fracto-emission may be due to several kinds of
mechanical stress: tensile, compression, and torsional
stress. Furthermore, it may be induced by friction, shock,
drilling, splitting, scaling, grinding, skimming etc.2,3 This
phenomenon is particularly strong in composite mate-
rials. In the frequency domain these processes are cha-
racterized as a flicker noise, especially in the low-fre-
quency range. Several authors put forward a physical
interpretation directly connected with particular
non-homogeneous structures.4
Electromagnetic emissions in the radio-frequency
region (EME) make up one of very important fracto-
emission components for material research in physics as
well as in engineering. In the past, great attention was
paid to the application of EME from rocks and minerals
being exposed to a mechanical stress, both in connection
with earthquake and volcanic activity predictions and in
rock mechanics.5–7 Although there are a number of
Materiali in tehnologije / Materials and technology 50 (2016) 4, 547–552 547
UDK 67.017:625.821.5:677.494.742 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(4)547(2016)
experimental papers dealing with various aspects of
EME, the physical origin of this phenomenon is not
sufficiently known for the time being.
Our diagnostic method designed for experimental
examinations of EME signals in composite materials is
based on an experimental fact, namely, that the forma-
tion of cracks in an electrically non-conducting material
is accompanied by the generation of an electromagnetic
field.8 The cracks’ generation in solids is accompanied
by the redistribution of the electric charge. The crack
walls are electrically charged and their vibrations pro-
duce time-variable electrical dipole moments. Hence, the
individual cracks become electromagnetic field sources,
which can be measured with the appropriate sensors.8
The signal of the acoustic emission (AE) is generated
simultaneously with the EME signal. Acoustic emission
appears due to the release of elastic energy during this
process and it is in the frequency range of ultrasonic
waves.9
The electromagnetic emission and acoustic emission
methods are promising methods for studying the gene-
ration and behavior of cracks. The main advantage of
EME and AE is their ability to be detected already in the
stressed stage, which prevents the macroscopic dete-
rioration in solids. A suitably designed methodology of
EME and AE signals measurement, processing and
evaluation allows us to observe the response of stressed
materials to an applied mechanical load continuously
and also allows us to obtain useful information about the
processes taking place in the cracks’ formation in
solids.10–12 For the time being, the EME method is the
only method suitable for studying the time development
of crack growth (crack-propagation speed, crack-face
movement speed, crack length and size, etc.).13,14
2 EXPERIMENTAL PART
For EME and AE observations the automatic
measurement system was developed. It is schematically
shown in Figure 1. The adjustable hydraulic press is
controlled by computer via a voltage that is set by card
NI PCI-6014. A press provides a mechanical load in the
range from 10 kN up to 110 kN. The measurement card
acquires the output voltage of the Wheatstone bridge
with a sensitive load cell that measures the mechanical
load. A deformation meter is used to measure the sam-
ple’s contraction during a compressive stress application.
The data from the deformation meter are read and
processed by computer.
For the EME detection we used capacitance sensors.
Our capacitance sensor was formed by a specially made
adjustable bracket with two electrodes, into which we
can easily insert the rectangular samples from the studied
material.
For AE capturing the piezoelectric sensors were used.
These sensors meet the requirements of the AE frequen-
cy band (at least up to 1 MHz). Beeswax was applied for
good mechanical coupling between the sensors and the
specimen.
The EME measurement channel consists of a capaci-
tance sensor, the dielectric of which is composed of the
stressed sample, a high-pass-filter-type load impedance
ZL, a low-noise preamplifier PA31, and an amplifier
AM22 with a set of filters. The total EME channel gain
is 60 dB, the frequency range is from 30 kHz to 1.2 MHz
and the sampling rate is 5 MHz.
The AE channel consists of a piezoelectric acoustic
sensor (30 kHz ~ 1 MHz), the low-noise preamplifier
PA31 and the amplifier AM22 with a set of filters. The
total AE channel gain is 40 dB, while the frequency
range is from 30 kHz to 1.2 MHz.
EME and AE Preamplifiers and Amplifiers:
• Preamplifier (3S SEDLAK PA31): This low-noise
preamplifier exhibits a bandwidth from 20 Hz to 10
MHz, a high input impedance 10 M/20 pF, a vari-
able gain of 6/20/40 dB and a noise voltage < 1.8
nV/vHz.
• Amplifier (3S SEDLAK AM22): This low-noise am-
plifier exhibits a bandwidth from 0.3 Hz to 1.2 MHz,
a set of high-pass and low-pass filers, an input impe-
dance of 1 M/50 pF and a noise voltage < 3
nV/vHz.
Both signals of the EME and AE are acquired by the
NI PCI 6111 card and stored in the PC. The stored data
is processed using our software. The whole measurement
system is controlled by a computer with the software
developed in LabVIEW. The software makes it possible
to find the typical events in the EME and AE. The evalu-
ation process for these parameters and the measurement
process are provided in real time and simultaneously.
Our study is focused on lightweight concrete with
polypropylene fibers. Classical lightweight concrete
(without polypropylene fibers) has been used as a refe-
rence sample. This fresh concrete mixture was prepared
from Liapor 4-8/600 lightweight aggregates (LWA),
heavyweight aggregates (HWA) of a 0–4-mm fraction,
CEM I – 42.5 R cement, fly ash, plasticizer and water.
The water and lightweight aggregates of a 4–8-mm frac-
tion were dosed by volume, the remaining components
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548 Materiali in tehnologije / Materials and technology 50 (2016) 4, 547–552
Figure 1: Experimental set-up
Slika 1: Eksperimentalni sestav
by weight. The composition of the fresh concrete mix-
ture is in Table 1.
Table 1: Composition of fresh concrete mixture (reference sample)
Tabela 1: Sestava sve`e me{anice betona (referen~ni vzorec)
Components Units Quantitiesper 1 m3
LWA Liapor 4-8/600 L 440
HWA 0-4 mm Brat~ice kg 580
Cement 42.5 R kg 400
Fly ash Trinec kg 50.0
Plasticizer Sika Viscocrete 1035 kg 5.00
Water L 206
Table 2: Dosage of polypropylene fibers, additional water and
plasticizer. The length of the fibers is shown in brackets.
Tabela 2: Odmerek polipropilenskih vlaken, dodatka vode in
plastifikatorja. Dol`ina vlaken je prikazana v oklepajih.
Components Quantities per 1 m3 (kg)
Forta Ferro (54 mm) – 9.0 6.0 4.0 2.0
Forta Econo-Net
(38 mm) – – 9.0 1.5 1.0 0.5
Forta Econo-Net
(19 mm) – – – 1.5 1.0 0.5
Additional water – 60 40 60 20 –
Additional plasticizer – – – 0.8 0.8 0.8
Designation of the
mixture
We also made five mixtures of fiber-lightweight con-
crete, which are mutually of a different type and amount
of polypropylene fibers. The compositions of these con-
cretes were based on the reference lightweight concrete –
to the basic components was added an appropriate dose
of fibers and the consistency of the mixtures was ad-
justed by adding water, possibly plasticizer. The dosage
of the fibers, additional water and plasticizer, for the
individual mixtures is shown in Table 2.
3 RESULTS AND DISCUSSION
Six groups of lightweight concrete samples with vari-
ous types and amounts of fibers were prepared for a
two-channel (EME and AE) measurement on our expe-
rimental set-up. The measured specimens were concrete
blocks of overall dimensions 100 mm × 100 mm × 100
mm. These blocks from each of the prepared groups
were measured for defined loading conditions (linearly
increasing the uniaxial compression up to a load of 110
kN with a rate of 27 N/s.). The steel rods with a square
cross-section of 8 mm × 8 mm were inserted between the
specimens and the hydraulic press jaws.
Figure 2 shows an example of the REF specimens’
loading conditions, the curve of the applied mechanical
load and the histogram describing the distribution of the
fracture events (triggered by the AE signals) with time.
Each bar in this chart describes the number of AE events
during a time interval of 30 s. The number of fracture
events in individual time intervals is almost constant
below the applied load of approximately 85 kN. The
number of fracture events exponentially increases above
this load level. The peak in the maximum value corres-
ponds to the start of the whole sample destruction. In
Figure 2 we can also see that the force versus time is
linear only for the range of 20 kN to 95 kN. This is due
to the hydraulic press.
Figures 3 and 4 show examples of the measured
signals that contain only separated EME and AE events.
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Figure 4: Separated EME and AE events, mechanical load 107.06 kN,
sample MIX2
Slika 4: Lo~eni EME in AE dogodki, mehanska obremenitev
107,06 kN, vzorec MIX2
Figure 2: Applied mechanical load and the distribution of the fracture
events (triggered by the AE signals) in time, sample REF
Slika 2: Uporabljena mehanska obremenitev in ~asovna razporeditev
pojavov loma (spro`enega z AE signali), vzorec REF
Figure 3: Separated EME and AE events, mechanical load 64.33 kN,
sample MIX2
Slika 3: Lo~eni EME in AE dogodki, mehanska obremenitev
64,33 kN, vzorec MIX2
The time delay between both signals is caused by the
different propagation velocities of the acoustic and
electromagnetic signals in the sample under examination.
The AE signal time latency to the EME signal arrival
provides information about the distance of the crack
from the AE sensor. In the case of the AE signal multi-
channel measurement, we can get useful information
about the crack’s position in the stressed material.12,13
The formation of the continuous EME and AE signals
(Figure 5) occurs just before the total destruction of the
whole sample. In the case of the EN38 sample, the maxi-
mum load (before the sample destruction) was approxi-
mately 82 kN.
The curves of the applied force versus sample deflec-
tion (contraction) for the individual measured concrete
samples are in Figures 6 to 11. The histograms in these
figures illustrate the distribution of the AE fracture
events depending on the increasing sample contraction.
The numbers of AE and EME events, the maximum
force and the maximum strain of the measured samples
are shown in Table 3. The cumulative numbers of de-
tected AE events for a defined applied force are in
Figure 12. Figure 13 shows the dependence of the
cumulative number of AE events on the total dosage of
polypropylene fibers in 1 m3 of lightweight concrete
mixture for a defined mechanical load.
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550 Materiali in tehnologije / Materials and technology 50 (2016) 4, 547–552
Figure 5: Continuous EME and AE events, mechanical load
76.23 kN, sample EN38
Slika 5: Neprekinjeni EME in AE dogodki, mehanska obremenitev
76,23 kN, vzorec EN38
Figure 9: Sample MIX1 – AE intensity
Slika 9: Vzorec MIX1 – intenzivnost AE
Figure 7: Sample FF54 – AE intensity
Slika 7: Vzorec FF54 – intenzivnost AE
Figure 6: Sample REF – AE intensity
Slika 6: Vzorec REF – intenzivnost AE
Figure 8: Sample EN38 – AE intensity
Slika 8: Vzorec EN38 – intenzivnost AE
Table 3: Numbers of AE and EME events, maximum force and maxi-
mum strain of measured samples
Tabela 3: [tevila dogodkov AE in EME, maksimalna sila in maksi-
malna napetost izmerjenih vzorcev
Designation of
the mixture REF FF54 EN38 MIX1 MIX2 MIX3
Number of AE
events 14199 1347 26873 11959 7123 16644
Number of EME
events 25 4 166 13 113 96
Maximum force
(kN) 100.43 105.57 82.62 110.49 107.90 108.09
Maximum strain
(mm) 0.348 0.358 0.723 0.504 0.475 0.437
4 CONCLUSIONS
For the EN38 sample it is evident that the intensity of
the detected AE signals increases from the beginning of
the mechanical loading (Figure 6) and the cumulative
number of AE events for a defined load reaches signifi-
cantly higher values compared to the rest of the samples.
This is only valid for a mechanical load of approximately
83 kN, during which the EN38 sample was destroyed
(Figure 12). By contrast, for the sample FF54 we re-
corded the lowest value of AE events for every load. We
can see in Figure 12 that the REF sample (without
polypropylene fibers) is located between the FF54 and
EN38 samples. The low strength and the high AE acti-
vity of sample EN38 is probably due to pulping fiber
bundles and their re-clustering already during mixing. In
the production of this sample we observed a noticeable
increase in the volume of the mixture.
From the cumulative number of AE events (Figure
12) we can plot the dependence of the total number of
AE events on the total dosage of polypropylene fibers for
a defined mechanical load (Figure 13). In this figure we
can see only the REF sample and the samples with mix-
tures of fibers. There is a clear minimum of AE activity
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Materiali in tehnologije / Materials and technology 50 (2016) 4, 547–552 551
Figure 13: Dependence of the cumulative number of AE events on the
total dosage of polypropylene fibers in 1 m3 of lightweight concrete
mixture for a defined mechanical load (REF – 0 kg, MIX3 – 3 kg,
MIX2 – 6 kg, MIX1 – 9 kg)
Slika 13: Odvisnost kumulativnega {tevila dogodkov pri AE od celot-
nega odmerka polipropilenskih vlaken v 1 m3 me{anice lahkega
betona pri dolo~eni mehanski obremenitvi ( REF – 0 kg, MIX3 – 3 kg,
MIX2 – 6 kg, MIX1 – 9 kg)
Figure 11: Sample MIX3 – AE intensity
Slika 11: Vzorec MIX3 – intenzivnost AE
Figure 10: Sample MIX2 – AE intensity
Slika 10: Vzorec MIX 2 – intenzivnost AE
Figure 12: Cumulative numbers of detected AE events for a defined
applied force
Slika 12: Kumulativno {tevilo zabele`enih dogodkov AE pri dolo~eni
uporabljeni sili
for the sample MIX2 (a total of 6 kg of polypropylene
fibers in 1 m3 of concrete mixture). A smaller dosage of
fibers does not have such a big impact – sample MIX3
(total 3 kg) and a larger dosage seems to be reflected
again in the clustering of polypropylene fibers – sample
MIX1 (total 9 kg).
In the case of electromagnetic emission the largest
total detected EME event was again for the EN38 sample
(Table 3). But the number of these events in all the
samples is too small for any conclusions to be drawn.
From the obtained results it can be concluded that the
generated cracks’ intensity is significantly affected by
the total dosage of polypropylene fibers and by their
length (and type). More detailed studies would require
additional measurements on a larger number of light-
weight concrete samples with defined compositions.
Acknowledgments
This paper has been written as part of 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 of Czech Science
Foundation project GA13-18870S and specific research
program at Brno University of Technology, project No.
FAST-S-13-2149.
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