L. TOPOLÁØ et al.: NON-TRADITIONAL NON-DESTRUCTIVE TESTING OF THE ALKALI-ACTIVATED SLAG ...
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NON-TRADITIONAL NON-DESTRUCTIVE TESTING OF THE
ALKALI-ACTIVATED SLAG MORTAR DURING THE HARDENING
NETRADICIONALNO NEPORU[NO PREIZKU[ANJE Z
ALKALIJAMI AKTIVIRANE MALTE MED STRJEVANJEM
Libor Topoláø, Peter Rypák, Kristýna Tim~aková-[amárková, Lubo{ Pazdera,
Pavel Rovnaník
Brno University of Technology, Faculty of Civil Engineering, Veveri 331/95, 602 00 Brno, Czech Republic
topolar.l@fce.vutbr.cz
Prejem rokopisa – received: 2014-07-30; sprejem za objavo – accepted for publication: 2015-01-30
doi:10.17222/mit.2014.130
This paper reports the results of the measurements of alkali-activated slag mortars made during the hardening and drying of
specimens. The alkali-activated slag is a material with a great potential for practical use. The main drawback of this material is
its high level of autogenous and, especially, drying shrinkage, which causes a deterioration in the mechanical properties. The
aim of this paper is to present the effects of the treatment method for mortars and the curing time on the microstructures of the
alkali-activated slag mortars. The knowledge of the microstructure/performance relationship is the key to a true understanding
of the material behaviour. The results obtained in the laboratory are useful for understanding the various stages of the
micro-cracking activity during the hardening process in quasi-brittle materials such as alkali-activated slag mortars and for their
extension to field applications. Non-destructive acoustic-analysis methods – the impact-echo method as a traditional method and
the acoustic-emission method as a non-traditional method for civil engineering – were used for the experiment. The principle of
the impact-echo method is based on analysing the response of an elastic-impulse-induced mechanical wave. Acoustic emission
is the term for the noise emitted by materials and structures when they are subjected to stress. The types of stress can be
mechanical, thermal or chemical. Ultrasound testing and the loss in mass were used as complementary methods for the tested
samples.
Keywords: acoustic-emission method, loss in mass, impact-echo method, ultrasound testing, alkali-activated slag mortars
^lanek obravnava rezultate meritev med strjevanjem in su{enjem vzorcev malt, aktiviranih z alkalijami. Z alkalijami aktivirana
`lindra je material, ki ima velik potencial za prakti~no uporabo. Glavna pomanjkljivost tega materiala je, da ima sam po sebi, {e
posebno pa pri su{enju, velik skr~ek, ki povzro~i poslab{anje mehanskih lastnosti. Namen tega ~lanka je predstavitev vpliva
metode obdelave malte in ~asa strjevanja na mikrostrukturo z alkalijami aktivirane malte. Razumevanje odvisnosti med
mikrostrukturo in zmogljivostjo je klju~ za pravilno razumevanje vedenja materiala. Rezultati, dobljeni v laboratoriju, so
koristni za razumevanje razli~nih stopenj nastajanja mikrorazpok med procesom strjevanja kvazikrhkega materiala, kot je malta
z `lindro, aktivirano z alkalijami, in za njihov prenos na gradbi{~e. Neporu{ne analizne metode z akusti~no emisijo in metoda
udarec – odmev kot tradicionalne ter metoda akusti~ne emisije kot netradicionalna metoda v gradbeni{tvu, so bile uporabljene
pri preizkusu. Princip metode udarec – odmev temelji na analizi odgovora elasti~nega impulznega mehanskega vala. Akusti~na
emisija je izraz za hrup, ki ga oddajata material in zgradba, ko sta izpostavljena napetosti. Napetosti so lahko mehanske,
termi~ne ali kemijske. Preiskava z ultrazvokom in izguba mase sta bili uporabljeni kot komplementarni metodi pri preizkusnih
vzorcih.
Klju~ne besede: metoda akusti~ne emisije, izguba mase, metoda udarec – odmev, preiskava z ultrazvokom, malta z `lindro,
aktivirano z alkalijami
1 INTRODUCTION
Alkali-activated aluminosilicate materials represent
an alternative to ordinary Portland-cement-based mate-
rials, reducing the impact of the building industry on the
environment and exhibiting new superior properties.
Alkali-activated slag (AAS) is based on granulated
blast-furnace slag that can be activated by alkali hydro-
xides, carbonates or, most preferably, by silicates.1 The
type and dosage of the activator as well as the way of the
curing have significant effects on the hydration course
and final mechanical properties.2 The major disadvantage
of AAS is an increased shrinkage during the hardening
period, caused by both the autogenous and drying
shrinkage, which finally results in a volume contraction,
micro-cracking and deterioration of tensile and bending
properties.3
The impact-echo method (IE) is a type of the non-
destructive testing method. A short-term mechanical
impact, generated by tapping a hammer against the
surface of a concrete structure, produces low-frequency
stress waves which propagate into the structure.4 A wave
generated in this way propagates through the specimen
structure and reflects from the defects located in the
volume of specimen or on its surface. Surface displace-
ments caused by the reflected waves are recorded by a
transducer located adjacent to the impact.5 The signal is
digitized via an analogue/digital data system and trans-
mitted to a computer’s memory. This signal describes the
transient local vibrations, caused by the mechanical-
wave 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.6
Materiali in tehnologije / Materials and technology 50 (2016) 1, 7–10 7
UDK 691:620.179.1 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)7(2016)
Acoustic emission (AE) is the term for the noise
emitted by materials and structures when they are sub-
jected to stress. The types of stresses can be mechanical,
thermal or chemical. This emission is caused by a rapid
release of the energy within a material due to the events
such as a crack formation and its subsequent extension
occurring under the applied stress, generating transient
elastic waves that can be detected by piezoelectric sen-
sors. The acoustic-emission system allows us to monitor
the changes in the material behaviour over a long time
and without moving one of its components, i.e., sensors.
This makes the technique quite unique along with the
ability to detect the crack propagation occurring not only
on the surface but also deep inside the material. The
acoustic-emission method is considered to be a "passive"
non-destructive technique, because it usually identifies
defects while they develop during the test.7
Ultrasonic testing is the name given to the study and
application of ultrasound, which is a sound too high to
be detected by the human ear, i.e., of the frequencies
greater than about 18 kHz. Ultrasonic waves have a wide
variety of applications. For example, ultrasound with
high intensity is used for cutting, cleaning and destroy-
ing a tissue in medicine. For the non-destructive testing
(NDT), ultrasound with a lower intensity is used. An
ultrasonic inspection can be used for a flaw detection/
evaluation, dimensional measurements, a material cha-
racterization and more. Ultrasonic testing (UT) is based
on the propagation of low-amplitude waves through a
material, measuring the time of travel or detecting any
change in the intensity over a given distance. Applica-
tions include distance gauging, flaw detection and
parameter measurement (such as the elastic modulus and
the grain size), all relating to the material structure.8
2 EXPERIMENTAL PART
2.1 Material
The mixture consisted of 450 g of fine-grained
granulated blast-furnace slag [tramberk 380 (a specific
surface area of 380 m2 kg–1), 180 g of sodium silicate
(water glass) with a modulus of 1.6, 1350 g of silica sand
and 95 mL of water. The AAS slurry was poured into
steel moulds (40 mm × 40 mm × 160 mm) to set and
after 24 h the samples were demoulded and immersed in
water for another (2, 6 and 27) d before the testing.
2.2 Experimental set-up
For the impact-echo method, a short mechanical
impulse (a hammer blow) was applied to the surface of a
specimen during the test and detected by means of a
piezoelectric sensor (Figure 1). The impulse reflected
from the surface and also from the micro-cracks and
defects present on the specimen under investigation. The
resonance frequency created in this way was determined
by means of a frequency analysis. Dominant frequencies
could be determined from the response signal by means
of the fast Fourier transform. A MIDI piezoelectric
sensor was used to pick up the response and the respec-
tive impulses were directed into the input of a TiePie
engineering oscilloscope, two-channel Handyscope HS3
with a resolution of 16 bits.
The initiation of cracks during the hardening was
monitored with the method of acoustic emission. AE
signals were detected by measuring equipment DAKEL
XEDO with four channels (Figure 2). The AE sensors
(type IDK09) were attached to the surface with beeswax.
The change in the mass during the hardening was
measured using equipment QuantumX with a Z6
bending-beam load cell for the maximum mass of 50 kg
by HBM.
Measuring equipment PUNDIT (portable ultrasonic
non-destructive digital indicating tester) Plus was used
for the ultrasonic testing. For the testing speed of the
sound through the mortar specimens, the coefficient of
variation for the repeated measurements at the same
location was 2 %. The accuracy of the pulse velocity was
L. TOPOLÁØ et al.: NON-TRADITIONAL NON-DESTRUCTIVE TESTING OF THE ALKALI-ACTIVATED SLAG ...
8 Materiali in tehnologije / Materials and technology 50 (2016) 1, 7–10
Figure 2: Photography of the acoustic-emission measurement
Slika 2: Posnetek meritve akusti~ne emisije
Figure 1: Photography of the impact-echo measurement
Slika 1: Posnetek meritve udarec – odmev
a direct function of the accuracy of the measured
distance between the transducer faces. The PUNDIT
instruments have a transit time resolution of 0.1 s. All
the measurements were carried out for 336 h (14 d)
immediately after the specimens were pulled out of the
immersion water.
3 RESULTS AND DISCUSSION
To evaluate the crack formation during spontaneous
drying, we focused on the activity of AE with respect to
the most used parameter, which is the number of signals
overshooting the pre-set threshold. The diagrams in
Figures 3 to 5 show the dependence of the number of
overshoots and the loss of mass versus the time of
measurement. It was assumed that the number of micro-
cracks could be inferred from the AE activity. Unfortu-
nately, the AE signals originate not only from the crack
formation but also from the process of water evaporation.
However, most of the AE activity was observed within
the first 24 h of spontaneous drying, which corresponds
to approximately 50 % loss in mass. Therefore, at the
beginning the AE signals could be attributed to both the
drying process and the crack formation, whereas after 24
h of drying the observed signals corresponded mainly to
the formation of microcracks. The highest number of
overshoots during the remaining time of the measure-
ment was detected for the specimen that was cured in
water for 2 d (Figure 3). The reason for such a diffe-
rence in comparison with the specimens cured for 6 d
and 27 d arise from a shorter hydration time. Three days
after the mixing, the hydration process was still not
complete and the weak basic structure was not able to
bear a heavy stress; therefore, the AAS matrix was more
susceptible to the cracking caused by drying shrinkage.
To evaluate the signals with the impact-echo method
the fast Fourier transform was used. The modification of
the dominant frequency during the drying process is dis-
played in Figure 6. The results show that the frequencies
decreased from the initial values of 10.10 kHz, 10.86
kHz, 12.12 kHz to the steady values of 5.90 kHz, 7.50
kHz, 8.44 kHz, respectively, for the specimens cured in
water for (2, 6 and 27) d, respectively. Similarly, the
ultrasonic velocity decreased from the initial values of
(3760, 3960, 4450) m s–1 to the steady values of (2010,
2470, 2970) m s–1, respectively, for the specimens
immersed in water for (2, 6 and 27) d, respectively (Fig-
ure 7). The pulse cannot travel across the material/air
interface, but it is able to travel from the transmitter to
the receiver by diffraction at the crack edge. As the travel
path is longer than the distance between the transducers,
the apparent pulse velocity is lower than through the
sound material.
L. TOPOLÁØ et al.: NON-TRADITIONAL NON-DESTRUCTIVE TESTING OF THE ALKALI-ACTIVATED SLAG ...
Materiali in tehnologije / Materials and technology 50 (2016) 1, 7–10 9
Figure 6: Change in the dominant frequency over time
Slika 6: Spreminjanje prevladujo~e frekvence s ~asom
Figure 4: Results for the specimen cured in water for 6 d
Slika 4: Rezultati vzorca, ki se je 6 d utrjeval v vodi
Figure 3: Results for the specimen cured in water for 2 d
Slika 3: Rezultati vzorca, ki se je 2 d utrjeval v vodi
Figure 5: Results for the specimen cured in water for 27 d
Slika 5: Rezultati vzorca, ki se je 27 d utrjeval v vodi
The frequencies as well as the ultrasonic velocity
exhibit a decreasing trend of the values. The initial, quite
steep drop corresponds to the evaporation of the water
absorbed in the pore system, leaving air voids that do not
transmit ultrasonic signals. The further decrease
connected with the crack formation caused by drying
shrinkage is bit more moderate. These results are in very
good accordance with the acoustic-emission measure-
ments. Higher absolute values of the ultrasonic velocity
for the specimens cured for longer times are associated
with a denser and more compact structure.
The comparison in the mass loss for variously cured
specimens is given in Figure 8. The loss in mass was
calculated relative to the steady state after spontaneous
drying. The relative mass of the steady state was set to 1.
The specimens cured for 27 d in water lost only 28 % of
mass, whereas the specimens immersed in the water bath
for 6 d and 2 d decreased their mass by 41 % and 46 %,
respectively. It can be assumed that the AAS specimens
that were not completely hydrated were more porous
and, hence, contained higher amounts of the evaporable
water.
4 CONCLUSIONS
The paper deals with the use acoustic non-destructive
methods for monitoring the alkali-activated slag mortars
during the process of drying and hardening. Volume
variations in the alkali-activated slag mortars are
connected with autogenous and drying shrinkage. The
loss in mass observed during the setting and hardening of
AAS is a result of the drying process. The rate of the
moisture release is in good accordance with the number
of signals detected with the AE method. The changes in
the dominant frequency towards lower values detected
with the impact-echo method for all three specimens are
visible and there is also a trend of a decrease in the
ultrasonic velocity, indicating that a large number of new
inhomogeneities appeared in the tested specimens during
their storage in air. It is assumed that most of these
changes can be attributed to the crack formation; there-
fore, it can be concluded that the main process leading to
a deterioration of the AAS binder is the drying shrink-
age.
Acknowledgement
This paper was elaborated with the financial support
of the Czech Science Foundation project CSF No.
13-09518S and the Ministry of Education, Youth and
Sports of the Czech Republic under the "National
Sustainability Programme I" (project No. LO1408
AdMaS UP), as an activity of the regional Centre
AdMaS (Advanced Materials, Structures and Tech-
nologies).
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10 Materiali in tehnologije / Materials and technology 50 (2016) 1, 7–10
Figure 8: Change in the loss in mass in relative units over time
Slika 8: Spreminjanje izgube mase v relativnih enotah s ~asom
Figure 7: Change in the ultrasonic velocity over time
Slika 7: Spreminjanje hitrosti ultrazvoka s ~asom