Construction and Building Materials 42 (2013) 1–4
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Heating asphalt mixtures with microwaves to promote self-healing
Juan Gallego a,⇑, Miguel A. del Val a, Verónica Contreras b, Antonio Páez b
a
b
Technical University of Madrid – UPM, Spain
Repsol Technology Centre, Spain
h i g h l i g h t s
" We present the heating of asphalt mixes to promote the self-healing process.
" The novelty is the use of microwaves instead of electromagnetic induction.
" Microwaves could be advantageous when compared to the electromagnetic induction.
" Content of additives are smaller than those in the case of electromagnetic induction.
" Microwaves appear to be a promising technique to in situ heat asphalt layers.
a r t i c l e
i n f o
Article history:
Received 31 July 2012
Received in revised form 26 November 2012
Accepted 4 December 2012
Available online 31 January 2013
Keywords:
Bituminous mixture
Self-healing
Microwaves
a b s t r a c t
This paper presents the results of a research into the technical viability of heating asphalt mixtures with
microwaves and how the microwaves influence the heating process of the different variables involved.
Various past studies have established that elevated temperatures during rest periods of an asphalt mixture promote self-healing. In addition, recent investigations have been done regarding the heating of
asphalt mixtures by means of electromagnetic induction: steel wool and graphite additives where introduced to the mixtures, improving their conductivity and thus increasing their susceptibility to electromagnetic induction.
In this study, heating to promote self-healing is achieved with microwaves. The optimal steel wool content established for this study is around ten times less than that recommended for heating by electromagnetic induction, which in practice could mean an important reduction in costs. Additionally, the
amount of electricity used by microwave devices is much less than that required to produce a similar
effect by electromagnetic induction.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
It has been observed that the microcracks found in asphalt mixtures heal to differing degrees depending on the type of mixture,
the rest periods given to the pavement and the temperature of
the pavement during these periods.
Little and Bhasin [1] state that this phenomenon can be
explained by the diffusion of the molecules between the two sides
of the crack, which would create connection points that partially
restore the continuity of the material. This explanation would
agree with the results obtained by Bonnaure et al. [2], and the
results reported by Daniel and Kim [3], according to whom temperatures of 50–60 °C, sufficient to soften the binder during rest
periods, lead to the self-healing seen in the material.
⇑ Corresponding author. Address: Escuela de Ingenieros de Caminos (UPM), Calle
Profesor Aranguren s/n, 28040 Madrid, Spain. Tel.: +34 91 336 64 34; fax: +34 91
336 66 54.
E-mail address: juan.gallego@upm.es (J. Gallego).
0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2012.12.007
Other authors, such as Liu et al. [4,5], have heated asphalt mixtures that had previously been cracked, verifying that heating to
relatively high temperatures of between 70 and 120 °C led, in some
cases, to up to 100% self-healing. These same authors used a novel
method to heat the mixtures: electromagnetic induction. It is a
promising technique, as it could be applied to in-service pavements, and thus achieve in situ self-healing. However, one of the
conditions of the technique of heating asphalt mixtures by electromagnetic induction is the need to incorporate metallic additives.
This increases the conductivity of the asphalt mixture, thereby
making it more susceptible to induction and enabling more efficient heating in terms of energy use. Some investigations – Wu
el al. [6], Liu et al. [7], and García et al. [8,9] – have concluded that
the optimum steel wool content is between 1.5% and 5% by weight
of the asphalt mixture, depending on the type of asphalt mixture
and the addition of other additives, such as graphite. García et al.
[10] have even designed a numerical model to determine the
induction heating speed as a function of the characteristics of the
bituminous mixture, the content of steel wool and other additives,
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J. Gallego et al. / Construction and Building Materials 42 (2013) 1–4
the underlying pavement layer, the meteorological conditions during the operation and the characteristics of the electromagnetic
induction generator.
The research presented in this paper studies the viability of
heating asphalt mixes with a different type of electromagnetic
waves: microwaves. Test samples of the asphalt mixture were prepared with different percentages of steel wool and were subjected
to heating in microwave ovens. It was found that the bituminous
mixture increases in temperature at a similar rate to that reported
by the researchers who used magnetic induction.
The advantages of the technique lie in the fact that the recommended steel wool content in the bituminous mixture is around
ten times less when using microwaves, resulting in important economic savings when compared with electromagnetic induction. In
addition, the device needed in the laboratory is much simpler in
the case of microwave heating, as, in order to obtain heating
speeds similar to those obtained by means of induction, all that
is necessary is a microwave oven with an output of 1.2 kW, as opposed to the 50 kW power supply used by other researchers to heat
specimens by induction [4,5,7–9].
Table 1
Grading curve of the asphalt mixture.
Sieve (mm)
% Passing
22
100
16
95
8
68
4
43
2
31
0.5
16
0.25
11
0.063
5.1
2. Materials used in the investigation
Fig. 1. Appearance of the coarse steel wool (CSW).
In order to make the laboratory test specimens, an asphalt mixture AC 16 surf
50/70 (UNE-EN 13108–1) [11] was used. It was made with limestone aggregates
and calcium carbonate as a filler. Its particle size composition is presented in Table
1.
The binder content of the mixture is 4.6% over the mass of the bituminous mixture, the maximum density (UNE-EN 12697-5) [12] is 2.340 g/cm3, the bulk density
(UNE-EN 12697-6) [13] is 2.210 g/cm3, and the void content (UNE-EN 12697-8) [14]
is 5.15%. As for the steel wool, it was used in low percentages of between 0.2 and
1.8% of the mass of the asphalt mixture. Two kinds of steel wool were used: one
which here has been termed ‘‘coarse steel wool’’ (CSW), with wires of 0.10–
0.12 mm thick, and another called ‘‘medium steel wool’’ (MSW), with a thickness
of 0.04–0.06 mm. There is a finer grade of steel wool (0.01–0.02 mm), but in the first
tests of incorporating it into a bituminous mixture, it was observed that during the
mixing process the wires tended to become tangled with one another, quickly forming clumps and impeding a homogenous distribution throughout the mass of the
bituminous mixture. For this reason, the finer steel wool was excluded from the
study.
Both the CSW and the MSW used in the tests had been previously cut into
pieces from the balls in which they are sold. The tests were conducted with wires
of 5 mm and 10 mm in length, as the lengths used by other researchers were in this
range [6–10]. Fig. 1 shows the appearance of the steel wool.
Fig. 2. Bituminous mixture in the microwave oven (2.45 GHz).
3. Microwaves. Laboratory procedures followed
Microwaves are electromagnetic waves of a similar nature to
radio, visible light and X-ray waves. What differentiates them from
the others is their wavelength (or, in other words, their frequency).
Thus, for example, visible light has a wavelength of between
4 10 7 m (violet) and 7 10 7 m (red), while microwaves have
wavelengths of between 3 mm and 3 m, which correspond to
frequencies of between 100 MHz and 100 GHz. A microwave oven
typically functions at 2.45 GHz, which corresponds to an approximate wavelength of 120 mm.
To heat the asphalt mixture in this study, a microwave oven
was used with an output of 1200 W and a 230 V, 50 Hz power
supply. The oven can produce microwaves of up to 800 W, with a
frequency of 2.45 GHz.
The asphalt mixture specimens, following the Marshall method
[15], were cut on a diametric plane, leaving two semi-cylindrical
halves. Each half of the specimen weighed approximately 530 g;
each was placed in the microwave oven (Fig. 2) and heated for
periods of 20 s. Between each two consecutive periods, the oven
was opened and the surface temperatures were taken using an
infrared gun (Fig. 3). Three temperatures were taken, randomly
chosen on the surface of the test sample, and the average was
calculated; then, the oven was turned on again. This process was
repeated six times for a total heating period of 120 s. Finally, as
the sample had softened due to the elevated temperature, a thermometer was introduced into the interior of the specimen to verify
its internal temperature (Fig. 4).
Three cylindrical test samples of each type of bituminous mixture were prepared and then cut in half diametrically, resulting in
six halves. The results presented below are the average of the individual results of the tests performed on the six halves of each type
of asphalt mixture.
4. Results and discussion
As has been indicated, the asphalt mixtures with the steel wool
were subjected to heating in the microwave oven for 120 s. The
surface temperature was taken every 20 s, as well as the internal
temperature after the 120 s by directly introducing a thermometer
into the interior of the half specimen. Below, four graphs have been
compiled, two of which illustrate the evolution of the surface temperature during the 120 s and the other two show the internal
temperatures reached after 120 s in the microwave oven. As can
be observed, the asphalt mixtures have been grouped into two
J. Gallego et al. / Construction and Building Materials 42 (2013) 1–4
3
Fig. 3. Measuring the surface temperature of the specimen.
Fig. 6. Internal temperature of specimens after 120 s in microwave oven (5 mm
steel wool).
Fig. 4. Measuring the internal temperature upon completion of the heating process.
families: the first containing steel wool cut into pieces 5 mm in
length, and the other with steel wool pieces of 10 mm.
Fig. 5 shows how the addition of the steel wool with a length of
5 mm increased the speed at which the temperature of the mixture
increased as compared to the reference mixture without steel wool
(No SW). It can be observed that the MSW is more effective than
the CSW: in 2 min, the surface temperatures reached 100–120 °C.
Fig. 5. Evolution of surface temperature (5 mm steel wool).
Fig. 7. Evolution of surface temperature (10 mm steel wool).
The internal temperatures of the test samples, after 120 s in the
microwave oven, are presented in Fig. 6. The internal temperatures
are somewhat higher than the surface temperatures (Fig. 5), which
can be explained by the fact that heat dissipation is greater on the
surface of a specimen than in its interior. In any case, one can observe that the temperatures reached in the test samples with MSW
are higher than those with CSW.
Asphalt mixtures with steel wool cut to a length of 10 mm were
also tested. Fig. 7 shows the surface temperatures of each specimen
during the heating process. These are about 20 °C higher than those
reached with the 5 mm steel wool pieces, despite the smaller proportions of steel wool in the mixtures. Thus, the energy efficiency
offered by the 10 mm steel wool is greater than that offered by the
steel wool cut to 5 mm. However, there is very little difference between the MSW and the CSW at a length of 10 mm.
The internal temperature of the specimens is presented in Fig. 8.
It can be observed that, as with the 5 mm steel wool, the internal
temperatures are some 20–30 °C higher than the average temperatures on the surface of the test samples. The explanation lies, as
before, in the greater heat dissipation that occurs on the surface
of the material. In this case, there is no relevant difference between
the internal temperatures of the samples with MSW and CSW.
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J. Gallego et al. / Construction and Building Materials 42 (2013) 1–4
This technique, then, is very promising, allowing for continued
research to develop full-scale applications that can promote
in situ self-healing of asphalt pavements.
Acknowledgements
This research was developed within the TRAINER Project,
‘‘Development of a new technology for autonomous, intelligent
self-healing of materials’’, financed by the Centre for Industrial
Technological Development (CDTI) of the Spanish Government
within the program supporting the Strategic National Consortiums
for Technical Investigation (CENIT).
The authors wishes to acknowledge to Raquel Casado Barrasa,
Ana Filipa Pereira y Carlos Martín-Portugues from Acciona their
contribution to this study.
Fig. 8. Internal temperature of specimens after 120 s in microwave oven (10 mm
steel wool).
Another conclusion that can be drawn from the above data is
that the use of MSW, cut to a length of 10 mm and in percentages
of 0.2% over the mass of the asphalt mixture, is sufficient to achieve
the desired results, as the susceptibility to the microwaves is not
significantly less than that attained with higher percentages.
The temperatures reached and the heating time of the mixture
with 0.2% of MSW are similar to those described by Wu et al. [6],
Liu et al. [7] and García et al. [8,9], although the percentages of
steel wool recommended by these researchers are between 1.5%
and 5% of the mass of the asphalt mixture. This difference seems
to indicate that significant savings could be made in steel wool,
which would make the microwave technique more suitable than
electromagnetic induction.
5. Conclusions
From the research presented here, one can draw some important conclusions:
– It is possible to employ microwaves to heat asphalt mixtures.
Among the possible applications of this technique is that of aiding
the self-healing of asphalts by heating them during rest periods.
– To improve the energy efficiency of the microwave heating process, it is necessary to add steel wool to the asphalt mixture, which
makes it more susceptible to the energy of the microwaves.
– The necessary percentages of steel wool are quite low. In this
study, it has been demonstrated that 0.2% of the mass of the
asphalt mixture is sufficient, preferably cut into pieces 10 mm in
length. This percentage is ten times less than the quantity recommended when employing electromagnetic induction for heating.
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