Construction and Building Materials 42 (2013) 1–4 Contents lists available at SciVerse ScienceDirect 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  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. , and the results reported by Daniel and Kim , 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: [email protected] (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. , Liu et al. , 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.  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, 2 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)  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)  is 2.340 g/cm3, the bulk density (UNE-EN 12697-6)  is 2.210 g/cm3, and the void content (UNE-EN 12697-8)  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 , 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. 4 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. , Liu et al.  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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