Healing of Porous Asphalt Concrete via Induction Heating

This art icle was downloaded by: [ Lib4RI ] On: 21 April 2013, At : 23: 54 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Road Materials and Pavement Design Publicat ion det ails, including inst ruct ions f or aut hors and subscript ion inf ormat ion: ht t p: / / www. t andf online. com/ loi/ t rmp20 Healing of Porous Asphalt Concrete via Induction Heating Quant ao Liu a , Erik Schlangen a , Mart in van de Ven b & Álvaro García a a Delf t Universit y of Technology, Facult y of Civil Engineering and Geosciences Micromechanics Laborat ory (MICROLAB), St evinweg 1, 2628, CN Delf t , The Net herlands E-mail: b Delf t Universit y of Technology, Facult y of Civil Engineering and Geosciences, Road and Railway Engineering, St evinweg 1, 2628, CN Delf t , The Net herlands E-mail: Version of record f irst published: 19 Sep 2011. To cite this article: Quant ao Liu , Erik Schlangen , Mart in van de Ven & Álvaro García (2010): Healing of Porous Asphalt Concret e via Induct ion Heat ing, Road Mat erials and Pavement Design, 11: sup1, 527-542 To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 14680629. 2010. 9690345 PLEASE SCROLL DOWN FOR ARTI CLE Full t erm s and condit ions of use: ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions This art icle m ay be used for research, t eaching, and privat e st udy purposes. Any subst ant ial or syst em at ic reproduct ion, redist ribut ion, reselling, loan, sub- licensing, syst em at ic supply, or dist ribut ion in any form t o anyone is expressly forbidden. The publisher does not give any warrant y express or im plied or m ake any represent at ion t hat t he cont ent s will be com plet e or accurat e or up t o dat e. The accuracy of any inst ruct ions, form ulae, and drug doses should be independent ly verified wit h prim ary sources. The publisher shall not be liable for any loss, act ions, claim s, proceedings, dem and, or cost s or dam ages what soever or howsoever caused arising direct ly or indirect ly in connect ion wit h or arising out of t he use of t his m at erial. Healing of Porous Asphalt Concrete via Induction Heating Downloaded by [Lib4RI] at 23:54 21 April 2013 Quantao Liu* — Erik Schlangen* — Martin van de Ven** Álvaro García* * Delft University of Technology, Faculty of Civil Engineering and Geosciences Micromechanics Laboratory (MICROLAB), Stevinweg 1 2628 CN Delft, The Netherlands [email protected] [email protected] [email protected] ** Delft University of Technology, Faculty of Civil Engineering and Geosciences Road and Railway Engineering, Stevinweg 1 2628 CN Delft, The Netherlands [email protected] ABSTRACT. The lifetime of porous asphalt pavement is only about 11 years. In this research, a porous asphalt concrete with long lifetime, based on a healing mechanism triggered by means of induction heating, is explained. Conductive fillers (steel fibers and steel wool) are added to porous asphalt concrete to enhance its electrical conductivity and induction heating is used to increase the temperature locally, just enough to increase the healing rate of asphalt concrete to heal the micro-cracks and to repair the bond between aggregates and binder. The main purposes of this research are to examine the electrical conductivity, particle loss resistance and induction heating speed of electrically conductive porous asphalt concrete and prove that damage in the material can be healed via induction heating. It is found that long fibers with small diameter are better than short fibers with bigger diameter to make porous asphalt concrete electrically conductive, induction heatable and have high particle loss resistance as well. Finally, it is also proved that damage in porous asphalt concrete can be healed via induction heating. KEYWORDS: Porous Asphalt Concrete, Conductive Fillers, Resistivity, Particle Loss, Induction Heating, Healing. DOI:10.3166/RMPD.11HS.527-542 © 2010 Lavoisier, Paris Road Materials and Pavement Design. EATA 2010, pages 527 to 542 528 Road Materials and Pavement Design. EATA 2010 Downloaded by [Lib4RI] at 23:54 21 April 2013 1. Introduction Most of the surface wearing courses of primary motorways in the Netherlands are paved with porous asphalt concrete (PA) due to its excellent performance in noise and spray reduction. However, the biggest defect of porous asphalt concrete is its poor performance in terms of durability compared with dense graded asphalt concrete. The average lifetime of porous asphalt concrete is about 11 years, much shorter than the lifetime of dense asphalt. The most common form of distress of porous asphalt concrete is loss of aggregates from the road surface named ravelling (Klomp, 1996). Ravelling is mainly caused by an increase of stiffness, reduction of relaxation capacity and formation of micro-cracks in the binder due to ageing. Traffic and thermal stresses, together with age hardening, are the main causes of ravelling. However, most of the ravelling damage is the result of the material properties itself (Eijssen et al., 2006). Small cracking on highway runway can mean the start of some big distress. Once the first stone is removed by a car wheel, the remaining stones around the gap will lack support from at least one direction, making it rather easy to remove the subsequent stones in the gap. So, more stones will be removed at a higher speed. Ravelling has a negative effect on the noise reduction capacity of porous asphalt concrete and requires early maintenance (Swart et al., 1997). So, finding a solution to avoid ravelling and to extend the lifetime of porous asphalt roads is one of the biggest concerns of pavement researchers. Asphalt concrete is known as a kind of self healing material (Little et al., 2007; Jo et al., 2001). The healing of asphalt concrete occurs due to rest periods. The problem comes because it is difficult to stop traffic circulation on a road to allow enough self healing recovery at ambient temperature. It is well known that the amount of healing increases when the material is subjected to a higher temperature (Jo et al., 2001; Bonnaure et al., 1982). Increasing the temperature will increase the healing effect of asphalt concrete; as a result of healing, the service lifetime of asphalt concrete will be increased. The idea is to add steel fibers to porous asphalt concrete and increase its temperature via induction heating to trigger the self healing process when micro-cracks occur in the material. Conductive asphalt concrete is a kind of functional material developed to achieve electrical conductivity for certain purposes. Sherif et al. made conductive concrete containing steel fibers and shavings for bridge deck de-icing (Sherif et al., 1999). Wu et al. (2006) described conductive asphalt concrete as a kind of structure selfmonitoring asphalt-based material for the resistance change can denote the variation of its interior structure. These authors did a lot of work concerning how to make conductive asphalt concrete by adding conductive carbon fiber, carbon black or graphite (Wu et al., 2005). Traditionally, in conductive roads, heat was generated due to the electrical resistance in the conductive particles when connected to a power source, but in this occasion, authors are trying to make porous asphalt concrete appropriate for induction heating and subsequently healing of cracks (Liu et al., 2010). Kim et al. (2002) did an elaborate study on the induction heating of Healing of PA via Induction Heating 529 carbon fiber reinforced thermoplastic composites. Induction heating was introduced to asphalt industry by García et al. (2009). In this method, the power supply sends alternating current through the coil, generating an alternating electromagnetic field. When the conductive asphalt specimen is placed under the coil, this electromagnetic field induces currents flowing through the conductive loops formed by the steel fibers and heat is generated by the Joule effect. Adding heat via induction energy in asphalt concrete will have a good future if the micro-cracks can be healed via induction heating. Downloaded by [Lib4RI] at 23:54 21 April 2013 The main objectives of this research are to examine the electrical conductivity, particle loss resistance and induction heating speed of porous asphalt concrete containing conductive fillers and prove that damage in this material can be healed by means of induction heating. 2. Experiments 2.1. Materials The aggregates used to prepare porous asphalt concrete specimens were quarry material (Bestone, Norway, size between 2.0 and 22.4 mm and density 2770 kg/m3), crushed sand (size between 0.063 and 2 mm and density 2688 kg/m3), and filler type Wigro 60K (size < 0.063 mm and density 2638 kg/m3). The bitumen used was 70/100 pen, obtained from Kuwait Petroleum, with density 1032 kg/m3, penetration 93 dmm and softening point 45.0 ºC. Besides, three different types of electrically conductive steel fibers were mixed in the porous asphalt mixture. The first one was steel fibers (from now, we will call them type 1) with length shorter than 1 mm and diameters between 29.6 µm and 191.1 µm. The second one was steel wool type 00 with length around 9 mm and diameters between 8.89 µm and 12.7 µm. The third one was steel wool type 000, with average length around 7 mm and diameters between 6.38 µm and 8.89 µm. All three types of fiber had an approximate density of 7.8 g/cm3 and an electrical resistivity of 7·10-5 :m. 2.2. Porous asphalt concrete composition Porous asphalt PA 0/16 was used in this research, because it is the mainly used surface wearing course material in the Netherlands. The materials composition was determined based on the Dutch Standard RAW 2005. Gyratory compactor was used to mould the specimens. The composition of PA 0/16 mixture for gyratory specimen is shown in Table 1. The maximum theoretical density of the mixture was 2.569 g/cm3. It was calculated as the total weight divided by the total volume of all the materials before compaction. The standard calls for a minimum of 20% of air voids content. In this 530 Road Materials and Pavement Design. EATA 2010 case, the air voids content was assumed to be 21%, Based on the maximum theoretical density, the assumed density of the specimens after compaction and the weight of the mixture, the gyratory compactor can control the height of the specimen to obtain the ultimate target density. The density of the mixture changes with the variation in the volume of conductive fillers in the mixture and can be computed according to the total weight and volume of all the materials in the mixture. Finally the air voids content can be calculated after moulding the specimens by determining their dimensions and weights. The air voids ratio for all specimens studied was around 21%. Downloaded by [Lib4RI] at 23:54 21 April 2013 Table 1. Composition of PA 0/16 mixture for gyratory specimen based on the Dutch standard (RAW 2005) Sieve size (mm) Density (g/cm3) RAWSpec. % retained Cumm % ret. % ret. by weight Weight (g) 22.4-16.0 16.0-11.2 11.2 - 8.0 8.0 - 5.6 5.6 - 2.0 2.0- 0.063 < 0.063 2.778 2.774 2.762 2.765 2.781 2.688 2.638 0-7 15-30 50-65 70-85 85 95.5 100 4 25 57 80 85 95.5 100 4 21 32 23 5 10.5 4.5 48 252 384 276 60 126 54 1200 Bitumen70/100 1.032 4.5% by wt. Total wt. 54 1254 2.3. Electrical resistivity test The electrical resistivity measurements were done at room temperature 21.5 ºC. The samples, with diameter 100 mm and thickness 50 mm were cut from the gyratory compacted specimens to get flat surfaces for resistance testing. After cutting, the samples were placed in an oven at 40 ºC for 8 hours to remove the moisture and prevent the steel fibers from corroding on the surface of the samples. Inside the sample, the steel fibers do not corrode, because they are completely coated with bitumen. A digital multimeter was used to measure the resistance below 36 x 106 :. A resistance tester was used to measure the resistance higher than this value. Two electrodes were made of 100 mm x 160 mm rectangular copper plates in conjunction with wires, which could be connected with the resistance tester when testing resistance. Both electrodes were placed at both ends of the test sample to measure the electrical resistance (Figure 1). Downloaded by [Lib4RI] at 23:54 21 April 2013 Healing of PA via Induction Heating 531 Figure 1. The situation for resistance testing A small pressure was applied to the copper electrodes to obtain a good contact with the surface of the sample. The total contact resistance between the two electrodes was about 0.4:, which was negligible with respect to the great resistances studied (higher than 100 K: in the samples). The electric field was assumed constant and the end-effects were considered negligible. The electrical resistivity was obtained from the second Ohm-law in Equation [1]: U RS L [1] Where U is the electrical resistance, L is the internal electrode distance in meter, S is the electrode conductive area in square meter and R is the measured resistance in omega. 2.4. Cantabro test Cantabro test was used to evaluate the particle loss resistance of porous asphalt concrete specimens containing conductive fibers. The test was done at room temperature (21.5 ºC) in a Los Angeles abrasion machine without steel ball, according to the European Standard EN 12697-17. Each specimen was initially weighed (W1) and placed separately into a Los Angeles drum. Then, each specimen was weighed again after 300 revolutions of the drum (W2) to determine the weight loss during testing. This weight loss is an indication of the cohesive properties of the mix. Lower weight loss means better cohesion and better particle loss resistance. The test results are expressed as a percentage of weight loss in relation to the initial weight: 532 Road Materials and Pavement Design. EATA 2010 PL W1 W2 u100 W1 [2] where, PL is the particle loss in percent, W1 is the initial specimen mass in gram and W2 is the final Specimen mass in gram. Five specimens of each composition were tested to check the reproducibility of the results obtained. The data shown in this section are the mean value of the results obtained in the five samples. Downloaded by [Lib4RI] at 23:54 21 April 2013 2.5. Induction heating test Induction heating tests were conducted on conductive porous asphalt concrete to prove that it could be easily heated with induction energy. For that, the effect of fiber volume content on the heating rate was studied. The principles of induction heating are electromagnetic induction and Joule heating. According to Faraday law of electromagnetic induction, the electromotive force can be produced around a closed path in a changing magnetic field and is proportional to the rate of change of the magnetic flux through any surface bounded by that path. Faraday’s law of electromagnetic induction states that: d ‡B dt H [3] where, 0 is the electromotive force (emf) in volt and B is the magnetic flux in weber. In practice, this means that an electrical current is induced through the fibers when the magnetic flux touches them. The electrical current generates heat when it flows through the conductive fibers. This is Joule heating, which can be explained by Joule’s first law: p I 2R [4] where, P is the heat generated per unit time by a constant current I flowing through a conductor of electrical resistance R. This law applies to any circuit that can be characterized by a resistance. Ohm’s law states that for a voltage 0 across a circuit of resistance R the current I will be: I H R [5] Healing of PA via Induction Heating 533 By substituting this formula for current into one or both factors of current in Joule’s law, the power dissipated can be written in the equivalent form: 2 §H · P ¨ ¸ R ©R¹ H2 R [6] Downloaded by [Lib4RI] at 23:54 21 April 2013 The induced electromotive force depends on the rate of change of the magnetic field flux. For this heating system, a constant frequency in the induction equipment will generate a constant electromotive force. The heating rate induced by Joule heating is in contrast with the resistance of the sample, the lower the resistance, the higher the heating rate. Figure 2. Heating the sample with the induction machine The induction heating experiment was performed by using an induction heating system with a capacity of 50 kW and at a frequency of 70 kHz (Figure 2). Although the system was not fully optimized, it had no influence on the objectives of this research. The heating samples were the same as the ones used in the resistance measurements. The cylindrical samples cut from gyratory specimens were used to avoid the problem of binder concentration on the surfaces of samples and to get a higher heating efficiency for that thinner samples mean less temperature difference between the top and the bottom of the samples. Each sample studied was heated for 3 minutes and its temperature variation was measured with a 640 x 480 pixel, full colour infrared camera. 2.6. Healing detection test It was expected that induction heating would increase the healing capability of porous asphalt concrete. To prove this, indirect tensile fatigue tests at 5 °C, with rest periods to apply the induction heating, were used to detect the induction healing 534 Road Materials and Pavement Design. EATA 2010 13000 Resilient modulus Mpa Downloaded by [Lib4RI] at 23:54 21 April 2013 effect. The idea was to use fatigue test to introduce damage to samples, then the samples were heated with induction heating and rested for some time to see if the damage can be healed or not. The fatigue test applied a 0.2 seconds haversine load followed by a rest period of 0.3 seconds. The maximum load was 0.82 MPa, which was 40% of the indirect tensile strength of plain samples. A Poisson’s ratio of 0.22 was assumed for this porous asphalt concrete to determine its resilient modulus. As is shown in Figure 3, fatigue test was stopped when the resilient modulus of the sample reduced to 70% or 80% of its original value. Then, the sample was heated with induction machine for 2 minutes and rested for 24 hours. Finally, the fatigue test was continued until the resilient modulus reduced to 70% or 80% of its initial value for a second time. After the tests, two numbers of loading cycles C1 and C2, were obtained (Figure 3). The healing index HI is defined as C2 divided by C1, where 100% means the entire healing of damage and 0% means no healing at all. 10000 7000 C2 C1 4000 1000 0 2000 4000 6000 8000 10000 Loading cycles Figure 3. Test procedure for healing detection with indirect tensile fatigue test 3. Results and discussions 3.1. Effect of the steel fiber volume content on the electrical resistivity of porous asphalt concrete Absolute fiber size determines the number of fibers per unit of batched weight and the number per cubic meter of matrix. Since the total weight rather than the Healing of PA via Induction Heating 535 absolute size reflects the material cost of the fibers, the question arises whether a large number of small fibers offer better conductive effectiveness than the same weight of a small number of large fibers. To answer this, three different types of steel fiber with different diameters are used to see which one is best to make porous asphalt concrete electrically conductive. 1,00E+11 type 1 type 00 1,00E+09 Resistivity Downloaded by [Lib4RI] at 23:54 21 April 2013 m type 000 1,00E+07 1,00E+05 1,00E+03 0 5 10 15 Fiber volume content % 20 25 Figure 4. Effect of the steel fiber volume content on the resistivity of porous asphalt concrete The effect of the steel fiber volume content on the resistivity of porous asphalt concrete samples is shown in Figure 4. Addition of all three types of steel fibers can decrease the electrical resistivity of porous asphalt concrete, which means a promising increase of the induction heating speed. These three electrical resistivity curves show three different stages: high resistivity stage, exhibiting insulating behaviour with resistances higher than 109 :m; transit stage, where the electrical resistivity of asphalt concrete suffers a sharp decrease from 109 :m to 104 :m; and low resistivity stage, exhibiting conductive behaviour with resistances of approximately 104 :m. This can be explained by means of the percolation theory: When a small amount of steel fibers are added to the mixture, they are uniformly distributed in the porous asphalt concrete samples and do not contact each other, having a similar resistivity to that of a plain sample without fibers. This is the high resistivity stage. When more steel fibers are added to the mixture, they start 536 Road Materials and Pavement Design. EATA 2010 Downloaded by [Lib4RI] at 23:54 21 April 2013 contacting each other, which causes a gradual decrease in the resistivity. If the fiber volume content reaches a certain value (percolation threshold), the first conductive paths are formed in the sample. This corresponds to a sharp decrease of resistivity and has been called transit stage. Beyond the percolation threshold, the conductive network develops and spreads gradually in three dimensions with the increase of the volume content of the steel fibers. When the fiber volume content reaches a certain value ( the optimal content), steel fibers contact each other in all directions and many conductive networks and passages are formed, corresponding to a very low value of resistivity at which adding more steel fibers doesn’t reduce the resistivity any more. This is the low resistivity stage. However, there are some differences among the three resistivity curves. Among the three different types of steel fiber, the diameter of steel fiber type 1 is biggest and the diameter of steel wool type 000 is smallest. The percolation threshold comes earlier and the transit stage of the resistivity curve is sharper for the type of fiber with smaller diameter. The optimal contents of fibers to make porous asphalt concrete conductive are 20%, 12% and 10% for steel fibers type 1, steel wool type 00 and steel wool type 000 respectively, which means that the smaller the diameter, the less volume of fibers is needed to make porous asphalt concrete conductive. So, steel wool type 000 is the best one to make porous asphalt concrete conductive. 3.2. Effect of steel fiber volume content on the particle loss resistance of porous asphalt concrete Particle loss tests were done to check if the addition of steel fibers for electrical conductivity purpose affects the particle loss resistance of porous asphalt concrete or not. The effect of the steel fiber volume content on the particle loss resistance of porous asphalt concrete is shown in Figure 5. The particle loss of plain samples (without fibers) is 14.84%. It can be seen that the particle loss of samples decreases with the increase of the fiber volume content and reaches a minimum, after which adding more steel fibers will result in an increase of the particles loss. The minimum particles losses are 8.77%, 8.01% and 8.27%, obtained with around 15%, 8% and 8% of steel fiber type 1, type 00 and type 000, respectively. From these data, steel wool type 00 is optimal to increase the particle loss resistance of porous asphalt concrete. At the contents for optimal conductivity (20% for steel fiber type 1, 12% for steel wool type 00 and 10% steel wool type 000), the particle losses are 10.31%, 15.83% and 13.66%, respectively. Adding 12% of steel wool type 00 to porous asphalt concrete for optimal conductivity purposes will decrease its particle loss resistance and adding 10% of steel wool type 000 or 20% steel fiber type 1 to porous asphalt concrete for optimal conductivity purpose will increase its particle loss resistance. Healing of PA via Induction Heating 537 17 type 1 15 type 00 Downloaded by [Lib4RI] at 23:54 21 April 2013 Particle loss % type 000 13 11 9 7 0 5 10 15 Fiber volume content % 20 Figure 5. Effect of the steel fiber volume content on the particle loss resistance of porous asphalt concrete 3.3. Effect of the steel fiber volume content on the induction heating speed of porous asphalt concrete All the samples studied were heated for 3 minutes and a full colour infrared camera was used to record the temperature variation. Figure 6 is an induction heating image of a sample with 8% steel wool type 000 at the end of the heating test. All samples have similar images like this. On the top surface of the sample, clusters of steel wool work as small heaters. This corresponds to the shining dots on the surface of the sample in Figure 6. The image shows uniform temperature in the horizontal direction. This is because the magnetic field is constant at the same distance from the coil. The temperature of the sample decreases from top to bottom. It was also found that samples had a higher heating rate when they were closer to the coil of the induction machine. The reason for this is that the magnetic field is stronger close to the coil. Finding the optimum distance between the pavement and the coil will be a topic for future study. The distance between sample and the coil of the induction machine hasn’t yet been optimized and was just fixed at a constant value of 32 mm in the test. 538 Road Materials and Pavement Design. EATA 2010 115 ºC 62 ºC 250 30s 60s 90s 120s 150s 180s 200 Temperature ºC Downloaded by [Lib4RI] at 23:54 21 April 2013 Figure 6. Induction heating image of the sample with 8% steel wool type 000 150 100 50 0 0 5 10 15 20 25 Steel fiber volume content % Figure 7. Temperatures variations of porous asphalt concrete containing steel fiber type 1 during 3 minutes induction heating Healing of PA via Induction Heating 539 Downloaded by [Lib4RI] at 23:54 21 April 2013 To analyze the effect of the steel fiber volume content on the heating speed of concrete, the temperatures of the tops of all porous asphalt concrete samples studied were recorded every 30 seconds during the heating process. The temperature variations of porous asphalt concrete samples containing steel fiber type 1 during 3 minutes induction heating is shown in Figure 7. In this figure, it can be appreciated how the maximum temperature reached in the samples increases with the volume of fibers in the mixture, but the heating speed (increase of temperature in a certain time during induction heating) can’t be increased any faster when the steel fiber volume content reaches a certain value (optimal content). The heating speeds of samples containing steel wool type 00 and type 000 have the same trend, which is not shown here. Porous asphalt concrete samples without conductive fibers almost cannot be induction heated. The optimum content of fiber to obtain the highest induction heating speeds and the maximum temperatures reached are shown in table 2 for the three types of fibers studied. The optimum volume contents are 20%, 12% and 10% for steel fiber type 1, steel wool type 00 and steel wool type 000, respectively. These values coincide with the optimal contents of fiber to obtain the minimum resistivity in Figure 4. The maximum temperatures reached in samples with optimum content of fibers are 207 ºC, 169 ºC and 137 ºC, respectively. Adding steel fibers above these contents doesn’t increase the induction heating speeds and will result in a decrease of the particle loss resistance of porous asphalt concrete, as shown in Figure 5. Table 2. Optimum volume contents of fiber for induction heating and maximum temperatures reached after 3 minutes induction heating Steel fiber type Steel fiber type 1 Steel wool type 00 Steel wool type 000 Optimal content 20% 12% 10% Maximum temperature 207 ºC 169 ºC 137 ºC 3.4. Healing of porous asphalt concrete via induction heating Porous asphalt concrete with 10% steel wool type 000 has optimal conductivity (Figure 4) and optimal induction heating speed (Table 2). At the same time, its particle loss resistance is better than that of samples containing 10% of the other two types of fiber studied. For this reason, 10% is proposed as an ideal content of fibers type 000 to obtain an optimum conductivity, induction heating speeds and good particle loss resistance in porous asphalt concrete. To prove that damage in porous asphalt concrete could be healed via induction heating, samples containing 10% of steel wool type 000 are used in the healing detection test. The healing index of plain samples (without fibers) and samples containing 10% steel wool type 000 after 540 Road Materials and Pavement Design. EATA 2010 Downloaded by [Lib4RI] at 23:54 21 April 2013 24-hour rest are shown in Table 3, which presents the averages of the results of three samples for each condition. In this table, HI70% and HI80% mean that fatigue tests were stopped for healing when the resilient modulus of the sample decreased to 70% and 80% of its original value, respectively. In both cases, samples with fibers have better healing effect than plain samples and the healing effect of conductive samples is increased after the induction heating is applied. In the case where fatigue test was stopped for healing when the resilient modulus of the sample decreased to 70% of its original value, after 24 hours rest, only 11.42% and 17.99% of the damage in plain samples and samples containing 10% steel wool type 000 were healed. But if a 2 minute induction heating was applied, 23.05% of the damage was healed in the samples with fibers. In this case, the healing is relatively low because of this level of damage, structural damage in the samples, as deformation or broken aggregates appears. Induction healing can repair micro damage, not structural damage, so in this case, it is too late to heal the damage in the asphalt concrete. But, if fatigue test was stopped when the resilient modulus had reduced to 80% of its original value (to avoid structural damage in the sample), 52.08% and 83.80% of the damage in plain samples and in samples containing 10% of fibers type 000 were healed after 24 hours rest. And, with 2 minutes induction heating, the damage in samples containing 10% steel wool type 000 could be completely healed. Table 3. Healing index of the samples studied Samples with 10% steel wool type 000 Samples Plain samples Without heating with heating HI70% 11.42% 17.99% 23.05% HI80% 52.08% 83.80% 100% 4. Conclusions Adding steel fiber to porous asphalt concrete can increase its electrical conductivity and particle loss resistance. Long steel wool (type 00 and type 000) with smaller diameter is more effective than short steel fibers (type 1) with bigger diameter to increase the electrical conductivity and particle loss resistance of porous asphalt concrete. Porous asphalt concrete samples containing steel fibers can be induction heated easily with induction energy. The volume of fibers needed to obtain the maximum conductivity is the same as the volume of fibers to obtain the maximum heating rates. 10% (by volume of bitumen) is proposed as an ideal content of steel wool type 000 to obtain optimal electrical conductivity, induction heating speed and good particle loss resistance in porous asphalt concrete. Besides, it is Healing of PA via Induction Heating 541 proved that the healing capability of porous asphalt concrete containing 10% of steel wool type 000 is increased with the induction heating. Finally, it is also found that when structural damage happens in porous asphalt concrete, it is too late to apply induction heating to heal damage. Acknowledgements Downloaded by [Lib4RI] at 23:54 21 April 2013 The scholarship from the China Scholarship Council is acknowledged. The technical support of Prof. Klaas van Breugel, Prof. A.A.A. Molenaar and Marco Poot are also appreciated. Furthermore, the authors would like to express thanks to Global Material Technologies for their technical expertise and advices about steel wool. 5. Bibliography Bonnaure F.P., Huibers A.H., Boonders A., “A laboratory investigation of the influence of rest periods on the fatigue characteristics of bituminous mixes”, Journal of the Association of Asphalt Paving Technologists, 51, 1982, p. 104-128. Eijssen M.L.J., Klarenaar W.H.M., “Zwart Mozaïek, evaluatie van bestaande kennis en kennisleemten als basis voor de levensduurverbetering van 2-laags ZOAB”, Intron-report A831100/R20060101/MEy, 2006. EN 12697-17: Bituminous mixtures. Test methods for hot mix asphalt, Part 17: Particle Loss of porous asphalt specimen, 2004. García A., Schlangen E., van de Ven M. and Liu Q., “Electrical conductivity of asphalt mortar containing conductive fibers and fillers”, Construction and Building Materials, Vol. 23, Vol. 10, 2009, p. 3175-3181. Kim H., Yarlagadda S., W. Gillespie J.R.J. et al., “A study on the induction heating of carbon fiber reinforced thermoplastic composites”, Advanced Composite Materials, Vol. 11, No. 1, 2002, p. 71-80. Hagos E.T., Molenaar A.A.A., van de Ven M., “The Effects of Aging of the Bituminous Mortar on the Ravelling of Porous Asphalt Surface Layers, A Literature Review”, 5th International PhD Symposium in Civil Engineering, Delft, The Netherlands, 2004. Jo S.D., Richard K.Y., “Laboratory evaluation of fatigue damage and healing of asphalt mixtures”, Journal of Materials in Civil Engineering, Vol. 13, No. 6, 2001, p. 434-440. Klomp A.J.G., “Life period of porous asphalt”, Dutch Road and Hydraulic Engineering Institute report, 1996. Little D.N., Bhasin A., “Exploring mechanisms of healing in asphalt mixtures and quantifying its impact”, Self Healing Materials an Alternative Approach to 20 Centuries of Materials Science, S. van der Zwaag (Ed.), Springer Series in Materials Science, 100, 2007, p. 205218. 542 Road Materials and Pavement Design. EATA 2010 Liu Q., Schlangen E., García A., van de Ven M., “Induction heating of electrically conductive porous asphalt concrete”, Construction and Building Materials, 2010, doi:10.1016/j.conbuildmat.2009.12.019 Swart J.H., “Experience with porous asphalt in the Netherlands”, European Conference on Porous Asphalt, Madrid, 1997. Sherif Y.H., Christopher Y.T., “Conductive concrete overlay for bridge deck deicing”, ACI Materials Journal, Vol. 96, No. 3, 1999, p. 382-390. Wu S., Liu X., Ye Q., Li N., “Self-monitoring electrically conductive asphalt-based composite containing carbon fillers”, Transactions of Nonferrous Metals Society of China, 16, 2006, p. 512-516. Downloaded by [Lib4RI] at 23:54 21 April 2013 Wu S., Mo L., Shui Z., Chen Z., “Investigation of the conductivity of asphalt concrete containing conductive fillers”, Carbon, Vol. 43, No. 3, 2005, p. 1358-1363.