Construction and Building Materials 24 (2010) 1207–1213

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Construction and Building Materials

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Induction heating of electrically conductive porous asphalt concrete

Quantao Liu a,*, Erik Schlangen a, Álvaro García a, Martin van de Ven b

a Delft University of Technology, Faculty of Civil Engineering and Geosciences, Micromechanics Laboratory (MICROLAB), Stevinweg 1, 2628 CN Delft, The Netherlandsb Delft University of Technology, Faculty of Civil Engineering and Geosciences, Road and Railway Engineering, Stevinweg 1, 2628 CN Delft, The Netherlands

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 August 2009Received in revised form 15 December 2009Accepted 16 December 2009Available online 12 January 2010

Keywords:Porous asphalt concreteSteel fibersSteel woolElectrical resistivityIndirect tensile strengthInduction heating

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.12.019

* Corresponding author. Tel.: +31 (0) 15 2788078.E-mail address: quantao.liu@tudelft.nl (Q. Liu).

In this research, an electrically conductive porous asphalt concrete, used for induction heating, was pre-pared by adding electrically conductive filler (steel fibers and steel wool) to the mixture. The main pur-pose of this paper is to examine the electrical conductivity and the indirect tensile strength of thisconductive porous asphalt concrete and prove that it can be heated via induction heating. It was foundthat, to make porous asphalt concrete electrically conductive, long steel wool with small diameter is bet-ter than short steel fibers with bigger diameter. However, steel fibers with short length and big diameterhave better strength reinforcement capability than steel wool with long length and small diameter. It wasalso proved that conductive porous asphalt concrete containing steel wool can be easily heated via induc-tion heating. Finally, 10% (by volume of bitumen) of steel wool type 000 was proposed as an optimal con-tent in porous asphalt concrete to obtain an optimal conductivity, a good induction heating rate and anacceptable indirect tensile strength. It is expected that the autogenous healing capacity of asphalt con-crete will be enhanced with the increase of temperature during induction heating.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Porous asphalt concrete is the most commonly used surfacewearing course in the Netherlands due to its excellent performancein noise reduction and water drainage. However, the biggest defectof porous asphalt concrete is its poor performance in terms ofdurability compared with dense graded asphalt concrete. The aver-age lifetime of porous asphalt is of approximately 11 years [1],much shorter than the lifetime of a dense asphalt mixture. Themain durability problem of porous asphalt concrete is called ravel-ing [2], which is the loss of aggregates from the surface layer of thepavement and has a negative effect on the general performance ofthe pavement [3]. Part of the damage due to raveling is the result oferrors in the production and processing methods, including poorcontrol of asphalt temperature during production, variations inthe mixture composition, the presence of weak aggregates in theasphalt mixture or insufficient filler fraction in the mixture [1].However, the majority of the raveling damage is the result of thematerial properties themselves [4]. Aggregates in the porous as-phalt mixture are hold together by the binder. Due to ageing hard-ening (temperature cycles, wetting–drying, ultra-violet and trafficloading) or due to extreme cold weather conditions, the stiffness ofthe binder increases, the relaxation capacity decreases, the binder

ll rights reserved.

becomes more brittle, the self healing potential and fracture resis-tance of the bitumen decreases, and cracking of the interface be-tween aggregates and the binder occurs. Small cracking onhighway runway can mean the start of some big distresses. Then,the traffic load will remove aggregate particles from the surfacelayer. After the first stones are removed, more stones will followat a higher rate [5]. This will increase the noise levels in the roadand reduce the traffic comfort.

It is well known that asphalt concrete is a self healing material[6,7]. As Little and Bhasin explain [6], healing occurs only after astress or strain is induced, which is sufficiently large to generatedamage. Immediately after the load that generated the damagehas been removed, and both faces of the crack are in contact, thediffusion of molecules from one face to the other starts. This pro-cess will happen, while there are not more loads, until the mi-cro-crack has completely disappeared and the repaired materialhas the level of strength of the original material. The problemcomes because it is difficult to stop traffic circulation on a roadto allow enough self healing recovery at ambient temperature. Itis also well known that the amount of healing increases whenthe material is subjected to a higher temperature during the restperiod [7,8]. Increasing the temperature will enhance the healingeffect of the asphalt concrete.

Conductive concrete is a category of concrete containing electri-cally conductive components to attain stable and high electricalconductivity. The first attempt of making an electrically conductiveconcrete road for deicing dates back to the 1950s. Besides, very

0

5

10

15

20

25

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Size of steel wool mm

Per

cen

tag

e %

a

b

0

10

20

30

40

50

60

70

Size of steel fiber mm

Per

cen

tag

e %

0 0.125 0.25 0.5 1.0 2.0

Fig. 1. Size distribution of steel fiber type 1 (a) and steel wool type 000 (b).

1208 Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213

recently Sherif et al. [9] and Tuan [10] made conductive concretecontaining steel fibers and shavings for bridge deck deicing.

Electrically conductive asphalt concrete is a relatively newfunctional material developed to achieve electrical conductivity.To reach this property, conductive filler or fibers should be addedto the mixture. Wu et al. [11] made the first electrically conductivemixture by adding conductive carbon fibers, carbon black or graph-ite to the asphalt concrete. They demonstrated that the conductiv-ity is proportional to the volume of conductive filler or fibersadded. An excess of conductive particles can cause the degradationof the asphalt concrete properties, such as the strength or theworkability of fresh materials. Finally, they showed that addingconductive fibers to the mixture is much more effective to increasethe conductivity than adding conductive filler. In addition, Garcíaet al. [12] show how the conductivity is proportional to the volumeof aggregates in the mixture and how there is an optimum volumeof electrically conductive particles for each mixture. These authorsexplain the conductive mechanism by means of the percolationtheory. With a low volume of conductive particles in the mixture,the resistivity of asphalt concrete is that of a non conductive mate-rial, but when the volume of conductive particles is above the per-colation threshold, the resistivity drops and the sample becomesconductive.

It is theoretically feasible to repair cracks in asphalt concretebefore they become too big to be healed. The use of fibers toreinforce binder has long been in practice. In this research, con-ductive steel fibers are added to the asphalt mixture and induc-tion heating is used to increase the temperature locally, justenough to increase the asphalt healing rates and repair micro-cracks or the bond between aggregates and binder. The mainobjective of using conductive fibers is to increase the conductiv-ity of binder so that micro-cracks can be healed via inductionheating. Induction heating in the asphalt industry is a novel tech-nique originally developed by García et al. [12]. In this method,the power supply sends alternating current through the coil, gen-erating an alternating electromagnetic field. When the conductiveasphalt specimen is placed under the coil, this electromagneticfield induces currents flowing along the conductive loops formedby steel fibers. The induced current dissipates heat by the Jouleeffect. This method can be repeated if damage returns. Tradition-ally, in conductive roads, heat was generated due to the electricalresistance in the conductive particles when connected to a powersource, but in this occasion, authors are trying to make porousasphalt concrete appropriate for induction heating and the subse-quent healing of cracks. This method has the advantage of havinghigh volumetric heating rates. Another advantage of using fibersis that they will probably improve the ageing behavior and theresistance to water damage of bitumen by achieving thicker bin-der film and by preventing drain-off of the mastic. This may im-prove significantly the raveling resistance of porous asphaltconcrete.

This will be the first time that this induction heating techniqueis applied to porous asphalt concrete. The objectives of this re-search are:

� To study the effect of steel fibers volume content on the electri-cal conductivity of porous asphalt concrete.

� To investigate the effect of the steel fibers volume content on theindirect tensile strength of porous asphalt concrete.

� To analyze the effect of steel wool volume content on the heat-ing rate of porous asphalt concrete.

� To explore the relationship between electrical conductivity andinduction heating in porous asphalt concrete.

� To optimize the type and volume content of steel fibers to obtaina good electrical conductivity and a high induction heating ratein porous asphalt concrete.

2. Experimental method

2.1. Materials

The aggregates used to make porous asphalt concrete specimens were quarrymaterial (Bestone, Bremanger Quarry, Norway) (size between 2.0 and 22.4 mmand density 2770 kg/m3), crushed sand (size between 0.063 and 2 mm and density2688 kg/m3), and filler type Wigro 60 K (size < 0.063 mm and density 2638 kg/m3).Finally, the bitumen used was 70/100 pen, obtained from Kuwait Petroleum, withdensity 1032 kg/m3.

Besides, two different types of electrically conductive fibers were mixed in theasphalt. The first one was steel fibers (called steel fiber type 1), with a diameter be-tween 0.0296 mm and 0.1911 mm. The second one was steel wool type 000 withdiameters between 0.00635 mm and 0.00889 mm. These fibers had to be choppedby hand, always by the same operator. Both types of fibers had an approximate den-sity of 7.8 g/cm3. These fibers had an electrical resistivity of 7 � 10�7 X cm. To findthe size distribution of the conductive fillers, more than 100 fibers of each typewere checked by taking photographs under the optical microscope and measuringtheir length with an image processing program, obtaining the distribution shown inFig. 1a and b.

2.2. Porous asphalt design

Porous asphalt PA 0/16, the most commonly used mixture in the Netherlands,was used in this research. The mixture composition was fixed based on the DutchStandard, RAW 2005 (Table 1) and Gyratory compactor was used to compact themixture. These specimens have a diameter of 100 mm and a variable thickness be-tween 77.40 and 84.22 mm.

The maximum theoretical density of the mixture was 2.569 g/cm3. It was calcu-lated as the total weight divided by the total volume of all the materials before com-paction. After mixing, the maximum theoretical density of the loose mixture wasmeasured using an ultrapycnometer 1000 (America, Quantachrome Instruments),

Table 1Composition of PA 0/16 mixture for Gyratory specimen based on the Dutch standard(RAW 2005).

Sieve size(mm)

Density(g/cm3)

RAW spec.% retained

Cumm.% ret.

% ret. byweight

Weight(g)

22.4–16.0 2.778 0–7 4 4 4816.0–11.2 2.774 15–30 25 21 25211.2–8.0 2.762 50–65 57 32 3848.0–5.6 2.765 70–85 80 23 2765.6–2.0 2.781 85 85 5 602.0–0.063 2.688 95.5 95.5 10.5 126<0.063 2.638 100 100 4.5 54

1200Bitumen 70/100 1.032 4.5% by wt. 54

Total wt. 1254

Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213 1209

which is an instrument for measuring the true volume and density of powders,foams and bulk solids. The maximum theoretical density measured with this appa-ratus was 2.571 g/cm3, very close to the previously calculated value. The maximumtheoretical density of the mixture changes with the variation in volume of steelwool in the mixture and can be computed according to the total weight and volumeof all materials in the mixture. The standard calls for a minimum of 20% of air voidscontent. In this case, the air voids content was assumed to be 21%. Based on themaximum theoretical density, the calculated density of the specimen after compac-tion and the weight of the mixture, the Gyratory compactor can control the heightof the specimen to obtain the ultimate target density. The air voids content can becomputed after molding the specimens by determining their dimensions (to com-pute their volumes) and weights. The maximum and assumed densities of the mix-tures with different volume contents of the fibers (the fiber–bitumen volume ratio)were calculated to obtain the same air void content in all samples studied. The airvoids content for all specimens after compaction was around 21%.

2.3. Electrical resistivity tests

The electrical resistivity measurements were done at room temperature 20 �C.The samples, with diameter 100 mm and thickness 50 mm were cut from the Gyra-tory compacted specimens. After cutting, the samples were placed in an oven at40 �C during 8 h to remove the moisture and prevent the steel fibers rusting onthe surface of the samples. Inside the sample, the steel fibers do not corrode, be-cause they are completely coated with bitumen. A digital multimeter was used tomeasure the resistance below 36 � 106 X. A resistance tester was used to measurethe resistance higher than this value. Two electrodes made of 100 mm square cop-per plates were connected with resistance tester when testing resistance. Both elec-trodes were placed at both ends of the test sample to measure the electricalresistance. A small pressure was applied to the copper electrodes to obtain a goodcontact with the surface of the sample. The total contact resistance between the twoelectrodes was about 0.4 X, which is negligible with respect to the great resistancesstudied (higher than 100 kX in the samples). The electrical resistivity was obtainedfrom the second Ohm-law by

q ¼ RSL

ð1Þ

where q is the electrical resistivity, L is the internal electrode distance in meter, R isthe measured resistance in omega and S is the electrode conductive area in squaremeter. The electric field is assumed constant and the end-effects considerednegligible.

2.4. Indirect tensile strength test

Indirect tensile strength is a parameter that indicates how strongly the bindercan bond the aggregates and can be used as an indicator of mixture cohesion[13,14]. In this study, indirect tensile tests (performed at 5 �C) were conducted to

Fig. 2. Induction he

characterize the effect of the steel wool volume content on the mechanical behaviorof porous asphalt concrete. Indirect tensile strength tests were conducted on cylin-drical samples cut from Marshall Specimens. The samples had a diameter of100 mm and a thickness of 50 mm. The equipment used in this part is a universaltesting machine (UTM-25). The indirect tensile strength of the specimen is deter-mined by applying a load at a rate of 50 mm per minute. The test procedures aredescribed in American Association of State Highway and Transportation Officials(AASHTO) provisional standard TP 9-95. To check the data reproducibility, threespecimens of each composition were tested and the average value was obtained.After the tests, the indirect tensile strength of the samples could be calculated usingthe following equation:

ITS ¼ 2FpDH

ð2Þ

where ITS is the indirect tensile strength in Pascal, F is the total applied vertical loadat failure in Newton, D is the diameter of specimen in meter and H is the height ofspecimen in meter.

2.5. Microscope image analysis

An optical microscope was used to check the fibers distribution in the conduc-tive porous asphalt concrete. Different images were taken in the broken specimenswith different volumes of fibers after indirect tensile tests.

2.6. Induction heating tests

Induction heating tests were conducted on conductive porous asphalt. For that,the effect of fiber volume content on the heating rate was studied. The principles ofinduction heating are electromagnetic induction and Joule heating. According toFaraday law of electromagnetic induction, the electromotive force can be producedaround a closed path in a changing magnetic field and is proportional to the rate ofchange of the magnetic flux through any surface bounded by that path. Faraday’slaw of electromagnetic induction states that:

e ¼ � dØBdt

ð3Þ

where e is the electromotive force (emf) in volts and B is the magnetic flux in webers.In practice, this means that an electrical current is induced through the fibers

when the magnetic flux touches them. The electrical current generates heat whenit flows through the conductive fibers. This is Joule heating, which can be explainedby Joule’s first law:

P ¼ I2R ð4Þ

where P is the heat generated per unit time by a constant current I flowing through aconductor of electrical resistance R. For a time t, the heat generated Q = I2Rt.

This law applies to any circuit that can be characterized by a resistance. Ohm’slaw states that for a voltage e across a circuit of resistance R the current will be:

I ¼ eR

ð5Þ

By substituting this formula for current into one or both factors of current in Joule’slaw, the power dissipated can be written in the equivalent form:

P ¼ eR

� �2R ¼ e2

Rð6Þ

The induced electromotive force depends on the rate of change of the magnetic fieldflux. For this heating system, a constant frequency in the induction equipment willgenerate a constant electromotive force. The heating rate induced by Joule heating isin contrast with the resistance of the sample. The lower the resistance, the higher theheating rate.

The induction heating experiment was performed by using an induction heatingsystem with a capacity of 50 kW and at a frequency of 70 kHz (Fig. 2). Although thesystem was not fully optimized, it had not influence on the research objectives. The

ating machine.

2.40

2.60

2.80

Pa

1210 Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213

heating samples were the same as the ones used to measure the electrical resistiv-ity. The cylindrical samples cut from Gyratory specimens were used to avoid theproblem of binder concentration on the surfaces of samples and to get a higherheating efficiency for that thinner samples mean less temperature difference be-tween the top and the bottom of the samples. The distance between the coil ofthe induction machine and the top surface of the heating sample was about32 mm. Each sample studied was heated for 3 min and its temperature changewas measured with a 640 � 480 pixel, full color infrared camera.

1.60

1.80

2.00

2.20

0 5 10 15 20 25Fiber volume content %

ITS

M

Fig. 4. Effect of steel fiber type 1 volume content on the ITS of porous asphaltconcrete.

3. Results and discussions

3.1. Effect of conductive fiber volume content on the electricalresistivity of porous asphalt concrete

The effect of steel fiber (type 1) volume content on the resistiv-ity of porous asphalt concrete samples is shown in Fig. 3. The resis-tivity of the samples shows three stages: high resistivity stage,with fiber volume content less than 10%; transit stage, with fibervolume content from 10% to 20%; and low resistivity stage, with fi-ber volume content higher than 20%. Samples with less than 10% offibers exhibited insulating behavior, with resistances higher than109 X m. During the transit stage (between 10% and 20% of fibers),the electrical resistivity of the sample suffered a sharp decreasefrom 109 X m to 4.7 � 104 X m. It was discovered that, for the steelfiber type 1, 20% fibers are needed to make the porous asphalt con-crete electrically conductive.

In addition, when drying the sample after cutting, it was foundthat plain samples, without steel fibers, glued a little bit to theplate bellow, showing drainage of binder in the sample. However,samples with steel fibers did not show this phenomenon, whichshows that the steel fibers could prevent the drainage of binder.

3.2. Effect of conductive fiber volume content on the indirect tensilestrength of porous asphalt concrete

The effect of steel fibers volume content on the indirect tensilestrength (ITS) of porous asphalt concrete is shown in Fig. 4. The ITSof the plain porous asphalt concrete studied was 2.06 MPa. The ITSof porous asphalt concrete increased with the volume of steel fi-bers until a maximum of 2.62 MPa with a steel fibers content of11%. Adding more fibers to the mixture results in a decrease ofthe ITS, because too many fibers reduce the mastic film thickness,causing a bad adhesion between the asphalt components. Finally,samples with 20% of steel fibers (the one with highest electrical

1,00E+04

1,00E+06

1,00E+08

1,00E+10

0 5 10 15 20 25Fiber volume content %

Res

isti

vity

Ω.m

Fig. 3. Effect of steel fiber type 1 volume content on the resistivity of porous asphaltconcrete.

conductivity), had a maximum tensile strength of 2.03 MPa, closeto that of the plain porous asphalt concrete.

3.3. Electrically conductive mechanism in porous asphalt concrete

Broken porous asphalt samples after indirect tensile test wereobserved under the microscope to check the fibers distribution.The conductive mechanism was explained according to the perco-lation theory [15]. Fiber distribution in samples with different con-tents of steel fibers is shown in Fig. 5. When 5% of steel fibers isadded to the mixture, they are uniformly distributed and do notcontact each other, having a similar resistivity to that of an asphaltconcrete sample without fibers. When more steel fibers are addedto the mixture, they start contacting each other, which causes to agradual decrease in the resistivity. If the fiber volume contentreaches more than 10% (percolation threshold), the first conductivepaths are formed in the sample. This corresponds to a sharp de-crease of resistivity. Beyond the percolation threshold, the conduc-tive network develops and spreads gradually in three dimensionswith the increase of the volume content of the steel fibers. Whenthe fiber volume content is more than 20%, steel fibers contact eachother in all directions and many conductive networks and passagesare formed, corresponding to a very low value of resistivity atwhich adding more steel fibers does not reduce the resistivity ofsample any more.

3.4. Effect of bitumen content on the electrical resistivity of porousasphalt concrete

When fibers are added to the mixture, the thickness film ofmastic around the aggregates is reduced. This can have a negativeeffect on the mechanical properties of asphalt concrete pavementsso, to ensure the durability of this porous asphalt, bitumen contentneeds to be increased. In this section, higher bitumen contentswere used to check how the electrical conductivity of porous as-phalt concrete was affected. The fiber content was fixed at 20% inthe mixture (for optimal electrical conductivity purpose) and thebitumen content was gradually increased from 4.5% to 5.4%. The ef-fect of bitumen content on the electrical resistivity of porous as-phalt concrete samples is shown in Fig. 6. An increase in thebitumen content causes a reduction in the electrical resistivity ofporous asphalt concrete. Furthermore, ITS reductions from2.03 MPa to 1.40 MPa were also observed with the increase onbitumen content.

Fig. 5. Steel fibers distribution in the porous asphalt concrete samples.

1,00E+03

1,00E+04

1,00E+05

4.2 4.5 4.8 5.1 5.4 5.7

Bitumen content %

Res

istiv

ity Ω

.m

1.3

1.5

1.7

1.9

2.1IT

S M

Pa

Resistivity

ITS

Fig. 6. Effect of bitumen content on the electrical resistivity and ITS of porousasphalt concrete.

1,00E+03

1,00E+05

1,00E+07

1,00E+09

1,00E+11

0 5 10 15 20 25Fiber volume content %

Res

isti

vity

Ω.m

Steel fiber type 1

Steel wool type 000

Fig. 7. Effect of fiber volume content on the resistivity of the porous asphaltconcrete.

1.60

1.80

2.00

2.20

2.40

2.60

2.80

0 5 10 15 20 25Fiber volume content %

ITS

MP

a

Steel fiber type 1

Steel wool type 000

Fig. 8. Effect of fiber volume content on the ITS of porous asphalt concrete.

Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213 1211

3.5. Effect of different type of fibers on the electrical resistivity and ITSof porous asphalt concrete

Fibers size determines the number of fibers per unit of batchedweight and the number of fibers per cubic meter of asphalt con-crete. Since the total weight, rather than the absolute size deter-mines the cost of the fibers, the question arises whether a largenumber of short fibers are more effective than the same weightof long fibers. To answer this, steel wool type 000 instead of steelfiber type 1 was used to check if it is better to improve both elec-trical conductivity and mechanical properties of porous asphaltconcrete.

Comparisons between the effect of steel wool type 000 and steelfiber type 1 volume content on the resistivity and indirect tensilestrength of porous asphalt concrete can be seen in Figs. 7 and 8,respectively. The high resistivity stage and the low resistivity stagehave almost the same values for both steel wool type 000 and steelfiber type 1 at 109 X m and 104 X m, respectively. However, thetransition of the steel wool type 000 curve begins at 8%, exhibitinga lower percolation threshold than that of the steel fiber type 1 at10%. Besides, the transition curve of steel wool type 000 is sharperthan the one of steel fiber type 1 and reaches the low resistivitystage at 10% of volume of steel wool type 000, much earlier thanthat of steel fiber type 1 at 20% of volume of steel fibers. So, steel

wool type 000 is much more effective than steel fibers type 1 to en-hance the electrical conductivity of porous asphalt concrete.

In Fig. 8, it can be seen that the indirect tensile strength curveversus volume of steel wool type 000, has the same shape as thecurve for steel fiber type 1. Additions of steel wool type 000 below4% by volume increase the ITS of the samples, but too much steel

Fig. 9. Induction heating image of a sample with 10% steel wool type 000.

0

20

40

60

80

100

120

140

160

Tem

pera

ture

ºC

30s 60s

90s 120s

150s 180s

1212 Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213

wool results in a decrease of the maximum strength of porous as-phalt concrete. Steel wool type 000 offers a much weaker rein-forcement than steel fiber type 1. The maximum ITS of samplescontaining steel wool type 000 happens at around 4% with a valueof 2.27 MPa, only slightly higher than the ITS of plain sampleswithout steel wool at a value of 2.06 MPa and much lower thanthe maximum ITS of samples containing about 11% steel fibers ata value of 2.62 MPa. It is very interesting to note that the ITS ofsamples with optimum electrical conductivity containing 10% ofsteel wool type 000 and 20% of steel fiber type 1 are 1.99 MPaand 2.03 MPa, very close to each other. So, steel wool type 000and steel fiber type 1 have the same effect on the ITS of porous as-phalt sample at their respective content for optimal electrical con-ductivity. From the point of view of the electrical conductivity,steel wool type 000 has better performance than steel fibers.

0 2 4 6 8 10 12 14 16Steel wool volume content %

Fig. 10. Effect of steel wool type 000 volume content on the heating rate of porousasphalt concrete.

3.6. Induction heating of porous asphalt concrete

To study the induction heating of porous asphalt concrete, sam-ples containing steel wool type 000 were used. Fig. 9 shows aninduction heating image of the sample with 10% steel wool. Allsamples have the similar images like this. On the top surface ofthe sample, clusters of steel wool work as small heaters. This cor-responds to the shining dots on the surface of the sample in Fig. 9.The image shows uniform temperature in the horizontal direction.This is because the magnetic field is constant at the same distancefrom the coil. The temperature of the sample decreases from top tobottom. It was also found that samples had a higher heating ratewhen they were closer to the coil of the induction machine. Thereason for this is that the magnetic field is stronger close to thecoil. Finding the optimum distance between the pavement andthe coil will be a topic for future study.

The temperatures of all the samples with different steel woolvolume contents were recorded and used to analyze the effect ofthe steel wool content on the heating rate of porous asphalt sam-ples (Fig. 10). The plain sample without steel wool was almost notheated in 3 min because of the very low heating speed. Below 10%of steel wool in the mixture, the temperature reached increasedwith the increase of the volume content of steel wool. The samplescould be heated, even if hey were not conductive, but the heatingrate was lower. The temperature cannot be increased any morewhen the steel wool volume content is above 10%. This value coin-cides with the minimum in the resistivity curve in Fig. 8. The min-imum resistivity will result in a maximum Joule heat in thesample; hence the temperature will be also a maximum. Addingsteel wool above 10% does not improve the heating and will resultin a decrease of the indirect tensile strength, as shown in Fig. 8. So,

10% is the optimal volume content of this type of steel wool to heateasily porous asphalt concrete.

4. Conclusions

It was found that long steel wool type 000, is better than shortsteel fiber type 1 to make porous asphalt concrete electrically con-ductive. However, short steel fibers have better strength reinforce-ment capacity than steel wool. The mechanisms of electricalconductivity and induction heating are explained in this paper.10% (by volume of bitumen) of steel wool type 000 is proposedas an ideal content to make porous asphalt concrete conductiveand for a fast induction heating. Besides, this value gives an accept-able indirect tensile strength to porous asphalt concrete. Addingmore bitumen for durability purpose can increase the electricalconductivity of porous asphalt concrete, but it can also decreaseits indirect tensile strength. It is expected that the self-healingcapacity of asphalt concrete will be enhanced with the inductionheating, although this is a matter for future study.

5. Further research plan

It has been proved that the electrically conductive porous as-phalt concrete can be heated quickly with induction energy. How-ever, work needs to be done to detect the healing effect of thisporous asphalt concrete. The idea is to introduce some damage

Q. Liu et al. / Construction and Building Materials 24 (2010) 1207–1213 1213

to the porous asphalt concrete samples, and then heat the sampleswith induction energy to see if the damage can be healed or not.Strength recovery can serve as a healing index and CT scanningcan serve as a method to see the cracks occurring and closing in-side the material. In addition, more researches related to practicalimplementation of this kind of material will be done.

Acknowledgements

The scholarship from the China Scholarship Council is acknowl-edged. The technical support of Prof. Klaas van Breugel, Prof. A.A.A.Molenaar and Marco Poot are also appreciated. Furthermore, theauthors would like to express thanks to Global Material Technolo-gies for their technical expertise and advices about steel wool.

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