Construction and Building Materials 49 (2013) 729–737

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Construction and Building Materials

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Investigation of induction healing effects on electrically conductiveasphalt mastic and asphalt concrete beams through fracture-healingtests

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.089

⇑ Corresponding author.E-mail addresses: qingdai@mtu.edu (Q. Dai), zigengw@mtu.edu (Z. Wang),

mmohdhas@mtu.edu (M.R. Mohd Hasan).

Qingli Dai ⇑, Zigeng Wang, Mohd Rosli Mohd HasanDepartment of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931-1295, United States

h i g h l i g h t s

� This paper evaluates the healing capacity of asphalt samples with induction energy.� Asphalt mastic and concrete beam samples were prepared by adding steel wool fibers.� The specimens were heated to different temperatures, including 60 �C, 80 �C and 100 �C.� Fracture-healing cycles via three point bending test and induction healing were used.� The cracks can be effectively healed even after six fracture-healing cycles.

a r t i c l e i n f o

Article history:Received 3 May 2013Received in revised form 29 August 2013Accepted 30 August 2013

Keywords:Asphalt concreteCracksInduction healingElectrical conductive fiberSteel wool fibers

a b s t r a c t

The purpose of this study is to evaluate the healing capacity of electroactive asphalt mastic and concretebeam samples with induction heating. The electrically conductive steel wool fibers were mixed withasphalt materials to heat the surrounding binders through induction energy. By introducing Newtonianbinder flow in the heated asphalt materials, the microcrack healing performance can be significantlyimproved with the accelerated process. To investigate the induction healing performance, asphalt masticand concrete beam samples were prepared by incorporating Type 1 steel wool fibers with an approximatelength of 6.5 mm. Subsequently, the mastic beams were tested with fracture-healing cycles under thethree-point bending test and induction healing process. Meanwhile, the concrete beams were tested withfracture-healing cycles using the modified three-point bending test with an elastic foundation support andhealing procedure. Prior to the fracture tests, all samples were conditioned in the freezer for 6 h at �20 �Cto limit the viscoelastic and unrecovered deformation. The samples were loaded until the peak value wasobtained for fracture tests. During the healing process, the samples were heated at different temperatures(60 �C, 80 �C and 100 �C) within a short period of time. The result shows that the mastic sample can be fullyhealed at these three temperatures based on the recovered peak loads from the cyclic test. For the asphaltconcrete beams, the test results showed that the healing performance increases with the heating temper-atures (60 �C, 80 �C and 100 �C). The temperature distribution in the samples at the end of the healing pro-cedure is also captured. Overall, it was found that the asphalt mixture samples still maintained at least halfof the original fracture strength after six fracture-healing cycles. The experimental results indicate that theinduction healing techniques have promises in elongating the pavement service life.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Asphalt mixtures are used on the surface for over 94% of allpavements [1] in the United States. As an essential component ofthe US pavement infrastructure system, the self-healing perfor-mance of asphalt mixtures has a significant impact on maintenance

costs. Bitumen or asphalt binder is used in asphalt concrete mixesto bind together aggregate particles. It is typically known as a vis-coelastic material which deforms at high temperatures and is brit-tle at low temperatures. Pavements surfaces must remain drivablefor a wide range of the traffic loads under different climatic condi-tions for an extended period of time. In order to maintain a driv-able pavement surface, asphalt concrete wearing courses shouldbe constantly maintained and repaired. Miniature cracking onhighway runways can cause the start of major forms of pavementdistress [2]. The abrasive action of vehicle wheels on the pavement

Table 1Aggregate gradation of sand mastic samples.

Sieve number Sieve size (mm) Retained (%) Mass (g)

No. 8 2.36 68.3 286.2No. 16 1.18 10.9 45.7No. 30 0.6 8.2 34.4No. 50 0.3 4.3 18.0No. 100 0.15 2.7 11.3No. 200 0.075 5.6 23.5

730 Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737

surface, especially on high stressed areas, can initiate raveling.Kneepkens et al. [3], described that raveling starts with the re-moval of the first stone, creating a gap, followed by surroundingstones, and resulted in higher rate of aggregate particles loss dueto weaker bonding or interlocking between particles in the asphaltpavement. When the first stone is removed by a vehicle wheel, theremaining aggregates around the gap lack support in at least onedirection, aggregates can easily plug out from the pavement, whichresults in uneven pavement surfaces. In the Netherlands [4], induc-tion healing was used as a method of preventive maintenance toincrease the healing rate and to prevent raveling of porous asphalt.This approach was developed at the Delft University of Technology(TU Delft) based on two intrinsic properties of asphalt concrete: (1)asphalt concrete is a self-healing material and (2) its’ healing capa-bility is better at elevated temperatures [4].

Shen et al. [5] defined the healing behavior as the self-recoverycapability of asphalt materials under certain loading and/or envi-ronmental conditions, especially during the rest time. Accordingto García work [6], asphalt concrete can be classified as a ther-mally-induced self-healing material whereas directly linked totemperature [7] and the rest periods [8,9]. Healing in asphalt mate-rials evolves complex behavior, which depend on the activation en-ergy in asphalt binder, capillary flow through the cracks, self-diffusion of molecules across the crack interface, crack phases,material modifications and confinement [6,10,11]. According to astudy by Qiu et al. [11], the modified bituminous mastic using Sty-rene–Butadiene–Styrene polymer resulted in lower healing capa-bility by comparing to a bituminous mastic made withconventional binder 70/100 penetration grade. Little and Bhasin[12] affirmed that the healing process between two surfaces of ananocrack can be classified into three primary steps: (1) wettingof the nanocrack surfaces, (2) diffusion of molecules from one sur-face to the other and (3) randomization of the diffused moleculesto attempt to reach the level of strength of the original material.This complete self-healing process will occur with sufficient resttime (no loading) and contacted crack surface. The lab test resultssupport that the healing processes increase when the asphaltmaterials are subjected to an elevated temperature during the resttime [13].

In a recent work [14], the asphalt binder behaves as a Newto-nian fluid by stimulating the activation energy at an adequate le-vel which can be predicted through the Arrhenius equation.Conductive steel fibers are added to the asphalt mixtures to gen-erate eddy current under an alternating magnetic field in order touse induction heating to heal the micro-crack, increase theasphalt binder healing rates and repair the bond between aggre-gates and binder. Theoretically, heat was generated due to thedominant Joule effects through the conductive components underinduction process [15]. The induction healing of asphalt compos-ites containing conductive fibers were investigated at TU Delft[2,4,15–19]. The induction healing effects were experimentallyinvestigated with the porous asphalt concrete specimens. Garcíaet al. [17] conducted Gel-Permeation Chromatography tests oninduction healed specimens and original specimens, and demon-strated the chemical properties of asphalt such as averagedmolecular weight did not change. Liu et al. [4] evaluated theinduction healing effect of porous asphalt concrete through thefour-point bending fatigue test and found that the fatigue life ofporous asphalt concrete can be extended by applying multipleinduction healing cycles. Liu et al. [15,18] concluded that theself-healing rate of porous asphalt concrete can be increased byinduction heating.

The main objective of this study is to investigate the self-heal-ing performance of conductive asphalt materials through cyclicpeak-to-peak fracture and induction healing tests, and to evaluatethe influence of different temperatures of induction heating on the

healing capability of dense-graded asphalt materials. The masticand asphalt concrete samples were prepared by adding Type 1steel wool fibers with an approximate length of 6.5 mm. These pre-pared samples were then tested under fracture-healing cycles byusing the three-point bending loading and modified three-pointbending loading tests (with an elastic foundation support) for theasphalt mastic and asphalt concrete beam samples, respectively.The healing performances of samples were evaluated with recov-ered peak loads under fracture-healing cycles.

2. Mixture design and sample preparation

2.1. Materials

The materials used in this study for the preparation of asphaltmastic and asphalt concrete beams were aggregates, asphalt bin-der and steel wool fibers. The aggregate were obtained from a localsource in Hancock, Michigan that contained natural sand andcrushed aggregate, with an average density of 2.72 g/cm3. Whilethe asphalt binder used was PG 58-28 with the density of1.024 g/cm3. The steel wool fibers were Type 1 with the approxi-mate length of 6.5 mm and the density of 7.6 g/cm3.

2.2. Asphalt mastic sample preparation

Asphalt mastic samples were prepared using the sand to bitu-men ratio 1.6 in volume for mixture design. As a conductive com-ponent, 5.66% steel wool by volume of asphalt binder wasincorporated in the mixture to obtain a high electrical conductivity,by referring to [2]. To obtain a homogenous mixture, aggregates(gradation listed in Table 1) and steel wool fibers were mixed usinga Hobart mixer for 10 min before mixing with the asphalt binderfor another 5 min. Finally, the mixture was hand-compacted in afabricated aluminum mold as shown in Fig. 3a. The dimensionsof each sample were 125 mm � 25 mm � 15 mm with a triangularshape notch (5.8 mm in bottom width and 5 mm in height) at thecenter of sample.

2.3. Asphalt concrete sample preparation

For the asphalt concrete beam preparation, the aggregate grada-tion listed in Table 2 was mixed with the same type of asphalt bin-der and steel wool used in the mastic sample preparation. Two-step mixing procedures were also involved as mentioned in masticsample preparation. The beam samples contained a percentage ofsteel wool which was approximately 8% of the binder volume. Thispercentage was determined by balancing the sample electricalresistivity and mixing workability. The mixture was first com-pacted using a slab kneading compactor and then cut into a beamwith the dimensions of 69 mm � 50 mm � 190 mm. Additionally,a notch with 3 mm width and 20 mm depth was sawn in the centerof the asphalt concrete beam samples.

Table 2Aggregate gradation of asphalt concrete mixture.

Sieve number Sieve size (mm) Retained (%) Mass (g)

3/4 19 1.3 124.01/2 12.5 12.2 1164.03/8 9.5 14.7 1402.5No. 4 4.75 20.4 1946.4No. 8 2.36 15.3 1459.8No. 16 1.18 10.6 1011.3No. 30 0.6 10.8 1030.4No. 50 0.3 7 667.9No. 100 0.15 2.3 219.4No. 200 0.075 5.4 515.2

Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737 731

3. Mechanism of induction heating of asphalt materials

The induction heating system includes alternating power gen-erator, induction coils, and electrical conductive steel fibers in as-phalt concrete sample as shown in Fig. 1. Under alternatingelectrical power supply, the copper coil generated the magneticfield with the same frequency. Each conductive steel fiber insideof asphalt mixtures was induced as indicated in the figure(Fig. 1). The induction heat was generated through the dominantJoule effect, dielectric hysteresis and contact resistance heating be-tween fibers. The binder in the samples exhibits Newtonian flowwhen the asphalt temperature is higher than 50 �C depending onthe source and composition of the asphalt binder [20–24]. The cap-illary pressure will drive the binder to flow over crack space. Thusthe micro-cracks will be healed with filled binder and recoveredbonding and adhesive strength [25].

3.1. Induction healing system setup

The induction healing systems for asphalt mastic and asphaltconcrete samples were used as shown in Fig. 2a and b, respectively.As mentioned, the induction coils connected with power generatorintroduced magnetic field for induction healing of asphalt samples.These experiments were performed using Ameritherm Solid Stateinduction heating equipment SP-5.0 with a capacity of 5 kW. Thefrequency range of 0.120–0.160 MHz was applied for the inductionhealing of asphalt samples in this study. A 125 mm by 105 mm pla-ner coil was fabricated to carry out the heating process. The coilwas place over the fracture area of the sample during healing. Toregulate and set the temperature being produced, a thermal couplewas used in conjunction with the induction heating machine. Thethermal couple used feed-back process to control the output volt-age with a range of desired temperatures. A hand held Fluke 62Mini IR Thermometer was used to detect the temperature changeson the surface of the sample during the healing process.

Fig. 1. Schematic demonstration of induction healing

3.2. Resistivity measurement of prepared samples

Electrical resistivity measurements were conducted on the pre-pared asphalt mastic (Fig. 3a) and asphalt concrete beam speci-mens (Fig. 3b) at the room temperature of 20 �C. Copperelectrodes with the size of the short end of the samples were usedto measure resistance (R). Graphite powder was used on the masticsample to ensure full contact with the electrode. Resistivity wasthen calculated using Ohm’s second law as shown in the followingequation:

q ¼ RSL

ð1Þ

where R is the measured electrical resistance (X), S is the electrodeconductivity (m2), L is the internal electrode distance (m) and q isthe electrical resistivity (X m).

The measured electrical resistivity of asphalt mastic and asphaltconcrete samples was plotted as logarithm values as shown inFig. 4a and b. Fig. 4a shows the measured volumetric resistivityof nine mastic samples used for the following fracture-healing cyc-lic studies. The average volume resistivity of the mastic sampleswas calculated about 2.4 � 1011 X m. The average volume resistiv-ity (Log (X m)) was 9.7 with a standard deviation about 1.55.Fig. 4b displays the measured volume resistivity of nine asphaltconcrete beams. These asphalt concrete beam samples were alsoused for healing performance study. The average volumetric resis-tivity of these asphalt concrete beam samples was about5.81 � 1011 X m. The average Logarithm value of volume resistiv-ity (Log (X m)) was 11.2 with a standard deviation of 0.66. The rel-ative larger deviation values were caused due to relatively higherfiber content in asphalt mastic samples, compared with asphaltconcrete samples.

3.3. Temperature distribution of induction heated concrete samples

Type 1 steel wool fibers were incorporated into the asphaltmastic and asphalt concrete samples to induction heat the samplesthrough different heating temperatures. During induction heating,the asphalt concrete sample was placed about 10 mm from theinduction heating coil. Once the sample surface temperaturereached the desired heating temperature (60 �C, 80 �C and100 �C), the sample was held for another 2 min. The infrared ther-mal camera (FLIR 8� Digital Zoom 640 � 480) was used to capturethe temperature distribution in each sample as shown in Fig. 5. Theimages were taken at the end of healing process. Fig. 5a–c showsthe temperature distribution of samples at three heating tempera-tures. In these images, the color bar at the right side represents thetemperature variation of the specimen. The bright yellow colorindicates the relatively high temperature while the dark blue color

mechanism for thermoplastic asphalt materials.

Fig. 2. Induction healing system with heating generator and induction coils (a) asphalt concrete sample healing and (b) asphalt mastic sample healing.

Fig. 3. Electrical resistivity measurement of prepared samples: (a) asphalt mastic samples; and (b) asphalt concrete samples.

(a)

(b)

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9

Log

(V

olum

e re

sist

ivity

, m

)

No. of Samples

Average = 9.7

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9

No. of Samples

Average = 11.2

ΩL

og (

Vol

ume

resi

stiv

ity,

m)

Ω

Fig. 4. Electrical resistivity values of prepared samples before loading: (a) asphaltmastic samples; and (b) asphalt concrete samples.

732 Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737

represents the relatively low temperature. Even though thestrength of alternating magnetic field frequency decreased fromtop surface to bottom surface of the samples, the induction energyand heating temperature have the same trend. The highest temper-ature can reach about 67.2 �C, 80.4 �C and 100 �C in the middle ofthe top surface for the specimen healed at 60 �C, 80 �C and100 �C, respectively. The figures indicate that the samples are prop-erly heated during the induction healing process.

4. Fracture-healing cyclic study of asphalt mastic samples withthree-point beam bending tests

4.1. Fracture-healing tests of asphalt mastic samples

The peak loads of three-point beam bending test of asphaltmastic samples were used to evaluate the healing performanceafter the fracture-healing cycles. A total of nine fracture-healingcycles were completed to measure the peak loads of asphalt masticsamples by following the same test procedure. The mastic sampleswere formerly conditioned at �20 �C about 6 h before the test tolimit the viscoelastic and unrecovered deformation. The sampleswere tested with a three-point bending setup at a loading rate of50 mm/min (as shown in Fig. 6a). When the loading force startedto decrease, the loading machine was stopped. The loading curvesand peak values were recorded for healing performance analysis.After fractured, the samples were rested at least for 15 min untilthey reached the room temperature about 20 �C. For the healingprocess, the samples were placed in a wooden mold to help main-tain the shape during healing. The samples were heated at a dis-tance of 35 mm under the planar induction heating coil. Once thesurface temperature reaches the desired values (60 �C, 80 �C and100 �C), the sample was held for heating about 1 min. Then, thesample in the mold was placed in the freezer for 15 min to main-tain the shapes of the samples. This allows the samples to cool

Fig. 5. Infrared images of induction heating temperature distribution with asphaltconcrete samples under heating temperature of (a) 60 �C, (b) 80 �C and (c) 100 �C.

Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737 733

down before being removed from the mold to prevent damagingthe mastic samples. This process was repeated several times withrest period and conditioning time to allow the samples to reach�20 �C, before being fractured in the next cycle.

4.2. Fracture-healing test results with asphalt mastic samples

The fracture-healing tests were conducted with three samplesat each heating temperature of 60 �C, 80 �C or 100 �C. During the

healing process, it can be observed that the binder flowed intocracks. Table 3 shows the peak loads of the original mastic samples.The peak loads were measured in the range from 61.23 N (sample3) to 154.25 N (sample 5). The fracture-healing test curves of as-phalt mastic samples (Samples 1, 2 and 3) at the heating tempera-ture of 60 �C are shown in Fig. 7. The figure shows that the peakloads of the samples are relatively identical after each fracture-healing cycle. The percentage of recovered peak loads after eachcycle for mastic samples were listed in Table 4. After nine cycles,the Sample 1 has the recovered strength about 117.8%, while Sam-ple 2 and 3 have increased the peak strength about 160.9% and209.3%. Since the mastic samples have relatively high binder con-tent, the notched crack in the bottom of the sample was filled withsome heated binder and sands after each healing process with labobservation. These caused the increased peak strength of asphaltmastic samples after several fracture-healing cycles.

The fracture-healing test curves of asphalt mastic samples(Samples 4 and 5) at the heating temperature of 80 �C are shownin Fig. 8. Again, the peak loads of the original mastic samples werelisted in Table 3. After nine cycles, Sample 4 and 5 have increasedthe peak strength about 160.9% and 209.3%. The test data of Sam-ple 6 was not included since the recovered peak load of cycle 9 wasvery high (about three times higher than the original peak load va-lue). This abnormal test data was probably caused by large amountbinder and sands flowing into the notched crack.

The fracture-healing test curves of asphalt mastic samples(Sample 7, 8 and 9) at the heating temperature of 100 �C are shownin Fig. 9. The peak loads of the original mastic samples are listed inTable 3. After nine cycles, Sample 7 and 9 have increased the peakstrength about 185.0% and 249.9%. Sample 8 was fractured as twopieces after cycle 6, with the recovered peak strength about133.9%. These mastic sample test results indicated that inductionhealing process can fully repair cracks and also increase peakstrength with cycles. The recovered peak strength has increasedwith heating temperatures. Since the viscosities of asphalt binderwere further reduced with heating temperatures, the binder’s flowrates were also increased to seal the cracks.

5. Fracture-healing investigation of asphalt concrete samplesusing beam bending test with elastic foundation

5.1. Fracture-healing tests of asphalt concrete samples

The measured peak loads of the asphalt concrete beams afterfracture-healing cycles were used to determine the healing capac-ity of asphalt mixture samples. The tests were conducted to deter-mine the recovered peak loads of the asphalt concrete samples byfollowing the same test procedures for the asphalt mastic test. Theasphalt concrete samples were initially conditioned at �20 �C.Then, the conditioned samples were fractured using a modifiedbeam bending test with an elastic rubber foundation support asshown in Fig. 6b. The low loading rate of 0.5 mm/min was appliedfor the beam fracture test. This test setup allowed the sample to befractured globally by limiting local damage. When the appliedforces started to reduce after reaching peak values, the loadingwere stopped. The loading curve and peak loads were recordedfor healing performance investigation. After beam bending test,the samples were rested for at least an hour until they reachedthe room temperature about 20 �C. During the induction healingprocess, the sample (top surface) was placed about 10 mm belowthe induction heating coil. The cracks were generated from the bot-tom surfaces of samples. This arrangement will demonstrate thatthe developed healing techniques can be applied to repair boththe bottom-up cracks (with reduced intensity of magnetic field)and the top-down cracks in pavements. Once the surface

Fig. 6. Demonstration of the test setup: (a) three-point beam bending test of mastic beams and (b) modified bending test of asphalt concrete beam with elastic foundation.

Table 3Peak load of virginal mastic samples.

Specimen 1 2 3 4 5 7 8 9

Peak load (N) 126.64 82.56 61.23 152.05 154.25 129.16 151.53 100.38

0

50

100

150

200

250

Loa

ding

For

ce, N

Loading Time, Sec

Cycle1

Cycle2

Cycle3

Cycle4

Cycle5

Cycle6 Cycle8

Cycle7 Cycle9

Sample 1

Sample 2

Sample 3

Fig. 7. Fracture-healing test data of asphalt mastic samples at heating temperatureof 60 �C.

Table 4Percentage of recovered peak loads after each fracture-healing cycle for masticsamples.

Specimen 1 (%) 2 (%) 3 (%) 4 (%) 5 (%) 7 (%) 8 (%) 9 (%)

Cycle 1 99.8 162.3 220.9 108.0 17.3 171.0 115.9 128.8Cycle 2 117.4 160.6 200.1 85.5 113.8 107.8 125.5 151.1Cycle 3 114.3 158.6 207.4 101.4 113.5 182.3 110.0 168.6Cycle 4 113.5 149.5 190.3 110.2 104.8 195.6 106.9 163.3Cycle 5 101.9 131.1 205.1 108.6 115.1 152.5 122.3 174.5Cycle 6 96.8 152.6 195.4 129.0 106.6 208.4 133.9 202.8Cycle 7 103.7 112.5 196.1 143.8 110.8 191.4 – 210.9Cycle 8 88.2 136.2 205.9 142.9 139.4 192.1 – 238.0Cycle 9 117.8 160.9 209.3 167.6 148.7 185.0 – 249.9

0

50

100

150

200

250

300

350

400L

oadi

ng F

orce

, N

Loading Time, Sec

Cycle1

Cycle2

Cycle3

Cycle4

Cycle5

Cycle6

Cycle7

Cycle8

Cycle9

Sample 4Sample 5

Fig. 8. Fracture-healing test data of asphalt mastic samples at heating temperatureof 80 �C.

0

50

100

150

200

250

300

350

400

Loa

ding

For

ce, N

Loading Time, Sec

Cycle1

Cycle2

Cycle3

Cycle4

Cycle5

Cycle6

Cycle7

Cycle8

Cycle9

Sample 7

Sample 8

Sample 9

Fig. 9. Fracture-healing test data of asphalt mastic samples at heating temperatureof 100 �C.

734 Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737

temperature reaches to the desired temperature (60 �C, 80 �C and100 �C), the sample was held for another two-minute heating pro-cess. Afterward, the samples were cooled down to the room tem-perature. Before the next fracture-healing cycle, the sampleswere conditioned in the freezer for 6 h to reach �20 �C.

5.2. Fracture-healing test results on asphalt concrete samples

As mentioned in the previous test section, the measured peakloads of the asphalt concrete beams after each fracture-healing cy-cle were used to determine healing performance. The peak loads of

the original asphalt concrete samples are listed in Table 5. Therange of the peak values is from 3500 N to 5000 N. The fracture-healing tests were conducted with three samples at each heatingtemperature of 60 �C, 80 �C or 100 �C.

Fig. 10 shows the fracture-healing test data of asphalt concretesamples at heating temperature of 60 �C for Sample 1, 2 and 3. Theoriginal peak loads of these asphalt concrete samples were within

Table 5Peak load of original asphalt concrete samples.

Specimen 1 2 3 4 5 6 7 8 9

Peak load (N) 4593.6 4512.2 5158.0 3608.9 4325.8 4961.5 4605.0 4862.0 4661.4

0

1000

2000

3000

4000

5000

6000

Loa

ding

For

ce, N

Loading Time, Sec

Sample 1

Sample 2

Sample 3

Sample 10

Sample 11

Cycle5

Cycle4 Cycle6 Cycle8

Cycle9Cycle3

Cycle2

Cycle1 Cycle7

Fig. 10. Fracture-healing test data of asphalt concrete samples at heating temper-ature of 60 �C.

0

1000

2000

3000

4000

5000

6000

Loa

ding

For

ce, N

Loading Time, Sec

Sample 4

Sample 5

Sample 6

Sample 10

Sample 11

Cycle2 Cycle4 Cycle6 Cycle8

Cycle1 Cycle3 Cycle5 Cycle7 Cycle9

Fig. 11. Fracture-healing test data of asphalt concrete samples at heating temper-ature of 80 �C.

0

1000

2000

3000

4000

5000

6000

Loa

ding

For

ce, N

Loading Time, Sec

Sample 7

Sample 8

Sample 9

Sample 10

Sample 11

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Cycle 6

Cycle 7

Cycle 8

Cycle 9

Fig. 12. Fracture-healing test data of asphalt concrete samples at heating temper-ature of 100 �C.

Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737 735

the range from 4500 N to 5000 N, and were reduced to the rangefrom 2000 N to 2500 N after nine fractured-healed cycles. Table 6lists the percentage of recovered peak loads of asphalt concretesamples after each fracture-healing cycle. Sample 1 and 2 has35.8% and 56.8% recovered peak loads after 9 cycles respectively.Sample 3 was fractured into two pieces in the seventh beam bend-ing loading test with 41.9% recovered fracture strength. It was ob-served that the cracks appeared visible after 3 or 4 cycles, and grewwith a length close to 20 mm and a width about 0.5 mm at the endof 8 or 9 cycles. It was also found that the fracture strength reducedto half of original values after few cycles at the heating tempera-ture of 60 �C.

The fracture-healing test data of asphalt concrete Sample 4, 5and 6 at the heating temperature of 80 �C were plotted in Fig. 11.The percentages of recovered peak loads of these samples aftereach fracture-healing cycle were also listed in Table 6. Sample 5and 6 has 51.8% and 45.5% recovered peak loads after 9 cyclesrespectively. Sample 4 was fractured into two pieces at the 9thbeam-bending loading after reaching about 48.3% recovered frac-ture strength. Overall, the healing performance of these sampleswas improved at the heating temperature of 80 �C, comparing tothe previous case. The test results show that the recovered fracturestrength maintained about half of the original values at the end offracture-healing cycles.

Fig. 12 shows that the fracture-healing test data of asphalt con-crete Sample 7, 8 and 9 at the heating temperature of 100 �C. Fromthe recovered peak load percentages as listed in Table 6, Sample 8

Table 6Percentage of recovered peak loads after each fracture-healing cycle for asphalt concrete

Specimen 1 (%) 2 (%) 3 (%) 4 (%)

Cycle 1 86.4 77.0 84.1 102.8Cycle 2 77.5 73.9 65.7 75.2Cycle 3 57.4 67.9 66.4 70.8Cycle 4 54.3 65.5 58.3 64.4Cycle 5 57.1 64.9 47.1 61.1Cycle 6 53.5 69.5 41.9 60.2Cycle 7 49.3 63.5 – 50.8Cycle 8 36.6 59.5 – 48.3Cycle 9 35.8 56.8 – –

and 9 maintained at 63.2% and 73.6% of original fracture strengthafter 9 fracture-healing cycles respectively. Sample 7 was fracturedinto two pieces in the 9th beam bending loading with about 32.7%recovered fracture strength. In the meanwhile, the asphalt con-crete beam samples (Sample 10 and Sample 11) were loaded withthe same beam bending test at �20 �C. These samples did not gothrough the healing process to compare with healed samples.The samples were also conditioned with �20 �C before the nextfracture test. The peak loads of these two samples were used asthe comparison values to demonstrate the effectiveness of the

samples.

5 (%) 6 (%) 7 (%) 8 (%) 9 (%)

102.5 87.5 79.1 91.0 104.585.8 80.1 77.6 91.9 90.686.5 78.4 73.5 81.5 81.984.8 68.8 63.9 74.7 80.284.6 63.9 54.7 70.8 78.776.2 57.7 50.5 67.5 76.963.2 56.4 44.4 62.8 74.057.6 51.7 32.7 64.5 75.151.8 45.5 – 63.2 73.6

736 Q. Dai et al. / Construction and Building Materials 49 (2013) 729–737

induction healing. The original peak loads of these two sampleswere about 4500 N. After the third-time loading, the peak loads de-crease to 1500 N and 2500 N for Sample 10 and 11 respectively, asindicated with the diamond and triangle markers in the figure. Andthe recovered fracture strength of these two samples reachedabout 55% and 33% of the original level after the third loading.Therefore, the micro-cracks in asphalt concrete samples with steelwool fibers can be effectively healed under induction healing andthe fracture strength of asphalt mixes can be significantly recov-ered. From the test data of Sample 8 and 9, the recovered fracturestrengths maintained about high portions of original values aftersix fracture-healing cycles at the heating temperature of 100 �C.

Therefore, the healing performance of asphalt concrete beamsamples was improved with the heating temperature as shownin the test data. For the tested asphalt mixture samples, the heatingtemperature of 100 �C can achieve optimum healing withoutaffecting the physical properties of asphalt materials.

5.3. Changes of electrical resistivity during fracture-healing cycles

Sample 7 was chosen to evaluate the change of electrical resis-tivity through four fracture-healing cycles (from fifth cycle toeighth cycle) as shown in Fig. 13. During each cycle, the electricalresistivity was measured at three different stages to investigate thepotential of self-sensing ability. At the first stage, the conditionedsample was loaded after the bending beam test and then wasplaced until the surface temperature reached the room tempera-ture about 20 �C. In the second stage, the induction healing processwas conducted and the sample was also placed to reach the roomtemperature. While at the third stage, the sample was put insidethe freezer and conditioned about 6 h to maintain �20 �C.

Comparing the first and the second stages at room temperature,the electrical resistivity of the sample was reduced after inductionhealing process. The value was further reduced at the stage 3 afterthe sample was conditioned in the freezer at subcooling tempera-ture �20 �C. The self-healing behavior during the freezing processcontributes this resistivity reduction by further repairing the mi-cro-cracks. In addition, the binder generates shrinkage deforma-tion to allow fibers to have better contacts at the subcoolingtemperatures. After the sample was loaded with the peak force un-der the beam bending test and returned to the room temperature,the electrical resistivity was increased significantly because themicro-crack behaviors cause the fibers to separate. This study indi-cates the electro-active asphalt mixtures with conductive fibershave potentials in damage-sensing ability for the structure healthmonitoring application. The electrical resistivity of the samples de-

9

9.5

10

10.5

11

4 5 6 7 8 9

No. of Cycles

After Break

After Healing

Frozen to -20

Log

(V

olum

e re

sist

ivity

, m

)Ω

Fig. 13. Change of electrical resistivity of one asphalt concrete sample duringseveral fracture-healing cycles.

creases with fracture-healing cycles because of the accumulatedself-or induction- healing behavior in the samples.

6. Conclusions

The induction healing performance of prepared asphalt masticand asphalt concrete samples were investigated through frac-ture-healing cyclic studies. The test data of asphalt mastic samplesindicate that micro-cracks in the mastic samples can be effectivelyhealed at the heating temperature of 60 �C. The sample fracturestrength was improved because the binder filled the notchedcracks at the bottom of the samples at the heating temperaturesof 80 �C and 100 �C. For the tested asphalt concrete samples withthree temperatures, the 100 �C heating temperature could be theoptimum option for the induction healing because the recoveredpeak load values were higher than the other two temperaturesafter several fracture-healing cycles. However, the heating temper-ature 60 �C and 80 �C are also acceptable since the recovered frac-ture strength is higher than 50% after six fracture-healing cycles.The induction healing effects on asphalt concrete samples was alsodemonstrated by comparing sample behaviors without healingprocess. During each fracture-healing cycle, both induction healingand self-healing behavior can decrease the electrical resistivity ofasphalt concrete samples. And the beam bending test can increasethe electrical resistivity by generating the internal damage orcracks. The electrical resistivity of the samples decreases with frac-ture-healing cycles because of the accumulated crack opening inthe samples.

Future work will be performed to consider the healing effectswith different mixture design of asphalt mastic or asphalt concretesamples, heating time-periods, and volume percentage and dimen-sion parameters of steel fibers. Especially, the micro-crack healingcapacity of asphalt concrete samples will be optimized by varyingthese design parameters for sustainable pavement applications. Itis expected that the effective induction healing system might re-duce huge pavement maintenance cost and the associated trafficjams. The successful applications also help to reduce the CO2 emis-sion and save energy with smooth interactions between pavementsurface layer and vehicle tires.

Acknowledgements

The authors would like to acknowledge the Michigan TechTransportation Institute Initiative Fund program for partial finan-cial support. The authors would like to thank Paul Fraley, RobertFritz, Morgan Hansen and David Porter at Michigan TechnologicalUniversity and Weina Meng at Missouri University of Science andTechnology for their help in preparing samples and experimentalsetup.

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