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Performance evaluation of asphalt mixture using brake pad waste as mineral filler
Construction and Building Materials 138 (2017) 410–417
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Performance evaluation of asphalt mixture using brake pad waste as mineral filler Xiaodi Hu, Ning Wang, Pan Pan ⇑, Tao Bai School of Resource and Civil Engineering, Wuhan Institute of Technology, Wuhan, Hubei 430073, People’s Republic of China
h i g h l i g h t s Brake pad waste (BPW) used as mineral filler in asphalt concrete. Hamburg wheel tracking and semi-circular bending tests conducted. Anti-moisture and anti-rutting properties improved by BPW powder. Asphalt mixture with BPW powder has longer fatigue life.
a r t i c l e
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Article history: Received 30 November 2016 Received in revised form 3 February 2017 Accepted 8 February 2017
Keywords: Brake pad waste Asphalt mixture Hamburg wheel tracking test Rheological property Anti-moisture property
a b s t r a c t The failed brake pads have been recognized as a kind of solid waste, which result in serious environmental problem. In order to provide an approach for treating the brake pad waste (BPW), this paper investigated the feasibility of using BPW as mineral filler in asphalt mixture. Firstly, BPW was laboratory manufactured into powder with a size lesser than 0.075 mm. Then, the effect of BPW powder on the physical and rheological properties of asphalt mortar was studied by the softening point test, the penetration test, the rotation viscosity test, and the dynamic shear rheological (DSR) test. The results showed that the addition of BPW powder could improve the viscosity and high temperature performance of asphalt mortar. Finally, the influence of BPW powder on the mechanical properties of asphalt mixture was analyzed by the Hamburg wheel tracking test, the accelerated freezing-thawing test, the dynamic uniaxial compression test, the semi-circular bending test and the four-point bending fatigue test. Although the low temperature property of WBP mixture was worse than the control asphalt mixture with limestone filler, BPW mixture has better anti-moisture, anti-rutting and fatigue properties. Asphalt mixture containing BPW filler showed satisfactory performance improvement. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction From the safety point of view, failed brake pads are common consumable of the prosperous automobile industry [1]. For instance, it is recommended to replace the brake pads of heavy duty trucks at least once a year. Generally, the weight loss of failed brake pad is about 40% compared to a new pad. For a biaxial wheel truck, the failed brake pads generated by a single replacement could be 35 kg. Since the global production of heavy-duty vehicles are 3.7 million per year, it is estimated that annual increment of brake pad waste (BPW) would be 129 thousand tons with just the new heavy-duty vehicles to be considered [2]. Unfortunately, failed brake pad could not be degraded naturally, which would cause enormous space waste and environmental ⇑ Corresponding author. E-mail address: [email protected] (P. Pan). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.031 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
pollution. The typical semi-metallic brake pad, has gained a considerable proportion in the international market due to its reliable performance, convenient maintenance and inexpensive cost [3]. Studies on the brake pad mainly focused on the development of new materials [4], such as microstructure analysis [5], mechanical behavior [6], wear performance [7], noise reduction [8] and environmental protection [9–11]. However, few studies have been conducted on recycling the BPW. Hot-mix asphalt (HMA) consists of asphalt, mineral filler, coarse and fine aggregates. In recent years, construction and maintenance of HMA pavement have consumed large amount of non-renewable resources [12]. Therefore, many researchers are dedicated to explore alternative materials for the purpose of substituting natural mineral aggregate and filler, such as oxygen furnace slag [13,14], demolition waste [15], fly ash [16,17], biomass ashes [18], etc. It is widely recognized that utilization of recycled wastes
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is a promising way to reduce the demand for natural resources in asphalt mixture. In addition, previous studies have investigated the effect of steel slag, rubber particle, fiber and graphite on the mechanical properties of asphalt mixture. The steel slag used as coarse aggregate could improve the mechanical and electrically conductive properties of asphalt mixture [19]. The rubber particle could also improve the mechanical properties of asphalt at high and low temperatures, and prolong the durability of pavement [20]. Fiber has a reinforcing and toughening effect on asphalt mixture, which results in a significant improvement of Marshall stability, rutting resistance, indirect tensile strength, and low temperature cracking resistance compared to conventional asphalt mixture [21]. Graphite powder could improve the thermal conductivity and anti-ageing property of asphalt binder [22]. Generally, brake pads are composed of various components, e.g. metal powder, rubber, fiber and graphite, etc. in order to improve the wear resistance and structure stability [23]. Since such materials also help to improve the performance of asphalt mixture, it is assumed that the brake pad waste could be used as road material and is of high potential to improve the properties of asphalt mixture. Meanwhile, utilization of BPW could also make a considerable contribution to mitigate the environment pollution problem. However, few literature are found to investigate the effect of BPW materials on the mechanical properties of asphalt mixture. The objective of this research is to study the feasibility of using BPW in asphalt mixture. Firstly, the BPW was laboratory manufactured into powder for substituting the mineral filler in asphalt mixture. Then, both asphalt mortar and asphalt mixture with different content of BPW powder were prepared. And the effect of BPW powder on the physical and rheological properties of asphalt mortar was investigated by the softening point test, the penetration test, the rotation viscosity test, and the dynamic shear rheological (DSR) test. Moreover, the influence of BPW powder on the performance of asphalt mixture was evaluated by the Hamburg wheel tracking test, the accelerated freezing-thawing test, the dynamic uniaxial compression test, the semi-circular bending test and the four-point bending fatigue test.
2. Materials and experimental method 2.1. Raw materials Basalt with the particle size lesser than 19 mm was obtained from Hong’an County, Hubei Province, China. The 60/80 penetration graded asphalt binder, obtained from the SK Co., Ltd. in Korea, with a softening point of 47.2 °C (ASTM D36) [24], a ductility of 156 cm (25 °C, ASTM D113) [25] and a penetration of 73 dmm (deci-millimetre, 25 °C, ASTM D5) [26], was used for this research. Limestone filler has a particle size of lesser than 0.075 mm and a density of 2.88 g/cm3. Characteristics of aggregates and asphalt binder were tested according to ASTM and AASHTO standards. BPW was obtained from the local truck garage in Wuhan. It was a kind of semi-metallic brake pad with arc shape. In this study, the BPW was processed into powder (lesser than 0.075 mm) by three steps. Firstly, recycled BPW was broken into coarse particles with different sizes less than 20 mm. Secondly, all the BPW particles were put into the Los Angeles abrasion tester for further crushing. Thirdly, the BPW powder with a particle size lesser than 0.075 mm was obtained by screening the BPW particles obtained in step 2. The BPW powder has a density of 2.79 g/cm3. Based on X-Ray Fluorescence analysis, the chemical composition of BPW powder was seen in Table 1. Loss on ignition mainly consists of the resin binder and the graphite. The oxidation of silica and calcium accounted for the main component, which can change
Table 1 Chemical composition of BPW powder. Component
Proportion (%)
Component
Proportion (%)
Loss on ignition SiO2 CaO MgO Fe2O3 Al2O3 SO3 BaO K2O Na2O Cr2O3
36.946 21.558 14.038 7.139 5.804 4.963 3.833 3.229 0.555 0.545 0.489
TiO2 ZnO SrO Cl MnO P2 O5 ZrO CuO NiO As2O3 PbO
0.292 0.178 0.083 0.081 0.08 0.054 0.044 0.041 0.034 0.009 0.006
the microstructure of asphalt mortar and bring useful characteristics such as high absorptive and chemical stability [18]. 2.2. Sample preparation and mix proportions In order to prepare asphalt mortar specimen, asphalt binder (200 g ± 5 g) was preheated to 165 ± 5 °C in an oil-bath heating container. Then, limestone filler (LF) and BPW powder were separately added slowly within 10 min, while the shear speed was kept at 2000 rp/m. After all the mineral filler and BPW powder were added, the asphalt mortar was sheared for another 30 min to make sure the homogeneously dispersing of additives in the asphalt. Table 2 presented the material composition of different asphalt mortars. In this study, three test repetitions were used for all the performance tests of asphalt mortar in Section 2.3, and the average value was used. Superpave procedure was adopted to design a mixture with the nominal maximum size of 12.5 mm in this research. The gradation curve of Superpave 12.5 was shown in Fig. 1. The upper and lower limits of gradation curves followed to the Superpave Specification AASHTO M323 [27]. The optimum bitumen content of asphalt mixture was 4.9% by weight of asphalt mixture, which was determined by the volume parameters (e.g. air void, voids in mineral aggregate VMA, and voids filled with asphalt VFA). The content of mineral filler was 3% by weight of aggregate. Table 2 also illustrated the mixture proportions by weight sum of Superpave design specimen (U150 mm h115 mm). In this study, six test repetitions were used for all the performance tests of asphalt mixture in Section 2.4, and the average value was used. 2.3. Performance tests of asphalt mortar 2.3.1. Physical properties tests The physical properties of asphalt mortar, including softening point, penetration (25 °C), and rotation viscosity (135 °C and 175 °C) were tested according to ASTM D36, ASTM D5, and ASTM D4402 [28], respectively. 2.3.2. Dynamic shear rheological test The dynamic shear rheological (DSR) test was applied to characterize the rheological properties of the asphalt mortar according to AASHTO T315 [29]. High temperature sweep test was performed under strain-controlled mode with a constant frequency of 10 rad/s. The testing temperature ranged from 30 °C to 80 °C. 2.4. Performance tests of asphalt mixture 2.4.1. Hamburg wheel tracking test Hamburg wheel tracking test (HWTT) was proposed to evaluate the moisture susceptibility of asphalt mixtures according to AASHTO T324 [30]. The size of HWTT specimen was 150 mm in
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Table 2 Details of the proportions of asphalt mortar and asphalt mixture. Specimen type
Asphalt mortar
Asphalt mixture
M-100LF M-80LF-20BPW M-60LF-40BPW M-40LF-60BPW M-20LF-80BPW M-100BPW C-100LF C-67LF-33BPW C-33LF-67BPW C-100BPW
Filler combinations/(g) Limestone filler
BPW powder
200 160 120 80 40 – 138.2 92.0 46.2 –
– 40 80 120 160 200 – 44.6 89.2 133.8
Asphalt binder/(g)
Aggregate/(g)
199.5 200.4 201.5 200.6 199.4 200.1 230.5 230.5 230.5 230.5
– – – – – – 4567.2 4568.8 4570.4 4571.6
2.4.4. Semi-circular bending test The semi-circular bending (SCB) test including monotonic loading and repeated loading was selected to evaluate the cracking resistance of asphalt mixture at low temperature (10 °C) and medium temperatures (20 °C) according to EN 12697-44 [34]. As seen in Fig. 2, the SCB specimen was a half cylinder, typically 150 mm in diameter with a notch depth of 6.35 mm, which was loaded by compression using a three-point flexural apparatus. The loading rate under monotonic mode was 1.27 mm/min at 10 °C and 20 °C. The classical fracture parameters measured from the SCB tests were shown in Fig. 2. Fracture energy referred to the shaded area surrounded by stress and strain, which can be calculated by Eq. (1). Additionally, the fracture energy index (FE Index) is defined as a parametric ratio of the fracture energy to the HMA tensile strength and tensile strain at peak failure load per unit crack length, which was calculated by Eq. (2) [35].
Z Fig. 1. Grading curve of asphalt mixture.
diameter and 62.5 mm in height, which was molded by Superpave gyratory compactor (SGC). The current HWTT specification requires a wheel speed of 52 cycles per minute up to 20,000 passes at 50 °C in a water bath. The HWTT test would stop if the rut depth reach to 12.5 mm or the wheeling number reaches to 20,000 cycles [31]. 2.4.2. Accelerated freezing-thawing test To evaluate the long term effect of moisture on the mechanical properties of asphalt mixture, accelerated freezing-thawing cycles (1, 3, 7, 12, 18 times) were applied to treat the asphalt mixture. A standard accelerated conditioning cycle was adjusted including 16 h at 18 °C in freezer and 8 h at 60 °C in a water bath, which is referred in the Modified Lottman Test (AASHTO T-283) [32]. The vacuum was also applied to make the specimen saturated before conditioning. 2.4.3. Dynamic uniaxial compression test Dynamic uniaxial compression test was conducted by UTM-100 to assess the permanent deformation of asphalt mixture according to ASTM D3497 [33]. Cylindrical specimens with a diameter of 150 mm and a height of 120 mm were prepared by SGC procedure. The compacted specimens were cored and cut to produce smaller ones with dimensions of U100 mm h100 mm. A haversine load with a pulse loading of 0.1 s and a rest time of 0.9 s was implemented to simulate the dynamic load. A loading of 0.5 MPa was applied under three temperatures of 40 °C, 50 °C, and 60 °C. The test continued until the accumulated microstrain attained 100,000 le.
GF ¼ 0
em
rðeÞde
FEIndex ¼ 1 105
ð1Þ
Gf et lcr rt
ð2Þ
Where GF is the fracture energy, lcr is the length traversed by the crack, rt is the tensile strength and et is the tensile strain at peak failure load. The repeated mode was established by loading the specimen at a frequency of 1 Hz. A test cycle included a 0.5 s of loading and a 0.5 s of rest. The testing temperature was 20 °C. The loading
Fig. 2. Parameters in the monotonic semi-circular bending test.
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waveform was the triangular wave with a peak load of 5.5 kN, which is 50% of the average failure load. 2.4.5. Four-point bending fatigue test Four-point bending fatigue test is one of the most representative methods to evaluate the fatigue performance of asphalt mixture according to AASHTO T321 [36]. All specimens were prepared into a beam with a size of 380 mm in length, 65 mm in width and 50 mm in height. A frequency of 1 Hz and strain levels of 400 le, 500 le, 600 le and 700 le were selected for this test. When the stiffness modulus decreases by 50%, corresponding load cycles is defined as the fatigue life. Generally, the fatigue performance of mixtures could be analyzed by the regression equation as below in Eq. (3):
log Nf ¼ log A n log e
ð3Þ
where Nf is the fatigue life of HMA mixture; e is the strain level; both A and n are regression coefficients. 3. Results and discussion 3.1. Asphalt mortar performance 3.1.1. Effect of BPW powder on physical property of asphalt mortar The effect of BPW powder on softening point and penetration of asphalt mortar was analyzed and the results were shown in Fig. 3. The limestone filler was replaced by BPW powder deliberately. While the content of BPW powder increased from 0% to 100%, the corresponding content of limestone filler decreased from 100% to 0%. As seen in Fig. 3, the penetration of asphalt mortar decreased with the increment of BPW powder while the softening point temperature correspondingly increased. Compared to the M100LF mortar, the penetration of M-100BPW mortar decreased by 67.7% from 42.1 dmm to 25.1 dmm while the softening point temperature increased by 20.1% from 52.4 °C to 63.4 °C. The results showed that BPW powder could improve the high temperature properties of asphalt mortar. The reason might due to the stiffening effect of BPW powder on asphalt mortar. Fig. 4 illustrated the effect of BPW powder on the viscosity of asphalt mortar. The rotation viscosity increased exponentially with the increment of BPW powder. The viscosity of M-100LF mortar were 1250 mPa s at 135 °C and 173 mPa s at 175 °C, respectively. When the limestone filler was replaced by BPW powder, the corresponding viscosity of M-100BPW mortar were 9750 mPa s and
Fig. 3. Effect of BPW powder on penetration and softening point temperature of asphalt mortar.
Fig. 4. Effect of BPW powder on rotation viscosity of asphalt mortar.
1295 mPa s, respectively. Compared to M-100LF mortar, the viscosity of M-100BPW mortar increased by 680% at 135 °C and 648% at 175 °C. It is known that filler dispersing in asphalt would improve the viscosity of asphalt mortar. However, the results in Figs. 3 and 4 indicated that there was a significant difference in the effect of limestone filler and BPW powder on physical properties of asphalt mortar. It might because that the BPW powder could transfer more free asphalt to structural asphalt due to their component and microstructure. Therefore, it is required to conduct further investigations on the effect mechanism of BPW powder on asphalt mortar.
3.1.2. Effect of BPW powder on rheological property of asphalt mortar Dynamic shear rheological (DSR) test was used to study the effect of BPW powder on the rheological properties of asphalt mortar. Complex modulus (G⁄) and phase angle (d) are the most important parameters to provide information about the rheological properties of asphalt binders during the shearing process. The curves of G⁄ and d at a temperature region ranging from 30 °C to 80 °C were shown in Fig. 5. It illustrated that BPW powder increased the complex modulus and reduced the phase angle of asphalt mortar. The phase angle implied the proportion of viscous and elastic properties of asphalt. Greater phase angle represents
Fig. 5. Effect of BPW powder on complex modulus and phase angle of asphalt mortar.
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asphalt mortar is more susceptible to permanent deformation. Therefore, it seems that BPW powder could improve the high temperature performance of asphalt mortar. Moreover, the differences in complex modulus and phase angle of asphalt mortar were more significant at high temperature as the content of BPW powder increased. Since asphalt mixture is susceptible to permanent deformation, rutting parameter (G⁄/sind) was adopted to evaluate the effect of BPW powder on high temperature performance of asphalt mortar and the results were shown in Fig. 6. Greater rutting parameter represents better anti-rutting property. As seen in Fig. 6, the rutting parameter of asphalt mortar increased with the addition of BPW powder in the whole temperature region. Moreover, the difference in rutting parameter gradually expanded as the temperature increased. For instance, the rutting parameters of M-100LF were 1006 MPa and 0.79 MPa at 30 °C and 80 °C, respectively, and the corresponding rutting parameters of M-100BPW were 1588 MPa and 1.38 MPa. It can be concluded that asphalt mortar with BPW powder showed better high temperature performance than the limestone filler mortar, which was in accordance with the softening point results in Fig. 3.
3.2. Asphalt mixture performance 3.2.1. Moisture damage resistance Since the HWTT test condition could simulate the combination of loading, water and temperature, the rutting depth and creeping rate were used to evaluate the anti-moisture property and high temperature stability of asphalt mixture. In HWTT test, lower rutting depth and lower creeping rate represents better anti-moisture and high temperature properties of asphalt mixture. Fig. 7 illustrated the HWTT rutting response curves of asphalt mixtures containing different composition of fillers. At the beginning period of the HWTT test, the rutting depth increased rapidly due to the second compaction of specimen. After several cycles, the change of rutting curves became steady. As seen in Fig. 7, the creeping rate of C-100BPW and C-100LF mixtures were 0.103 mm/cycle and 0.369 mm/cycle, respectively. The former was just one third of the latter. After 20,000 load passes, the rutting depth of asphalt mixture decreased with the increment of BPW powder. The rutting depth of C-100BPW mixture was the lowest among the four kinds of asphalt mixtures in this study. This implied that BPW powder
Fig. 6. Effect of BPW on rutting parameter of asphalt mortar.
Fig. 7. HWTT result of asphalt mixture with BPW powder and limestone filler.
could not only enhance the anti-moisture property, but also improve the high temperature performance of asphalt mixture. In this research, accelerated freezing-thawing test was employed to study the effect of moisture and temperature on mechanical properties of asphalt mixture in a long term and the results were shown in Figs. 8 and 9, respectively. Indirect tensile strength and tension strength ratio were adopted to evaluate the anti-moisture properties of asphalt mixture. Fig. 8 illustrated the evolution of indirect tensile strength of different asphalt mixtures after the accelerated freezing-thawing test. For the same freezingthawing cycles, the indirect tensile strength of asphalt mixture increased with the addition of BPW powder. For example, indirect tensile strength of C-100LF was 0.82 MPa after 1 cycle, while the corresponding result of C-100BPW was 1.25 MPa. It also implied that BPW powder could improve the anti-moisture property of asphalt mixture. For all the asphalt mixtures, it was noticeable that stripping and cracking occurred in the specimen after 12 cycles. This is because that phase change of water during the freezingthawing process lead to volume expansion of specimen and serious performance degradation of asphalt mixture. The effect of BPW powder on tensile strength ratio of asphalt mixture during the accelerated freezing-thawing process was analyzed and the results were shown in Fig. 9. After 1 cycle of
Fig. 8. Effect of BPW powder on indirect tensile strength of asphalt mixture during the accelerated freezing-thawing process.
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Fig. 9. Effect of BPW powder on tensile strength ratio of asphalt mixture during accelerated freezing-thawing process.
freezing-thawing, the tensile strength ratio of all the specimens were greater than the criterion of 75%. For the same freezingthawing cycles, the tensile strength ratio of asphalt mixture increased with the addition of BPW powder. This also implied that BPW powder could improve the anti-moisture property of asphalt mixture. It was interesting that there was no significant difference in the results of C-100LF, C-67LF-33BPW and C-33LF-67BPW mixtures, especially after 7 freezing-thawing cycles. When the limestone filler was replaced by 100% BPW powder, a noticeable increment of tensile strength ratio occurred for C-100BPW mixture. Therefore, the results of HWTT test and indirect tensile test after freezing thawing indicated that asphalt mixture with BPW powder has better anti-moisture property. This might because BPW powder helps to improve the bonding characteristic between asphalt and aggregate. 3.2.2. High-temperature stability Dynamic uniaxial compression test was employed to evaluate the high temperature performance of asphalt mixture and the results were shown in Fig. 10. For the same testing temperature, greater loading repetition cycles represents better high temperature property. As seen in Fig. 10, the loading repetition cycles of asphalt mixture increased with the increment of BPW powder. The terminate repetition numbers of C-100LF mixture at 40 °C, 50 °C, 60 °C were 1517 times, 389 times, and 119 times, respectively. And the corresponding results of C-100BPW mixture were 4680 times, 1126 times, and 323 times, which were increased by about 208.5%, 189.5%, and 171.4% compared with the results of C-100LF mixture. It indicated that BPW powder improved the anti-rutting property of asphalt mixture, which was in accordance with the results in Fig. 7. Since BPW powder could improve the viscosity and softening point temperature of asphalt mortar, the BPW powder mixture was less susceptible to permanent deformation than the LF mixture. Therefore, the BPW mixture exhibits better anti-rutting performance than that of LF mixture. 3.2.3. Cracking resistance Table 3 summarized the results of the monotonic SCB test. Generally, the tensile strength and tensile strain were widely accepted to assess the cracking resistance of the different materials. As shown in Table 3, the tensile strength and modulus of C-100LF were lower than that of the C-100BPW mixture, while the tensile strain of the former was greater than the latter at the test temperatures of 10 °C and 20 °C. This implied that the anti-cracking
Fig. 10. Dynamic creep stain of asphalt mixture with different fillers under different temperature. (a) 40 °C, (b) 50 °C, (c) 60 °C.
property of C-100LF mixture was superior to the C-100BPW mixture with aspect of fracture strain. It might be concluded that the addition of BPW powder might attenuate the cracking resistance of asphalt mixture. However, Fig. 2 showed that the tensile strain et and tensile modulus Et were determined once the tensile load reached to the peak point, i.e. rt. And the cracking period after peak loading was not taken into account. Therefore, it could not completely indicate the cracking resistance performance of asphalt mixture. The fracture energy and FE index were also adopted to evaluate the cracking resistance of asphalt mixture. From the fracture energy point of view, the whole energy produced by loading would
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Table 3 Effect of BPW powder on cracking resistance of asphalt mixture. Mixture type
Temperature/(°C)
Tensile strength/(MPa)
Tensile strain/(mm/mm)
Tensile modulus/(MPa)
Fracture energy/(J/m2)
FE-Index
C-100LF
10 20 10 20
5.21 4.87 6.21 5.79
0.0137 0.0145 0.0105 0.0119
403.10 335.59 591.8 486.47
1124 170 1241 489
4.44 114 3.09 172
C-100 BPW
be dissipated by cracking formation and propagation. Fracture energy values of the C-100LF and C-100BPW mixtures were relatively comparative at 10 °C. However, the fracture energy of C100BPW samples was 188% higher than that of C-100LF samples at 20 °C. The results of fracture energy indicated that BPW powder could improve the cracking resistance of asphalt mixture, which was different with the analysis on tensile strain and tensile modulus. Additionally, FE Index was proposed to simultaneously consider the tensile strain and fracture energy concept, which was calculated as Eq. (2). As showed in Table 3, it can be concluded that the addition of BPW improved the crack propagation resistance in medium temperature and reduced the mixture flexibility in low temperature at the same time. Furthermore, the repeated SCB test at 20 °C was also employed to evaluate the cracking propagation of asphalt mixtures and the results were shown in Fig. 11. In this test, greater terminal cycles represents better anti-cracking performance. The terminal cycles of asphalt mixture increased as the content of BPW powder increased. The terminal repetition number of C-100LF specimen at 20 °C was 2974 times. The corresponding results of asphalt mixture with 33%, 67%, and 100% BPW powder were 3151 times, 3438 times, and 3885 times, which were increased by about 6.0%, 15.6%, and 30.6%, respectively. The results of fracture energy also confirmed that the addition of BPW could improve the crack propagation resistance of asphalt mixture under medium temperature condition. 3.2.4. Fatigue property Fatigue of asphalt mixture is defined as the damage caused by repeated stresses and strains due to environmental condition and traffic loading. The bonding property of asphalt binder contributes largely to the fatigue performance of the asphalt mixture. The testing results of different asphalt mixtures were shown in Fig. 12. It was observed that the fit curve of C-100BPW samples moved
Fig. 12. Relationship between strain level and fatigue life of asphalt mixture.
upwards, compared with C-100LF samples. The fatigue life of asphalt mixture increased with the addition of BPW powder at the same strain level. Therefore, it could be concluded that BPW powder as mineral filler improved the fatigue life of asphalt mixture. Using the data collected from this investigation, Fig. 12 also illustrated a regression analysis of the influence of different mineral filler on fatigue parameters. Several studies pointed out that power law regression can be fitted well with the test data, which was shown as Eq. (3) [37,38]. Greater A value represents better fatigue property of asphalt mixture, while greater n value means the fatigue life of asphalt mixture is more sensitive to the microstrain level. According to the fitting analysis, the parameter A and n tend to increase as the content of BPW powder increased. It implied that BPW powder could improve the fatigue performance of asphalt mixture. With the increment of BPW powder, asphalt mixture became more sensitive to the strain level. At a relatively low strain level, the fatigue life of C-100BPW mixture was much higher than that of the C-100LF mixture. For instance, the fatigue life of C100BPW and C-100LF mixtures were 427,184 cycles and 123,590 cycles at 400 le microstrain level. While the strain increased, the enhancement effect of BPW powder on asphalt mixture became weakening. However, the C-100BPW specimen exhibited the best fatigue performance at each strain level of this test.
4. Conclusions
Fig. 11. Displacement development of asphalt mixtures with the number of load cycles.
In this research, the feasibility of utilizing brake pad waste as mineral filler in asphalt mixture was investigated. From the experimental evaluations carried out on some of the most important parameters affecting pavement performance, the following conclusions can be drawn.
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(1) BPW powder could increase the softening point, rotation viscosity and decrease the penetration of asphalt mortar, which implied better high temperature stability compared with that of LF mortar. Rheological characteristics also proved that the BPW improves the elastic response of asphalt mortar at high temperature. (2) BPW mixtures could improve the anti-moisture damage and anti-rutting resistance, compared to the LF mixture. Accelerated freezing-thawing test results indicated that less moisture destruction occurred in BPW mixture. The permanent deformation of BPW mixture was less than the LF mixture, which was in accordance with DSR result of asphalt mortar. (3) BPW powder could improve the fatigue property of asphalt mixture, but correspondingly degrade the low temperature performance. Compared to LF mixture, the BPW mixture was more sensitive to the changes of strain levels. Cracking propagation resistance of BPW asphalt mixture at a medium temperature was better than that of LF asphalt mixture. However, BPW powder degraded the flexibility of asphalt mixture at low temperature. Due to the problems caused by cracking, the BPW powder was not recommended in cold regions. Asphalt mixture containing BPW filler showed satisfactory performance improvement. The successful utilization of brake pad waste as mineral filler in pavement construction can provide a more effective approach for sustainable resources, and mitigate the problems of solid wastes to the environment. In order to promote the application of BPW in pavement, it is recommended to focus on the feasibility of the coarse BPW particles substituting for mineral aggregate, as well as the method to improve the low temperature performance of BPW asphalt mixture. Further research can be conducted on the interaction mechanism of BPW asphalt mortar from microstructure point of view. Additional laboratory tests with more mixture type along with correlations and validation with field performance date are strongly recommended as well. Acknowledgments The authors acknowledge the support provided by the National Natural Science Foundation of China (No. 51278389), the Science Foundation of Wuhan Institute of Technology (No. 16QD23), and the Graduate Innovative Fund of Wuhan Institute of Technology (No. CX2015031). References [1] J.R. Laguna-Camacho, G. Juárez-Morales, C. Calderón-Ramón, V. VelázquezMartínez, I. Hernández-Romero, J.V. Méndez-Méndez, M. Vite-Torres, A study of the wear mechanisms of disk and shoe brake pads, Eng. Fail. Anal. 56 (2015) 348–359. [2] D. Kodjak, B. Sharpe, O. Delgado, Evolution of heavy-duty vehicle fuel efficiency policies in major markets, Mitig. Adapt. Strat. Glob. Change 20 (2015) 755–775. [3] A.V. Pavlov, S.P. Kudelnikova, A.N. Vicharev, On the corrosion resistance of half-metallic composite brake pads for railroad cars, J. Frict. Wear 36 (2015) 123–126. [4] M.C. Lagel, L. Hai, A. Pizzi, M.C. Basso, L. Delmotte, S. Abdall, A. Zahed, F.M. AlMarzouki, Automotive brake pads made with a bioresin matrix, Ind. Crops Prod. 85 (2016) 372–381. [5] G.Y. Bian, H.Z. Wu, Friction and surface fracture of a silicon carbide ceramic brake disc tested against a steel pad, J. Eur. Ceram. Soc. 35 (2015) 3797–3807. [6] M. Eriksson, F. Bergman, S. Jacobson, On the nature of tribological contact in automotive brakes, Wear 252 (2002) 25–36. [7] A.K. Ilanko, S. Vijayaraghavan, Wear behavior of asbestos-free eco-friendly composites for automobile brake materials, Friction 4 (2016) 144–152.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Performance evaluation of asphalt mixture using brake pad waste as mineral filler Xiaodi Hu, Ning Wang, Pan Pan ⇑, Tao Bai School of Resource and Civil Engineering, Wuhan Institute of Technology, Wuhan, Hubei 430073, People’s Republic of China
h i g h l i g h t s Brake pad waste (BPW) used as mineral filler in asphalt concrete. Hamburg wheel tracking and semi-circular bending tests conducted. Anti-moisture and anti-rutting properties improved by BPW powder. Asphalt mixture with BPW powder has longer fatigue life.
a r t i c l e
i n f o
Article history: Received 30 November 2016 Received in revised form 3 February 2017 Accepted 8 February 2017
Keywords: Brake pad waste Asphalt mixture Hamburg wheel tracking test Rheological property Anti-moisture property
a b s t r a c t The failed brake pads have been recognized as a kind of solid waste, which result in serious environmental problem. In order to provide an approach for treating the brake pad waste (BPW), this paper investigated the feasibility of using BPW as mineral filler in asphalt mixture. Firstly, BPW was laboratory manufactured into powder with a size lesser than 0.075 mm. Then, the effect of BPW powder on the physical and rheological properties of asphalt mortar was studied by the softening point test, the penetration test, the rotation viscosity test, and the dynamic shear rheological (DSR) test. The results showed that the addition of BPW powder could improve the viscosity and high temperature performance of asphalt mortar. Finally, the influence of BPW powder on the mechanical properties of asphalt mixture was analyzed by the Hamburg wheel tracking test, the accelerated freezing-thawing test, the dynamic uniaxial compression test, the semi-circular bending test and the four-point bending fatigue test. Although the low temperature property of WBP mixture was worse than the control asphalt mixture with limestone filler, BPW mixture has better anti-moisture, anti-rutting and fatigue properties. Asphalt mixture containing BPW filler showed satisfactory performance improvement. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction From the safety point of view, failed brake pads are common consumable of the prosperous automobile industry [1]. For instance, it is recommended to replace the brake pads of heavy duty trucks at least once a year. Generally, the weight loss of failed brake pad is about 40% compared to a new pad. For a biaxial wheel truck, the failed brake pads generated by a single replacement could be 35 kg. Since the global production of heavy-duty vehicles are 3.7 million per year, it is estimated that annual increment of brake pad waste (BPW) would be 129 thousand tons with just the new heavy-duty vehicles to be considered [2]. Unfortunately, failed brake pad could not be degraded naturally, which would cause enormous space waste and environmental ⇑ Corresponding author. E-mail address: [email protected] (P. Pan). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.031 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
pollution. The typical semi-metallic brake pad, has gained a considerable proportion in the international market due to its reliable performance, convenient maintenance and inexpensive cost [3]. Studies on the brake pad mainly focused on the development of new materials [4], such as microstructure analysis [5], mechanical behavior [6], wear performance [7], noise reduction [8] and environmental protection [9–11]. However, few studies have been conducted on recycling the BPW. Hot-mix asphalt (HMA) consists of asphalt, mineral filler, coarse and fine aggregates. In recent years, construction and maintenance of HMA pavement have consumed large amount of non-renewable resources [12]. Therefore, many researchers are dedicated to explore alternative materials for the purpose of substituting natural mineral aggregate and filler, such as oxygen furnace slag [13,14], demolition waste [15], fly ash [16,17], biomass ashes [18], etc. It is widely recognized that utilization of recycled wastes
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is a promising way to reduce the demand for natural resources in asphalt mixture. In addition, previous studies have investigated the effect of steel slag, rubber particle, fiber and graphite on the mechanical properties of asphalt mixture. The steel slag used as coarse aggregate could improve the mechanical and electrically conductive properties of asphalt mixture [19]. The rubber particle could also improve the mechanical properties of asphalt at high and low temperatures, and prolong the durability of pavement [20]. Fiber has a reinforcing and toughening effect on asphalt mixture, which results in a significant improvement of Marshall stability, rutting resistance, indirect tensile strength, and low temperature cracking resistance compared to conventional asphalt mixture [21]. Graphite powder could improve the thermal conductivity and anti-ageing property of asphalt binder [22]. Generally, brake pads are composed of various components, e.g. metal powder, rubber, fiber and graphite, etc. in order to improve the wear resistance and structure stability [23]. Since such materials also help to improve the performance of asphalt mixture, it is assumed that the brake pad waste could be used as road material and is of high potential to improve the properties of asphalt mixture. Meanwhile, utilization of BPW could also make a considerable contribution to mitigate the environment pollution problem. However, few literature are found to investigate the effect of BPW materials on the mechanical properties of asphalt mixture. The objective of this research is to study the feasibility of using BPW in asphalt mixture. Firstly, the BPW was laboratory manufactured into powder for substituting the mineral filler in asphalt mixture. Then, both asphalt mortar and asphalt mixture with different content of BPW powder were prepared. And the effect of BPW powder on the physical and rheological properties of asphalt mortar was investigated by the softening point test, the penetration test, the rotation viscosity test, and the dynamic shear rheological (DSR) test. Moreover, the influence of BPW powder on the performance of asphalt mixture was evaluated by the Hamburg wheel tracking test, the accelerated freezing-thawing test, the dynamic uniaxial compression test, the semi-circular bending test and the four-point bending fatigue test.
2. Materials and experimental method 2.1. Raw materials Basalt with the particle size lesser than 19 mm was obtained from Hong’an County, Hubei Province, China. The 60/80 penetration graded asphalt binder, obtained from the SK Co., Ltd. in Korea, with a softening point of 47.2 °C (ASTM D36) [24], a ductility of 156 cm (25 °C, ASTM D113) [25] and a penetration of 73 dmm (deci-millimetre, 25 °C, ASTM D5) [26], was used for this research. Limestone filler has a particle size of lesser than 0.075 mm and a density of 2.88 g/cm3. Characteristics of aggregates and asphalt binder were tested according to ASTM and AASHTO standards. BPW was obtained from the local truck garage in Wuhan. It was a kind of semi-metallic brake pad with arc shape. In this study, the BPW was processed into powder (lesser than 0.075 mm) by three steps. Firstly, recycled BPW was broken into coarse particles with different sizes less than 20 mm. Secondly, all the BPW particles were put into the Los Angeles abrasion tester for further crushing. Thirdly, the BPW powder with a particle size lesser than 0.075 mm was obtained by screening the BPW particles obtained in step 2. The BPW powder has a density of 2.79 g/cm3. Based on X-Ray Fluorescence analysis, the chemical composition of BPW powder was seen in Table 1. Loss on ignition mainly consists of the resin binder and the graphite. The oxidation of silica and calcium accounted for the main component, which can change
Table 1 Chemical composition of BPW powder. Component
Proportion (%)
Component
Proportion (%)
Loss on ignition SiO2 CaO MgO Fe2O3 Al2O3 SO3 BaO K2O Na2O Cr2O3
36.946 21.558 14.038 7.139 5.804 4.963 3.833 3.229 0.555 0.545 0.489
TiO2 ZnO SrO Cl MnO P2 O5 ZrO CuO NiO As2O3 PbO
0.292 0.178 0.083 0.081 0.08 0.054 0.044 0.041 0.034 0.009 0.006
the microstructure of asphalt mortar and bring useful characteristics such as high absorptive and chemical stability [18]. 2.2. Sample preparation and mix proportions In order to prepare asphalt mortar specimen, asphalt binder (200 g ± 5 g) was preheated to 165 ± 5 °C in an oil-bath heating container. Then, limestone filler (LF) and BPW powder were separately added slowly within 10 min, while the shear speed was kept at 2000 rp/m. After all the mineral filler and BPW powder were added, the asphalt mortar was sheared for another 30 min to make sure the homogeneously dispersing of additives in the asphalt. Table 2 presented the material composition of different asphalt mortars. In this study, three test repetitions were used for all the performance tests of asphalt mortar in Section 2.3, and the average value was used. Superpave procedure was adopted to design a mixture with the nominal maximum size of 12.5 mm in this research. The gradation curve of Superpave 12.5 was shown in Fig. 1. The upper and lower limits of gradation curves followed to the Superpave Specification AASHTO M323 [27]. The optimum bitumen content of asphalt mixture was 4.9% by weight of asphalt mixture, which was determined by the volume parameters (e.g. air void, voids in mineral aggregate VMA, and voids filled with asphalt VFA). The content of mineral filler was 3% by weight of aggregate. Table 2 also illustrated the mixture proportions by weight sum of Superpave design specimen (U150 mm h115 mm). In this study, six test repetitions were used for all the performance tests of asphalt mixture in Section 2.4, and the average value was used. 2.3. Performance tests of asphalt mortar 2.3.1. Physical properties tests The physical properties of asphalt mortar, including softening point, penetration (25 °C), and rotation viscosity (135 °C and 175 °C) were tested according to ASTM D36, ASTM D5, and ASTM D4402 [28], respectively. 2.3.2. Dynamic shear rheological test The dynamic shear rheological (DSR) test was applied to characterize the rheological properties of the asphalt mortar according to AASHTO T315 [29]. High temperature sweep test was performed under strain-controlled mode with a constant frequency of 10 rad/s. The testing temperature ranged from 30 °C to 80 °C. 2.4. Performance tests of asphalt mixture 2.4.1. Hamburg wheel tracking test Hamburg wheel tracking test (HWTT) was proposed to evaluate the moisture susceptibility of asphalt mixtures according to AASHTO T324 [30]. The size of HWTT specimen was 150 mm in
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Table 2 Details of the proportions of asphalt mortar and asphalt mixture. Specimen type
Asphalt mortar
Asphalt mixture
M-100LF M-80LF-20BPW M-60LF-40BPW M-40LF-60BPW M-20LF-80BPW M-100BPW C-100LF C-67LF-33BPW C-33LF-67BPW C-100BPW
Filler combinations/(g) Limestone filler
BPW powder
200 160 120 80 40 – 138.2 92.0 46.2 –
– 40 80 120 160 200 – 44.6 89.2 133.8
Asphalt binder/(g)
Aggregate/(g)
199.5 200.4 201.5 200.6 199.4 200.1 230.5 230.5 230.5 230.5
– – – – – – 4567.2 4568.8 4570.4 4571.6
2.4.4. Semi-circular bending test The semi-circular bending (SCB) test including monotonic loading and repeated loading was selected to evaluate the cracking resistance of asphalt mixture at low temperature (10 °C) and medium temperatures (20 °C) according to EN 12697-44 [34]. As seen in Fig. 2, the SCB specimen was a half cylinder, typically 150 mm in diameter with a notch depth of 6.35 mm, which was loaded by compression using a three-point flexural apparatus. The loading rate under monotonic mode was 1.27 mm/min at 10 °C and 20 °C. The classical fracture parameters measured from the SCB tests were shown in Fig. 2. Fracture energy referred to the shaded area surrounded by stress and strain, which can be calculated by Eq. (1). Additionally, the fracture energy index (FE Index) is defined as a parametric ratio of the fracture energy to the HMA tensile strength and tensile strain at peak failure load per unit crack length, which was calculated by Eq. (2) [35].
Z Fig. 1. Grading curve of asphalt mixture.
diameter and 62.5 mm in height, which was molded by Superpave gyratory compactor (SGC). The current HWTT specification requires a wheel speed of 52 cycles per minute up to 20,000 passes at 50 °C in a water bath. The HWTT test would stop if the rut depth reach to 12.5 mm or the wheeling number reaches to 20,000 cycles [31]. 2.4.2. Accelerated freezing-thawing test To evaluate the long term effect of moisture on the mechanical properties of asphalt mixture, accelerated freezing-thawing cycles (1, 3, 7, 12, 18 times) were applied to treat the asphalt mixture. A standard accelerated conditioning cycle was adjusted including 16 h at 18 °C in freezer and 8 h at 60 °C in a water bath, which is referred in the Modified Lottman Test (AASHTO T-283) [32]. The vacuum was also applied to make the specimen saturated before conditioning. 2.4.3. Dynamic uniaxial compression test Dynamic uniaxial compression test was conducted by UTM-100 to assess the permanent deformation of asphalt mixture according to ASTM D3497 [33]. Cylindrical specimens with a diameter of 150 mm and a height of 120 mm were prepared by SGC procedure. The compacted specimens were cored and cut to produce smaller ones with dimensions of U100 mm h100 mm. A haversine load with a pulse loading of 0.1 s and a rest time of 0.9 s was implemented to simulate the dynamic load. A loading of 0.5 MPa was applied under three temperatures of 40 °C, 50 °C, and 60 °C. The test continued until the accumulated microstrain attained 100,000 le.
GF ¼ 0
em
rðeÞde
FEIndex ¼ 1 105
ð1Þ
Gf et lcr rt
ð2Þ
Where GF is the fracture energy, lcr is the length traversed by the crack, rt is the tensile strength and et is the tensile strain at peak failure load. The repeated mode was established by loading the specimen at a frequency of 1 Hz. A test cycle included a 0.5 s of loading and a 0.5 s of rest. The testing temperature was 20 °C. The loading
Fig. 2. Parameters in the monotonic semi-circular bending test.
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waveform was the triangular wave with a peak load of 5.5 kN, which is 50% of the average failure load. 2.4.5. Four-point bending fatigue test Four-point bending fatigue test is one of the most representative methods to evaluate the fatigue performance of asphalt mixture according to AASHTO T321 [36]. All specimens were prepared into a beam with a size of 380 mm in length, 65 mm in width and 50 mm in height. A frequency of 1 Hz and strain levels of 400 le, 500 le, 600 le and 700 le were selected for this test. When the stiffness modulus decreases by 50%, corresponding load cycles is defined as the fatigue life. Generally, the fatigue performance of mixtures could be analyzed by the regression equation as below in Eq. (3):
log Nf ¼ log A n log e
ð3Þ
where Nf is the fatigue life of HMA mixture; e is the strain level; both A and n are regression coefficients. 3. Results and discussion 3.1. Asphalt mortar performance 3.1.1. Effect of BPW powder on physical property of asphalt mortar The effect of BPW powder on softening point and penetration of asphalt mortar was analyzed and the results were shown in Fig. 3. The limestone filler was replaced by BPW powder deliberately. While the content of BPW powder increased from 0% to 100%, the corresponding content of limestone filler decreased from 100% to 0%. As seen in Fig. 3, the penetration of asphalt mortar decreased with the increment of BPW powder while the softening point temperature correspondingly increased. Compared to the M100LF mortar, the penetration of M-100BPW mortar decreased by 67.7% from 42.1 dmm to 25.1 dmm while the softening point temperature increased by 20.1% from 52.4 °C to 63.4 °C. The results showed that BPW powder could improve the high temperature properties of asphalt mortar. The reason might due to the stiffening effect of BPW powder on asphalt mortar. Fig. 4 illustrated the effect of BPW powder on the viscosity of asphalt mortar. The rotation viscosity increased exponentially with the increment of BPW powder. The viscosity of M-100LF mortar were 1250 mPa s at 135 °C and 173 mPa s at 175 °C, respectively. When the limestone filler was replaced by BPW powder, the corresponding viscosity of M-100BPW mortar were 9750 mPa s and
Fig. 3. Effect of BPW powder on penetration and softening point temperature of asphalt mortar.
Fig. 4. Effect of BPW powder on rotation viscosity of asphalt mortar.
1295 mPa s, respectively. Compared to M-100LF mortar, the viscosity of M-100BPW mortar increased by 680% at 135 °C and 648% at 175 °C. It is known that filler dispersing in asphalt would improve the viscosity of asphalt mortar. However, the results in Figs. 3 and 4 indicated that there was a significant difference in the effect of limestone filler and BPW powder on physical properties of asphalt mortar. It might because that the BPW powder could transfer more free asphalt to structural asphalt due to their component and microstructure. Therefore, it is required to conduct further investigations on the effect mechanism of BPW powder on asphalt mortar.
3.1.2. Effect of BPW powder on rheological property of asphalt mortar Dynamic shear rheological (DSR) test was used to study the effect of BPW powder on the rheological properties of asphalt mortar. Complex modulus (G⁄) and phase angle (d) are the most important parameters to provide information about the rheological properties of asphalt binders during the shearing process. The curves of G⁄ and d at a temperature region ranging from 30 °C to 80 °C were shown in Fig. 5. It illustrated that BPW powder increased the complex modulus and reduced the phase angle of asphalt mortar. The phase angle implied the proportion of viscous and elastic properties of asphalt. Greater phase angle represents
Fig. 5. Effect of BPW powder on complex modulus and phase angle of asphalt mortar.
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asphalt mortar is more susceptible to permanent deformation. Therefore, it seems that BPW powder could improve the high temperature performance of asphalt mortar. Moreover, the differences in complex modulus and phase angle of asphalt mortar were more significant at high temperature as the content of BPW powder increased. Since asphalt mixture is susceptible to permanent deformation, rutting parameter (G⁄/sind) was adopted to evaluate the effect of BPW powder on high temperature performance of asphalt mortar and the results were shown in Fig. 6. Greater rutting parameter represents better anti-rutting property. As seen in Fig. 6, the rutting parameter of asphalt mortar increased with the addition of BPW powder in the whole temperature region. Moreover, the difference in rutting parameter gradually expanded as the temperature increased. For instance, the rutting parameters of M-100LF were 1006 MPa and 0.79 MPa at 30 °C and 80 °C, respectively, and the corresponding rutting parameters of M-100BPW were 1588 MPa and 1.38 MPa. It can be concluded that asphalt mortar with BPW powder showed better high temperature performance than the limestone filler mortar, which was in accordance with the softening point results in Fig. 3.
3.2. Asphalt mixture performance 3.2.1. Moisture damage resistance Since the HWTT test condition could simulate the combination of loading, water and temperature, the rutting depth and creeping rate were used to evaluate the anti-moisture property and high temperature stability of asphalt mixture. In HWTT test, lower rutting depth and lower creeping rate represents better anti-moisture and high temperature properties of asphalt mixture. Fig. 7 illustrated the HWTT rutting response curves of asphalt mixtures containing different composition of fillers. At the beginning period of the HWTT test, the rutting depth increased rapidly due to the second compaction of specimen. After several cycles, the change of rutting curves became steady. As seen in Fig. 7, the creeping rate of C-100BPW and C-100LF mixtures were 0.103 mm/cycle and 0.369 mm/cycle, respectively. The former was just one third of the latter. After 20,000 load passes, the rutting depth of asphalt mixture decreased with the increment of BPW powder. The rutting depth of C-100BPW mixture was the lowest among the four kinds of asphalt mixtures in this study. This implied that BPW powder
Fig. 6. Effect of BPW on rutting parameter of asphalt mortar.
Fig. 7. HWTT result of asphalt mixture with BPW powder and limestone filler.
could not only enhance the anti-moisture property, but also improve the high temperature performance of asphalt mixture. In this research, accelerated freezing-thawing test was employed to study the effect of moisture and temperature on mechanical properties of asphalt mixture in a long term and the results were shown in Figs. 8 and 9, respectively. Indirect tensile strength and tension strength ratio were adopted to evaluate the anti-moisture properties of asphalt mixture. Fig. 8 illustrated the evolution of indirect tensile strength of different asphalt mixtures after the accelerated freezing-thawing test. For the same freezingthawing cycles, the indirect tensile strength of asphalt mixture increased with the addition of BPW powder. For example, indirect tensile strength of C-100LF was 0.82 MPa after 1 cycle, while the corresponding result of C-100BPW was 1.25 MPa. It also implied that BPW powder could improve the anti-moisture property of asphalt mixture. For all the asphalt mixtures, it was noticeable that stripping and cracking occurred in the specimen after 12 cycles. This is because that phase change of water during the freezingthawing process lead to volume expansion of specimen and serious performance degradation of asphalt mixture. The effect of BPW powder on tensile strength ratio of asphalt mixture during the accelerated freezing-thawing process was analyzed and the results were shown in Fig. 9. After 1 cycle of
Fig. 8. Effect of BPW powder on indirect tensile strength of asphalt mixture during the accelerated freezing-thawing process.
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415
Fig. 9. Effect of BPW powder on tensile strength ratio of asphalt mixture during accelerated freezing-thawing process.
freezing-thawing, the tensile strength ratio of all the specimens were greater than the criterion of 75%. For the same freezingthawing cycles, the tensile strength ratio of asphalt mixture increased with the addition of BPW powder. This also implied that BPW powder could improve the anti-moisture property of asphalt mixture. It was interesting that there was no significant difference in the results of C-100LF, C-67LF-33BPW and C-33LF-67BPW mixtures, especially after 7 freezing-thawing cycles. When the limestone filler was replaced by 100% BPW powder, a noticeable increment of tensile strength ratio occurred for C-100BPW mixture. Therefore, the results of HWTT test and indirect tensile test after freezing thawing indicated that asphalt mixture with BPW powder has better anti-moisture property. This might because BPW powder helps to improve the bonding characteristic between asphalt and aggregate. 3.2.2. High-temperature stability Dynamic uniaxial compression test was employed to evaluate the high temperature performance of asphalt mixture and the results were shown in Fig. 10. For the same testing temperature, greater loading repetition cycles represents better high temperature property. As seen in Fig. 10, the loading repetition cycles of asphalt mixture increased with the increment of BPW powder. The terminate repetition numbers of C-100LF mixture at 40 °C, 50 °C, 60 °C were 1517 times, 389 times, and 119 times, respectively. And the corresponding results of C-100BPW mixture were 4680 times, 1126 times, and 323 times, which were increased by about 208.5%, 189.5%, and 171.4% compared with the results of C-100LF mixture. It indicated that BPW powder improved the anti-rutting property of asphalt mixture, which was in accordance with the results in Fig. 7. Since BPW powder could improve the viscosity and softening point temperature of asphalt mortar, the BPW powder mixture was less susceptible to permanent deformation than the LF mixture. Therefore, the BPW mixture exhibits better anti-rutting performance than that of LF mixture. 3.2.3. Cracking resistance Table 3 summarized the results of the monotonic SCB test. Generally, the tensile strength and tensile strain were widely accepted to assess the cracking resistance of the different materials. As shown in Table 3, the tensile strength and modulus of C-100LF were lower than that of the C-100BPW mixture, while the tensile strain of the former was greater than the latter at the test temperatures of 10 °C and 20 °C. This implied that the anti-cracking
Fig. 10. Dynamic creep stain of asphalt mixture with different fillers under different temperature. (a) 40 °C, (b) 50 °C, (c) 60 °C.
property of C-100LF mixture was superior to the C-100BPW mixture with aspect of fracture strain. It might be concluded that the addition of BPW powder might attenuate the cracking resistance of asphalt mixture. However, Fig. 2 showed that the tensile strain et and tensile modulus Et were determined once the tensile load reached to the peak point, i.e. rt. And the cracking period after peak loading was not taken into account. Therefore, it could not completely indicate the cracking resistance performance of asphalt mixture. The fracture energy and FE index were also adopted to evaluate the cracking resistance of asphalt mixture. From the fracture energy point of view, the whole energy produced by loading would
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Table 3 Effect of BPW powder on cracking resistance of asphalt mixture. Mixture type
Temperature/(°C)
Tensile strength/(MPa)
Tensile strain/(mm/mm)
Tensile modulus/(MPa)
Fracture energy/(J/m2)
FE-Index
C-100LF
10 20 10 20
5.21 4.87 6.21 5.79
0.0137 0.0145 0.0105 0.0119
403.10 335.59 591.8 486.47
1124 170 1241 489
4.44 114 3.09 172
C-100 BPW
be dissipated by cracking formation and propagation. Fracture energy values of the C-100LF and C-100BPW mixtures were relatively comparative at 10 °C. However, the fracture energy of C100BPW samples was 188% higher than that of C-100LF samples at 20 °C. The results of fracture energy indicated that BPW powder could improve the cracking resistance of asphalt mixture, which was different with the analysis on tensile strain and tensile modulus. Additionally, FE Index was proposed to simultaneously consider the tensile strain and fracture energy concept, which was calculated as Eq. (2). As showed in Table 3, it can be concluded that the addition of BPW improved the crack propagation resistance in medium temperature and reduced the mixture flexibility in low temperature at the same time. Furthermore, the repeated SCB test at 20 °C was also employed to evaluate the cracking propagation of asphalt mixtures and the results were shown in Fig. 11. In this test, greater terminal cycles represents better anti-cracking performance. The terminal cycles of asphalt mixture increased as the content of BPW powder increased. The terminal repetition number of C-100LF specimen at 20 °C was 2974 times. The corresponding results of asphalt mixture with 33%, 67%, and 100% BPW powder were 3151 times, 3438 times, and 3885 times, which were increased by about 6.0%, 15.6%, and 30.6%, respectively. The results of fracture energy also confirmed that the addition of BPW could improve the crack propagation resistance of asphalt mixture under medium temperature condition. 3.2.4. Fatigue property Fatigue of asphalt mixture is defined as the damage caused by repeated stresses and strains due to environmental condition and traffic loading. The bonding property of asphalt binder contributes largely to the fatigue performance of the asphalt mixture. The testing results of different asphalt mixtures were shown in Fig. 12. It was observed that the fit curve of C-100BPW samples moved
Fig. 12. Relationship between strain level and fatigue life of asphalt mixture.
upwards, compared with C-100LF samples. The fatigue life of asphalt mixture increased with the addition of BPW powder at the same strain level. Therefore, it could be concluded that BPW powder as mineral filler improved the fatigue life of asphalt mixture. Using the data collected from this investigation, Fig. 12 also illustrated a regression analysis of the influence of different mineral filler on fatigue parameters. Several studies pointed out that power law regression can be fitted well with the test data, which was shown as Eq. (3) [37,38]. Greater A value represents better fatigue property of asphalt mixture, while greater n value means the fatigue life of asphalt mixture is more sensitive to the microstrain level. According to the fitting analysis, the parameter A and n tend to increase as the content of BPW powder increased. It implied that BPW powder could improve the fatigue performance of asphalt mixture. With the increment of BPW powder, asphalt mixture became more sensitive to the strain level. At a relatively low strain level, the fatigue life of C-100BPW mixture was much higher than that of the C-100LF mixture. For instance, the fatigue life of C100BPW and C-100LF mixtures were 427,184 cycles and 123,590 cycles at 400 le microstrain level. While the strain increased, the enhancement effect of BPW powder on asphalt mixture became weakening. However, the C-100BPW specimen exhibited the best fatigue performance at each strain level of this test.
4. Conclusions
Fig. 11. Displacement development of asphalt mixtures with the number of load cycles.
In this research, the feasibility of utilizing brake pad waste as mineral filler in asphalt mixture was investigated. From the experimental evaluations carried out on some of the most important parameters affecting pavement performance, the following conclusions can be drawn.
X. Hu et al. / Construction and Building Materials 138 (2017) 410–417
(1) BPW powder could increase the softening point, rotation viscosity and decrease the penetration of asphalt mortar, which implied better high temperature stability compared with that of LF mortar. Rheological characteristics also proved that the BPW improves the elastic response of asphalt mortar at high temperature. (2) BPW mixtures could improve the anti-moisture damage and anti-rutting resistance, compared to the LF mixture. Accelerated freezing-thawing test results indicated that less moisture destruction occurred in BPW mixture. The permanent deformation of BPW mixture was less than the LF mixture, which was in accordance with DSR result of asphalt mortar. (3) BPW powder could improve the fatigue property of asphalt mixture, but correspondingly degrade the low temperature performance. Compared to LF mixture, the BPW mixture was more sensitive to the changes of strain levels. Cracking propagation resistance of BPW asphalt mixture at a medium temperature was better than that of LF asphalt mixture. However, BPW powder degraded the flexibility of asphalt mixture at low temperature. Due to the problems caused by cracking, the BPW powder was not recommended in cold regions. Asphalt mixture containing BPW filler showed satisfactory performance improvement. The successful utilization of brake pad waste as mineral filler in pavement construction can provide a more effective approach for sustainable resources, and mitigate the problems of solid wastes to the environment. In order to promote the application of BPW in pavement, it is recommended to focus on the feasibility of the coarse BPW particles substituting for mineral aggregate, as well as the method to improve the low temperature performance of BPW asphalt mixture. Further research can be conducted on the interaction mechanism of BPW asphalt mortar from microstructure point of view. Additional laboratory tests with more mixture type along with correlations and validation with field performance date are strongly recommended as well. Acknowledgments The authors acknowledge the support provided by the National Natural Science Foundation of China (No. 51278389), the Science Foundation of Wuhan Institute of Technology (No. 16QD23), and the Graduate Innovative Fund of Wuhan Institute of Technology (No. CX2015031). References [1] J.R. Laguna-Camacho, G. Juárez-Morales, C. Calderón-Ramón, V. VelázquezMartínez, I. Hernández-Romero, J.V. Méndez-Méndez, M. Vite-Torres, A study of the wear mechanisms of disk and shoe brake pads, Eng. Fail. Anal. 56 (2015) 348–359. [2] D. Kodjak, B. Sharpe, O. Delgado, Evolution of heavy-duty vehicle fuel efficiency policies in major markets, Mitig. Adapt. Strat. Glob. Change 20 (2015) 755–775. [3] A.V. Pavlov, S.P. Kudelnikova, A.N. Vicharev, On the corrosion resistance of half-metallic composite brake pads for railroad cars, J. Frict. Wear 36 (2015) 123–126. [4] M.C. Lagel, L. Hai, A. Pizzi, M.C. Basso, L. Delmotte, S. Abdall, A. Zahed, F.M. AlMarzouki, Automotive brake pads made with a bioresin matrix, Ind. Crops Prod. 85 (2016) 372–381. [5] G.Y. Bian, H.Z. Wu, Friction and surface fracture of a silicon carbide ceramic brake disc tested against a steel pad, J. Eur. Ceram. Soc. 35 (2015) 3797–3807. [6] M. Eriksson, F. Bergman, S. Jacobson, On the nature of tribological contact in automotive brakes, Wear 252 (2002) 25–36. [7] A.K. Ilanko, S. Vijayaraghavan, Wear behavior of asbestos-free eco-friendly composites for automobile brake materials, Friction 4 (2016) 144–152.
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