The application of baghouse fines in Taiwan

The application of baghouse fines in Taiwan

Resources, Conservation and Recycling 46 (2006) 281–301 The application of baghouse fines in Taiwan Deng-Fong Lin a,∗ , Jyh-Dong Lin b , Shun-Hsing C...

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Resources, Conservation and Recycling 46 (2006) 281–301

The application of baghouse fines in Taiwan Deng-Fong Lin a,∗ , Jyh-Dong Lin b , Shun-Hsing Chen c a

Department of Civil and Ecological Engineering, I-Shou University, 1 Section 1, Hsueh-Cheng Road, Ta-Hsu Hsiang, Kaohsiung County 84008, Taiwan, ROC b Department of Civil Engineering, National Central University, No. 300, Jhongda Road, Jhongli City, Taoyuan County 32001, Taiwan, ROC c Northern District Engineer’s Office, Taiwan Area National Freeway Bureau, Taiwan, ROC Received 29 June 2004; accepted 4 August 2005 Available online 23 September 2005

Abstract Strict environmental regulations in Taiwan require baghouse fines (BHFs) to be collected during hot mix asphalt production. In an attempt to utilize this byproduct, baghouse fines have been incorporated into asphalt concrete mixtures, which were applied to county roads and other light traffic roads despite a lack of research. This study examines 14 types of fines, including 9 baghouse fines, 2 mineral fillers, fly ash, cement, and lime. The baghouse fines that were collected from asphalt plants in different regions represent generic fine types from various aggregate sources. Comprehensive laboratory tests were performed to determine the impact of different types and quantities of fines on mechanical properties and moisture resistance of the asphalt concrete mixtures. Resulting data indicates that the amount of stiffening is not uniquely related to fines and that gradation and miner properties alone cannot explain the stiffening effect of fines. Increased stiffness, due to the addition of the filler, is represented by an increase in the softening point, in viscosity, and in complex shear modulus (G* ), as well as a decrease in penetration. The stiffening effects of baghouse fines vary greatly. Performance in terms of stiffness and resistance to moisture related damage for asphalt binders with fines (baghouse fines, lime, cement, mineral filler) was better than AC20 asphalt without fines. The best performer among baghouse fines was A4 (Group 1), with the least amount of SiO2 . The authors believe that Group 1 baghouse fines, including A1, A2, A3, A4 and A9, are superior to mineral filler and can be used on highways or expressways with heavy traffic. Groups 2 (A6) and Group 3 (A5, A7, A8)



Corresponding author. Tel.: +886 7 6577711x3320; fax: +886 7 6577461. E-mail address: [email protected] (D.-F. Lin).

0921-3449/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2005.08.002

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baghouse fines should be used on light traffic roads, such as county roads. To increase moisture resistance, the addition of 1–2% lime should be considered. © 2005 Elsevier B.V. All rights reserved. Keywords: Baghouse; Asphalt cement; Viscosity

1. Introduction Environmental pollution has become a major concern in many countries, particularly those undergoing rapid industrial growth. Many countries are imposing stringent regulations on industrial manufacturing facilities to control discharge of pollutants in the atmosphere. For example, the requirement set by Environmental Protection Agency (2004) for SO3 emission in Taiwan has been tighten from a requirement of 2000 ppm in 1973 to 300 ppm in 1999. Many industries are required to install additional pollution control systems to comply with these new regulations. During the production of hot mix asphalt, fines are generated from the heating of mineral aggregate. Many new hot mix asphalt plants are equipped with a baghouse for dust collection, whereas older plants have been retrofitted with a baghouse to comply with the regulations. Before the introduction of the baghouse, the finer fraction of the dust was wasted to the wet scrubber or vented into the atmosphere. Baghouses consist of several rows or compartments of fabric filters that collect the fines/dusts during the operation of a hot mix asphalt plant. To avoid accumulation of a waste product and to help offset the production cost; many asphalt plants are using the collected baghouse fines either as a mineral filler or a fine aggregate substitute in asphalt paving mixture. Since these fines are derived from naturally occurring aggregates (crushed stone or sand and gravel), their properties are generally similar to those of commonly used mineral fillers. However, this practice has led to considerable controversy. The use of baghouse fine (BHF)/dust has been connected to different researchers with poor compaction, bleeding, flushing and tender mixes (Hesp et al., 2001; Kandahl, 1981; Anderson, 1987a, 1987b). Some research claims that the baghouse fines might be detrimental to the quality of the asphalt mixture. For example, Dukatz and Anderson (1980) reported that some baghouse fines have a considerable stiffening effect on the asphalt and makes the mixture brittle and/or difficult to compact in the field. Eick and Shook (1978) illustrated that some baghouse fines make the asphalt concrete mixture susceptible to moisture-induced damage such as stripping (separation of asphalt binder and aggregate). Crawford (1987) documented his concern that the introduction of baghouse fines without a proper check on the design properties of the mix could possibly be a cause of tender mixes. Some of states in United States have considered eliminating the use of the baghouse fine because of this problem. Ironically, the commercial mineral fillers purchased by these agencies may well be baghouse fines collected in another process or at another hot mix asphalt plant (Anderson, 1987a, 1987b). Asphalt concrete mixtures are composed of mineral aggregates bond together by asphalt binder. The mineral aggregates are composed of different sizes ranging from coarse to fine. The asphalt binder acts as a film that coats the mineral aggregates. When the diameter of the filler is smaller than the thickness of the film, the filler becomes embedded in the films, thereby extending the asphalt binder. In contrast, if the diameter of the filler is greater than the thickness of the film, the filler will increase the voids in the mineral aggregates and the

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demand of the mixture for asphalt cement. Kandahl (1981) concluded that fines play a dual role in asphalt mixture. First, fines are part of the mineral aggregate as they fill the interstices and provide contact points among larger aggregates. Second, when fines are mixed with asphalt, they cement larger aggregates together. Thus, the addition of baghouse fines to an asphalt concrete mixture may reduce asphalt demand and result in economic savings. However, fines can also act as a stiffener when added to asphalt. The amount of stiffening can affect the compactibility and stiffness of the mixture. Anderson (1987b) and Kim et al. (2003) reported that the role of the filler was more than void filling. Variations in the stiffening effects of baghouse fines are not fully explained by either the fineness or the gradation of a particular dust source. It is necessary to identify the detrimental baghouse fines in terms of quality and quantity. Thus, the quality of the asphalt mixture will not suffer and the pavement durability can be insured. In contrast to the considerable amount of research done on fillers over the past decades, little is known about the effects of fines on asphalt mixtures. The impact of fines on the compactibility of asphalt concrete mixtures, the stiffening effect of baghouse fines on pavement fatigue, and the dynamics of mechanical properties of asphalt paving mixtures when fines are added, have yet to be understood. Almost all hot mix asphalt plants in Taiwan are batch plants with a total annual production of approximately 1.3 million metric tons of asphalt paving material. Simplified diagrams of batch mix plant operations are presented in Fig. 1. It is estimated that approximately 3900 metric tons of baghouse fines are generated annually in the highly populated country of Taiwan (23 million people in an island of 30,000 m2 ). The baghouse fines are only permitted for use on county roads or other low volume roads. Commercially available mineral fillers can

Fig. 1. Typical batch plant type hot mix asphalt production facility.

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only be used for highways and expressways. As high-quality mineral filler become scarce, alternatives must be found. By using recyclable materials that would otherwise have been taken to landfills, two major expenses have been reduced. Due to the significant expenditures on pavement construction ($14 billion was budgeted for roadway work in 2001), a research project was funded to study the applicability of baghouse fines on roadways.

2. Objectives The objectives of this study include the following. 1. To determine the gradation and physical properties of fines (baghouse fine, mineral filler, fly ash, lime, and cement). 2. To determine the effect of the type and quantity of fines on the mechanical properties of fines-asphalt binders. 3. To determine the effects of fines on mix resistance of moisture-induced damage. 4. To determine the effects of fines on the mechanical properties of asphalt concrete mixtures (such as stiffness/modulus, etc.). This study examines 14 different fines including nine baghouse fines from different regions, one fly ash, one cement, two mineral fillers (MF), and one lime. The nine baghouse fines were collected from asphalt plants in different regions, as shown in Fig. 2. These samples represent different generic types and aggregate sources. For comparison, cement, lime, fly ash and commercially available mineral filler were included in the test matrix.

3. Properties of the fines evaluated 3.1. Particle size Only the minus No. 200 sieve (0.075 mm) fractions of the fines were evaluated in this study. Particle size distributions in Table 1 were determined by the hydrometer analysis of fines using American Association of State Highway and Transportation Officials (AASHTO) T88 specification. Significant variations in gradation were observed for the fine samples from the various plants, as shown in Table 1. The standard specification of mineral fillers for bituminous paving mixtures (ASTM D 242-85) has the following grading requirements: 100% passing 600 ␮m, 95–100% passing 300 ␮m, and 75–100% passing 75 ␮m. AASHTO M17-83 (1986) specification has similar requirements for passing 600 and 300 ␮m but it requires 70–100% to pass 75 ␮m. According to Table 1, A4, A6 and B4 materials would not pass both AASHTO M17-83 and ASTM D 242-85 grading requirements. The NAPA (1980) report demonstrated that plants with identical equipment and operating conditions, but that utilize different aggregates, produce baghouse fines of different quantity and size distribution. Anderson et al. (1982) reported that fineness alone is not sufficient for defining how a fine will behave in an asphalt mixture. They concluded that different fillers and fines reacted differently with different asphalts. Anderson (1987a) illustrated that the size of the

%Pass

A1 BHF

A2 BHF

A3 BHF

A4 BHF

A5 BHF

A6 BHF

A7 BHF

A8 BHF

A9 BHF

B1 Fly ash

B2 C

B3 MF

B4 MF

B5 Lime

75 ␮m 50 ␮m 30 ␮m 20 ␮m 10 ␮m 5 ␮m 3 ␮m 1 ␮m cm2 /g pH SG

80 53 43 18 10 7 3 1 8031 8.26 2.56

100 98 95 90 80 73 48 23 9212 9.32 2.64

78 77 76 75 52 34 27 14 10410 8.22 2.53

56 53 44 27 21 14 12 10 9263 9.66 2.55

96 88 75 46 16 15 13 11 4953 10.65 2.55

49 45 36 29 23 20 17 15 5533 8.01 2.67

79 67 58 46 34 27 21 18 5886 10.11 2.58

98 97 95 86 70 52 40 16 6448 8.16 2.62

100 98 97 95 77 51 36 19 8167 9.32 2.5

91 89 31 13 12 11 10 10 2827 11.64 2.19

– – – – – – – – 3177 12.01 3.15

77 59 52 40 29 24 22 14 1807 10.04 2.75

58 43 33 27 10 9 8 7 2335 10.31 2.72

100 98 42 10 8 8 7 7 2717 12.11 2.26

BHF = bag house fine; C = cement; MF = miner filler; SG = specific gravity.

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Table 1 Physical properties of fines

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Fig. 2. Sources of the fines in Taiwan island.

fine affected rheological behavior but the source of the asphalt and the mineralogy of the fine also had a significant effect on rheologocal behavior. In order to properly measure the stiffening effect of a mineral filler, it is necessary to test the specific asphalt and mineral filler in a mixture so that asphalt–mineral interaction can be accounted for. Anderson and Tarris (1983) studied the effect of physico-chemical properties of the filler on performance. They

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Table 2 Mineral compositions

A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3 B4 B5

SiO2

A12 O3

Fe2 O3

CaO

MgO

SO3

Na2 O

K2 O

57.38 53.88 56.2 56.95 64.5 79.42 68.28 60.48 57.46 50.52 20.32 2.02 1.02 1.34

19.04 28.7 24.65 16.87 15.01 5.58 14.27 16.93 20.82 27.28 5.39 0.61 0.24 0.52

2.36 7.85 6.5 6.98 5.89 3.27 5.23 7.42 6.89 6.32 4.51 0.25 0.1 0.24

1.79 1.43 1.57 2.01 1.91 0.4 2.25 1.77 1.4 6.02 62.78 41.42 53.79 67.11

1.6 1.76 1.42 1.55 1.48 0.77 1.54 1.42 2.07 1.68 1.73 11.01 1.04 4.21

0.27 0.45 0.45 0 0 0 0.11 0.93 0.54 0 1.89 0.1 0.05 0.29

1.31 1.32 1.23 1.17 1.50 0.81 1.30 1.31 1.11 0.5 0.14 0.03 0 0.1

2.74 3.49 3.18 3.33 2.59 1.61 2.23 2.83 3.52 1.4 0.54 0.09 0.03 0.02

also concluded that the mineral filler stiffens asphalt and that stiffening varies significantly between different fillers. 3.2. Surface area, pH, and mineral composition Surface area (cm2 /gm) was determined using the Blaine air permeability apparatus following the ASTM C204 specification. Of the 14 samples tested, baghouse fines generally have the highest surface area and mineral filler had the lowest, as shown in Table 1. In addition, pH values of fines were determined after mixing the fines with an equal weight of water (pH 7) devoid of dissolved ions (Table 1). Mineral compositions of the baghouse fines, fly ash, cement, mineral fillers, and lime were obtained by X-ray diffraction (Table 2). Baghouse fines A1–A4 were obtained from the southern region. The SiO2 contents range from 53.8–57.4% and CaO contents range from 1.4–1.8%. As shown in Table 2, the mineral compositions (SiO2 , CaO, MgO, K2 O, etc) for A1 to A4 are similar because they were from the same source (Kaping river). This research reconfirms the study done by Kandahl (1981) that the chemical properties of baghouse dust can be expected to reflect the properties of the fed aggregate. A6 was from the Tato River with the SiO2 and CaO contents of 79.42 and 0.4%, respectively. As shown in Table 2, the mineral compositions of the baghouse fines can be divided into 3 major groups; Group 1 includes A1, A2, A3, A4 and A9, Group 2 contains A6, and Group 3 includes A5, A7 and A8.

4. Properties of asphalt–mineral filler mixture 4.1. Penetration and viscosity Penetration and viscosity have been used for years to depict the physical properties of asphalt binders. The terms asphalt binder and asphalt mineral filler mixture are used interchangeable in this paper. Penetration tests are a measure of hardness and viscosity

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that represent the fluid’s resistance to flow. Both properties were employed to study the stiffening effects of adding fines to asphalt mixture. Based on ASTM D3515 requirements for asphalt concrete mixture with a nominal size of 12.7 mm, the mineral filler passing 75 ␮m consists of approximately 2–10% by weight. Note that the asphalt concrete mixture includes the coarse aggregate and asphalt mineral filler mixture. Three different levels of fines (3, 6, and 9%) were selected to mix with 5.5% of asphalt cement to study the effects of different amounts (%) of fines on asphalt mineral filler mixture. The ratio by weight between fines and asphalt cement are 0.545, 1.09, and 1.635. In terms of volume, the ratios are approximately between 0.1 and 0.76, depending on the type of fines. Table 3 presents the volume ratios between the fines and asphalt cement for the materials investigated. The asphalt cement adopted in this study is a commonly used AC20 (60/70 grade by penetration). Anderson (1987b) recommended that the fines/asphalt ratio should be closely monitored during the mix design to limit the bulk volume of fines to less than 50% (volume ratios between the fines and asphalt cement). The particle size distribution of the baghouse fines should be well graded, with some of the dust finer than 0.010–0.020 mm. The percent of free asphalt should be kept at approximately 40% since excessive amounts of baghouse fines as filler are likely to result in an asphalt mix that will be difficult to compact. The fines/asphalt ratio is a better control criterion than seeking an upper limit or the percentage of baghouse fines in the mix. As indicated in NAPA (1980) report, fillers with void volume greater than 60% produce mixture that can exhibit plastic or brittle behavior. Fig. 3 indicates that an increase in the fines/asphalt ratio results in an almost linear decrease in the penetration value of the resultant asphalt binder mixture. The viscosity and softening point of fines/asphalt blends increase or stiffen as the fines/asphalt ratio is increased, as shown in Figs. 4 and 5, respectively. The penetration and viscosity tests were conducted at 25 ◦ C and 60 ◦ C. It was observed from Figs. 3–5 that lime (B5) had the most

Fig. 3. Penetration values for asphalt binders with different percentage of fines at 25 ◦ C.

3%-all (0.545) 6%-all (1.09) 9%-all (1.635)

A1 BHF

A2 BHF

A3 BHF

A4 BHF

A5 BHF

A6 BHF

A7 BHF

A8 BHF

A9 BHF

B1 Fly ash

B2 C

B3 MF

B4 MF

B5 Lime

66.1 42.2 24.6

66.5 43.0 25.3

63.5 37.8 18.9

60.0 32.1 11.5

67.72 45.1 28.5

75.56 58.1 45.1

70.09 50.4 35.2

64.7 39.8 21.1

59.1 30.7 9.69

65.1 42.0 25.3

68.2 44.6 26.4

76.26 59.26 46.43

75.1 57.3 43.9

49.1 14.78 –

Note: 3, 6 and 9% are the quantities in asphalt concrete mixture by weight. 0.545, 1.09 and 1.635 are the ratios by weight between fines and asphalt cement.

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Table 3 Volume ratios (%) between fines and asphalt cement

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Fig. 4. Viscosity values for asphalt binders with different percentage of fines at 60 ◦ C.

stiffening effects, the baghouse fines A4 ranked second and the miller filler (B4) had the least. Fig. 6 clearly shows the stiffening effect of fines as compared to the AC20 asphalt at different shear stress levels. Complex shear modulus for AC20 asphalt is the lowest compared to all other mixtures with an addition of 3% fines by weight. Test conditions were kept at 60 ◦ C and 1 Hz. In view of Fig. 6, asphalt mixture with lime (B5) had the highest complex shear moduli (G* ) at all shear stress levels. Also, asphalt mixture with lime (B5) had the steepest slope, indicating that the asphalt mixture with lime was sensitive to the test condition. The asphalt mixtures with mineral filler (B4) and cement (B2) had the least stiffening effect. The asphalt mixtures with fines (including baghouse fines, mineral

Fig. 5. Softening point for asphalt binders with different percentage of fines.

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Fig. 6. Comparisons of complex shear modulus for various of fines (tests conducted at 60 ◦ C and 1 Hz).

filler, cement, etc) had significantly higher (>20%) complex shear modulus than that for AC20. Fig. 7 illustrates the comparison between AC20 asphalt and asphalt mixtures with 3% fines at different test frequencies (up to 10 Hz). The test conditions were set at 60 ◦ C and for a shear stress of 1000 Pa. It can be observed from Fig. 7 that the stiffening effects were relatively small at low-test frequency, but were large at high test frequency. Similar to previous

Fig. 7. Comparisons of complex shear modulus for various of fines (tests conducted at 60 ◦ C and shear stress of 1000 Pa).

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observations, AC20 asphalt had the least complex shear moduli at all test frequencies. The least effective fines in terms of complex shear modulus was mineral filler (B4). 4.2. Resistance to moisture damage Taiwan is located in a region of high precipitation. Therefore the mixture’s resistance to moisture damage directly impacts the life of the paving mixture. Using a high percentage of fines or misusing fines causes adhesion to become even more critical as the paving mixture is exposed to water and water vapor. Several studies conducted in Germany, Japan and the United States have revealed that an asphalt paving mixture can be vulnerable to water when certain fines are used as mineral filler (Kandahl, 1981; Eick and Shook, 1978; Harris and Stuart, 1995). In this study, the mix resistance to moisture damage was evaluated by examining the stripping area through digitized images. Seven fines including baghouse fines A4, A6 and A7, fly ash (B1), cement (B2), mineral filler (B4) and lime B5 were selected. A4, A6, and A7 were selected because they represent the three major groups of the baghouse fines as aforementioned. Three different levels of fines (3, 6 and 9%) were mixed with 5.5% asphalt cement to form the asphalt binder. Approximately, 0.26–0.30 g of asphalt binders were evenly spread on micro slide glass and then submerged in water at a high temperature (80 ◦ C) in order to accelerate stripping. Pictures were taken at three different time periods as shown in Fig. 8. Micro slide glass was used as a function of the aggregate to facilitate greater consistency and uniformity. The aggregate is composed of a high SiO2 content similar to the micro slide glass. As illustrated in Fig. 8, the white area represents the separation of asphalt binders and micro slide glass in water of high temperature (80 ◦ C). This separation is indicative of stripping or moisture related damages. Images were digitized to quantify the stripped area, as best as can be represented by gray scale from 0 (black) to 255 (white). For an intensity image, the image data can be stored in a single two-dimensional matrix, with each element of the matrix corresponding to one image pixel. The size of the matrix is the size of the image by pixel. Gray intensity of 100 was used as the threshold. Pixels were counted as a stripped area when the gray intensity exceeded 100. The ability of the asphalt binder to resist moisture resulted in damage of 3% filler content by weight (Fig. 9). At 3% filler content, cement (B2) and lime (B5) were best able to resist stripping, and the performance of mineral filler (B4) and baghouse fines A6 and A7 were in the same range. The best performer among baghouse fines was A4 with the least amount of SiO2 . Similarly, Fig. 10 shows the filler content at 6%. Asphalt binder with lime (B5) and mineral filler (B4) were the best and the worst performers, respectively. As illustrated in Fig. 10, even though asphalt binder with mineral filler (B5) was the worst performer, it performed slightly better than the pure AC20 binder. A comparison of Figs. 7 and 10 shows that the asphalt binder with baghouse fines A6 and A7 performed better than mineral filler B4 in terms of stiffness, even though it’s ability to resist moisture is lower than mineral filler B4. Fig. 11 uses baghouse fines sample A6 to illustrate the relationship between percent fines and stripping. The amount of stripping decreased when fines content is increased. At a fine content of 9%, stripping is no longer a concern. As seen in Fig. 11, even at 3% fines, the ability to resist stripping is higher than the pure AC20 (0%) without filler.

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Fig. 8. Photos showing stripping areas at three different submerging times.

5. Properties of asphalt concrete mixture It was found that free asphalt decreased to zero when the lime content was 9% or when the fine and asphalt cement weight ratio was at 1.635 (refer to Table 3). Fig. 12 illustrates the definition of free asphalt as was first defined by Rigden (1954). When asphalt is added to the fine, the asphalt first fills the voids. Asphalt within these voids is called fixed asphalt because it is fixed within the void structure. Asphalt in excess of the fixed asphalt is called free asphalt because it is free to lubricate the larger particles. The free asphalt pushes the particles apart, lubricating the fine/asphalt mixture and thereby enhancing its fluidity. It is the ratio or percentage of the volume of free asphalt compared to the total asphalt

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Fig. 9. Comparison of stripping areas for asphalt binders with different fines (at 3% fine contents).

content that has proven significant in predicting the stiffness of the mastic. Therefore, the mineral filler contents were changed to 2, 3.5 and 5% during the preparation of asphalt concrete mixture. The particle size distributions for asphalt concrete mixture are presented in Table 4. Three different asphalt cement contents were selected: 5.8, 5.3 and 4.8%. The combination produced mineral filler and asphalt cement weight ratios of 0.34, 0.66 and 1.04, respectively. By volume, the ratios between mineral filler and asphalt cement were approximately 0.12, 0.24 and 0.36. The asphalt concrete specimens were prepared following

Fig. 10. Comparison of stripping areas for asphalt binders with different fines (at 6% fine contents).

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Fig. 11. Effects of fine contents on stripping for baghouse fine A6 (0% means pure AC20).

ASTM D3387 requirements. The asphalt concrete specimens were tested at 10 ◦ C and 25 ◦ C with 5 different frequencies of 0.01, 0.025, 0.1, 0.25 and 1 Hz to determine the dynamic modulus. Following are observations made from the experiments: (1) In all cases, the higher test frequency yields higher dynamic modulus. (2) Depending on the source of fines, the higher mineral filler contents may not necessary yield higher dynamic modulus. For example, the dynamic modulus for asphalt concrete mixture with lime (B5) decreased with increasing lime contents. In contrast, the dynamic modulus for cement (B2) increased with increasing cement content, as shown in Fig. 13.

Fig. 12. Schematic illustrating fixed and free asphalt.

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Fig. 13. Dynamic modulus for asphalt concrete mixtures with different percentage of fines at 10 ◦ C.

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Table 4 Particles size distribution for asphalt concrete mixture Sieve-%passing

ASTM D3515

Fines-2% by weight

Fines-3.5% by weight

Fines-5% by weight

3/4 1/2 3/8 #4 #8 #16 #30 #50 #100 #200

100 90–100

100 93 84 50 30 21 15 9 5 2

100 93 84 50.5 31 22.5 16.5 10.5 6.6 3.5

100 93 84 51 32 24 18 12 8 5

44–74 28–58

5–21 2–10

In view of Fig. 13, some fines had the highest modulus values at fine content of 5% while others performed best at fine content of 3.5%. The behavior changes slightly when the test temperatures changed to 25 ◦ C, as reflected in Fig. 14. (3) For a mineral filler content of 3.5% and a given test frequency, asphalt concrete mixture with lime (B5) yielded the highest dynamic modulus, the baghouse fine (A4) rank the second and the miller filler (B4) last. The ranking yielded a similar trend as that for asphalt–mineral filler mixtures that have no coarse aggregates. Dukatz and Anderson (1980) also found that the stiffness of the asphalt concrete samples was significantly affected by the stiffness of the asphalt–mineral filler mixture. Both the fineness of the filler as well as the source of the asphalt and filler affected the creep compliance. The stiffer filler asphalt mixtures produced stiffer asphalt concrete mixtures.

6. Discussion Strict air pollution control codes and the regulation of emission of particles into the atmosphere result in the increased use of dust collection systems. This research was initiated in high-priority areas to study the replacement of mineral fillers with baghouse fines in pavement construction. It is well known that the success of a recycling program is strongly dependent upon the marketability of the materials from the waste stream for recycling. Cost and performance are the two most important factors in the decision making process. Cost saving in this case can be easily realized as baghouse fines are derived from naturally occurring aggregates, their properties are generally similar to those of commonly used mineral fillers. Although baghouse fines have been successfully reintroduced to hot mix asphalt mixtures, some failures have occurred due to poor compaction, bleeding, flushing and tender mixes. For a given volume of mineral filler, “improper” mineral fillers either stiffen asphalt binders too much (which may lead to poor workability, low temperature cracking, or fatigue cracking) or do not stiffen asphalt binders enough (which may lead to shoving or bleeding). Performance related tests can indicate whether a problem exists with the use of a trial filler in a mix; however, determining physical and empirical properties of the filler can lead to an understanding of the mechanisms governing the contributions of the filler to the overall performance of the mix.

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Fig. 14. Dynamic modulus for asphalt concrete mixtures with different percentage of fines at 25 ◦ C.

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Extensive tests were conducted to evaluate the type and quantity of fines on asphalt binder and asphalt concrete mixture performance. Viscosity, softening point, penetration complex shear modulus, and dynamic modulus have been commonly used and were adopted in this study as performance indicators. Currently, baghouse fines are only allowed in county or low volume roads. Commercially available mineral fillers (e.g. B4) can only be used in highways or expressways. Test results indicated that baghouse fines A4 are superior to mineral filler B4 in terms of stiffness and the ability to resist moisture related damage. Thus, baghouse fines A4 should be recommended for use in highways or expressways that carry high volume traffic. A4 represents Group 1 including A1, A2, A3, and A9, all of which are believed to have similar properties. Asphalt binder with baghouse fines A6 and A7 performed better than mineral filler B4 in terms of stiffness, but the ability to resist moisture related damage is lower than mineral filler B4. Also, the ability to resist moisture related damage decreases with increasing fine content. Results show that the fine content for A6 and A7 should not exceed 5% as the ability to resist moisture related damage decreases when the fine content is above 6%. Asphalt binder with baghouse fines A6 and A7 performed better than pure AC20 (without fine), thus they can be used in county or low volume roads. To strength the ability to resist moisture related damage, 1–2% of lime addition should be considered. A7 represents Group 3 and also includes A5, A8. A6 is Group 2 by itself. As indicated above, properties within the same group are expected to be similar. It was found that when the fine contents increased to 9%, there is no significant increase on ability to resist moisture related damage. At such high fine content, the surface area increases and it requires higher asphalt content that leads to higher cost. Hence, for cost and performance consideration, the optimum fine content for Groups 2 and 3 was found to be 5%. Tayebali et al. (2003) conducted extensive indirect-tensile-strength tests on mixes with baghouse fines and found significant improvement. Although crack resistant tests were not performed in this study, authors believe with inclusion of baghouse fines or mineral filler the asphalt concrete’s fatigue life will be improved. The speculation is further justified by increasing G* observed in this study. It has been known that the complex shear modulus (G* ) is closely related to the fatigue life.

7. Conclusion With strict environmental requirements to reduce air pollution, baghouse fines are collected during hot-mix asphalt production. Before the introduction of the baghouse, the finer fraction of the dust was wasted to the wet scrubber or vented to the atmosphere. Most asphalt producers try to recycle as much as possible the baghouse fines back into their own paving mixes. The environmental and economical impacts to recycle the baghouse fines as mineral fillers is expected to be significant. Observations and conclusions are as follows: • Baghouse fines vary considerably from plant to plant. Baghouse fines can have a coarseness that exceeds the limit generally accepted for mineral filler (ASTM D 242-85 and AASHTO M17-83) as two baghouse fines have less than 70% passability through a seive size of 75 ␮m.

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• The data collected as part of this study indicates that the amount of stiffening is not uniquely related to the fineness and that gradation and miner properties alone cannot explain the stiffening effect of fines. • The increase in stiffness, due to the addition of the filler, is represented by an increase in softening point, a decrease in penetration, an increase in viscosity and an increase in complex shear modulus (G* ). The stiffening effects of baghouse fines vary greatly. • Performance in terms of stiffness and resistance to moisture related damage for asphalt binder with fines (baghouse fines, lime, cement, mineral filler) was better than the AC20 asphalt without fines. • The best performer among baghouse fines was A4 (Group 1), with the least amount of SiO2 . The authors believe that Group 1 baghouse fines, (A1, A2, A3, A4 and A9) are superior to the mineral filler B4 and can be used in mixtures for high volume roadways such as highways and expressways. • Groups 2 (A6) and Group 3 (A5, A7, A8) baghouse fines should be used in mixtures for county roads and other low volume roads. To increase moisture resistance, the addition of 1–2% of lime should be considered. An optimum content of 5% is recommended.

Acknowledgements The authors wish to thank Prof. E.C. Ting, Prof. H. L. Luo, and Dr. D.H. Chen for their consultants and discussions regarding this research.

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