The application of baghouse fines in Taiwan

Deng-Fong Lina,∗, Jyh-Dong Linb, Shun-Hsing Chenc

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 NationalFreeway Bureau, Taiwan, ROC

Abstract

Strict environmental regulations in Taiwan require baghouse fines (BHFs) to be collected during hotmix asphalt production. In an attempt to utilize this byproduct, baghouse fines have been incorporatedinto asphalt concrete mixtures, which were applied to county roads and other light traffic roads despitea 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 differentregions represent generic fine types from various aggregate sources. Comprehensive laboratory testswere performed to determine the impact of different types and quantities of fines on mechanicalproperties and moisture resistance of the asphalt concrete mixtures. Resulting data indicates that theamount of stiffening is not uniquely related to fines and that gradation and miner properties alonecannot explain the stiffening effect of fines. Increased stiffness, due to the addition of the filler, isrepresented by an increase in the softening point, in viscosity, and in complex shear modulus (G* ), aswell as a decrease in penetration. The stiffening effects of baghouse fines vary greatly. Performance interms of stiffness and resistance to moisture related damage for asphalt binders with fines (baghousefines, lime, cement, mineral filler) was better than AC20 asphalt without fines. The best performeramong baghouse fines was A4 (Group 1), with the least amount of SiO2. The authors believe thatGroup 1 baghouse fines, including A1, A2, A3, A4 and A9, are superior to mineral filler and canbe 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: dflin@isu.edu.tw (D.-F. Lin).

baghouse fines should be used on light traffic roads, such as county roads. To increase moistureresistance, the addition of 1–2% lime should be considered.

Keywords: Baghouse; Asphalt cement; Viscosity

1. Introduction

Environmental pollution has become a major concern in many countries, particularlythose undergoing rapid industrial growth. Many countries are imposing stringent regulationson industrial manufacturing facilities to control discharge of pollutants in the atmosphere.For example, the requirement set byEnvironmental Protection Agency (2004)for SO3emission in Taiwan has been tighten from a requirement of 2000 ppm in 1973 to 300 ppm in1999. Many industries are required to install additional pollution control systems to complywith these new regulations. During the production of hot mix asphalt, fines are generatedfrom the heating of mineral aggregate. Many new hot mix asphalt plants are equipped witha baghouse for dust collection, whereas older plants have been retrofitted with a baghouseto comply with the regulations. Before the introduction of the baghouse, the finer fractionof 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 thefines/dusts during the operation of a hot mix asphalt plant. To avoid accumulation of a wasteproduct and to help offset the production cost; many asphalt plants are using the collectedbaghouse fines either as a mineral filler or a fine aggregate substitute in asphalt pavingmixture. Since these fines are derived from naturally occurring aggregates (crushed stoneor sand and gravel), their properties are generally similar to those of commonly used mineralfillers. 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 theasphalt mixture. For example,Dukatz and Anderson (1980)reported that some baghousefines have a considerable stiffening effect on the asphalt and makes the mixture brittle and/ordifficult to compact in the field.Eick and Shook (1978)illustrated that some baghouse finesmake the asphalt concrete mixture susceptible to moisture-induced damage such as stripping(separation of asphalt binder and aggregate).Crawford (1987)documented his concern thatthe introduction of baghouse fines without a proper check on the design properties of the mixcould possibly be a cause of tender mixes. Some of states in United States have consideredeliminating the use of the baghouse fine because of this problem. Ironically, the commercialmineral fillers purchased by these agencies may well be baghouse fines collected in anotherprocess or at another hot mix asphalt plant (Anderson, 1987a, 1987b).

Asphalt concrete mixtures are composed of mineral aggregates bond together by asphaltbinder. 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 ofthe 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 thanthe thickness of the film, the filler will increase the voids in the mineral aggregates and the

demand of the mixture for asphalt cement.Kandahl (1981)concluded that fines play a dualrole in asphalt mixture. First, fines are part of the mineral aggregate as they fill the intersticesand provide contact points among larger aggregates. Second, when fines are mixed withasphalt, they cement larger aggregates together. Thus, the addition of baghouse fines toan 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 stiffeningcan affect the compactibility and stiffness of the mixture.

Anderson (1987b)andKim et al. (2003)reported that the role of the filler was more thanvoid filling. Variations in the stiffening effects of baghouse fines are not fully explained byeither the fineness or the gradation of a particular dust source. It is necessary to identifythe detrimental baghouse fines in terms of quality and quantity. Thus, the quality of theasphalt mixture will not suffer and the pavement durability can be insured. In contrast tothe considerable amount of research done on fillers over the past decades, little is knownabout the effects of fines on asphalt mixtures. The impact of fines on the compactibility ofasphalt concrete mixtures, the stiffening effect of baghouse fines on pavement fatigue, andthe 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 productionof approximately 1.3 million metric tons of asphalt paving material. Simplified diagrams ofbatch mix plant operations are presented inFig. 1. It is estimated that approximately 3900metric tons of baghouse fines are generated annually in the highly populated country of Tai-wan (23 million people in an island of 30,000 m2). The baghouse fines are only permitted foruse on county roads or other low volume roads. Commercially available mineral fillers can

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

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 beentaken to landfills, two major expenses have been reduced. Due to the significant expendi-tures on pavement construction ($14 billion was budgeted for roadway work in 2001), aresearch 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 offines-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 mix-

tures (such as stiffness/modulus, etc.).

This study examines 14 different fines including nine baghouse fines from differentregions, one fly ash, one cement, two mineral fillers (MF), and one lime. The nine baghousefines were collected from asphalt plants in different regions, as shown inFig. 2. Thesesamples 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 thisstudy. Particle size distributions inTable 1were determined by the hydrometer analy-sis of fines using American Association of State Highway and Transportation Officials(AASHTO) T88 specification. Significant variations in gradation were observed for thefine samples from the various plants, as shown inTable 1. The standard specification ofmineral fillers for bituminous paving mixtures (ASTM D 242-85) has the following grad-ing requirements: 100% passing 600�m, 95–100% passing 300�m, and 75–100% passing75�m. AASHTO M17-83 (1986) specification has similar requirements for passing 600 and300�m but it requires 70–100% to pass 75�m. According toTable 1, A4, A6 and B4 mate-rials would not pass both AASHTO M17-83 and ASTM D 242-85 grading requirements.TheNAPA (1980)report demonstrated that plants with identical equipment and operatingconditions, but that utilize different aggregates, produce baghouse fines of different quantityand size distribution.

Anderson et al. (1982)reported that fineness alone is not sufficient for defining howa fine will behave in an asphalt mixture. They concluded that different fillers and finesreacted differently with different asphalts.Anderson (1987a)illustrated that the size of the

Table 1Physical properties of fines

%Pass A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3 B4 B5BHF BHF BHF BHF BHF BHF BHF BHF BHF Fly ash C MF MF Lime

75�m 80 100 78 56 96 49 79 98 100 91 – 77 58 10050�m 53 98 77 53 88 45 67 97 98 89 – 59 43 9830�m 43 95 76 44 75 36 58 95 97 31 – 52 33 4220�m 18 90 75 27 46 29 46 86 95 13 – 40 27 1010�m 10 80 52 21 16 23 34 70 77 12 – 29 10 85�m 7 73 34 14 15 20 27 52 51 11 – 24 9 83�m 3 48 27 12 13 17 21 40 36 10 – 22 8 71�m 1 23 14 10 11 15 18 16 19 10 – 14 7 7cm2/g 8031 9212 10410 9263 4953 5533 5886 6448 8167 2827 3177 1807 2335 2717pH 8.26 9.32 8.22 9.66 10.65 8.01 10.11 8.16 9.32 11.64 12.01 10.04 10.31 12.11SG 2.56 2.64 2.53 2.55 2.55 2.67 2.58 2.62 2.5 2.19 3.15 2.75 2.72 2.26

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

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

fine affected rheological behavior but the source of the asphalt and the mineralogy of thefine also had a significant effect on rheologocal behavior. In order to properly measure thestiffening effect of a mineral filler, it is necessary to test the specific asphalt and mineral fillerin 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

Table 2Mineral compositions

SiO2 A12O3 Fe2O3 CaO MgO SO3 Na2O K2O

A1 57.38 19.04 2.36 1.79 1.6 0.27 1.31 2.74A2 53.88 28.7 7.85 1.43 1.76 0.45 1.32 3.49A3 56.2 24.65 6.5 1.57 1.42 0.45 1.23 3.18A4 56.95 16.87 6.98 2.01 1.55 0 1.17 3.33A5 64.5 15.01 5.89 1.91 1.48 0 1.50 2.59A6 79.42 5.58 3.27 0.4 0.77 0 0.81 1.61A7 68.28 14.27 5.23 2.25 1.54 0.11 1.30 2.23A8 60.48 16.93 7.42 1.77 1.42 0.93 1.31 2.83A9 57.46 20.82 6.89 1.4 2.07 0.54 1.11 3.52B1 50.52 27.28 6.32 6.02 1.68 0 0.5 1.4B2 20.32 5.39 4.51 62.78 1.73 1.89 0.14 0.54B3 2.02 0.61 0.25 41.42 11.01 0.1 0.03 0.09B4 1.02 0.24 0.1 53.79 1.04 0.05 0 0.03B5 1.34 0.52 0.24 67.11 4.21 0.29 0.1 0.02

also concluded that the mineral filler stiffens asphalt and that stiffening varies significantlybetween different fillers.

3.2. Surface area, pH, and mineral composition

Surface area (cm2/gm) was determined using the Blaine air permeability apparatus fol-lowing the ASTM C204 specification. Of the 14 samples tested, baghouse fines generallyhave the highest surface area and mineral filler had the lowest, as shown inTable 1. Inaddition, pH values of fines were determined after mixing the fines with an equal weight ofwater (pH 7) devoid of dissolved ions (Table 1).

Mineral compositions of the baghouse fines, fly ash, cement, mineral fillers, and limewere obtained by X-ray diffraction (Table 2). Baghouse fines A1–A4 were obtained from thesouthern region. The SiO2 contents range from 53.8–57.4% and CaO contents range from1.4–1.8%. As shown inTable 2, the mineral compositions (SiO2, CaO, MgO, K2O, etc) forA1 to A4 are similar because they were from the same source (Kaping river). This researchreconfirms the study done byKandahl (1981)that the chemical properties of baghouse dustcan be expected to reflect the properties of the fed aggregate. A6 was from the Tato Riverwith the SiO2 and CaO contents of 79.42 and 0.4%, respectively. As shown inTable 2, themineral compositions of the baghouse fines can be divided into 3 major groups; Group 1includes 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 propertiesof asphalt binders. The terms asphalt binder and asphalt mineral filler mixture are usedinterchangeable in this paper. Penetration tests are a measure of hardness and viscosity

that represent the fluid’s resistance to flow. Both properties were employed to study thestiffening effects of adding fines to asphalt mixture.

Based on ASTM D3515 requirements for asphalt concrete mixture with a nominal sizeof 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 mineralfiller 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 mineralfiller mixture. The ratio by weight between fines and asphalt cement are 0.545, 1.09, and1.635. In terms of volume, the ratios are approximately between 0.1 and 0.76, depending onthe type of fines.Table 3presents the volume ratios between the fines and asphalt cementfor the materials investigated. The asphalt cement adopted in this study is a commonly usedAC20 (60/70 grade by penetration).Anderson (1987b)recommended that the fines/asphaltratio should be closely monitored during the mix design to limit the bulk volume of finesto less than 50% (volume ratios between the fines and asphalt cement). The particle sizedistribution of the baghouse fines should be well graded, with some of the dust finer than0.010–0.020 mm. The percent of free asphalt should be kept at approximately 40% sinceexcessive amounts of baghouse fines as filler are likely to result in an asphalt mix that willbe difficult to compact. The fines/asphalt ratio is a better control criterion than seeking anupper limit or the percentage of baghouse fines in the mix. As indicated inNAPA (1980)report, fillers with void volume greater than 60% produce mixture that can exhibit plasticor brittle behavior.

Fig. 3 indicates that an increase in the fines/asphalt ratio results in an almost lineardecrease in the penetration value of the resultant asphalt binder mixture. The viscosityand softening point of fines/asphalt blends increase or stiffen as the fines/asphalt ratio isincreased, as shown inFigs. 4 and 5, respectively. The penetration and viscosity tests wereconducted at 25◦C and 60◦C. It was observed fromFigs. 3–5that lime (B5) had the most

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

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

A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3 B4 B5BHF BHF BHF BHF BHF BHF BHF BHF BHF Fly ash C MF MF Lime

3%-all (0.545) 66.1 66.5 63.5 60.0 67.72 75.56 70.09 64.7 59.1 65.1 68.2 76.26 75.1 49.16%-all (1.09) 42.2 43.0 37.8 32.1 45.1 58.1 50.4 39.8 30.7 42.0 44.6 59.26 57.3 14.789%-all (1.635) 24.6 25.3 18.9 11.5 28.5 45.1 35.2 21.1 9.69 25.3 26.4 46.43 43.9 –

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.

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 theleast.

Fig. 6 clearly shows the stiffening effect of fines as compared to the AC20 asphaltat different shear stress levels. Complex shear modulus for AC20 asphalt is the lowestcompared to all other mixtures with an addition of 3% fines by weight. Test conditionswere kept at 60◦C and 1 Hz. In view ofFig. 6, asphalt mixture with lime (B5) had thehighest complex shear moduli (G* ) at all shear stress levels. Also, asphalt mixture withlime (B5) had the steepest slope, indicating that the asphalt mixture with lime was sensitiveto the test condition. The asphalt mixtures with mineral filler (B4) and cement (B2) had theleast stiffening effect. The asphalt mixtures with fines (including baghouse fines, mineral

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

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 forAC20.

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 fora shear stress of 1000 Pa. It can be observed fromFig. 7that the stiffening effects were rela-tively 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 of1000 Pa).

observations, AC20 asphalt had the least complex shear moduli at all test frequencies. Theleast 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 tomoisture damage directly impacts the life of the paving mixture. Using a high percentage offines or misusing fines causes adhesion to become even more critical as the paving mixtureis exposed to water and water vapor. Several studies conducted in Germany, Japan and theUnited States have revealed that an asphalt paving mixture can be vulnerable to water whencertain fines are used as mineral filler (Kandahl, 1981; Eick and Shook, 1978; Harris andStuart, 1995).

In this study, the mix resistance to moisture damage was evaluated by examining thestripping area through digitized images. Seven fines including baghouse fines A4, A6 andA7, 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 finesas 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 binderswere 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 asshown inFig. 8. Micro slide glass was used as a function of the aggregate to facilitate greaterconsistency and uniformity. The aggregate is composed of a high SiO2 content similar tothe micro slide glass. As illustrated inFig. 8, the white area represents the separation ofasphalt binders and micro slide glass in water of high temperature (80◦C). This separationis indicative of stripping or moisture related damages.

Images were digitized to quantify the stripped area, as best as can be represented by grayscale from 0 (black) to 255 (white). For an intensity image, the image data can be stored in asingle two-dimensional matrix, with each element of the matrix corresponding to one imagepixel. The size of the matrix is the size of the image by pixel. Gray intensity of 100 was usedas 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 contentby weight (Fig. 9). At 3% filler content, cement (B2) and lime (B5) were best able to resiststripping, and the performance of mineral filler (B4) and baghouse fines A6 and A7 werein the same range. The best performer among baghouse fines was A4 with the least amountof SiO2. Similarly, Fig. 10shows the filler content at 6%. Asphalt binder with lime (B5)and mineral filler (B4) were the best and the worst performers, respectively. As illustratedin Fig. 10, even though asphalt binder with mineral filler (B5) was the worst performer, itperformed slightly better than the pure AC20 binder. A comparison ofFigs. 7 and 10showsthat the asphalt binder with baghouse fines A6 and A7 performed better than mineral fillerB4 in terms of stiffness, even though it’s ability to resist moisture is lower than mineralfiller B4.

Fig. 11uses baghouse fines sample A6 to illustrate the relationship between percent finesand stripping. The amount of stripping decreased when fines content is increased. At a finecontent of 9%, stripping is no longer a concern. As seen inFig. 11, even at 3% fines, theability to resist stripping is higher than the pure AC20 (0%) without filler.

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 whenthe fine and asphalt cement weight ratio was at 1.635 (refer toTable 3). Fig. 12illustratesthe definition of free asphalt as was first defined byRigden (1954). When asphalt is addedto the fine, the asphalt first fills the voids. Asphalt within these voids is called fixed asphaltbecause it is fixed within the void structure. Asphalt in excess of the fixed asphalt is calledfree asphalt because it is free to lubricate the larger particles. The free asphalt pushesthe 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

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, themineral filler contents were changed to 2, 3.5 and 5% during the preparation of asphaltconcrete mixture. The particle size distributions for asphalt concrete mixture are presentedin Table 4. Three different asphalt cement contents were selected: 5.8, 5.3 and 4.8%. Thecombination produced mineral filler and asphalt cement weight ratios of 0.34, 0.66 and1.04, respectively. By volume, the ratios between mineral filler and asphalt cement wereapproximately 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).

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◦Cwith 5 different frequencies of 0.01, 0.025, 0.1, 0.25 and 1 Hz to determine the dynamicmodulus.

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 concretemixture with lime (B5) decreased with increasing lime contents. In contrast, the dynamicmodulus for cement (B2) increased with increasing cement content, as shown inFig. 13.

Fig. 12. Schematic illustrating fixed and free asphalt.

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

Table 4Particles size distribution for asphalt concrete mixture

Sieve-%passing ASTM D3515 Fines-2% by weight Fines-3.5% by weight Fines-5% by weight

3/4 100 100 100 1001/2 90–100 93 93 933/8 84 84 84#4 44–74 50 50.5 51#8 28–58 30 31 32#16 21 22.5 24#30 15 16.5 18#50 5–21 9 10.5 12#100 5 6.6 8#200 2–10 2 3.5 5

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 slightlywhen the test temperatures changed to 25◦C, as reflected inFig. 14.

(3) For a mineral filler content of 3.5% and a given test frequency, asphalt concrete mixturewith lime (B5) yielded the highest dynamic modulus, the baghouse fine (A4) rank thesecond and the miller filler (B4) last. The ranking yielded a similar trend as that forasphalt–mineral filler mixtures that have no coarse aggregates.Dukatz and Anderson(1980)also found that the stiffness of the asphalt concrete samples was significantlyaffected by the stiffness of the asphalt–mineral filler mixture. Both the fineness of thefiller as well as the source of the asphalt and filler affected the creep compliance. Thestiffer filler asphalt mixtures produced stiffer asphalt concrete mixtures.

6. Discussion

Strict air pollution control codes and the regulation of emission of particles into theatmosphere result in the increased use of dust collection systems. This research was initiatedin high-priority areas to study the replacement of mineral fillers with baghouse fines inpavement construction. It is well known that the success of a recycling program is stronglydependent 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 naturallyoccurring aggregates, their properties are generally similar to those of commonly usedmineral fillers. Although baghouse fines have been successfully reintroduced to hot mixasphalt mixtures, some failures have occurred due to poor compaction, bleeding, flushingand tender mixes. For a given volume of mineral filler, “improper” mineral fillers eitherstiffen asphalt binders too much (which may lead to poor workability, low temperaturecracking, or fatigue cracking) or do not stiffen asphalt binders enough (which may lead toshoving or bleeding). Performance related tests can indicate whether a problem exists withthe use of a trial filler in a mix; however, determining physical and empirical properties ofthe filler can lead to an understanding of the mechanisms governing the contributions of thefiller to the overall performance of the mix.

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

Extensive tests were conducted to evaluate the type and quantity of fines on asphalt binderand asphalt concrete mixture performance. Viscosity, softening point, penetration complexshear modulus, and dynamic modulus have been commonly used and were adopted in thisstudy as performance indicators.

Currently, baghouse fines are only allowed in county or low volume roads. Commerciallyavailable mineral fillers (e.g. B4) can only be used in highways or expressways. Test resultsindicated that baghouse fines A4 are superior to mineral filler B4 in terms of stiffness and theability to resist moisture related damage. Thus, baghouse fines A4 should be recommendedfor use in highways or expressways that carry high volume traffic. A4 represents Group 1including 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 B4in terms of stiffness, but the ability to resist moisture related damage is lower than mineralfiller B4. Also, the ability to resist moisture related damage decreases with increasing finecontent. Results show that the fine content for A6 and A7 should not exceed 5% as theability 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 (withoutfine), thus they can be used in county or low volume roads. To strength the ability to resistmoisture related damage, 1–2% of lime addition should be considered. A7 represents Group3 and also includes A5, A8. A6 is Group 2 by itself. As indicated above, properties withinthe same group are expected to be similar. It was found that when the fine contents increasedto 9%, there is no significant increase on ability to resist moisture related damage. At suchhigh fine content, the surface area increases and it requires higher asphalt content that leadsto higher cost. Hence, for cost and performance consideration, the optimum fine content forGroups 2 and 3 was found to be 5%.

Tayebali et al. (2003)conducted extensive indirect-tensile-strength tests on mixes withbaghouse fines and found significant improvement. Although crack resistant tests were notperformed in this study, authors believe with inclusion of baghouse fines or mineral fillerthe asphalt concrete’s fatigue life will be improved. The speculation is further justified byincreasingG* 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 col-lected during hot-mix asphalt production. Before the introduction of the baghouse, the finerfraction of the dust was wasted to the wet scrubber or vented to the atmosphere. Most asphaltproducers try to recycle as much as possible the baghouse fines back into their own pavingmixes. The environmental and economical impacts to recycle the baghouse fines as mineralfillers is expected to be significant. Observations and conclusions are as follows:

• Baghouse fines vary considerably from plant to plant. Baghouse fines can have a coarse-ness that exceeds the limit generally accepted for mineral filler (ASTM D 242-85 andAASHTO M17-83) as two baghouse fines have less than 70% passability through a seivesize of 75�m.

• The data collected as part of this study indicates that the amount of stiffening is notuniquely related to the fineness and that gradation and miner properties alone cannotexplain the stiffening effect of fines.

• The increase in stiffness, due to the addition of the filler, is represented by an increasein softening point, a decrease in penetration, an increase in viscosity and an increase incomplex shear modulus (G* ). The stiffening effects of baghouse fines vary greatly.

• Performance in terms of stiffness and resistance to moisture related damage for asphaltbinder with fines (baghouse fines, lime, cement, mineral filler) was better than the AC20asphalt without fines.

• The best performer among baghouse fines was A4 (Group 1), with the least amount ofSiO2. The authors believe that Group 1 baghouse fines, (A1, A2, A3, A4 and A9) aresuperior to the mineral filler B4 and can be used in mixtures for high volume roadwayssuch as highways and expressways.

• Groups 2 (A6) and Group 3 (A5, A7, A8) baghouse fines should be used in mixtures forcounty roads and other low volume roads. To increase moisture resistance, the additionof 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 theirconsultants and discussions regarding this research.

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