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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Baghouse fines from dust collection devices at asphalt mixing plants are routinely recycled as all or part of the mineral filler portion in hot mix asphalt paving mixtures. Since these fines are derived from naturally occurring aggregates (crushed stone or sand and gravel), their properties are ordinarily quite similar to those of commonly used mineral fillers, such as stone dust or hydrated lime. Baghouse fines from a particular plant and aggregate type should only be used at that plant and with the aggregate type from which it was derived, since baghouse fines from different aggregates vary in gradation, chemical composition, and affinity for asphalt.
There is a limited amount of documented field performance data related to the use of baghouse dust as a mineral filler. Between 1975 and 1980, the Pennsylvania Department of Transportation (PennDOT) monitored the performance of 12 pavement sections in western Pennsylvania that contained different sources of baghouse fines. Examination of core samples from these pavements indicated that unusually high or inconsistent incorporation of baghouse fines resulted in mixes that were stiff, brittle, and difficult to compact.(1)
Furthermore, a number of the mixes monitored exhibited high air voids contents that accelerated asphalt hardening and contributed to premature distress in the form of ravelling, loss of fines, and reduced pavement durability. These problems appeared to be more prevalent when baghouse fines from slag aggregates were used, compared with baghouse fines from crushed stone aggregates. The pavement cores for projects using slag fines all showed an excessive amount of fines and high air voids contents (from 9 to 14 percent), which led to poor mix compaction and accelerated age hardening of the asphalt.(1)
A number of state transportation agencies and/or trade associations have also investigated the use of baghouse dust or baghouse fines as a mineral filler in asphalt mixtures. Many of these studies were laboratory investigations that focused on the variability of baghouse dust and the effect of such variability on mixture behavior.
In 1976, the California Department of Transportation studied the effects of six different sources of baghouse dust on Hveem mixture design parameters. The report concluded that a maximum baghouse fines content of 2.0 percent has little effect on the stability of asphalt mixtures and was found to be beneficial to mixture cohesion.(2)
In 1978, the Asphalt Institute evaluated the properties of asphalt mixtures containing baghouse fines from a number of sources. The properties of the mixes with baghouse fines were compared to known properties of commercial mineral fillers and filler-asphalt mixtures. Although significant variations in gradation were found in the dust sampled from different plants, it was concluded that the quality of baghouse fines is satisfactory for use in asphalt mixtures as long as the quality of the parent aggregate is satisfactory.(3)
The West Virginia Department of Highways studied 16 different sources of baghouse fines with a wide variety of particle size ranges and physical and chemical properties. The study found that fine dust particles (0.020 mm and smaller) will combine with the asphalt binder and act as an extender. The report concluded that baghouse dust is not harmful to a paving mixture and can be successfully reintroduced into an asphalt mixture.(4)
The Washington State Department of Transportation investigated the grain size distribution of 12 different baghouse dust sources and added 3 of those dusts to asphalt mixes. The gradation of the baghouse fines varied considerably from plant to plant. The viscosity of the dust-asphalt mixtures varied considerably, and temperature susceptibilities and hardening were different for the different dusts and the two asphalt cements. Little correlation was found between particle size and consistency of the dust-asphalt mixtures.(5)
MATERIAL PROCESSING REQUIREMENTS
Baghouse dust from the primary collection system (cyclone), if present, is generally returned to the hot elevator. Baghouse dust from the baghouse hopper can be returned to the hot elevator, the No. 1 hot bin, or the weigh box. In either case, the dust is dry and further processing is unnecessary.
Some of the properties of baghouse fines that are of particular interest when baghouse fines are used as mineral filler in asphalt pavements include gradation, organic impurities, and plasticity index. Mineral filler requirements, which address these properties, are specified in American Association of State Highway and Transportation Officials (AASHTO) M17(6) and are shown in Table 2-2.
Gradation: Whether baghouse dust will comply with AASHTO gradation specifications depends largely on whether the asphalt plant is equipped with a primary dust collector (cyclone). The primary collector captures the coarser particles, thus ensuring that the remainder of the dust will be sufficiently well graded and fine enough to consistently satisfy AASHTO M17 gradation requirements.(7)
Table 2-2. AASHTO M17-83 specification requirements for mineral filler for use in bituminous paving mixtures.
Organic Impurities: In an National Cooperative Highway Research Program (NCHRP) study on baghouse dust, little to no clay was found in 26 different baghouse dust samples. Organic impurities are seldom detected in baghouse dust, except possibly in asphalt plants that burn oil.(8)
Plasticity Index: In the same referenced NCHRP study, the plasticity indices of 23 samples were all less than 4.(8)
The properties of the asphalt concrete that could be affected by baghouse dust include cement penetration and viscosity, stability, resilient modulus, and moisture sensitivity.
Penetration and Viscosity: Previous studies indicate that an increase in the fines/asphalt ratio (from 0.2 to 0.5 by volume) can be expected to result in an almost linear decrease in the penetration value of the resultant asphalt binder material.(7) The viscosity of fines/asphalt blends can be expected to increase or stiffen as the fines/asphalt ratio is increased.
Stability: The bulk volume of fines in a mix will have a direct effect on Marshall stability. As the fines/asphalt ratio increases, the Marshall stability can also be expected to increase, until peak stability is reached at or around 55 percent bulk volume of fines in the binder.(1)
Resilient Modulus: Laboratory tests indicate that the resilient modulus can be expected to increase with increasing dust content.(9)
Moisture Sensitivity: Moisture sensitivity is a possible concern in asphalt mixes in which baghouse fines are used as all or part of the mineral filler. It is recommended that the bulk volume of fines should be less than 50 percent in order to minimize the potential for moisture damage or stripping of dense graded asphalt paving mixtures.(1)
Asphalt mixes containing baghouse fines can be designed using standard laboratory procedures.
Moisture sensitivity of the design mixes, as determined in the laboratory, should be determined using the Marshall immersion-compression test (ASTM D1075), with a minimum 75 percent retained strength.(10)
The fines/asphalt ratio should be closely monitored during the mix design to limit the bulk volume of fines to less than 50 percent. The particle size distribution of the baghouse fines should be well graded, with some of the dust finer than 0.010 to 0.020 mm. The percent free asphalt should be kept at approximately 40 percent, 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.(16)
The asphalt mix design must also take into account the location (or locations) where the dust is being collected and added into the paving mix at the asphalt plant. If dust is being added from a primary collection device (such as a cyclone) and/or a baghouse, the relative proportions of dust collected from each source must be known beforehand and closely monitored during asphalt production to ensure that they remain consistent. The dust type and gradation have been found to have a significant influence on the mechanical properties of an asphalt mix. Well-graded dusts tend to behave in the most predictable manner. Anomalous behavior can be expected when the dust is highly uniform (one-sized) and finer than 0.010 to 0.015 mm, or when the dust is coarse and lacking material finer than 0.010 to 0.020 mm.(9)
The aggregate source from which the baghouse dust is derived may also have an effect on asphalt binder properties. Baghouse fines from gneiss, traprock, sand and gravel, and slag aggregates were reported to result in larger viscosity increases (and corresponding decreases in ductility) when introduced at a high fines/asphalt ratio (0.3 to 0.5) than fines from carbonate aggregates.(7) The bulk volume concentration of fines in the fines/asphalt system, however, appears to be the primary factor causing stiffening of the asphalt binder.(1)
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements containing baghouse fines.
Material Handling and Storage
Baghouse dust can be fed directly from the baghouse into the plant mix or stored in a silo prior to use. The location where the baghouse fines are to be added to the plant mix depends on whether the plant is a batch mixing plant or a drum mix plant. In many cases, baghouse dust may be only a part of the mineral filler component of the mixture.
In a batch mixing plant, baghouse fines can be returned to the paving mix at three different locations: (1) the hot elevator, (2) the No. 1 hot bin, or (3) the weigh box. The preferred method is direct return of the dust to the hot elevator or the No. 1 hot bin, if proper control of uniformity can be obtained. A surge bin and a positive feed system may be added to improve metering uniformity. If these systems do not ensure uniformity in the quantity of fines, it may be necessary to meter the dust into the weigh hopper.(11)
In a drum mix plant, baghouse dust can be returned at one of four possible locations: (1) the cold feed conveyor, (2) the drum entrance, (3) the drum discharge, or (4) at the point where the asphalt cement is introduced. This latter location is the location that has been most widely recommended by state transportation agencies and manufacturers of drum mix plants.(7) The introduction of the dust simultaneously with the asphalt cement eliminates re-entrainment of the dust in the system gas and provides a good distribution of the dust through the coating zone. If the dust is introduced to the cold feed or at the drum entrance, it may be recycled through the system gas. The least desirable method is blowing the dust into the mixture at the drum discharge. Introduction of the dust together with the asphalt cement is the recommended method and is also the most widely accepted by state highway agencies and manufacturers of drum mix plants.(7)
Mixing, Placing, and Compacting
The same methods and equipment used to mix, place, and compact conventional asphalt paving mixes are applicable for baghouse fines. However, if the filler/asphalt ratio with baghouse fines is too high, a stiffer mix may result and the specified level of compaction may be difficult to obtain. Inadequate compaction appears to be more of a problem when very fine baghouse fines are used as filler.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing baghouse fines. Mixes should be sampled in accordance with AASHTO T168,(12) and tested for specific gravity in accordance with ASTM D2726(13) and in-place density in accordance with ASTM D2950.(14)
There are wide variations in the stiffening effects of baghouse fines, which are not fully explained by either the fineness or the gradation of a particular dust source. The effect of dusts from different types of aggregates on the compactibility of asphalt concrete mixtures needs to be more fully understood. The influence of the stiffening effect of baghouse fines on the fatigue and mechanical properties of asphalt paving mixtures also requires further study.
The range of dust properties that enhance asphalt extension and the effects of asphalt extension on the stiffness, fatigue, and aging characteristics of the asphalt cement binder are also worthy of investigation.(7)
There is some 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.(15) Tender mixes are characterized in one of two ways. One is that the asphalt mix is difficult to compact when normal construction techniques are used, which is usually manifested by shoving of the material under a steel wheel roller. The other type of tenderness relates to the slow setting of the asphalt mixture after placement, making it sensitive to wheel turning or concentrated loading, especially during periods of hot weather.(16) Some further investigation of this concern may be warranted.