Short-Term Laboratory Conditioning of Asphalt Mixtures (2015)

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B-1 A P P E N D I X B Objective and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1 Aging of Asphalt Binder . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2 Factors that Affect Binder Aging During Plant Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-3 Batch Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-6 Parallel-Flow Drum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-8 Counter-Flow Drum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-10 Plant Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-13 Storage Silos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-13 Asphalt Binder Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . B-15 Checklist for Mixture to be Collected . . . . . . . . . . . . . . . B-15 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-17 The properties of asphalt binder change as it is heated and mixed with aggregates in an asphalt mixing plant . The heat- ing and mixing action drives off some of the lighter ends of the asphalt binder, facilitates absorption of the asphalt binder into the aggregate, and results in oxidation of the binder (Anderson and Bonaquist 2012; Glover et al . 2009; Morian et al . 2011) . Increased mixing times and tempera- tures are generally believed to result in increased aging . It is important to be able to simulate these changes in binder properties in the laboratory so that mixtures can be con- ditioned prior to testing to represent field properties . This is necessary to be able to better predict performance of the asphalt mixture in the field . The thin-film oven test was developed to simulate this aging that occurs when HMA is produced . This test is con- ducted under a single set of conditions (temperature, heating time, film thickness, etc .) so at best it is a rough estimate of the changes in properties since mixtures are produced in the field over a range of temperatures, mixing times, and other conditions . With new materials such as warm mix asphalt (WMA), the mixing temperatures are much lower than that for hot mix asphalt (HMA), likely resulting in more variabil- ity in the recovered binder properties (Rand and Lee 2012) . There has been much interest in the use of WMA since it was introduced to the United States in 2004, and many state departments of transportation (DOTs) have begun to use a significant amount of WMA . The National Asphalt Pave- ment Association (NAPA), on behalf of the FHWA, has esti- mated that during 2010, the amount of WMA produced in the United States was approximately 47 million tons, which is approximately 10 percent of all dense-graded asphalt mix- tures (Nadeau 2012) . The amount has increased since then and many believe that within the next few years, the amount of WMA that is used will exceed the amount of HMA used . Since WMA is produced at temperatures lower than the HMA temperatures, it is clear that the oxidation, loss of lighter ends, and absorption of the binder could be significantly affected . There is a need to evaluate present methods for estimat- ing the properties of asphalt binders and mixtures after plant production and to determine an improved method for simulating the aging that occurs during mixture production . There are a number of asphalt plant types and many different operating conditions for each plant type . Asphalt mixtures are also produced over a range of temperatures even within a particular plant type, which will affect the changes in binder properties . The factors during plant production that most widely affect aging and absorption of the asphalt binder need to be identified so that they can be considered when investi- gating effects of mixing plants on binder property changes . It is very likely that plant design is a contributing factor to the aging of asphalt mixtures . Objective and Scope The objective of the report presented in this appendix is to identify the operating characteristics of the various types of asphalt plants that affect the oxidation, loss of lighter ends, and absorption properties of an asphalt binder . This study was performed through a literature search on aging and absorption of asphalt binders . While there has Effect of Plant Type on Binder Aging

B-2 been a significant amount of work on aging of asphalt bind- ers, there has been very little work investigating the effects of different types of asphalt plants on aging characteristics of asphalt binders . Much of the information on the effect of plant operations on asphalt aging was developed based on the experience of the research team . The research team has also collected information from plant manufacturers about the plant types and characteristics . This study did not include any new testing but primarily involved collecting the information available in the literature along with the experience of the research team . The results of this study were used to develop a test plan for evaluating the aging and absorption of asphalt binders under various condi- tions during the production of WMA and HMA . Aging of Asphalt Binder Asphalt aging is generally caused by loss of some of the volatiles in the asphalt binder, oxidation of the asphalt binder, and selective absorption of the asphalt binder into the aggre- gate pores (Anderson and Bonaquist 2012; Glover et al . 2009; Morian et al . 2011) . Aging is at its most severe condition when the asphalt binder comes in contact with oxygen, at high tem- peratures and in thin films . Some mixtures tend to have a thinner asphalt film thickness (FT) and these mixtures may have a higher rate of aging . The source of asphalt binder can have a significant effect on the aging characteristics (Rand and Lee 2012) . Hence, care must be used when comparing the effect of plant production on aging when using asphalt binders from different sources . Aging is exacerbated when the binder is absorbed into the aggregate particles . This absorption is believed to be selec- tive resulting in some of the lighter ends of the binder being absorbed into the aggregate leaving a stiffer binder on the aggregate surface . This results in a stiffer mixture . For a given aggregate, more absorption occurs at higher mixture temperatures (lower asphalt binder viscosity) and when the mixture is held at a higher temperature for a longer time (Houston et al . 2005) . Experience has shown that the amount of absorption of asphalt binder into the aggregate pores is generally affected more by the aggregate absorption (AASHTO T 84 and T 85) and binder viscosity than by the plant operating characteristics . Once the asphalt mixture has been produced, placed, and compacted, the amount and rate of aging is affected by the climate and the in-place mixture properties such as film thickness (FT) and voids in the mixture . Dickinson (1980) showed, as expected, the rate of hardening is higher in hot climates than in cold climates and that the higher the air voids in the mixture the higher the viscosity of the recovered binder . Corbett and Merz (1975) showed that the location of the asphalt layer in the pavement structure also influences the level of aging in the mixture . Hardening occurs more quickly near the surface of a pavement because of greater exposure to air and sunlight . Coons and Wright (1968) evaluated the hardening of asphalt binder with depth in actual pavement cores . A total of 14 projects were considered in this study with core ages ranging from 4 months to 12 years . They found that the rate of change in viscosity was highly variable in the top 1 .5 in . of the pavement and that below that level in the pave- ment, there is little difference in the rate of change in viscosity with depth . The rate of change in viscosity is much higher for the asphalt mixture closest to the surface . This finding is shown in Figure B-1, where the average viscosity of all sam- ples of different ages was plotted as a function of depth . At a depth of approximately 1 .5 in . and greater, the viscosity can be seen to be approximately the same . Houston et al . (2005) developed an aging model that was incorporated into the Mechanistic–Empirical Pavement Design Guide (MEPDG) . This model shows that the most significant changes in binder viscosity in asphalt pavement occur in the top few inches of the pavement . Prapaitrakul (2009) indicates that one difficulty with the assumptions in this model is that the solvent recovery process could leave enough solvent in the recovered binder to soften its properties considerably . This remaining solvent could be greater for the more heavily aged binders, which tend to be stiffer . Because of this, stiffer bind- ers could have a higher impact caused by residual solvent, leading to inaccurate relative viscosity values when binder properties are monitored over time . Glover et al . (2005) conducted a study in which a large number of Texas pavements were cored and the binder extracted, recovered, and tested to determine binder stiffness as a function of age . This study showed that binders do age in pavements below the surface and that the hardening of binder continues over time . There is little that can be done about the climate but the air void (AV) content in the in-place mixture and other mixture properties are controlled during the production and com- paction process . Most specifications require some minimum compaction level to ensure that the mixture has adequate density to begin with and that the asphalt mixture maintains acceptable properties for an extended period of time . While high in-place AV content may not have a significant effect on short-term performance, these high void levels do result in more rapid oxidation of the binder and may result in long- term durability issues such as cracking and raveling . The focus of this report is on short-term aging as affected by asphalt plant type; hence, the primary concern is the oxida- tive aging process during mixture production . Material prop- erties that may have an effect on aging during production are also discussed . However, the effect of asphalt plant type on short-term aging cannot be determined without some under- standing of the effect of binder and mixture properties on

B-3 this aging process . Knowing the effect of binder and mixture properties on short-term aging will allow these components to be controlled so that the effect of plant operations can be more accurately determined . Factors that Affect Binder Aging During Plant Production The aging (hardening) of asphalt binder takes place in two phases: short-term aging during plant production and lay- down and long-term in-service aging that occurs during the life of the pavement (Houston et al . 2005) . The short-term aging is primarily due to conditions such as mixing tem- perature, mixing time, and other factors that occur during the production of the mixture . The long-term aging is more affected by items such as AV in the compacted in-place mix- ture and climatic conditions . Short-term aging occurs due to volatilization and oxidation, as well as other factors, such as absorption of asphalt binder into the aggregate particles that are responsible for the changes in binder properties . Oxidation is the change in chemical composition of the binder due to the reaction with oxygen . Volatilization is the loss of lighter oil components of the binder due to evaporation . Oxidation and volatilization are expedited when the temperature of the asphalt binder is increased . They are also accelerated by agitation during mixing in a plant or in remixing at the job site, although mixing is critical to ensure the uniformity of binder distribution and aggregate distribu- tion in the mixture as well as the uniformity of temperature . Houston et al . (2005) stated that 10 to 30 percent of the total ultimate hardening of asphalt binder occurs in a short period of time during mixing and laydown operations . Mixing Temperature Houston et al . (2005) suggested that mixing temperature and the source of asphalt binder are the most important fac- tors affecting short-term aging . Heating the mixture to a higher temperature during production results in a higher loss of volatile materials, more absorption of asphalt binder into the aggregate particles, and increased oxidation of the binder . Experience has shown that the trend, up until a few years ago, had been a gradual increase in the temperature of the mixture during plant production . One reason for these higher tem- peratures has been the increased use of polymer-modified asphalt . These modified asphalts are stiffer and require higher temperatures to obtain a workable mixture . There has also been an increased emphasis on using angular aggregates and coarser gradations which tend to produce stiffer mixtures, again requiring a higher mixing temperature . Figure B-1. Average viscosity of field samples as a function of depth. Source: Coons and Wright (1968).

B-4 These higher temperatures may provide for a more work- able mixture and a safety factor in mixture temperature in case there is a delay in the construction operation . It is reasonable to believe that this increase in temperature has increased the rate of mixture aging resulting in some loss in durability . These higher mixture temperatures have also caused an increase in mixture cost due to the requirement for more energy to produce these hotter mixtures . Another effect of these higher temperatures is higher emissions and this is one of the reasons for the increasing use of WMA . A study conducted by Kennedy and Huber (1985) evalu- ated engineering properties of binders and mixtures pro- duced over a range of mixing temperatures and stockpile moisture contents in both drum mix and batch plants . The mixing temperatures were varied from 325°F to 175°F, which is much lower than normal operating temperatures for HMA . The levels of moisture content in the stockpiles were denoted as dry, wet, and saturated . The penetration of the extracted and recovered binder was measured to estimate the harden- ing that occurred under the experimental conditions . The amount of asphalt hardening was not significantly affected within the ranges of mixing temperatures selected; however, the higher mixing temperature resulted in a penetration reduction of 5 to 10 units . There was some variability in the data, which resulted in a drop of 5 to 10 penetration units not being statistically significant . It was difficult to obtain uniform coating of the aggregates with mixing temperatures below 200°F . No significant difference was identified between mixtures produced in the batch plant and the drum plant for all conditions of mixing temperature and stockpile moisture, but an increase in moisture content caused a reduction in asphalt hardening during mixing . In conclusion, the study was not able to identify a clear relationship between mixing temperature, moisture content, and the properties of asphalt mixtures produced in either batch or drum plants for the aggregates studied . Some differences were noted, but these differences were not determined to be statistically significant . Asphalt Binder Grade Asphalt binder grade is also a critical factor in the aging process . Softer binders are more likely to experience signifi- cantly higher changes in binder properties than stiffer bind- ers for a given temperature . Many times stiffer binders are heated to a higher temperature during production, which may increase the changes in properties for the stiffer binder . Another property that is dependent on viscosity of the asphalt binder is binder absorption . Binders at lower tempera- tures will have higher viscosities resulting in lower absorption of the binder into the pores of the aggregate . Accordingly, at high temperatures, there will be more of the binder absorbed into the aggregate pores . Lee et al . (1990) describes some of the problems that may result if asphalt absorption values in asphalt mixture tested in the laboratory do not match the absorption values during production: 1 . Incorrect measurement of AV, voids in mineral aggregates (VMAs), or voids filled with asphalt, which may lead to durability or stability problems with the mixture . 2 . Low effective binder content that may cause raveling, cracking, or stripping . 3 . Premature age hardening and low-temperature cracking as a result of changes in asphalt properties . 4 . Construction problems such as segregation and tender mixtures . Mixing Time Mixing time will have an influence on the hardening of the asphalt binder . During mixing, there is more opportunity for the coated particles to react with oxygen, increasing oxida- tion . Longer mixing time will cause greater hardening due to the increased loss of volatile material and more oxidation . Consequently, mixing time is generally minimized (just long enough to obtain even coating of the asphalt binder and the aggregate particles) to keep oxidation at a minimum and to reduce the amount of breakdown of the aggregate during the mixing process . The Hot-Mix Asphalt Paving Handbook (US Army Corps of Engineers 2000) states the following: • If the binder is properly distributed, additional mixing time does not improve the coating and only hardens the binder by exposing it to air . • In batch plants, for a short wet-mixture time of 28 to 35 sec- onds, the penetration will decrease 30 to 45 percent for an average binder; also, viscosity will increase approximately the same percentage . If the mixing time is increased up to 45 seconds, the penetration of the binder may decrease up to 60 percent and the viscosity may increase up to four times its value . • For drum mix plants (DMPs), the hardening of the binder with respect to batch plants is variable and depends on different factors such as binder composition and thick- ness of the asphalt cement film around the aggregate par- ticles . A decrease in mixture discharge temperature and an increase in the volume of aggregate and production rate will generally produce less hardening of the binder in a parallel-flow DMP . • Considerably less hardening will occur in the binder in a counter-flow drum than in a parallel-flow DMP . This reduced oxidation is likely because the counter-flow plant keeps the asphalt binder away from the flame and hot gases while, in a parallel-flow drum, the asphalt binder does have some contact with the flame and the hot gases .

B-5 Plant Type Lund and Wilson (1984) compared the aging in asphalt binder between batch mix plants (BMPs) and DMPs . They concluded that there was a significant difference, at a 90 per- cent confidence level, between the two types of plants . The drum mixers result in less hardening of asphalt binder, which some considered detrimental in terms of tenderness of the mixture . An approach to quantify the aging of the asphalt binder was needed . For this evaluation, the percent change in asphalt viscosity (C) at the time of paving was used, as described in Eq . (B-1): = − − × R A B A C 100% Eq . (B-1) Where: A = absolute viscosity (Oregon Department of Transpor- tation [ODOT] TM 417) of original asphalt binder used in production of the mixture . B = absolute viscosity (ODOT TM 417) of rolling thin- film oven residue for asphalt used in production of the mixture . R = absolute viscosity (ODOT TM 417) of asphalt recov- ered from the mixture . This ratio was used to judge potential for failures due to aging based on field observations . When C values were above 50 percent, no problems were experienced; when C values were between 30 and 50 percent, some problems were expe- rienced; and when C values were 30 percent or less, pavement problems were always experienced . Another observation during this study was that low temperature in the mixing or aggregate drying process, particularly in DMPs, will produce less aging and may produce poor combustion of burner fuel oil, which may result in fuel contamination of the mixture . The low mixing temperature that was used in DMPs in the early 1980s did result in some issues and the temperatures were soon raised to be similar to that used in batch plants . At the time this report was prepared, the operating temperature of DMPs was very similar to that used for batch plants . Another study was conducted by Chollar et al . (1989) to identify possible differences in asphalt binders in drum plants versus batch plants and to compare changes induced by various laboratory conditioning methods versus those tak- ing place in drum plants . A total of 27 virgin asphalts were subjected to different conditioning that included thin-film oven, rolling thin-film oven (RTFO), small steam distillation (SSD), forced-air distillation (FAD) and rolling forced-air distillation (RFAD) . Two of these methods, FAD and RFAD, involved blowing air over asphalt binder and collecting the volatile matter removed . The SSD technique involved steam bubbled through the asphalt that removes volatile asphalt components from the residue . Different physical and chemi- cal properties were evaluated and compared with extracted and recovered asphalt binder from drum plant operations . The authors found that the recovered asphalt binders from drum plants were slightly harder with lower penetration val- ues at 25°C and higher viscosity values at 60°C than those from batch plant mixing and that they were more oxidized with higher carbonyl and oxidized nitrogen contents . This is opposite the information that many other researchers, including Lund and Wilson mentioned previously, have found . Chollar et al . also concluded that the binders recov- ered from the asphalt mixtures produced at a DMP, at the time the report was published (1989), were stiffer than bind- ers recovered in previous years for mixtures produced in a parallel-flow DMP . As mentioned earlier, originally drum mix asphalt plants heated the asphalt mixture to a much lower temperature than batch plants . For example, many drum mix projects, espe- cially in the early days of DMPs, produced asphalt mixture at a temperature of approximately 240°F (116°C) while most batch plant mixtures were produced at over 300°F (149°C) . This difference in temperature was true in the early 1980s, but by the end of the 1980s, the temperatures used to produce mixture in the two types of plants were approximately equal and these temperatures continued to be approximately equal at the time this report was written . There are not many studies that looked at the effect of aging between the two types of plants, but most studies were performed when there was a significant temperature differ- ence between the two plants . It is believed that the Lund and Wilson (1984) study was before the temperatures in DMPs had changed and it is believed that the Chollar et al . (1989) study was performed after the drum mix tempera- tures had increased to approximately equal that in a batch plant . Thus, it is not surprising that the two studies found opposite results . Aggregate Gradation Aggregate gradation can have a significant effect on aging of the asphalt mixture . The gradation of the aggregate is directly related to the surface area for a given amount of material . If the gradation is on the finer side of the require- ments for a given nominal maximum aggregate size (NMAS), the surface area is larger resulting in more exposure of the asphalt film to oxygen and increased oxidation if the asphalt content is the same . The percentage of aggregate passing the finer sieves, such as the No . 200 sieve, will have a much greater effect on the aggregate surface area than coarser material . The fines can make the binder stiffer, modify the moisture resistance of the mixture, and affect the aging characteristics . Fine

B-6 particles that are less than approximately 20µm in diameter may become part of the asphalt binder, causing an increase in hardening of the binder and a stiffer mixture . An increase in the dust percentage will normally fill some of the voids in the aggregate matrix and hence decrease the VMAs . Increas- ing the amount of material passing the No . 200 sieve results in an increase in the total surface area of the aggregate and typically a reduction in the optimum asphalt content, hence, resulting in a thinner FT that may lead to mixture durability issues (Chadbourn et al . 1999) . Reclaimed Asphalt Pavement The use of RAP in the asphalt mixture during production will make it more difficult to determine aging of the vir- gin asphalt binder . Most asphalt contractors now use RAP in their mixtures as a standard practice . The asphalt binder in the RAP has typically hardened to the point that it is sig- nificantly harder than the virgin binder in the asphalt mix- ture . For relatively small amounts (up to 15 to 20 percent) of RAP, there is typically no change in the virgin asphalt binder grade that is used . However, even at these small RAP percent- ages, there is some stiffening of the properties of the binder recovered from a mixture as a result of the aged binder in the RAP . The properties of the RAP binder and the virgin binder are considerably different making it difficult to know if the change in binder properties between two mixtures produced in two different plants is caused by the addition of RAP or aging of the asphalt binder during plant production . Also, the mixing temperature is sometimes increased to improve workability when RAP is used resulting in more aging of the binder during the production process . In most types of plants, the aggregate has to be superheated when mixed with the RAP to ensure that the combined temperature of the asphalt mixture is acceptable . Aggregate Absorption The absorption of asphalt binder into the aggregate will generally result in some stiffening of the effective binder due to selective absorption of lighter fractions of the binder (Lee et al . 1990) . Effective binder is defined as that which is not absorbed into the aggregate . When the lighter ends of the binder are absorbed by the aggregate pores, the effective binder remaining on the surface of the aggregate is stiffer . Some aggregates have high water absorption values (3 to 4 percent) and other aggregates have low absorption val- ues (less than 1 percent) . If the water absorption (AASHTO T 84 and T 85) of the aggregate is small, then the amount of asphalt that will be absorbed during production will be small; however, for highly absorptive aggregates, the amount of asphalt binder absorbed can be significant . This selec- tive absorption of asphalt binder can result in significant changes in the properties of the effective asphalt binder . Use of Lime as Anti-strip Lime, which may be added as mineral filler, is often used to improve the resistance of asphalt mixtures to moisture damage . As a result of lime’s ability to improve moisture sus- ceptibility, many state DOTs require the use of lime in their asphalt mixtures . While lime improves resistance to mois- ture, research by Plancher et al . (1976) has also shown that lime reduces aging of the asphalt binder during production of the mixture . Hence, if lime is being used to improve mois- ture susceptibility, it will likely affect the properties of the recovered binder . If lime is used on one project and not on another project, it may be difficult to evaluate the effect of plant operations on the recovered properties of the binder . Incomplete Ignition of Fuel There have been concerns expressed over the years about the possibility of incomplete combustion of the burner fuel during production (US Army Corps of Engineers 2000) in a DMP . There have been cases where this unburned fuel ends up being collected in the baghouse . There have even been cases where fire has occurred as a result of this problem . If combustion is incomplete, some of the unburned fuel will react with the asphalt in the mixture to soften the asphalt binder and likely result in a mixture with a tender surface . This is likely a more significant problem when using a parallel- flow DMP, but it potentially can be a problem with all types of plants . This has not been a significant problem but it has occurred and does need to be considered . Batch Plants Today, the percentage of batch plants being used to pro- duce HMA in the United States is on the decline, but there is still a significant amount of this plant type being used . The basic components and operations of a batch plant (Fig- ure B-2) include aggregate storage and cold feeders, aggre- gate dryers, screening and storage of hot aggregate, storage and heating of the binder, scales for weighing the proper amount of each aggregate hot bin and the asphalt binder, a pug mill for mixing of aggregate and binder, and likely a stor- age silo . The aggregate is removed from stockpiles and placed in the cold-feed bins, which add the different aggregates in the proper proportions to the collector belt . From here, the aggregates are carried to the dryer where the aggregate is heated and dried . For most HMA projects, the aggregate is heated to approximately 300°F or higher . The aggregate is then transported to the top of the mixing tower by a bucket

B-7 elevator, referred to as the hot elevator, and discharged from the elevator onto a set of vibrating screens where it is sepa- rated by size and deposited into hot bins . The correct aggregate proportion to be used from each bin is determined by weight by its introduction into a weigh hop- per . If RAP is used, it is typically fed directly into the weigh hopper and is heated primarily by the virgin aggregate, which has to be heated to a significantly higher temperature (super- heated) if RAP is used . After weighing in the proper amount of material from each hot bin, the materials are dropped into a pug mill where they are mixed with the asphalt binder that has been weighed separately . Prior to adding the asphalt binder to the mixture, the binder is stored in a binder stor- age tank where it is kept at an elevated temperature until being pumped into the binder weigh bucket . The aggregate is mixed dry ahead of the binder for approximately 5 seconds . The asphalt binder that has been weighed is then discharged into the pug mill, and the aggregate and asphalt binder are mixed for a specified amount of time . This wet mixing time should be no longer than needed to get complete coating of the aggregate . A typical wet mixing time is 25 to 35 seconds . If the asphalt mixture is agitated too long, it will result in exces- sive oxidation of the asphalt binder and excessive breakdown of the aggregate . After the mixing is completed, the mixture is discharged from the pug mill into a waiting truck or conveyed to a hot storage silo . On most projects, the asphalt mixture is conveyed to a hot storage silo where it is stored until used, typically a few hours or less . Factors that Could Cause Significant Hardening of the Binder (Generally Assumed to be Caused by Oxidation) Mixing Temperature and Mixing Time The aggregates are delivered from the cold-feed bins to the dryer to remove moisture from the aggregates and to heat the material to the required discharge temperature . The burner is adjusted to provide sufficient heat to dry and heat the material . If the rate of feed is increased, the burner will have to consume more fuel to bring the aggregate to the same temperature . Since 92 to 96 percent, by weight, of the asphalt mixture is aggregate, the aggregates control the temperature of the mixture that is ultimately obtained after mixing in the pug mill . Excessive heating of the aggregates can cause hard- ening of the asphalt binder during the mixing process . On the other hand, under-heated aggregates are difficult to coat evenly and the mixture will be difficult to handle and place . In a batch plant, there is direct control of the dry and wet mixing time of the mixture . In a DMP, this mixing time is controlled Figure B-2. Schematic of typical batch plant. Source: US Army Corps of Engineers (2000).

B-8 indirectly by drum diameter, length of mixing unit, slope, and speed of rotation of the drum . Changes in mixing time and temperature will definitely affect the aging of the binder . Production of Recycled Mixture There are three variables that dictate the temperature the new aggregate needs to be heated to so that sufficient heat transfer with the ambient RAP temperature results in the desired mixture temperature: (1) the moisture content of the RAP, (2) the discharge temperature of the mixture, and (3) the amount of RAP used (US Army Corps of Engi- neers 2000) . The new aggregate needs to be superheated when using RAP to a higher-than-normal temperature, so the resulting mixture temperature is satisfactory . This super- heated temperature of the aggregate can be significantly higher than that required to produce a conventional mixture with little or no RAP . When RAP is incorporated, batch plants may need some modification since excessive smoking and material buildup problems may occur . Also for higher amounts of RAP, the aggregate must be heated to a very high temperature and this may create problems in some plants . Modifications may be needed in the aggregate dryer, burner, hot bins, or dryer exhaust system . The methods for batch plant operations to add RAP are briefly described below (Kandhal and Mallick 1997): • Method 1—The superheated aggregate and RAP are introduced into the boot of the hot elevator and the mixed material is screened and stored in hot bins . The scavenger system in the tower removes the water evaporated from the RAP . With this method there are no problems with emissions, but only low percentages of RAP may be used because excessive asphalt binder may blind the screens . • Method 2—In this method the batch tower requires one hot bin to hold the mixture components (screens are removed) . The hot aggregate and RAP are introduced at the foot of the hot elevator and stored in the hot bin with- out screening . This method allows the use of up to 40 per- cent RAP . • Method 3—This method, also called the Maplewood method, uses cold prescreened RAP that is delivered directly to the weigh hopper of the batch tower with the superheated virgin aggregate from the hot bins . The RAP is sandwiched between superheated aggregate thus reducing the time needed for the RAP to become adequately heated . • Method 4—This method has a new control feed system . RAP is weighed separately and dropped intermittently into the pug mill with a feeder that introduces RAP over a 20- to 30-second period . This method allows more control of the steam that is generated . • Method 5—In this system RAP material is preheated in a separate dryer before mixing with the aggregate . The RAP is conveyed to a separate heated storage bin with weigh hopper . RAP material is weighed separately and conveyed to the pug mill to produce the recycled mixture . Batch Size The batch size that is used is a function of the pug mill size and other parameters . The capacity of the plant to produce asphalt mix ranges from 1 ton up to approximately 10 tons in a single batch . Regardless the batch size, the total required mixing time is approximately the same . For a given pug mill size, a smaller batch is likely to result in more oxidation of the binder than a larger batch . However, the batch size is not expected to be a significant problem since most asphalt is produced with a batch size within the limits recommended by the manufacturer . Material and Mixture Properties It is also important to ensure that the most common material and mixture issues that will affect the oxidation of the asphalt binder are controlled . These issues include use of lime, aggre- gate absorption, gradation of aggregate, and use of RAP . The best approach is to set up the test plan to monitor the use of lime and RAP and carefully document these to understand their potential effects on the outcome . Parallel-Flow Drum A parallel-flow DMP is a continuous mixing process that uses proportioning cold feeds to control the aggregate grad- ing . The main components are the cold-feed system, asphalt cement supply, drum mixer, storage silos, and emission- control equipment . The major difference between this pro- cess and the batch process is that, in a parallel-flow drum, the dryer is used to both heat and dry the aggregate and mix these aggregates with the asphalt binder . Also, the parallel- flow DMP is a continuous mixing operation and the batch plant produces mixture in batches . The aggregate is proportioned through the cold-feed system and introduced to the drum at the high end of the drum next to the burner . The drum rotates and the aggre- gate moves toward the low end of the drum traveling in parallel with the air flow (hence, parallel-flow) . Flights lift up the aggregate particles and then drop them through the hot air flow resulting in drying . The temperature of the aggregate increases as the aggregate moves down the drum until the aggregate reaches a point where the temperature is relatively constant . The heat generated by the burner

B-9 and the air flow evaporate the moisture in the aggregate . Because of this, the time the aggregate temperature remains constant depends in part on its moisture and its porosity (US Army Corps of Engineers 2000) . In general, moisture from porous material takes longer to be removed from the internal pores . When the moisture is removed, the tempera- ture of the aggregates rises, asphalt cement is injected, and the flights tumble the mixture putting the material in contact with the exhaust gases . Finally, the mixture achieves the required discharge temperature at the end of the drum . This process indicates that the mixing time and mixture temperature are highly dependent on the moisture in the aggregates . Some of the factors that influence the time required for an aggregate particle to travel through the drum include the length, diameter, and slope of the drum, the number and type of flights, the speed of rotation of the drum, and the size of the aggregate particles . A typical duration ranges from 4 to 8 minutes for an aggregate to reach the discharge end of the drum (US Army Corps of Engineers 2000) . This duration time in the drum controls the mixing time for the mixture for a given plant . Another factor that will affect the mixing time is the location where the asphalt binder is added to the drum . In a parallel-flow drum plant, the asphalt binder is added about halfway down the drum while, in one type of counter- flow plant, the mixing occurs near the end of the drum . For some counter-flow plants, there is a coater or unitized drum that allows mixing to take place outside the drum . All of these approaches for adding the asphalt binder and mixing will affect the mixing time . The binder is introduced to the mixture components through a pipe in the rear of the mixing chamber . The point of discharge of the binder varies but is generally between the midpoint and two-thirds of the way down the drum length . Once the coating of the aggregate occurs, the mixture is dis- charged and conveyed to either a surge bin or HMA storage silo, where it is loaded into transport trucks . Most plants use a storage silo for storing the asphalt mixture, where it is typically stored for a few hours but may be stored up to 72 hours . Factors that Could Cause Significant Oxidation Mixing Time and Temperature The most likely causes of oxidative aging are the tempera- ture of the mixture and mixing time . The mixing time is a function of slope of dryer, diameter of dryer, rotation of dryer, etc . All of these parameters will affect the mixing time . Mixing time and temperature have to be considered in any test plan that is established to determine the effect of produc- tion on short-term aging of the asphalt binder . Production of Recycled Mixture When a recycled mixture is produced using a parallel-flow DMP, the most common method is to add the RAP to the aggregate from its feeder bin and conveyor system into an entry point located near the center of the drum . By adding the RAP with this split feed system, there is better control of emissions since the RAP is protected from the high-temperature exhaust gases by the veil of aggregate upstream of the RAP entry point (US Army Corps of Engineers 2000) . If high-RAP content is used, less virgin aggregate is placed into the drum, reducing the veil of aggregate upstream of the RAP and decreasing the amount of heat transferred from the exhausted gases to the aggregate . This may cause the RAP to come in direct contact with the flame, possibly resulting in the generation of smoke and the further oxidation of RAP binder . When 20 percent or less of RAP is used, minimal emissions occur, the veil of aggregate is satisfactory to protect the RAP, and the discharge temperature of the mixture is adequate . Higher RAP percent- ages may be possible only if production is carefully monitored . Figure B-3 shows a typical configuration of the mixing opera- tion in a parallel-flow DMP . Production Rate The production rate will depend upon several factors such as aggregate temperature, mixture discharge temperature, drum diameter, fuel type, exhausted gas velocity and vol- ume, estimated air leakage into the system, average aggregate moisture, and gradation . One of the most important factors on plant production is the average moisture content of the coarse and fine aggregates . When the average percentage of moisture in the aggregate increases, the production rate of a DMP for a particular diameter decreases . On the other hand, for a constant average moisture content, the production rate increases when the drum diameter becomes larger . Plant operators of DMPs usually develop charts for drum mix pro- duction rates as a function of drum diameter and length and average moisture content . If RAP is used, the production rate can have a significant effect on the aging of the asphalt binder . Design of Flights The main functions of the flights in the different sections of the drum are to lift up the aggregate and drop it through the hot air flow so that it (1) shields the asphalt binder and RAP, if used, from the flames; (2) exposes the aggregate to the heat to dry the aggregate; (3) increases the aggregate tem- perature; and (4) facilitates coating of the aggregate with the binder . The first flights at the upper end of the drum direct the aggregate into the drum beyond the flame . The next flights

B-10 lift some of the aggregate from the bottom of the drum and tumble the material through the exhaust gases . The aggregate then moves down the drum; near the center of the drum, a veil of aggregate develops across the cross-sectional area . This veil allows the heat transfer between the exhausted gases and the aggregate . The asphalt cement is added at the end of the drum and mixing flights combine the aggregate with the asphalt cement and allow the heat transfer between the material and exhausted gases to be completed and also allows the material to reach the required discharge temperature . The condition of these flights can significantly affect the oxidation of the asphalt binder . There are also adjustments that can be made inside the drum to reduce the speed of material through the drum . These adjustments can affect the mixing time and result in changes to the aging characteris- tics of the binder . It is important to ensure that the flights are in good operating condition . The mixing time must be estimated so that factors other than plant operating charac- teristics can be quantified . Incomplete Burning of Fuel There are a number of types of fuel that can be used in the burner for drying and heating the aggregates and asphalt materials (US Army Corps of Engineers 2000) . Depending on the operating condition of the burner, how it is set to operate, and the type of fuel being used, there are cases where com- plete combustion of the fuel does not occur . In these cases, the unburned fuel can contaminate the asphalt binder resulting in some softening of the binder . This is expected to be more of a problem in a parallel-flow DMP than in the other plant types, because the asphalt binder will come in direct contact with any fuel that is not completely burned . This might cause some problems in other plant types since it is possible that this unburned fuel will attach to the aggregate as it moves through the dryer and then be coated with asphalt binder when added . However, in other plant types, the air flow will remove much of any unburned fuel from the flow of materials before the asphalt is added so contamination should be minimized . Material and Mixture Properties The material and mixture properties are expected to have approximately the same effect in the parallel-flow DMP as in the BMP as discussed previously . Counter-Flow Drum The counter-flow drum plant became popular in the 1980s . It utilizes a counter-flow aggregate dryer similar to the dryer used in batch plants . Steps have to be taken to ensure that the asphalt binder is protected from the burner flames . In this type of plant, the materials move in the drum in an oppo- site direction or counter-flow to the exhausted gases . The aggregates enter the high side of the drum and flow by grav- ity in the sloped drum toward the burner, which is located near the low end of the drum . In counter-flow plants, the aggregate passes the burner and is protected from the actual burner flames . There are three different types of counter-flow drum mix plants that are generally used . The first one has a Figure B-3. Parallel-flow drum mix plant. Source: US Army Corps of Engineers (2000).

B-11 mixing unit inside the drum but located on the end of the drum behind the flame . The second type refers to a drum mixer most commonly referred to as a double-barrel plant that has the mixing unit folded back around the aggregate dryer portion of the drum . The third type of counter-flow plant is a plant that dries the aggregate with the counter-flow drum after which the aggregate moves into an external mixer (sometimes referred to as a dryer-coater) where the asphalt binder is added and mixed with the aggregate . In contrast to a parallel-flow drum mix plant, no asphalt binder is added inside the drying portion of the counter-flow drum mixer . Drum Mixer In this type of plant, the burner is embedded into the drum and the mixing occurs in the bottom portion of the drum (Figure B-4) . After the aggregate is heated and dried, it passes the flame and then is mixed with the asphalt binder in a sepa- rate chamber . Once the aggregate has entered the mixing unit of the drum, the RAP, if used, is added to the aggregate . Since the RAP is added to the drum behind the burner, it does not come in contact with the burner flame or hot gases . This helps to avoid problems with emissions . Heat transfer from the new aggregate occurs when the two materials come together at the end of the mixing unit . After the RAP is added into the mixing portion of the drum, any other additives that are required such as baghouse dust and mineral filler, are also added at or near that point . The mixing of the RAP, filler and baghouse fines, and new aggregate begins when the materials are introduced into the mixing chamber . After all of the materials have been mixed, the asphalt binder is added and coating of the aggre- gate takes place . Depending on the angle of the drum and the number, rotation speed, and type of flights in the mixing unit, the mixing time typically ranges between 45 and 60 seconds . Factors that Could Cause Significant Oxidation A number of factors in a counter-flow DMP are likely to affect oxidative aging . The most likely causes of oxidative aging are the temperature of the mixture and mixing time as with the other plant types that have been discussed . The mix- ing time is a function of slope of the dryer, diameter of dryer, speed of rotation of dryer, etc . All of these parameters will affect the mixing time . Mixing time and temperature have to be considered in any test plan that is established to determine the effect of production on short-term aging of the asphalt binder . As with the other plant types, it is important to ensure that the most common material and mixture issues that will affect the oxidation of the asphalt binder be closely moni- tored . These issues include use of lime, aggregate absorption, gradation of aggregate, and use of RAP . Unitized Drum Mixer (Double Barrel) This mixing unit has the mixing chamber folded back around the aggregate drying drum . The outer shell is a non- rotating unit . The heated and dried aggregate is discharged Figure B-4. Dryer mixing drum. Source: US Army Corps of Engineers (2000).

B-12 from the inner drum into the nonrotating outer shell or drum (US Army Corps of Engineers 2000) . Once the aggre- gate is fed into the mixing chamber, asphalt binder and RAP, if used, are added into the external drum and away from the flame . The material blends with the superheated aggregate and the heat transfer between aggregate and RAP takes place . Similar to the process in a conventional counter-flow drum mixer, any moisture in the RAP and emissions that is gener- ated during the heating process is sent back into the dryer unit by the exhaust fan . The moisture is carried into the emission- control equipment and the hydrocarbon emissions from the RAP, if any, are incinerated by the burner (US Army Corps of Engineers 2000) . Any additives, such as mineral filler and baghouse fines, are also added into the outer shell . Finally, binder is added into the mixture and the mixing takes place . Mixing takes place by a series of paddles in the outside of the aggregate dryer . The size and production capacity of the unit will determine the mixing time that is typically less than 60 seconds . The paddles are actually attached to the rotating inner drum and these rotating paddles perform the mixing and facilitate the movement of the asphalt mixture toward the high end of the drum and out the exit chute . Figure B-5 shows a typical unitized mixer configuration . Factors that Could Cause Significant Oxidation A number of items in a unitized DMP are likely to affect oxidative aging . The most likely causes of oxidative aging are the temperature of the mixture and mixing time . The mixing time is a function of the slope of drum, production rate, and size of the mixing unit . All of these parameters will affect the mixing time . Mixing time and temperature have to be consid- ered in any test plan that is established to determine the effect of production on short-term aging of the asphalt binder . It is also important to ensure that the most common material and mixture issues that will affect the oxidation of the asphalt binder are monitored and documented . These issues include use of lime, aggregate absorption, gradation of aggregate, and use of RAP . Mixing Outside of Dryer The last type of asphalt plant to be discussed involves a mixer outside the drum mixer . This approach involves heat- ing the aggregate inside the dryer and then mixing the aggre- gate with the asphalt binder in the external mixer . This setup separates the flame from the asphalt binder and minimizes potential damage to the binder . A schematic of the drum mixer with the mixing unit separated from the dryer is shown in Figure B-6 . This technique is similar to the unitized drum mixer since the asphalt cement is mixed with the mixture out- side the drum . The external mixer keeps the asphalt binder away from the flame and solves problems that may occur due to the flame coming in contact with the binder . Factors that Could Cause Significant Oxidation A number of factors in a DMP with external mixing unit are likely to affect oxidative aging . The most likely causes of oxidative aging are the temperature of the mixture and Figure B-5. Unitized drum mixer. Source: US Army Corps of Engineers (2000).

B-13 mixing time . The mixing time is controlled by the rate of production, slope of mixer, and the capacity of the mixer . Dividing the mixer capacity by the production rate will give an estimate of the mixing time . Mixing time and temperature have to be considered in any test plan that is established to determine the effect of production on short-term aging of the asphalt binder . It is also important to ensure that the most common material and mixture issues that will affect the oxidation of the asphalt binder are monitored and documented . The primary difference between a parallel-flow plant and a counter-flow plant is that the asphalt mixture is exposed to hot, steam-laden gases in the mixing area of a parallel-flow plant, while the asphalt cement is introduced outside steam- laden exhaust gases in a counter-flow plant . Plant Characteristics A number of plant characteristics have been identified that may affect asphalt binder oxidation . Some of those charac- teristics are listed in Table B-1 . The manufacturers shown in Table B-1 are the major producers of asphalt plants in the United States . The plants described in Table B-1 provide good examples of the various characteristics that affect oxidation . As mentioned earlier the primary factors that affect oxidation and absorption are mixing temperature and mixing time . Many of the characteristics of asphalt plants are shown in Table B-1, but many components are custom built to meet owners’ needs so some of these characteristics shown may be adjusted for specific plants that are manufactured . It is clear from Table B-1 that the counter-flow plants have gained in popularity and significantly exceed the number of parallel- flow plants . Also, there are many more models of drum mix plants than batch plants . Storage Silos The primary purpose of a storage silo is to provide tem- porary storage space for the asphalt mixture during produc- tion to help minimize the number of trucks needed and to improve the overall efficiency during construction . Without this storage space, trucks would have to operate continuously to collect the mixture being produced and to haul the mixture to the job site . This would require that the contractor have more trucks available for use than actually needed in case some are delayed while traveling or there are breakdowns . On some projects the mixture is required to be used the same day that it is produced, but for other projects the mix- ture can be stored for up to 2 to 3 days or longer, and the con- tractor still has the ability to place and adequately compact the mixture . Of course, one concern is how much oxidation of the mixture occurs in the silo during short-term storage, but it is even more important for long-term storage . Most silos are insulated and sealed when extended storage is uti- lized, and many believe that this reduces the potential for sig- nificant oxidation . The binder is normally not recovered and Figure B-6. Drum mixer with external mixer. Source: US Army Corps of Engineers (2000).

B-14 Table B-1. Description of asphalt plants produced by selected plant manufacturers. Equipment Supplier Plant Model Plant Type Description Tons/Hour Other Information ADM Milemaker Counter-flow drum mix Dual-drum counter-flow. The first portion of the drum heats the aggregate and the second portion, located behind the flame, adds filler, asphalt binder, RAP, etc. and mixes. Hence, flame is kept away from binder. 160–425 Roadbuilder Parallel-flow drum mix Conventional parallel-flow drum plant. 110–350 SPL Series Parallel-flow drum mix Conventional parallel-flow drum plant designed and built for lower production quantity. 60–160 Ex Series Counter-flow drum mix Mixing chamber behind isolation ring at burner nose. 100–425 ALMIX Compact Series Counter-flow drum mix First portion of the drum heats and dries the aggregate. Mixing with asphalt binder, RAP, and filler occurs in mixing unit behind burner. 60–140 Duo Drum “CF” Counter-flow drum mix Dual-drum counter-flow. The first drum heats the aggregate and the second drum adds filler, asphalt binder, RAP, etc. and mixes. Hence, flame is kept away from binder. 120–600 32-ft drying drum and 16-ft mixing drum Batch mix Counter-flow batch Typical batch plant where flame is kept away from asphalt binder. 50–450 Uniflow Counter-flow drum mix Single-drum plant. Mixing occurs in mixing area behind burner. 160–400 Dratch Counter-flow drum mix/batch Counter-flow drum with a batching tower and mixing drum so it can be operated as a batch plant or a dual-drum asphalt plant. 1- to 8-ton batch Gencor Ultra Plant Counter-flow drum mix Drum plant with counter-flow drum. Mixing chamber behind flame. Can include advanced RAP entry, which allows for some heating and drying of RAP prior to entering the mixing area. Can be used in conjunction with batch tower (Convertible Ultra Plant) to offer both drum mix and batch plant capabilities. 100–800 Astec, Inc. Six pack portable plant Counter-flow drum mix Counter-flow double barrel that heats and dries the aggregate in the inner drum and then adds binder, RAP, and filler to the aggregate as it feeds in between the inner and outer drums. 200–400 M Pack Relocatable Counter-flow drum mix Counter-flow double barrel. 200–600 Stationary Drum Counter-flow drum mix Counter-flow double barrel. Stationary Batch Counter-flow batch Batch plant. 45–500 Recycle Batch Plant Counter-flow batch plant Batch plant. RAP is mixed with superheated aggregate at the bottom of the hot elevator. Batch drum plant Counter-flow drum mix/batch Double-barrel drum is set up along with batch tower. Provides versatility in production by allowing use of batch or DMP. Coater II Plants Counter-flow with coater for batch or continuous Batch plant with coater drum. Can mix in batches or produce mix continuously. Up to 725 Heatherington and Berner Batch Plants Counter-flow batch Batch plant. Terex E225P/E275P Counter-flow drum mix Mixing unit behind flame. Drum 7 ft diameter by 39 ft 4.5 in. E3-300 Counter-flow drum mix Includes an early-entry RAP preheating zone. Mixing unit is behind the burner. 300 Drum 7 ft 4 in. diameter by 42 ft long E3-400 Counter-flow drum mix Includes an early-entry RAP preheating zone. Mixing unit is behind the burner. 400 Drum 8 ft 4 in. diameter by 47 ft 6 in. E3-500 Counter-flow drum mix Includes an early-entry RAP preheating zone. Mixing unit is behind the burner. 500 Drum 9 ft 3 in. diameter by 47 ft 6 in.

B-15 tested during the construction operation, so it is not known if the properties change significantly or not . Surprisingly, there is not much literature that looks at the amount of oxidation, if any, that occurs in the storage silo . Middleton et al . (1967) discusses research from the time when storage silos were first beginning to be used . Middleton et al . found a significant change in binder properties during the mixing operation but only small changes during storage . Storage times up to 100 hours were evaluated . To ensure that any effect of mix- ture storage is minimized, the storage time should be held constant when the short-term aging characteristics of an asphalt plant are evaluated . Asphalt Binder Storage There has not been much emphasis on evaluating the qual- ity of the asphalt binder in the binder storage tank . It is gener- ally assumed that the properties in the tank are approximately the same as the properties of the asphalt binder that is deliv- ered . Tests on the asphalt binder are typically conducted by the material supplier and the results provided to the quality assurance staff of the owner agency . Samples are often taken at the storage tank at the asphalt plant, but in most cases, no testing is done . Arambula et al . (2005) evaluated different factors that can contribute to the changes in asphalt binder even before it is added to the asphalt mixing operation . The testing plan was designed to simulate the effect of storage time and storage temperature on the dynamic shear rheometer parameter G*/ sin d after RTFO . The plan considered three levels of stor- age time—1 week, 1 month, and 2 months—and two storage temperature levels—168°C and 191°C . Samples were stored in small containers prepared to simulate the properties at the binder storage tank at the plant . The study found that the material responses to the two levels of storage temperature were statistically significant for 1 week and 1 month, but for 2 months, there was no significant effect . It is generally believed that very little aging occurs in the asphalt binder storage at the plant (stored in large volumes), but based on this study prolonged storage and high temperatures can result in some aging of the binder . Again, this assumes that the small sam- ples tested accurately simulate the conditions of the asphalt binder in much larger tanks . It is not believed that significant aging occurs in the plant binder storage tanks, but the possi- bility should be considered in any study of short-term binder aging during asphalt mixture production . Checklist for Mixture to be Collected There are many variables affecting binder oxidation that cannot be controlled . These mixture properties and plant characteristics, shown in Tables B-2 and B-3, must be mea- sured and documented during production . Storage of asphalt binder between 300°F and 320°F for not longer than 30 days is recommended . These temperature and time limits are within normal operation procedures and Table B-2. Mixture properties at mixing plant. Sample Number Mixture Type Agg. Water Abs. Grad. Asphalt Content Agg. Asphalt Abs. Calculated Film Thickness Mixing Temp. RAP % PG 1 2 3 4 Table B-3. Plant characteristics. Plant Type Set Mixing Time Mixing Unit Length Slope Mixing Unit Revolutions per Minute Mixing Unit Calculated Mixing Time Storage Time Production Rate Batch Drum Parallel Flow Double Barrel Dual Drum Mixing Unit Behind Burner Coater

B-16 will help ensure that excess oxidation has not occurred to the asphalt binder at the time of mixing . Storage of asphalt for 6 hours or less is recommended . The use of RAP can significantly affect the properties of the recovered binder and must be controlled to determine plant effects on the short-term aging of the binder . A real- istic approach to account for RAP, which most contractors routinely use, is to document the amount of RAP and, if pos- sible, attempt to compare a mixture with no RAP to that same mixture with RAP . The use of lime or other anti-stripping agents may signifi- cantly affect the aging characteristics . It will be important to monitor their use and document the projects in which they happen . Mixing time for DMPs should be determined by stop- ping the addition of asphalt binder and measuring how long it takes for the aggregate going through the plant to show a significant reduction in coating . Alternatively, it can be cal- culated from the length of the mixing zone and the rate of production . Summary From the literature, it is clear that there have not been many studies conducted that document the effect of plant operations on the aging properties of asphalt binders . How- ever, there have been many studies that have evaluated binder aging in general and the research team attempted to adapt the results of these studies to what would be expected to be observed at plants . Further, there has been a lot of testing dur- ing construction that allow researchers to have some under- standing of the effects of the plant operations on aging . A total of five plant types were discussed in this report: batch plant, parallel-flow DMP, and three types of counter- flow DMPs . The three types of counter-flow plants included mixing behind burner in drum, unitized plant, and external mixer . Based on the way that each of these plants operate, certain common or unique factors can affect the aging of asphalt binders during the construction process, and these are summarized in checklists provided for the plant and for the asphalt mixture being produced . It is also possible that the storage of the asphalt binder can result in some aging if it is stored at high temperatures for long periods of time . This is not believed to be a significant problem, but it should be considered during the study if there is reason to believe that it could be a problem . The storage of the asphalt mixture may be a greater concern as the asphalt binder is in thin films when stored with the aggregate and this might expedite aging . Certainly, if this is an issue, it will be affected by storage time and temperature . As a result, the asphalt mixture in the storage silo should be used within a few hours to minimize any potential for aging . Batch Plant For a batch plant, the primary items that need to be con- trolled are mixture temperature and mixing time . These items can be directly controlled and easily measured during the construction process . While the limited literature on stor- age silos indicates that there is not significant change while being stored, this certainly depends on a number of factors . Hence, placing of asphalt mixture on the same day it is pro- duced is recommended . Using asphalt mixture that was pro- duced on previous days is likely to result in more aging and might present a problem in the analysis of short-term aging during production and placement . Parallel-Flow Drum Mix Plant A number of factors can affect the aging that occurs in a parallel-flow DMP beyond those expected to have an effect with the counter-flow DMP . The primary reason for this is that, in a parallel-flow DMP, all of the components are mixed in the drum and exposed to the flame . In the counter-flow plants, methods are used to separate the asphalt binder from the flame and air flow and this should reduce the potential for oxidation . Mixture temperature and mixing time are important to control . Mixing time is more difficult to measure since it is a continuous-flow operation; however, it can be calculated based on production rates, slope of mixing unit, drum diam- eter, flight design, and the length of the mixing zone . Also there are times that methods are used inside a drum to slow the flow of material down to get more mixing time . The air flow is believed to affect the aging characteristics in a parallel- flow plant since the air will be flowing through the asphalt binder and will likely increase oxidation . Incomplete ignition of the burner fuel can cause contamination of the asphalt binder resulting in a change in binder properties . While this can be an effect in other plant types, the probability of this being a problem in other plant types is significantly reduced . The flight design can significantly affect aging in a parallel- flow plant through its effect on the veil of aggregate protect- ing the binder . It is important to note that parallel-flow plants are becom- ing a much smaller portion of the total population of asphalt mixture plants and their inclusion in this study proved dif- ficult for that reason . Counter-Flow Plant The two primary factors that affect the aging of the asphalt binder in a counter-flow plant are mixture temperature and mixing time . The additional factors that were noted for a parallel-flow plant are not expected to be a significant problem in a counter-flow plant .

B-17 References Anderson, D . A . and R . Bonaquist (2012) . NCHRP Report 709: Investi- gation of Short-Term Laboratory Aging of Neat and Modified Asphalt Binders. Transportation Research Board, Washington, D .C . Arambula, E ., A . Epps Martin, E . S . Park, C . Spiegelman, and C . J . Glover (2005) . “Factors Affecting Binder Properties Between Production and Construction .” Journal of Materials in Civil Engineering, ASCE, Vol . 17, No . 1, pp . 89–98 . Chadbourn, B . A ., E . L . Skok, Jr ., D . E . Newcomb, B . L . Crow, and S . Spindle (1999) . “The Effect of Voids in Mineral Aggregate on Hot Mix Asphalt Pavements .” MN/RC-2000-13, University of Minnesota, Department of Engineering, Minneapolis, Minnesota . Chollar, B . H ., J . A . Zenewitz, J . G . Boone, K . T . Tran, and D . T . Anderson (1989) . “Changes Occurring in Asphalts in Drum Dryer and Batch (Pug Mill) Mixing Operations .” Transportation Research Record 1228, TRB, National Research Council, Washington, D .C ., pp . 145–155 . Coons, R . F . and P . H . Wright (1968) “An Investigation of the Hardening of Asphalt Recovered from Pavements of Various Ages .” Journal of Association of Asphalt Paving Technologists, Vol . 37, pp . 510–528 . Corbett, L . W . and P . E . Merz (1975) . “Asphalt Binder Hardening in the Michigan Test Road After 18 Years of Service .” Transportation Research Record 544, TRB, National Research Council, Washington, D .C ., pp . 27–34 . Dickinson, E . J . (1980) . “The Hardening of Middle East Petroleum Asphalts in Pavement Surfacing .” Journal of the Association of Asphalt Paving Technologists, Vol . 49, pp . 30–63 . Glover, C ., R . R . Davison, C . H . Domke, Y . Ruan, P . Juristyarini, D . B . Knorr, and S . H . Jung (2005) . “Development of a New Method for Assessing Asphalt Binder Durability with Field Valida- tion .” Report FHWA/TX-03/1872-2 . Texas Transportation Institute, College Station, Texas . Glover, C ., A . Epps Martin, N . Prapaitrakul, X . Jin, and J . Lawrence (2009) . “Evaluation of Binder Aging and Its Influence in Aging of Hot Mix Asphalt Concrete: Literature Review and Experimental Design .” Report 0-6009-1 . Texas Transportation Institute, Texas A&M University . Houston, W ., M . W . Mirza, C . E . Zapata, and S . Raghavendra (2005) . NCHRP Web-Only Document 113: Environmental Effects in Pave- ment Mix and Structural Design Systems. Transportation Research Board of the National Academies, Washington, D .C . Kandhal, P ., and R . Mallick (1997) “Pavement Recycling Guidelines for State and Local Governments, Participant’s Reference Book .” FHWA- SA-98-042 . National Center for Asphalt Technology, Washington, D .C . Kennedy, T . and G . A . Huber (1985) . “Effect of Mixing Temperature and Stockpile Moisture on Asphalt Mixtures .” Transportation Research Record 1034, TRB, National Research Council, Washington, D .C ., pp . 35–46 . Lee, D . Y ., J . A . Guinn, P . S . Khandhal, and R . L . Dunning (1990) . “Absorption of Asphalt into Porous Aggregates .” SHRP-A/UIR-90- 009 . TRB, National Research Council, Washington, D .C . Lund, J . W . and J . E . Wilson (1984) . “Evaluation of Asphalt Aging in Hot Mix Plants .” Journal of the Association of Asphalt Paving Technologists, Vol . 53, pp . 1–18 . Middleton, S . C ., J . C . Goodknight, and J . S . Eaton (1967) . “The Effects of Hot Storage on an Asphaltic Concrete Mix .” Journal of the Asso- ciation of Asphalt Paving Technologists, Vol . 36, pp . 180–205 . Morian, N ., E . Y . Hajj, C . J . Glover, and P . Sebaaly (2011) . “Oxidative Aging of Asphalt Binders in Hot-Mix Asphalt Mixtures .” Transpor- tation Research Record: Journal of the Transportation Research Board, No. 2207, Transportation Research Board of the National Academies, Washington, D .C ., pp . 107–116 . Nadeau, G . (2012) . “Warm Mix and the ‘Every Day Counts’ Initiative .” Asphalt Pavement, Vol . 17, No . 1, pp . 16–19 . Plancher, H ., E . L . Green, and J . C . Petersen (1976) . “Reduction of Oxi- dative Hardening of Asphalts by Treatment with Hydrated Lime— A Mechanistic Study .” Journal of the Association of Asphalt Paving Technologists, Vol . 45, pp . 1–24 . Prapaitrakul, N . (2009) . “Toward an Improved Model of Asphalt Binder Oxidation in Pavements .” Ph .D . Dissertation, Texas A&M University . Rand, D . and R . Lee (2012) . “Warm Mix Asphalt .” Lone Star Roads, Texas Department of Transportation and FHWA, Issue 1 . US Army Corps of Engineers (2000) . Hot-Mix Asphalt Paving Hand- book, Second Edition .