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5 CHAPTER 1 Introduction 1.1 Background at lower temperatures (3). Lubrication rather than viscosity reduction may be the primary mechanism by which many Warm mix asphalt (WMA) refers to asphalt concrete mix- WMA processes improve workability and compactability at tures that are produced at temperatures approximately 50F lower temperatures. (28C) lower (or more) than temperatures typically used in the Sasobit is the wax that has been used most extensively for production of hot mix asphalt (HMA). The goal with WMA is WMA projects in the United States. Sasobit is a Fischer- to produce mixtures with similar strength, durability, and per- Tropsch wax that is produced from coal gasification. It is formance characteristics as HMA using substantially reduced supplied in pellet form and is typically added at the rate of production temperatures. There are important environmental 1.5 percent by weight of binder. The pellet can be added to and health benefits associated with reduced production tem- the binder at the asphalt terminal or in the plant supply tank, peratures including lower greenhouse gas emissions, lower fuel or it can be added to the mixture by blowing it into the drum consumption, and reduced exposure of workers to asphalt in a manner similar to the addition of fibers to stone matrix fumes. Lower production temperatures can also potentially asphalt (SMA). improve pavement performance by reducing binder aging, Several WMA processes use foaming to permit coating and providing added time for mixture compaction, and allowing provide workability at lower production temperatures. When improved compaction during cold weather paving. small amounts of water are added to hot asphalt, the water WMA technologies were first introduced in Europe in the vaporizes and the vapor is encapsulated in the binder. This late 1990s as a measure to reduce greenhouse gas emissions. produces a foaming action in the binder, temporarily increas- Since then, a number of WMA processes have been devel- ing the volume of the binder and lowering its viscosity, which oped in Europe and the United States. Brief descriptions of improves coating and workability. Foamed asphalt has been several of these processes are presented here. The National used for over 50 years to produce cold mixes (4). Early drum Asphalt Pavement Association (NAPA) publication, Warm- mix plants also took advantage of foaming that resulted from Mix Asphalt: Best Practices (2) presents more detailed informa- incomplete drying of aggregates to produce mixtures at lower tion on many of these processes including the types of plant temperatures (5). modifications that are needed with each. Table 1 summa- A variety of methods are used to produce foamed asphalt. rizes the various WMA processes identified under NCHRP Aspha-min and Advera are synthetic zeolites. Zeolites are Project 09-43. minerals that have approximately 20 weight percent water The earliest WMA processes developed in Europe were trapped in their porous structure. Upon heating to approxi- based on using either waxes or foamed asphalt. Waxes are mately 185F (85C), the water is released, and when this is added to the binder to reduce its viscosity and improve lubri- done in the presence of asphalt binder, foamed asphalt is pro- cation. These materials typically have melting points below duced. Synthetic zeolite additives are typically added at the normal HMA production temperatures. At temperatures rate of 0.25 percent by weight of the asphalt mixture. A vari- above the melting point, these materials reduce the viscosity ety of methods can be used to add synthetic zeolites at the of the asphalt binder. Below the melting point, these materi- plant. To be effective, it is critical that the additive is quickly als tend to increase the stiffness of the binder. Recent research encapsulated in the asphalt binder and not lost in the exhaust suggests that wax additives also improve the binder's lubrica- air stream of the plant. Zeolites have been used on several tion capability resulting in improvement in mix workability projects in the United States.

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6 Table 1. Summary of WMA processes identified during NCHRP Project 09-43. Name Process/Additive Company Website Accu-Shear Dual Warm Foaming system Stansteel Mix Additive System Adesco/Madsen Static Foaming system Adesco/Madsen Inline Vortex Mixer 20Vortex%20Mixer%20Brochure.pdf Advera Zeolite PQ Corporation Maxam Equipment AQUABLACK Foaming system Company, Inc. AquaFoam Foaming system Reliable Asphalt Products Asphaltan B Montan wax Romonta Aspha-min Zeolite Eurovia Cecabase RT Unspecified additive Ceca warm_coated_material/ Double Barrel Green Foaming system Astec, Inc. id=117&Itemid=188 Emulsion with unspecified Evotherm ET MeadWestvaco additives Evotherm DAT Unspecified additive Evotherm 3G Unspecified additive Licomont BS-100 Fatty acid derivative Clariant BF00484894/$FILE/20100203_Clariant_LowEmissionModifierBoosts.pdf Sequential coating using McConnaughay Low Emission Asphalt wet fine aggregate and Technologies unspecified additive Meeker Warm Mix Foaming system Meeker Equipment Asphalt System Rediset WMX Unspecified additive Akzo Nobel 0907.PDF Sasobit Fischer Tropsch wax Sasobit Terex Warm Mix Foaming system Terex Roadbuilding Asphalt Thipoave Sulfur plus compaction aid Shell Trinidad Lake Asphalt plus Lake Asphalt of Trinidad TLA-X modifiers and Tobago technology.html Ultrafoam GX Foaming system Gencor Industries, Inc. Soft binder followed by Kolo Veidekke, Shell WAM Foam hard foamed binder Bitumen Asphalt foaming is also used in the low emission asphalt These systems produce foamed asphalt by directly injecting (LEA) process. In the LEA process, the coarse aggregate and a water into the hot asphalt binder at the mixing drum. Water portion of the fine aggregate are heated to normal HMA tem- is added at the rate of approximately 1 to 2 percent by weight peratures and mixed with the binder. A coating and adhesion of binder. The systems are designed to provide the appropri- additive (approximately 0.5 percent by weight of binder) is ate ratio of water to asphalt binder, which governs the prop- added to the binder in the asphalt supply line to the plant. After erties of the resulting foam. The primary reported benefits of the heated portion of the aggregate is coated, cold, wet, fine these systems are the following: (1) there is no change in the aggregate or a blend of fine aggregate and recycled asphalt mixing process, and (2) special additives are not required. pavement (RAP) are added. The wet portion of the mixture has Foaming systems have been used on numerous projects in the a moisture content of 3 to 4 percent. When heated, this mois- United States. ture is liberated as steam, which causes the asphalt coating to Foamed asphalt is also used in the two-stage WAM Foam foam and encapsulate the uncoated fine aggregate. LEA is a process. This process adds a soft binder and a hard, foamed complex thermodynamic process where the temperature of the binder at different times during the mixing process. In the first mixture drops rapidly as the moisture in the wet portion of the stage, a soft binder is used to fully coat the coarse aggregate. aggregate turns to steam. The final discharge temperature is The soft binder is typically 20 to 30 percent of the total binder slightly less than 212F (100C), which allows some of the content of the mixture. In the second stage, a hard binder is steam to condense into water that aids in the workability and foamed onto the pre-coated aggregate. The grades of the two compaction of the mixture. The LEA process has been used on binders are selected to produce a blended binder that satisfies several projects in New York and Pennsylvania. the performance grade requirement for the project location. Recently, major asphalt plant and equipment suppliers in The WAM Foam process has not been used to date in the the United States have introduced various foaming systems. United States.