National Academies Press: OpenBook

Mix Design Practices for Warm-Mix Asphalt (2011)

Chapter: Chapter 1 - Introduction

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Page 5
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Page 5
Page 6
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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Page 6
Page 7
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2011. Mix Design Practices for Warm-Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14488.
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51.1 Background Warm mix asphalt (WMA) refers to asphalt concrete mix- tures that are produced at temperatures approximately 50°F (28°C) lower (or more) than temperatures typically used in the production of hot mix asphalt (HMA). The goal with WMA is to produce mixtures with similar strength, durability, and per- formance characteristics as HMA using substantially reduced production temperatures. There are important environmental and health benefits associated with reduced production tem- peratures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes. Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving. WMA technologies were first introduced in Europe in the late 1990s as a measure to reduce greenhouse gas emissions. Since then, a number of WMA processes have been devel- oped in Europe and the United States. Brief descriptions of several of these processes are presented here. The National Asphalt Pavement Association (NAPA) publication, Warm- Mix Asphalt: Best Practices (2) presents more detailed informa- tion on many of these processes including the types of plant modifications that are needed with each. Table 1 summa- rizes the various WMA processes identified under NCHRP Project 09-43. The earliest WMA processes developed in Europe were based on using either waxes or foamed asphalt. Waxes are added to the binder to reduce its viscosity and improve lubri- cation. These materials typically have melting points below normal HMA production temperatures. At temperatures above the melting point, these materials reduce the viscosity of the asphalt binder. Below the melting point, these materi- als tend to increase the stiffness of the binder. Recent research suggests that wax additives also improve the binder’s lubrica- tion capability resulting in improvement in mix workability at lower temperatures (3). Lubrication rather than viscosity reduction may be the primary mechanism by which many WMA processes improve workability and compactability at lower temperatures. Sasobit is the wax that has been used most extensively for WMA projects in the United States. Sasobit is a Fischer- Tropsch wax that is produced from coal gasification. It is supplied in pellet form and is typically added at the rate of 1.5 percent by weight of binder. The pellet can be added to the binder at the asphalt terminal or in the plant supply tank, or it can be added to the mixture by blowing it into the drum in a manner similar to the addition of fibers to stone matrix asphalt (SMA). Several WMA processes use foaming to permit coating and provide workability at lower production temperatures. When small amounts of water are added to hot asphalt, the water vaporizes and the vapor is encapsulated in the binder. This produces a foaming action in the binder, temporarily increas- ing the volume of the binder and lowering its viscosity, which improves coating and workability. Foamed asphalt has been used for over 50 years to produce cold mixes (4). Early drum mix plants also took advantage of foaming that resulted from incomplete drying of aggregates to produce mixtures at lower temperatures (5). A variety of methods are used to produce foamed asphalt. Aspha-min and Advera are synthetic zeolites. Zeolites are minerals that have approximately 20 weight percent water trapped in their porous structure. Upon heating to approxi- mately 185°F (85°C), the water is released, and when this is done in the presence of asphalt binder, foamed asphalt is pro- duced. Synthetic zeolite additives are typically added at the rate of 0.25 percent by weight of the asphalt mixture. A vari- ety of methods can be used to add synthetic zeolites at the plant. To be effective, it is critical that the additive is quickly encapsulated in the asphalt binder and not lost in the exhaust air stream of the plant. Zeolites have been used on several projects in the United States. C H A P T E R 1 Introduction

Asphalt foaming is also used in the low emission asphalt (LEA) process. In the LEA process, the coarse aggregate and a portion of the fine aggregate are heated to normal HMA tem- peratures and mixed with the binder. A coating and adhesion additive (approximately 0.5 percent by weight of binder) is added to the binder in the asphalt supply line to the plant. After the heated portion of the aggregate is coated, cold, wet, fine aggregate or a blend of fine aggregate and recycled asphalt pavement (RAP) are added. The wet portion of the mixture has a moisture content of 3 to 4 percent. When heated, this mois- ture is liberated as steam, which causes the asphalt coating to foam and encapsulate the uncoated fine aggregate. LEA is a complex thermodynamic process where the temperature of the mixture drops rapidly as the moisture in the wet portion of the aggregate turns to steam. The final discharge temperature is slightly less than 212°F (100°C), which allows some of the steam to condense into water that aids in the workability and compaction of the mixture. The LEA process has been used on several projects in New York and Pennsylvania. Recently, major asphalt plant and equipment suppliers in the United States have introduced various foaming systems. These systems produce foamed asphalt by directly injecting water into the hot asphalt binder at the mixing drum. Water is added at the rate of approximately 1 to 2 percent by weight of binder. The systems are designed to provide the appropri- ate ratio of water to asphalt binder, which governs the prop- erties of the resulting foam. The primary reported benefits of these systems are the following: (1) there is no change in the mixing process, and (2) special additives are not required. Foaming systems have been used on numerous projects in the United States. Foamed asphalt is also used in the two-stage WAM Foam process. This process adds a soft binder and a hard, foamed binder at different times during the mixing process. In the first stage, a soft binder is used to fully coat the coarse aggregate. The soft binder is typically 20 to 30 percent of the total binder content of the mixture. In the second stage, a hard binder is foamed onto the pre-coated aggregate. The grades of the two binders are selected to produce a blended binder that satisfies the performance grade requirement for the project location. The WAM Foam process has not been used to date in the United States. 6 Name Process/Additive Company Website Accu-Shear Dual Warm Mix Additive System Foaming system Stansteel http://www.stansteel.com/sip.html Adesco/Madsen Static Inline Vortex Mixer Foaming system Adesco/Madsen http://www.asphaltequipment.com/documents/Static%20Inline% 20Vortex%20Mixer%20Brochure.pdf Advera Zeolite PQ Corporation http://www.pqcorp.com/products/AdveraWMA.asp AQUABLACK Foaming system Maxam Equipment Company, Inc. http://maxamequipment.com/AQUABlackWMA.htm AquaFoam Foaming system Reliable Asphalt Products http://www.reliableasphalt.com/Default.asp Asphaltan –B Montan wax Romonta http://www.romonta.de/ie4/english/romonta/i_wachse.htm Aspha-min Zeolite Eurovia http://www.eurovia.fr/en/produit/135.aspx?print=y Cecabase RT Unspecified additive Ceca http://www.cecachemicals.com/sites/ceca/en/business/bitumen_additives/ warm_coated_material/warm_coated_material.page Double Barrel Green Foaming system Astec, Inc. http://www.astecinc.com/index.php?option=com_content&view=article&id=117&Itemid=188 Evotherm ET Emulsion with unspecified additives Evotherm DAT Unspecified additive Evotherm 3G Unspecified additive MeadWestvaco http://www.meadwestvaco.com/Products/MWV002106 Licomont BS-100 Fatty acid derivative Clariant http://clariant.com/C12576850036A6E9/A0F44E23B922E21CC12576BF00484894/$FILE/20100203_Clariant_LowEmissionModifierBoosts.pdf Low Emission Asphalt Sequential coating using wet fine aggregate and unspecified additive McConnaughay Technologies http://www.mcconnaughay.com/lowemissionasphalt_intro.php Meeker Warm Mix Asphalt System Foaming system Meeker Equipment http://www.meekerequipment.com/new_warmmixad1.html Rediset WMX Unspecified additive Akzo Nobel http://www.surfactants.akzonobel.com/asphalt/pdf/Rediset%20Brochure_0907.PDF Sasobit Fischer Tropsch wax Sasobit http://www.sasolwax.us.com/sasobit.html Terex Warm Mix Asphalt Foaming system Terex Roadbuilding http://www.terexrb.com/default.aspx?pgID=308 Thipoave Sulfur plus compaction aid Shell http://www.shell.com/home/content/sulphur/your_needs/products/in_roads/ TLA-X Trinidad Lake Asphalt plus modifiers Lake Asphalt of Trinidad and Tobago http://www.trinidadlakeasphalt.com/home/products/tla-x-warm-mix- technology.html Ultrafoam GX Foaming system Gencor Industries, Inc. http://gencorgreenmachine.com WAM Foam Soft binder followed by hard foamed binder Kolo Veidekke, Shell Bitumen http://www.shell.com/home/content/bitumen/products/shell_wam_foam/ Table 1. Summary of WMA processes identified during NCHRP Project 09-43.

A few processes that rely on chemical additives have been developed in the United States and Europe. The manufactur- ers do not disclose specific information on the chemicals used in these processes. The first chemical additive process used in the United States was the Evotherm process developed by MeadWestvaco and introduced in 2005. The active ingredients in Evotherm are chemical additives that reportedly improve coating, workability, and adhesion at lower temperatures. Ini- tially, Evotherm was supplied as a high residue emulsion, cur- rently referred to as Evotherm ET (Emulsion Technology). The emulsion contained approximately 70-percent asphalt binder by weight. The water in the emulsion vaporizes when mixed with hot aggregates leaving the residual asphalt and chemical additives. A number of projects were constructed in the United States using the Evotherm ET process. MeadWestvaco then introduced a process where the chemical additives are injected as a solution directly into the asphalt line at the plant. This process is referred to as Evotherm DAT (Dispersed Addi- tive Technology). It has the advantage that much less water is added to the mixture compared to the emulsion process. MeadWestvaco has recently introduced a third-generation process referred as Evotherm 3G, which is a water-free warm mix technology developed jointly by Ergon Asphalt and Emul- sions, Inc., and Mathy Construction Company. This process allows the additive to be mixed with the binder at a terminal and distributed to asphalt plants using the normal binder distribu- tion process. Because of their improved convenience, Evotherm DAT and Evotherm 3G have largely replaced Evotherm ET. Rediset WMX is a chemical process that was introduced in the United States in 2007. Rediset WMX is produced by Akzo Nobel and is marketed as a warm mix additive with adhesion- promoting properties. It is supplied as a pellet and added at the rate of 1.5 to 2.5 percent by weight of the asphalt binder. The pellets can be added to the binder at the asphalt terminal or in the plant supply tank, or they can be added to the mix- ture by blowing them into the drum in a manner similar to the addition of fibers to SMA. 1.2 Problem Statement and Objective NAPA has been instrumental in bringing WMA technolo- gies into practice in the United States. Numerous demonstra- tion projects have been constructed since 2004. These projects have demonstrated the feasibility of using warm mix processes in the United States. Pavements have been successfully con- structed using various warm mix processes with only minimal changes to equipment and quality control practices. These proj- ects have served the important functions of introducing WMA to agency and contractor personnel; demonstrating the con- structability of WMA; and providing initial data on energy usage, emissions, and pavement performance. The success of these demonstration projects has led some state highway agen- cies to allow WMA to be used routinely on paving projects. One of the critical issues facing WMA is the lack of a formal mixture design procedure. For most WMA projects con- structed in the United States, WMA has been substituted into a mixture designed as HMA with no change to the job mix for- mula. If warm mix is to replace hot mix in the future, a labora- tory mixture design procedure for WMA must be established. The objective of NCHRP Project 09-43 was to develop mixture design and analysis procedures that can be used with the wide range of warm mix processes that are currently available or may likely become available in the future. 7

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 691: Mix Design Practices for Warm-Mix Asphalt explores a mix design method tailored to the unique material properties of warm mix asphalt technologies.

Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 50°F (28°C) or more cooler than typically used in the production of hot mix asphalt (HMA). The goal of WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures.

There are important environmental and health benefits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes.

Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving.

Appendices to NCHRP Report 691 include the following. Appendices A, B, and D are included in the printed and PDF version of the report. Appendices C and E are available only online.

• Appendix A: Draft Appendix to AASHTO R 35: Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

• Appendix B: Commentary to the Draft Appendix to AASHTO R 35

Appendix C: Training Materials for the Draft Appendix to AASHTO R 35

• Appendix D: Proposed Standard Practice for Measuring Properties of Warm Mix Asphalt (WMA) for Performance Analysis Using the Mechanistic-Empirical Pavement Design Guide Software

Appendix E: NCHRP Project 09-43 Experimental Plans, Results, and Analyses

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