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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
×
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Suggested Citation:"I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)." National Academies of Sciences, Engineering, and Medicine. 2011. Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14615.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

This report presents special mixture design considerations and methods used with warm mix asphalt (WMA) and is a supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt. In this report, all references to chapters refer to the corresponding chapters in NCHRP Report 673. Although the procedures described have been specifically selected for use in designing dense-graded mixtures, most can be applied to the design of other mix types with little or no modification. Before reading this report, engineers and technicians should make certain they have a thorough understanding of the dense-graded mix design process presented in Chapter 8 and the procedures for incorporating RAP into hot mix asphalt (HMA) discussed in Chapter 9 of NCHRP Report 673. NCHRP Report 714 is based on research conducted in NCHRP Project 9-43, “Mix Design Practices for Warm Mix Asphalt,” which concluded that only minor modifications of cur- rent mix design practice are needed to address WMA. These modifications are discussed in detail herein. Part II provides a commentary to support the proposed design considerations and methods. What is WMA? WMA refers to asphalt concrete mixtures produced at temperatures approximately 50°F (28°C) or more cooler than typically used in the production of HMA. The goal with WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA while using sub- stantially reduced production temperatures. There are important environmental and health bene- fits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes. Lower production tem- peratures can also improve pavement performance by (1) reducing binder aging, (2) providing added time for mixture compaction, and (3) allowing improved compaction during cold weather paving. For these reasons, many WMA technologies may also be incorporated in the production of HMA at typical production temperatures during cold weather paving. WMA technologies were first introduced in Europe in the late 1990s as one measure to reduce greenhouse gas emissions. Since then, many WMA processes have been developed in Europe and the United States. At the time this report was completed (2011), approximately 20 WMA processes were marketed in the United States. These processes included chemical, wax, and syn- thetic zeolite additives that can be blended with the binder or added to the mixture during pro- duction; plant foaming systems; and sequential mixing processes. The National Asphalt Pavement Association (NAPA) publication, Quality Improvement Series 125, “Warm-Mix Asphalt: Best Practices,” presents more detailed information on many of these processes including the types of plant modifications needed with each. Overview of WMA Design The design of WMA is very similar to the design of HMA, following the 11 steps described in Chapter 8 for the design of dense-graded HMA. Table 1 summarizes the differences between WMA and HMA for each of the 11 steps. 1 I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

Specimen fabrication in Step 10, Evaluate and Refine Trial Mixtures, is the primary differ- ence for the design of WMA compared with HMA. Procedures for specimen fabrication are process specific; therefore, information on the WMA process that will be used and the planned production and compaction temperatures must be collected in Step 1 at the beginning of the WMA mix design process. Given that binder absorption is lower in WMA mixtures, the lower absorption should be accounted for when estimating the target binder content in Step 6. Another important difference between WMA and HMA design occurs in the selection of binders in Step 2. The high-temperature grade of the recycled binders should be lower than the planned WMA compaction temperature to promote mixing of the new and recycled binders. When using blending charts, the low-temperature grade of the new binder may be improved due to the lower aging that occurs at WMA temperatures. The following sections provide step-by-step discussions of the similarities and differences between WMA and HMA. These are followed by an example WMA design. Step 1. Gather Information The design of WMA requires the same information about the design traffic level, the climate at the place of construction, available aggregates and binders, anticipated lift thickness, and pavement type (that is, surface, intermediate, or base course) as the design of HMA. In addition, WMA design requires information on the WMA process and the planned mixing and compaction temperatures because the fabrication of WMA specimens in the laboratory is process specific, simulating in an approximate manner, the production of the mixture in the field. Table 2 summarizes the informa- tion that should be collected for designing WMA mixtures and compares this information to that required for designing HMA mixtures. 2 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Step Description Major WMA Differences 1 Gather Information 1. WMA process, 2. Additive rates, 3. Planned production temperature, 4. Planned compaction temperature. 2 Select Asphalt Binder 1. Recommended limit on high-temperature stiffness of recycled binders. 2. May consider low-temperature grade improvement when using blending charts. 3 Determine Compaction Level Same as HMA 4 Select Nominal Maximum Aggregate Size Same as HMA 5 Determine Target VMA and Design Air Voids Value Same as HMA 6 Calculate Target Binder Content 1. Lower asphalt absorption due to lower temperatures. 7 Calculate Aggregate Volume Same as HMA 8 Proportion Aggregate Blends for Trial Mixtures Same as HMA 9 Calculate Trial Mixture Proportions by Weight and Check Dust/Binder Ratio Same as HMA 10 Evaluate and Refine Trial Mixtures 1. WMA process specific specimen fabrication procedures, 2. Lower short-term aging temperature, 3. Evaluate coating and compactability in lieu of viscosity-based mixing and compaction temperatures. 11 Compile Mix Design Report Same as HMA Table 1. Steps in design of dense-graded HMA and WMA.

WMA process selection is best made by the producer in consultation with the specifying agency and WMA process suppliers considering (1) available performance data, (2) cost of the required warm mix additives, (3) planned production and compaction temperatures, (4) planned produc- tion rates, (5) existing plant capabilities, and (6) plant and laboratory modifications required to successfully use the WMA process. For the purposes of mixture design, the various WMA processes can be grouped into four generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. Specimen fabrication techniques are somewhat different for each of these categories. Given that viscosity-based mixing and compaction temperatures are not applicable to many WMA processes, the planned production and compaction temperatures are used in the WMA mixture design process to evaluate coating and the compactability/workability of the WMA. It should be empha- sized that the optimal production and compaction temperatures are different for the various WMA processes and should be carefully considered when selecting production and compaction temperatures to be used in the WMA design process. I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 3 Type of Information Detail WMA HMA Site Geographic Location X X Climate Relating to Binder Grade X X Design Traffic Level X X Design Life X X Unusual Performance Requirements X X Construction Lift Thickness X X Haul Time X X Construction Temperatures X X Unusual Specification Requirements X X Unusual Construction Requirements X X Pavement Mix Type X X Distance from Pavement Surface X X Aggregate Nominal Maximum Size X X Gradation X X Specific Gravity and Absorption X X Specification Properties X X Binder Performance Grade X X PG Plus Properties, if applicable X X Type of Modification, if applicable X X Continuous Performance Grade for Blending Chart Analysis X X Mixing and Compaction Temperatures NA X RAP Binder Content X X Continuous Performance Grade X X Nominal Maximum Size X X Gradation X X Specific Gravity and Absorption X X Specification Properties X X Anti-Strip Additives Type X X Dosage Rate X X WMA WMA Process X NA Additive Type X NA Additive Dosage Rate X NA Production Temperature X NA Compaction Temperature X NA Table 2. Information required for WMA and HMA design.

Step 2. Select Asphalt Binder The grade of binder used in WMA mixtures with less than 15% recycled binder is the same as that for an HMA mixture designed for the same conditions. The change in the high- and low- temperature properties of the binder due to lower WMA temperatures is not sufficient to warrant a change in the grade of the binder used in the mixture. The binder grade used in WMA should be either (1) the grade required by the specifying agency for HMA or (2) selected as discussed in Chapter 8 considering 1. The climate at the project location, 2. High-temperature grade adjustment required for traffic level and speed, and 3. High-temperature grade adjustments for temporary construction. When a recycled binder is used in WMA, it is recommended that the continuous high- temperature grade of the recycled binder be equal to or lower than the planned compaction tem- perature to ensure adequate mixing of the new and recycled materials. This recommendation will generally not affect the use of recycled asphalt pavement (RAP) in WMA. The high-temperature grade of RAP in the United States ranges from about 82°C in colder climates to about 100°C in hotter climates. Planned compaction temperatures for most warm mix processes are greater than 212°F (100°C). This recommendation will, however, limit the use of recycled asphalt shingles (RAS) in many WMA processes. Many RAS binders have high-temperature grades exceeding 124°C, indicating that these materials should not be used in WMA processes where the planned compaction temperature is less than about 255°F (124°C). When adding more than 15% recycled binder to WMA, the blending chart analysis described in Chapter 9 for HMA should also be used. If permitted by the specifying agency, the low-temperature continuous grade of the new binder may be improved somewhat to account for the lower WMA process temperatures. The recommended improvement depends on the new binder grade and the production temperature. Table 3 presents recommended low-temperature binder grade improvements developed in NCHRP Project 9-43 for some common binder grades. For a WMA process having a production temperature of 250°F, the low-temperature grade improvement ranges from 0.3 to 0.7°C. For a typical blending chart analysis this translates to 5 to 10% additional RAP based on low-temperature binder grade considerations. As discussed in Chapter 9, binder grade is one of several considerations affect- ing the amount of RAP that can be added to a mixture. In summary, the grade of binder used in most WMA mixtures will be the same as that used in a comparable HMA mixture. When the WMA mixture incorporates a recycled binder, it is rec- ommended that the high-temperature grade of the recycled binder be equal to or lower than the planned compaction temperature to ensure adequate mixing of the new and recycled materials. If permitted by the specifying agency, it is reasonable to improve the low-temperature grade of the new binder somewhat when performing blending chart analyses for higher RAP content mix- tures. Although small, this improvement may permit 5 to 10% additional RAP to be added, based on low-temperature binder grade considerations. Step 3. Determine Compaction Level Although it is well documented that WMA mixtures are generally easier to compact than HMA, the same design compaction level should be used with WMA and HMA. In NCHRP Project 9-43, several mixtures with less than 1% binder absorption designed as WMA and HMA following the procedures given in this manual had optimum binder contents and volumetric properties that were essentially the same. The recommended design compaction levels for both WMA and HMA 4 Special Mixture Design Considerations and Methods for Warm Mix Asphalt

are summarized in Table 4. As discussed in Chapter 8, these compaction levels are under review and could be modified soon. Step 4. Select Nominal Maximum Aggregate Size The same nominal maximum aggregate size mixture should be used when designing the mix- ture as WMA or HMA. Usually, the nominal maximum size is given by the specifying agency. When it is not, the recommendations in Chapter 8 on the ratio of lift thickness to nominal maxi- mum aggregate size should be followed. These recommendations are reproduced in Table 5. As with HMA, smaller aggregate sizes should be used for wearing course mixtures and where extra durability is desired; this will help provide a mix that compacts easily, has low permeability, and resists fatigue cracking. Step 5. Determine Target VMA and Design Air Voids Values In the design procedure presented herein, the target VMA and design air void content are used to initially calculate the design binder content for the mixture using an assumed value for binder absorption. Trial mixtures are then prepared using the design binder content to deter- mine an aggregate gradation that provides the design air void content. Minor adjustments to the I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 5 Virgin Binder PG Grade 58-28 58-22 64-22 64-16 67-22 Average HMA Production Temperature, oF 285 285 292 292 300 Rate of Improvement of Virgin Binder Low- Temperature Grade per oC Reduction in Plant Temperature 0.035 0.025 0.025 0.012 0.025 WMA Production Temperature, oF Recommended Improvement in Virgin Binder Low- Temperature Continuous Grade for RAP Blending Chart Analysis, oC 300 NA NA NA NA 0.0 295 NA NA NA NA 0.1 290 NA NA 0.0 0.0 0.1 285 0.0 0.0 0.1 0.0 0.2 280 0.1 0.1 0.2 0.1 0.3 275 0.2 0.1 0.2 0.1 0.3 270 0.3 0.2 0.3 0.1 0.4 265 0.4 0.3 0.4 0.2 0.5 260 0.5 0.3 0.4 0.2 0.6 255 0.6 0.4 0.5 0.2 0.6 250 0.7 0.5 0.6 0.3 0.7 245 0.8 0.6 0.7 0.3 0.8 240 0.9 0.6 0.7 0.3 0.8 235 1.0 0.7 0.8 0.4 0.9 230 1.1 0.8 0.9 0.4 1.0 225 1.2 0.8 0.9 0.4 1.0 220 1.3 0.9 1.0 0.5 1.1 215 1.4 1.0 1.1 0.5 1.2 210 1.5 1.0 1.1 0.5 1.3 205 1.6 1.1 1.2 0.6 1.3 200 1.7 1.2 1.3 0.6 1.4 Table 3. Recommended improvement in virgin binder low-temperature continuous grade for RAP blending chart analysis for WMA production temperatures. Design Traffic (Million ESALs) Ndesign < 0.3 50 0.3 to < 3 75 3 to < 10 100 10 to < 30 100 30 125 Table 4. Recommended design compaction levels for dense-graded HMA mixtures.

design binder content may also be necessary to account for differences between the initially assumed binder absorption and the actual absorption in the trial mixtures. For the design of WMA mixtures, the same minimum, maximum, and target VMA values discussed in Chapter 8 for HMA should be used. These values are reproduced in Table 6. Higher design VMA will increase the binder content of the mixture, thereby improving compactability, durability, and resistance to fatigue damage, but decreasing the resistance to rutting. Decreas- ing the design VMA will have the opposite effect on compactability, durability, resistance to fatigue damage, and resistance to rutting. The target air void content for WMA mixtures should be 4.0% with an acceptable range of 3.5 to 4.5%. Lower design air voids will increase the design binder content of the mixture, thereby improving compactability, durability, and resistance to fatigue damage, but decreasing the resistance to rutting. Higher design air voids will have the opposite effect on compactability, durability, resistance to fatigue damage, and resistance to rutting. Step 6. Calculate Target Binder Content The target binder content by volume for WMA is calculated in the same manner as described in Chapter 8 for HMA: target VMA minus design air voids plus volume of binder absorbed. The lower temperatures for WMA mixtures result in less binder absorption compared with HMA. In NCHRP Project 9-43, the binder absorption for WMA mixtures was about 90% of that for HMA 6 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Application Recommended NMAS, mm Minimum Lift Thickness, mm Fine-Graded Mixtures Coarse-Graded Mixtures Leveling course mixtures 4.75 15 to 25 20 to 25 9.5 30 to 50 40 to 50 Wearing course mixtures 4.75 15 to 25 20 to 25 9.5 30 to 50 40 to 50 12.5 40 to 65 50 to 65 Intermediate course mixtures 19.0 60 to 100 75 to 100 25.0 75 to 125 100 to 125 Base course mixtures 19.0 60 to 100 75 to 100 25.0 75 to 125 100 to 125 37.5 115 to 150 150 Rich base course mixtures 9.5 30 to 50 40 to 50 12.5 40 to 65 50 to 65 Table 5. Recommended nominal maximum aggregate sizes for dense-graded HMA mixtures. Aggregate NMAS (mm) Minimum VM A A (% ) Ma xi mum VMA A (% ) Target VMA (% ) 4.75 16.0 18.0 17.0 9.5 15.0 17.0 16.0 12.5 14.0 16.0 15.0 19.0 13.0 15.0 14.0 25.0 12.0 14.0 13.0 37.5 11.0 13.0 12.0 A The specifying agency ma y increase the m ini mu m and ma xim um values for VMA by up to 1.0% to obtain mi xtures with incr eased asphalt binder content, which can im pr ove field com paction, fatigue resistance, and general durability. Care should be taken to ensure that th e resulting HMA m ixtures ma in tain adequate rut resistance for their in tended application. Table 6. VMA requirements for dense-graded mixtures.

mixtures designed using the same binder and aggregates. A reasonable estimate of the volume of binder absorbed in WMA mixtures is 45% of the volume of water absorbed by the aggregates used in the mixture. This estimate is given in Equation 1 and is used in the software program HMA Tools (available for download at http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=967) to estimate the binder content by volume for WMA mixtures. where Vb = total asphalt content by volume % VMA = target voids in the mineral aggregate, vol. % VA = design air voids, vol. % Gsb = aggregate bulk specific gravity Pwa = water absorption of the aggregate, weight % As with HMA mixture design, the binder content by volume computed at this point is an esti- mate that will be refined during Step 10, Evaluate and Refine Trial Mixtures, of the design process. For batching, the binder content by volume must be converted to binder content by weight using the specific gravity of the binder and the aggregates in the mixture. These calculations were pre- sented in Chapter 8 and are performed by HMA Tools. Step 7. Calculate Aggregate Content by Volume The total aggregate content by volume is calculated in the same manner as described in Chapter 8 for HMA: 100% minus target VMA. Determination of the total aggregate content by weight will depend on the aggregate specific gravity values, and the specific blend of aggregates used in each mixture. These calculations were presented in Chapter 8 and are performed by HMA Tools. Step 8. Proportion Aggregates for Trial Mixtures Proportioning aggregates for trial WMA mixtures is the same as described in Chapter 8 for design of HMA mixtures. The mix design procedure presented in this manual sets the binder content at a value that will provide the proper VMA once the design air void content is met. Therefore, proportioning aggregates can be thought of as determining the blend of aggregates that will provide the proper air void content for the mixture. Note that the control points given in Chapter 8 are considered guidelines and not specification requirements. Chapter 8 presented a graphical procedure for aggregate blending that uses the continuous maximum density (CMD) plot. This plot quantifies how much various aggregate gradations deviate from the maximum density gradation and is effective at identifying those changes that potentially affect the VMA of the mixture. This procedure should be followed when designing new WMA mixtures. Most WMA mixture design work will be able to adapt a specific WMA process to an existing HMA design. For this type of design, there is no need to change the aggregate proportions from those used in the HMA design; unless the binder absorption is very high, the volumetric prop- erties of the WMA and HMA mixtures will be very similar. When performing a WMA design of an existing HMA mixture in HMA Tools, enter the aggregate and RAP data (if used) in worksheets “Aggregates,” and “RAP_Aggregates.” Then, the aggregate blend for the existing mix is entered V VMA VA VMA G P b sb wa = − + − ⎛⎝⎜ ⎞⎠⎟ ⎛⎝⎜ ⎞⎠⎟1 100 2 2 1. ( ) I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 7

in worksheet “Trial_Blends.” The design then proceeds as described in Chapter 8 for HMA, deter- mining the air void content and VMA for trial batches, and then making further refinements in the aggregate gradation as needed, until the desired mix properties are met. The aggregates used in WMA should meet the aggregate specification properties given in Chap- ter 8 for HMA. There are four aggregate specification properties: (1) coarse aggregate fractured faces (CAFF); (2) flat and elongated particles in the coarse aggregate; (3) fine aggregate angularity (FAA); and (4) clay content of the fine aggregate (sand equivalent). These requirements are presented in Tables 7 to 10. Note that the aggregate specification requirements apply to the blended aggregates and should be measured on the final design blend of aggregates. For trial batches, they can be 8 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Design ESALs (million) Percentage of Particles with at Least One/Two Fractured Faces, for Depth of Pavement LayerA, mm 0 to 100 Below 100 < 0.30 55 / --- --- / --- 0.3 to < 3 75 / --- 50 / --- 3 to < 10 85 / 80 60 / --- 10 to < 30 95 / 90 80 / 75 30 or more 98 / 98B 98 / 98B ADepth of pavement layer is measured from pavement surface to surface of pavement layer. BThe CAFF requirement for design traffic levels of 30 million ESALs or more may be reduced to 95/95 if experience with local conditions and materials indicate that this would provide HMA mixtures with adequate rut resistance under very heavy traffic. Table 7. Coarse aggregate fractured faces (CAFF) requirements. Design ESALs (million) Maximum Percentage of Flat andElongated Particles at 5:1 < 0.30 --- 0.3 to < 3 10 3 to < 10 10 10 to < 30 10 30 or more 10 Criteria are presented as percent flat and elongated particles by mass. Table 8. Criteria for flat and elongated particles. Design ESALs (million) Depth of Pavement Layer from Surface, mm 0 to 100a Below 100a < 0.30 ---b --- 0.3 to < 3 40 --- 3 to < 10 45c 40 10 to < 30 45c 45c 30 or more 45c 45c Criteria are presented as percent air voids in loosely compacted fine aggregate. b Although there is no FAA requirement for design traffic levels below million ESALS, consideration should be given to requiring a minimum uncompacted void content of 40 percent for 4.75 mm nominal maximum aggregate size mixes. c The FAA requirement of 45 may be reduced to 43 if experience with local conditions and materials indicate that this would produce HMA mixtures with adequate rut resistance under the given design traffic level. 0.30 a If Less than 25 percent of a construction lift is within 100 mm of the surface, the lift may be considered to be below 100 mm for mixture design purposes. Table 9. Fine aggregate angularity (FAAAQ15) requirements.

I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 9 Design ESALs (million) Minimum Sand Equivalency Value < 0.30 40 0.3 to < 3 40 3 to < 10 45 10 to < 30 45 30 or more 50 Criteria are presented as sand equivalent value. Table 10. Maximum clay content requirements. Mi x Aggregate NMAS, mm Allowable Ran ge for Dust/Binder Rati o, by We ig ht > 4.75 0.8 to 1. 6 A 4.75 0.9 to 2.0 A The specifying agency ma y lower the allowable range for dust/binder ratio to 0. 6 to 1.2 if warra nted by local conditions and ma terials. The dust/binder ratio should however not be lowered if VMA requirem ents are increased above the standard values as listed in Table 13-6. Table 11. Requirements for dust/binder ratio. estimated from the specification property values for the individual aggregates using HMA Tools. These estimates should then be verified by measurements on the final design blend. Step 9. Calculate Trial Mix Proportions By Weight and Check Dust-to-Binder Ratio This step in the design of WMA mixtures is identical to that described in Chapter 8 for HMA mixtures. This step involves calculating the following for each trial blend of aggregates: 1. Bulk specific gravity of the aggregate blend, 2. Weight percentage of binder in the mixture, 3. Weight percentage of total aggregate in the mixture, 4. Effective binder content by weight, 5. Weight percentage of each aggregate in the mixture, 6. Weight percentage of mineral dust in the mixture, and 7. Dust to effective binder ratio. These calculations are performed automatically by HMA Tools and when RAP is included in the mixture, binder from the RAP is properly accounted for in the calculations. WMA mixtures should meet the requirements for the ratio of dust to effective binder content given in Chapter 8 for HMA. These requirements are reproduced in Table 11. Step 10. Evaluate and Refine Trial Mixtures This step involves the preparation and evaluation of laboratory specimens of WMA. The pro- cedure follows that described in Chapter 8 for HMA with slight modification. Table 12 summa- rizes the steps for WMA and HMA design. The modifications required for WMA design are 1. For some processes, the WMA additive must be calculated. 2. Viscosity-based mixing temperatures are not used with WMA. Laboratory mixing is done at the planned production temperature.

3. The short-term conditioning temperature for WMA is the planned compaction temperature. 4. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction is done at the planned compaction temperature. 5. WMA design includes an evaluation of coating and compactability using the planned pro- duction and compaction temperatures. These modifications are discussed in the sections that follow. Calculate Batch Weights Some WMA processes require an additive to be added either to the binder or to the mixture. The amount of additive needed may be specified by the WMA process supplier as percent by weight of binder or total mixture. The “Additive” sheet in HMA Tools allows the user to specify the dosage rate for up to three additives and whether the dosage rate is based on binder or total mixture weight. Batch Aggregates For most WMA processes, aggregate batching is identical to that for HMA. HMA Tools pro- vides a convenient tool for calculating batch weights for various specimens and degrees of aggre- gate processing. In one WMA process, water is added to a portion of the fine aggregate, then this wet, fine aggregate is added cold to the mixture during the mixing process. For this process, treat the wet portion of the fine aggregate as a separate fine aggregate in HMA Tools. Compute the dry aggregate batch weight for this aggregate, then add the required weight of water to the dry aggregate, mix, cover, and let stand 2 hours before using it in the mixing process. Heat Aggregates and Asphalt Binder The most notable differences between the design of WMA and HMA occur in the specimen fabrication process, which begins with this step and continues through the next three steps. Viscosity-based mixing and compaction criteria cannot be used with the wide range of WMA 10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Step Description HMA WMA Comment 1 Calculate batch weights X X Must calculate WMA additive content for some processes 2 Batch aggregates X X Must batch WMA additive for some processes 3 Heat aggregates and asphalt binder X X Use planned production temperature for WMA 4 Mix aggregates and binder X X Procedure is WMA process specific 5 Mixture conditioning X X WMA uses lower temperature 6 Compact laboratory specimens X X WMA uses lower temperature 7 Calculate volumetric composition of laboratory specimens X X 8 Adjust aggregate proportions to meet volumetric requirements X X 9 Evaluate coating and compactability NA X Used in WMA design in place of viscosity- based mixing and compaction temperatures 10 Conduct performance testing X X Moisture sensitivity for all mixtures, rutting resistance for design traffic levels of 3 m ESALs or greater procedures Table 12. Comparison of trial specimen fabrication procedures for WMA and HMA design.

processes available. In fact, research in progress suggests that enhanced lubrication, not viscosity reduction, is the primary mechanism governing the success of WMA processes. The design of WMA mixtures is done using the planned field production and compaction temperatures. The aggregates and binder that will be used are heated in an oven to approximately 27°F (15°C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP, if used, should be heated the minimum time necessary to reach this target temperature. Mix Aggregates and Binder For mixture design purposes, the various WMA processes can be grouped into the following generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. This section describes laboratory procedures for preparing each of these types of WMA mixtures. Some WMA processes may include elements from two or more of these processes. The laboratory equipment needed to produce the mixtures is generally the same as that required for HMA. A mechanical mixer capable of mixing 10 to 45 lb (5 to 20 kg) batches is needed for all WMA processes. The mixing times presented later are based on a planetary mixer with a wire whip. Bucket mixers are less efficient than planetary mixers; therefore, the mixing times may need to be increased. For WMA processes requiring the additive to be blended in the binder, a low-shear mechanical stirrer with appropriate size impeller is needed to homoge- neously blend the additive in the binder. Finally, for foamed asphalt mixtures, a laboratory-scale foamed asphalt plant capable of producing consistent foamed asphalt at the water content used in field production is needed. An example of such a device is shown in Figure 1. The device should be capable of producing foamed asphalt for laboratory batches ranging in size from approximately 20 to 45 lb (10 to 20 kg). Note that laboratory foaming plants designed for cold mix applications will require a more precise flow controller to allow foamed asphalt production I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 11 Figure 1. A foaming device for preparing WMA in the laboratory.

at the lower water contents used in WMA. Also, because these machines are designed to produce large quantities of foamed asphalt, it will be necessary to produce larger batches of WMA and then split the material needed for the various tests required. Additives Blended in the Binder For WMA processes that require the WMA additive to be blended in the binder, the additive must be blended into the binder before the WMA mixture can be produced. The required dosage rate will be provided by the WMA process supplier who usually will also provide instructions for blending the additive in the binder. HMA Tools will compute the mass of additive to add for a given batch size. If instructions for blending the additive are not provided, use the following procedure: 1. Follow the manufacturer’s instruction for storage of the additive (e.g., temperature and humidity) particularly after opening the manufacturer’s packaging. 2. Weigh the required amount of the additive into a small container. The additive is typically specified as a percent by weight of binder. For mixtures containing RAP, determine the weight of additive based on the total binder content of the mixture. 3. Heat the asphalt binder in a covered container in an oven set at 135°C until the binder is sufficiently fluid to pour. During heating, occasionally stir the binder manually to ensure homogeneity. 4. Add the required amount of additive to the binder and stir with a mechanical stirrer until the additive is totally dispersed in the binder. 5. Store the binder with WMA additive at room temperature in a covered container until needed for use in the mixture design. Some binders are being supplied with the WMA additive pre-blended into the binder. For these binders, it is not necessary to blend the additive, and the preparation of the WMA mixture proceeds as outlined below. Once the WMA additive has been added to the binder, the preparation of the WMA mixture proceeds in a similar manner as that for HMA. The following steps summarize the mixture preparation process: 1. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 2. If a liquid antistrip is required, add it to the binder per the manufacturer’s instructions. 3. Place the hot mixing bowl on a scale and zero the scale. 4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture where wt = target total weight wi = oven dry weight from Step a Pbnew = % by weight of total mix of new binder in the mixture c. Add new binder to the bowl to reach wt w w p t i bnew = − ⎛⎝⎜ ⎞⎠⎟1 100 12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt

6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 7. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Additives Added to the Mixture Some WMA processes specify that the additive be added to the mixture during plant mixing. The additive dosage rate may be given as a percent of the total mixture mass or a percentage of the binder in the mixture. If the mixture contains RAP and the dosage rate is as a percentage of the binder, remember to include the RAP binder contribution when computing the amount of additive. HMA Tools will compute the mass of additive to add for a given batch size. For these processes, the following mixing procedure should be followed: 1. Follow the manufacturer’s instruction for storage of the additive (e.g., temperature and humidity) particularly after opening the manufacturer’s packaging. 2. Weigh the required amount of the additive into a small container. 3. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 4. If a liquid antistrip is required, add it to the binder per the manufacturer’s instructions. 5. Place the hot mixing bowl on a scale and zero the scale. 6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture where wt = target total weight wi = oven dry weight from Step a Pbnew = % by weight of total mix of new binder in the mixture c. Add new binder to the bowl to reach wt 8. Pour the WMA additive into the pool of new asphalt binder. 9. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 10. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Figure 2 shows a WMA additive being added to a mixture in the laboratory. Wet Aggregate Mixtures One WMA process uses cold, wet fine aggregate to produce asphalt concrete at significantly lower discharge temperatures. In this process, a portion of the total aggregate is added wet. The coarse aggregate and dry portion of the fine aggregate are mixed with the binder at normal HMA production temperatures. The percentage of the fine aggregate that will be added wet, the mois- ture content of that portion of the fine aggregate, and the initial mixing temperature are recom- mended by the WMA process supplier. An additive is also added to the binder following the steps described above for Additives Blended in the Binder. In HMA Tools, treat the portion of the fine w w p t i bnew = − ⎛⎝⎜ ⎞⎠⎟1 100 I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 13

aggregate that will be added wet as a separate aggregate. Compute the dry aggregate batch weight for this aggregate, then the weight of water to add (based on the recommended moisture content), and then proceed as follows: 1. Add the required moisture to the wet fraction of the aggregate, mix thoroughly, then cover and let stand for at least 2 hours before mixing with the heated fraction. 2. Heat the aggregate, RAP, binder, and mixing tools to approximately 27°F (15°C) above the initial mixing temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 3. Place the hot mixing bowl on a scale and zero the scale. 4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the heated aggregates and RAP, wi b. Determine the target total weight of the mixture: where wt = target total weight wi = oven dry weight from Step a wdwf = oven dry weight of the wet fraction from the batch sheet Pbnew = percent by weight of total mix of new binder in the mixture c. Determine the target weight of the heated mixture: w w wthm t dwf= − w w w p t i dwf bnew = +( ) − ⎛⎝⎜ ⎞⎠⎟1 100 14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Figure 2. Adding a WMA additive to a mixture in the laboratory.

where wthm = target weight of the heated mixture wt = target total weight wdwf = oven dry weight of the wet fraction from the batch sheet d. Add new binder to the bowl to reach wthm 6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 30 sec. 7. Stop the mixer and immediately add the wet fraction. 8. Restart the mixer and continue to mix for 60 sec. 9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. 10. Check the temperature of the mixture in the pan. It should be between 90 and 100°C. Foamed Asphalt The preparation of foamed asphalt mixtures requires special asphalt binder foaming equip- ment that can produce foamed asphalt using the amount of moisture that will be used in field production. The procedure for preparing foamed asphalt mixtures is as follows: 1. Prepare the asphalt binder foaming equipment and load it with binder per the manufacturer’s instructions. 2. If a liquid antistrip is required, add it to the binder in the foaming equipment per the manu- facturer’s instructions. 3. Heat the aggregate, RAP, and mixing tools to approximately 27°F (15°C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 4. Prepare the foamed asphalt binder per the instructions for the foaming equipment. 5. Place the hot mixing bowl on a scale and zero the scale. 6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7. Form a crater in the blended aggregate and add the required amount of foamed asphalt into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of foamed binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture where wt = target total weight wi = oven dry weight from Step a Pbnew = percent by weight of total mix of new binder in the mixture c. Add foamed binder to the bowl to reach wt The laboratory foaming equipment uses a timer to control the amount of foamed binder provided. Make sure the batch size is large enough that the required amount of foamed binder is within the calibrated range of the foaming device. This may require producing one batch for the two gyratory specimens and the maximum specific gravity specimen at a given asphalt content and then splitting the individual samples. 8. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. w w p t i bnew = − ⎛⎝⎜ ⎞⎠⎟1 100 I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 15

9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Mixture Conditioning Procedures AASHTO R 30 describes three different procedures for mixture conditioning in a forced draft oven: (1) mixture conditioning for volumetric mix design; (2) short-term conditioning for mixture mechanical property testing; and (3) long-term conditioning for mixture mechanical property testing. The last procedure, long-term conditioning for mixture mechanical property testing, is generally not used in the design and testing of WMA mixtures and is not addressed here. WMA mixtures for both volumetric mixture design and mechanical property testing (performance evaluation) should be conditioned for 2 hours at the planned compaction temperature. The conditioning should follow AASHTO R 30, using a preheated forced-draft oven, a mixture thickness of 1 to 2 in (25 to 50 mm), and stirring of the mixture after 1 hour. The conditioning procedure for volumetric mix design is essentially identical for WMA and HMA, the only difference being in the definition of compaction temperature for the two types of mixes. Short-term conditioning for performance testing differs in that only 2 hours is required for WMA mixtures, while 4 hours is required when conditioning HMA. Also, WMA mixtures for performance testing are conditioned at the planned compaction temperature, whereas AASHTO R 30 specifies a conditioning temperature of 135°C for HMA mixtures for mechanical testing. Compact Laboratory Specimens WMA specimens are compacted in the same manner as described in Chapter 8 for HMA using a properly calibrated and maintained Superpave gyratory compactor (see Figure 3). Compact duplicate specimens in accordance with AASHTO T 312. Calculate Volumetric Composition of Laboratory Specimens Volumetric analysis of compacted WMA specimens is the same as described in Chapter 8 for HMA. Recall that the procedure used in this manual sets the binder content at a value that 16 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Figure 3. Compacting a WMA specimen in a Superpave gyratory compactor.

will provide the proper VMA once the design air void content is met, and the gradation was selected to provide an acceptable ratio of dust to effective binder content by mass. Thus the air void content is the primary volumetric factor used to determine the acceptability of the trial mixture. Given that the binder content was initially set using an assumed binder absorp- tion, the effective binder content and VMA of the trial mixture should also be analyzed. The following equations (which were derived in Chapter 5) are used to perform the volumetric analysis. Air Void Content where VA = Air void content, volume % Gmb = Bulk specific gravity of compacted mixture Gmm = Maximum theoretical specific gravity of loose mixture Total Binder Content by Volume where VB = Total asphalt binder content, % by total mix volume Pb = Total asphalt binder content, % by mix mass Gmb = Bulk specific gravity of the mixture Gb = Specific gravity of the asphalt binder Absorbed Binder by Volume where VBA= Absorbed asphalt content, % by total mixture volume Gmb = Bulk specific gravity of the mixture Pb = Total asphalt binder content, % by mix mass Gb = Specific gravity of the asphalt binder Ps = Total aggregate content, % by mix mass, equal to 100 − Pb Gsb = Average bulk specific gravity for the aggregate blend Gmm = Maximum specific gravity of the mixture Effective Binder Content by Volume where VBE = Effective asphalt content, % by total mixture volume VB = Asphalt binder content, % by mix volume VBA = Absorbed asphalt content, % by total mixture volume VBE VB VBA= − ( )5 VBA G P G P G G mb b b s sb mm = ⎛⎝⎜ ⎞⎠⎟ + ⎛⎝⎜ ⎞⎠⎟ − ⎛⎝⎜ ⎞⎠⎟⎡ 100 ⎣⎢ ⎤ ⎦⎥ ( )4 VB P G G b mb b = ( )3 VA G G mb mm = − ⎛⎝⎜ ⎞⎠⎟⎡⎣⎢ ⎤ ⎦⎥100 1 2( ) I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 17

Effective Binder Content by Mass where Pbe = Effective asphalt binder content, % by total mass Pb = Asphalt binder content, % by total mass VBE = Effective asphalt binder content, % by total mixture volume VB = Asphalt binder content, % by mix volume Voids in Mineral Aggregate where VMA = Voids in the mineral aggregate, % by total mixture volume VA = Air void content, % by total mix volume VBE = Effective binder content, % by total mixture volume Dust Proportion where D/B = dust/binder ratio, calculated using effective binder content P0.075 = mineral dust content, % by total mix weight Pbe = effective binder content, % by total mix weight HMA Tools performs the volumetric analysis using data on the bulk and maximum specific gravity of the trial mixture. The design procedure in this manual provides acceptable ranges for three volumetric factors: (1) air void content, (2) VMA, and (3) dust proportion. The acceptable ranges are summarized in Table 13. These are the same ranges used for the design of HMA. Adjust Aggregate Proportions to Meet Volumetric Requirements Adjusting WMA mixtures to meet volumetric requirements is the same as discussed in Chapter 8 for HMA. The procedure presented in this manual is designed to provide accept- D B P Pbe = 0 075. ( )8 VMA VA VBE= + ( )7 P P VBE VB be b= ⎛⎝⎜ ⎞⎠⎟ ( )6 18 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Aggregate NMAS (mm) VMA A Air Voids Dust Proportio n B Minimum (% ) Maximum (% ) Minimum (% ) Maximum (% ) Minimu m M ax imum 4.75 16.0 18.0 3.5 4.5 0.9 2.0 9.5 15.0 17.0 3.5 4.5 0.8 1.6 12.5 14.0 16.0 3.5 4.5 0.8 1.6 19.0 13.0 15.0 3.5 4.5 0.8 1.6 25.0 12.0 14.0 3.5 4.5 0.8 1.6 37.5 11.0 13.0 3.5 4.5 0.8 1.6 AThe specifying agency may increase the minimum and maximum values for VMA by up to 1.0% to obtain mixtures with increased asphalt binder content, which can improve field compaction, fatigue resistance, and general durability. Care should be taken to ensure that the resulting HMA mixtures maintain adequate rut resistance for their intended application. BThe specifying agency may lower the allowable range for dust/binder ratio to 0.6 to 1.2 for NMAS mixtures of 9.5 mm and greater if warranted by local conditions and materials. The dust/binder ratio should, however, not be lowered if VMA requirements are increased using Note A above. Table 13. Acceptable range for volumetric factors.

able volumetric properties when specimens of the trial mixture meet the design air void content. After preparing trial specimens, it may be necessary to make minor adjustments to the binder content to account for differences between the assumed and actual binder absorption. If the air voids of the trial specimens are more than a few tenths of a percent out- side the design range, then the aggregate gradation should be adjusted to change the VMA of the mixture. The general rule for adjusting aggregate blends to meet VMA requirements is that the closer an aggregate gradation is to a maximum density gradation, the lower will be its VMA. Evaluate Coating and Compactability The viscosity-based mixing and compaction temperatures used in the design of HMA can- not be used with the wide range of WMA processes currently available. For WMA, the design procedure, therefore, includes an evaluation of coating at the planned production temperature and an evaluation of compactability at the planned compaction temperature. The sections that follow describe these evaluations. Coating Coating is evaluated at the planned production temperature by preparing loose mix of the design mixture following the specimen fabrication procedures presented earlier and evaluating the coating of the coarse aggregate particles using AASHTO T 195, Standard Method of Test for Determining Degree of Particle Coating of Bituminous-Aggregate Mixtures. This test method consists of separating out the coarse aggregates of the mixture and determining the percentage of the coarse aggregate particles that are fully coated. The recommended criterion is 95% of the coarse aggregates fully coated. It should be noted that this criterion and the mixing times given earlier were developed using a planetary mixer with a wire whip. Bucket mixers are not as effi- cient as planetary mixers; therefore, laboratory mixing times may need to be increased if a bucket mixer is used. Compactability Compactability is evaluated by compacting two specimens at the planned compaction temperature and two specimens at 30°C below the planned compaction temperature. The num- ber of gyrations required to reach 8% air voids is determined for both sets of specimens. It is recommended that the increase in gyrations to 8% air voids between the planned compaction temperature and 30°C below the planned compaction temperature should be less than 25% of the number of gyrations at the planned compaction temperature. The procedure is described in detail below: 1. Prepare a sufficient quantity of the design mixture for four gyratory specimens and one maximum specific gravity measurement using the appropriate WMA fabrication procedure. 2. Determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209. 3. Compact duplicate specimens at the planned compaction temperature to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration. 4. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 5. Allow the mixture to cool to 30°C below the compaction temperature. Compact duplicate specimens to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration 6. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 7. For each specimen determine the height at a relative density of 92.0% using Equation 9. I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 19

where h92 = height at a relative density of 92% hd = height at Ndesign, as measured by the gyratory compactor %Gmmd = relative density at Ndesign 8. For each specimen, determine the number of gyrations to reach 92% relative density. This can be done by looking at the output from the gyratory compactor giving specimen height as a function of gyrations—simply find the number of gyrations at a height where the rela- tive density is 92%, h92 as determined in Step 7 above. 9. Determine the gyration ratio using Equation 10. where Ratio = gyration ratio (N92)T-30 = gyrations to 92% relative density at 30°C below the planned compaction temperature. (N92)T = gyrations to 92% relative density at the planned compaction temperature 10. The compactability is acceptable if the gyration ratio is less than 1.25. Conduct Performance Testing Like HMA, the final stage of laboratory work in a WMA design is evaluating the performance of the mixture. Performance evaluation for WMA is the same as HMA and includes evaluation of the resistance to moisture damage for all mixtures and evaluation of rutting resistance for mix- tures designed for traffic levels of 3 million ESALs and higher. As discussed above, the primary difference between WMA and HMA is in mixture conditioning—HMA mixtures are condi- tioned for 4 hours at 135°C, whereas WMA mixtures for performance testing are conditioned at the planned compaction temperature for 2 hours. Otherwise, the procedure for short-term con- ditioning for mixture mechanical testing as described in AASHTO R 30 should be followed when preparing WMA specimens for performance testing. The resistance to moisture damage is evaluated using AASHTO T 283, Resistance of Com- pacted Asphalt Mixture to Moisture-Induced Damage. Like HMA, WMA mixtures have accept- able resistance to moisture damage if the tensile strength ratio is equal to or greater than 80% and there is no visual evidence of stripping in the conditioned test specimens. When redesign- ing an existing HMA mixture using a WMA process that does not include an antistrip additive, it is not uncommon to find that the WMA mixture is less resistant to moisture damage. For these mixtures, the use of hydrated lime or an antistrip additive will usually provide acceptable results. Many WMA processes include an antistrip additive. For these processes, consult the process sup- plier for recommendations to improve resistance to moisture damage if unacceptable results are obtained at normal WMA additive dosage rates. The rutting resistance of WMA is evaluated for mixtures designed for 3 million ESALs and higher using the flow number test, AASHTO TP 79, Determining the Dynamic Modulus and Flow Number of Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT). The same testing conditions used for HMA are used with WMA. Ratio N N T T = ( ) ( ) − 92 30 92 ( )10 h h G d mmd 92 92 = ⎛⎝⎜ ⎞⎠⎟ % ( )9 20 Special Mixture Design Considerations and Methods for Warm Mix Asphalt

1. Specimens compacted to 7.0 ± 0.5% air voids, 2. Test temperature equal to the 50% reliability, 7-day maximum pavement temperature as determined using LTPPBind version 3.1. For surface courses, compute the test temperature at a depth of 20 mm. For intermediate and base courses, compute the test temperature at the top of the layer. 3. Unconfined testing with a repeated deviator stress of 87 psi (600 kPa) and a contact deviator stress of 4.4 psi (30 kPa). Lower criteria are used for WMA compared with HMA because of the reduced short-term con- ditioning. Recall that HMA used in making specimens for performance testing is conditioned for 4 hours at 275°F (135°C) while WMA used for making specimens for performance testing is con- ditioned for only 2 hours at the planned compaction temperature, which is usually lower than 275°F (135°C). The shorter conditioning time and lower condition temperature result in less aging for the binder in the WMA specimens. The WMA flow number criteria are listed in Table 14. The rutting resistance of WMA mixtures can be improved using the same adjustments described in Chapter 8 for HMA. These include • Increasing the binder high-temperature grade • Adding RAP to the mixture • If the binder is not modified, considering using a polymer-modified binder of the same grade or one high-temperature grade lower • If the binder is polymer-modified, trying a different type of modified binder • Increasing the amount of mineral filler in the mix and adjusting the aggregate gradation if necessary to maintain adequate VMA • Decreasing the design VMA value, if possible, by adjusting the aggregate gradation • Replacing part or all of the aggregate (fine or coarse or both) with a material or materials having improved angularity If a different asphalt binder is used in the mix, the volumetric composition should not change. However, if other aspects of the mix design are changed, the volumetric composition might change significantly which will require further refinement of the mix prior to further rut resistance testing. Step 11. Compile Mix Design Report The mix design report is compiled in the same manner as described for HMA. In many states, standard forms must be filled out by producers and submitted to the appropriate state agency or office for approval. In some cases, engineers or technicians may wish to develop their own mix design reports, for internal purposes or for use on private jobs. In such cases, the following information should be included in the report: • The organization that performed the mix design • The name of the technician or engineer responsible for developing the mix design I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 21 Traffic Level, Million ESALs Minimum Flow Number < 3 NA 3 to < 10 30 10 to < 30 105 30 415 Table 14. Flow number criteria for WMA mixtures.

• The date the mix design was completed • The name of the client for which the mix design was developed • The name of the project for which the mix design was developed (if applicable) • General mix design information, including the type of mix (surface course, intermediate course, base course), the nominal maximum aggregate size, the design traffic level, the Ndesign value, and any special requirements • Complete aggregate information, including for each aggregate the producer, the size designation of the aggregate, gradation, specific gravity, and all applicable specification properties • Binder information, including the binder PG grade and the name of the supplier • Composition of the mixture, including the design air void content, the design VMA, the design VBE, the mineral filler content, the target dust/binder ratio and the estimated unit weight for the mix • The planned production and compaction temperatures • The results of the evaluation of coating • The results of the compactability analysis • The results of moisture resistance testing • The results of rut resistance testing, if applicable (generally for mixtures designed for traffic levels of 3 million ESALs and over) HMA Tools includes a comprehensive mix design report that contains all of this information and additional information on the results of trial mixtures evaluated during the mix design process. This HMA Tools report might be useful to some engineers and technicians for internal purposes and might also serve as a template for those wishing to develop their own customized mix design reports. Example WMA Mix Design A 12.5-mm WMA mix is to be designed, using a foaming process. The gradation of the aggregates to be used is listed in Table 15; the table includes the gradation of the RAP to be included in the mix design. Other test properties for the four aggregates are listed in Table 16. The RAP was separated into fine and coarse fractions for testing, and the results are also included in Table 16. The specified binder grade is PG 64-22, while the grade of the binder extracted from the RAP is PG 76-16. Binder test data are given in Table 17. The binder content of the RAP is 5.2% by total weight. The design traffic level is 6 million ESALs. Referring to Table 1, the steps in a WMA mix design are as follows: 1. Gather information 2. Select asphalt binder 22 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Sieve Size (mm) % Passing by Weight No. 67 Stone 1B Stone Screenings Sand RAP 19.000 100.0 100.0 100.0 100.0 100.0 12.500 72.0 91.0 100.0 100.0 100.0 9.500 48.0 48.0 100.0 100.0 95.0 4.750 8.0 10.0 81.0 100.0 76.0 2.360 6.0 5.0 45.0 91.0 56.0 1.180 4.0 3.0 30.0 48.0 42.0 0.600 4.0 3.0 20.0 36.0 28.0 0.300 3.0 3.0 16.0 21.0 19.0 0.150 3.0 2.8 8.0 12.0 14.0 0.075 2.6 2.2 5.8 7.1 11.4 Table 15. Aggregate gradations for WMA example.

3. Determine compaction level 4. Select nominal maximum aggregate size 5. Determine target VMA and air voids values 6. Calculate target binder content 7. Calculate aggregate content 8. Proportion aggregate blends for trial mixtures 9. Calculate trial mix proportions by weight and check dust/binder ratio 10. Evaluate and refine trial mixtures 11. Compile mix design report In this example, much of Steps 1, 2, and 4 have been completed and the pertinent infor- mation listed above. The purpose of this example is to illustrate those parts of the mix design process that differ from normal HMA mix design as outlined in Chapter 8 of this manual. Therefore, those steps in the mix design that do not differ from standard HMA mix design practice are not discussed in detail in this example; readers uncertain of these steps should review Chapter 8. One of the important differences in the WMA mix design process is the limits on production and compaction temperatures. In this case, these limits have been established by the producer: 132°C for production and 124°C for compaction. In order to ensure proper compaction, the high-temperature grade of the RAP binder (81.4°C) must be less than the specified WMA com- paction temperature—124°C in this case, so the RAP binder is acceptable. The mix design in this example requires the use of two liquid additives in the mix design: an antistrip additive and a recycling agent. The antistrip additive has a specific gravity of 1.030 and is to be added at 0.50% by binder weight. The recycling additive has a specific gravity of 1.020 and is to be added at 1.00% by total mix weight. After entering the binder grading data in HMA Tools, the blended binder grade is given as a PG 64-22, with an intermediate grading temperature of 25°C. This meets the specified require- ment. Note that HMA Tools includes the low-temperature grade adjustment as described in I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 23 Property No. 67 Stone 1B stone Screenings Sand RAP, Fine RAP, Coarse Bulk Specific Gravity 2.635 2.639 2.771 2.630 2.631 2.605 Apparent Specific Gravity 2.699 2.702 2.856 2.760 2.682 2.694 Water Absorption 0.90 0.88 1.07 1.79 0.72 1.27 CAFF, One Fractured Face, % 96.0 96.0 N/A N/A N/A 100.0 CAFF, Two Fractured Faces, % 91.0 93.0 N/A N/A N/A 95.0 Flat & Elongated, % 0.8 0.0 N/A N/A N/A 1.5 FAA, Uncompacted Voids N/A N/A 48.0 43.0 44.2 N/A Sand Equivalent N/A N/A 58.0 89.0 N/A N/A Table 16. Aggregate test properties for WMA example. Property PG 64-22 Binder RAP Binder Specific Gravity 1.030 1.030 Continuous high-temperature grade, °C 59.0 81.4 Continuous intermediate-temperature grade, °C 13.0 27.4 Continuous low-temperature grade, °C -30.1 -19.7 Table 17. Binder properties for WMA example.

Table 3. Details on the calculations used to estimate blended binder grades for HMA and WMA containing RAP are given in AASHTO M 323. At a traffic level of 6 million ESALs, Ndesign is 100 gyrations (Table 8-2). Given the NMAS of 12.5 mm, the minimum VMA is 15.0% and the maximum 17.0%; therefore, a target VMA of 16.0% is selected. A target air void content of 4.0% is also selected for the mix design. For this example, an aggregate gradation somewhat below maximum density is selected, with aggregate proportions as shown in Table 18. Calculation of the volumetric composition for the trial blend is complicated by the use of liquid additives. The suggested procedure involves working through the known composition by volume, and then working back and forth between weight and volume calculations until the total binder volume and weight can be calculated. Then, the weights of the additives can be calculated. The various steps are shown in Table 19. The volume compositions in Table 19 are given in percents, which are equivalent to cm3 per 100 cm3 total volume. The weights are then calculated simply by multiplying this volume by the appropriate specific gravity. (Note that the component weights are not percentages but are in units 24 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Aggregate Wt. % in Aggregate Blend No. 67 Stone 20 1B Stone 25 Screenings 10 Manufactured Sand 15 RAP (aggregate only) 30 Table 18. Aggregate proportions of WMA example. Step Calculation Formula Result 1. VMA Given 16.00 2. Volume % of air voids (VA) Given 4.00 3. Binder effective volume % (VBE) VBE=VMA–VA 12.00 4. Volume % of aggregate (VS) VS=100 VMA 84.00 5. Weight of aggregate (Ps), g/100 cm3 Ps = VS Gsb 222.06 6. Weight of RAP aggregate (Psr), g/100 cm3 Psr = Ps % RAP in Aggregate Blend 66.62 7. Weight of RAP binder (Pbr), g/100 cm3 Pbr = Psr RAP binder content 3.46 8. Volume % RAP binder (VBR) VBR=Pbr/RAP binder specific gravity 3.36 9. Volume % of effective new binder and liquid additives (VBEN) VBEN=VBE−VBR 8.64 10. Approximate weight of effective new binder and liquid additives (Pben), g/100 cm3 Pben=VBA/specific gravity of new binder 8.86 11. Weight of absorbed binder (Pba), g/100 cm3 Pba=Ps water absorption of aggregate 0.45 1.09 12. Weight of new binder, RAP binder and liquid additives (Pb), g/100 cm3 Pb=Pbr+Pben+Pba 13.42 13. Total weight of mix (Ptot), g/100 cm3 Sum all weights 235.47 14. Weight of antistrip additive, g/100 cm3 = (0.50/100) Pb 0.066 15. Weight of recycling additive, g/100 cm3 = (1.0/100) Ptot 2.309 16. Weight % of various components =Wt. in g/100 cm3 / Ptot 100 % Varies Table 19. Calculation of volume percentages and weights for WMA example.

of g/100 cm3 volume.) In the final step (Step 16), these weights are converted to percentages by divid- ing by the total mix weight and multiplying by 100%. In order to simplify calculations, the volume and weight of new binder and liquid additives are lumped together and assumed to have the same specific gravity—that of the new binder. Because the amount of liquid additives should be very small and their specific gravity values close to that of the binder, the error in this assumption is quite small. Note that in the calculation of absorbed asphalt (Step 11), the water absorption is multiplied by 0.45 to estimate the asphalt binder absorption; in HMA designs, water absorption is mul- tiplied by 0.50 rather than 0.45. The last step in this procedure is calculation of composition in weight percent, which is done by dividing the weight of the various components by the total mix weight. The final volumetric composition of the trial mixture is summarized in Table 20. Although this procedure appears complicated, it can be performed using HMA Tools (or other similar spreadsheet programs), simplifying the calculations and reducing the chance for errors. In preparing laboratory specimens, parts of the procedure are similar to those for HMA practice. Batch weights are calculated in the same way. Because this is a foamed asphalt, a laboratory foaming unit is used to foam the binder (with antistrip additive) prior to mix- ing it with the hot aggregate and RAP. In this case, the recycling additive is added to the hot aggregate and RAP and mixed for a few seconds prior to the addition of the foamed asphalt binder. The mixed WMA is short-term oven conditioned for 2 hours at the planned com- paction temperature, 124°C. It is then compacted in the Superpave gyratory compactor for 100 gyrations. Provided the volumetrics of the trial mix are acceptable, the WMA, just as for HMA, must be evaluated for moisture resistance according to AASHTO T 283. However, the WMA must also be evaluated for coating and compactability. For the particle coating test (ASTM T 195), some of the freshly mixed WMA is set aside and spread out on a metal pan to cool. The coarse aggregate par- ticles are then separated out and the degree of coating determined. The number of completely coated, partially coated, and uncoated particles is counted. In this case, there are 145 completely coated particles, 6 particles that are partially coated, and none that are completely uncoated. The percentage of coated particles is then 145/(145 + 6) × 100% = 96%. Since this is greater than 95%, this WMA passes the coating test. Calculations for the compactability test are shown in Table 21. As explained previously, four specimens are compacted—two at the planned compaction temperature of 124°C and two at a temperature 30°C below this, 94°C. Then, the relative density and height and Ndesign are I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 25 Mix Component Percent by Total Mix Volume Bulk Specific Gravity Percent by Aggregate Weight Percent by Total Mix Weight Air 4.00 --- --- --- New Asphalt Binder 7.34 1.026 --- 3.19 RAP Asphalt Binder 3.37 1.030 --- 1.47 Liquid Antistrip Additive 0.066 1.020 --- 0.028 Recycling Additive 2.31 1.030 --- 1.00 No. 67 Stone 16.67 2.635 20 18.86 1B Stone 20.81 2.639 25 23.58 Screenings 7.90 2.771 10 9.43 Manufactured Sand 12.36 2.630 15 14.15 RAP Aggregate 25.13 2.619 30 28.29 Note: Calculations may not agree exactly because of rounding. Table 20. WMA mix composition by weight percentage from volume percentage and specific gravity values.

determined for each specimen. Then, Equation 9 is used to calculate the height at a relative den- sity of 92%. The gyratory output files are then examined to determine the number of gyrations required to reach a relative density of 92%, N92. The gyration ratio is then calculated as the ratio of N92 at TC-30/TC: In this example, the gyration ratio is 1.14, which is below the maximum allowable value of 1.25 so the mix passes this test. Because the design traffic level in this case—6 million ESALs—is greater than or equal to 3 million ESALs, performance testing using the AMPT is required as a final step in the mix design. After short-term oven conditioning for 2 hours at 116°C, two gyratory specimens are prepared and tested for flow number. In this example, the 7-day maximum pavement temperature at a depth of 20 mm and at 50% reliability is determined to be 58.5°C. The two specimens are tested at this temperature and produce flow number values of 27 and 29, giving an average value of 28. Unfortunately, this is below the minimum required value of 30 (Table 14). The mix must be adjusted to provide better rut resistance. A second trial mix design is made, increasing the RAP content from 30% to 40%; given that the asphalt binder in RAP is relatively stiff, this should increase the rut resistance of the mix. The resulting mix meets all requirements for volumetric composition, moisture resistance, coating, and compactability. Performance testing results in an average flow number of 33, meeting the minimum requirements. The laboratory mix design is completed and a report prepared. A Note on Using HMA Tools to Perform WMA Mix Designs HMA Tools has several features designed specifically for use in designing WMA. In work- sheet “General,” there is a section for entering information generally required for WMA, such as production and compaction temperature, and whether or not to allow adjustment in the low-temperature grade of the virgin binder. If allowed, HMA Tools will make this adjustment automatically. There is also a worksheet for recording data from the two mixture tests required for WMA—the coating and compactability tests. The worksheet on liquid additives, although not exclusively for use in designing WMA, allows for rigorous inclusion of such materials in the volumetric analyses performed in HMA Tools. Gyration Ratio = +( ) +( ) = 42 39 2 33 38 2 1 14. ( )11 26 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Property 124 C 94 C Specimen 1 Specimen 2 Specimen 3 Specimen 4 Density @ Ndesign 96.0 95.9 96.1 96.0 Height @ Ndesign, mm 116.7 115.3 114.1 113.0 Height @ Relative Density = 92% 121.8 120.2 119.2 117.9 Gyrations at 92% Relative Density 33 38 42 39 Gyration Ratio 1.14 < 1.25 Pass Table 21. Calculation of gyration ratio for compactability test.

References AASHTO Standards M 320, Standard Specification for Performance-Graded Asphalt Binder R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA) T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens T 209, Theoretical Maximum Specification Gravity and Density of Bituminous Paving Mixtures T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyra- tory Compactor TP79, Determining the Dynamic Modulus and Flow Number of Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT) Other Publications Bonaquist, R., “Mix Design Practices for Warm Mix Asphalt,” NCHRP Report 691, National Cooperative Highway Research Program, Washington, D.C., 2011. Christensen, D. W., “A Manual for the Design of Hot Mix Asphalt with Commentary,” NCHRP Report 673, National Cooperative Highway Research Program, Washington, D.C., 2010. Prowell, B. D., and Hurley, G. C., “Warm-Mix Asphalt: Best Practices,” Quality Improvement Series 125, National Asphalt Pavement Association, Lanham, MD, 2007. I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 27

Next: II. Commentary on Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) »
Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 714: Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary presents special mixture design considerations and methods used with warm-mix asphalt.

NCHRP Report 714 is a supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt. All references to chapters in NCHRP Report 714 refer to the corresponding chapters in NCHRP Report 673.

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