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Mix Design Practices for Warm-Mix Asphalt (2011)

Chapter: Chapter 2 - Research Approach

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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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Suggested Citation:"Chapter 2 - Research Approach." 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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82.1 Overview The general approach taken in NCHRP 09-43 to develop mix design and analysis procedures for WMA was to adapt as many of the current methods used with HMA as possible and to concentrate development efforts on areas where WMA and HMA differ substantially. Figure 1 presents a flow chart for the project. The project was divided into two phases. In Phase I, a preliminary mixture design and analysis procedure was developed based on a review of the literature and research in progress. The preliminary procedure was then revised based on the results of several laboratory studies directed at elements of the mixture design process that were expected to be differ- ent for WMA as compared to HMA. In Phase II, the revised preliminary procedure was evaluated through a laboratory sensitivity study designed to test the engineering reasonable- ness, sensitivity, and practicality of the mixture design proce- dure and a field validation study using mixtures from paving projects. Phase II also included a study to evaluate fatigue characteristics of WMA, the development of draft standards for WMA, and the development of workshop materials for the proposed WMA mixture design methods. 2.2 Differences Between the Design of WMA and HMA HMA mixture design and analysis generally consists of five major steps: (1) materials selection, (2) design aggregate struc- ture, (3) design binder content selection, (4) evaluate mois- ture sensitivity, and (5) performance analysis. Criteria for Steps 1 through 4 for HMA are contained in AASHTO M 323, Standard Specification for Superpave Volumetric Mix Design. AASHTO R 35, Standard Practice for Superpave Volumetric Design for Hot Mix Asphalt (HMA), provides procedures for Steps 1 through 4. Although there is not a standard practice addressing performance testing of HMA, several performance tests have been developed and have received some level of acceptance by industry. Performance tests are available for measuring mixture modulus, rutting resistance, and resis- tance to fatigue cracking and thermal cracking. The new mix design manual being developed under NCHRP Project 09-33 includes performance testing to ensure that mixtures sub- jected to traffic levels greater than 3 million equivalent single axle loads have adequate rutting resistance (6). Several modifications to current HMA mix design proce- dures are needed to address the wide range of WMA processes currently available and likely to become available in the future. The first step in NCHRP Project 09-43 was to identify poten- tial areas of the HMA mixture design process requiring mod- ification for WMA. These are summarized in Table 2 and discussed below for the major steps in the mixture design and analysis process. 2.2.1 Materials Selection Some elements of materials selection may require modifi- cation for WMA. Aggregate requirements for warm mix will not be different than requirements for hot mix, but it may be necessary to select different binder grades for WMA. The lower temperatures used in WMA as compared to HMA prob- ably result in less aging during plant mixing and construction; therefore, a stiffer high-temperature binder grade may be needed for satisfactory rutting performance. This effect, how- ever, may be offset by the addition of warm mix additives and the effect that these additives and water have on binder aging. The lower production temperatures may also limit the types and quantity of recycled asphalt materials that can be used in WMA. Design of HMA assumes substantial mixing of new and recycled binders, which may not be possible at the lower production temperatures used in warm mix. Lower produc- tion temperatures may also limit the effectiveness of some anti-strip additives. Finally, WMA design will require the selec- tion of an appropriate warm mix additive and dosage rate. Although dosage rates may be provided by warm mix additive C H A P T E R 2 Research Approach

9Review Literature and Research in Progress Develop Preliminary Mixture Design and Analysis Procedure Design and Execute Phase I Laboratory Studies Revise Preliminary Mixture Design and Analysis Procedure Design and Execute Laboratory Mix Design Study Design and Execute Field Verification Study Prepare Draft Mixture Design and Analysis Procedures Prepare Workshop Materials Prepare Final Report Phase I Phase II Design and Execute Laboratory Fatigue Study Figure 1. Flow chart for NCHRP Project 09-43. Step Item Special Warm Mix Considerations Binder Selection • Potentially less aging during mixing and construction due to lower production temperatures. • Effect of any warm mi x additives and warm mix processing on binder properties. Aggregate Properties • None Recycled Asphalt Pave me nt • Effect of production te mp erature on the degree of commingling of recycled and new binders. • Effect of warm mix additives and warm mix processing on the degree of commingling of recycled and new binders. Materials Selectio n Additives • Warm-mix additive selection. • Effect of lower production temperatures and warm mix additives on anti-strip additives. Nom inal Maxi mu m Aggregate Size • None Trial Gradations • None Batching • WMA process specific Mixing • WMA process specific • Method to determine appropriate mi xing temperatures for warm mix processes. • Method to assess workability of WMA. Conditioning • Verify that short-term conditioning per AASHTO R 30 applies to WMA processes. Compaction • Method to determ ine appropriate co mp action te mp eratures for warm mix processes. • Verification of com paction levels. Design Aggregate Structure Volumetric Analysis and Criteria • None Specim en Preparation • See considerations above for laboratory batching, mi xing, conditioning, and co mp action. Design Binder Content Selection Volumetric Analysis and Criteria • None Specim en Preparation • See considerations above for laboratory batching, mi xing, conditioning, and co mp action. Evaluate Moisture Sensitivity Testing and Analysis • None Specim en Preparation • See considerations above for laboratory batching, mi xing, conditioning, and co mp action. Performance Analysis Testing and Analysis • None Table 2. Areas of HMA mixture design and analysis potentially requiring modification for WMA.

suppliers, agencies should have a procedure to ensure that the recommended dosage rate is appropriate. 2.2.2 Design Aggregate Structure The design of the aggregate structure may also require some modifications for WMA. Since the goal of WMA is to produce mixtures with strength and performance characteristics simi- lar to those of HMA, the volumetric criteria used in design should not differ from those used for HMA. However, the pro- cedures used to fabricate and condition specimens may require some modification. Most WMA process developers have prepared laboratory procedures for specimen fabrication. Additionally, mixture coating, workability, and compactabil- ity must be evaluated directly instead of using viscosity- based mixing and compaction temperatures. In many WMA processes, it is impossible to directly measure the viscosity of the binder. Additionally, there is increasing evidence that the temperature reductions associated with many WMA processes are not related to the change in viscosity of the binder (3, 7). 2.2.3 Design Binder Content Selection The selection of the design binder content should not require substantial modification other than specimen-fabrication as discussed above. In NCHRP Project 09-25, “Requirements for Voids in Mineral Aggregate for Superpave Mixtures” and NCHRP Project 09-31, “Air Void Requirements for Superpave Mix Design,” relationships between mixture volumetric prop- erties and pavement performance were developed (8). These relationships confirm the importance of many of the volumet- ric criteria included in the Superpave mixture design method. An important step in achieving WMA with performance char- acteristics comparable to HMA is to use the same volumetric criteria in the design of both mixtures. 2.2.4 Evaluate Moisture Sensitivity and Performance Analysis Evaluation of the mixture for moisture sensitivity and per- formance also will not require substantial modification other than specimen fabrication. Although there is concern that some WMA may exhibit greater moisture sensitivity than HMA (9, 10, 11), AASHTO T 283, Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced Damage, is a fairly reli- able indicator of moisture-induced adhesive failure, which is the mechanism of greatest concern for WMA. The major consideration in the preparation of moisture sensitivity and performance specimens will be replicating the mechanical properties of field-mixed material in laboratory-prepared spec- imens. The same tests and criteria that are used for performance evaluation of HMA should be used with WMA. 2.3 Preliminary WMA Mixture Design and Analysis Procedure 2.3.1 Overview Based on a review of available literature for the various WMA processes and discussions with WMA process develop- ers, a preliminary mixture design and analysis procedure was developed. The preliminary procedure served two purposes. First, the preliminary procedure provided a starting point for the WMA mixture design and analysis procedure for review and comment by the project panel and WMA process develop- ers. Second, the preliminary procedure focused the Phase I testing and analysis effort on the areas of mixture design and analysis that required additional development to properly address WMA. The preliminary procedure was revised based on the findings of the Phase I testing and analysis and the com- ments received on the preliminary procedure. The revised pre- liminary procedure was further modified based on the findings of the Phase II laboratory mix design study, field validation study, and fatigue study to produce the draft standards that were the primary products of NCHRP Project 09-43. The preliminary WMA mixture design and analysis proce- dure was based on AASHTO R 35, Standard Practice for Super- pave Volumetric Design for Hot Mix Asphalt (HMA). The preliminary procedure referred to AASHTO M 323, Stan- dard Specification for Superpave Volumetric Mix Design, and AASHTO M 320, Standard Specification for Performance- Graded Asphalt Binder, for criteria for materials selection, vol- umetric design, and moisture sensitivity evaluation. Table 3 summarizes the areas where the preliminary procedure dif- fered from AASHTO R 35. The differences are discussed below. 2.3.2 WMA Process Selection A section in the preliminary mixture design and analysis procedure for WMA addressed WMA process selection. It advised that WMA process selection should be done in consul- tation with the specifying agency and technical assistance per- sonnel from WMA process suppliers. This section alerts users that when selecting a WMA process, consideration should be given to a number of factors including (1) available perfor- mance data, (2) the cost of any warm mix additives, (3) planned plant mixing and field compaction temperatures, (4) planned production rates, (5) plant capabilities, and (6) modifications required to successfully use the WMA process with available field and laboratory equipment. 2.3.3 Binder Grade Selection and RAP For the preliminary procedure, it was hypothesized that the WMA production temperature would be a consideration in the selection of the high-temperature binder grade and the allow- 10

able RAP content of the mixture. Because the lower produc- tion temperatures in WMA result in reduced binder aging, the preliminary procedure provided a conceptual table for bumping the high-temperature binder grade based on the planned production temperature. Similarly, since the degree of mixing of RAP and new binders in mixtures containing RAP is likely to be temperature dependent, the preliminary procedure provided a second conceptual table for limiting the RAP content of mixtures based on the production tempera- ture and the compatibility of the RAP and new binder. An appendix was added to provide procedures for measuring the compatibility of two binders with ASTM D6703, Standard Test Method for Automated Heithaus Titrimetry. Experi- ments to flesh out the conceptual tables for binder grade bumping and RAP mixing were included in the Phase I test- ing and analysis. In addition to the above, the preliminary procedure pro- vided more detailed information on how to characterize RAP materials for mixture design. This information was provided in an appendix and was consistent with the recommendations for RAP analysis that were included in the mix design manual for HMA being developed under NCHRP Project 09-33 (6). 2.3.4 Specimen-Fabrication Procedures The preliminary design procedure documented specimen- fabrication procedures for several WMA processes. These pro- cedures identify the equipment and methods that are needed to prepare WMA specimens in the laboratory. These specimen- fabrication procedures were included in an appendix to the preliminary mixture design and analysis procedure for WMA. Short-term aging of WMA was tentatively set at 2 h at the planned field compaction temperature based on limited research performed by some WMA process developers. 2.3.5 Process Temperature Since binder viscosity-temperature relationships cannot be developed for many WMA processes, mixing and compaction temperatures cannot be used to control coating, workability, and compactability of WMA. The preliminary procedure proposed evaluating coating, workability, and compactability directly during the evaluation of trial blends. This is accom- plished by preparing trial blends using the planned production temperature and compacting the trial blends using the planned field compaction temperature. Coating is evaluated using AASHTO T 195, Determining Degree of Particle Coating of Bituminous-Aggregate Mixtures. A standard procedure for evaluating workability is not available; therefore, as part of the Phase I testing and analysis several possible workability tests were evaluated. In the preliminary procedure for WMA, it was envisioned that the density at Ninitial in the gyratory compactor would serve as a measure of compactability. The use of the den- sity at Ninitial as a measure of compactability was evaluated dur- ing the Phase I testing and analysis. 2.3.6 Required Performance Testing Evaluation of the moisture sensitivity of the design mixture in the preliminary mixture design and analysis procedure for WMA is the same as that for HMA in AASHTO R 35. AASHTO T 283, Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced Damage, is used except that the mixture conditioning procedure is the same as that used in the volumetric design, tentatively 2 h at the compaction temper- ature. The minimum tensile strength ratio is 0.80 as specified in AASHTO M 323 for HMA. The preliminary procedure for WMA mixture design and analysis also included a mandatory evaluation of the design 11 Key Element Included in Preliminary Procedure WMA Process Selection Key considerations for WMA process selection. Binder Grade Selection Concept of high-temperature grade bumping based on WMA production temperature. Recycled Asphalt Pavement Materials Concept of limiting RAP content based on production temperature and compatibility of new and recycled binders. Specimen-Fabrication Procedures Process-specific fabrication procedures provided for major WMA processes. Process Temperature Direct evaluation of coating, workability, and compactability. Required Performance Testing Flow number in addition to moisture sensitivity testing in mixture design. Optional Performance Testing Recommended methods for measuring dynamic modulus, resistance to fatigue cracking, and resistance to thermal cracking included in mixture analysis. Table 3. Major new elements included in the preliminary WMA mixture design and analysis procedure.

mixture for rutting resistance using the flow number test devel- oped in NCHRP Project 09-19 (12). This test is conducted using the Asphalt Mixture Performance Tester (AMPT) on specimens that have been conditioned according to the volu- metric design procedure, tentatively 2 h at the compaction temperature. The AMPT was formerly called the Simple Per- formance Test (SPT) system. The flow number test is con- ducted in accordance with AASHTO TP 79, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT). The test is conducted unconfined with a repeated deviatoric stress of 87 psi (600 kPa) and a contact deviatoric stress of 4.4 psi (30 kPa). The test temperature is the design high pavement temperature at 50-percent reliability as deter- mined using LTPPBind Version 3.1 (13). The temperature is computed at a depth of 0.79 in. (20 mm) for surface courses, and the top of the pavement layer for intermediate and base courses. Flow number criteria for various traffic levels are given in Table 4. These are the same criteria being recommended for HMA in the mix design manual being developed under NCHRP Project 09-33 (6). 2.3.7 Optional Mixture Analysis Tests The preliminary mixture design and analysis procedure for WMA included optional performance tests to evaluate the dynamic modulus, resistance to fatigue cracking, and resis- tance to thermal cracking. Performance tests are not included in AASHTO R 35 for HMA. The optional performance tests can be used with the Mechanistic-Empirical Pavement Design Guide (MEPDG) to predict the performance of pavements incorporating WMA (1). The following performance tests and equipment were selected for the preliminary procedure: • Dynamic Modulus. Dynamic modulus master curves for use in pavement structural design and performance analysis using the MEPDG (1) can be developed using the AMPT in accordance with AASHTO PP 61, Developing Dynamic Modulus Master Curves for Hot-Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester. • Fatigue Cracking. Fatigue characteristics of WMA are eval- uated using simplified continuum damage analysis of cyclic direct tension-compression tests. This procedure was devel- oped in NCHRP Projects 09-25 and 09-31 to quickly char- acterize the fatigue resistance of a mixture using a limited amount of testing (14). The same geometry specimen as used for the dynamic modulus and flow number can be used in the direct tension-compression fatigue testing. With appro- priate tension grips, the test can be performed with the AMPT. • Thermal Cracking. The recommended method for analysis of thermal cracking in flexible pavements requires measure- ment of compliance and strength properties of the mixture at low temperatures. These properties are then used in a thermo-viscoelastic stress analysis to estimate the thermal stresses induced in the pavement during cooling cycles. Mixture compliance and strength properties are obtained by testing specimens in the indirect tensile (IDT) mode in accordance with AASHTO T 322, Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Device. Two software programs are available to perform the thermo-viscoelastic stress analysis. The first is the MEPDG, which includes a model to predict the extent of thermal cracking in a flexible pavement consid- ering environmental conditions at the project site and the thickness and properties of the asphalt concrete used in the pavement. This model has been calibrated using data from several in-service pavements (1). The second is an Excel Workbook called “LTSTRESS.xls,” which was developed at the Northeast Center for Excellence in Pavement Technol- ogy to reduce data from AASHTO T 322 and perform a sim- plified thermal cracking analysis (15). The output of this analysis is a critical cracking temperature, the temperature where the computed thermal stresses for a specified cooling rate exceed the tensile strength of the mixture. LTSTRESS.xls has not been calibrated to observed cracking and should be used for comparative evaluation of mixtures. 2.4 Phase I Laboratory Studies In developing the preliminary mixture design and analysis procedure for WMA, several areas were identified where lab- oratory testing and analysis was needed to develop criteria for the procedure. This section describes the laboratory studies that were conducted and analyzed during Phase I of the proj- ect. Detailed results and analyses for each study are presented in Appendix E and summarized in Chapter 3. The preliminary procedure was then revised based on the results of these studies, and the revised preliminary procedure was used in the mix design, field validation, and fatigue studies. The resulting draft standards for WMA mixture design and analysis are dis- cussed in Chapter 3. 2.4.1 Phase I Data Sources Data for the Phase I studies were collected from four sources: the FHWA Mobile Asphalt Laboratory, FHWA Turner- 12 Traffic Level, Million ESALs Minimum Flow Number < 3 --- 3 to < 10 53 10 to < 30 190 ≥ 30 740 Table 4. Minimum flow number requirements.

Fairbank Highway Research Center, McConnaughay Tech- nologies, and a WMA project on I-70 in Colorado that was sampled by the research team. The FHWA Mobile Asphalt Laboratory and McConnaughay Technologies provided mix- ture modulus data that were used to evaluate the effect of sample reheating on the mechanical properties of WMA. The FHWA Turner-Fairbank Highway Research Center provided data from an experiment that used the Rolling Thin Film Oven Test (RTFOT) to evaluate the effect of temperature on the short-term aging of asphalt binders. These data were used to develop preliminary recommendations for binder grade selection for WMA as a function of production temperature. Samples of loose mix and component materials from the Col- orado I-70 WMA project were used to evaluate short-term oven conditioning and the mixing of RAP at WMA tempera- tures. The Colorado I-70 project included an HMA control mix and three WMA processes: Advera, Evotherm, and Saso- bit. Table 5 presents the approved mixture design for the Col- orado I-70 HMA. The three WMA processes used this same mix design without modification. 2.4.2 Sample Reheating Study Since some of the planned experiments involved mechani- cal property tests on specimens prepared from loose mix, a study was conducted to determine if sample reheating sig- nificantly affected the mechanical properties of WMA. The response variable used in this study was the mixture dynamic modulus because it is very sensitive to changes in binder stiff- ness, and it was expected that sample reheating might result in additional stiffening of the binder in the mixture. The effect of sample reheating was evaluated for a control HMA and four WMA processes: Aspha-min, Evotherm ET, LEA, and Sasobit. The data for the control HMA, Aspha-min, Evotherm ET, and Sasobit mixtures were provided by the FHWA Mobile Asphalt Laboratory. The FHWA provided data for a WMA project constructed in St. Louis, Missouri, where modulus tests were performed for three conditions: (1) samples prepared at the time of construction and immediately tested, (2) samples pre- pared at the time of construction, but tested weeks later, and (3) reheated samples. McConnaughay Technologies prepared dynamic modulus specimens for the LEA process during con- struction and the research team prepared an additional set of dynamic modulus specimens after reheating. Both sets of LEA specimens were tested by the research team. All of the dynamic modulus tests were conducted in accordance with AASHTO PP 61. Table 6 summarizes the sample reheating study. The data analysis consisted of comparing dynamic modulus mas- ter curves for the various sample preparation and testing conditions. 2.4.3 Binder Grade Study The lower production temperatures used with WMA pro- duce less aging of the binder during construction. This reduced aging may result in increased rutting of pavements produced using WMA processes and it may also result in improved resis- tance to fatigue and low-temperature cracking. NCHRP Project 09-43 included analysis of an experiment conducted by the FHWA where the effects of WMA production tempera- tures were simulated using the RTFOT (AASHTO T 240). In this experiment, binders were short-term aged in the RTFOT, at temperatures of 325°F, 266°F, and 230°F (163°C, 130°C, and 110°C). The high-temperature properties of the binders were then measured in accordance with AASHTO T 315 at multiple temperatures to determine the continuous RTFOT high- temperature grade of the binder. Low-temperature properties for several of the binders were measured during NCHRP Proj- ect 09-43 for RTFOT temperatures of 325°F and 230°F (163°C and 110°C). Low-temperature properties were measured in accordance with AASHTO T 313 at two temperatures to deter- mine the continuous low-temperature grade of the binder. The RTFOT aged binders were further aged in the pressure aging vessel (PAV) in accordance with AASHTO R 28 at a tem- perature of 100°C prior to bending beam rheometer testing. 13 Property Sieve Size I-70 Colorado Control HMA 1/2 in 100.0 3/8 in 95.0 #4 73.0 #8 54.0 #16 40.0 #30 29.0 #50 18.0 #100 11.0 Gradation (% passing) #200 6.7 Asphalt Content, % 6.2 Ndesign 75.0 Design Air Voids, % 3.9 Design VMA, % 16.9 Design VFA, % 77.0 Fines to Effective Asphalt Ratio 1.0 Fractured Faces (one face/two faces), % 100/99 Fine Aggregate Angularity (FAA) 48.6 Aggregate Water Absorption, % 0.8 Dry Tensile Strength, psi 64.0 Conditioned Tensile Strength, psi 58.0 Tensile Strength Ratio, % 91.0 Binder Grade PG 58-28 Table 5. Phase I project mix design data. Mixture Immediate Delayed Reheated HMA Control X X X Aspha-min X X X Evotherm ET X X X LEA X X Sasobit X X X Table 6. Summary of the sample reheating study.

Table 7 summarizes the RTFOT temperature experiment. The continuous high-temperature grade data for these binders were used to develop preliminary production temperature lim- its below which consideration should be given to increasing the high-temperature grade of the binder. The continuous low- temperature grade data were used to develop preliminary rec- ommendations for low-temperature binder grade selection based on production temperature. 2.4.4 Short-Term Oven Conditioning Study An important step in mixture design and analysis is short- term oven conditioning of laboratory-prepared loose mix prior to compaction. Short-term oven conditioning simulates the binder absorption and aging that occurs during construc- tion. The short-term oven conditioning recommended for HMA at the end of the Strategic Highway Research Program (SHRP) was 4 h at 275°F (135°C) for both volumetric design and performance testing (16). This was included in AASHTO PP 2, Practice of Short and Long Term Aging of Hot Mix Asphalt (HMA), which later became AASHTO R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA). To expedite the mixture design process and reduce the number of ovens required for mixture design, the FHWA Mixtures and Aggre- gates Expert Task Group (ETG) reviewed data concerning the effect of conditioning time and temperature on the volumetric properties of asphalt mixtures. The ETG ultimately recom- mended that the short-term oven conditioning time for mix- ture design be changed to 2 h at the compaction temperature for aggregates with water absorption less than 4.0 percent. For aggregates with greater water absorption and for performance testing, the short-term oven conditioning time remained 4 h at 275°F (135°C). AASHTO R 30 was eventually modified to reflect the ETG’s recommendation. Short-term conditioning of 2 h at the compaction tempera- ture has been recommended by some WMA process develop- ers for mixture design. No recommendations have been made for short-term conditioning of WMA for performance testing. In Phase I of NCHRP Project 09-43, an experiment was under- taken to establish short-term oven conditioning times for both volumetric design and performance testing. The approach that was used was to compare the maximum specific gravity, dynamic modulus, and tensile strength of laboratory-prepared mixtures with those from field mixtures. For convenience and to properly assess the effect of WMA process temperature, the short-term conditioning temperature was selected to be equal to the compaction temperature. Conditioning times of 2 h and 4 h were included in the experiment. The short-term oven con- ditioning experiment was completed for the Colorado I-70 mixtures, and a tentative short-term conditioning time was selected. This tentative conditioning time was then verified in the Phase II field validation study. 2.4.5 RAP Study The primary concern when using RAP in WMA is whether the RAP and new binders mix at the lower temperatures used in WMA. In the preliminary mixture design procedure, it was hypothesized that the allowable RAP content of WMA mix- tures would decrease as the production temperature decreased. Two experiments were conducted in Phase I of NCHRP Proj- ect 09-43 in an attempt to determine production temperatures below which it may be necessary to limit the RAP content of WMA to some amount less than the amount allowed in HMA. The first experiment included measurements of interfacial mixing to determine whether thin films of new binder on RAP binder actually mix and measurements of binder compatibil- ity to determine the effect of mixing on the properties of the combined binder. The interfacial mixing measurements used atomic force microscope imaging of “film-on-film” interface contact lines. Asphalt binders including Advera and Sasobit WMA additives were used in the interfacial mixing measure- ments. Thin films of these WMA binders were cast onto a film of binder that was previously aged in the PAV to simulate an aged RAP binder. The specific procedures for the “film- on-film” imaging were developed by the Western Research Institute during Phase I of the project. The compatibility measurements were performed in accordance with ASTM D6703, Standard Test Method for Automated Heithaus Titrimetry. As the compatibility of an asphalt binder changes, the physical properties change. Less compatible binders tend to have more structure and more elastic properties. Compati- 14 Binder Source HighTemperature Low Temperature B-6354 Missouri WMA PG 70-22 X X B-6348 Hawaii PG 70-16 X X B-6328 Venezuelan PG 64-22 X X AAM-1 SHRP MRL X X AAM-2 SHRP MRL X X AAG-1 SHRP MRL X Not Tested AAD-1 SHRP MRL X X B6272 ALF PG 70-22 Control X X B6272+1.5% Sasobit ALF PG 70-22 Control + Sasobit X Not Tested B6272+3.0% Sasobit ALF PG 70-22 Control + Sasobit X Not Tested Table 7. Binders used in FHWA RTFOT temperature experiment.

bility measurements were made for three neat asphalt binders, two WMA additives (Advera and Sasobit), one RAP binder, and four RAP percentages. Table 8 summarizes the compati- bility testing. The second experiment in the RAP study was a laboratory mixing experiment designed to assess the degree of mixing between RAP and new binders at WMA process tempera- tures. This experiment used an approach that was developed by Advanced Asphalt Technologies, LLC, for the Maryland State Highway Administration and the Pennsylvania Department of Transportation to evaluate the acceptability of plant mixing of mixtures containing RAP and recycled asphalt shingles (RAS) (17). The approach involves comparing dynamic moduli mea- sured on mixture samples with dynamic moduli estimated using the properties of the binder recovered from the mixture sam- ples. The dynamic modulus test is very sensitive to the stiffness of the binder in the mixture, and adding RAP will increase the dynamic modulus significantly when the RAP is properly mixed with the new materials. The dynamic modulus for the as-mixed condition was measured in accordance with AASHTO PP 61. The dynamic modulus for the fully blended condition was esti- mated using the Hirsch model (18) from the shear modulus of binder recovered from the dynamic modulus specimens. Table 9 summarizes the experimental design for the labora- tory mixing experiment. The experimental design included testing a control HMA and three WMA processes: Advera, Evotherm, and Sasobit. Each of the four mixtures was tested at two temperatures and three aging times. Each mixture was mixed at the mixing temperatures listed in Table 9, then short- term oven aged at the compaction temperature listed in Table 9 prior to compaction. Duplicate dynamic modulus spec- imens were prepared and tested for each mixture in accordance with AASHTO PP 61. The binder from one of the specimens was recovered in accordance with ASTM D5404. Dynamic shear rheometer (DSR) frequency sweep tests were performed on the recovered binders in accordance with AASHTO T 315 to determine binder modulus input values for the Hirsch model. 2.4.6 Workability Study Phase I also included a screening study to select an appro- priate workability device for use in WMA mixture design. To accommodate the wide range of WMA processes currently available and expected in the future, the preliminary procedure proposed evaluating coating, workability, and compactability directly during the evaluation of trial blends and during the optimum binder content selection. Six potential workability tests were identified by the research team. Table 10 presents a summary of key elements of these devices. After careful review of the workability devices in Table 10, the following devices were selected for the Phase I screening test: • UMass Workability Device • Nynäs Workability Device • University of New Hampshire Workability Device • Gyratory Compactor with Shear Stress Measurement The UMass, Nynäs, and University of New Hampshire work- ability devices are shown in Figures 2, 3, and 4, respectively. These devices measure either the torque (UMass and Univer- sity of New Hampshire) or force (Nynäs) required to move a blade through the mixture. The University of New Hampshire device is very simple, consisting of a handheld drill with variable torque chuck clutch. The UMass and Nynäs devices are much more complex. Some gyratory compactors are equipped with devices that measure the force required to apply the gyratory compaction angle. This measurement may be provided as a force or con- verted to stress based on the geometry of the equipment. The specific compactor used in the workability screening study was an Intensive Compaction Tester Model ICT –150R/RB manu- factured by Invelop Oy of Finland and shown in Figure 5. 15 AAB-1 AAG-1 Yellowstone National Park RAP Content (%) Neat 1.5 % Sasobit 5 % Advera Neat 1.5 % Sasobit 5 % Advera Neat 1.5 % Sasobit 5 % Advera 0 X X X X X X X X 5 X X 15 X X 25 X X X X X 50 X X X X X Blank cells were not tested. Table 8. Summary of compatibility testing. Conditioning Time (h)Process Mixing/Compaction Temperatures (°F) 0.5 1.0 2.0 280/255 X X X Control 248/230 X X X 248/230 X X X Advera 230/212 X X X 248/230 X X X Evotherm 230/212 X X X 248/230 X X X Sasobit 230/212 X X X Table 9. Experimental design for the laboratory RAP mixing experiment.

16 Device Measurement Modification of Procedure Needed for WMA Advantages Disadvantages NCAT Prototype Workability Device Torque to rotate paddle at constant speed. None • Measure workability during mixing. • Previous research. Requires new mixer. UMass Prototype Workability Device Torque to rotate an auger at constant speed. None • Measure workability during mixing. • Augur may better represent field movement. Requires new mixer. Modified Nynäs Workability Device Force to push a blade into a loose mix sample. Temperature control at WMA placement and compaction temperatures. • Simulates screed action. • Relatively inexpensive. Requires new device. ASTM D6704 Force to push a blade into a loose mix sample. Temperature control at WMA placement and compaction temperatures. • Simple and inexpensive. • Uses existing equipment. May not represent field conditions. Gyratory Shear Stress Shear stress during gyratory compaction. None for gyratory compactors with this capability. • Measure workability during compaction. Requires gyratory compactor with shear stress measurement. University of New Hampshire Torque using blade attached to hand drill with adjustable torque settings. None • Simple and inexpensive. • Can easily be performed after mixing or prior to compaction. Blade and drill torque settings need to be standardized. Table 10. Key elements of potential workability devices for WMA. Figure 2. UMass workability device. Figure 3. Nynäs workability device. The primary concern in the initial screening study was the effect of temperature and WMA additive on the workability of the mixture. The Phase I screening experiment is summarized in Table 11. It consisted of performing workability tests on a single mixture produced with three binders: PG 64-28 control, PG 64-28 with Sasobit, and PG 64-28 with Advera. Table 12 presents pertinent properties of the mixture used in the exper- iment. Sasobit and Advera were selected as the warm mix addi- tives because these additives are easy to use in the laboratory. Duplicate workability tests were made with each device at three temperatures. Analysis of variance was used to evaluate the sensitivity of the test to changes in temperature and WMA additive. The sensitivity of the test along with ease of integra- tion into the WMA design procedure were the factors consid- ered in the final selection.

2.6 Phase II Studies In Phase II of NCHRP Project 09-43, three studies were con- ducted to evaluate the revised preliminary procedure. The Phase II studies included (1) a laboratory mixture design study, (2) a field validation study, and (3) a WMA fatigue study. The sections that follow describe these studies. 17 Figure 4. University of New Hampshire workability device. Figure 5. Gyratory compactor with shear stress measurement. 2.5 Revised Preliminary Mixture Design Procedure The preliminary mixture design procedure was modified based on the findings of the Phase I studies. These modifica- tions generally involved substituting tentative criteria devel- oped from the Phase I studies into the appropriate sections of the preliminary mixture design procedure. The criteria that were developed are discussed in Chapter 3. No modifications were made to the mixture analysis portion of the procedure. Factor Levels Details Mixtures 1 12.5 mm Binders 3 PG 64-28 control PG 64-28 with Sasobit PG 64-28 with Advera Workability Tests 5 UMass Prototype (auger) Modified Nynäs Gyratory Shear Stress University of New Hampshire Temperatures 3 300°F 245°F 190°F Replicates 2 — Note. — = No qualifying details for replicates. Table 11. Screening study for workability tests. Property Sieve Size Value 3/4 in 100.00 1/2 in 99.00 3/8 in 86.00 #4 57.00 #8 40.00 #16 28.00 #30 20.00 #50 12.00 #100 6.00 Gradation (% Passing) #200 3.20 Asphalt Content, % 5.40 Ndesign 75.00 Design Air Voids, % 3.70 Design VMA, % 14.60 Design VFA, % 74.50 Fines to Effective Asphalt Ratio 0.69 Table 12. Mixture used in the workability study.

2.6.1 Laboratory Mixture Design Study The objective of the laboratory mixture design study was to test the engineering reasonableness, sensitivity, and practicality of the revised preliminary mixture design and analysis proce- dure for WMA. The study was designed to compare properties of WMA mixtures designed according to the revised prelimi- nary procedure with those of corresponding HMA mixtures designed according to AASHTO R 35. As previously mentioned, the underlying principle for the mixture design procedure for WMA is to produce mixtures with strength and performance properties similar to those of HMA. The experimental design for the mix design study was a paired difference experiment. This design is commonly used to compare population means— in this case, the properties of properly designed WMA and HMA mixtures for the same traffic level, using the same aggre- gates with the same gradation. In this design, differences between the properties for WMA and HMA are computed for each mixture included in the experiment. If the two design procedures produce mixtures with the same properties, then the average of the differences will not be significantly different from zero. The difference for an individual mixture may be positive or negative, but the average difference over several mixtures should be zero. A t-test is used to assess the statistical significance of the average difference as summarized below. Null hypothesis Alternative hypo WMA HMA: μ μ− = 0 thesis or as appro WMA HMA WMA HMA: ( μ μ μ μ− > − <0 0 priate Test statistic Rejectio ) : t d s n d = ⎛⎝⎜ ⎞⎠⎟ n region Reject the null hypothesis and ac: cept the alternative hypothesis if fot t> α r degrees of freedom.n −1 where μWMA = population mean for WMA mixtures, μHMA = population mean for HMA mixtures, d – = average of the differences between WMA and HMA mixtures, sd = standard deviation of the differences, and n = number of mixtures compared. Table 13 presents the experimental design for the laboratory mix design study. In this study, various properties for WMA and corresponding HMA mixtures were evaluated using paired difference comparisons. Comparisons were made for Advera, Evotherm, and Sasobit. For the WMA processes, two mixing and compaction temperatures were used: one above the prelim- inary grade bumping temperature from the Phase I binder grade study and one below. The HMA mixtures and the WMA mixtures above the grade bumping temperature were made with PG 64-22 binder. Also, the WMA mixtures with RAP and Sasobit below the grade bumping temperature were made with PG 64-22 because both RAP and Sasobit increase the high- temperature grade of the binder. The Advera and Evotherm WMA mixtures below the grade bumping temperature were made with PG 70-22 binder. All mixtures were short-term con- ditioned for 2 h at the compaction temperature. The six mix- tures were selected to provide a range of gyratory compaction levels and aggregate absorptions. One half of the mixtures included RAP at 25 percent. A total of 24 mixture designs were prepared using either AASHTO R 35 for HMA mixtures or the revised preliminary WMA mixture design procedure. For the experimental design in Table 13, separate compar- isons were made between the properties of HMA and each of the WMA processes. Comparisons were made for the follow- ing properties: • Design air voids, vol % • Design VMA, vol % • Effective binder content (VBE), vol % 18 Mixture Identification Process No. N design Aggregate Absorption RAP 2 HMA Advera WMA Evotherm G3 WMA Sasobit WMA 1 50 High 3 Yes 320/310 4 225/215 225/215 270/260 2 50 Low 5 No 320/310 270/260 270/260 225/215 3 75 Low Yes 320/310 270/260 225/215 270/260 4 75 High No 320/310 225/215 270/260 225/215 5 100 High Yes 320/310 270/260 270/260 225/215 6 100 Low No 320/310 225/215 225/215 270/260 1 Low-temperature Advera and Evotherm WMA use PG 70-22; all other mixtures use PG 64-22. All mixtures short-term conditioned 2 h at the compaction temperature. 2 RAP content 25 percent in all mixtures containing RAP. 3 High absorption > 2.0 percent. 4 XXX/XXX, e.g., 320/310, denotes mixing/compaction temperatures, °F. 5 Low absorption < 1.0 percent. Table 13. Mix design experiment.1

• Binder absorption, wt % • Design binder content, wt % • Effective binder content, wt % • Coating • Gyrations to 8% air voids at the compaction temperature • Gyrations to 8% air voids at the compaction temperature minus 54°F (30°C) • Density at Nmax • Dry tensile strength • Conditioned tensile strength • Tensile strength ratio • Flow number • Rutting resistance These properties are all obtained as part of the WMA mix- ture design process. The HMA mixtures required design in accordance with AASHTO R 35, flow number testing, and assessment of compactability at the lower temperature as pro- posed in the WMA mixture design procedure. Table 14 presents the six mixtures that were included in the mix design study. The volumetric properties presented for these mixtures are those obtained from conducting an HMA mixture design in accordance with AASHTO R 35 and AASHTO M 323. The low-absorption mixtures were composed of limestone or diabase aggregate from Virginia. The high-absorption mixtures were composed of gravel and limestone from Pennsylvania. The gravel material in these mixtures was selected for its historically high absorption, but the material supplied had lower absorp- tion than expected, which resulted in lower water absorptions for the planned high-absorption mixtures. For the 50 and 75 gyration designs, the high-absorption mixtures have approx- imately twice the water absorption of the low-absorption mixtures. For the 100 gyration design, the planned low- and high-absorption mixtures have approximately the same water absorption. This difference was taken into account when per- forming statistical analysis of the experiment results. The same RAP was used in the three mixtures that incorporated RAP. Table 15 presents the gradation and binder content of the RAP material that was used. The RAP binder had a continuous per- formance grading of PG 95.9 (33.9) -13.1. The RAP was obtained from Loudoun County Asphalt in Leesburg, VA. All of the RAP mixtures used 25 percent RAP, which resulted in an RAP binder contribution of approximately 1.1 percent by weight. NuStar Asphalt Refining, LLC, provided the binders for this study from their Paulsboro, NJ, refinery. The dosage rate of the Sasobit was 1.5 percent by weight of the total binder (virgin plus RAP) in the mixture. The dosage rate of the Advera was 0.25 percent by total mix weight. Binders containing the Evotherm G3 were provided premixed by NuStar Asphalt Refining, LLC, and the Evotherm dosage rate was not adjusted 19 1 2 3 4 5 6 50 50 75 75 100 100 1. 5 0 .8 1. 0 1 .6 1. 2 1.3 Ye s N o Y es No Ye s No 9. 5 mm 9. 5 mm 9. 5 mm 9. 5 mm 9. 5 mm 9. 5 mm Co ar se PA Gr av el RA P VA Li me st on e VA Di ab as e RA P PA Gr av el PA Gr av el RA P VA Diab as e Fi ne PA Li me st on e PA Gr av el RA P VA Li me st on e VA Di ab as e Na tu ra l Sa nd RA P PA Li me st on e PA Gr av el PA Li me st on e RA P VA Diab as e Na tur al Sa nd RA P LC A, L ees bu rg , VA No ne LC A, L ees bu rg , VA No ne LC A, L ees bu rg , VA No ne Si eve Si ze , mm 12 .5 100 100 100 100 100 100 9. 5 97 94 94 98 97 98 4.75 61 50 54 63 63 53 2.36 43 32 38 44 41 40 1.18 32 22 28 32 26 31 0. 6 25 14 21 22 17 22 0. 3 13 10 12 12 11 12 0.15 6 7 8 5 7 7 0.075 3. 8 5 .4 5. 2 3 .0 4. 6 4.8 FAA 44.1 45.8 46.1 43.5 45.4 48.3 CA A2 98/95 100/100 100/99 98/95 98/95 100/100 Fl at & El on gate d 4. 5 1 .6 2. 7 7 .4 4. 4 7.6 Sa nd E qui va le nt 93.2 75.0 59.4 80.2 91.9 76.7 6. 4 6 .8 5. 5 6 .3 6. 0 5.7 5. 6 6 .1 4. 8 5 .3 5. 4 4.7 3. 6 4 .0 3. 9 4 .3 4. 0 3.7 16.4 18.0 15.9 16.3 16.4 15.1 12.8 14.0 12.0 12.0 12.4 11.4 78.0 77.8 75.5 73.6 75.6 75.5 0. 7 0 .9 1. 1 0 .6 0. 9 1.0 Effective Binder Content, vol % Voids Filled With Asphalt, % Dust to Effective Asphalt Ratio 1 NMAS = Nominal maximum aggregate size. 2 CAA = Coarse aggregate angularity. Gradation Binder Content, wt % Effective Binder Content, wt % Air Voids, vol % Voids in Mineral Aggregate, vol % Aggregate Sources Aggregate Properties Mix Number Design Gyrations Aggregate Water Absorption, % RAP NMAS1 Table 14. Mixtures used in the mix design experiment.

for the RAP binder in the Evotherm RAP mixtures. The mix- tures incorporating gravel required an anti-strip additive. Akzo- Nobel WETFIX 312 was used in the HMA, Sasobit, and Advera mixtures. The dosage rate for the anti-strip additive was 0.25 percent by weight of the total binder in the mixture. Rep- resentatives of Evotherm recommended that the anti-strip not be added when using the Evotherm G3 additive. 2.6.2 Field Validation Study The objective of the field validation study was to use prop- erties of laboratory- and field-produced WMA to validate selected parts of the revised preliminary WMA mixture design and analysis procedure. The parts of the revised preliminary procedure addressed by the validation included the following: • Binder grade selection • RAP • Short-term oven conditioning • Specimen fabrication and compactability • Moisture sensitivity • Rutting resistance Table 16 summarizes the mixtures that were included in the validation study. Materials from a total of 16 mixtures from six projects were sampled. The validation mixtures included a wide range of processes. Four mixtures were HMA control; three mixtures used the Advera WMA process; two mixtures used the Evotherm WMA process; two mixtures used the LEA process; two mixtures used plant foaming processes; and three mixtures used Sasobit. The WMA production temperatures ranged from 210°F to 275°F (99°C to 135°C), and the WMA compaction temperatures ranged from 195°F to 250°F (90°C to 121°C). Most of the WMA mixtures were produced around 250°F (121°C) and compacted around 230°F (121°C). The mixes included PG 58 and PG 64 binders. Only one mixture included RAP. Table 17 summarizes the analyses that were completed in the validation study. Initial validation of the findings from the Phase I binder grade study was completed using recovered binder grading and estimates of rutting from the MEPDG rut- ting model using measured dynamic moduli from plant mix- tures (1). Recovered binder grading data were collected on all of the 16 validation mixtures. Rutting estimates were made only for the mixtures included in the Colorado I-70, Yellow- stone National Park, and New York Route 11 projects. 20 Property Sieve Size (mm) Value 12.5 100 9.5 92 4.75 63 2.36 44 1.18 32 0.6 24 0.3 17 0.15 11 Gradation (% Passing) 0.075 7.8 Asphalt Content, wt % 4.4 Continuous Performance Grade PG 95.9 (33.9) -13.1 Aggregate Bulk Specific Gravity 2.877 Aggregate Water Absorption, % 1.01 Fine Aggregate Angularity, % 44.4 Crushed Aggregate Fractured Faces (1 Face), % 99.3 Crushed Aggregate Fractured Faces (2 Faces), % 94.3 Flat and Elongated Particles, % 0.5 Table 15. Properties of RAP used in the mixture design experiment. Temperature ( °F) Project Process Production Compaction Mix Type HMA Control 280 260 Advera 250 230 Evotherm DAT 250 230 Colorado I-70 Sasobit 250 230 9.5 mm, PG 58-28, 75 gyrations HMA Control 325 315 Advera 275 250 Yellowstone National Park Sasobit 275 245 19 mm, PG 58-34, Hveem NY Route 11 LEA 210 205 9.5 mm, PG 64-22, 65 gyrations HMA Control 320 300 PA SR2007 Evotherm DAT 250 230 9.5 mm, PG 64-22, 50 gyrations HMA 310 275 Advera 250 230 Gencor Ultrafoam GX 250 230 LEA 210 195 PA SR2006 and PA SR2012 Sasobit 250 230 9.5 mm, PG 64-22, 75 gyrations Monroe, North Carolina Astec Double Barrel Green 275 260 9.5 mm, PG 64-22 with 30% RAP, 75 gyrations Table 16. Field validation mixtures.

A binder mixing analysis using dynamic modulus and recovered binder testing on plant mix from the North Carolina project was used to validate that RAP and new binders mix at WMA process temperatures. The North Carolina project was the only project in the field validation study that included RAP. The short-term oven conditioning process recommended in the Phase I short-term oven conditioning study was validated by comparing the maximum specific gravity of plant mixtures with laboratory-prepared mixtures and comparing the ten- sile strength of plant-mixed, laboratory-compacted samples with the tensile strength of laboratory-mixed, laboratory- compacted samples. Fifteen of the 16 validation mixtures were included in the analysis. The New York Route 11 LEA mixture was not included because the LEA additive used on the project was not available. The process-specific specimen-fabrication procedures for WMA contained in the preliminary WMA mixture design pro- cedure and the compactability criteria developed in the Phase I workability study were validated by preparing laboratory WMA mixtures replicating the field mixtures. Volumetric properties of the laboratory-prepared specimens were used to validate the process-specific specimen-fabrication procedures. Addition- ally, for the two projects that used plant foaming processes, a WMA mixture design was completed using a Wirtgen WLB-10 laboratory foaming plant to assess the practicality of using this type of equipment for mixture design work. The compactabil- ity of the HMA and WMA mixtures from the field validation study was used to validate the tentative compactability criteria developed in the Phase I workability study. Finally, specimens of WMA and HMA prepared from labo- ratory mixtures were subjected to moisture sensitivity and flow number testing as required by the preliminary WMA mixture design procedure. A comparison was made between the results of the HMA control and the results of the WMA mixture for each project. 2.6.3 Fatigue Study One of the potential benefits of WMA mixtures is improved fatigue characteristics compared to HMA mixtures due to the reduced aging that occurs during plant mixing at the lower WMA process temperatures. Phase II included a brief study to evaluate the fatigue resistance of WMA compared to HMA. The experimental design for this study is presented in Table 18. Two of the mixtures from the mix design experiment were used in this study. Continuum damage fatigue tests were 21 Component Phase I Study Validation Analyses Binder Grade Selection Binder Grade Selection Recovered binder grading. Estimated rutting using MEPDG rutting model. Mixing of RAP and New Binders RAP Study Mixing analysis of plant-produced WMA with RAP. Short-Term Conditioning Short-Term Conditioning Study Compare maximum specific gravity and tensile strength of plant mixtures with laboratory mixtures. Process-Specific Specimen- Fabrication Procedures Literature Review and Research in Progress Volumetric properties of WMA mixtures. WMA mixture design for plant foaming processes. Compactability Workability Study Compare compactability of field mixtures to reported workability. Moisture Sensitivity Literature Review and Research in Progress Compare moisture sensitivity results for HMA control and WMA mixtures. Rutting Resistance Literature Review and Research in Progress Compare flow numbers for HMA control and WMA mixtures. Table 17. Summary of validation study analyses. Mixture Identification Process No. N design Aggregate Absorption RAP HMA WMA Organic WMA Foam WMA Chemical 4 75 High No 320/310 2 250/240 250/240 250/240 6 100 Low No 320/310 250/240 250/240 250/240 1 Mixtures from Table 14. All mixtures use PG 64-22 binder. All mixtures short-term conditioned 2 h at the compaction temperature. All mixtures long-term conditioned 120 h at 185°F. 2 XXX/XXX, e.g., 320/310, denotes mixing/compaction temperatures, °F. Table 18. Experimental design for the WMA fatigue study.1

performed on the HMA control and WMA mixtures produced using Advera, Evotherm, and Sasobit. All mixtures used PG 64- 22 binder. The HMA mixture was prepared at the recom- mended viscosity-based mixing and compaction temperatures. The WMA mixtures were prepared at the midpoint of the tem- peratures used in the mix design experiment. After com- paction, all specimens were long-term oven aged in accordance with AASHTO R 30 to simulate the effects of long-term aging. The data from the study were evaluated to determine whether the fatigue characteristics of WMA mixtures are significantly improved over HMA mixtures. 2.7 Draft Standards for WMA The mixture design portion of the revised preliminary procedure was further modified based on the findings of the Phase II studies. The final modifications are discussed in detail in Chapter 3. The mixture design portion of the revised preliminary procedure was reformatted to be in the form of an appendix to AASHTO R 35 highlighting spe- cial mixture design considerations and procedures for ad- dressing WMA during mixture design. This document is included in Appendix A of this report. Appendix B is a commentary that provides supporting information for use in adoption and future revision of the mix design consid- erations and methods for WMA. Training materials for in- troducing the recommended WMA methods are included in Appendix C. The mixture analysis portion of the proce- dure was reformatted to be a standard practice for measur- ing properties of WMA for performance analysis using the MEPDG (1). This proposed standard practice is included in Appendix D. 22

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Mix Design Practices for Warm-Mix Asphalt Get This Book
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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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