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1 SUMMARY Mix Design Practices for Warm Mix Asphalt Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at lower temperatures than the temperatures typically used in the production of hot mix asphalt (HMA) (50F [28C] lower or more). The goal with WMA is to produce mixtures with sim- ilar 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 temper- atures can also potentially improve pavement performance by reducing binder aging, pro- viding added time for mixture compaction, and allowing improved compaction during cold weather paving. WMA technologies were first introduced in Europe in the late 1990s as a measure to reduce greenhouse gas emissions. Since then, a number of WMA processes have been developed in Europe and the United States. At the time that NCHRP Project 09-43 was completed, there were approximately 20 WMA processes being marketed in the United States. These processes include chemical, wax, and synthetic zeolite additives; plant foaming systems; and sequential mixing processes. The objective of NCHRP Project 09-43 was to develop mixture design and analysis proce- dures that can be used with the wide range of WMA processes that are currently available or that are likely to become available in the future. The research conducted during NCHRP Proj- ect 9-43 included the following: 1. Development of a preliminary procedure based on a review of the literature and research in progress. 2. A first phase of testing and analysis to investigate critical aspects of the preliminary pro- cedure including (1) effect of sample reheating, (2) binder grade selection, (3) mixture of recycled asphalt pavement (RAP) and new binders at WMA process temperatures, (4) appropriate short-term oven conditioning for WMA, and (5) evaluation of devices to measure workability. 3. Revisions to the preliminary procedure based on the findings of the first phase of testing and analysis. 4. A second phase of testing and analysis to evaluate the revised preliminary procedure. This phase included (1) a mix design study to test the engineering reasonableness, sensitivity, and practicality of the revised preliminary procedure; (2) a field validation study that used prop- erties of laboratory- and field-produced WMA to validate the procedure; and (3) a fatigue study to investigate whether lower WMA temperatures improve mixture fatigue properties. 5. Final revision of the preliminary procedure based on the findings of the second phase of testing and analysis.

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2 The primary products of NCHRP Project 09-43 are (1) a draft appendix to AASHTO R 35 titled Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) (presented as Appendix A of this report) and (2) a draft standard practice titled Standard Practice for Measuring Properties of Warm Mix Asphalt (WMA) for Performance Analysis Using the Mechanistic-Empirical Pavement Design Guide Software (presented as Appendix D of this report). Training materials and a commentary for the draft appendix to AASHTO R 35 were developed to aid in implementing the research conducted under NCHRP Project 09-43. The following are the major conclusions drawn from the research completed in NCHRP Project 09-43: 1. Volumetric Properties. For HMA mixtures with 1.0 percent binder absorption or less, the volumetric properties of WMA designed with the procedures developed under NCHRP Project 09-43 were essentially the same as those obtained from an HMA design. This conclusion supports the current practice of substituting a WMA process into an approved HMA mixture design. However, the compactability, moisture sensitivity, and rutting resis- tance of the WMA may be significantly different than those of the HMA. Each of these (compactability, moisture sensitivity, and rutting resistance) is evaluated directly in the methods included in the draft appendix to AASHTO R 35. 2. Binder Grade Selection. The same grade of binder should be used in WMA and HMA mixtures designed for the same project location. Recovered binder test data from projects sampled and tested under NCHRP Project 09-43 indicated that only extremely low pro- duction temperatures resulted in a significant decrease in the stiffness of the recovered binder from the mixture. Additionally, WMA production temperatures resulted in a minor improvement in the low-temperature grade of the binder. The draft appendix to AASHTO R 35 (included herein as Appendix A), therefore, recommends that the same grade of binder be used in both WMA and HMA mixtures. High-temperature grade bumping may be necessary for WMA processes with extremely low production tempera- tures to meet the flow number rutting resistance requirements included in the draft appen- dix to AASHTO R 35. 3. RAP in WMA. RAP and new binders do mix at WMA process temperatures provided the mixture is held at elevated temperatures for a sufficient length of time. Because the mixing is time dependent, it appears that the new binder added to the mixture coats the virgin aggregate and RAP; then, during storage at elevated temperature, the two binders continue to mix. In the laboratory mixing studies that were conducted, 2 h of conditioning at the compaction temperature resulted in substantial mixing of RAP and new binders when the compaction temperature exceeded the high-temperature grade of the "as recovered" RAP binder. To ensure good mixing of RAP and new binders, the draft appendix to AASHTO R 35 recommends that the planned field compaction temperature for WMA exceed the high-temperature grade of the "as recovered" RAP binder. 4. Short-Term Oven Conditioning. Short-term oven conditioning is included in mixture design to simulate the absorption and aging of the binder that occurs during construc- tion. For WMA, it is appropriate to use 2 h of oven conditioning at the compaction temperature--the same short-term conditioning that is used for design of HMA mixtures. The degree of binder aging that occurs, however, is less than that obtained using the AASHTO R 30 conditioning for performance testing--4 h at 275F (135C). 5. Coating, Workability, and Compactability. For the wide range of WMA processes avail- able, viscosity-based mixing and compaction temperatures cannot be used to control coat- ing, workability, and compactability. The draft appendix to AASHTO R 35 uses direct measures of coating and compactability on laboratory-prepared mixtures. The degree of coating obtained in the laboratory depends on the type of mixer that is used. The mixing

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3 times included in the draft appendix to AASHTO R 35 were developed using a planetary mixer with a wire whip. If bucket mixers are used, appropriate WMA mixing times should be established by evaluating the coating of HMA mixtures prepared for various mixing times at the appropriate viscosity-based mixing temperature specified in Section 8.2.1 of AASHTO T 312. Several workability devices were evaluated under NCHRP Project 09-43. These devices, which measure the torque or force required to move an auger or blade through the mix- ture, were able to measure differences between HMA and WMA mixtures, but only when temperatures dropped to the compaction range of WMA. At these temperatures, differ- ences in air voids also were evident in gyratory compacted specimens. The draft appendix to AASHTO R 35 uses the change in the number of gyrations to 92-percent relative density when the compaction temperature is decreased 54F (30C) to characterize the compaction temperature sensitivity of the WMA processes. Increases that exceed 25 percent indicate that the WMA is more temperature sensitive than HMA. This measure of compactability was sensitive to the compaction temperature, the WMA process, and the presence of RAP in the mixture. The combination of RAP and low WMA process and compaction temper- atures, may lead to WMA mixtures that are more sensitive to changes in temperature than similar HMA mixtures. 6. Moisture Sensitivity. Moisture sensitivity as measured by AASHTO T 283 will likely be dif- ferent for WMA and HMA mixtures designed using the same aggregates and binder. WMA processes that included anti-strip additives improved the tensile strength ratio of some of the mixtures included in the NCHRP Project 09-43 testing and analysis. Of the nine WMA mixtures that used a WMA process that included an anti-strip additive, the tensile strength ratio remained the same or improved in 67 percent of the mixtures. For WMA mixtures produced using processes that did not include anti-strip additives, the tensile strength ratio never improved and decreased in 79 percent of the mixtures. The draft appendix to AASHTO R 35 includes evaluation of moisture sensitivity using AASHTO T 283. 7. Rutting Resistance. The draft appendix to AASHTO R 35 includes an evaluation of the rutting resistance of WMA using the flow number test. The test is conducted on speci- mens that have been short-term conditioned for 2 h at the compaction temperature to simulate the binder absorption and stiffening that occurs during construction. Because lower short-term conditioning temperatures are used for WMA mixtures as compared to HMA mixtures, binder aging in WMA mixtures is less, resulting in lower flow num- bers for WMA mixtures produced with the same aggregates and binder as compared to HMA mixtures. Current criteria for the flow number and other rutting tests for HMA are based on 4 h of short-term conditioning at 275F (135C). The short-term conditioning study completed under NCHRP Project 09-43 shows that this level of conditioning rep- resents the stiffening that occurs during construction as well as some time in service. Since it is inappropriate to condition WMA mixtures at temperatures exceeding their production temperature, the criteria for evaluating the rutting resistance of WMA mix- tures were reduced compared to those currently recommended for HMA mixtures con- ditioned for 4 h at 275F (135C). 8. Performance Evaluation. The research completed under NCHRP Project 09-43 showed that for the same aggregates and binders, WMA mixtures designed in accordance with the draft appendix to AASHTO R 35 will have similar properties as HMA mixtures. Volumet- ric properties will essentially be the same, but the stiffness of the WMA mixture will prob- ably be lower for as-constructed conditions. Since the differences between HMA and WMA are relatively small, an analysis of the performance of pavements constructed with WMA can be made using the Mechanistic-Empirical Pavement Design Guide (MEPDG) and appropriate material properties (1). A draft standard practice for fabricating WMA test

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4 specimens and performing dynamic modulus master curves and low-temperature compliance and strength testing was developed to aid in the performance analysis of WMA using the MEPDG. The research conducted under NCHRP 09-43 has shown that only minor changes to cur- rent mixture design practice are needed to design WMA mixtures. Although volumetric prop- erties for HMA and WMA will be similar when binder absorption is 1.0 percent or less, the compactability, moisture sensitivity, and rutting resistance of WMA mixture will likely be dif- ferent than the compactability, moisture sensitivity, and rutting resistance of an HMA mix- ture designed with the same aggregates and binders. Therefore, it is recommended that the procedures for WMA mixture design developed under NCHRP 09-43 be used when designing WMA mixtures. The draft appendix to AASHTO R 35 should be used on a trial basis by agencies and pro- ducers to provide additional data to further refine the WMA mixture design methods and criteria before being considered for adoption. Elements that would benefit from additional evaluation and possible refinement include the process-specific specimen-fabrication proce- dures, and the criteria for coating, compactability, and rutting resistance. At the time that NCHRP Project 09-43 was completed, two additional projects on WMA were initiated by NCHRP: NCHRP Project 09-47A, "Properties and Performance of Warm Mix Asphalt Technologies" and NCHRP 09-49, "Performance of WMA Technologies: Stage I--Moisture Susceptibility." NCHRP Project 09-47A will include an evaluation of the field performance of WMA mixtures, and NCHRP Project 09-49 will address the moisture sus- ceptibility of WMA in detail. The findings of NCHRP Project 09-43 support the need for these studies addressing field performance and moisture sensitivity. There are, however, two elements of the WMA mixture design process that require addi- tional research that is not currently planned. First, the WMA specimen-fabrication procedures included in the draft appendix to AASHTO R 35 should be expanded to include bucket mix- ers, which are more readily available in most production mix design laboratories. Second, additional research is needed to develop a short-term conditioning procedure for specimens used for the evaluation of moisture sensitivity and rutting resistance that is equally applicable to both WMA and HMA. A two-step conditioning process should be considered. In the first step, the mixture would be conditioned for 2 h at the compaction temperature to simulate the binder absorption and stiffening that occurs during construction. In the second step, the mix- ture would be further conditioned for an extended time at a representative high in-service pavement temperature to simulate a short period of time in service. Based on an analysis of data collected under NCHRP Project 09-13, it appears that the second step will require less than 16 h of additional conditioning.