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62 the mixtures. The draft appendix to AASHTO R 35 includes 4.2 Recommendations evaluation of moisture sensitivity using AASHTO T 283. The research conducted under NCHRP Project 09-43 has shown that only minor changes to current mixture design 4.1.7 Rutting Resistance practice are needed to design WMA mixtures. Although volu- The draft appendix to AASHTO R 35 includes an evaluation metric properties for HMA and WMA will be similar when of the rutting resistance of WMA using the flow number test. binder absorption is 1.0 percent or less, the compactability, The test is conducted on specimens that have been short-term moisture sensitivity, and rutting resistance of WMA mixtures conditioned for 2 h at the planned field compaction temper- will likely be different than HMA mixtures designed with the ature to simulate the binder absorption and stiffening that same aggregates and binders. Therefore, it is recommended occurs during construction. Because lower short-term condi- that the procedures for WMA mixture design developed under tioning temperatures are used for WMA mixtures than are NCHRP Project 09-43 be used when designing WMA mix- used for HMA mixtures, binder aging in WMA mixtures is tures. For the mixtures studied under NCHRP Project 09-43, less, resulting in lower flow numbers for WMA mixtures pro- compactability was sensitive to the WMA process and temper- duced with the same aggregates and binder. Current criteria ature, particularly for mixtures incorporating RAP. The com- for the flow number and other rutting tests for HMA are bination of low WMA temperatures and RAP yielded mixtures based on 4 h of short-term conditioning at 275F (135C). with compactability that was more temperature sensitive than The short-term conditioning study completed in NCHRP HMA mixtures. Moisture sensitivity as measured by AASHTO Project 09-43 shows that this level of conditioning represents T 283 will likely be lower for WMA mixtures than HMA mix- the stiffening that occurs during construction as well as some tures unless the WMA process includes an anti-strip additive. time in service. Since it is inappropriate to condition WMA Finally, very low WMA temperatures may lead to mixtures mixtures at temperatures exceeding their production temper- with inadequate rutting resistance. All of these issues can be ature, the criteria for evaluating the rutting resistance of evaluated using the methods included in the draft appendix to WMA mixtures were changed from those currently recom- AASHTO R 35. mended for WMA (conditioned for 4 h at 275F [135C]). To aid in the implementation of this recommendation, a Based on an analysis of data from NCHRP Project 09-13, it draft appendix to AASHTO R 35 titled Special Mixture Design appears feasible that WMA can reach approximately the same Considerations and Methods for Warm Mix Asphalt (WMA), level of binder stiffening that occurs in 4 h at 275F (135C) was developed to address differences between the design of by using a two-step aging process: (1) 2 h of conditioning at WMA and HMA. This appendix covers the following: the compaction temperature to simulate construction effects and (2) extended loose mix conditioning at a representative WMA Process Selection, high in-service pavement temperature to represent early in- Binder Grade Selection, service aging. The duration of the extended conditioning will RAP in WMA, likely be less than 16 h. Process-Specific Specimen-Fabrication Procedures, Evaluation of Coating, Evaluation of Compactability, 4.1.8 Performance Evaluation Evaluation of Moisture Sensitivity, The research completed under NCHRP Project 09-43 has Evaluation of Rutting Resistance, and shown that for the same aggregates and binders, WMA mix- Adjusting the Mixture to Meet Specification Requirements. tures designed in accordance with the draft appendix to AASHTO R 35 will have similar properties to HMA mix- The draft appendix should be used on a trial basis by agen- tures. Volumetric properties will essentially be the same, but cies and producers to provide additional data to further refine the stiffness of the WMA mixture will probably be lower for the WMA mixture design methods and criteria before being as-constructed conditions. Since the differences between considered for adoption. Elements that would benefit from HMA and WMA are relatively small, an analysis of the per- additional evaluation and possible refinement include the formance of pavements constructed with WMA can be made process-specific specimen-fabrication procedures and the using the MEPDG and appropriate material properties (1). criteria for coating, compactability, and rutting resistance. A draft standard practice for fabricating WMA test speci- Additionally, agencies and producers should encourage the mens and performing dynamic modulus master curves and manufacturers of plant foaming equipment to develop labo- low-temperature creep compliance and strength testing was ratory foaming equipment that can be used to design foamed developed to aid in the performance analysis of WMA using asphalt WMA mixtures in the laboratory. The laboratory the MEPDG. foaming equipment that was used in NCHRP Project 09-43

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63 was designed for preparing laboratory samples of foamed sta- ditioning procedure that is applicable to both WMA and bilized bases, not WMA. Although it is feasible to design WMA HMA for the specimens used to evaluate moisture sensitivity mixtures for plant foaming processes using this equipment, and rutting resistance. Research completed under NCHRP devices specifically designed to replicate the WMA foaming Project 09-43 concluded that 2 h of oven conditioning at the process and produce the smaller quantities of foamed asphalt field compaction temperature reasonably reproduces the used in mix design batches without extensive cleaning are binder absorption and stiffening that occurs during construc- needed to make the design process efficient. tion for both WMA and HMA mixtures. WMA mixtures that At the time that NCHRP Project 09-43 was completed, are conditioned 2 h at the field compaction temperature have three additional projects on WMA were initiated by NCHRP: binder that is less stiff than similarly conditioned HMA mix- (1) NCHRP 09-47A, "Engineering Properties, Emissions, and tures because of the lower conditioning temperature. Current Field Performance of Warm Mix Asphalt Technologies," criteria for evaluating moisture sensitivity and rutting resis- (2) NCHRP 09-49, "Performance of WMA Technologies: Stage tance are based on mixtures that have been aged to a greater I--Moisture Susceptibility," and (3) NCHRP Project 09-49A, degree. The conditioning originally specified in AASHTO "Performance of WMA Technologies: Stage II--Long-Term T 283 for moisture sensitivity testing was 16 h at 140F (60C). Field Performance." NCHRP Projects 09-47A and 09-49A will Additionally, most rutting criteria are based on 4 h of condi- include an evaluation of the field performance of WMA mix- tioning at 275F (135C). Under NCHRP Project 09-13, mix- tures, and NCHRP Project 09-49 will address the moisture tures were conditioned for 2 h at 275F (135C), 4 h at 275F susceptibility of WMA in detail. The findings of NCHRP (135C), and 16 h at 140F (60C). From analysis of this data, Project 09-43 support the need for these studies addressing the NCHRP Project 09-43 research team concluded that 16 h field performance and moisture sensitivity. at 140F (60C) resulted in somewhat more aging than 4 h at There are, however, two elements of the WMA mixture 275F (135C). The difference in aging between 2 h and 4 h design process that require additional research that is not cur- at 275F (135C) was not statistically significant. To simu- rently planned. First, mixing procedures for laboratory mix- late both WMA and HMA, a two-step conditioning process tures have not been standardized. For design of HMA, mixing should be considered for specimens used to evaluate mois- can be done manually or with a mechanical mixer. Two types ture sensitivity and rutting resistance. In the first step, the of mechanical mixers are available: planetary mixers and mixture would be conditioned for 2 h at the field compaction bucket mixers. To use coating of laboratory mixtures as a temperature to simulate the binder absorption and stiffening design criterion, a mechanical mixer must be used, and the that occurs during construction. In the second step, the mix- mixing process must be standardized. Coating evaluations ture would be further conditioned for an extended time at a performed during NCHRP Project 09-43 indicate that there representative high in-service pavement temperature to sim- is a significant difference in the efficiency of planetary and ulate a short period of time in service. Only specimens used bucket mixers. The mixing times included in the draft appen- to evaluate moisture sensitivity and rutting resistance would dix to AASHTO R 35 are based on a planetary mixer with a receive the second conditioning step. Volumetric design would wire whip. Since bucket mixers are probably more readily be based only on the first step. The temperature and duration available in most production mix design laboratories, addi- of the extended conditioning would be selected based on tem- tional mixing studies should be conducted to establish mix- peratures from LTPPBind and typical laboratory working ing times for WMA specimen fabrication for bucket mixers. hours. Most likely, the second step would require conditioning The draft appendix to AASHTO R 35 should then be modi- specimens overnight. The extended conditioning temperature fied to include mixing times for bucket mixers. and time would be selected so that HMA conditioned using the The second element of WMA mix design that requires two-step process would have a similar stiffness to HMA condi- additional research is the development of a short-term con- tioned for 4 h at 275F (135C).