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60 4.1 Conclusions The objective of NCHRP Project 09-43 was to develop mix- ture design and analysis procedures that can be used with the wide range of WMA processes that are currently available or are likely to become available in the future. The research con- ducted under NCHRP Project 09-43 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 procedure, including (1) the effect of sample reheating, (2) binder grade selection, (3) mixing of 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 find- ings 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 pro- cedure; (2) a field validation study that used properties of laboratory- and field-produced WMA to validate the pro- cedure; 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. 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) and (2) a draft standard practice titled Standard Practice for Measuring Properties of Warm Mix Asphalt (WMA) for Per- formance Analysis Using the Mechanistic-Empirical Pavement Design Guide Software. Training materials and a commen- tary for the draft appendix to AASHTO R 35 were developed to aid in implementing the research conducted in NCHRP Project 09-43. The draft appendix to AASHTO R 35 addresses the most widely used WMA processes, including (1) additives that are added to the binder, (2) additives that are added to the mixture during production, (3) wet aggregate mixtures, and (4) plant foaming systems. The following unique aspects of WMA mixture design are addressed by the draft appendix to AASHTO R 35: ⢠Process-specific specimen-fabrication procedures, ⢠An evaluation of coating at the planned production temperature, ⢠An evaluation of compactability at the planned field com- paction temperature and lower using the Superpave gyra- tory compactor, and ⢠A check on rutting resistance using the flow number test. The standard practice for measuring performance proper- ties for WMA describes how to prepare WMA performance test specimens and conduct dynamic modulus and low- temperature creep compliance and strength tests to obtain material properties for analysis using the MEPDG (1). The sections that follow describe specific conclusions from the research completed in NCHRP Project 09-43. 4.1.1 Volumetric Properties A major conclusion drawn from the research conducted under NCHRP Project 09-43 was that the volumetric prop- erties of properly designed WMA and HMA mixtures are very similar. For HMA mixtures with 1.0-percent binder absorp- tion or less, the volumetric properties of WMA designed with the procedures developed in NCHRP Project 09-43 were essentially the same as those obtained from an HMA design. This conclusion supports the current practice of substitut- ing a WMA process into an approved HMA mixture design. C H A P T E R 4 Conclusions and Recommendations
61 However, the compactability, moisture sensitivity, and rut- ting resistance of the WMA may be significantly different than those of the HMA. Each of these is evaluated directly in the recommended WMA mixture design method. 4.1.2 Binder Grade Selection The same grade of binder should be used in WMA and HMA mixtures designed for the same project location. Although the RTFOT experiment that was conducted in Phase I of NCHRP Project 09-43 showed a significant effect of temperature on the high-temperature grade of the binder, recovered binder test data from projects sampled and tested in Phase II of the proj- ect indicated that only extremely low production tempera- tures resulted in a significant decrease in the stiffness of the recovered binder from the mixture. WMA production tem- perature showed a minor improvement in the low-tempera- ture grade of binders in both the RTFOT experiment and the recovered binder testing. The draft appendix to AASHTO R 35, 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 temperatures to meet the flow number rutting resistance requirements included in the draft appendix. 4.1.3 RAP in WMA RAP and new binders do mix at WMA process tempera- tures 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 mix- ture 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 com- paction temperature exceeded the high-temperature grade of the âas recoveredâ RAP binder. To ensure good mixing of RAP and new binders, the draft appendix to AASHTO R 35 recommends that the planned field compaction tempera- ture for WMA exceed the high-temperature grade of the âas recoveredâ RAP binder. 4.1.4 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 construction. For WMA, it is appropriate to use 2 h of oven conditioning at the planned field compaction tempera- ture, 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 testingâ4 h at 275°F (135°C). 4.1.5 Coating, Workability, and Compactability For the wide range of WMA processes available, viscosity- based mixing and compaction temperatures cannot be used to control coating, 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 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 54°F (30°C) to characterize the compaction temperature sensitiv- ity of the WMA processes. Increases that exceed 25 percent indicate that the WMA is more temperature sensitive than HMA. This measure of compactability is sensitive to the com- paction temperature, the WMA process, and the presence of RAP in the mixture. The combination of RAP and low WMA production and compaction temperatures may lead to WMA mixtures that are more sensitive to changes in temperature than similar HMA mixtures. 4.1.6 Moisture Sensitivity Moisture sensitivity as measured by AASHTO T 283 will likely be different 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 per- cent 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
62 the mixtures. The draft appendix to AASHTO R 35 includes evaluation of moisture sensitivity using AASHTO T 283. 4.1.7 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 specimens that have been short-term conditioned for 2 h at the planned field compaction temper- ature to simulate the binder absorption and stiffening that occurs during construction. Because lower short-term condi- tioning temperatures are used for WMA mixtures than are used for HMA mixtures, binder aging in WMA mixtures is less, resulting in lower flow numbers for WMA mixtures pro- duced with the same aggregates and binder. Current criteria for the flow number and other rutting tests for HMA are based on 4 h of short-term conditioning at 275°F (135°C). The short-term conditioning study completed in NCHRP Project 09-43 shows that this level of conditioning represents 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 temper- ature, the criteria for evaluating the rutting resistance of WMA mixtures were changed from those currently recom- mended for WMA (conditioned for 4 h at 275°F [135°C]). Based on an analysis of data from NCHRP Project 09-13, it appears feasible that WMA can reach approximately the same level of binder stiffening that occurs in 4 h at 275°F (135°C) by using a two-step aging process: (1) 2 h of conditioning at the compaction temperature to simulate construction effects and (2) extended loose mix conditioning at a representative high in-service pavement temperature to represent early in- service aging. The duration of the extended conditioning will likely be less than 16 h. 4.1.8 Performance Evaluation The research completed under NCHRP Project 09-43 has shown that for the same aggregates and binders, WMA mix- tures designed in accordance with the draft appendix to AASHTO R 35 will have similar properties to HMA mix- tures. Volumetric properties will essentially be the same, but the stiffness of the WMA mixture will probably be lower for as-constructed conditions. Since the differences between HMA and WMA are relatively small, an analysis of the per- formance of pavements constructed with WMA can be made using the MEPDG and appropriate material properties (1). A draft standard practice for fabricating WMA test speci- mens and performing dynamic modulus master curves and low-temperature creep compliance and strength testing was developed to aid in the performance analysis of WMA using the MEPDG. 4.2 Recommendations The research conducted under NCHRP Project 09-43 has shown that only minor changes to current mixture design practice are needed to design WMA mixtures. Although volu- metric properties 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 mixtures will likely be different than HMA mixtures designed with the same aggregates and binders. Therefore, it is recommended that the procedures for WMA mixture design developed under NCHRP Project 09-43 be used when designing WMA mix- tures. For the mixtures studied under NCHRP Project 09-43, compactability was sensitive to the WMA process and temper- ature, particularly for mixtures incorporating RAP. The com- bination of low WMA temperatures and RAP yielded mixtures with compactability that was more temperature sensitive than HMA mixtures. Moisture sensitivity as measured by AASHTO T 283 will likely be lower for WMA mixtures than HMA mix- tures unless the WMA process includes an anti-strip additive. Finally, very low WMA temperatures may lead to mixtures with inadequate rutting resistance. All of these issues can be evaluated using the methods included in the draft appendix to AASHTO R 35. To aid in the implementation of this recommendation, a draft appendix to AASHTO R 35 titled Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA), was developed to address differences between the design of WMA and HMA. This appendix covers the following: ⢠WMA Process Selection, ⢠Binder Grade Selection, ⢠RAP in WMA, ⢠Process-Specific Specimen-Fabrication Procedures, ⢠Evaluation of Coating, ⢠Evaluation of Compactability, ⢠Evaluation of Moisture Sensitivity, ⢠Evaluation of Rutting Resistance, and ⢠Adjusting the Mixture to Meet Specification Requirements. The draft appendix should be used on a trial basis by agen- cies and producers 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 procedures and the criteria for coating, compactability, and rutting resistance. Additionally, agencies and producers should encourage the manufacturers of plant foaming equipment to develop labo- ratory foaming equipment that can be used to design foamed asphalt WMA mixtures in the laboratory. The laboratory foaming equipment that was used in NCHRP Project 09-43
63 was designed for preparing laboratory samples of foamed sta- bilized bases, not WMA. Although it is feasible to design WMA mixtures for plant foaming processes using this equipment, devices specifically designed to replicate the WMA foaming process and produce the smaller quantities of foamed asphalt used in mix design batches without extensive cleaning are needed to make the design process efficient. At the time that NCHRP Project 09-43 was completed, three additional projects on WMA were initiated by NCHRP: (1) NCHRP 09-47A, âEngineering Properties, Emissions, and Field Performance of Warm Mix Asphalt Technologies,â (2) NCHRP 09-49, âPerformance of WMA Technologies: Stage IâMoisture Susceptibility,â and (3) NCHRP Project 09-49A, âPerformance of WMA Technologies: Stage IIâLong-Term Field Performance.â NCHRP Projects 09-47A and 09-49A will include an evaluation of the field performance of WMA mix- tures, and NCHRP Project 09-49 will address the moisture susceptibility 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 additional research that is not cur- rently planned. First, mixing procedures for laboratory mix- tures have not been standardized. For design of HMA, mixing can be done manually or with a mechanical mixer. Two types of mechanical mixers are available: planetary mixers and bucket mixers. To use coating of laboratory mixtures as a design criterion, a mechanical mixer must be used, and the mixing process must be standardized. Coating evaluations performed during NCHRP Project 09-43 indicate that there is a significant difference in the efficiency of planetary and bucket mixers. The mixing times included in the draft appen- dix to AASHTO R 35 are based on a planetary mixer with a wire whip. Since bucket mixers are probably more readily available in most production mix design laboratories, addi- tional mixing studies should be conducted to establish mix- ing times for WMA specimen fabrication for bucket mixers. The draft appendix to AASHTO R 35 should then be modi- fied to include mixing times for bucket mixers. The second element of WMA mix design that requires additional research is the development of a short-term con- ditioning procedure that is applicable to both WMA and HMA for the specimens used to evaluate moisture sensitivity and rutting resistance. Research completed under NCHRP Project 09-43 concluded that 2 h of oven conditioning at the field compaction temperature reasonably reproduces the binder absorption and stiffening that occurs during construc- tion for both WMA and HMA mixtures. WMA mixtures that are conditioned 2 h at the field compaction temperature have binder that is less stiff than similarly conditioned HMA mix- tures because of the lower conditioning temperature. Current criteria for evaluating moisture sensitivity and rutting resis- tance are based on mixtures that have been aged to a greater degree. The conditioning originally specified in AASHTO T 283 for moisture sensitivity testing was 16 h at 140°F (60°C). Additionally, most rutting criteria are based on 4 h of condi- tioning at 275°F (135°C). Under NCHRP Project 09-13, mix- tures were conditioned for 2 h at 275°F (135°C), 4 h at 275°F (135°C), and 16 h at 140°F (60°C). From analysis of this data, the NCHRP Project 09-43 research team concluded that 16 h at 140°F (60°C) resulted in somewhat more aging than 4 h at 275°F (135°C). The difference in aging between 2 h and 4 h at 275°F (135°C) was not statistically significant. To simu- late both WMA and HMA, a two-step conditioning process should be considered for specimens used to evaluate mois- ture sensitivity and rutting resistance. In the first step, the mixture would be conditioned for 2 h at the field 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 sim- ulate a short period of time in service. Only specimens used to evaluate moisture sensitivity and rutting resistance would receive the second conditioning step. Volumetric design would be based only on the first step. The temperature and duration of the extended conditioning would be selected based on tem- peratures from LTPPBind and typical laboratory working hours. Most likely, the second step would require conditioning specimens overnight. The extended conditioning temperature and time would be selected so that HMA conditioned using the two-step process would have a similar stiffness to HMA condi- tioned for 4 h at 275°F (135°C).