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10 suppliers, agencies should have a procedure to ensure that the 2.3 Preliminary WMA Mixture recommended dosage rate is appropriate. Design and Analysis Procedure 2.3.1 Overview 2.2.2 Design Aggregate Structure Based on a review of available literature for the various The design of the aggregate structure may also require some WMA processes and discussions with WMA process develop- modifications for WMA. Since the goal of WMA is to produce ers, a preliminary mixture design and analysis procedure was mixtures with strength and performance characteristics simi- developed. The preliminary procedure served two purposes. lar to those of HMA, the volumetric criteria used in design First, the preliminary procedure provided a starting point for should not differ from those used for HMA. However, the pro- the WMA mixture design and analysis procedure for review cedures used to fabricate and condition specimens may require and comment by the project panel and WMA process develop- some modification. Most WMA process developers have ers. Second, the preliminary procedure focused the Phase I prepared laboratory procedures for specimen fabrication. testing and analysis effort on the areas of mixture design and Additionally, mixture coating, workability, and compactabil- analysis that required additional development to properly ity must be evaluated directly instead of using viscosity- address WMA. The preliminary procedure was revised based based mixing and compaction temperatures. In many WMA on the findings of the Phase I testing and analysis and the com- processes, it is impossible to directly measure the viscosity of ments received on the preliminary procedure. The revised pre- the binder. Additionally, there is increasing evidence that the liminary procedure was further modified based on the findings temperature reductions associated with many WMA processes of the Phase II laboratory mix design study, field validation are not related to the change in viscosity of the binder (3, 7). study, and fatigue study to produce the draft standards that were the primary products of NCHRP Project 09-43. 2.2.3 Design Binder Content Selection The preliminary WMA mixture design and analysis proce- dure was based on AASHTO R 35, Standard Practice for Super- The selection of the design binder content should not require pave Volumetric Design for Hot Mix Asphalt (HMA). The substantial modification other than specimen-fabrication as preliminary procedure referred to AASHTO M 323, Stan- discussed above. In NCHRP Project 09-25, "Requirements dard Specification for Superpave Volumetric Mix Design, and for Voids in Mineral Aggregate for Superpave Mixtures" and AASHTO M 320, Standard Specification for Performance- NCHRP Project 09-31, "Air Void Requirements for Superpave Graded Asphalt Binder, for criteria for materials selection, vol- Mix Design," relationships between mixture volumetric prop- umetric design, and moisture sensitivity evaluation. Table 3 erties and pavement performance were developed (8). These summarizes the areas where the preliminary procedure dif- relationships confirm the importance of many of the volumet- fered from AASHTO R 35. The differences are discussed below. ric criteria included in the Superpave mixture design method. An important step in achieving WMA with performance char- 2.3.2 WMA Process Selection acteristics comparable to HMA is to use the same volumetric criteria in the design of both mixtures. 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- 2.2.4 Evaluate Moisture Sensitivity tation with the specifying agency and technical assistance per- and Performance Analysis sonnel from WMA process suppliers. This section alerts users Evaluation of the mixture for moisture sensitivity and per- that when selecting a WMA process, consideration should be formance also will not require substantial modification other given to a number of factors including (1) available perfor- than specimen fabrication. Although there is concern that some mance data, (2) the cost of any warm mix additives, (3) planned WMA may exhibit greater moisture sensitivity than HMA plant mixing and field compaction temperatures, (4) planned (9, 10, 11), AASHTO T 283, Resistance of Compacted Hot Mix production rates, (5) plant capabilities, and (6) modifications Asphalt (HMA) to Moisture-Induced Damage, is a fairly reli- required to successfully use the WMA process with available able indicator of moisture-induced adhesive failure, which field and laboratory equipment. is the mechanism of greatest concern for WMA. The major consideration in the preparation of moisture sensitivity and 2.3.3 Binder Grade Selection and RAP performance specimens will be replicating the mechanical properties of field-mixed material in laboratory-prepared spec- For the preliminary procedure, it was hypothesized that the imens. The same tests and criteria that are used for performance WMA production temperature would be a consideration in the evaluation of HMA should be used with WMA. selection of the high-temperature binder grade and the allow-
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11 Table 3. Major new elements included in the preliminary WMA mixture design and analysis procedure. Key Element Included in Preliminary Procedure Key considerations for WMA process WMA Process Selection selection. Concept of high-temperature grade bumping Binder Grade Selection based on WMA production temperature. Concept of limiting RAP content based on Recycled Asphalt Pavement production temperature and compatibility of Materials new and recycled binders. Process-specific fabrication procedures Specimen-Fabrication Procedures provided for major WMA processes. Direct evaluation of coating, workability, and Process Temperature compactability. Flow number in addition to moisture Required Performance Testing sensitivity testing in mixture design. Recommended methods for measuring dynamic modulus, resistance to fatigue Optional Performance Testing cracking, and resistance to thermal cracking included in mixture analysis. able RAP content of the mixture. Because the lower produc- 2.3.5 Process Temperature tion temperatures in WMA result in reduced binder aging, the preliminary procedure provided a conceptual table for Since binder viscosity-temperature relationships cannot be bumping the high-temperature binder grade based on the developed for many WMA processes, mixing and compaction planned production temperature. Similarly, since the degree temperatures cannot be used to control coating, workability, of mixing of RAP and new binders in mixtures containing and compactability of WMA. The preliminary procedure RAP is likely to be temperature dependent, the preliminary proposed evaluating coating, workability, and compactability procedure provided a second conceptual table for limiting the directly during the evaluation of trial blends. This is accom- RAP content of mixtures based on the production tempera- plished by preparing trial blends using the planned production ture and the compatibility of the RAP and new binder. An temperature and compacting the trial blends using the planned appendix was added to provide procedures for measuring the field compaction temperature. Coating is evaluated using compatibility of two binders with ASTM D6703, Standard AASHTO T 195, Determining Degree of Particle Coating of Test Method for Automated Heithaus Titrimetry. Experi- Bituminous-Aggregate Mixtures. A standard procedure for ments to flesh out the conceptual tables for binder grade evaluating workability is not available; therefore, as part of the bumping and RAP mixing were included in the Phase I test- Phase I testing and analysis several possible workability tests ing and analysis. were evaluated. In the preliminary procedure for WMA, it was In addition to the above, the preliminary procedure pro- envisioned that the density at Ninitial in the gyratory compactor vided more detailed information on how to characterize RAP would serve as a measure of compactability. The use of the den- materials for mixture design. This information was provided sity at Ninitial as a measure of compactability was evaluated dur- in an appendix and was consistent with the recommendations ing the Phase I testing and analysis. for RAP analysis that were included in the mix design manual for HMA being developed under NCHRP Project 09-33 (6). 2.3.6 Required Performance Testing Evaluation of the moisture sensitivity of the design mixture 2.3.4 Specimen-Fabrication Procedures in the preliminary mixture design and analysis procedure The preliminary design procedure documented specimen- for WMA is the same as that for HMA in AASHTO R 35. fabrication procedures for several WMA processes. These pro- AASHTO T 283, Resistance of Compacted Hot Mix Asphalt cedures identify the equipment and methods that are needed (HMA) to Moisture-Induced Damage, is used except that the to prepare WMA specimens in the laboratory. These specimen- mixture conditioning procedure is the same as that used in the fabrication procedures were included in an appendix to the volumetric design, tentatively 2 h at the compaction temper- preliminary mixture design and analysis procedure for WMA. ature. The minimum tensile strength ratio is 0.80 as specified Short-term aging of WMA was tentatively set at 2 h at the in AASHTO M 323 for HMA. planned field compaction temperature based on limited The preliminary procedure for WMA mixture design and research performed by some WMA process developers. analysis also included a mandatory evaluation of the design