Because of its many benefits, warm-mix asphalt (WMA) is becoming increasing popular with producers, paving crews and specifying agencies.
The goal with WMA is to produce mixtures with strength, durability and performance characteristics similar to or better than hot-mix asphalt (HMA) using lower production temperatures. There are important environmental and health benefits associated with lower production temperatures, including lower greenhouse-gas emissions, lower fuel consumption and reduced exposure of workers to asphalt fumes. Lower production temperatures also can potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction and allowing improved compaction during cold-weather paving.
Advanced Asphalt Technologies LLC has just completed a major new guidance document developed with funding from the National Cooperative Highway Research Program (NCHRP) Project 9-43. The publication, titled “Mix Design Practices for Warm Mix Asphalt,” will assist agencies and asphalt-pavement producers in designing and testing WMA mixtures.
The design of WMA requires some changes to the current HMA mix-design practice, AASHTO R35, Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt (HMA). NCHRP Project 9-43 was conducted to develop mixture design procedures that are applicable to the 20 or so warm-mix processes currently available in the U.S. The research and development work completed in NCHRP Project 9-43 included:
Development of a preliminary mixture design procedure based on a review of the literature and research in progress;
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 reclaimed 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;
Revisions to the preliminary procedure based on the findings of the first phase of testing and analysis;
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 properties 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; and
Final revision of the preliminary procedure based on the findings of the second phase of testing and analysis.
Dissecting an appendix
The primary product of NCHRP Project 9-43 is a recommended appendix to AASHTO R35 titled “Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA).” Training materials and a commentary for the recommended appendix for WMA design also were developed to aid in implementing the research conducted in NCHRP Project 9-43. The following highlights key elements of the recommended appendix for WMA design:
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 during NCHRP Project 9-43 indicated that only extremely low production 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 WMA design appendix, therefore, recommends that the same grade of binder be used in both WMA and HMA mixtures;
Volumetric criteria: The same compaction levels and volumetric design criteria that are used in the design of HMA should be used with WMA;
RAP in WMA: RAP and new binders do mix at WMA process temperatures, provided that 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, and then, during storage at elevated temperatures, the two binders continue to mix. In the laboratory mixing studies that were conducted in NCHRP 9-43, two hours 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 WMA design appendix recommends that the planned field-compaction temperature for WMA exceed the high-temperature grade of the “as-recovered” RAP binder. The high-temperature grade of RAP ranges from about PG 82 in northern areas to PG 100 in the Southwest, resulting in corresponding minimum WMA field compaction temperatures ranging from 180°F to 212°F;
Process specific specimen fabrication procedures: The wide range of WMA processes currently available requires special specimen fabrication procedures to be used during laboratory mixture design. The draft WMA design appendix includes specimen fabrication procedures for four generic WMA process types: (1) additives added to the binder, (2) additives added to the mixture, (3) wet aggregate mixtures and (4) foamed-asphalt mixtures. These procedures describe how to perform laboratory mixing. Short-term oven conditioning and compaction are the same as currently used with HMA: two hours of conditioning at the planned field compaction temperature, and compaction to Ndesign gyrations in a Superpave gyratory compactor;
Coating, workability and compactability: For the wide range of WMA processes that are currently available, viscosity-based mixing and compaction temperatures cannot be used to control coating, workability and compactability.
The draft WMA design appendix uses direct measures of coating and compactability on laboratory-prepared specimens. Coating is measured on laboratory-prepared mixtures using AASHTO T 195, Standard Method of Test for Determining Degree of Particle Coating of Bituminous-Aggregate Mixtures. Compactability is evaluated by measuring the change in the number of gyrations to 92% relative density when the compaction temperature is decreased by 54°F. Increases that exceed 25% indicate that the WMA is more temperature-sensitive than typical HMA; and
Moisture sensitivity and rutting resistance: The draft WMA design appendix includes evaluation of the moisture sensitivity and rutting resistance of the mixture using two tests developed for HMA: (1) AASHTO T 283, Standard Method of Test for Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced Damage, for moisture sensitivity and (2) the flow-number test from AASHTO TP 79, Provisional Standard Method for Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester, for rutting resistance. The moisture-sensitivity criteria are the same as those for HMA; the flow number criteria are different accounting for the reduced aging that occurs in WMA.
Hot and warm
It is expected that much of the early use of the draft WMA design appendix will be to adapt current HMA mixture designs to one or more WMA processes. NCHRP Project 9-43 included a mix-design study where six different HMA mixtures were redesigned as WMA for three different WMA processes using the procedures included in the draft WMA design appendix. The following describes differences between the HMA and WMA designs:
Optimum binder content: For HMA mixtures with 1% binder absorption or less, the optimum binder content of WMA designed using the WMA mixture-design appendix was essentially the same as that obtained from an HMA design. Binder absorption is less for WMA than for HMA. For mixtures with greater than 1% binder absorption, there may be some difference in the optimum binder content, and the difference will be greater for mixtures with higher binder absorption;
Compactability: WMA mixtures, at their lower compaction temperatures, generally exhibited compactability similar to HMA. The exception was one WMA process when 25% RAP was included. The WMA Process B mixtures with RAP are less compactable than HMA; the other mixtures have compactability similar to HMA;
Moisture sensitivity: Like HMA, moisture sensitivity is material- and proc-ess-specific. WMA Process B included an antistrip additive, and there was no significant difference in the resistance to moisture damage as measured by AASHTO T 283 for WMA mixtures produced with this process and corresponding HMA mixtures. For the other processes, which did not include antistrip additives, the resistance to moisture damage was generally poorer; and
Rutting resistance: WMA mixtures generally have lower flow numbers than HMA mixtures made with the same aggregates and binder. The lower flow numbers are the result of less aging in WMA than in HMA. This has been accounted for in the criteria for the flow-number test that are included in the draft WMA design appendix.
At the time that NCHRP Project 9-43 was completed, three additional projects on WMA were initiated by NCHRP: (1) NCHRP 9-47A, Properties and Performance of Warm Mix Asphalt Technologies, (2) NCHRP 9-49, Performance of WMA Technologies: Stage I—Moisture Susceptibility, and (3) NCHRP Project 9-49A, Performance of WMA Technologies: Stage II—Long-Term Field Performance. NCHRP Projects 9-47A and 9-49A will include an evaluation of the field performance of WMA mixtures, and NCHRP Project 9-49 will address the moisture susceptibility of WMA in detail. The findings of NCHRP Project 9-43 support the need for these studies addressing field performance and moisture sensitivity. AT