By Dwight Walker, P.E.
As most everyone working with asphalt knows, warm mix asphalt (WMA) is a group of processes that allow a reduction in the temperature at which an asphalt mix is produced and placed.
A temperature reduction of 50¿F is fairly typical, with reductions up to 100¿F documented. Numerous advantages have been attributed to using WMA, including reduced mixing plant fuel consumption and decreased emissions. As a result, there has been tremendous interest in WMA.
WMA technologies use a variety of means to lowering the mixing and paving temperature. Some of the technologies are readily duplicated in the lab, while others present a challenge in dealing with mix designs and evaluations of WMA mixes. In an article published in the Fall 2010 issue of this magazine, John Duval and Bob Humer identified some of these challenges. This article describes progress in designing and evaluating warm mix asphalt.
Recent design experience
At the 2008 International Warm Mix Conference in Nashville, the recommended practice for designing warm mixes could be roughly described as, “Substitute a WMA technology into an existing HMA design.” A refinement to that practice suggested performing a mix design with the WMA technology at the anticipated production temperature. Some agencies required trial mixing runs and/or test sections to establish the actual production temperature and evaluate the coating and compactability or density.
NCHRP Project 09-43
It was recognized that a comprehensive lab design procedure was needed, and a major research effort was undertaken to develop mix design and analysis procedures which could be used for WMA. This research, NCHRP Project 09-43, “Mix Design Practices for Warm Mix Asphalt” was led by Dr. Ramon Bonaquist of Advanced Asphalt Technologies, LLC.
The goal of this project was to develop a mix design method that AASHTO could consider as a recommended practice. This method was to be based on the existing Superpave (AASHTO M323) procedure, to include performance tests and to be applicable to all WMA processes.
This work has been completed and documented in NCHRP Report 691, also titled “Mix Design Practices for Warm Mix Asphalt.” The report includes a draft appendix to AASHTO R35, “Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt (HMA).” The appendix is titled, “Special Mixture Considerations and Methods for Warm Mix Asphalt (WMA).” The report also includes a draft practice for measuring WMA properties, training materials, and a commentary to aid in implementing the 9-43 findings.
9-43 report findings
The report covers the following steps in the mix design process:
- volumetric properties
- binder grade selection
- RAP in WMA
- specimen conditioning
- coating, workability and compactability
- moisture sensitivity
- rutting resistance
- performance evaluation.
The lab studies of 9-43 found that a separate mix design procedure for WMA is not needed; WMA can be designed with only minor changes to the Superpave procedure.
The study found that, for mixes with less than 1 percent asphalt absorption, the volumetric properties of WMA mixes are typically very similar to those of HMA designs. The WMA mixes averaged 0.1 percent less binder than HMA mixes with the same materials.
However the compactability, susceptibility to moisture damage and rutting resistance of WMA may be significantly different from HMA. These properties were sensitive to the type of WMA process and the WMA process temperature. Procedures were developed to directly evaluate the differences in these properties.
The RAP work found that if the mix is held a sufficient length of time at elevated temperatures, the RAP and virgin binder do mix at WMA processing temperatures. To ensure mixing of the recycled and new binders, the report recommends that the field compaction temperature of the WMA exceed the high temperature grade of the ?as-recovered? RAP binder. To simulate the absorption and aging of the binder, a two-hour short-term aging period is included in the mix design process.
Additional procedural adjustments
Other procedural adjustments recommended by 9-43 include changes in specimen preparation procedures, and moisture sensitivity and rutting resistance evaluations.
Specimen preparation
The WMA mix design procedure uses process-specific fabrication procedures to simulate the actual production conditions. For plant-foaming systems, foamed asphalt had to be produced in the lab. When the 9-43 work was conducted, the Wirtgen WLB-10 foaming device was the only commercially available lab foamer. An additional manufacturer, Pavement Technology Inc., is now producing a lab foaming device, “The Foamer.”
The report recommends 2 hours of mixture conditioning at the planned field compaction temperature for volumetric design, moisture susceptibility testing, and flow number testing. It recommends process-specific procedures for fabricating specimens.
The study found that the type of lab mixer has a significant effect on the coating of the coarse aggregate particles. For example, the mixing time with a wire-whip planetary mixer may vary from that for a bucket mixer.
The study found that the compactability of WMA can be evaluated using the gyratory compactor to compare the number of gyrations required to reach 92 percent relative density at the planned field compaction temperature to the number of gyrations needed to reach 92 percent density at 54ºF (30ºC). A limit of a 25 percent increase in gyrations at the reduced temperature is recommended.
Moisture Sensitivity
The moisture sensitivity of WMA was evaluated in AASHTO T283, the procedure commonly used with HMA. The test is performed on specimens that have been conditioned for 2 hours at the compaction temperature. Moisture sensitivity from T283 may be different for WMA and HMA with the same aggregates and asphalt. The dry tensile strengths, conditioned (wet) tensile strengths and the tensile strength ratios of WMA was found to be lower than for HMA. The WMA process type affected the TSR. One WMA process, which included an antistripping additive, had no reduction in TSR.
Rutting resistance
The 691 report recommends evaluating rutting resistance by using the flow number, AASHTO TP 79, “Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt Using the Asphalt Mixture Performance Tester (AMPT).” Mixes were conditioned for 2 hours at the planned field compaction temperature before measuring the rutting resistance. The research found that the flow numbers of WMA are significantly lower (more susceptible to rutting) than for similar HMA mixes. The average reduction was about 40 percent and the rutting resistance was similar for all the WMA processes tested.
The study found, for WMA and HMA mixes using the same materials and having similar volumetric properties, the fatigue properties are similar.
Further studies
The author of the 691 report suggests that agencies use the recommendations of the 9-43 effort on a trial basis before full adoption. Some areas are identified as needing additional evaluation and/or refinement, including specimen fabrication procedures; criteria for coating, compactability and rutting resistance; and lab foaming equipment. Additional WMA-related research is currently underway. Research to further evaluate WMA performance includes:
- NCHRP 09-47, “Properties and Performance of Warm Mix Asphalt Technologies.” 9-47 began in March 2008 and Phase I has been completed.
- NCHRP 09-49, “Performance of Warm Mix Technologies: Stage I – Moisture Sensitivity.” Work is being done on Phase II.
- NCHRP 09-49A, “Performance of Warm Mix Technologies: Stage II – Long-term Field Performance.” Work is underway on Phase I.
Other research work is suggested. These potential studies include establishing standardized lab mixing procedures for specimen preparation and further development of the short-term conditioning of specimens for evaluating moisture sensitivity and rutting resistance.
All-in-all, the 9-43 study provides workable information on improving the WMA lab mix design procedure. It is a valuable document for agencies, producers, consultants and testing labs needing guidance on designing warm mix asphalt.