BACKGROUND

Traditionally, hot mix asphalt (HMA) is produced in either batch or drum plants at a discharge temperature of between 280° and 320°F. These temperatures are needed to dry the aggregate, coat it with the asphalt binder, and achieve the desired workability. Mix production temperatures are set according to local practice or AASHTO Test Procedure T 316. Higher temperatures are often associated with the use of polymer modified binders.

The HMA industry is embarking on a program to substantially reduce mix production temperatures. Reduced mix production and paving temperatures would (1) decrease the energy required to make HMA; (2) reduce emissions and odors from plants, and (3) improve the working conditions at the plant and paving site.

The term warm mix asphalt (WMA) refers to technologies, including various proprietary products and processes, that allow substantially reduced HMA mix production temperatures. Because these technologies were often originally intended to enhance compaction, they may also have positive impacts on HMA performance. Such technologies should make in-place density easier to achieve because they improve the workability of the mix. The majority of aging in an asphalt mixture takes place during mix production when it is exposed to elevated temperatures. By reducing mix production temperature, less oxidative hardening will take place, which should reduce the asphalt mixture's susceptibility to cracking. Reduced hardening during WMA production can increase its susceptibility to permanent deformation, but strategies such as the use of higher asphalt binder performance grades and stone matrix mixes can address this issue.

While the technologies available to reduce HMA mix production and paving temperatures offer promising results, much needs to be learned about mix design and performance testing of WMA mixes. Because of the variety of proprietary products and processes involved, the conventional methods of mix design may not be appropriate. For instance, establishment of laboratory mix and compaction temperatures would need to be investigated because of the reduced field temperatures. Because of these reduced temperatures, aging procedures in the laboratory need to be reviewed and adjusted, if necessary. The increased workability should result in easier compaction, and thus, the laboratory compaction effort might need to be refined.

OBJECTIVE

The objective of this research is to develop a mix design method for warm mix asphalt (WMA) in the form of a draft AASHTO-recommended practice for use by engineers and technicians in the public and private sectors. This method shall (1) be based on Superpave mix design methodology, (2) include a suite of performance tests to assess whether a WMA mix design will provide satisfactory field service, and (3) be applicable to any WMA technology used to lower mixing and compaction temperatures.

Accomplishment of the project objectives will require the following tasks.

TASKS

Phase I  (1.) Evaluate the current state of WMA mix design in the United States and worldwide through a critical review of (1) technical literature and research in progress, (2) relevant methods for mix design and performance testing, and (3) reports of WMA field and full-scale accelerated pavement tests. Coordinate with relevant FHWA and NCHRP projects to evaluate the applicability of methods for HMA mix design and performance testing to WMA.(2.) Based on the results of Task 1, develop a preliminary volumetric mix design method for WMA. Identify appropriate mixing and compaction temperatures; asphalt binder grade selection criteria; asphalt binder, aggregate, and mixture conditioning protocols; and laboratory compaction efforts. Assess the need to modify the HMA volumetric criteria in AASHTO M 323 for WMA. The design procedure must be applicable to the use of recycled asphalt pavement (RAP). Compare the volumetric properties of designs produced with the preliminary WMA methods with those prepared by current methods for several well-documented U.S. and European field projects using various WMA technologies. Investigate the cause of any significant differences in volumetric properties between designs produced by the two methods and revise the preliminary WMA method as needed. (3.) Identify a suite of performance-test methods and criteria to assess the durability and potential for rutting, fatigue cracking, thermal cracking, and moisture sensitivity of volumetric WMA mix designs produced by the method developed in Task 2. At a minimum, consider the test methods and criteria developed in NCHRP Projects 9-19, 9-25, 9-29, 9-31, 9-33, and 9-36 as well as loaded-wheel test methods reported in NCHRP Project 9-17 and NCAT Report 06-02. (4.) Based on the results of Tasks 1 through 3, submit within 9 months of the effective date of the contract an interim report presenting (1) a proposed mix design method for WMA in the form of a draft AASHTO-recommended practice and (2) a plan for a controlled laboratory experiment to evaluate the engineering reasonableness, reliability, sensitivity, and practicality of the proposed design method for WMA mixes prepared with a range of WMA technologies, binder performance grades, aggregate types and gradations, and RAP. The contractor will be expected to meet with the NCHRP panel approximately 1 month after submission of the report; work shall not begin on Phase II until NCHRP approval is received.

Phase II (5.) Conduct the laboratory experiment approved in Task 4. Based on its results, prepare a final WMA mix design method with satisfactory engineering reasonableness, reliability, sensitivity, and practicality. The method shall yield final mix designs based on both volumetric and performance testing criteria. 6.) Validate the WMA mix design method developed in Task 5 using materials and data from new and previously constructed field projects. (7.) Prepare a 1/2-day workshop on the features and use of the WMA mix design method intended for materials engineers from state DOTs and industry. Present the workshop to the NCHRP project panel and revise as directed. (8.) Prepare a final report that presents the results and summarizes the findings and recommendations of the project. In the report, present (1) the Task 5 WMA mix design method in the form of an AASHTO-recommended practice with reference to the necessary standard or provisional test methods, (2) course materials for the 1/2-day workshop, and (3) a CD-ROM with a database of all experimental results.

STATUS: Work is underway on Phase II.