Warm Mix Asphalt: European Practice

Chapter 7: Summary of Challenges and Recommendations

Throughout the scan, the team identified a number of challenges that need to be addressed as WMA is implemented in the United States. These challenges, enumerated throughout the report, are summarized below. Based on the findings of the scanning study and the challenges identified, the team developed a series of recommendations to help move WMA toward implementation in the United States.

Challenges

Based on the summary and the team's experience, the following represents the challenges that need to be addressed as the United States moves forward with WMA:

1. Make sure that the overall performance of WMA is truly as good as HMA. On a life-cycle basis, if WMA does not perform as well, there will not be long term environmental benefits or energy savings. A number of test sections have been constructed. Monitoring of these and future sections combined with laboratory performance testing should provide this information.
2. Address initial product approval. How do we sort out good innovative products from poor products? Accepted performance tests are needed to separate the good from the bad. The traditional practice of products and technologies being approved on a state-by-state basis needs to be changed. Products and technologies should be approved on a national, or at least a regional, basis.
3. Address issues with existing specifications that may prohibit the use of WMA. Examples include allowing blending of binders, minimum production and placement temperatures, and minimum ambient temperatures or cutoff dates.
4. Adapt WMA products and technologies from low-production batch and drum plants frequently used in Europe to higher production plants used in the United States. Trial projects have been completed in the United States with higher production plants using the appropriate modifications.
5. Coarse aggregate must be dry. Aggregates with low water absorption, less than 2 percent, are used to produce WMA in Europe. Aggregates with much higher water absorptions are used in parts of the United States. WMA processes must be adapted to produce dry aggregates in the mix. Best practices for drying and minimizing moisture in aggregates should be encouraged, including paving under stockpiles and in certain conditions covering stockpiles.
6. Individual contractors need to determine what products and technologies will work over the widest range of applications. In the past, agencies have mandated changes. In Europe, contractors have staffs who routinely conduct research to develop new products and processes. In the United States, contractors generally do not have these resources available in their own organizations. Such resources in the United States are generally found in research institutions and consultancies.

Recommendations

The United States has already made great strides in evaluating WMA. A WMA Technical Working Group (TWG) has been established to oversee the implementation of WMA. A large number of trial sections and demonstration projects have been completed. In some cases, WMA has been used in production paving. During the WMA scanning study, the team learned that a wide variety of techniques are used to produce WMA, wider than the team was previously aware. One key element the team observed was that production temperatures were generally higher than expected. In part, this was done to ensure that the coarse aggregate particles were dry.

Based on the team's experience, there are no long-term barriers to the use of WMA in the United States. Many elements of WMA still need to be investigated. The consensus among team members is that WMA is a viable technology and that U.S. agencies and the HMA industry should cooperatively pursue this path. Implementation goals include the following:

• WMA should be an acceptable alternative to HMA at the contractor's discretion, provided the WMA meets applicable HMA specifications.
• An approval system needs to be developed for new WMA technologies. The approval system must be based on performance testing and supplemented by field trials. The WMA TWG should lead the development of a performance-based evaluation plan for new WMA products. Realistically, such a system is needed for a broader range of modifiers and technologies used in HMA.
• The WMA scan team should provide technology transfer of the information gained through presentations, articles, and reports. An international workshop will be organized to promote WMA in 2008.
• Best practices need to be implemented for production handling and aggregate storage to minimize moisture content, burner adjustment, and WMA in general or specific technologies.
• More WMA field trials with higher traffic are needed. The field trials need to be large enough to allow a representative sample of the mixture to be produced. The trials should be built in conjunction with a control. The WMA Technical Working Group has developed guidelines that describe minimum test section requirements and data collection guidelines. The guidelines are at www.warmmixasphalt.com. Agency commitment is needed to monitor the project for a minimum of 3 years. More information on WMA, upcoming trials, and the WMA Technical Working Group is at www.fhwa.dot.gov/pavement/asphalt/wma.cfm.
• The factors affecting the economic viability of WMA need to be identified and tracked. Potential factors include additive costs, plant modifications, asphalt costs, fuel costs, costs of emission compliance equipment such as low-NO_x burners and fugitive emissions containment systems, and costs related to worker exposure.