In summary, the term warm mix asphalt (WMA) refers to technologies and systems that allow for the substantial reduction in production and compaction temperatures of hot mix asphalt. The original intent of utilizing WMA was to provide better workability and compaction of asphalt mixtures at significantly lower temperatures. In addition, WMA was developed to reduce emissions and energy production usage and their associated production energy costs. Furthermore, the production and compaction at substantially lower temperatures can allow for longer mixture hauling distances/times and may prolong the paving season particularly in colder regions of the US and Canada. Ideally, an asphalt pavement that is easier to compact should also experience an extension in its in-service performance life in terms of all major asphalt distresses: rutting, fatigue, low temperature damage, thermal cracking, and moisture damage. It is well known that asphalt pavements compacted to proper densities often have superior fatigue and rutting performance. A thorough analysis of this can be found in detail in NCHRP Report 567, Volumetric Requirements for Superpave Mix Design (Christensen and Bonaquist, 2006).

Since its initial demonstration project at the annual World of Asphalt Trade Show and Conference in 2004, the use of WMA in the United States has ranged from 200 ton pilot projects to specifying 20,000 ton interstate projects. To date, the reported performance on these projects has been generally good with premature failures often being classified as construction issues or plant malfunctions. However, it has been consistently reported on a number of documented WMA projects that; 1) WMA often shows greater potential for rutting than conventional HMA when evaluated using conventional laboratory procedures and 2) WMA often shows greater potential for moisture damage than conventional HMA when evaluated using conventional laboratory procedures. These differences in performance may be explained by the lower production temperatures not oxidizing the asphalt binder resulting in a mixture with lower stiffness and lesser aggregate drying and possible creating a mixture more sensitive to stripping and rutting. Figure 1 and Table 1 provides an example of how the relative change in production temperature and initial aggregate moisture content can create these potential issues in a laboratory setting (Bennert et al., 2010).

A compounding issue to the influence of production temperature reduction and the possibility of residual aggregate moisture is the number of WMA technologies/processes currently on the market. According to the Federal Highway Administration (FHWA), there exists over twenty different WMA technologies/processes in North America, although they can generally be broken down into three distinct categories; 1) Organic/wax additives, 2) Chemical additives, and 3) Water-based foaming processes. Each one of these technologies/processes results in a slightly different modification to the final asphalt mixture. For example, the Sasobit wax will have a tendency to increase the high temperature PG grade, thus aid in rutting resistance. While the Rediset WMX and Evotherm 3G additives are surfactants containing anti-stripping agents to aid in reducing moisture damage potential. Therefore, for WMA to be successfully and faithfully implemented by federal, state, and local agencies, it is extremely important that a thorough and comprehensive acceptance testing program be evaluated and implemented to ensure WMA performs in similar manner to HMA with respect to rutting and moisture susceptibility properties.