Reduced Fuel and Energy Usage
Reports indicated that burner fuel savings with WMA typically range from 20 to 35 percent.
These levels could be higher if burner tuning was completed to allow the burner to run at lower settings.
Fuel savings could be higher (possibly 50 percent or more) with processes such as low-energy asphalt concrete (LEAB) and low-energy asphalt (LEA), in which the aggregates (or a portion of the aggregates) are not heated above the boiling point of water. It does not appear that any change in electrical usage to mix and move the material through the plant has been considered in the analysis of potential fuel savings. No specific study was referenced for the suggested fuel savings.
Although paving benefits may not have been a driving force in the development of WMA technologies, they may be particularly attractive to contractors and agencies. Several paving-related benefits were discussed, including the following:
- Ability to pave in cooler temperatures and still obtain density
- Ability to haul the mix longer distances and still have workability to place and compact
- Ability to compact mixture with less effort (assuming typical conditions, not cold weather or long haul)
- Ability to incorporate higher percentages of RAP
- Ability to place thick lifts and open to traffic in a short time period
Case studies were presented in Germany in which paving was completed with various technologies when ambient temperatures were between -3 and 4 °C. Base, binder, and an SMA surface course were placed in Germany.
Actual production temperatures for WMA mixes produced during cool weather vary, depending on the WMA technology, ambient conditions, and haul distance. In most cases the production temperatures will most likely be reduced compared to HMA produced under the same conditions. In some cases, the production temperatures may be closer to that of HMA.
Similar to the potential for extending the paving season using WMA technologies, longer hauls may be facilitated by the reduced rate of cooling of WMA and the reduced viscosity of WMA at lower temperatures. The Department also believes that WMA technologies can be used for longer hauls while maintaining workability.
Several studies provided data to show that the WMA technologies acted as compaction aids and reduced the required compactive effort. Some of the technologies were initially used for their stiffening effect at high in-service pavement temperatures. During this use it was observed that the materials reduced viscosity at compaction temperatures, particularly when compared to other types of modifiers.
Finally, WMA technologies can be used to facilitate deep patches, such as those placed when repaving the Frankfurt Airport. Sasobit was used in the repaving of Frankfurt Airport. Twenty-four inches of HMA were placed in a 7.5-hour window. The runway was then reopened to jet aircraft at a temperature of 85 °C (185 °F). This may have significant implications either for trench patching or when rehabilitation strategies require multiple lifts to be placed in the same night.
Reduced Worker Exposure
Enforcement of a new European Union regulation called Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) was implemented in June 2007. It requires chemical suppliers to provide information to workers on potential exposure and to set derived noneffect levels (DNEL).
Asphalt binders are included under these regulations. Research has shown a strong correlation between production temperatures and asphalt fume production. It is anticipated the DNEL levels will set asphalt application temperatures at less than 200 °C. While this is well above the temperature at which HMA is placed, particularly in the United States, it is lower than temperatures used for the production of mastic asphalt. Although mastic asphalt usage is relatively small, it is a technology that European agencies want to continue to specify. This seems to be a driving force toward WMA in areas where mastic asphalt is routinely used.
French, German, and Italian data were presented that indicated reduced worker exposure when placing WMA. Direct comparisons of measurements of fumes and aerosols are difficult since different testing protocols and sampling periods are used in different countries. It should be noted that all of the exposure data for HMA were below the acceptable exposure limits. Tests for asphalt aerosols/fumes and polycyclic aromatic hydrocarbons (PAHs) indicated significant reductions compared to HMA. Data presented by the Bitumen Forum appear to result in a 30 to 50 percent reduction.(9) Preliminary data from a forthcoming Italian study indicate even larger reductions.
In addition to reducing worker exposure, the lower mix temperatures also provide a more comfortable working environment. This may aid in worker retention. In Germany, one contractor also observed greater worker productivity when placing WMA compared to HMA.