Hot mix asphalt (HMA) is produced at temperatures between 280° and 320°F. These temperatures ensure that the aggregate is dry, the asphalt binder coats the aggregate, and the HMA mix has a suitable workability. HMA needs to be workable so it can be transferred into storage silos, transported, placed, and compacted. Even higher temperatures are used for HMA mixtures containing polymer-modified asphalt binders and crumb rubber asphalt binders.
Technology is now available to decrease HMA production temperature by 30° to over 100°F. These relatively new processes and products use various physicochemical means to reduce the shear resistance of the HMA at construction temperatures while reportedly maintaining or improving pavement performance.
While the current state of the practice for producing HMA has been shown to comply with existing environmental, health and safety standards, reducing HMA production and placement temperatures will provide several benefits, including reduced emissions, fumes, and odors, and a cooler work environment. An energy savings from lower production temperatures is evident with the use of warm mix asphalt (WMA) technology.
The quality of the HMA construction and performance may also be improved when production temperatures are lower. Workability improvements may result in higher in-place density. This decrease in in-place air voids decreases the permeability of the HMA and the long-term or in-service hardening of the asphalt binder as well as reducing water damage that can occur in the HMA.
Workability improvements also have the potential to extend the construction season and the time available for placement of the asphalt mixture during a given day. Due to enhanced workability of the HMA, it may be placed under cooler weather conditions.
A significant amount of asphalt binder aging occurs during the mixing and placing of HMA. Lower production temperatures for asphalt paving mixtures will decrease the aging of the asphalt binder during production. This decrease in aging can improve thermal and fatigue cracking resistance.
The use of WMA technology has some potential engineering challenges. Since the asphalt binders may not harden as much at the lower production temperatures, a softer binder will likely be in the HMA mixtures when the pavement is opened to traffic and the mixture may have a greater potential for rutting. In addition, traffic may not be allowed on the pavement at the conclusion of the compaction process until the mixture cools beyond what is normally required for conventional HMA. Because binders may be softer and some WMA technologies use water as a workability aid, WMA may be more susceptible to moisture damage. The relationship among engineering properties of such mixes and field performance needs to be investigated to facilitate the implementation of this technology.
The objectives of this project were to (1) establish relationships among engineering properties of WMA binders and mixes and the field performance of pavements constructed with WMA technologies, (2) determine relative measures of performance between WMA and conventional HMA pavements, (3) compare production and laydown practices and costs between WMA and HMA pavements, and (4) provide relative emissions measurement of WMA technologies as compared to conventional HMA technologies. Project deliverables shall include (1) recommended modifications to the preliminary WMA mix design and analysis procedure under development in NCHRP Project 9-43, (2) a protocol for laboratory evaluation of WMA performance, (3) guidelines for WMA production and construction, and (4) an updated emissions measurement protocol.